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Influence of Activated Charcoal on Desorption Kinetics and Biodegradation of Phenanthrene in Soil
Abstract
The observed strong sorption of polycyclic aromatic hydrocarbons (PAHs) to black carbon (BC) presents potential implications for PAH bioaccessibility in soils. The effects of BC on the desorption kinetics and mineralisation of phenanthrene in four soils was investigated after 1, 25, 50 and 100d soil-PAH contact time, using sequential hydroxypropyl-β-cyclodextrin (HPCD) extractions in soils amended with 0, 0.1, 1 and 5% (dry wt. soil) activated charcoal (AC, a form of BC). The Rrapidly (%Frap) and slowly (%Fslow) desorbing phenanthrene fractions and their rate constants were determined using a first-order two-compartment (biphasic) desorption model. A minimum 7.8-fold decrease in %Frap occurred when AC was increased from 0-5%, with a corresponding increase in %Fslow. Desorption rate constants followed the progression krap(% h-1) > kslow(% h-1) and were in the order of 10-1 to 10-2 and 10-3 to 10-4, respectively. Linear regressions between %Frap and the fractions degraded by a phenanthrene inoculum (% Fmin) indicated that slopes did not approximate 1 at concentrations greater than 0% AC; %Fmin often exceeded %Frap, indicating a fraction of sorbed phenanthrene (%Fslow) remained microbially accessible. Therefore, sorption-HPCD-desorption kinetics alone may not be an adequate basis for the prediction of the bioaccessibility of PAHs to microorganisms and/or bioremediation potential in AC amended soils.
... There is a plethora of contemporary studies that have reported the mechanisms of biochar-microorganism interactions and ecotoxicological effects of biochar. However, the current state of knowledge on the multifunctional roles of biochar in supporting and altering microbial community structure under different biochar production conditions and physicochemical properties is inadequate Rhodes et al., 2012;Yu et al., 2020). As one of the first review articles of its kind, this paper will help to improve the existing knowledge base on the interactions of biochar with microorganisms, immobilization of contaminants on biochar through various interspecific reactions, variation in microbial colonization due to spatio-temporal heterogeneity of biochar surface, and transformation and biodegradation of contaminants as a result of the above interactions. ...
... There is relatively little information available on the impact of biochar on soil microorganisms that play key roles in organic pollutant degradation . Semple et al. (2013) and Rhodes et al. (2012) reported the role of microbial electron shuttling in the degradation of phenanthrene, PAHs and dioxins, where attachment of microbial cells to biochar surfaces through cross-linking, covalent bonding, and encapsulation took place. The total and bioavailable concentration of such contaminants depended on the recalcitrance and physical structure of biochar adsorbents, which also indirectly influenced the toxicity potential of HOCs. ...
... The water-soluble and rapidly desorbable phases of the contaminant would determine their bioavailability and eco-toxicity level to the microorganisms. On the contrary, the biochar-to-water distribution coefficient (K biochar ) of PAHs showed a higher value (≥400 times) than the soil C-to-water partition coefficient (K OC ) Rhodes et al., 2012;Yu et al., 2020). The bioavailability and mobility of HOCs and their fractional distribution from biochar might depend on the kinetics of desorption, combination of biotic and abiotic factors, and decomposition rate of biochar Ogbonnaya et al., 2014aOgbonnaya et al., , 2014b. ...
Numerous harmful chemicals are introduced every year in the environment through anthropogenic and geological activities raising global concerns of their ecotoxicological effects and decontamination strategies. Biochar technology has been recognized as an important pillar for recycling of biomass, thereby contributing to the carbon capture and bioenergy industries, and remediation of contaminated soil, sediments and water. This paper aims to critically review the application potential of biochar with a special focus on the synergistic and antagonistic effects on contaminant-degrading microorganisms in single and mixed-contaminated systems. Owing to the high specific surface area, porous structure, and compatible surface chemistry, biochar can support the proliferation and activity of contaminant-degrading microorganisms. A combination of biochar and microorganisms to remove a variety of contaminants has gained popularity in the recent years, other than traditional chemical and physical remediation technologies. The microbial compatibility of biochar can be improved by optimizing the surface parameters so that toxic pollutant release is minimized, biofilm formation is encouraged, and microbial populations are enhanced. Biocompatible biochar thus shows potential in the bioremediation of organic contaminants by harboring microbial populations, releasing contaminant-degrading enzymes, and protecting beneficial microorganisms from immediate toxicity of surrounding contaminants. This review recommends that biochar-microorganism co-deployment holds a great potential for the removal of contaminants thereby reducing the risk of organic contaminants to human and environmental health.
... The application of larger amounts (1.0% > 0.5%) of both biochar types reduced 14 C-mineralisation in soil. Even though biochar has been reported to have intrinsic ability to biodegrade organic contaminants (Anyika et al., 2015), the sorptive properties of black carbon, including biochars, can reduce mass transfer and bioaccessibility of PAHs to soil microorganisms (Rhodes et al., 2008(Rhodes et al., , 2012Anyika et al., 2015;Ogbonnaya et al., 2016). This further suggests the likely effect that is seen in this investigation occurs in soils with higher biochar amendments. ...
... It may be hypothesised that the phenanthrene may have been more strongly sorbed to the soil-biochar complex in the soils which received larger amendments of 0.5 and 1.0%. This may have caused a slower or irreversible desorption of the PAH or rapidly depleted desorbable fractions of the contaminant over time (50-100 d) for microbial uptake and biodegradation (Rhodes et al., 2008(Rhodes et al., , 2012. For example, Rhodes et al. (2012) noticed a 7.8-fold decrease in rapidly (%F rap ) and a corresponding increase in slowly (%F slow ) desorbing fractions of phenanthrene in soils receiving larger amount of black carbon (a form of pyrolysed carbon) and with increase soil contact time (1 d to 100 d). ...
... This may have caused a slower or irreversible desorption of the PAH or rapidly depleted desorbable fractions of the contaminant over time (50-100 d) for microbial uptake and biodegradation (Rhodes et al., 2008(Rhodes et al., , 2012. For example, Rhodes et al. (2012) noticed a 7.8-fold decrease in rapidly (%F rap ) and a corresponding increase in slowly (%F slow ) desorbing fractions of phenanthrene in soils receiving larger amount of black carbon (a form of pyrolysed carbon) and with increase soil contact time (1 d to 100 d). Further, Rhodes et al. (2012) found that there were 50% reductions in the rates of 14 C-phenanthrene mineralised with increasing amount of black carbon from 0 to 5% after 20 d soil-PAH contact time. ...
... After spiking and venting, soil was placed in sealed sterilized amber jars and incubated in the dark at 21 ± 1 • C in a controlled environment room until needed. 14 C-activity in the soil was assessed at the beginning of every time point through sample oxidation as described by Rhodes et al. (2012). In brief, soil samples (1 g dw, n = 3) were placed into cellulose combustion cones and combusted for 3 min using a Sample Oxidizer (Packard 307) previous addition of combustaid (200 μL). ...
... After 1, 21 and 42 days root-soil contact time, subsamples of the root amended and control soils were sampled for the assessment of mineralisation and bioaccessibility of 14 C-phenanthene using complete randomized arrangements. Additional subsamples were taken from the control soils at each time point for the measurement of the initial 14 C-activity through sample oxidation (Model 307, Packard) as described by Rhodes et al. (2012). ...
... An aliquot (5 ml) was then mixed with 15 ml liquid scintillation cocktail and assessed by LSC as previously described . Residual activity in the soil was assessed by sample oxidation as described by Rhodes et al. (2012). ...
Plants and their roots, in particular, may be important in stimulating microbial degradation of organic contaminants in soil. It is also known that bioaccessibility is important in controlling the biodegradation of organic contaminants in soil. This study aimed to assess the impact of plant roots on the biodegradation and bioaccessibility of 14C-phenanthrene in soil in the presence of plant root biomass. To do this, changes in the 14C-phenanthrene mineralisable and hydroxypropyl-β-cyclodextrin (HPCD) extractable fractions were evaluated. After 1, 25, 50 and 100 days of PAH-soil contact time, 12/14C-phenanthrene-spiked soil was amended with roots from two grasses and one legume. Mineralisation and HPCD extractability of 14C-phenanthrene were measured after 1, 21 and 42 days PAH-root-soil contact time. Findings revealed that mineralisation of 14C-phenanthrene was enhanced following the amendment with plant root biomass regardless their species or form, especially in the soils that been aged for ≥50 days soil-PAH contact time. Overall, this study showed that greater levels of phenanthrene biodegradation may be achieved by incorporating root biomass into the contaminated soil, particularly where indigenous catabolic activity has developed. The findings reported here contribute to the understanding of how plant root biomass may impact on contaminant bioaccessibility and stimulate the more effective indigenous biodegradation of hydrocarbons in soil.
... The application of larger amounts (1.0% > 0.5%) of both biochar types reduced 14 C-mineralisation in soil. Even though biochar has been reported to have intrinsic ability to biodegrade organic contaminants (Anyika et al., 2015), the sorptive properties of black carbon, including biochars, can reduce mass transfer and bioaccessibility of PAHs to soil microorganisms (Rhodes et al., 2008(Rhodes et al., , 2012Anyika et al., 2015;Ogbonnaya et al., 2016). This further suggests the likely effect that is seen in this investigation occurs in soils with higher biochar amendments. ...
... It may be hypothesised that the phenanthrene may have been more strongly sorbed to the soil-biochar complex in the soils which received larger amendments of 0.5 and 1.0%. This may have caused a slower or irreversible desorption of the PAH or rapidly depleted desorbable fractions of the contaminant over time (50-100 d) for microbial uptake and biodegradation (Rhodes et al., 2008(Rhodes et al., , 2012. For example, Rhodes et al. (2012) noticed a 7.8-fold decrease in rapidly (%F rap ) and a corresponding increase in slowly (%F slow ) desorbing fractions of phenanthrene in soils receiving larger amount of black carbon (a form of pyrolysed carbon) and with increase soil contact time (1 d to 100 d). ...
... This may have caused a slower or irreversible desorption of the PAH or rapidly depleted desorbable fractions of the contaminant over time (50-100 d) for microbial uptake and biodegradation (Rhodes et al., 2008(Rhodes et al., , 2012. For example, Rhodes et al. (2012) noticed a 7.8-fold decrease in rapidly (%F rap ) and a corresponding increase in slowly (%F slow ) desorbing fractions of phenanthrene in soils receiving larger amount of black carbon (a form of pyrolysed carbon) and with increase soil contact time (1 d to 100 d). Further, Rhodes et al. (2012) found that there were 50% reductions in the rates of 14 C-phenanthrene mineralised with increasing amount of black carbon from 0 to 5% after 20 d soil-PAH contact time. ...
Biochar is a by-product from the pyrolysis of biomass and has a great potential in soil amendment due to its carbon and nutrient-rich properties. The aim of this study was to investigate the impact of increasing amounts (0, 0.01, 0.1, 0.2, 0.5 and1.0%) of two types of biochar (so-called enhanced and non-enhanced) to soil on the biodegradation of14C-phenanthrene. Enhanced biochar contains inoculants which are designed to potentially stimulate microbial activity and promote biological function in soil. After 100 d of incubation, the addition of 0.5% and 1% enhanced (EbioC) and non-enhanced biochars (NEbioC) led to longer lag phases, reduced rates and extentsof14C-phenanthrene in amended soil. However, in soils amended with 0.01%, 0.1% and0.2% amendments, extents of mineralisation of14C-phenanthrene increased and were found to be higher in the EBioC — as compared to the NEbioC-amended soils. Increasing soil-phenanthrene contact time also increased14C-phenanthrene mineralisation in soil which had received smaller amounts of EBioC. Application of both EbioC and NEbioC also enriched the soil microbial populations during the incubation. However, it was found that phenanthrene-degrading microbial populations declined as soil contact time increased; this was particularly true for soils receiving larger amounts due to reduction in the mobile/bioaccessible fraction of the phenanthrene in soil. The findings revealed theimportance of the type and amount of biochar that may be added to soil to stimulate or enhance organic contaminant biodegradation.
... However, soils and sediments are very complex and heterogeneous matrices with a wide range of sorbent phases. Sorbent phases, such as black carbon, other carbonaceous materials and clay minerals, can also contribute to entrapping HOCs [3,4,11,[21][22][23][24][25]. Hence, the sole use of K oc to predict or explain the nature of sorption, HOC sorption, is rather too simplistic [26,27]. ...
... For instance, it has been shown that microbial mineralisation of phenanthrene in soil terminated because the amounts of readily available or desorbable phenathrene plateaued, regardless of the presence of catabolically active microbes and enough nutrients [46]. To understand contaminant release and bioavailability (and bioaccessibility), a clear understanding of how much of the sorbed contaminant can be desorbed from soils now or in the future [11,47,48], including associated rates and extents [24,33]. Non-exhaustive extractions (explained in detail later) of aged soils containing sorbed HOC residues and subsequent modelling of associated desorption behaviour have shown that desorption occurs in two or three phases [24,49,50], as depicted in Fig. 4. The first phase is rapid and describes the fast desorption of dissolved or weakly sorbed HOCs into surrounding soil pore water. ...
... To understand contaminant release and bioavailability (and bioaccessibility), a clear understanding of how much of the sorbed contaminant can be desorbed from soils now or in the future [11,47,48], including associated rates and extents [24,33]. Non-exhaustive extractions (explained in detail later) of aged soils containing sorbed HOC residues and subsequent modelling of associated desorption behaviour have shown that desorption occurs in two or three phases [24,49,50], as depicted in Fig. 4. The first phase is rapid and describes the fast desorption of dissolved or weakly sorbed HOCs into surrounding soil pore water. Because the weakly sorbed or rapidly desorbing fractions are readily bioavailable, they are likely to be considerably depleted, particularly in long-term contaminated soils, due to biotic and/or abiotic losses and extensive sequestration. ...
Many publications on contaminant bioavailability in soils often state that the use of total contaminant concentrations in risk assessment is an overly conservative approach. Such conservatism makes traditional risk assessment approaches and contaminated land decision-making expensive. The risk-based approach to contaminated land management strives to identify and manage the potential risks of significant harm being caused to humans and ecological receptors, following exposure to contaminated land. Risk-based approaches are more cost-effective than the traditional approaches from the perspective of contaminated land management. Contaminant bioavailability or bioaccessibility is one of the critical concepts that underpins risk-based approaches to contaminated land management. Bioavailability describes the fraction of the total contaminant concentration that desorbs from soil and is immediately available to cause harm to a living organism, after passing through the organism’s membrane. Bioaccessibility describes what is available and potentially available under natural environmental conditions and during realistic timeframes. The reliable measurements of either contaminant bioavailability or bioaccessibility is therefore critical; in this regard, a thorough understanding of contaminant sequestration and desorption behaviour is required. This chapter discusses the fate of HOCs in soils, bioavailability and bioaccessibility of organic contaminants and their associated desorption-based measurements.
... μm) inaccessible to microorganisms may reduce degradation of pollutants (Johnsen et al. 2005, Kołtowski et al. 2016, Ahmad et al. 2014. Several studies found decreased bioaccessibility of PAHs after biochar addition to soil due to effective sorption mechanisms (Rhodes et al. 2012, Sneath et al. 2013. In contrast, Marchal et al. (2013) observed a declining density of degrader bacteria over the course of their experiment, but found no inhibitory effects on biodegradation d e s p i t e s t r o n g s o r p t i o n o f P H E t o c h a r c o a l . ...
... Finally, microorganisms may have preferred easily available root exudates as a carbon source over the remaining 13 C-labeled PHE, especially if it was sorbed to soil amendments. Rhodes et al. (2012) reported competition for sorption sites preventing PAH sequestration to biochar in a soil rich in organic matter. Sorption to plant root surfaces could have reduced PAH availability (Meagher 2000) and led to slower degradation in the cultivated treatments compared to the unplanted control, although this effect was reported to be negligible elsewhere (Joner andLeyval 2003, Wick et al. 2011). ...
... Soil microorganism has access only to the water-dissolved fraction of PAHs (Ogbonnaya and Semple 2013). Therefore, adequate water supply is required to enable microbial degradation of PAHs, reducing the half-life of PAHs in soil (Rhodes et al. 2012). ...
Co-contaminations of soils with organic and inorganic pollutants are a frequent environmental problem. Due to their toxicity and recalcitrance, the heterogeneous pollutants may persist in soil. The hypothesis of this study was that degradation of polycyclic aromatic hydrocarbons (PAHs) is enhanced if heavy metals in soil are immobilized and their bioavailability reduced. For metal immobilization and enhanced biodegradation, distinct mineral and organic soil amendments (iron oxides, gravel sludge, biochar) were deployed in an incubation batch experiment. The second part of the experiment consisted of a greenhouse pot experiment applying fast-growing and pollution-tolerant woody plants (willow and black locust). Soil amendments initially immobilized NH4NO3-extractable zinc, cadmium, and lead; after 100 days of incubation, soil amendments showed reductions only for cadmium and a tendency to enhance arsenic mobility. In order to monitor the remediation success, a ¹³C-phenanthrene (PHE) label was applied. ¹³C-phospholipid fatty acid analysis (¹³C-PLFA) further enabled the identification of PHE-degrading soil microorganisms. Both experiments exhibited a similar PLFA profile. Gram-negative bacteria (esp. cy17:0, 16:1ω7 + 6, 18:1ω7c) were the most significant microbial group taking up ¹³C-PHE. Plants effectively increased the label uptake by gram-positive bacteria and increased the biomass of the fungal biomarker, although their contribution to the degradation process was minor. Plants tended to prolong PAH dissipation in soil; at the end of the experiment, however, all treatments showed equally low total PAH concentrations in soil. While black locust plants tended not to take up potentially toxic trace elements, willows accumulated them in their leaves. The results of this study show that the chosen treatments did not enhance the remediation of the experimental soil.
... The ability of biochar to sorb in a nonlinear fashion to hydrophobic organic contaminants (HOCs) is well established 1, 2 . Much of current research has emphasized the important role of carbonaceous materials in reducing contaminant aqueous availability and, consequently, biodegradation and/or bioaccumulation of sediment-bound contaminants, for the purpose of remediation [3][4][5] . Sopeña et al. 6 demonstrated how HOC desorption processes in the presence of biochar are intimately related to HOC biodegradation by the indigenous sediment/soil microbiota. ...
... Previous studies on the effects of biochar on the biodegradation of HOC focused mainly on HOC bioavailability [3][4][5] . For example, Rhodes et al. 5 found that activated carbon (AC) depressed the mineralization of phenanthrene ...
... Previous studies on the effects of biochar on the biodegradation of HOC focused mainly on HOC bioavailability [3][4][5] . For example, Rhodes et al. 5 found that activated carbon (AC) depressed the mineralization of phenanthrene ...
The observed strong sorption of hydrophobic organic contaminants (HOCs) to biochar presents potential implications for HOCs bioavailability and bioaccessibility in sediments, while biochar could impact sediment microbial ecology. However, the comprehensive study on the effects of biochar on HOC biodegradation coupled with bioavailability and microbial ecology are rarely documented. In this paper, the effects of biochar on the biodegradation of nonylphenol (NP) were investigated using 3 different NP concentrations (20, 50 and 500 mg/Kg) in sediments amended with different percentage of rice straw biochar (RC). Results showed that the influence of RC on NP biodegradation varied with different NP concentrations. At low NP concentrations, RC suppressed NP biodegradation by reducing NP bioavailability, while at high NP concentrations, moderate RC addition promoted biodegradation by reducing toxicity of NP to microbes. The effects of NP on microbial community structures were significant (P < 0.01), but those of RC were not significant (P > 0.05). The RC affected microorganisms through altering NP toxicity, microbial quantity and activity, but not microbial community structures. This study indicated that there could be an optimal biochar percentage in biochar-sediment systems at different HOC concentrations, which strengthened HOC biodegradation process and accelerated biodegradation rate, forming adsorption-biodegradation coupled bioremediation.
... Additionally, soil factors such as the dominant fine texture (clay-loam) or the presence of organic matter together with the aged diesel (Yu et al., 2018), could restrict the bioavailability of the hydrocarbon, thus limiting the role of the biochar in the process (Galitskaya et al., 2016;Zhang et al., 2010). The hydrocarbon degradation obtained with the biochar treatment could be improved by means of a greater diversity or abundance of the involved taxa (Aziz et al., 2020), and by increasing the bacterial colonization of the biochar and therefore a potential mineralization and/or desorption of hydrocarbons (Rhodes et al., 2012). In any case, a longer time than the 12 weeks would be necessary to confirm this assumption. ...
... The most efficient treatment in the degradation of the petroleum hydrocarbon was the combination of biochar and bioaugmentation in both soils A and B. In freshly polluted soil A, after the first six weeks, this treatment was more effective than the other treatments (Table 1, Fig. 2), suggesting a synergistic effect. This effect could be explained by the ability of the biochar to adsorb the hydrocarbon on its surface, favoring the adhesion of the consortium bacteria to the biochar, and thus facilitating the degradation of the hydrocarbon (Marchal et al., 2013a(Marchal et al., , 2013bRhodes et al., 2008Rhodes et al., , 2012. In this way, the weekly addition of the bacterial consortium to both soils could progressively improve their survival and their bioaccessibility to hydrocarbons, thus favoring the degradation of the pollutant. ...
In order to contribute to a minimum impact on soil biocenosis during the application of in-situ bioremediation techniques, this work assessed the efficiency of a scarcely used combination of biochar and a bioaugmentation based on an autochthonous bacterial consortium. Bioaugmentation-biochar combination was assessed by using soil samples from a polluted site with two pollution scenarios: a) soil with aged diesel, and b) clean soil to which fresh diesel was later added simulating a recent pollution event. The autochthonous consortium, isolated from the aged-diesel soil, was genetically, taxonomically and functionally characterized by these authors in a previous work. The biochar used was obtained from tree pruning residues. In both scenarios, four treatments were carried out under short-term test conditions: i) natural attenuation, ii) biochar, iii) bioaugmentation, and iv) biochar-bioaugmentation combination. Our results show that the bioaugmentation-biochar combination was significantly more effective than the simple treatments. This combination produced more than 20% diesel degradation in both scenarios over twelve weeks. Simultaneously, an increase in bacterial diversity was observed in that period. Therefore, using biochar combined with bioaugmentation suggests synergies that lead to a highly efficient and environmentally friendly bioremediation processes.
... Polycyclic Aromatic Hydrocarbons (PAHs) are hydrophobic compounds that are com- posed of two or more fused benzene rings and they are classified as carcinogenic and muta- genic (Lee, 2012;Schwarzenbach, Gschwend, and Imboden, 2005). Soil remediation technologies are usually constrained by PAHs low water solubility. ...
... Some studies on desorption kinetics of contaminants removal from soils have shown that they occurred in a rapid stage followed by a much slower stage (Barnier, Ouvrard, Robin, and Morel, 2014;Bezza and Nkhalambayausi-Chirwa, 2015;Fonseca, Pazos, Figueiredo, Tavares, and Sanroman, 2011;Rhodes, Riding, McAllister, Lee, and Semple, 2012). As a con- taminated soil ages, PAHs and heavy metals tend to move into deeper fraction of soil par- ticles. ...
Soils contaminated simultaneously with polycyclic aromatic hydrocarbons (PAHs) and heavy metals pose major threat to human health and environment by getting released from soil into water environment. The purpose of this study was to evaluate simultaneous desorption and desorption kinetics of PAHs (phenanthrene and anthracene) and heavy metals (lead, nickel, and zinc) from artificially contaminated kaolinite soils with different organic matter content. Batch desorption tests were conducted using single and combined enhancing agents containing Triton X-100 and Tween 80 as non-ionic surfactants, Ethylenediaminetetraacetic acid (EDTA) as a chelating agent, and citric acid as an organic acid. The solution with the highest removal efficiency was the combined solution of Triton X-100 (10% w/w) + EDTA (0.01 M). Removal levels around 92, 46, 92, 95, and 96% were obtained for phenanthrene, anthracene, lead, nickel, and zinc, respectively, by using this combination. Batch desorption kinetics experiments were performed using the mentioned combination. During the first 24 h, desorption kinetics were rapid, followed by a plateau until the end. The data obtained from desorption kinetics experiments were fitted with four kinetics models: pseudo-second-order equation, empirical power function, elovich, and parabolic diffusion. The correlation coefficient of the pseudo-second-order equation was higher than that of other functions. Moreover, batch experiments have showed inverse correlations between removal efficiency and organic matter content of soil.
... Previous studies investigated the bioavailable, bioaccessible and non-bioaccessible fractions of PAHs to microorganisms [8][9][10] and plants [11][12][13][14][15] in laboratory and field contaminated soils. Plant uptake of PAHs from soils varies significantly and is affected not only by the types of plants and physical-chemical conditions of soils but also by the physical and chemical properties of the contaminants. ...
... As the number of PAH rings increased, the affinity to humin increased and resulted in a decreasing affinity to FA. This was apparently due to increasing hydrophobicity, Koc value, the chemical structure of the sorbate, and presence of black carbon materials [10]. However, for 2-, 3-, and 4-benzene ring PAHs, the FA-bound PAHs were significantly higher than HA-bound PAHs in the study region (p < 0.05). ...
... Extents of 14 C-naphthalene mineralisation were consistently lower as the concentration of biochar amendments increased (0% > 0.1% > 0.5% > 1%). Rhodes et al. (2008aRhodes et al. ( , 2012, Marchal et al. (2013) and Ogbonnaya et al. (2014) confirmed that the addition of AC and biochar to soils reduced the extents of 14 C-PAH mineralisation through sorption and reduction of the PAH in aqueous phase. Similarly, biochar reduced extents of 14 C-naphthalene mineralisation and the reduction was more pronounced in the BioC2 amended soils; thus, the degree of sorption differs amongst biochar materials. ...
... BioC1 initially sustained rate of mineralisation but increasing biochar concentrations and contact time accompanied increases in lag phases and reductions in the rates and extents of biodegradation (Ogbonnaya and Semple, 2013). Reduction in extents of 14 C-naphthalene mineralisation with increase in soil-PAH contact time is in agreement with other related studies (Rhodes et al., 2008a;Allan et al., 2006;Ogbonnaya et al., 2014;Rhodes et al., 2010Rhodes et al., , 2012. Semple et al. (2007) clearly described bioavailability as a good descriptor of the rate of biodegradation of an organic contaminant; whilst bioaccessibility described the biodegradation end-point. ...
This study investigated the impact of two different wood biochars (BioC1 and BioC2) on the extractability and biodegradation of C-naphthalene in soil. Both biochars had contrasting properties due to difference in feedstocks and pyrolytic conditions (450–500 C and 900–1000 C, designated as BioC1 and BioC2, respectively). This study investigated effects of biochar on the relationship between C-naphthalene mineralisation and calcium chloride (CaCl ), hydroxypropyl- -cyclodextrin (HPCD) or methanol extraction in soil amended with 0%, 0.1%, 0.5% and 1% BioC1 and BioC2 after 1, 18, 36 and 72 d contact times. Total extents of C-naphthalene mineralisation and extraction were reduced with increasing concentrations of biochar; however, BioC2 showed greater sorptive capacity. Good linear correlation existed between total extents of C-naphthalene mineralisation and HPCD extractions in BioC1 (slope 0.86, r 0.92) and BioC2 (slope 0.86, r 0.94) amended soils. However CaCl and methanol extractions underestimated and overestimated extents of mineralisation, respectively. These results indicate that biochar can reduce the bioaccessibility of PAHs and the corresponding risk of exposure to biota, whilst HPCD extraction estimated the bioaccessible fraction of PAHs in soil. Bioaccessibility assessment is vital in evaluation of biodegradation potential and suitability of bioremediation as a remediation option.
... The amendment of soil with strongly sorbing carbonaceous materials as a low-cost in-situ remediation approach for soil contaminated with pollutants such as polycyclic aromatic hydrocarbons, polychlorinated biphenyls, and pesticides has been discussed (Marchal et al., 2013;Wang et al., 2014). Among these sorbents, biochar, a carbon-rich product obtained by heating biomass under oxygen-limited conditions (Kookana et al., 2011), has attracted scientific attention because of its very high sorption affinities for a wide range of organic pollutants, due to its large surface area and high microporosity, among other physicochemical properties (Chen et al., 2008a;Kookana et al., 2011;Rhodes et al., 2012). As a consequence it can reduce the mobility, reactivity, and toxicity of xenobiotics in soil (Kookana, 2010;Beesley et al., 2011;Kookana et al., 2011;Xu et al., 2012) and their bioavailability and bioaccessibility to soil organisms (Yang et al., 2010;Marchal et al., 2013;Semple et al., 2013;Wang et al., 2014), subsequently enhancing their persistence in soil (Yang et al., 2006;Rhodes et al., 2008;Wen et al., 2009). ...
... Mineralization of DCP in and Phe was significantly inhibited (P < 0.05) with increasing biochar application rate (Fig. 1), The inhibitory effects of the biochar on the transformation of DCP and Phe can be understood primarily as reflecting the strong sorption of these compounds and of their metabolites to the biochar (Table S3), in agreement with previous studies on Phe degradation (Rhodes et al., 2008(Rhodes et al., , 2012. In our study, biochar (without the presence of earthworms) had a markedly higher inhibitory effect on DCP mineralization than on Phe mineralization, with a relative reduction in mineralization of 89% and 55%, respectively (SI Fig. S1). ...
Little is known about the effects of biochar on the fate and behavior of micropollutants in soil, especially in the presence of soil macrofauna. Using a 14C-tracer, we studied the fate of 2,4-dichlorophenol and phenanthrene, after 30 days in soil in the presence of a biochar (0-5%, dry weight) produced from China fir at 400 °C and/or the earthworm Metaphire guillelmi. Application of the biochar significantly reduced the degradation and mineralization of both pollutants and strongly increased the accumulation of their metabolites in soil. The earthworm had no significant effects on the degradation of parent molecules of the pollutants but it significantly reduced the mineralization of the pollutants independent of the presence of the biochar. Although at an application rate of <1% the biochar strongly sorbed both pollutants, it did not significantly decrease the bioaccumulation of free dichlorophenol and phenanthrene and their metabolites by the earthworm. Our results demonstrate the complex effects of biochar on the fate, transformation, and earthworm bioaccumulation of organic pollutants in soil. They show that biochar application may not be an appropriate strategy for treating soil contaminated with hydrophobic organic pollutants and underline the importance of soil-feeding earthworms in risk assessments of biochar effects on soil remediation.
... Although a linear relationship was established between the residual contaminants after microbial degradation and the extracted contaminants, the fractional range of the best-fit slope varied widely. Due to the presence of steric hindrance and carbonaceous materials such as activated carbon, the prediction of bioaccessibility of HPCD extraction for highcyclic PAHs is insufficient (Rhodes et al. 2008;Rhodes et al. 2012). ...
More evidence shows that bioaccessibility instead of total concentrations based on exhaustive extraction methods can better reflect the actual risk level of petroleum hydrocarbon contaminated sites, so it is essential to establish an effective assessment method for bioaccessibility. This study utilized Tenax extraction, butanol extraction, hydroxypropyl-β-cyclodextrin (HPCD) extraction, and a composite extraction method involving HPCD with LMWOAs (citric acid, CA) and surfactants (rhamnolipid, RL; Tween80, TW80; sodium dodecyl sulfate, SDS) at varying concentrations. These methods were employed to predict the bioaccessibility of earthworms to soil at different aging time of petroleum hydrocarbons. The results showed that traditional extraction methods such as Tenax 6h extraction and n-butanol extraction were ineffective in evaluating petroleum hydrocarbons’ bioaccessibility. In contrast, the composite extraction of HPCD and solubilizer enhanced the extraction efficiency of HPCD greatly, and the extraction results showed a significant positive correlation with earthworm accumulation. By the comparison of the extraction results of different fractions of petroleum hydrocarbons, heavy fractions of petroleum hydrocarbons (C29-C40) are essential factors affecting chemical extraction effects. The correlation coefficients of four composite extraction methods and total petroleum hydrocarbons (TPH) of earthworm accumulation by linear regression analysis ranged from 1.1797 to 1.7990, and the slopes ranged from 0.8727 to 0.9792. Among them, the combined extraction method of 50 mmol/L HPCD and 0.5 mmol/L rhamnolipid had the best effect (r² = 0.9792, slope = 1.1797), which could be used as an evaluation method suitable for the bioaccessibility of petroleum hydrocarbons in soil. This study could provide a new method for evaluating the bioaccessibility of organic pollutants and technically supporting risk assessment and bioremediation of complex petroleum hydrocarbons in soil.
... And desorption can reflect the possibility of the re-release of organic pollutants from sorbent under actual environmental conditions, and thus can assess the releasing risk of organic pollutants effectively (Ortega-Calvo et al., 2015). Furthermore, the desorption behavior of organic pollutants from soil/sludge can be divided into three fractions: rapid, slow, and resistant desorption fraction (Rhodes et al., 2010(Rhodes et al., , 2012, where the slow and resistant desorption fractions may be related to the presence of hard carbon (such as black carbon, kerogen, HM), while the rapid desorption fraction refers to the weakly adsorbed part such as soft carbon (such as HA, FA) with a more sparse structure. It shows that the composition and content of OM affect the desorption behavior and bioavailability of organic pollutants in soil/sludge, as well as the correlation between sorption and desorption (Wang et al., Water Air Soil Pollut (2023) 234:242 Page 3 of 13 242 ...
Sludge composting has been widely used as one of the most economical and environmentally friendly methods to solve the waste sludge problem. However, the effects of compost on the releasing risk of organic pollutant in soil when sewage sludge for land use have rarely been investigated. In this study, pentachlorophenol (PCP), a pesticide with a high detection rate in soil, was chosen as the target contaminant, and the desorption kinetics of PCP from compost systems and different organic matters were conducted assisted by Tenax@TA. The results showed that the composting process could increase humic acid (HA) content and decreased fulvic acid (FA) and humin (HM) content. The rapid desorption fraction (Frap) of PCP from compost, HM+HA, and HM+HA+FA systems increased by 8.0%, 10.8%, and 13.8% while decreased by 12.9% and 17.2% in HM and HA systems. The adsorption capacity of compost systems determined the desorption capacity for PCP. And the pore filling and the interaction between oxygen-containing functional groups of OM and PCP were the dominant mechanisms controlling the desorption of PCP. Frap of PCP from compost systems can be represented by 12-h single point Tenax desorption amounts, which can be used to assess the bioavailability of PCP in compost systems. Sludge composting can control the releasing risk of organic pollutant in soil to some extent, which was beneficial to the land use of sewage sludge.
Statement of Novelty
The composted sludge can be widely used as an organic soil amendment due to its high organic matter (OM) contents and nutrients, and strong affinity of OMs for organic pollutants, which can reduce the releasing risk of organic pollutants in soil. However, there are seldom research exploring the effect of composted sludge and OMs on the releasing of organic pollutants, as well as the dominant mechanisms controlling the release of organic pollutants. Therefore, variation of organic matters during sludge composting process was analyzed and releasing risk of pentachlorophenol (PCP) from sludge compost systems was investigated. We found that HM in sludge could inhibit the desorption of PCP and this inhibitory effect was enhanced by the composting process. And the pore filling and the interaction between oxygen-containing functional groups of HM and PCP were the dominant mechanisms controlling the desorption of PCP. Consequently, sludge composting can control the releasing risk of organic pollutant in soil to some extent, which was beneficial to the land use of sewage sludge. The research was expected to provide a theoretical basis for the application of composted sludge to control the releasing risk of organic pollutants in soil.
Graphical Abstract
... aging effects) is also important and predicted bioaccessibility for historically contaminated soils is much reduced compared with recently contaminated and spiked soils (Khan et al. 2011;Kreitinger et al. 2007). Non-exhaustive extraction techniques such as HPCD (Cuypers et al. 2002;Doick et al. 2006) and potassium persulfate (KPS) oxidation (Cuypers et al. 2000;Juhasz, Waller, and Stewart 2005) have shown promising relationships for bioremediation endpoint prediction and PAH biodegradation, but are often tested on spiked soils that do not reflect the PAH behavior in naturally contaminated aged soils (Khan et al. 2011;Rhodes et al. 2012;Wang et al. 2019;Ye et al. 2019;Yu et al. 2016). In this regard, 5-6 ring PAH biodegradability in historically contaminated soils is significantly over-estimated using HPCD extraction and KPS oxidation (Cuypers et al. 2000(Cuypers et al. , 2002Juhasz, Waller, and Stewart 2005;Rostami and Juhasz 2013). ...
This study compared polycyclic aromatic hydrocarbon (PAH) extractable fractions determined using six in-vitro methods, typically used for bioaccessibility/bioavailability predictions, for manufactured gas plant (MGP) historically contaminated soils, and evaluated the influence of soil properties. Methods used included depletive approaches for the bioaccessible fraction including butanol extraction (BuOH), non-buffered and buffered hydroxypropyl-β-cyclodextrin extractions (HPCD, Buf-HPCD), potassium persulfate oxidation (KPS) and solid-phase extraction using Tenax resin (Tenax); and a non-depletive polyoxymethylene solid phase extraction (POM) approach to determine the freely dissolved fraction. The KPS extraction removed the most ∑16 PAHs (1.5–77.1 fold more than other methods), while POM extracted the smallest ∑16 PAHs (average of 3.9–77.1 fold less than the other 5 methods). Soil properties were generally inconclusive as indicators of extractable fraction for the historically contaminated soils. Neither total nor organic carbon were important but soil particle size was most influential. Toxicity values and biodegradation endpoints derived using bioaccessibility-biodegradability linear regression models on the extracted PAH fractions ranged widely for the different soils, and illustrated the high variability for predicted ecotoxicity and bioremediation outcomes in risk managment application. The results demonstrated the importance of validation with intended soil end use and receptors for in-vitro assessment of bioaccessibility and bioavailability, to fully inform regulatory decision-making and risk management.
... In addition, this could be linked to high sorptive capacity of organic materials for PAH and subsequent decrease in bioavailability of PAH for microbial degradation (Rhodes et al., 2008). More so, the degree of contaminant sorption and their rapidly/slowly desorbing fractions in amended soils are important factors that determine the extent of microbial sequestration and transformation (Rhodes et al., 2012). The decrease in the catabolic response by the higher amendment (2:1) agrees with previous studies on organic additives addition to PAHs contaminated soils (Namkoong et al., 2002;Semboung Lang et al., 2016). ...
This study investigated the impact of spent brewery grains and spent mushroom compost on the development of phenanthrene biodegradation in soil. Two aspects were considered: (i) the influence of increasing waste-to-soil ratios (1:10, 1:5, 1:2, 1:1 & 2:1) and (ii) the impact of soil-PAH contact time (1–100 d). Biodegradation was quantified by measuring changes in the lag phase, the fastest rates and extents of mineralization of ¹⁴C-phenanthrene, as well as changes in the number of total heterotrophic and phenanthrene degrading bacteria and fungi. The amendment of smaller amounts of the wastes (1:10 & 1:5) resulted in greatest levels of biodegradation. Microbial numbers increased in all of the amended soils but phenanthrene-degrading numbers in most amended soils did not correlate with the rates and extents of ¹⁴C-phenanthrene mineralization. This investigation highlighted the value of waste organic materials as nutrient sources to stimulate microbial degradation of contaminants in soil.
... Results show that PAH removal at each 2-pv step were higher for the first 10-12 pv, after which the incremental removals started to decrease step-by-step, reaching a plateau. Though this change in the rate of removal with total flushing volume (or time) is not very significant, some previous researchers (Bezza and Nkhalambayausi-Chirva, 2015;Barnier et al., 2014;Rhodes et al., 2012;Fonseca et al., 2011) reported two-step kinetics of PAH desorption, related to the nature of the desorption of contaminants from soil, particularly for soils containing great amounts of organic matter. Heavy metals and PAHs adsorbed onto the surface of soil particles could be removed more easily and rapidly than contaminants absorbed at depth onto soil particles and organic matter (Cornelissen et al., 1998;Bezza and Nkhalambayausi-Chirva, 2015). ...
Simultaneous removal of three polycyclic aromatic hydrocarbons (acenaphthene, fluorene and fluoranthene) co-existing with three heavy metals (Ni, Pb and Zn) in artificially contaminated soil from the vicinity of an oil refinery was examined by column flushing of solutions containing Triton X-100 + Ethylenediaminetetraacetic acid (EDTA) and Tween 80 + EDTA at three levels of surfactant concentrations. While the effectiveness of both combined solutions in removal of heavy metals did not differ significantly, Triton X-100 + EDTA was more efficient in removing PAHs. Results showed that after 21 pore volume flushing of enhancing solution (Triton X-100 7.5% + EDTA 0.01 M) at flow rate of 0.534 mL min-1 through the column with hydraulic conductivity of 8.5 × 10-5 cm s-1, 54, 47 and 40% of acenaphthene, fluorene, and fluoranthene were removed simultaneously. At the same conditions, 75, 85 and 90% of Pb, Ni and Zn, were also simultaneously removed. Increasing the flow rate of flushing solution decreased the removal efficiency of the contaminants.
... The decreases in B[a]P extractability through aging indicate that B[a]P became progressively sequestered in the soil matrix [36,37]. Hard OC and clay-sized particle fractions in soils can sequester PAHs [29,32,[38][39][40], through physical adsorption of PAHs to surfaces and occlusion or entrapment in pores [10,13,29,41]. In this study, the fractions of hard OC relative to TOC (hard OC/TOC) in soils (0.45 -0.58), as well as the contents of very fine-textured particles (10.7-30.9%) ...
There is a lack of understanding about the potential for remobilisation of polycyclic aromatic hydrocarbons (PAHs) residues in soils, specifically after the removal of readily available fractions, and the likelihood to cause harm to human and environmental health. Sequential solvent extractions, using butanol (BuOH), dichloromethane/acetone, and methanolic saponification were used to investigate the time-dependent remobilisation of B[a]P residues in aged soils, after removal of readily available or total-extractable fractions. After 120 d of aging, BuOH-remobilised B[a]P were small or extremely small ranging from 2.3 ± 0.1 mg/kg to 4.5 ± 0.5 mg/kg and from 0.9 ± 0.0 mg/kg to 1.0 ± 0.1 mg/kg, after removal of readily available and total-extractable fractions, respectively. After removal of readily available fractions, the remobilisation rates of B[a]P residues were constant over 5 re-equilibration times, as shown by first-order kinetics. The amounts of B[a]P remobilised significantly (p < 0.05) decreased with aging, particularly in hard organic carbon-rich soils. After 4 years of aging, BuOH- and total-remobilised B[a]P were generally < 5% of the initially spiked 50 mg/kg. Based on the findings of this study, the potential or significant potential for B[a]P NERs in soils to cause significant harm to human and environmental health are minimal.
... PAHs emitted into the atmosphere can enter surface soil by wet and dry deposition (Ahad et al. 2015;Lau et al. 2010;Yunker et al. 2002). As environmental conditions change, soil may serve as repositories of PAHs, which when perturbed, may release or increase bioaccessible and bioavailable fractions which were initially non-bioavailable (Luca et al. 2004;Neff et al. 2005;Nyarko and Klubi 2011;Ogbonnaya et al. 2017;Rhodes et al. 2012;Tao et al. 2008;Wang et al. 2001). Therefore, it is essential for human health research to evaluate the potential risks of PAHs in soils . ...
The Yulin National Energy and Chemical Industry Base is widely known for its rich mineral resources and multi-type fossil chemical industry, yet little is known regarding the level of contaminants. Therefore, this study investigates the spatial distributions and potential exposure risk of ubiquitous polycyclic aromatic hydrocarbons (PAHs) contamination in this region and apportions PAHs source and source-oriented risk using two mathematical models, principal component analysis-multiple linear regression (PCA-MLR) model and positive matrix factorization (PMF) model coupling human health exposure risk. Results showed that ∑16PAHs concentrations ranged from 110 to 4934 μg/kg dw in 38 soil sampling sites. Compared with PCA-MLR model, PMF model is preferred method for source apportionment. Source apportionment results derived from PMF model indicated that the dominant contribution to ∑16PAHs was from coal-derived sources (34% for coke oven emissions and 33% coal combustion source), followed by wood combustion (22%) and vehicular emission (11%). The human health exposure risk of each source category was quantitatively calculated for three exposure routes by combining the total carcinogenic risk (Total-CR) and total hazard index (Total-HI) values with identified source contributions. The results showed that increased Total-CR was highly apportioned from coke oven emissions source and coal combustion was identified as the major cause of increased Total-HI, even though it was less contributed to ∑16PAHs. Moreover, the distributions of Total-CR and Total-HI apportionment for each source were significantly influenced by land utilization types.
Graphical Abstract
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... However, the lipophilic organic contaminants like AHs and PAHs are preferentially adsorbed onto/into soil organic matter (SOM) composed of fulvic acid (FA), humic acid (HA), and humin, as well as black carbon, which are not extractable with organic solvents. Therefore, many previous studies have removed the endogenetic soil humus fractions or added exogenous soil humus fractions and black carbon to explore the effects of SOM on the contaminant behavior [8][9][10][11][12]. However, these methods changed the soil internal structure and soil endogenetic fractions. ...
Particulate polycyclic aromatic hydrocarbons (PAHs) have the characteristics of high carcinogenicity, degradation resistance, and long-distance transport. It is necessary to determine the critical sources that contribute to PAHs in atmospheric particulate matter (PM), soils, and sediments. In the present study, particulate PAHs were collected from Chinese cooking, biomass combustion, waste incineration, coal combustion, and petroleum combustion in different regions of the Chinese mainland. Through linear regression analysis, the relationships have been investigated between different measurement units, including the mass of particulate PAHs per mass of particle matter, mass of particulate PAHs per volume of air, and emission factors of particulate PAHs. This information helps to effectively evaluate the contamination level of PAHs among different studies. On the scale of the entire Chinese mainland, the top three sources of particulate PAHs are industrial coal combustion (11,244 t/year, 44.45%) > domestic coal combustion (7,797 t/year, 30.82%) > straw combustion (6,015 t/year, 23.78%). Efforts should be made to develop clean coal utilization. Although cooking contributes to a small amount of PAHs in China, it is extremely harmful for cooks [benzo(a)pyrene: 1.64 μg/m³]. Principal component analysis (PCA) is an effective method to identify various pollution sources. Using PCA, Chinese cooking, biomass combustion, waste incineration, domestic coal combustion, coking coal combustion, coal combustion for power generation, and diesel/gasoline engines that possess distinct PAH compositions are effectively discriminated from one another. Moreover, the characteristics of particulate PAHs emitted from human activities in China are not totally consistent with those in other countries. It is critical for pollution control to determine the distinctive characteristics of particulate PAHs emitted from typical production processes and life in China.
... Result of independent samples group t test showed that there were significant differences between differently treated groups (P < 0.05) except for EWC-10% and EWC-15% (P = 0.073 > 0.05), which indicated that the degradation of NP was comparatively more sensitive to the increase of the EWC application rate at low concentration stage (≤ 10%). The EWC-induced enhanced sorption and attenuated desorption of NP in soil as demonstrated by the sorption-desorption tests here are comparable to earlier studies of Gu et al. (2016) and Rhodes et al. (2012) who reported potential negative impacts of EWC on the bioaccessibility of organic pollutants in soil and considered them to be mainly responsible for inhibiting the degradation of 2,4-dichlorophenol and phenanthrene in soil. In addition, EWC-induced soil modification might exert an adverse side effect on the community composition and activity of contaminant-degrading microorganisms (Gu et al. 2016;Hobbie 2008;Liu et al. 2014). ...
Up to hundreds of milligrams per kilogram (dry weight) of nonylphenol (NP) reportedly entered the soil and sediment through the agricultural reuse of biosolids, pesticide application, etc. Organic pollutants in soil could not only further trigger groundwater contamination via leaching (that highly depends upon sorption-desorption and degradation phenomena) but also harm food safety by crop uptake (that mainly rests with the bioavailability of pollutants in soil). Thus, we first investigated the effects of earthworm casts (EWCs) on the sorption-desorption, degradation, and bioavailability of NP in soil under laboratory microcosm conditions, and then, analyzed the FT-IR spectra of EWC and soil samples (with and without EWC). The application of EWC could notably increase the sorption capacity of soils for NP and in turn significantly inhibited the desorption of NP from soil; responsively lengthened the half-time of NP in the soil; and reduced the uptake and translocation of NP in tomato seedlings and promoted their growth during the first 3 weeks. Finally, FT-IR spectra of EWC and soil samples indicated that the application of EWC increased the content of N, P, and organic matter in soil.
... However, the lipophilic organic contaminants like AHs and PAHs are preferentially adsorbed onto/into soil organic matter (SOM) composed of fulvic acid (FA), humic acid (HA), and humin, as well as black carbon, which are not extractable with organic solvents. Therefore, many previous studies have removed the endogenetic soil humus fractions or added exogenous soil humus fractions and black carbon to explore the effects of SOM on the contaminant behavior [8][9][10][11][12]. However, these methods changed the soil internal structure and soil endogenetic fractions. ...
Aliphatic hydrocarbons (AHs) are major petroleum contaminants in the environment. In this study, the AHs bound to various soil endogenetic humus fractions were separated through successive extraction. Most of the AHs (46.1%) in soils were adsorbed onto/into humic acids (HA) and a small quantity of AHs (9.6%) were organic solvent extractable. AHs in B. chinensis were also analyzed since their potential risks to the residents through ingestion. AHs from C21 to C34, so called high molecular weight AHs (HMWAHs), were dominant AHs in B. chinensis (85.5%) and soils (70.4%), followed by AHs from C16 to C21, whose mobility can be enhanced via binding to fulvic acids and then can be taken up by plant root lipids (soil-plant pathway). HMWAHs were mainly HA-bound and then were detained in the top soil layers. HMWAHs associated with fine topsoil particles could be transported to B. chinensis via the soil-air-plant pathway, including resuspension and aboveground plant cuticle capture. Results from Principal Component Analysis combined with Regression Analysis supported this assumption due to the positive correlations between HMWAHs concentration in B. chinensis and fine particle contents in soils. This work presents the distributions of petroleum contaminants that result from previously described behavior mechanisms.
... The desorption kinetics data of BaP were described using a twofraction model (Fig. 3). The shape of the desorption kinetics curve was similar to previous investigations using the same or similar mild extraction techniques (Cornelissen et al., 2001;Greenberg et al., 2005;Kleineidam et al., 2004;Rhodes et al., 2012). In the first phase, the desorption rate is rapid, which is followed by a second phase with a slower rate. ...
The desorption kinetics and mechanism were investigated using a Tenax extraction technique on different sediments spiked with radiocarbon-labeled benzo[a]pyrene (BaP). Five sedimentary fractions were sequentially fractionated, and the only nonhydrolyzable organic carbon fractions (NHC) were characterized using advanced solid-state ¹³C nuclear magnetic resonance spectroscopy (NMR), improved six end-member model, and a CO2 gas adsorption technique. The sediments contained high percentages of algaenan and/or sporopollenin but low percentages of black carbon and lignin. A first-order, two-compartment kinetics model described the desorption process very well (R² > 0.990). Although some of the organic carbon fractions were significantly related to the desorption kinetics parameters, the NHC fractions showed the highly significant correlation. Moreover, the nanoporosity or specific surface area (SSA) of the NHC fractions was highly related to their OC contents and aliphatic C (R² = 0.960, p < 0.01). The multiple regression equations among the desorption kinetics parameters, structural parameters, and nanoporosity were well established (R²=>0.999). Nanoporosity and aromatic C were the dominant contributors. Furthermore, the enhanced percentages of desorbed BaP at elevated temperatures significantly showed a linear regression with the structure and nanoporosity. To our knowledge, the above evidence demonstrates for the first time that the transfer (or diffusion) of BaP in the nanopores of condensed aromatic components is the dominant mechanism of the desorption kinetics of BaP at organic matter particle scale.
... Very little information is available on how CNMs act within soil matrices, especially in relation to their adsorption to organic fractions, organic pollutants and their subsequent toxicity (Dinesh et al., 2012). With an angelus sorbents such as black carbon (BC), elevated mineralisation of a phenanthrene substrate has been observed as a direct result of BC addition to soil, which was tentatively attributed to microbial sorption and utilisation of phenanthrene from the sorbed phase (Rhodes et al., 2008a;Rhodes et al., 2012). Only one study has identified an increase in contaminant mineralisation in soils 15 following the addition of CNMs. ...
The exceptional sorptive ability of carbon nanomaterials (CNMs) for hydrophobic organic contaminants (HOCs) is driven by their characteristically large reactive surface areas and highly hydrophobic nature. Given these properties, it is possible for CNMs to impact on the persistence, mobility and bioavailability of contaminants within soils, either favourably through sorption and sequestration, hence reducing their bioavailability, or unfavourably through increasing contaminant dispersal. This review considers the complex and dynamic nature of both soil and CNM physicochemical properties to determine their fate and behaviour, together with their interaction with contaminants and the soil micro-flora. It is argued that assessment of CNMs within soil should be conducted on a case-by-case basis, and further work to assess the long-term stability of sorbed contaminants and the toxicity of CNMs is required before their sorptive abilities can be applied to remedy environmental issues.
... The soil sample collected from the A horizon (0e30 cm) of a loamy soil at the Gatton Horticulture Research Station, in the Lockyer Valley, Australia (27 32 0 S, 152 19 0 E) ( Table 1). The soil is classified as black Vertosol equal to Udic Haplustert according to the USDA Soil Taxonomy (Soil Survey Staff, 1999) and the region has a subtropical climate (Rhodes et al., 2012). The soil was air-dried and passed through a 2-mm sieve. ...
Char as a carbon-rich material, can be produced under pyrolytic conditions, wildfires or prescribed burn offs for fire management. The objective of this study was to elucidate mechanistic interactions of copper (Cu²⁺) and nickel (Ni²⁺) with different chars produced by pyrolysis (green waste, GW; blue-Mallee, BM) and forest fires (fresh-burnt by prescribed fire, FC; aged char produced by wild fire, AC). The pyrolytic chars were more effective sorbents of Cu²⁺ (∼11 times) and Ni²⁺ (∼5 times) compared with the forest fire chars. Both cross-polarization (CPMAS-NMR) and Bloch decay (BDMAS-NMR) ¹³C NMR spectroscopies showed that forest fire chars have higher woody components (aromatic functional groups) and lower polar groups (e.g. O-alkyl C) compared with the pyrolytic chars. The polarity index was greater in the pyrolytic chars (0.99–1.34) than in the fire-generated chars (0.98–1.15), while aromaticity was lower in the former than in the latter. Fourier transform infrared (FTIR) and Raman spectroscopies indicated the binding of carbonate and phosphate with both Cu²⁺ and Ni²⁺ in all chars, but with a greater extent in pyrolytic than forest fire-generated chars. These findings have demonstrated the key role of char's oxygen-containing functional groups in determining their sorption capacity for the Cu²⁺ and Ni²⁺ in contaminated lands.
... The key observations from these studies were that the 3-parameter biphasic (i.e. one that assumes quick equilibration for F rapid ) and 3-parameter biphasic polymer diffusion models were recommended as preferable for predicting HOC desorption kinetics in contaminated soils compared to others, although rate limiting site-specific conditions must be considered. Importantly, these studies revealed that rates of slow desorption (k slow ) from strongly sorbed pools could range from between 10 −2 to 10 −4 h −1 (Cornelissen et al., 1997a;Rhodes et al., 2010;, depending on stage of SOM diagenesis and hydrophobicity of the HOCs of concern (Rhodes et al., 2008a;Rhodes et al., 2008c;Rhodes et al., 2012). Generally, k slow was reported to be up to 2 orders of magnitude lower than k rapid (Doick et al., 2005a). ...
... Overall, these results were in agreement with previous studies by Rhodes et al. (2008Rhodes et al. ( , 2010. These trends have been attributed to the strong sorptive capacity of BC (Cornelissen et al., 2005;Rhodes et al., 2012). Consequently, the aqueous concentration and biodegradation of target compound was reduced, as the microbial uptake of organic substances mainly takes place in soil aqueous phase (Cerniglia, 1992;Johnsen et al., 2005). ...
The presence of black carbon (BC) in soil drastically reduced the mineralization of C-phenanthrene and its extractability by hydroxylpropyl- -cyclodextrin (HPCD) extractions. This study also tested the effects of pH on the HPCD extraction of C-phenanthrene in soils with BC. Extractions using 60 mM HPCD solutions prepared in deionized water (pH 5.89) and phosphate buffers (pH 7 and 8) were conducted on C-phenanthrene-spiked soils amended with three different types of BC (1% dry weight) after 1, 25, and 50 d of ageing. Biodegradation assays using a Pseudomonassp. strain were also carried out. Results showed that after 1 and 25 d, HPCD at pH 7 extracted significantly more C-phenanthrene (p 0.05) from BC-amended soils than the other two solutions (un-buffered and pH 8), while HPCD at pH 8 extracted statistically similar (p 0.05) amounts of phenanthrene compared to the un-buffered solution. At 50 d, HPCD at pH 8 generally extracted more C-phenanthrene from all treatments. It was proposed that higher pH promoted the dissolution of soil organic matter (SOM), leading to a greater solubility of phenanthrene in the solvent phase and enhancing the extractive capability of HPCD solutions. Although correlations between extractability and biodegradability of C-phenanthrene in BC-amended soils were poor, increasing pH was demonstrated a viable approach to enhancing HPCD extractive capability from the C-PAH from soil.
... However, the slowly and very slowly desorbing rate constants herein were much lower in comparison with the results reported by Liu et al. (2011a). This was probably due to the much longer aging time herein (100 d) relative to their 14 d aging time, making it more difficult for the BDE-47 to desorb from sediments in this research (Rhodes et al., 2012). ...
... Hu et al. [7] have employed bioremediation and have reported a total PAH removal of 69% in 7 days at an initial concentration of 556 ng/mg. However, as reported in other studies [8], the removal of PAHs by biotreatment may take more days or even months. For these reasons, the combination of chemical oxidation and bioremediation can possibly lead to savings in cost and time, as chemical oxidation contributes to the improvement of the biodegradation of pollutants and the increase of the removal efficiency. ...
Polycyclic aromatic hydrocarbons (PAHs), which are detrimental to the environment and human health, are frequently found in soils. A pulsed dielectric barrier discharge (DBD) plasma system was employed for the remediation of phenanthrene (PHE)-contaminated soil. The role of active species was investigated by evaluating the effects of air flow rate and discharge voltage on the removal efficiency for their influence on the formation and transfer of these active species produced during plasma treatment. At an air flow rate of 0.6 L/min and a voltage of 110 V, the removal rate could reach 87.3% in 20 min with an energy efficiency of 0.01 mg/kJ. Radicals including O3, O, OH, OH+, N2∗, N2O+, and NO were identified by optical emission spectroscopy (OES). The contribution of ozone towards degradation was investigated by comparing ozone treatment with plasma treatment: ozone treatment was observed to account for 82.8% of the whole oxidation process. Theoretical calculations by Gaussian were employed to understand the degradation processes in combination of FTIR, GC–MS, and IC analysis and a possible degradation pathway of PHE was also proposed. Plasma technology was observed to be able to improve the biodegradability of PHE-contaminated soils. It was suggested that plasma treatment could be followed up by bio-remediation by taking advantage of the fast and effective plasma treatment (83.7% of PHE was removed in 20 min) for heavily polluted organic soils.
... In terms of rate constants for BDE-47 desorption from various CMs, they all followed the ranking of k r > k s » k vs , and the k r and k s values were generally in the range of 10 −1 -10 −2 and 10 −3 -10 −4 , respectively ( Table 2). These results were in line with other studies on desorption of pentachlorophenol and phenanthrene from CMcontaining sediments [70,71]. It is notable that the k r for BDE-47 desorption from MWCNT-1 was around two orders of magnitude lower than other CMs, and meanwhile it also had the largest SSA (159 m 2 g −1 ) as well as the greatest pore volume (0.870 cm 3 g −1 ) of all the CMs (Table 1). ...
In this study, we investigated the influence of various types of carbonaceous materials (CMs) on the bioavailability of 2,2',4,4'-tetrabromodiphenyl ether (BDE-47) to polybrominated diphenyl ether (PBDE)-degrading microorganisms in CM-amended sediments. The microbial debromination ratio of BDE-47 was reduced by 92.8%-98.2% in the 5.0% CM-amended sediment compared with in sediment without CM amendment after 100 d of anaerobic incubation. The concentrations of lower brominated products also decreased when the content of CMs increased from 0.2% to 5.0%. The inhibitory effects of CMs on BDE-47 debromination were CM content- and characteristic-specific. The reciprocals of BDE-47 debromination ratios and lower brominated product concentrations showed positive linear correlations with CM contents in sediments (p<0.01), and the slopes of linear regression fitting generally correlated with specific surface areas (SSAs) of CMs. Desorption of BDE-47 from CMs indicated the declined desorbing fraction of BDE-47 was responsible for the reduction in BDE-47 bioavailability to microorganisms, thus decreasing its debromination in sediments amended with CMs. This study revealed that CM amendment could reduce the PBDE bioavailability to PBDE-degrading microorganisms in sediments, and it is expected to help deepen our understanding of the environmental behaviors and risks of PBDEs.
... 33 Partition coefficients for biochar are generally in a similar order of magnitude as for soot 47 when K D values determined at the same aqueous PAH concentration range are compared. It was therefore expected that HPCD solutions would also have insufficient capacity for bioaccessibility extractions of biochar, which is in line with a study by Rhodes et al. 48 Simple partitioning calculations were carried out prior to experimental work to assess whether the HPCD solutions would have sufficient sink capacity with regards to the two biochars, and then the insufficient capacity was experimentally confirmed by the results of the sorptive bioaccessibility extractions (SBE; see below). Consequently, no HPCD extractions were conducted in the present study. ...
Biochars are obtained by pyrolysing biomass materials and are increasingly used within the agricultural sector. Owing to the production process, biochars can contain polycyclic aromatic hydrocarbons (PAHs) in the high mg/kg range, which makes the determination of the environmental exposure of PAHs originating from biochars relevant. However, PAH sorption to biochar is characterized by very high (104-106 L/kg) or extreme distribution coefficients (KD) (>106 L/kg), which makes the determination of exposure scientifically and technically challenging. Cyclodextrin Extractions, Sorptive Bioaccessibility Extractions, Tenax Extractions, Contaminant Traps and Equilibrium Sampling were assessed and selected methods used for the determination of bioavailability parameters for PAHs in two model biochars. Results showed: (1) Distribution coefficients of typically 106-109 L/kg made the biochars often act as sinks, rather than sources, of PAHs. (2) Equilibrium Sampling yielded freely dissolved concentrations (pg-ng/L range) that were below or near environmental background levels. (3) None of the methods were found to be suitable for the direct measurement of the readily desorbing fractions of PAHs (i.e. bioacessibility) in the two biochars. (4) The Contaminant Trap method yielded desorption resistant PAH fractions of typically 90-100%, implying bioaccessibility in the high µg/kg to low mg/kg range.
... The shape of the desorption kinetics obtained here with sediment was similar to those observed by previous authors using the same or similar mild extraction techniques either with model sorbents (Cornelissen et al. 1998b), sediments (Cornelissen et al. 2001;Cornelissen et al. 1997a;Greenberg et al. 2005;Spasojević et al. 2015) or with contaminated soils (Hawthorne et al. 2002;Rhodes et al. 2012;Swindell and Reid 2006). ...
In this study, wastewater from municipal services, such as a port wastewater reception facility (PRF-WW) and a municipal solid waste plant (MSWP), was tested for the presence of the suspected endocrine-disrupting compounds phthalates (PAEs) and bisphenol A (BPA). PAEs and BPA were found in this study in high concentrations in raw wastewater obtained from passenger ships (RMT-WWs) (up to 738 μg/L and 957 μg/L, respectively) collected in the Port of Gdynia and in landfill leachates (LLs) (up to 536 μg/L and up to 2202 μg/L, respectively) from a MSWP located near Gdynia. In particular, the presence of reprotoxic di(2-ethylhexyl) phthalate (DEHP, up to 536 μg/L in LLs and up to 738 μg/L in RMT-WWs) requires further action because if this compound, as well as other PAEs and BPA, is not degraded by activated sludge microorganisms, it may reach receiving waters and adversely impact aquatic organisms. Therefore, PAEs and BPA should be removed either during the onsite pretreatment of tested industrial wastewater or during tertiary treatment at municipal wastewater treatment plants (WWTPs, representing end-of-pipe technology).
Graphical abstract
... The changes in form of sorbed nonylphenol are shown in Fig 4. Across experimental conditions, the desorption fractions followed the progression f r > f rap % f slow and were generally in order of 10 -2 to 10 -1 , 10 -2 and 10 -3 to 10 -2 , respectively. Among these results, f r was significantly greater than the other two fractions (P < 0.01), suggesting that f r dominated in residual nonylphenol [14,36]. A slow decrease occurred in f rap , f slow and f r over the course of biodegradation experiments, representing the transformation of all three fractions. ...
Biochar adsorption presents a potential remediation method for the control of hydrophobic organic compounds (HOCs) pollution in the environment. It has been found that HOCs bound on biochar become less bioavailable, so speculations have been proposed that HOCs will persist for longer half-life periods in biochar-amended soil/sediment. To investigate how biochar application affects coupled adsorption-biodegradation, nonylphenol was selected as the target contaminant, and biochar derived from rice straw was applied as the adsorbent. The results showed that there was an optimal dosage of biochar in the presence of both adsorption and biodegradation for a given nonylphenol concentration, thus allowing the transformation of nonylphenol to be optimized. Approximately 47.6% of the nonylphenol was biodegraded in two days when 0.005 g biochar was added to 50 mg/L of nonylphenol, which was 125% higher than the relative quantity biodegraded without biochar, though the resistant desorption component of nonylphenol reached 87.1%. All adsorptive forms of nonylphenol (frap, fslow, fr) decreased gradually during the biodegradation experiment, and the resistant desorption fraction of nonylphenol (fr) on biochar could also be biodegraded. It was concluded that an appropriate amount of biochar could stimulate biodegradation, not only illustrating that the dosage of biochar had an enormous influence on the half-life periods of HOCs but also alleviating concerns that enhanced HOCs binding by biochar may cause secondary pollution in biochar-modified environment.
Biochar has great potential as a soil conditioner and as a carrier of beneficial microorganisms that support the removal of pollutants, influence the circulation of nutrients, and support plant growth. This review summarizes and discusses factors shaping the physicochemical properties of biochar, including feedstock, pyrolysis conditions, and accompanying processes used as post-pyrolysis modification to improve the functionality of biochar. Key physical and chemical properties such as high porosity and specific surface area, nutrient content, pH, and biochar functional groups are discussed in detail to show biochar’s potential as a carrier for microorganisms. This review also discusses and summarizes biological indicators that allow for assessing the quality and efficiency of the microbiological modifiers. Finally, this paper presents the benefits and limitations of biochar application to agriculture and provides recommendations for future research to improve the quality and expand the applicability of biochar-based inoculants.
Research investigating the desorptive behaviour of PAHs from contaminated soils often overlooked the effects of source materials, especially coal tar and coal tar pitch and materials alike. In this study, a refined experimental approach was adopted to establish a simple-to-complex continuum of systems that allow the investigation of desorption kinetics of benzo(a)pyrene (BaP) and 3 other carcinogenic PAHs (cPAHs) over an incubation period of 48 d. By comparing the modelled desorption parameters, elucidation of the effects of PAH source materials on their desorptive behaviour was achieved. Desorption of cPAHs from coal tar and pitch was enhanced when they were added to soils, with rapidly desorbing fraction (Frap) of BaP increased from 0.68% for pitch to 1.10% and 2.66% for pitch treated soils, and from 2.57% for coal tar to 6.24% for coal tar treated soil G and 8.76% for coal tar treated sand (1 d). At 1 d, desorption of target cPAHs from solvent and source material spiked soils generally followed the order of solvent > coal tar > pitch. Increases in Frap of cPAHs were observed in coal tar-treated soils after 48 d soil incubation (0.33%-1.16% for soil M, p ≥ 0.05, 6.24%-9.21% for soil G, p < 0.05) and was attributed to the continuous migration of coal tar as a non-aqueous phase liquid (NAPL) into soil pore structures. Slow desorption was dominated by source materials, whereas the extents and rates of rapid desorption (Frap and krap) were more controlled by the quantity of soil organic matter (SOM), rather than quality of SOM (as in solvent-spiked soils). The results of this study challenged the role of PAH source materials as 'sinks' and led to the proposed roles of coal tar and pitch and source materials alike as 'reservoirs' with a risk-driven perspective.
Microbial remediation is an eco-friendly and promising approach for the decontamination of wastewater containing sulfamethoxazole (SMX) and Zn²⁺. However, free bacteria are sensitive to environmental variation, limiting the application. In this study, biochar-immobilized Bacillus paramycoides SDB4 (ISDB4) was used to overcome the disadvantages. The results showed that the removal by ISDB4 was significantly increased under different pH conditions and initial combined contaminant concentrations. Notably, at pH 7, SMX (20 mg·L⁻¹) and Zn²⁺ (100 mg·L⁻¹) were completely removed by ISDB4, and their levels were 29.28% and 92.34% higher than that of SDB4, respectively. Besides, ISDB4 showed excellent reusability with 43.24% of SMX and 50.34% of Zn²⁺ removal efficiencies after five continuous cycles. Such improvements might be related to enhanced biotransformation of SMX from 45.81% with SDB4 to 89.98% with ISDB4. Compared with SDB4, the contributions of ion exchange and complexation of ISDB4 with Zn²⁺ increased by 9.53% and 6.72%, respectively. In case of ISDB4, SEM-EDS and FTIR showed that biochar provided sufficient space for the attachment and proliferation of SDB4, and the removal of combined contaminants was enhanced by SDB4 due to the presence of functional groups and light metals on biochar. This study provides a green and effective strategy for the bioremediation of antibiotic and heavy metal co-contaminated aquaculture wastewater.
Knowledge of microbial degradation of biochar-adsorbed organic pollutants is essential for recovering adsorption performance of biochars and reducing secondary pollution in soil remediation. In previous study, desorption of organic compounds from biochars was perceived as a prerequisite for the microbial degradation. However, microbial degradation of the nondesorbable organic compounds on biochars has not been studied. Therefore, degradation of nondesorbable naphthalene (NAPH), phenanthrene (PHEN) and pyrene (PYR) on a wood chip-derived biochar (WBC700) by Sphingobium yanoikuyae B1 was investigated. Significant microbial degradations of nondesorbable organic compounds were observed and followed the order of NAPH < PHEN < PYR. It was newly observed in this study that the microbial degradation of nondesorbable organic compounds on WBC700 was mainly attributed to the •OH in extracellular fluid of Sphingobium yanoikuyae B1. The extracellular •OH was produced through a Fenton-like reaction involved siderophore, H2O2 and iron ions, which could be significantly enhanced by WBC700. Microbial degradation was higher for larger organic compound (e.g., PYR), because larger molecules were adsorbed in relatively larger micropores of WBC700 and thus could be accessible to more extracellular •OH for degradation. The obtained results could provide a new insight into the microbial degradation of biochar-adsorbed organic pollutants in soil remediation.
There is a strong need for certified reference materials in the quality assurance of nonionic soil contaminant bioavailability estimations through physicochemical methods. We applied desorption extraction, a method recently standardized as ISO16751, to determine the bioavailable concentration of the most commonly regulated polycyclic aromatic hydrocarbon (PAH), benzo(a)pyrene (BaP), in the reference industrial soil BCR-524 with a certified BaP total concentration of 8.60 mg kg⁻¹. This concentration represented BaP levels found in many PAH-polluted soils. The method, based on single-point extraction of the analyte desorbed into the aqueous phase by a receiving phase (Tenax or cyclodextrin), was applied ten times. The data fulfilled highly demanding quality criteria based on recovery and repeatability. The bioavailable BaP concentration detected through Tenax extraction, 1.82 mg kg⁻¹, was comparable to bioavailable concentrations determined in field-contaminated soils and to environmental quality standards based on previously observed total BaP concentrations. There was good agreement (Student's t-test, P ≤ 0.05) with the bioavailable BaP concentration determined by cyclodextrin extraction (1.53 mg kg⁻¹). The methods were extended to four other certified 4- and 5-ringed PAHs for comparative purposes. We suggest ways of improving of the ISO16751 standard related to further systematic assessment of the Tenax-to-soil ratio and incorporation of mass balances. Furthermore, BCR-524 is suitable for quality-assurance protocols with these methods when used in site-specific risk assessments of PAH-polluted environments.
This study investigated the extractability, bioaccessibility and biodegradation of ¹⁴C-phenanthrene and ¹⁴C-octacosane in two soils from former oil refinery facilities over 341 days. The impact of biostimulation and bioaugmentation treatments was also evaluated. At 0, 31, 62, 124 and 341 days, the loss and extractability (using dichloromethane, methanol:water and hydroxypropyl-β-cyclodextrin (HPCD)) of the ¹⁴C-hydrocarbons were measured. Further at each time point, the mineralisation of the ¹⁴C-hydrocarbons was measured respirometrically under the different conditions. In general, extractions with methanol: water and HPCD were similar for both hydrocarbons in the different treatments; however, these values were less that those measured with DCM. Overall, significantly higher (p ≤ 0.05) amounts of ¹⁴C-phenanthrene were lost, readily extracted and mineralised in the soils, with treatments having little impact upon the degradation of this hydrocarbon over 341 days. Conversely, bioaugmentation significantly increased the loss of ¹⁴C-octacosane residues from soils and sustained degradation after 31 days. Surprisingly, HPCD and methanol:water both under-predicted the extent to which the contaminants were degraded at each time point. Determining the likelihood of effective biodegradation by the stimulation of indigenous microorganisms or through bioaugmentation needs to be assessed by both chemical and biological measurements of bioaccessibility, rather than just by that which is totally extractable from soil. However, soils which have high loadings of organic matter and/or organic contaminants may prevent accurate assessment of contaminant bioaccessibility, as measured by HPCD.
Biochars can strongly sorb hydrophobic organic contaminants in soils. However, contribution of contaminants in different binding sites to their biodegradation in biochar-amended soils is not clear. In this work, wheat straw biochars were prepared at pyrolysis temperatures of 400 °C (BC400) and 700 °C (BC700). During a 42-day experiment, degradation rate constant of phenanthrene in soils was in the order of treatment without biochar (1.64 × 10⁻² d⁻¹) > treatment with BC700 (0.96 × 10⁻² d⁻¹) > treatment with BC400 (0.30 × 10⁻² d⁻¹). At the beginning, amendment of BC400 and BC700 reduced the rapidly desorbing fraction of phenanthrene in soils by 44.8% and 92.5%, respectively. At the end, both phenanthrene and microbial biomass highly concentrated on the biochar separated from soils. The results of a coupled model of desorption and biodegradation revealed that only phenanthrene in rapidly desorbing sites was degraded in BC400-amended soils, whereas degradation of phenanthrene in both rapidly and slowly desorbing sites occurred in BC700-amended soils, contributing 24.4% and 75.6% of the degradation, respectively. High fraction (>95%) of biodegradable phenanthrene in slowly desorbing sites was the key reason for higher biodegradation rate of phenanthrene in soils with BC700 than in soils with BC400.
The interaction between root exudates and soil microbes has been hypothesised as the primary mechanism for the biodegradation of organic pollutants in the rhizosphere. However, the mechanisms governing this loss process are not completely understood. This study aimed to investigate the effect of two important compounds within root exudates (citric and malic acid) on ¹⁴C-phenanthrene desorption and bioaccessibility in soil. Overall results showed that the presence of both citric and malic acid (>100 mmol l⁻¹) enhanced the desorption of ¹⁴C-phenanthrene; this appeared to be concentration dependant. Increases in extractability were not reflected in a higher bioaccessibility. Despite enhancing the desorption of ¹⁴C-phenanthrene in soil, there is no direct evidence indicating that citric or malic acid have the ability to promote the biodegradation of ¹⁴C-phenanthrene from soil. Results from this study provide a novel understanding of the role that substrates, typically found within the rhizosphere due to root exudation, play in the bioaccessibility and biodegradation of hydrocarbons in contaminated soil.
The present study aimed to investigate the relationship between the desorption and biodegradability of phenanthrene sorbed to biochars by employing two approaches that may change the desorption and biodegradability: the use of powdered biochars and nonionic surfactants. Biochars derived from two feedstocks (rice husk and sewage sludge; pyrolyzed at 500 °C but showing different aromaticity) were used. When the biochars were powdered to obtain particles <250 μm the mass fractions of the desorbed phenanthrene at ∼80 days (f des ) increased from 0.303 to 0.431 for sewage sludge biochars. On the other hand, f des for rice husk biochars remained virtually unchanged (from 0.264 to 0.255). The mass fractions of the biodegraded phenanthrene (f bio ) increased from 0.191 to 0.306 for rice husk biochars and from 0.077 to 0.168 for sewage sludge biochars. When a nonionic surfactant was added at the sub-critical micelle concentration (CMC), f bio increased by 4.7 times and 8.3 times for rice husk and sewage sludge biochars. For both types of biochars, f bio was larger than f des when the surfactant was added. This study suggests that the addition of nonionic surfactants can be considered if the inhibition of microbial activity is of concern in soils and sediments treated by biochar.
The fate, impacts and significance of polycyclic aromatic hydrocarbon (PAH) non-extractable residues (NERs) in soils remain largely unexplored in risk-based contaminated land management. In this study, 7 different methanolic and non-methanolic alkaline treatments, and the conventional methanolic saponification, were used to extract benzo[a]pyrene (B[a]P) NERs that had been aged for 180 d from four contrasting soils. Up to 16% and 55% of the amount of B[a]P spiked (50 mg/kg) into soils was non-extractable after 2 d and 180 of aging, respectively; indicating rapid and progressive B[a]P sequestration in soils over time. The recovery of B[a]P from soils after 180 d of aging was increased by up to 48% by the 7 different alkaline extractions, although the extraction efficiencies of the different alkaline treatments did not differ significantly (p > 0.05). Approximately 40% of B[a]P NERs in the sandy-clay-loam organic matter-rich soil was recovered by the exhaustive alkaline extractions after 180 d of aging, compared to only 10% using conventional methanolic saponification. However, the amounts of B[a]P NERs recovered depend on soil properties and the amounts of NERs in soils. A significant correlation (R2 = 0.69, p < 0.001) was also observed between the amounts of B[a]P recovered by each of the 7 alkaline extractions in the contrasting soils, and corresponding NERs at 180 d of aging, indicating a potential association warranting further investigations. Extraction techniques that estimate the amounts of PAH NERs recoverable in soil can help give a better understanding of the fate of NERs in soil.
The presence of exogenous carbonaceous materials (ECMs) in organic contaminated soil is widespread because of their intentional application as carbonaceous amendments (e.g. biochar and activated carbon) or unintentional discharge (e.g. carbon nanomaterials). Most research so far has focused on the sorption behaviors of ECMs in soil. However, the impact of ECMs on the bioavailability of organic pollutants (OPs) and their ecological damages remain unclear. This paper presents an overview on how the ECMs affect bioavailability of OPs to different organisms, such as microorganisms, plants and earthworms. This is affected by different biological response and properties of ECMs. Moreover, the possible risks of ECMs on soil biota are also discussed at different level. This review presents a unique insight into risk assessment of ECMs. Further researches should focus on possible change in physicochemical characteristics of ECMs when exposed to the natural environment and the consequent influence on their sorption ability and ecotoxicity outcomes.
This study investigated the use of a hydrophobic resin, amberlite XAD, as a tool for assessing the biodegradation potential of 14C-phenanthene in soil. The method was optimised in terms of soil/XAD ratio, shaking, extraction time and eluting solvent. The most effective method was then tested on selected XADs, and the performance compared with cyclodextrin (HP-β-CD) and dichloromethane (DCM) extractions suitability to predict phenanthrene biodegradation in soil over 100 d. Results showed that the optimum conditions for the XAD extraction technique are a 2:1 soil/XAD ratio, 100 rpm mixing for 22 h and elution using a DCM:methanol solution (1:1). Mineralisation of 14C-phenanthrene was accurately predicted by HP-β-CD (r2=0.990, slope = 0.953, intercept = 1.374) and XAD-4 extractions (r2=0.989, slope = 0.820, intercept = 6.567), while DCM overestimated the bioaccessibility of 14C-phenanthrene (r2=0.999, slope = 1.328, intercept =−49.507). This investigation showed that XAD extraction can be considered a suitable non-exhaustive technique for estimating biodegradability of phenanthrene in soil.
In this study, the indigenous microbial mineralisation of 14C-phenanthrene in seven background soils (four Norwegian woodland and three UK (two grassland and one woodland)) was investigated. ∑PAHs ranged from 16.39 – 285.54 ng g-1 dw soil. Lag phases (time before 14C-phenanthrene mineralisation reached 5%) were longer in all of the Norwegian soils and correlated positively with TOC, but negatively with ∑PAHs and phenanthrene degraders for all soils. Phenanthrene mineralisation in the soils varied due to physic-chemical properties. Results show that indigenous microorganisms can adapt to 14C-phenanthrene mineralisation following diffuse PAH contamination. Considering the potential of soil as a secondary PAH source, these findings highlight the important role of indigenous microflora in the processing of PAHs in the environment.
The present study aimed to investigate biodegradation mechanisms of black carbon (BC)-bound contaminants in BC-amended sediment when BC was applied to control organic pollution. The single-point Tenax desorption technique was applied to track the species changes of nonylphenol (NP) during biodegradation process in the rice straw carbon (RC)-amended sediment. And the correlation between the biodegradation and desorption of NP was analyzed. Results showed that microorganisms firstly degraded the rapid-desorbing NP (6 h Tenax desorption) in RC-amended sediment. The biodegradation facilitated the desorption of slow-desorbing NP, which was subsequently degraded as well (192 h Tenax desorption). Notably, the final amount of NP degradation was greater than that of NP desorption, indicating that absorbed NP by RC amendment can be degraded by microorganisms. Finally, the residual NP amount in RC-amended sediment was decided by RC content and its physicochemical property. Moreover, the presence of the biofilm was observed by the confocal laser scanning microscope (CLSM) and scanning electron microscope (SEM) so that microorganisms were able to overcome the mass transfer resistance and directly utilized the absorbed NP. Therefore, single-point Tenax desorption alone may not be an adequate basis for the prediction of the bioaccessibility of contaminants to microorganisms or bioremediation potential in BC-amended sediment.
Cell immobilization technology has been considered as an effective method for bioremediation of hydrocarbon-contaminated soil. However, bacteria immobilized by a single method often encounter some problems, e.g., cell leakage, cellular damage and no reproduction. In this study, a biomimetic hybrid material was constructed by pre-immobilization of bacteria on sawdust followed by coating a silica layer through vapor deposition (Silica-IC). The viability and metabolic activity of Silica-IC were investigated. Results showed that the silica layer covering the bacterial agent could significantly reduce cell leakage from sawdust without losing reproductive capacity on nutrient plates. A viability assay by SYTO9/PI in flow cytometry indicated that the proportion of live cells was decreased 30% and injured cells was increased 23.9%, while that of dead cells was still below 2.5% during storage at 4 °C for 15 days, i.e., membrane permeability of Silica-IC was increased, indicating bacterial cells in Silica-IC were able to maintain long-term storage stability and shelf life. The metabolic activity of Silica-IC toward phenanthrene (Phe) was enhanced both in liquid and soil. Phe degradation kinetics of Silica-IC in liquid medium well fitted an adsorption-degradation model, suggesting that the silica layer did not inhibit Phe diffusion. Moreover, the Phe removal percentage of Silica-IC in soil was up to 93.4% on day 2. Silica-IC in soil grew well and the growth was closely related to the residual amount of Phe. This work provides a route to develop a wide range of bio-materials for bioremediation.
Activated carbon (AC) amendment for reduction of contaminant exposure in polluted soils and sediments has recently emerged as a promising remediation technique. Here, we provide a short overview of the state-of-the-art in activated carbon (AC) amendment to such sites. Most studies not only in sediments but also in soils were carried out in the laboratory and only a few in the field. Consequently, practical experience at the field scale is largely lacking, and feasible engineering approaches for AC amendment still need to be developed, especially for soils. The effectiveness of the AC treatment was evaluated by comparison of pollutant concentration reduction in the various endpoints with those in the unamended control matrix. Endpoints in use comprise pollutant concentrations in benthic organisms, depletive and non-depletive methods to quantify pollutant exposures, as well as various toxicity endpoints of plants. Half of the studies in soils and 68% of the studies in sediments showed reduced pollutant availabilities of >50% after AC amendment. Observed low reductions (<50%) might be due to low exposure time, insufficient equilibrium time for coke breeze, biochar, and granulated AC, overload of AC material, different uptake pathways of benthic organisms, and pollutant reductions outside the dynamic range of toxicity endpoints. Further research is needed to establish ideal AC amendment conditions to sediments and soils, leading to significantly reduced pollutant bioavailability.
Traditionally, soil extraction techniques have been concerned with the determination of “total” organic contaminant concentrations, following an “exhaustive” extraction. However, in light of the increasing body of knowledge relating to organic contaminant availability and aging, such methods have little relevance to the amount of contaminant that may pose an ecological risk i.e., the “bioavailable” portion. Less exhaustive techniques have therefore been the subject of more recent approaches in the hope that they may access the “labile” or bioavailable pool. The use of an aqueous-based extraction technique utilizing hydroxypropyl-β-cyclodextrin (HPCD) is presented here for the extraction of PAHs from soil. The optimization of the method is described in terms of HPCD concentration, extraction time, and solution buffering. The procedure is then tested and validated for a range of 14C-labeled PAHs (phenanthrene, pyrene, and benzo[a]pyrene) added at a range of concentrations to a range of soil types. The amounts of soil-associated phenanthrene mineralized by catabolically active microorganisms were correlated with total residual phenanthrene concentrations (r 2 = 0.889; slope of best fit line = 0.763; intercept = −5.662; n = 24), dichloromethane (DCM)-extractable phenanthrene concentrations (r 2 = 0.986; slope of best fit line = 0.648; intercept = 0.340; n = 24), butan-1-ol (BuOH)-extractable phenanthrene concentrations (r 2 = 0.957; slope of best fit line = 0.614; intercept = 0.544; n = 24), and HPCD-extractable phenanthrene concentrations (r 2 = 0.964; slope of best fit line = 0.997; intercept = 0.162; n = 24). Thus, in this study, the microbially bioavailable concentrations of soil-associated phenanthrene were best predicted using the optimized HPCD extraction technique. In contrast, the DCM Soxhlet extraction and the BuOH shake extraction both overestimated phenanthrene bioavailability by, on average, >60%.
Activated carbon (AC) can help overcome toxicity of pollutants to microbes and facilitate soil bioremediation. We used this approach to treat a Histosol and an Alluvial soil historically contaminated with PCB (4190 and 1585 mg kg(-1), respectively; primarily tri-, tetra- and pentachlorinated congeners). Results confirmed PCB persistence; reductions in PCB extractable from control and AC-amended soils were mostly due to a decrease in tri- and to some extent tetrachlorinated congeners as well as formation of a bound fraction. Mechanisms of PCB binding by soil and AC were different. In addition to microbial degradation of less chlorinated congeners, we postulate AC catalyzed dechlorination of higher chlorinated congeners. A large decrease in bioavailable PCB in AC-amended soils was demonstrated by greater clover germination and biomass. Phytotoxicity was low in treated soils but remained high in untreated soils for the duration of a 39-month experiment. These observations indicate the utility of AC for remediation of soils historically contaminated with PCB.
This study investigated the use of an aqueous hydroxypropyl-beta-cyclodextrin (HPCD) shake extraction to predict the degree of microbial degradation of polycyclic aromatic hydrocarbons (PAHs) in soils. Three different aged PAH-contaminated soils were studied: A soil from a former coke works (CW) and two artificially contaminated soils (AC1 and AC2). First, the catabolic activity of the indigenous soil microflora was assessed with 14C-respirometry, using a range of 14C-labeled aromatic compounds. Extensive mineralization of several compounds occurred in the CW and the AC2 soils, suggesting that both soils contained catabolically active microorganisms. No significant mineralization occurred in the AC1 soil, implying that either it did not contain an indigenous PAH-degrading microbial population or that degradation, but not mineralization, occurred. The soils then were subjected to three sets of analyses: dichloromethane (DCM) soxhlet extraction, six-week biodegradation assay followed by DCM extraction, and extraction with HPCD followed by DCM extraction. A general decrease in PAHs present in the soils occurred after the biodegradation assay. In the CW and the AC1 soils, strong correlations were observed between the amount of PAHs biodegraded and the fraction of PAHs removed from the soils using the HPCD extraction. However, the AC2 soil showed a more modest correlation between the biodegradable fraction and the HPCD extractable fraction, with the HPCD extraction slightly underestimating the extent of PAH biodegradation. The results of this study indicated that an aqueous HPCD extraction may be a useful tool in assessing the microbial availability of aged contaminant mixtures in soils, although further validation is required.
This research investigates the mechanisms contributing to the slow desorption of hydrophobic organic compounds from water-saturated mineral solids. The mechanisms investigated were adsorption-retarded aqueous diffusion, micropore diffusion, high-energy micropore adsorption, and micropore blockage by precipitated minerals. To reduce the potential confounding effects of adsorbent heterogeneity, a set of homogeneous silica gel and glass bead adsorbents were used in the investigation. Desorption rates for the slow-desorbing fractions of chloroform (CF), trichloroethylene (TCE), and perchloroethylene (PCE) from silica gel did not conform to the pore-diffusion model for adsorption-retarded aqueous diffusion. This indicated that diffusion through adsorbent mesopores was not responsible for slow desorption from silica gel. Micropore-diffusion modeling of TCE desorption from three silica gels and microporous glass beads indicated that pores less than 2 nm in diameter were responsible for slow desorption. Desorption rates for CF, TCE, and PCE from silica gel were also measured in methanol solutions. Under methanol extraction conditions, desorption rates for all three compounds were 1−2 orders of magnitude less than under water-saturated conditions. This indicated that high-energy adsorption was not responsible for the slow-desorbing fraction, and suggested that mineral precipitation leads to blockage of intragranular micropores. The activation energy for TCE desorption from water-saturated silica gel was measured using temperature-programmed desorption. The TCE desorption activation energy of 15 kJ/mol was close to the dissolution enthalpy for silica gel of 13 kJ/mol. This supported the hypothesis that micropore blockage by precipitated minerals may be limiting contaminant desorption rates under water-saturated conditions.
The desorption kinetics of PCBs and chlorobenzenes have been studied at 5, 20, and 60 °C for model sorbents in which either micropore diffusion (zeolite, montmorillonite, and XAD-8) or organic matrix diffusion/entrapment (rubbery polyacetal and glassy polystyrene) could occur. Also, a sediment was studied whose organic matter (OM) had been completely removed. All sorbents exhibited slow desorption (rate constants (1−5 × 10-3 h-1). The sediment without OM showed significantly smaller slowly desorbing fractions (factor 3−8) than the original sediment (about 6% OM). Sorbent−water distribution ratios of the microporous sorbents and the sediment without OM were 10−100 times lower than the ones of the original sediment. So, although the presence of both mineral micropores and/or OM can result in slow desorption behavior of organic compounds from soils and sediments, OM is more important for slow desorption than mineral micropores in sediments with more than about 0.1−0.5% OM. The sorption and desorption parameters measured for the sorbents were compared to the ones measured for sediment. This analysis showed that the observations for XAD-8 (in which slow desorption is assumed to be caused by slow diffusion along hydrophobic pore walls) were most similar to the ones for the sediment, indicating that diffusion through pores in the organic matter or pores coated with organic material play roles in slow desorption.
In the present study, the desorption kinetics of 15 PAHs (two to six rings) from sediments were determined before and after bioremediation in a bioreactor or landfarm. Desorption kinetics were measured with a method in which the water phase was kept PAH-free by Tenax TA beads. For almost all degraded PAHs, rapidly desorbing fractions (desorption rate constants > 0.1 h-1) were much smaller after bioremediation than before treatment whereas the slowly desorbing amounts remained unchanged. Thus, mainly the rapidly desorbing PAHs are degraded during bioremediation. The extent of possible PAH degradation could be roughly predicted from the initial rapidly desorbing fraction. For nondegraded PAHs, the rapidly desorbing fractions were substantial (up to 55%) and remained unchanged by remediation. The magnitude of the rapidly desorbing fractions of the nondegraded PAHs suggests that their persistence is due to microbial factors, not bioavailability.
Existing field data indicate that soot may significantly affect the environmental speciation of polycyclic aromatic hydrocarbons (PAHs). To expand hydrophobic partition models to include soot partitioning, we need to quantify fsc, the soot fraction of the solid matrix, and Ksc, the soot-carbon-normalized partition coefficient. To this end, we have developed a method that allows quantification of soot carbon in dilute and complex sedimentary matrices. Non-soot organic carbon is removed by thermal oxidation, and inorganic carbonates are removed by acidification, fol lowed by CHN elemental analysis of the residual soot carbon. The selectivity of the soot carbon method was confirmed in tests with matrices of known compostion. The soot quantification technique was applied to two sets of natural sediments, both previously analyzed for PAHs. The input histories of PAHs and soot recorded in a lacustrine sediment core followed the same general trends, and we thus infer a coupling between the two. Our measures of fsc and calculations of Ksc, approximated from studies of PAH sorption onto activated carbon, were applied to rationalize previously generated in situ Kocvalues. Intriguingly, we find that the elevated PAH Kd values of two marine sediment−porewater systems are now quantitatively explain able through the extended, soot-partioning inclusive, distribution model. The importance of the soot phase for PAHs in the environment has implications for how we perceive (and should test) in situ bioavailability and, consequently, also for the development of sediment quality criteria.
The relative rates of desorption and mineralization for spiked concentrations of [14C]phenanthrene and [14C]chrysene preloaded on two previously contaminated soils (foc, 0.029 and 0.0026) were investigated using static, slurry phase microcosms (Vwater/Vsoil = 10). Desorption rates of [14C]phenanthrene and [14C]chrysene preloaded on the contaminated soils were much faster than observed mineralization rates, whereas the desorption rates of native polynuclear aromatic hydrocarbons (PAH) in the higher organic content contaminated soil were equal to or slower than mineralization rates. This suggests that the desorption of aged PAH may control their degradation and may explain the persistence of PAH even in soils containing a large and active community of PAH-degrading microorganisms. In addition, using 14C-spiked PAH in contaminated soils to measure desorption and biodegradation rates may lead to misleading interpretations of the environmental fate of soil-bound polynuclear aromatic hydrocarbons.
The mechanism of formation and release of highly immobilized residues of some small nonpolar halogenated hydrocarbons in two surface soils, a stream sediment and an aquifer sediment was investigated. The labile sorbed fractions of compound were removed from treated soils by purging to infinite dilution in aqueous suspensions for 96 h. The remaining slowly reversible (residual) fraction of compound in the soil was quantitated as a function of prepurge sorption conditions. The residual increased nonlinearly with sorption equilibration time and applied concentration. Residuals in whole soils and whole soils pretreated with H2O2 were correlated with soil organic carbon. Among wet-sieved particle size fractions, however, the organic carbon-based concentrations followed the order, sand > silt >> clay. Also, some residual was associated with undecomposed plant matter. Release of the residual into water was greatly increased by pulverization of the soil and by acidification of the soil suspension. The results indicate that the slow release of the residual fraction is caused by molecular diffusion from remote sites in the soil organic matter matrix. Mineral surface and clay interlayer adsorption were ruled out by the finding of low residuals in the clay-sized particles. However, the mineral fraction plays an important role by shielding some of the organic matter in interstitial pores of particle aggregates from equilibrium with bulk solution.
This paper describes the validation and application of a simple flask-based 14C-respirometer system designed to assess mineralisation of 14C-labelled substrates under defined conditions. Validation of this respirometer system indicated stoichiometric CO2 trapping up to a maximum of 400 μmol of CO2 (in a single trap). Polycyclic aromatic hydrocarbon (PAH)-degrading bacteria were used to measure growth-linked biodegradation of [14C]naphthalene to 14CO2. A 14C activity balance of 101.7±8.9% (n=6), after 74 h incubation time and 10 respirometer-opening events, indicated the suitability of the system for monitoring substrate mineralisation. This respirometric apparatus was then successfully applied to assess: (i) the PAH catabolism of microbes in a field contaminated soil, where naphthalene and phenanthrene were rapidly mineralised and (ii) soil-associated organic contaminant bioavailability, where increased soil–phenanthrene contact time resulted in a reduction in phenanthrene mineralisation in the soil. The described respirometer system differs from existing respirometer systems in that the CO2 trap can be removed and replaced quickly and easily. The system is efficient, reproducible, adaptable to many situations, easy to construct and simple to use, it therefore affords advantages over existing systems.
A technique using Tenax TA® beads as “sink” for desorbed solute was employed to measure the kinetics of desorption of chlorobenzenes, polychlorinated biphenyls, and polycyclic aromatic hydrocarbons from laboratory-contaminated sediment. First-order rate constants of rapid and slow desorption were in the order of 10−1/h and 10−3/h, respectively. The rate constants of slow desorption correlate well with the molecular volumes of the compounds used and decrease between 2 and 34 d of equilibration. Slowly desorbing fractions increase with both increasing solute hydrophobicity and increasing equilibration time.
The adsorption of dysprosium ions onto activated charcoal from aqueous solution has been investigated in relation to pertinent variables, such as shaking time, pH, concentration of dysprosium ions, and temperature. The conditions leading to maximum adsorption have been established. The adsorption of dysprosium ions obeys the Langmuir and the Dubinin-Radushkevich isotherm equations. Thermodynamic quantities, namely AH and AS, have been calculated from the slopes and intercepts of plots of In (KD) versus . The results indicate that the adsorption of dysprosium ions on activated charcoal is an endothermic process. The influence of different cations and anions on the adsorption of dysprosium ions has been examined. The adsorption of other metal ions on activated charcoal has been measured under specified conditions to evaluate their selectivity. Approximately 98% of the dysprosium adsorbed on the activated charcoal was recovered using 40 ml of 3M HNO3 solution.
The widely existing fly ash and soot produced during the process of combustion, which are often known as waste but also an important source of black carbon (BC) in the environment, were treated by HCl and HF solution for this study, and recorded as FC and SC, respectively. A series of experiments were carried out to investigate the toxicity of pentachlorophenol (PCP) in sediment, influence of various BCs in sediment with different contents (0%, 0.5%, 1%, 2%, 5% and 10%) on the extractability and toxicity of PCP (50mg/kg), and toxicity of various BC in sediment. The results demonstrated that the PCP exposure to wheat seed exhibited a dose-dependent behavior, and the extractability and toxicity of PCP decreased with the increasing content of BC in sediment. The PCP extractable rate was significantly (P<0.01) influenced by the higher content of BCs. Noticeably, each BC had no toxic but stimulative effect on root elongation and early seedling growth. Furthermore, it was found that the inhibitive effect on the extractability and toxicity of PCP and the stimulative effect on root elongation and early seedling growth caused by SC were more evident than FC.
Sorption isotherms (pg-ng/L) were measured for 11 polychlorinated biphenyls (PCBs) of varying molecular planarity from aqueous solution to two carbonaceous geosorbents, anthracite coal and traffic soot. All isotherms were reasonably log-log-linear, but smooth for traffic soot and staircase-shaped for coal, to which sorption was stronger and more nonlinear. The isotherms were modeled using seven sorption models, including Freundlich, (dual) Langmuir, and Polanyi-Dubinin-Manes (PDM). PDM provided the best combination of reliability and mechanistically-interpretable parameters. The PDM normalizing factor Z appeared to correlate negatively with sorbate molecular volume, dependent on the degree of molecular planarity. The modeling results supported the hypothesis that maximum adsorption capacities (Q(max)) correlate positively with the sorbent's specific surface area. Q(max) did not decrease with increasing sorbate molecular size, and adsorption affinities clearly differed between the sorbents. Sorption was consistently stronger but not less linear for planar than for nonplanar PCBs, suggesting surface rather than pore sorption.
The desorption of polycyclic aromatic hydrocarbons (PAHs) often exhibits a biphasic profile similar to that observed for biodegradation whereby an initial rapid phase of degradation or desorption is followed by a phase of much slower transformation or release. Most investigations to-date have utilised a polymeric sorbent, such as Tenax, to characterise desorption, which is methodologically unsuitable for the analysis of soil. In this study, desorption kinetics of (14)C-phenanthrene were measured by consecutive extraction using aqueous solutions of hydroxypropyl-beta-cyclodextrin (HPCD). The data indicate that the fraction extracted after 24 h generally approximated the linearly sorbed, rapidly desorbing fraction (F(rap)), calculated using a three-compartment model. A good linear correlation between phenanthrene mineralised and F(rap) was observed (r(2) = 0.89; gradient = 0.85; intercept = 8.20). Hence HPCD extraction (24 h) and first-order three-compartment modelling appear to provide an operationally straightforward tool for estimating mass-transfer limited biodegradation in soil.
Consecutive and single Tenax extractions were applied to characterize the effectiveness of activated charcoal (AC) amendments to reduce the phytoavailability of dieldrin in a natively contaminated horticultural soil. Dieldrin desorption from untreated and 800 mg(AC) kg(-1) soil was well described by a model with three dieldrin fractions of different kinetics: a rapidly (F(rap)), slowly (F(slow)), and very slowly (F(v.slow)) desorbing fraction. The AC amendment resulted in a transfer of dieldrin from the F(slow) to the F(v.slow) fraction. The F(v.slow) increased by nearly 10% compared to the control soil. Dieldrin extractability by Tenax from AC amended soils was not influenced by the cultivation of cucumber plants indicating the stability of this remediation technique. Dieldrin extractability by Tenax at the beginning of plant growth correlated only weakly with the dieldrin content of the cucumbers at harvest. Therefore, the potential of Tenax extractions to predict the uptake of dieldrin by cucumbers appears to be limited.
This paper describes the validation and application of a simple flask-based (14)C-respirometer system designed to assess mineralisation of (14)C-labelled substrates under defined conditions. Validation of this respirometer system indicated stoichiometric CO(2) trapping up to a maximum of 400 micromol of CO(2) (in a single trap). Polycyclic aromatic hydrocarbon (PAH)-degrading bacteria were used to measure growth-linked biodegradation of [(14)C]naphthalene to (14)CO(2). A (14)C activity balance of 101.7+/-8.9% (n=6), after 74 h incubation time and 10 respirometer-opening events, indicated the suitability of the system for monitoring substrate mineralisation. This respirometric apparatus was then successfully applied to assess: (i) the PAH catabolism of microbes in a field contaminated soil, where naphthalene and phenanthrene were rapidly mineralised and (ii) soil-associated organic contaminant bioavailability, where increased soil-phenanthrene contact time resulted in a reduction in phenanthrene mineralisation in the soil. The described respirometer system differs from existing respirometer systems in that the CO(2) trap can be removed and replaced quickly and easily. The system is efficient, reproducible, adaptable to many situations, easy to construct and simple to use, it therefore affords advantages over existing systems.
A simple method to determine the availability of sediment-sorbed organic contaminants was developed and validated. For 10 polycyclic aromatic hydrocarbons, 4 polychlorinated biphenyls, and 9 chlorobenzenes in 6 sediments, we measured the fraction extracted by Tenax in 6 and 30 h. These fractions were compared with the rapidly desorbing fractions determined by consecutive Tenax extraction. Extraction by Tenax for 30 h completely removed the rapidly desorbing fraction plus some part of the slowly desorbing fraction. The fraction removed after 30 h was about 1.4 times the rapidly desorbing fraction. The fraction extracted by Tenax after 6 h is about 0.5 times the rapidly desorbing fraction for chlorobenzenes (CBs)/polychlorinated biphenyls (PCBs) and polycyclic aromatic hydrocarbons (PAHs). The rapidly desorbing fraction probably represents the fraction of sorbed organic compound that poses actual risks for transport to (ground) water and determines the uptake by organisms and that can be microbially degraded. Extraction by Tenax for 6 h provides an easy way to address these issues more accurately than does the measurement of total concentrations.
A study was conducted to investigate whether cyclodextrins and surfactants can be used to predict polycyclic aromatic hydrocarbon (PAH) bioavailability in contaminated sediments. Two sediment samples were extracted with aqueous solutions of hydroxypropyl-beta-cyclodextrin (HPCD) and Triton X-100. PAH removal during extraction was compared with PAH removal during biodegradation and solid-phase extraction. The latter two methods were used as reference methods to establish which part of the PAHs could be biodegraded and to what extent biodegradation was governed by bioavailability limitations. It was demonstrated that HPCD extraction followed solid-phase extraction and removed primarily readily bioavailable PAHs, while Triton X-100 extracted both readily and poorly bioavailable PAHs. Moreover, HPCD did not affect the degradation of PAHs in biodegradation experiments, while Triton X-100 enhanced the degradation of low molecular weight PAHs. It was concluded that HPCD extraction may provide a good method for the prediction of PAH bioavailability. Triton X-100 extraction is unfit for the prediction of PAH bioavailability.
A comprehensive wet chemical procedure was developed by combining acid demineralization, base extraction, and dichromate oxidation for fractionation and quantitative isolation of soil/sediment organic matter (SOM) into four fractions: (1) humic acids + kerogen + BC (HKB); (2) kerogen + BC (KB); (3) humic acid (HA); and (4) BC. The soil/sediment samples tested were collected from the suburban areas of Guangzhou, a rapidly developing city of China. The results show that BC and kerogen constitute 57.8-80.6% of the total organic carbon (TOC) and that the relative content of BC ranges from 18.3% to 41.0% of the TOC, indicating that both BC and kerogen are major organic components in soils and sediments from this industrialized region. Systematic characterization of the isolated SOMs shows that both BC and kerogen have sizes ranging from a few microns to above 100 microm, relatively low O/C and H/C atomic ratios, and low contents of oxygen-containing functional groups. The isolated BC has unique fusinite and semifusinite macerals, highly porous nature, and structures indicative of its possible origins. The study indicates that SOM is highly heterogeneous and that humin, the nonextractable humus fraction, consists mainly of kerogen and BC materials in the tested soil/sediment samples. The presence of these materials in soils and sediments may have significant impacts on pollutant mass transfer and transformation processes such as desorption and bioavailability of less polar organic chemicals in surface aquatic and groundwater environments.
Soot contains high levels of toxic compounds such as polycyclic aromatic hydrocarbons (PAHs). Extraction of PAHs from soot for quantitative analysis is difficult because the compounds are extremely tightly bound to the sorbent matrix. This study was designed to investigate the effect of solvent type on PAH extraction yield, to identify the most optimal solvent for PAH extraction from soot, and to gain insight into the mechanism of PAH sorption to soot in aquatic environments. To that end, different types of soot as well as coal, charcoal, and sediments containing soot-like material were extracted with seven organic solvents. Large differences in extraction recoveries were observed among solvents, with relative values as low as 16% as compared to the best extracting solvent. These differences were much larger for soot than for sediments. Dichloromethane, which to date is the most widely used solvent for soot and sediment extractions, appeared to be the overall worst extractant, whereas toluene/methanol (1:6) gave the best results. Based on extraction yields and solvent properties, extraction of PAHs from soot was explained by a two-step mechanism involving swelling of the sorbent matrix and subsequent displacement of sorbates by solvent molecules. Due to the low displacement capacity of water, desorption of PAHs from soot in the aquatic environment will be strongly limited. Moreover, a certain fraction of the total PAH mass on soot is suggested to be physically entrapped, making it unavailable for partitioning to the aqueous phase.
We hypothesized that the sorption of polycyclic aromatic hydrocarbons (PAHs) to natural sediments and soils should consider both absorption into a biogenic/diagenetic organic carbon (OC) fraction and adsorption onto a combustion-derived, black carbon (BC) fraction. Here, two sets of literature data were reevaluated to illustrate that an OC absorbent and a BC adsorbenttogether can (1) account for sediment--pore-waterdistribution coefficients observed in the field that are greater than predicted by a simple f(OC)K(OC) partitioning model and (2) explain a group of nonlinear phenanthrene isotherms observed in the laboratory with a single value for the BC-normalized distribution coefficient (log K(BC) = 6.1 i 0.04) and a Freundlich exponent (n approximately 0.6 if log K(OC) = 4.0) that is strongly dependent on the K(OC) value selected.
Laboratory based studies on the fate of organic contaminants in soil typically requires the test compound(s) to be spiked into the test medium. Consequently, such studies are inherently dependent on the homogeneity of the contaminant within the spiked soil. Three blending methods were compared for the addition of a phenanthrene-transformer oil mixture into field-wet soil. Spiking homogeneity, reproducibility and artefacts were assessed based on dichloromethane and hydroxypropyl-beta-cyclodextrin chemical extractability, and bacterial mineralization. Spiking using a stainless-steel spoon, consistently produced good spike homogeneity as determined by sample oxidation, chemical extraction and mineralization, and was consistently more reliable than either the Waring blender or modified bench drill. Overall, neither transformer oil-concentration nor blending method influenced chemical extractability or mineralization of the PAH following 1 day equilibration. In general, spiking procedures require validation prior to use, as homogeneity cannot be assured.
Strong sorption to soot- and charcoal-like material (collectively termed black carbon or BC) in soils and sediments is possibly the reason for recent observations of elevated geosorbent-water distribution ratios, slow desorption, limited uptake, and restricted bioremediation. We evaluated the role of environmental BC in the sorption of phenanthrene (PHE) to a polluted lake sediment from a Rhine River sedimentation area. Sorption isotherms were determined over a wide concentration range (0.0005-6 microg/ L) for the original sediment (with organic matter or OM, native sorbates, and BC), sediment from which we had stripped > 90% of the native sorbates (only OM and BC), and sediment combusted at 375 degrees C (only BC). The sorption isotherms of the original and stripped sediments were almost linear (Freundlich coefficient or n(F) > 0.9), whereas the isotherm of the BC remaining after the sediment combustion was highly nonlinear (n(F) = 0.54). At low concentrations (ng/L range), PHE sorption to BC in the combusted sediment was found to exceed the total PHE sorption in the original and stripped sediments. This implies that it may not be possible to use a BC-water sorption coefficient measured in combusted sediment to estimate total sorption to the original sediment. This "intrinsic" BC-water sorption coefficient after combustion was calculated to be 9 times larger than the "environmental" one in the untreated sediment. Competition between the added PHE and the native PAHs and/or OM may explain this difference. It appears that, at low aqueous PHE concentrations (ng/L and below), BC is the most important geosorbent constituent with respect to sorption. At higher concentrations (microg/L), BC sorption sites become saturated and BC sorption is overwhelmed by sorption to the other OM constituents. Because sorption is a central process affecting contaminant behavior and ecotoxicity, understanding this process can strongly contribute to risk assessment and fate modeling.
This paper presents the first attempt to quantify the production, cycling, storage and loss of PAHs in the UK environment. Over 53 000 tonnes of sigmaPAHs (sum of 12 individual compounds) are estimated to reside in the contemporary UK environment, with soil being the major repository. If soils at contaminated sites are included, this estimate increases dramatically. Emission of PAHs to the UK atmosphere from primary combustion sources are estimated to be greater than 1000 tonnes sigmaPAHs per annum, with over 95% coming from domestic coal combustion, unregulated fires and vehicle emissions. It is estimated that approximately 210 tonnes of sigmaPAH are delivered to terrestrial surfaces each year via atmospheric deposition. Therefore, inputs of PAHs to the UK atmosphere outweigh the outputs by a factor of over 4. This may be explained by enhanced particulate deposition near point sources, PAH degradation in the atmosphere and transport away from the UK with prevailing winds. Disposal of waste residues is estimated to contribute a further 1000 tonnes of sigmaPAH per year to the terrestrial environment. It is illustrated that the use of creosote has the potential to release considerable quantities of PAHs to the UK environment. Temporal trends in PAH cycling are then considered. There is good evidence to suggest that air concentrations and fluxes to the UK surface are now lower than at any time throughout this century. Nonetheless, the UK sigmaPAH burden is still increasing at the present time, principally through retention by soils. However, there are marked differences in the behaviour of individual compounds: there is evidence, for example, that phenanthrene concentrations in soils have declined since the 1960s, although soil concentrations of benzo[a]pyrene and other heavier PAHs have continued to increase through this century. Volatilisation of low molecular weight PAHs accumulated in soils over previous decades may be making an important contribution to the current atmospheric burden. The major uncertainties identified by data on this budget are: (1) the lack of PAH concentrations in some environmental matrices; (2) the possible importance of contaminated soils as a major repository and source of PAHs; (3) the lack of emission data (especially vapour phase releases) for some PAH sources; (4) the importance of biodegradation and volatilisation as loss mechanisms for low molecular weight PAHs in soils; and (5) the importance of creosote use in the PAH cycle.
The bioavailability of 38 individual polycyclic aromatic hydrocarbon (PAH) compounds was determined through calculation of biota-sediment-accumulation factors (BSAF). BSAF values were calculated from individual PAH concentrations in freshwater mussel, marine clam, and sediment obtained from field and laboratory bioaccumulation studies. Sediment that was amended with different types of soot carbon (SC) was used in some of the bioaccumulation experiments. BSAF values for petrogenic PAH were greater than those for pyrogenic PAH (e.g., 1.57 +/- 0.53 vs 0.25 +/- 0.23, respectively), indicating that petrogenic PAH are more bioavailable than pyrogenic PAH (p < 0.05). This trend was consistent among marine and freshwater sites. Increased SC content of sediment resulted in a linear decrease in the bioavailability of pyrogenic PAHs (r2 = 0.85). The effect of increasing SC content on petrogenic PAH was negligible. SC was considered as an additional sorptive phase when calculating BSAF values, and using PAH-SC partition coefficients from the literature, we obtained unreasonably large BSAF values for all petrogenic PAH and some pyrogenic PAH. This led us to conclude that a quantitative model to assess bioavailability through a combination of organic carbon and soot carbon sorption is not applicable among field sites with a wide range of soot carbon fractions and PAH sources, at least given our current knowledge of PAH-SC partitioning. Our data offer evidence that many factors including analysis of a full suite of PAH analytes, PAH hydrophobicity, sediment organic carbon content, sediment soot carbon content, and PAH source are importantto adequately assess PAH bioavailability in the environment.
Soot and charcoal, collectively termed "black carbon" or BC, can exhibit extremely strong sorption of many hydrophobic organic compounds. In order to include BC sorption in fate models, it is important to know BC nanopore surface areas. In addition, it is useful to know for which compounds BC sorption can be expected to be important. By nitrogen adsorption measurements at ultralow pressures on sediment that was strongly enriched in BC by HF treatment and/or chemothermal oxidation at 375 degrees C, we found that environmental BC has nanoporosity in the <4-10 A size range. The nanopore surface area (<50 A) of BC in Lake Ketelmeer (The Netherlands) sediment was approximately 58 m2/g. We measured sorption isotherms over a wide concentration interval for four compounds with the same Kow (10(46+/-0.1): planar anthracene (ANT), phenanthrene (PHE), and 4-chlorobiphenyl (4-PCB) along with nonplanar 2,2'-dichlorobiphenyl (2,2'-PCB). The environmental BC sorption coefficients of these iso-Kow compounds decreased in the order ANT > PHE approximately 4-PCB > 2,2'-PCB and spanned a factor of 50-200, depending on concentration. Nonplanar 2,2'-PCB showed much more linear BC sorption (nF = 0.92) than the planar compounds (nF = 0.54-0.70). This shows that steric hindrance strongly attenuates BC-sorbate interactions for a nonplanar PCB. Thus, BC is more important for environmental sorption of planar compounds (>50% sorbed to BC in the nanogram per liter range) than for nonplanar ones (<10-20%). Using the measured BC nanopore surface area, a close agreement between modeled and measured BC sorption data could be found.
Interest in the biodegradation mechanisms and environmental fate of polycyclic aromatic hydrocarbons (PAHs) is motivated by their ubiquitous distribution, their low bioavailability and high persistence in soil, and their potentially deleterious effect on human health. Due to high hydrophobicity and solid-water distribution ratios, PAHs tend to interact with non-aqueous phases and soil organic matter and, as a consequence, become potentially unavailable for microbial degradation since bacteria are known to degrade chemicals only when they are dissolved in water. As the aqueous solubility of PAHs decreases almost logarithmically with increasing molecular mass, high-molecular weight PAHs ranging in size from five to seven rings are of special environmental concern. Whereas several reviews have focussed on metabolic and ecological aspects of PAH degradation, this review discusses the microbial PAH-degradation with special emphasis on both biological and physico-chemical factors influencing the biodegradation of poorly available PAHs.
The sorption of polycyclic aromatic hydrocarbons (PAHs) to soot carbon in marine sediments has been hypothesized to reduce PAH bioavailability. This hypothesis was tested for eight species of marine benthic invertebrates (four polychaete worms, Clymenella torquata, Nereis virens, Cirriformia grandis, and Pectinaria gouldii, and four bivalve mollusks, Macoma balthica, Mulinia lateralis, Yoldia limatula, and Mya arenaria) that span a wide range of feeding behavior, ability to metabolize PAHs, and gut chemistry. Organisms were exposed for 20 d to two PAH-spiked sediments, one with soot and one without soot. The soot treatment generally resulted in lower bioaccumulation than the no soot treatment, though the differences between treatments were not significant for all species. All but one species accumulated significant PAH concentrations in their tissues from the soot treatment, indicating that soot-bound PAH cannot be dismissed as unavailable to infaunal benthic biota. Bioaccumulation factors were correlated negatively to both the organisms' ability to metabolize PAHs and the gut fluid contact angle, supporting the hypotheses that high PAH metabolism results in lower bioaccumulation factors and bioavailability of PAHs may be limited partially by PAH solubilization in the gut lumen. The variability in bioaccumulation due to the soot treatment was much less than the variability between species and between PAH analytes. Comparatively low bioaccumulation was observed in Nereis virens, a species commonly used in bioaccumulation tests. These results suggest that more effort is needed in understanding the salient characteristics of species present in a threatened environment, rather than focusing solely on the sediment geochemistry (e.g., soot and organic carbon content) and contaminant characteristics when predicting ecological risk of PAH-contaminated sediments.
The roles and relative importance of nonpyrogenic organic carbon (NPOC) and black carbon (BC) as binding phases of polycyclic aromatic hydrocarbons (PAHs) were assessed by their ability to estimate pore water concentrations and biological uptake in various marine sediments. Sediment bioaccumulation tests were performed with the marine polychaete Nereis virens, using a polyethylene device to estimate pore water concentrations of PAHs. Using existing partitioning data for pyrene and phenanthrene, it was found that the traditional Equilibrium Partitioning model, which assumes all organic carbon is NPOC (EqP(OC)), overestimated the measured pore water concentrations in the test sediments by one to three orders of magnitude. Instead, the measured pore water concentrations were better predicted from a distribution scenario that uses both BC and NPOC (EqP(NPOc,BC)) When comparing actual worm body burdens of pyrene and phenanthrene with the two model estimates of worm tissue concentrations, the EqP(OC) model tended to overestimate actual body burdens by three orders of magnitude, while the EqP(NPOC,BC) model came much closer to the true body burden values. The observed distribution of PAHs in the test sediments was used to calculate BC partition coefficients for five PAHs, which were one to two orders of magnitude higher than their corresponding organic carbon-normalized distribution coefficients, or K(OC)s. Together, these results suggest that, in certain situations, adding black carbon to distribution models may be necessary to predict accurately the bioavailability of PAHs.
Soot, soot-amended sediment, and unamended sediment spiked with hydrophobic organic contaminants (HOC) were subjected to laboratory desorption and assimilation efficiency experiments in an effort to assess and compare the importance of soot in controlling HOC desorption and deposit-feeder assimilation efficiency. Three contaminants, naphthalene (NAP), benzo[a]pyrene (BaP), and hexachlorobenzene (HCB) were sorbed to sediments, sediments amended with soot (2-4% dry wt), and soot for a period of 34 d. Desorption of all three contaminants into seawater from the three prepared sorbates was then monitored, and Nereis succinea assimilation efficiency experiments on the BaP- and HCB-contaminated sorbates were conducted. Both NAP and BaP desorption rates for soot and soot-amended sediments were reduced by at least a factor of two relative to unamended sediment. Hexachlorobenzene desorption rates were similar for both the soot-amended and the unamended sediments. Results of N. succinea assimilation efficiency experiments indicate a trend similar to the desorption experiments: higher assimilation of BaP from unamended relative to soot-amended sediment and little difference in assimilation between treatments for HCB. However, soot was more effective in reducing BaP desorption than assimilation efficiency, which would be consistent with the hypothesis that the gut fluid dissolution by deposit feeders may partially decouple biological availability from chemical availability.
Bioaccumulation of hydrophobic organic chemicals from sediments containing soot or sootlike materials has been hypothesized to be limited by strong sorption of the chemicals to the soot matrixes. To test this hypothesis, we quantified bioaccumulation of 11 polychlorinated biphenyls (PCBs) into the aquatic oligochaete Limnodrilus sp. exposed to spiked sediment with and without the sootlike materials coal and charcoal. In addition, sorption experiments with sediment containing varying amounts of coal or charcoal were performed to elucidate the accumulation mechanism. Results showed that coal and charcoal (at realistic levels of 1.5% on a dry-wt basis) reduced PCB accumulation in worms 1.2 to 8.5 times when expressed on a mass basis. Moreover, whereas bioaccumulation from pure sediment increased with molecular planarity of the PCBs (toxic potency), it decreased in case of sediments containing coal and charcoal. In contrast to this advantageous effect, it was hypothesized that coal and charcoal had an adverse influence on the habitat quality of oligochaetes: Organisms inhabiting sediment containing coal or charcoal had significantly reduced lipid contents as compared to organisms from pure sediment. Because of these reduced lipid contents, lipid-normalized PCB concentrations in worms and biota-to-sediment accumulation factors (BSAFs) for most PCBs were higher in sediments containing the sootlike materials as compared to those for reference sediment. Also, measured BSAFs for coal- and charcoal-containing sediments appeared to be much higher than estimated on the basis of equilibrium partitioning theory. Sorption experiments revealed that this was caused by much weaker sorption to the sediment-coal/charcoal mixture than calculated assuming linear additivity of sorption capacities of the distinct phases. It was hypothesized that this weaker sorption resulted from competition between PCBs and dissolved organic carbon molecules for sorption sites on coal/charcoal. This points to a sorption process that is much more complicated than generally assumed.
Recent studies demonstrate that sedimentary black carbon (BC) affects the sorption of some hydrophobic organic contaminants (HOCs) to a greater extent than sedimentary organic carbon (OC). Among HOC, polycyclic aromatic hydrocarbons (PAHs) are known to interact extensively with BC. Currently, data on the sorption of various kinds of HOCs to different types of BC are limited. In this study, we amended a marine sediment with BC from several different sources, humic acid, and inert sand. Equilibration studies with 14C fluoranthene and the polychlorinated biphenyl (PCB) 3H 2,4'-dichlorinated biphenyl were performed to determine the magnitude of sorption as a function of contaminant and BC type. The magnitude of sorption to the BC-amended sediments was greater for the PAH than the PCB as compared to the sediment alone, humic acid, and sand. For example, differences between the log partition coefficient (K(P)) for the PAH and PCB ranged from 0.41 to 0.69 log units for humic acid and sand treatments, while differences ranged from 0.88 to 1.57 log units for the BC-amended sediments. As a result, BC-normalized partition coefficients (log K(BC)) for the PAH averaged 6.41, whereas the PCB log K(BC) values averaged 5.33. These results demonstrate that PAH sorption and most likely bioavailability are influenced strongly by the presence of BC of different types, while sorption of a nonplanar PCB was affected to a lesser degree.
The first paper of this series reported that soil/sediment organic matter (SOM) can be fractionated into four fractions with a combined wet chemical procedure and that kerogen and black carbon (BC) are major SOM components in soil/sediment samples collected from the industrialized suburban areas of Guangzhou, China. The goal of this study was to determine the sorptive properties forthe four SOM fractions for organic contaminants. Sorption isotherms were measured with a batch technique using phenanthrene and naphthalene as the sorbates and four original and four Soxhlet-extracted soil/sediment samples, 15 isolated SOM fractions, and a char as the sorbents. The results showed that the sorption isotherms measured for all the sorbents were variously nonlinear. The isolated humic acid (HA) exhibited significantly nonlinear sorption, but its contribution to the overall isotherm nonlinearity and sorption capacity of the original soil was insignificant because of its low content in the tested soils and sediments. The particulate kerogen and black carbon (KB) fractions exhibited more nonlinear sorption with much higher organic carbon-normalized capacities for both sorbates. They dominate the observed overall sorption by the tested soils and sediments and are expected to be the most important soil components affecting bioavailability and ultimate fate of hydrophobic organic contaminants (HOCs). The fact that the isolated KB fractions exhibited much higher sorption capacities than when they were associated with soil/sediment matrixes suggested that a large fraction of the particulate kerogen and BC was not accessible to sorbing HOCs. Encapsulation within soil aggregates and surface coverage by inorganic and organic coatings may have caused large variations in the accessibility of fine kerogen and BC particles to HOCs and hence lowered the sorption capacity of the soil. This variability posts an ultimate challenge for precisely predicting HOC sorption by soils from the contents of different types of SOM.
The solid-water distribution ratios (Kd values) of "native" PAHs, PCBs, and PCDDs in Boston and New York Harbor sediments were determined using small passive polyethylene samplers incubated for extended times in sediment-water suspensions. Observed solid-water distribution coefficients exceeded the corresponding f(oc)Koc products by 1-2 orders of magnitude. It was hypothesized that black carbon (fBC), measured in the Boston harbor sediment at about 0.6% and in the New York harbor sediment at about 0.3%, was responsible for the additional sorption. The overall partitioning was then attributed to absorption into the organic carbon and to adsorption onto the black carbon via Kd = f(oc)Koc + f(BC)K(BC)C(w)n-1 with Cw in microg/L. Predictions based on published Koc, K(BC), and n values for phenanthrene and pyrene showed good agreement with observed Kd,obs values. Thus, assuming this dual sorption model applied to the other native PAHs, PCBs, and PCDDs, black carbon-normalized adsorption coefficients, K(BC)S, were deduced forthese contaminants. Log K(BC) values correlated with sorbate hydrophobicity for PAHs in Boston harbor (log K(BC) approximately 0.83 log gamma w(sat) - 1.6; R2 = 0.99, N= 8). The inferred sorption to the sedimentary BC phase dominated the solid-water partitioning of these compound classes, and its inclusion in these sediments is necessary to make accurate estimates of the mobility and bioavailability of PAHs, PCBs, and PCDDs.
Desorption kinetics were determined for fluoranthene (FLU) and trifluralin (TF) spiked onto Lake Erie and Lake Huron, USA, sediments at three concentrations (10, 40, 100 mg/kg dry wt). Following four months of equilibration, desorption was measured by extraction with Tenax and the data were fit to a first-order three-compartment kinetic model. The rate constants of the rapidly (k(rap)), slowly (k(slow)), and very slowly (k(vs)) desorbing fractions were on the order of 10(-1)/h, 10(-2-3)/h, and 10(-4)/h, respectively. The t99.9 (time required for 99.9% of the FLU and TF to desorb from each pool value) for each compartment indicated that FLU and TF desorption from rapid, slow, and very slow compartments were on the order of hours, days, and years, respectively. Higher rates of desorption were observed for FLU and TF from the Lake Huron sediments and this was not apparently related to the total organic carbon (TOC), particle size distribution, or polarity (carbon-to-nitrogen ratio) of the sediments. In general, the total fraction of the initial contaminant amounts that desorbed over the time course was directly related to concentration, which we hypothesized was due to the combined effects of saturation of high-energy (slow and very slow) binding sites in the organic carbon matrix and hysteresis. In extrapolations to field conditions, FLU and TF were predicted to persist in the sediments for years due to the very slow desorption of an estimated 31 to 53% of the bulk concentrations. Based on the rapidly desorbing fractions, the bioavailable amounts of the contaminants were predicted to be between 31 to 55% of bulk sediment concentrations.
The aim of this paper was to estimate the contribution to total phenanthrene sorption from unburned coal and black carbon (BC; soot and charcoal) in sediment. We determined sorption isotherms for five Argonne Premium Coal standards over a wide concentration interval (0.01-10 000 ng/L). The coals showed strong and nonlinear sorption (carbon-normalized K(F) = 5.41-5.96; nF = 0.68-0.82). Coal sorption appeared to become more nonlinear with increasing coal maturity. The coal's specific surface area appeared to influence K(F). On the basis of the current coal sorption observations combined with earlier petrographic analyses and BC sorption experiments, we calculated for one particular sediment that coal, BC, and "other" OC were all important to PHE sorption in the environmentally relevant nanogram per liter range. This indicates that it is important to consider strong sorption to coal in the risk assessment of coal-impacted geosorbents (e.g., river beds) where coal is mined/shipped and manufactured gas plant sites.
Biota-to-sediment accumulation factors (BSAFs) sometimes show large variations between sites for a given planar hydrophobic organic compound (pHOC) in a given organism. We present a framework in which strong adsorption to soot/charcoal (black carbon [BC]) explains such BSAF variations. Recently constrained parameters on environmental BC sorption were used in a dual-mode sorption model of BSAE Variations in sedimentary BC content are shown to explain variations in BSAF of one to two orders of magnitude. In addition, strong BC sorption can explain the often-observed difference in BSAF between polycyclic aromatic hydrocarbons (PAHs) and polychlorinated biphenyls (PCBs). The strong nonlinearity of BC sorption can render BSAF values strongly concentration-dependent for a given sediment/organism/compound combination, so it is of paramount importance to carry out uptake and toxicity tests at the relevant environmental concentrations.
In bioavailability studies, the biota sediment accumulation factor (BSAF) is invoked to describe the thermodynamic partitioning of a hydrophobic organic contaminant (HOC) between the organism lipid and the organic carbon fraction of the sedimentary matrix and accounts for differences in bioavailability among sediments. Bioaccumulation experiments were performed with Lumbriculus variegatus and Diporeia species exposed in seven sediments dosed with 2,4,5,2',4',5'-hexachlorobiphenyl (HCBP) and benzo[a]pyrene (BaP) or pyrene (PY) and 3,4,3',4'-tetrachlorobiphenyl (TCBP). The BSAF values for the nonplanar HCBP were consistent with equilibrium partitioning theory (EQP) and averaged 2.87 for L. variegatus and 1.45 for Diporeia, while the BSAF values for the planar compounds (BaP, PY, TCBP) were generally lower than estimated from EQP (<1). Correcting the BSAF values of the planar compounds for enhanced sorption due to black carbon improved the BSAF values for L. variegatus, generally resulting in values consistent with EQP, but substantial variation remained for Diporeia. The BSAF values for the planar compounds showed significant positive correlations with plant-derived carbon in sediments (lignin and pigments) but were more consistent for L. variegatus than for Diporeia. These correlations imply that compounds sorbed to plant-derived carbon are more bioavailable since this material is more likely ingested providing a second exposure route.
Molecular interactions controlling the sorption of pollutants to environmental black carbons (soot, charcoal) are not well-resolved. Sorption of a series of aromatic compounds was studied to wood charcoal and nonporous graphite powder as a model adsorbent. Issues of concern were the possible involvement of pi-pi electron donor-acceptor (EDA) interactions of electron-poor and electron-rich solutes with the graphene (polycyclic aromatic) surface and size exclusion effects. Sorption of pi-acceptors, benzonitrile (BNTL), 4-nitrotoluene (MNT), 2,4-dinitrotoluene (DNT), and 2,4,6-trinitrotoluene (TNT), and to a lesser extent pi-donor solutes, naphthalene (NAPH) and phenanthrene (PHEN), was greater than predicted by hydrophobic driving forces in accord with their acceptor or donor strength. Hydrophobic effects were estimated using a concentration-dependent free energy relationship between adsorption and partitioning into an inert solvent (n-hexadecane or benzene) for a non-donor/non-acceptor calibration set (benzene and chlorinated and methylated benzenes). Molecular complexation between acceptors and model graphene donors, NAPH, PHEN, and pyrene (PYR), in chloroform and benzene was tracked by ring-current induced upfield shifts in the 1H NMR spectrum and by charge-transfer bands in the UV/visible spectrum. The EDA component of graphite-water adsorption for the acceptors correlated with the NMR-determined complexation constant with the model donors in chloroform, which, in turn, correlated with pi-acceptor strength (TNT > DNT > MNT > BNTL) and pi-donor strength (PYR > PHEN > NAPH). Charcoal-graphite isotherms calculated from charcoal-water and graphite-water isotherms indicated molecular sieving effects on charcoal for tetrasubstituted benzenes (tetramethylbenzenes and TNT) and some trisubstituted benzenes (1,3,5-trichlorobenzene, possibly DNT). When steric effects are taken into account, the order in adsorption among acceptors was qualitatively similar for graphite and charcoal. The results suggest pi-pi EDA interactions of the acceptors-and possibly donors, although the calibration set may underestimate the hydrophobic effect for fused ring systems-with both graphite and charcoal surfaces. For graphite, it is postulated that pi-acceptors interact with electron-rich regions of the basal plane near edges and defects and that pi-donors interact with electron-depleted regions further away. A similar mechanism may operate on the charcoal but would be modified by the (mostly) electron-withdrawing effects of 0 functionality on the edges of graphene sheets.
It is becoming increasingly clear that the products of incomplete combustion (soot and charcoal, collectively termed black carbon or BC) can be responsible for as much as 80 - 90% of the total sorption to sediments of aromatic, planar, and hydrophobic compounds such as polycyclic aromatic hydrocarbons or planar polychlorinated biphenyls. In the present study, it was investigated whether a nonpolar aliphatic compound (hexachloroethane) and three nonplanar bipolar compounds with different functional groups [free electron pairs but no aromatic ring (butylate) or free electron pairs and an aromatic ring (diuron, atrazine)] would also show strong and nonlinear sorption to a BC-enriched sediment. At a concentration of 1 ng/L, the extent of elevated BC sorption compared to total organic carbon (TOC) sorption increased in the order atrazine < hexachloroethane < butylate < diuron. Rationalization of the differences between the sorbates was attempted in terms of dispersive and steric effects. This study shows that the effects of strong BC sorption apply to a broader range of organic contaminants than previously thought, and the results will aid in a better understanding of BC sorption mechanisms and improved fate modeling of contaminants in the environment.
Recent laboratory studies show that mixing activated carbon with contaminated sediment reduces the chemical and biological availability of hydrophobic organic contaminants. In this study, we test the effects of varying the activated carbon dose and particle size in reducing the aqueous availability of polychlorinated biphenyls (PCBs) and polycyclic aromatic hydrocarbons (PAHs) and the uptake of PCBs by two benthic organisms. We mixed PCB- and PAH-contaminated sediment from Hunters Point Naval Shipyard, San Francisco Bay (CA, USA), for one month with activated carbon, at doses of 0.34, 1.7, and 3.4% dry mass basis. We found that increasing the carbon dose increased the effectiveness in reducing PCB bioaccumulation. In 56-d uptake tests with the benthic organisms Neanthes arenaceodentata and Leptocheirus plumulosus, PCB bioaccumulation was reduced by 93 and 90%, respectively, with 3.4% carbon. Increasing the dose also increased the effectiveness in reducing PCB and PAH aqueous concentrations and uptake by semipermeable membrane devices and quiescent flux of PCBs to overlying water. Decreasing activated carbon particle size increased treatment effectiveness in reducing PCB aqueous concentration, and larger-sized activated carbon (400-1,700 microm) was ineffective with a contact period of one month. We invoke a numerical model based on intraparticle diffusion in sediment and activated carbon particles to help interpret our experimental results. This model was useful in explaining the trends for the effect of activated carbon dose and particle size on PCB aqueous concentrations in well-mixed systems.
Recently, it has become apparent that the use of total contaminant concentrations as a measure of potential contaminant exposure to plants or soil organisms is inappropriate and that bioavailability of contaminants is a better measure of potential exposure. In light of this, non-exhaustive extraction techniques are being investigated to assess their appropriateness in determining bioavailability. In this study, phenanthrene extractability using hydroxypropyl-beta-cyclodextrin (HPCD) and desorption kinetics using butan-1-ol (BuOH) were determined in three dissimilar spiked soils. The soils were extracted after 1 d, 40 d and 80 d of soil-compound contact time. The amount of phenanthrene extracted by HPCD was compared to the rapidly desorbed fraction removed by BuOH. Further experiments using the same soils and extraction methods to assess the relative extractability of phenanthrene, pyrene and benzo(a)pyrene were conducted. Overall, the extraction methods used in this study had different extraction efficiencies. Results suggest that as compound hydrophobicity increased, BuOH became a more exhaustive extractant with respect to HPCD, especially for soils with high clay and organic matter content. These results are important as they highlight differences between two contrasting non-exhaustive extraction techniques both of which have been suggested to be appropriate in the assessment of bioavailability.
The aim of this study was to characterize the behaviour of a PAH-degrading bacterium to determine whether mineralization plateaus as a result of substrate removal, a decrease in microbial activity or nutrient availability in sterile soils over time. To investigate this, the mineralization of 14C-phenanthrene was measured until it plateaued; subsequently, additional 14C-phenanthrene, catabolic inocula or nutrients were introduced and mineralization was measured for a further 10 d. Cell numbers were also measured together with 14C-uptake into microbial biomass. Freshly added 14C-phenanthrene was rapidly metabolised by the microorganisms. Neither the addition of a catabolic inoculum nor nutrients affected the extent of 14C-phenanthrene mineralization. Cell numbers remained constant over time, with only a small amount of the 14C-activity incorporated into the microbial biomass. This study indicated that the termination of mineralization was due to the removal of available phenanthrene and not decreasing cellular activity or cell death. The mineralization values also correlated with 14C-phenanthrene extractability using beta-cyclodextrin.
Evidence is accumulating that sorption of organic chemicals to soils and sediments can be described by "dual-mode sorption": absorption in amorphous organic matter (AOM) and adsorption to carbonaceous materials such as black carbon (BC), coal, and kerogen, collectively termed "carbonaceous geosorbents" (CG). Median BC contents as a fraction of total organic carbon are 9% for sediments (number of sediments, n approximately 300) and 4% for soils (n = 90). Adsorption of organic compounds to CG is nonlinear and generally exceeds absorption in AOM by a factor of 10-100. Sorption to CG is particularly extensive for organic compounds that can attain a more planar molecular configuration. The CG adsorption domain probably consists of surface sites and nanopores. In this review it is shown that nonlinear sorption to CG can completely dominate total sorption at low aqueous concentrations (<10(-6) of maximum solid solubility). Therefore, the presence of CG can explain (i) sorption to soils and sediments being up to 2 orders of magnitude higher than expected on the basis of sorption to AOM only (i.e., "AOM equilibrium partitioning"), (ii) low and variable biota to sediment accumulation factors, and (iii) limited potential for microbial degradation. On the basis of these consequences of sorption to CG, it is advocated that the use of generic organic carbon-water distribution coefficients in the risk assessment of organic compounds is not warranted and that bioremediation endpoints could be evaluated on the basis of freely dissolved concentrations instead of total concentrations in sediment/soil.
The emission of black carbon is known to cause major environmental problems. Black carbon particles contribute to global warming, carry carcinogenic compounds and cause serious health risks. Here, we show another side of the coin. We review evidence that black carbon may strongly reduce the risk posed by organic contaminants in sediments and soils. Extremely efficient sorption to black carbon pulls highly toxic polycyclic aromatic hydrocarbons, polychlorinated biphenyls, dioxins, polybrominated diphenylethers and pesticides into sediments and soils. This increased sorption is general, but strongest for planar (most toxic) compounds at environmentally relevant, low aqueous concentrations. Black carbon generally comprises about 9% of total organic carbon in aquatic sediments (median value of 300 sediments), and then may reduce uptake in organisms by up to two orders of magnitude. This implies that current environmental risk assessment systems for these contaminants may be unnecessarily safe.
When assessing the potential of a contaminated site for bioremediation, it is desirable to know how much of the contaminant(s) is available for microbial degradation, thus allowing the likelihood of successful bioremediation to be predicted. The aims of this study were to investigate the degradation of PAHs in two soils by a catabolic inoculum and indigenous soil microflora and link this to the cyclodextrin extractable fraction in the presence of transformer oil (0.05, 0.01, or 0.005%). This study showed very little difference between indigenous and inoculum-derived degradation for phenanthrene in laboratory-aged soil, and strong relationships were also observed between both of the microbial degradative conditions and the amount of phenanthrene extracted by cyclodextrin. Furthermore, the indigenous degradation of PAHs in a field-contaminated soil showed significant linear correlations with the cyclodextrin extractable fraction, with gradients approximating to 1. There are several novel facets to this study. First, in aged, contaminated soils, indigenous microflora gave an equally sensitive determination of degradative availability as that measured by the catabolic inoculum. Second, this is the first time intrinsic biodegradation of PAHs has been predicted by the cyclodextrin extraction in laboratory-spiked and field-contaminated soils. The cyclodextrin extraction technique represents a powerful tool for predicting the extent of intrinsic and augmented microbial degradation and will be useful in the assessment of contaminated land prior to bioremediation.
In this study, an aqueous-based hydroxypropyl-beta-cyclodextrin (HPCD) extraction technique was assessed for its capacity to determine the microbially degradable fraction of mono- and polycyclic aromatic hydrocarbons in four dissimilar soils. A linear relationship (slope=0.90; R2=0.89), approaching 1:1 between predicted and observed phenanthrene mineralization, was demonstrated for the cyclodextrin extraction; however, the water only extraction underestimated the microbially available fraction by a factor of three (slope=3.35; R2=0.64). With respect to determining the mineralizable fraction of p-cresol in soils, the cyclodextrin extraction (slope=0.94; R2=0.84) was more appropriate than the water extraction (slope=1.50; R2=0.36). Collectively, these results suggested that the cyclodextrin extraction technique was suitable for the prediction of the mineralizable fraction of representative PAHs and phenols present in dissimilar soils following increasing soil-contaminant contact times. The assessment of the microbial availability of contaminants in soils is important for a more representative evaluation of soil contamination.
There is currently considerable scientific interest in finding a chemical technique capable of predicting bioavailability; non-exhaustive extraction techniques (NEETs) offer such potential. Hydroxypropyl-beta-cyclodextrin (HPCD), a NEET, is further validated through the investigation of concentration ranges, differing soil types, and the presence of co-contaminants. This is the first study to demonstrate the utility of the HPCD-extraction technique to predict the microbial availability to phenanthrene across a wide concentration range and independent of soil-contaminant contact time (123 d). The efficacy of the HPCD-extraction technique for the estimation of PAH microbial availability in soil is demonstrated in the presence of co-contaminants that have been aged for the duration of the experiment together in the soil. Desorption dynamics are compared in co-contaminant and single-PAH contaminated spiked soils to demonstrate the occurrence of competitive displacement. Overall, a single HPCD-extraction technique proved accurate and reproducible for the estimation of PAH bioavailability from soil.