Chapter

Bioavailability and Bioaccessibility of Hydrophobic Organic Contaminants in Soil and Associated Desorption-Based Measurements

Authors:
To read the full-text of this research, you can request a copy directly from the authors.

Abstract

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.

No full-text available

Request Full-text Paper PDF

To read the full-text of this research,
you can request a copy directly from the authors.

... This means that the extent of biodegradation in PAHs-contaminated sites is limited by bioavailability (Semple et al., 2003;White et al., 1999), i.e., the fraction of the soil that can be taken up or used by microorganisms in a given time period. However, as soil contact time with PAHs increases, the bioavailability of PAHs decreases, a phenomenon known as "aging," in which sequestration and sorption are key processes (Umeh et al., 2020). Sequestration is a series of physical processes that lead to the presence of chemicals in soil, involving the capture of contaminants into soil micropores or nanopores through molecular diffusion. ...
... Sequestration is a series of physical processes that lead to the presence of chemicals in soil, involving the capture of contaminants into soil micropores or nanopores through molecular diffusion. Sorption is a combination of physical adsorption, partitioning into organic matter, and chemical binding of HOCs to soil matrices (Umeh et al., 2020). Typically, aging goes through three stages, (1) rapid-contaminants are easily desorbed and returned to pore water, (2) slow-reversible but with a long desorption time, and (3) irreversible-contaminants are tightly bound to the soil and will not be biodegraded (Cipullo et al., 2018;Wang et al., 2017). ...
... In general, the adsorption nature and extent of PAHs in water is controlled by the organic carbon-water partition coefficient (K oc ) and the soil organic matter content (SOM). When K oc is high, PAHs are mostly adsorbed in soil rather than dissolved in pore water, and vice versa (Umeh et al., 2020). However, PAHs are hydrophobic organic pollutants (HOCs), while the aliphatic and aromatic structures of SOM have high adsorption affinity for PAHs (Kobayashi et al., 2009;Sun et al., 2013), making most of PAHs adsorbed in the soil. ...
Article
Full-text available
Bioavailability is one of the important concepts affecting bioremediation of PAHs-contaminated site but often ignored in reaction transport models, resulting in failure of site repair prediction and incorrect evaluation of important parameters. A reaction transport model considering bioavailability is constructed that uses transition probability and Lagrangian methods to analyze the transport and reaction of PAHs in soil rather than the conventional reaction transport model. This method avoids the numerical problems associated with solving the transmission problem directly. The effects of soil physicochemical properties and biodegradation rate on the reaction and migration were evaluated using phenanthrene as model pollutant. It shows that low biodegradation rates are detrimental, may lead to elevated PAHs concentrations in liquids, prolong PAHs bioaccessibility, and create a risk of leakage, so the appropriate addition of reagents such as enzyme preparations is effective, but simply increasing the biodegradation rate is insufficient for contaminant removal from soil. Through sensitivity analysis, it was determined that the most important factor affecting the residual PAHs in soil should be the physical and chemical properties of soil, which provides a basis for the addition of surfactants, while it is also pointed out that the toxicity evaluation of surfactants is of great significance in soil remediation.
... The distinction between bioaccessibility, the portion of the pollutant that is available for uptake, and bioavailability is whether the bioavailable chemical is biologically incorporated. Here "bioaccessibility" is defined as the portion of a compound released and accessible for absorption by organisms (Umeh et al. 2020). Thus, for a MP additive to potentially induce toxicity effects, the chemical must first be bioaccessible, for example, within the gastrointestinal tract following ingestion and after leaching from the polymer resin matrix, and then be bioavailable to the organism, by crossing epithelium membranes to reach systemic circulation and/or enter cells and to become available at sites of biological activity (Umeh et al. 2020). ...
... Here "bioaccessibility" is defined as the portion of a compound released and accessible for absorption by organisms (Umeh et al. 2020). Thus, for a MP additive to potentially induce toxicity effects, the chemical must first be bioaccessible, for example, within the gastrointestinal tract following ingestion and after leaching from the polymer resin matrix, and then be bioavailable to the organism, by crossing epithelium membranes to reach systemic circulation and/or enter cells and to become available at sites of biological activity (Umeh et al. 2020). The release of leachates (additives released from the polymer matrix to the environment) from MPs and other plastic items can consequently result in a chemical-led toxicological effect, rather than induced impact of the physical particle, if substances are made bioaccessible, and consequentially bioavailable. ...
Chapter
Microplastics (MPs, <5 mm in size) are considered to be one of today’s major environmental problems. They are a ubiquitous persistent pollutant group that has reached into all parts of the environment – from the highest mountain tops to the lowest depths of the ocean. In their production, plastics have added to them a number of chemical additives in the form of plasticizers, colorants, fillers, and stabilizers, some of which with known toxicological effects to biota. When released into nature, MPs are also likely to encounter and sorb to their surfaces a variety of established pollutants including hydrophobic organic contaminants, trace metals, and pharmaceuticals. Importantly, MPs have been shown to be readily ingested by a wide range of organisms and it is this combination of biotic ingestion and chemical association that gives credence to the notion that MPs may impact the bioavailability and toxicity of both endogenous and exogenous pollutants. This chapter provides an overview of the literature that has examined the role of MPs as chemical carriers to biota, with particular focus on aquatic organisms. The influence and interactions of MPs with endogenous and exogenous chemicals are reviewed before a more critical view of the relative importance of MPs as pathway for chemical transfer is provided. Lastly, the current state of the literature is placed into context of the needs of risk assessment highlighting the challenges to assessing the risk of MPs as chemical vehicles.
... Here, different analytical approaches are commonly applied to differentiate between the total and bioavailable fractions of the organic contaminant. Such methods can be roughly separated into those that target the microbially available fraction of contaminant in the matrix (e.g., mild extractions or desorption-type methods) [10] and partitioning approaches that specifically target the freely dissolved fraction (e.g., passive sampling) [1,11]. ...
... In our proposal, it would be possible to assess the removal of the bioavailable fraction (however it is measured) instead of or as well as of the total amount of chemical. For example, desorption extraction methods, recently standardized as ISO/TS 16751 [68] and described in detail in another chapter of this series [10], can be used in simulation tests with soils (OECD 307, see Table 1). This is similar to previous attempts to use standardized desorption extraction as a tool to evaluate the performance of bioremediation using bioavailability assessments in a wide variety of PAH-contaminated soils [21,22]. ...
Chapter
Full-text available
Biodegradable chemicals may become persistent due to reductions in their bioavailability thereby impacting on the rate and extent of biodegradation in soils and sediments. This chapter examines this – commonly neglected – contradictory face of persistence assessments from the light of the latest advancements in bioavailability science. They include the microbial influences on bioavailability, the different sorption capacities of carbonaceous components of soils and sediments, and the dissimilar bioavailability shown by chemicals when they are present as non-extractable residues. We also discuss possible pathways to improve the realism in persistence assessments from standardized biodegradation tests by incorporating new bioavailability-based approaches. Innovations of the standard tests are possible through the modified chemical application of enhanced dispersion and passive dosing. In addition, we offer a proposal for integrating bioavailability measurements into standard simulation tests with soils and sediments, by using desorption extraction and passive sampling methods to assess the removal of the bioavailable fractions, in addition to the total extractable concentration of the chemical.
... Sorption of these co-contaminants onto MPs has led to consider them as vectors of hazardous chemicals into aquatic biota (Avio et al., 2015;Batel et al., 2018). The toxicological consequences are mainly related to MPs-associated chemicals that desorbing from them may become bioaccessible and then bioavailable to the organisms (Umeh et al., 2020). Biota can sorb chemical contaminants transported by MPs by several pathways (e.g., via the skin, gills, gut) (Endo et al., 2005). ...
Article
Microplastics (MPs) in the marine environment undergo complex weathering factors that can affect their ability to interact with different coexisting environmental contaminants (termed here co-contaminants). In this study, the influence of artificially aging using UV on the sorption of a complex mixture of co-contaminants onto MPs was investigated in order to provide meaningful hypotheses on their individual and combined toxicities on sea urchin embryos. A mixture of artificially aged MPs (PS particles and PA microfibers) combined with 2,2′,4,4′-tetrabromodiphenyl ether (BDE-47), or Cd or Cu, both alone and in a mix, were used to expose embryos of Paracentrotus lividus. The effects of polymer aging on co-contaminants bioavailability were assessed by measuring changes in the transcriptional profile of genes involved in oxidative-stress response and skeletogenic and endo-mesodermal specification. Changes in the sorption ability of MPs to co-contaminants in the aqueous phase highlighted that aging did not affect the sorption of BDE-47 and Cd on MPs, although a certain influence on Cu sorption was found. Despite no morphological effects in embryos at the gastrula stage after MPs/contaminants combinatorial exposure emerged, the greatest influence of the aging process was mainly found for combined exposures which included BDE-47. Finally, the exposure to multiple contaminants generated transcriptional profiles poorly related to those activated by single contaminant, at times suggesting a mixture-dependent different aging influence. These results open new scenarios on the controversial role of vector of co-contaminants for MPs, especially when complex and different types of mixtures were considered.
... Apart from the pyrene concentration in soil and the additional variables cited above, the environmental and ecotoxicological properties of pyrene strongly depend on the content of natural organic matter in soil (Cachada et al., 2016). Indeed, hydrophobicity and bioaccumulating characteristics make pyrene highly prone to be adsorbed onto suspended particulates and biota, and accumulated in soil and sediments Umeh et al., 2020). ...
Article
Soil contamination by pyrene has increased over the years due to human-related activities, urgently demanding for remediation approaches to ensure human and environment safety. Within this frame, phytoremediation has been successfully applied over the years due to its green and cost-effectiveness features. The scope of this review includes the main phytoremediation mechanisms correlated with the removal of pyrene from contaminated soils and sediments to highlight the impact of different parameters and the supplement of additives on the efficiency of the treatment. Soil organic matter (SOM), plant species, aging time, environmental parameters (pH, soil oxygenation, and temperature) and bioavailability are among the main parameters affecting pyrene removal through phytoremediation. Phytoextraction only accounts for a small part of the entire phytoremediation process, but the addition of surfactants and chelating agents in planted soils could increase pyrene accumulation in plant tissues by 20% as a consequence of the increased pyrene bioavailability. Rhizodegradation is the main phytoremediation mechanism involved due to the activity of bacteria capable of degrading pyrene in the root area. Inoculated-planted soil treatments have the potential to decrease pyrene accumulation in shoots and roots by approximately 30 and 40%, respectively, further stimulating the proliferation of pyrene-degrading bacteria in the rhizosphere. Plant-fungi symbiotic association results in an enhanced accumulation of pyrene in shoots and roots of plants as well as a higher biodegradation. Finally, pyrene removal from soil can be improved in the presence of amendments, such as natural non-ionic surfactants, biochar, and bacterial mixtures.
Article
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.
Chapter
The quality of soils and sediments is commonly evaluated on the basis of total contaminant concentrations within prospective as well as in retrospective risk assessment. From common practice, the perception has arisen that performing risk evaluation by measuring or modelling total concentrations often leads to either over- or underestimation of the risks: too often there is an indication of risk whereas in reality the ecosystem seems to be unaffected, whereas on the other hand, there are examples in which adverse effects are observed in realistic field settings at levels well below toxicity levels generated in a laboratory setting. This calls for properly accounting for the impact of the binding capacity of soils and sediments on the availability and risks of contaminants in soils and sediments.
Article
Full-text available
The gastrointestinal mobilization and oral bioaccessibility of polycyclic aromatic hydrocarbon (PAH) nonextractable residues (NERs) from soils remain unexplored, including associated incremental lifetime cancer risks. This study investigated the gastrointestinal mobilization of PAHs and their NERs from contrasting soils, using a physiologically based extraction test that incorporates a silicone-rod (Si-Org-PBET) as PAH sink. Associated cancer risks following soil ingestion were also evaluated. Four solvent-spiked and aged soils, and four long-term contaminated manufactured gas plant (MGP) soils, were utilized. Total-extractable PAH concentrations were measured after exhaustive solvent extractions of soils. We evaluated the PAH sorption efficiency of the silicone rods and associated sorption kinetics, using PAH-spiked silica sand as the contaminated matrix. We then assessed gastrointestinal mobilization of benzo[a]pyrene and benzo[a]pyrene NERs from the solvent-spiked soils, and mobilization of six PAHs and their NERs from the MGP soils. PAH oral bioaccessibility was determined. The incremental lifetime cancer risks (ILCRs), using Si-Org-PBET- and total-extractable PAH concentrations from the MGP soils, were calculated. Sorption kinetics modelling showed that 95% of mobilized PAHs sorbed to the silicone rods within 2-19 h, depending on PAH physico-chemical properties. Total-extractable and Si-Org-PBET extractable PAH concentrations exceeded health investigation levels (3 mg/kg based on benzo[a]pyrene toxic equivalent quotients) in soils. PAH oral bioaccessibility approached 100% for solvent-spiked soils, but only 24-36% for the MGP soils. Associated ILCRs exceeded target levels (10-5) for one MGP soil, particularly for 2-3 year olds, despite oral bioaccessibility considerations. In contrast, mobilized PAH NERs did not exceed health investigation and ILCR levels, as the NERs were highly sequestered, especially in the MGP soils. PAH nonextractable residues in long-term contaminated soils are unlikely to be mobilized in concentrations that pose cancer risks to humans following soil ingestion, and do not need to be considered in risk assessments.
Article
Full-text available
Biodegradation of polycyclic aromatic hydrocarbons (PAHs) and total petroleum hydrocarbons (TPH) in sediment and soil has been monitored on seven experimental fields during periods up to 25 years. With this unique dataset, we investigated long-term very slow biodegradation under field conditions. . The data show that three biodegradation rates can be distinguished for PAHs: 1) rapid degradation during the first year, 2) slow degradation during the following 6 years and 3), subject of this paper, a very slow degradation after 7 years until at least 25 years. Beside 2-, 3- and 4-ring PAHs, also 5- and 6-ring PAHs (aromatic rings) were degraded, all at the same rate during very slow degradation. In the period of very slow degradation, 6% yr-1 of the PAHs present were removed in five fields and 2% yr-1 in two other fields, while in the same period no very slow degradation of TPH could be observed. The remaining petroleum hydrocarbons were high boiling and non-toxic. Using the calculated degradation rates and the independently measured bioavailability of the PAHs (Tenax-method), the PAHs degradation curves of all seven monitored fields could be modelled. Applying the model and data obtained with the Tenax-method for fresh contaminated material, results of long-term biodegradation can be predicted, which can support the use of bioremediation in order to obtain a legally acceptable residual concentration.
Article
Full-text available
The bioavailability of organic chemicals in soil and sediment is an important area of scientific investigation by environmental scientists, but it is still poorly understood by regulators and industry working in the environmental sector. Regulators are starting to consider bioavailability within retrospective risk assessment frameworks for organic chemicals: by doing this, realistic decision-making on polluted environments is achievable, rather than relying on the traditional approach of using total extractable concentrations. However, implementation is not straightforward because the developments of bioavailability science have not always been translated into ready-to-use approaches for regulators. Similarly, bioavailability still remains largely unexplored within prospective regulatory frameworks dealing with approval and regulation of organic chemicals. This article discusses bioavailability concepts and methods, as well as possible pathways for the implementation of bioavailability into risk assessment and regulation, and it offers a simple, pragmatic and justifiable approach within retrospective and prospective fields.
Article
Full-text available
This review provides a comprehensive overview about nonextractable residue (NER) formation and attempts to classify the various types. Xenobiotic NER derived from parent pesticides (or other environmental contaminants) and primary metabolites sorbed or entrapped within the soil organic matter (Type I) or covalently bound (Type II) pose a considerably higher risk than those derived from productive biodegradation. However, biogenic nonextractable residues (bioNER) (Type III) resulting from conversion of carbon (or nitrogen) from the compounds into microbial biomass molecules do not pose any risk. Experimental approaches to clearly distinguish between the types are provided, and a model to prospectively estimate bioNER formation in soil is proposed.
Article
Full-text available
Persistent organic pollutants (POPs), such as polycyclic aromatic hydrocarbons (PAHs), may be found in high concentrations in soils of former industrial sites including manufactured gas plants or coking plants. Techniques using moderate solvent extraction, biological tests or solid phase extraction have proved useful for pollution availability estimation. However, more accurate and reliable measurement tools specifically adapted to low concentrations are still needed. Based on a solid-liquid extraction using a Tenax® resin, we suggest a protocol to assess the bioavailability of PAHs, dedicated to aged industrial wasteland soils. Desorption kinetics were measured on three representative contaminated industrial soils. Results were modeled using a first order two-compartment model that provided an estimate of the rapidly desorbing fraction, which was considered to be available, over a 30h extraction period. In conclusion, this method, allowing the measurement of the available fraction, might prove more relevant than the total concentration value when assessing soil contamination related risks. It may also predict achievable bioremediation performances.
Article
Full-text available
This chapter analyzes the underlying basis for risk-based land management. The protection and management of soils and groundwater has slowly climbed the environmental agenda. In common with many international jurisdictions, and in order to protect the users of land and water, environmental regulations have gradually evolved. While legislation differs amongst the regional Asia-Pacific countries, the underlying principle of environmental regulation is to ensure that land and water is fit for use and does not pose an unacceptable risk to human health or the environment. In this chapter, sources of contaminants and their interactions in soils are discussed. Potential risks to the environment and human health and cost-effective strategies including RBLM that could be adopted to minimize environmental impact of such pollutants are elaborated. Also, the details of risk-based land management are provided. An overview of bioavailability and risk management is presented and risk management policy is also explained. Historical approach to remediation is elaborated. Concepts related to contaminant interaction, and key contaminants are provided in the chapter. Selecting optimal risk-management strategies is described in detail. A case study is presented for the application of risk-based land management in case of arsenic‑contaminated soil.
Article
Full-text available
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%.
Article
Bioaccessibility extractions are increasingly applied to measure the fraction of pollutants in soil, sediment and biochar, which can be released under environmentally or physiologically relevant conditions. However, the bioaccessibility of hydrophobic organic chemicals (HOCs) can be markedly underestimated when the sink capacity of the extraction medium is insufficient. Here, a novel method called "Membrane Enhanced Bioaccessibility Extraction" (MEBE) applies a semipermeable membrane to physically separate an aqueous desorption medium that sets the desorption conditions from an organic medium that serves as acceptor phase and infinite sink. The specific MEBE method combines HOC (1) desorption into a 2-hydroxypropyl-β-cyclodextrin solution, (2) transfer through a low-density polyethylene (LDPE) membrane and (3) release into ethanol, serving as analytical acceptor phase. The surface to volume ratio within the LDPE membrane is maximized for rapid depletion of desorbed molecules, and the capacity ratio between the acceptor phase and the environmental sample is maximized to achieve infinite sink conditions. Several experiments were conducted for developing, optimizing and pre-testing the method, which was then applied to four soils polluted with polycyclic aromatic hydrocarbons. MEBE minimized sample preparation and yielded a solvent extract readily analyzable by HPLC. This study focused on the proof-of-principle testing of the MEBE concept, which now can be extended and applied to other samples and desorption media.
Article
Bioaccessibility measurements of polycyclic aromatic hydrocarbons (PAHs) in soils are significant for exposure risk assessment. The current physicochemical methods require tedious operation processes, underestimate the actual risks, or are unsuitable for high organic content soils. In this work, an efficient and convenient method based on polydopamine-coated polyethylene sieve plate (PDA@PESP) and hydroxypropyl-β-cyclodextrin (HPCD) was developed to predict the bioaccessibility of PAHs in multi-type soils. The PDA@PESP can be prepared via in situ self-polymerization, allowing to extract PAHs from HPCD solution quantitatively and rapidly. When applied to evaluate the bioaccessibility with PDA@PESP as an adsorption sink and HPCD as a diffusive carrier, the proposed method can significantly improve the extractable fraction of PAHs compared to single HPCD extraction in particular for high organic carbon content soil and high-ring PAHs. The desorption kinetics data indicated that the method can predict the bioaccessible fraction of PAHs. In addition, the method predicted a satisfactory accumulation into earthworms (Eisenia fetida) with a slope statistically approximated to 1. A highly significant linear regression (R2 = 0.95) was also found between the proposed method and Tenax desorption in historically contaminated soils, demonstrating that the method is an efficient and convenient approach for the bioaccessibility prediction of PAHs in soils.
Article
The potential for bioaccumulation and associated genotoxicity of nonextractable residues (NERs) of polycyclic aromatic hydrocarbon (PAHs) in long-term contaminated soils have not been investigated. Here we report research in which earthworms, Eisenia fetida, were exposed to a soil containing readily available benzo[a]pyrene (B[a]P) and highly sequestered B[a]P NERs aged in soil for 350 days. B[a]P bioaccumulation was assessed and DNA damage (as DNA single strand breaks) in earthworm coelomocytes were evaluated by comet assay. The concentrations of B[a]P in earthworm tissues were generally low, particularly when the soil contained highly sequestered B[a]P NERs, with biota-soil accumulation factors ranging from 0.6 to 0.8 kgOC/kglipid. The measurements related to genotoxicity, that is percentage (%) of DNA in the tails and olive tail moments, were significantly greater (p < 0.05) in the spiked soil containing readily available B[a]P than in soil that did not have added B[a]P. For example, for the soil initially spiked at 10 mg/kg, the percentage of DNA in the tails (29.2%) of coelomocytes after exposure of earthworms to B[a]P-contaminated soils and olive tail moments (17.6) were significantly greater (p < 0.05) than those of unspiked soils (19.6% and 7.0, for percentage of DNA in tail and olive tail moment, respectively). There were no significant (p > 0.05) differences in effects over the range of B[a]P concentrations (10 and 50 mg/kg soil) investigated. In contrast, DNA damage after exposure of earthworms to B[a]P NERs in soil did not differ from background DNA damage in the unspiked soil. These findings are useful in risk assessments as they can be applied to minimise uncertainties associated with the ecological health risks from exposure to highly sequestered PAH residues in long-term contaminated soils.
Article
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.
Article
The environmental and health risks associated with ‘non-extractable’ residues (NERs) of polycyclic aromatic hydrocarbons in soils and their potential for remobilisation remain largely unexplored. In this novel study, sequential solvent extractions were employed to interrogate time-dependent remobilisation of benzo[a]pyrene (B[a]P) NERs and associated kinetics after re-equilibration (REQ) periods lasting 30 d in four artificially-spiked soils aged for up to 200 days. Following sequential extractions of the re-equilibrated soils, remobilisation of B[a]P NERs was observed and further confirmed by decreases in the absolute amounts of B[a]P recovered following methanolic saponification after REQ. Remobilisation may occur through slow intercompartmental partitioning of more sequestered into less sequestered B[a]P fractions in soils. The amounts of B[a]P remobilised in soils decreased throughout aging following first-order kinetics and the rates of decrease were slow but 2 to 4 times faster than those of extractable B[a]P before re-equilibration. Sandy-clay-loam soils with large amounts of hard organic carbon exhibited less NER remobilisation compared to sandy soils. The amounts of remobilised B[a]P decreased significantly (p < 0.05) with aging. Specifically, butanol-remobilised B[a]P in soils spiked at 10 mg/kg and 50 mg/kg B[a]P ranged from 0.15 to 0.39 mg/kg and 0.67 to 2.30 mg/kg, respectively, after 200 d of aging.
Article
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.
Article
The fate and behaviour of polycyclic aromatic hydrocarbons (PAHs) in soil is of interest in the risk assessment of contaminated lands and are usually based on determinations of fractions extracted from soil. For decades, either single or sequential solvent extractions have been used to determine PAH extractability in soils; however, there is a lack of certainty as to which fractions are being extracted by these techniques. This study is the first report of changes and similarities in extractability of benzo[a]pyrene (B[a]P) in four contrasting soils (sandy loam, loamy sand, clayey loam, and sandy) when determined, simultaneously using both single (dichloromethane-DCM/acetone-Ace mixture) and sequential solvent (butanol followed by DCM/Ace) extractions. Residues after extraction were subjected to methanolic saponification (MeKOH). Butanol (BuOH)- and total-extractability of B[a]P, following sequential solvent extraction, decreased significantly (p < 0.05) with time after addition of B[a]P. The decrease in BuOH extractability was particularly marked in the organic matter-rich clayey loam soil which also had the largest (> 40%) amounts of non-extractable residues. The cumulative amounts of B[a]P extracted in each soil by single and sequential solvent extractions were similar (p > 0.05) at each aging period, which indicate access to similar B[a]P fractions in soil by both solvent extractions. The similarity in the amounts of B[a]P non-extractable residues recovered by MeKOH of pre-extracted soils, through either of the extraction methods, confirms that the total-extractable B[a]P fractions from both methods are similar.
Article
Predicting mammalian bioavailability of PAH mixtures from in vitro bioaccessibility results has proven to be an elusive goal. In an attempt to improve in vitro predictions of PAH soil bioavailability we investigated how energetic input influences PAH bioaccessibility by using a high and low energetic shaking method. Co-inertia analysis (COIA), and Structural Equation Modeling (SEM) were also used to examine PAH-PAH interactions during ingestion. PAH bioaccessibility was determined from 14 historically contaminated soils using the fed organic estimation of the human simulation test (FOREhST) with inclusion of a silicone rod as a sorption sink and compared to bioavailability estimates from the juvenile swine model. Shaking method significantly affected PAH bioaccessibility in the FOREhST model, with PAH desorption from the high energy FOREhST almost an order of magnitude greater compared to the low energy FOREhST. PAH-PAH interactions significantly influenced PAH bioavailability and when these interactions were used in a linear model, the model predicted benzo(a)anthracene bioavailability with an slope of 1 and r(2) of 0.66 and for benzo(a)pyrene bioavailability has a slope of 1 and r(2) of 0.65. Lastly, to confirm the effects as determined by COIA and SEM, we spiked low levels of benzo(a)anthracene into historically contaminated soils, and observed a significant increase in benzo(a)pyrene bioaccessibility. By accounting for PAH interactions, and reducing the energetics of in vitro extractions, we were able to use bioaccessibility to predict bioavailability across 14 historically contaminated soils. Our work suggests that future work on PAH bioavailability and bioaccessibility should focus on the dynamics of how the matrix of PAHs present in the soil interact with mammalian systems. Such interactions should not only include the chemical interactions discussed here but also the interactions of PAH mixtures with mammalian uptake systems.
Article
The bioavailability and bioaccessibility of polycyclic aromatic hydrocarbons (PAHs) in soil underpin the risk assessment of contaminated land with these contaminants. Despite a significant volume of research conducted in the past few decades, comprehensive understanding of the factors controlling the behaviour of soil PAHs and a set of descriptive soil parameters to explain variations in PAH bioavailability and bioaccessibility are still lacking. This review focuses on the role of source materials on bioavailability and bioaccessibility of soil PAHs, which is often overlooked, along with other abiotic factors including contaminant concentration and mixture, soil composition and properties, as well as environmental factors. It also takes into consideration the implications of different types of risk assessment (ecological and human health) on bioavailability and bioaccessibility of PAHs in soil. We recommend that future research should (1) account for the effects of source materials on bioavailability and bioaccessibility of soil PAHs; (2) adopt non-disruptive methods to analyse soil components controlling PAH sequestration; (3) integrate both natural organic matter (NOM) and xenobiotic organic matter (XOM) while evaluating the influences of soil organic matter (SOM) on the behaviour of PAHs; and (4) consider the dissimilar desorption scenarios in ecological risk assessment and human health risk assessment while assessing PAH bioavailability and bioaccessibility.
Article
Cleanup goals for sites contaminated with persistent organic pollutants (POPs) are often established based on total contaminant concentrations. However, mounting evidence suggests that understanding contaminant bioavailability in soils is necessary for accurate assessment of contaminant exposure to humans via oral ingestion pathway. Animal-based in vivo tests have been used to assess contaminant bioavailability in soils; however, due to ethical issues and cost, it is desirable to use in vitro assays as alternatives. Various in vitro methods have been developed, which simulate human gastrointestinal (GI) tract using different digestion fluids. These methods can be used to predict POP bioavailability in soils, foods, and indoor dust after showing good correlation with in vivo animal data. Here, five common in vitro methods are evaluated and compared using PAHs and PBDEs as an example of traditional and emerging POPs. Their applications and limitations are discussed while focusing on method improvements and future challenges to predict POP bioavailability in different matrices. The discussions should shed light for future research to accurately assess human exposure to POPs via oral ingestion pathway.
Article
PAH bioaccessibility in contaminated soil was determined using the organic physiologically based extraction test with the inclusion of a sorption sink. Silicone cord was incorporated into the assay in order to overcome the limited capacity of the in vitro medium to accommodate desorbable PAHs. Initially, silicone cord sorption efficacy was determined by assessing sorption kinetics using PAH-spiked sand (phenanthrene, pyrene and benzo[a]pyrene; 10-1000mgkg-1). Irrespective of PAH and concentration, >95% of the initial PAH mass partitioned into the silicone cord within 12h although rates were lower at higher concentration and with increasing hydrophobicity. When PAH bioaccessibility was assessed in contaminated soil (n=18), contamination source (i.e. pyrogenic versus petrogenic) influenced PAH bioaccessibility. Individual PAH bioaccessibility ranged up to 81.7±2.7% although mean values ranged from 2.1 (acenaphthalene) to 20.8% (benzo[k]fluoranthene) with upper 95% confidence intervals of the means of 4.5 and 28.3% respectively. Although a PAH in vivo-in vitro correlation is yet to be established, bioaccessibility approaches incorporating sorption sinks represent a robust approach for estimating PAH bioavailability as the desorbable fraction may be a conservative measure of the absorbable fraction.
Article
It is well documented that using exhaustive chemical extractions is not an effective means of assessing exposure of hydrophobic organic compounds in sediments and that bioavailability-based techniques are an improvement over traditional methods. One technique that has shown special promise as a method for assessing the bioavailability of hydrophobic organic compounds in sediment is the use of Tenax-extractable concentrations. A 6-h or 24-h single-point Tenax-extractable concentration correlates to both bioaccumulation and toxicity. This method has demonstrated effectiveness for several hydrophobic organic compounds in various organisms under both field and laboratory conditions. In addition, a Tenax bioaccumulation model was developed for multiple compounds relating 24-h Tenax-extractable concentrations to oligochaete tissue concentrations exposed in both the laboratory and field. This model has demonstrated predictive capacity for additional compounds and species. Use of Tenax-extractable concentrations to estimate exposure is rapid, simple, straightforward, and relatively inexpensive, as well as accurate. Therefore, this method would be an invaluable tool if implemented in risk assessments. Environ Toxicol Chem 2015;34:1445-1453. © 2015 SETAC. © 2015 SETAC.
Article
Polycyclic aromatic hydrocarbons (PAHs) associated with soot or black carbon can enter the human digestive tract by unintentional ingestion of soil or other particles. This study investigated the bioaccessibility of 11 PAHs in a composite fuel soot sample using an in vitro digestive model that included silicone sheet as an absorptive sink during the small intestinal digestion stage. The sheet was meant to simulate the passive transfer of PAHs in lumen fluid across the small intestinal epithelium which was postulated to promote desorption of labile PAHs from the soot by steepening the soot-fluid concentration gradient. We show that the presence of silicone sheet during a 4-h default digestion time significantly increased the apparent bioaccessible fraction (Bapp, %), defined as the sum in the sheet and digestive fluid relative to the total PAH determined. The ability to increase Bapp for most PAHs leveled off above a sheet-to-soot ratio of 2.0 g per 50 mg, indicating that the sheet is an effective absorptive sink and promotes desorption in the mentioned way. Enhancement of Bapp by the sheet correlated positively with octanol-water partition coefficient (Kow), even though the partition coefficient of PAH between sheet and digestive fluid (which contains bile acid micelles) correlated negatively with Kow. It was hypothesized that PAHs initially in the soot exist in labile and nonlabile states. The fraction of labile PAH still sorbed to the soot residue after digestion and the maximum possible (limiting) bioaccessibility (Blim) could be estimated by varying the sheet-to-soot ratio. We show conclusively that the increase in bioccessibility due to the presence of the sheet is accounted for by a corresponding decrease in fraction of labile PAH still sorbed to the soot. The Blim ranged from 30.8 to 62.4%, independent of molecular size. The nonlabile fraction of individual PAHs (69.2 - 37.6% in this case) is therefore large and needs to be taken into account in risk assessment.
Article
Former studies on human oral bioaccessibility of polycyclic aromatic hydrocarbons (PAH) from natural soil samples using human in vitro digestive tract models (physiologically based extraction tests, PBET) show highly variable results (0-100 % of mobilized PAH). Apart from other factors, the type and amount of present geosorbents are assumed to be significant for the degree of desorption/release of PAH into the digestive juice. Therefore, in this study, the reference geosorbents pure quartz sand, Na-montmorillonite clay, Pahokee peat, and charcoal "Sommerhit" were spiked with selected deuterated PAH and employed as single materials in a PBET. Lowest bioaccessibility was determined in charcoal, representing black carbon (0.1 ± 0.1 % for ∑10 PAH-d) in contrast to higher bioaccessibility in peat (6.4 ± 2.2 %) and clay (4.8 ± 1.1 %). Highest bioaccessibility was determined in sand (26.9 ± 7.5 %). The results show a systematic impact of heterogeneous geosorbents on human oral bioaccessibility of PAH and particularly black carbon acting as a very strong geosorbent that reduces human health risk.
Article
Characterizing sediment-associated hydrophobic contaminants is problematic, because assessing the total amount of a compound available for chemical exchange with an organism is difficult. To address this, contaminant concentrations have been normalized for specific sediment characteristics (including organic C content) or the chemical activity has been estimated using passive samplers. Another approach to assess compound availability is to determine the extent of readily desorbed compound using resin extractions of sediment slurries. The present paper reviews the literature that uses Tenax® TA, a 2,6-diphenylene-oxide polymer as an extraction tool to measure bioavailability of hydrophobic organic contaminants in sediment. Some work has assessed the extent of desorption with sequential extractions to characterize the maximum rate and pool sizes for different desorbing fractions of bound contaminant. As such, the rapidly desorbing fraction has been well correlated with the extent of degradation, bioaccumulation, and toxicity of hydrophobic contaminants. A shortcut to measuring the full desorption curve to determine the rapidly desorbing compound is to use a single-point extraction, with 6 h or 24 h extractions being the most common. The Tenax extraction has been shown to be effective with laboratory-spiked sediments, field-collected sediments, laboratory-exposed organisms, field-collected organisms, and studies among laboratories. Furthermore, a literature-based model has described the bioaccumulation of polychlorinated biphenyls from independently measured field-collected sediments. Despite the success of this approach, applying the Tenax method to manage contaminated sediments is limited by the absence of a standard set of conditions to perform the extractions, as well as standard methods for using field sediments. Integr Environ Assess Manag 2015;11:208–220. © 2014 SETAC Key Points The current paper reviews the literature using Tenax TA, a p-2,6-diphenylphyleneoxide polymer. The Tenax extraction has been shown to be effective with laboratory-spiked sediments, field-collected sediments, laboratory-exposed organisms, field-collected organisms and studies among laboratories, and a literature-based model has been developed that described the bioaccumulation of polychlorinated biphenyls from independently measured field-collected sediments. Despite the success of this approach, application of the Tenax method for management of contaminated sediments is limited by the absence of a standard set of conditions for performing the extractions and standard methods for using field sediments.
Article
The chemical interactions of hydrophobic organic contaminants (HOCs) with soils and sediments (geosorbents) may result in strong binding and slow subsequent release rates that significantly affect remediation rates and endpoints. The underlying physical and chemical phenomena potentially responsible for this apparent sequestration of HOCs by geosorbents are not well understood. This challenges our concepts for assessing exposure and toxicity and for setting environmental quality criteria. Currently there are no direct observational data revealing the molecular-scale locations in which nonpolar organic compounds accumulate when associated with natural soils or sediments. Hence macroscopic observations are used to make inferences about sorption mechanisms and the chemical factors affecting the sequestration of HOCs by geosorbents. Recent observations suggest that HOC interactions with geosorbents comprise different inorganic and organic surfaces and matrices, and distinctions may be drawn along these lines, particularly with regard to the roles of inorganic micropores, natural sorbent organic matter components, combustion residue particulate carbon, and spilled organic liquids. Certain manipulations of sorbates or sorbent media may help reveal sorption mechanisms, but mixed sorption phenomena complicate the interpretation of macroscopic data regarding diffusion of HOCs into and out of different matrices and the hysteretic sorption and aging effects commonly observed for geosorbents. Analytical characterizations at the microscale, and mechanistic models derived therefrom, are needed to advance scientific knowledge of HOC sequestration, release, and environmental risk.
Article
In this study, PAH bioavailability was assessed in creosote-contaminated soil following bioremediation in order to determine potential human health exposure to residual PAHs from incidental soil ingestion. Following 1000days of enhanced natural attenuation (ENA), a residual PAH concentration of 871±8mgkg(-1) (∑16 USEPA priority PAHs in the <250μm soil particle size fraction) was present in the soil. However, when bioavailability was assessed to elucidate potential human exposure using an in vivo mouse model, the upper-bound estimates of PAH absolute bioavailability were in excess of 65% irrespective of the molecular weight of the PAH. These results indicate that a significant proportion of the residual PAH fraction following ENA may be available for absorption following soil ingestion. In contrast, when PAH bioavailability was estimated/predicted using an in vitro surrogate assay (FOREhST assay) and fugacity modelling, PAH bioavailability was up to 2000 times lower compared to measured in vivo values depending on the methodology used.
Article
The evaluation of the available fraction of hydrophobic organic contaminants (HOCs) is extremely important for assessing their risk to the environment and human health. This available fraction, which can be solubilized and/or easily extracted, is believed to be the most accessible for bioaccumulation, biosorption and/or transformation by organisms. Based on this, two main types of chemical methods have been developed, closely related to the concepts of bioaccessibility and freely available concentrations: non-exhaustive extractions and biomimetic methods. Since bioavailability is species and compound specific, this work focused only in one of the most widespread group of HOCs in soils: polycyclic aromatic hydrocarbons (PAHs). This study aims at producing a state of the art knowledge base on bioavailability and chemical availability of PAHs in soils, clarifying which chemical methods can provide a better prediction of an organism exposure, and which are the most promising ones. Therefore, a review of the processes involved on PAHs availability to microorganisms, earthworms and plants was performed and the outputs given by the different chemical methods were evaluated. The suitability of chemical methods to predict bioavailability of the 16 US EPA PAHs in dissimilar naturally contaminated soils was not yet demonstrated, being especially difficult for high molecular weight compounds. Even though the potential to predict microbial mineralization using non-exhaustive extractions is promising, it will be very difficult to achieve for earthworms and plants, due to the complexity of accumulation mechanisms which are not taken into account by chemical methods. Yet, the existing models could be improved by determining compound, species and site specific parameters. Moreover, chemical availability can be very useful to understand the bioavailability processes and the behavior of PAHs in soils. The inclusion of chemical methods on risk assessment has been suggested and it is promising, despite some methods overpredict risks.
Article
The relative bioaccessibility leaching procedure (RBALP) is a simple, reproducible, and rapid in vitro procedure for estimating the in vivo (juvenile swine) relative bioavailability (RBA) of lead in solid media. Control of pH, temperature, and agitation are the most critical parameters of this in vitro procedure. The performance of the method was evaluated by triplicate analyses of each of 19 different test substances by the author and three independent laboratories, and comparison of the results to relative bioavailability (RBA) values measured in vivo. The results indicate that the RBALP measurements are strongly correlated with the in vivo RBA values (r = 0.924, p < 0.0001), with an average absolute error of 10% and an average predictive error of 20%. Comparison of results within and between laboratories indicates that the procedure is highly reproducible, with inter-and intra-laboratory coefficients of variation of 4% and 6%, respectively, and within-sample precision of approximately 7%. Based on the results reported here, the RBALP can be effective in providing reliable estimates of lead RBA as predicted by the immature swine model. The method may also be valuable in evaluating site-specific differences in bioaccessibility, assessing remedial technologies intended to reduce lead RBA, providing a screening mechanism for futurein vivo studies, and providing insight into the chemical and physical factors that control lead bioavailability.
Article
The sorption and desorption kinetics of polycyclic aromatic hydrocarbons (PAHs) by soils in mix-solvent systems were studied. The rate of PAH uptake or desorption from the solid phase was dependent upon the composition of the co-solvent/water mixture. Generally, higher co-solvent volume fractions resulted in a significant increase in rates of PAH sorption/desorption. A simplified retarded-radial diffusion model (RRDM) was applied to model the effects of co-solvents on the sorption kinetics of hydrophobic organic compounds. This approach assumes that the diffusion of an organic molecule into soil arregates is retarded by tortuosity effects and partitioning between the porefluids and sorbent organic matter. In the presence of co-solvents, the diffusional mobility of hydrophobic organic contaminants would presumably be enhanced. The RRDM was calibrated using sorption/desorption rate data collected for one PAH (phenanthrene), and was capable of estimating the effects of co-solvents on the sorption/desorption kinetics of the other two analytes. In all cases the RRDM was able to show that both sorption and desorption rates were significantly enhanced with increasing of co-solvent in the solution phase. 26 refs., 12 figs., 2 tabs.
Article
The ability of carbonaceous geosorbents (CGs) such as black carbon (BC) to extensively sorb many common environmental contaminants suggests that they potentially possesses qualities useful to the sequestration of harmful xenobiotics within contaminated land. Presently, however, there is limited understanding of the implications for the bioaccessibility, mobility and environmental risk of organic contaminants while sorbed to BC in soil and sediment, in addition to the inherent toxicity of BC itself to terrestrial flora and fauna. We review both the processes involved in and factors influencing BC sorption characteristics, and ultimately consider the impacts BC will have for bioavailability/bioaccessibility, toxicity and risk assessment/remediation of contaminated land. We conclude that while the application of BC is promising, additional work on both their toxicity effects and long-term stability is required before their full potential as a remediation agent can be safely exploited.
Article
Risk assessment and remediation of contaminated land is inherently dependent on the contaminants present and their availability for interaction with soil biota. An ever-growing body of evidence suggests that current regulatory procedures over-estimate the 'true' fraction available to biota. Thus, a procedure that predicts the 'bioavailable fraction' would be useful for predicting 'actual' exposure limits and provide a more relevant basis for risk assessment. The aim of this paper is to address several important questions: "How should bioavailability be defined?" "What factors affect bioavailability measurement?" "To what extent have existing protocols measured bioavailability?" "What is actually measured by chemical techniques purported to determine bioavailability?" We offer two definitions (namely 'bioavailability' and 'bioaccessibility') and review commonly employed chemical extraction techniques to measure putative bioavailability. Relative advantages and disadvantages of the techniques are highlighted to elucidate underlying factors for the wide range of conclusions observed in the literature. Although the concept of bioavailability is implicit to contaminated land risk assessment and remediation, explicit reference to and use of adjustment factors is rare amongst regulatory bodies and remediators. Use of chemical determinants for bioavailability, applicable within current legislation and due consideration to inherent variability, are proposed and barriers to their implementation discussed.
Article
A former coking works in England is being remediated to enable its redevelopment. We have developed site specific human health risk assessment criteria (SSAC) for benzo[a]pyrene (BaP) based on detailed characterisation of red shale proposed for reuse in an area allocated for residential development. Samples of red shale were analysed using the Colon-enhanced Physiologically Based Extraction Test (CEPBET) to estimate the bioaccessibility of BaP. The results indicate that between 5% and 28% of the red shale BaP is bioaccessible. A cautious value of 50% was used in calculating an SSAC. The CLEA human health assessment model was used to calculate the SSAC and any changes to the default input parameters have been discussed and justified. Based on the fact that BaP appears to be a point-of-contact, rather than a systemic, carcinogen in animal studies, individual assessment criteria were estimated for the oral, inhalation and dermal routes of entry in-line with current UK guidance. An overall SSAC of 2.5 mg/kg for BaP in red shale for the residential with home-grown produce land use was then derived as the lowest of the three route-specific assessment criteria. This value is specific to both the site and the material being assessed. The approach adopted however can be applied on other sites and for other materials to inform detailed quantitative risk assessments.
Article
A radial dual-mode diffusion model is proposed for mass transfer of hydrophobic compounds in soil organic matter (SOM) that is able to predict competitive and concentration effects on sorption and desorption rates. On the basis of dual-mode sorption theory for glassy polymers the model assumes a population of specific adsorption sites (``holes'') interspersed uniformly in the dissolution (partition) domain of SOM. It further assumes Fickian diffusion in the dissolution domain and immobilization in the holes, with microscopic local equilibrium between the two domains. The model is solved numerically (Crank-Nicolson implicit method). Using parameters from single-solute equilibrium and kinetic experiments, the model adequately predicts batch transient sorption and desorption of phenanthrene (primary solute) as a function of pyrene (cosolute) concentration, and batch transient sorption of phenanthrene as a function of its own concentration, in two soils. The model shows that phenanthrene sorption approaches equilibrium faster with increasing cosolute or self-concentration owing to the concentration dependence of the apparent diffusivity, as predicted by a simple hole-plugging mechanism (i.e., fewer and fewer holes are available). Simulations show the effect to be greatest under infinite bath uptake conditions. Under finite bath conditions this positive effect on rate may be opposed by a batch process temporal bias present when the water:soil ratio is kept constant in a series of experiments. The bias is due to gradient driving force effects that slow the rate as a result of the decrease in percent of solute finally taken up by the solid as cosolute or concentration increases.
Article
Chemical extraction techniques like non-exhaustive extraction with Tenax or hydroxypropyl-β-cyclodextrin (HPCD) have been shown to measure the biodegradable fraction of aromatic contaminants like PAHs in soil. However, there is little research on the chemical prediction of aliphatic hydrocarbon degradation. The aim of this study was to investigate the potential for HPCD and Tenax extractions to predict PAH and petroleum hydrocarbon biodegradation in soil. 11 historically contaminated soils with PAH concentrations between 74 and 680mg/kg and concentrations of petroleum hydrocarbons from 330 to 4704mg/kg were analysed. Both non-exhaustive extraction procedures showed promising results for estimating the available contaminant fraction of both contaminant groups concerning the feasibility, reproducibility and correlation with soil biodegradation applying single point testing. Both methods have the potential to be used to assess the biodegradable hydrophobic organic pollutant fraction in contaminated soils. In a direct comparison of the two extraction procedures, Tenax extraction is assessed to be more time-consuming than HPCD extraction. Furthermore, a sufficient soil/Tenax ratio has to be considered.
Article
The oral bioaccessibility of soil contaminants is increasingly assessed with Physiologically Based Extraction Tests (PBETs): the contaminant fraction that is desorbed into simulated digestive fluids is measured and classified as bioaccessible. However, this approach can lead to underestimations if the capacity of the fluids is insufficient to provide infinite sink conditions. Desorption will then progressively decrease and finally stop when equilibrium between soil and medium is reached. To circumvent this artefact, we incorporated a silicone rod as an absorption sink into the PBET to continuously absorb mobilized contaminants and maintain the desorption gradient. Polycyclic aromatic hydrocarbons served as model contaminants and the colon extended PBET as the extraction model. The inclusion of the silicone rod sink (1) increased the extraction capacity of the test by orders of magnitude, (2) ensured (near) infinite sink conditions and (3) allowed for simple back-extraction of PAHs for their quantification by GC-MS. The silicone rod provided fast enrichment when applied to the stomach and small intestine compartment, but was somewhat slower in the richer colon compartment. Finally, the sorptive-PBET was applied to wood soot and a kindergarten soil. The present article provides the basis for how an absorption sink can be integrated into PBET models.
Article
In vitro bioaccessibility testing is gaining popularity as a tool to estimate the oral bioavailability of contaminants in soil for human health risk assessment (HHRA). Bioaccessibility tests are used to measure the bioaccessible fraction of a contaminant in soil, which can then be used to estimate the bioavailable fraction. Inherent uncertainties are associated with bioaccessibility tests. Various test parameters need to be carefully considered in their development, including the liquid to soil (L/S) ratio employed. We used L/S ratios (v:wt) ranging from 25 ml:1 g to 1,000 ml:1 g in a modified relative bioaccessibility extraction procedure to investigate the effects on bioaccessibility of lead and arsenic in field and reference soils. General trends of increased percent bioaccessibility of lead and arsenic with increasing L/S ratio were observed in the reference soil. A similar positive relationship was observed for lead in the field soil; soluble arsenic concentrations were below the detection limit and data were insufficient to observe a trend. Percent bioaccessibility was significantly affected at each extreme of the L/S ratios tested (p < .05). Biological relevance, technical feasibility, and mathematical relationships with in vivo results should be considered when selecting an appropriate L/S ratio for bioaccessibility testing.
Article
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.
Article
Many important environmental contaminants are hydrophobic organic contaminants (HOCs), which include PCBs, PAHs, PBDEs, DDT and other chlorinated insecticides, among others. Owing to their strong hydrophobicity, HOCs have their final destination in soil or sediment, where their ecotoxicological effects are closely regulated by sorption and thus bioavailability. The last two decades have seen a dramatic increase in research efforts in developing and applying partitioning based methods and biomimetic extractions for measuring HOC bioavailability. However, the many variations of both analytical methods and associated measurement endpoints are often a source of confusion for users. In this review, we distinguish the most commonly used analytical approaches based on their measurement objectives, and illustrate their practical operational steps, strengths and limitations using simple flowcharts. This review may serve as guidance for new users on the selection and use of established methods, and a reference for experienced investigators to identify potential topics for further research.
Article
The recently developed SHIME reactor (Simulator of the Human Intestinal Microbial Ecosystem) was validated by analysing a number of microorganism-associated activities. Data from the reactor were compared with values from the literature and results obtained from the analysis of faecal material of eight healthy persons. The fermentation patterns of four polysaccharides were studied. Arabinogalactan, xylan and pectin gave fermentation patterns in vitro indistinguishable from in vivo. Five different enzymatic activities were compared with in vivo experiments. All had activities which were of the order of those observed in vitro. The pro-drug sulphasalazine, whose active compound is released through microbial modification in the large intestine, was used as a reference compound. A fair correlation between in vivo and in vitro was found for the latter transformation. In addition the effect of Lactobacillus plantarum on the autochthonous microbial populations was investigated. Administration of L plantarum resulted in an in vitro decrease in gram-negative anaerobic bacteria and total anaerobes. The same effect has been reported in vivo. The results obtained by the various validation experiments indicate that the reactor can be used to study the microbial communities of the gastrointestinal tract.
Article
A study was conducted to determine the effect of concentration on sequestration and bioavailability of phenanthrene and pyrene in soil. The compounds at 1.0, 10, and 100 mg/kg of soil became increasingly resistant to a mild solvent extraction and progressively less bioavailable to earthworms (Eisenia foetida) as a result of aging for 120 days. Aging also resulted in both compounds at 1.0 and 10 mg/kg and phenanthrene but not pyrene at 100 mg/kg becoming more resistant to microbial degradation. Increasing the concentration led to an increase in the percentages of the unaged and aged compounds that were susceptible to microbial degradation. Some of each of the two compounds was still available to earthworms following biodegradation. The data show that sequestration of the polycyclic aromatic hydrocarbons occurs at both low and high concentrations.
Article
Numerous correlations have been developed between the organic carbon/water partition coefficient KOC and various molecular properties and descriptors, but most notably the octanol/water partition coefficient KOW and water solubility. From an analysis of the theory underlying in this partitioning and an examination of the existing database, it is suggested that the preferred approach is to correlate the quantity log(KOC/KOW), which is essentially the ratio of activity coefficients in octanol and organic carbon, with a molecular property or with log KOW rather than log KOC with log KOW. Such an approach is presented for estimating KOC for hydrophobic chemicals, including an expression of uncertainty limits, based on a correlation derived between log(KOC/KOW) and log KOW. In its simplest form the correlation is that KOC = 0.35KOW subject to variation by a factor of 2.5 in either direction. It is suggested that new experimental data be assessed for consistency and achievement of true equilibrium by comparison with this correlation.
Article
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.