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Reciprocal interference of clay minerals and nanoparticulate zero-valent iron on their interfacial interaction with dissolved organic matter
Abstract
With increasing environmental application of nanoparticulate zero-valent iron (nZVI), it is essential to explore the interaction of nZVI with dissolved organic matter (DOM) and clay mineral particles (CMPs) and its potential effect on the formation of DOM-mineral complex that may impact the carbon sequestration. The aggregation and adsorption behaviors of nZVIs (two bare nZVIs of different sizes and one carboxymethyl cellulose coated nZVI (CMC-nZVI)) and CMPs (kaolinite and montmorillonite) coexisting in DOM (humic acid and fulvic acid) solutions were systematically investigated. The bare nZVIs more easily formed heteroaggregates with montmorillonite than kaolinite in DOM solutions, while the CMC-nZVI tended to attach on kaolinite surface. The heteroaggregation and competition between nZVIs and CMPs could change their interfacial interaction with DOM and the ultimate immobilization of DOM was determined by the formed nZVI-CMP complexes, irrelevant to the addition sequence of nZVIs and CMPs. Compared with the individual CMPs alone, the formed bare-nZVIs-CMP heteroaggregates promoted the sequestration of DOM especially its aromatic carbon fractions, while the CMC-nZVI had no such effect. These findings will be helpful for the understanding of nZVI interaction with DOM and CMPs and the effect on the immobilization of organic carbon in the environment.
... X-ray diffraction (XRD, D8 Advance, Bruker, Germany) and FTIR spectroscopy were used to confirm the characteristic peaks of these two minerals. [54,55] The isoelectric point of goethite was previously reported as being close to ∼9.0-9.4, whereas kaolinite has an isoelectric point of ∼2.7-4.1 for the overall surface and ∼7.3 for the surface edges. ...
Microplastic pollution in terrestrial ecosystems threatens to destabilize large soil carbon stocks that help to mitigate climate change. Carbon‐based substrates can release from microplastics and contribute to terrestrial carbon pools, but how these emerging organic compounds influence carbon mineralization and sequestration remains unknown. Here, microcosm experiments are conducted to determine the bioavailability of microplastic‐derived dissolved organic matter (MP‐DOM) in soils and its contribution to mineral‐associated carbon pool. The underlying mechanisms are identified by estimating its spectroscopic and molecular signatures and comparing its sorption properties on model minerals with natural organic matter (NOM). The results show that MP‐DOM leads to 21–576% higher CO2 emissions and 34–83% lower mineral‐associated organic carbon in soils than NOM, depending on the type of plastic polymer. DOM from biodegradable microplastics induces higher CO2 emissions than conventional microplastics. It is found that MP‐DOM is 7.96 times more labile than NOM, making it more accessible for microbial utilization. The lower degree of humification, fewer polar functional groups, and higher H/C ratios in MP‐DOM also led to 3.96 times less sorption with mineral particles. The findings provide insights into the effects of microplastics on soil carbon storage and highlight their consequences for wider terrestrial carbon cycling and climate warming.
... However, nZVI also faces some challenges that limit its application potential, such as its tendency to aggregate and react with dissolved oxygen in wastewater, resulting in poor fluidity, short reaction life, and low reactivity to target contaminants (Fan et al., 2019). To overcome these limitations and enhance the performance of nZVI, one of the effective methods is to load it onto support materials, such as biochar (BC), graphene, clay minerals, and animal bones (Liu, Li, et al., 2021b;Song et al., 2019;Wang et al., 2020). Due to the advantage of low-costing, wide range of raw materials, porous structure, and abundant functional groups, BC can be used as an excellent support material for dispersing nZVI particles and further facilitating the removal of pollutants (Yang, Zhang, et al., 2018a;Zhou et al., 2014). ...
Cadmium (Cd) pollution in aqueous solution has caused great threat to human health. A novel chitosan and biochar supported sulfide-modified nZVI composite (CB-S-nZVI) was synthesized by liquid phase reduction method and applied to the removal of Cd2+ from wastewater. The synthesized materials were characterized by SEM, BET, XRD, FTIR, and zeta potential. Batch experiments showed that the optimum synthesis conditions of CB-S-nZVI were the S/Fe molar ratio of 0.56 and the Fe/BC mass ratio of 2. The removal capacity of Cd2+ by CB-S-nZVI increased with the increase of pH and reached the maximum of 249.92 mg/g at pH of 9. Among the competitive cations, Pb2+ and Cu2+ had a strong inhibition to the removal of Cd2+. The removal process of Cd2+ conformed to the pseudo-second-order kinetic model and the Freundlich model, and it was exothermic. XPS analysis suggested that the removal mechanism of Cd2+ was mainly through electrostatic attraction, precipitation, and complexation. Overall, these findings provide new insights into the development of new nZVI-modified material, and CB-S-nZVI is promising in removing heavy metals from wastewater.
... After composting, the FA content ranged from 25.4 to 39.5 g/kg DM. During the 50 days of composting, the content of FA in treatments T1 to T4 degraded by 51.9 %, 50.9 %, 46.7 %, and 41.6 %, respectively, which may be attributed to the higher chemical stability of FA surface groups catalysed by NZVI (Wang et al., 2020a). ...
The effectiveness of nano-zero-valent-iron (NZVI) addition during composting of pig manure (PM) was investigated. Different dosages of NZVI were mixed with PM substrate during a 50 days composting process. The results revealed that the higher share of NZVI addition, the higher OM degradation rate is. On contrary, it was observed that the higher share of NZVI addition, the lower the fulvic acid and the humin degradation rate is. Meanwhile, NZVI amendment increased the CO2 and CH4 emissions by 29-47% and 53-57%, respectively. The in-depth analysis showed that NZVI addition increased the activity of Sphaerobacter and Luteimonas, which eventually led to the degradation of hard-to-degrade OM faster. Additionally, NZVI was found to increase the filtration of microorganisms, reducing the toxicity and hygiene of compost products. No significant improvement in humic substance enhancement was observed during composting with NZVI addition but improved OM degradation.
... A major challenge in understanding the behaviour of iron NPs is the poorly understood way in which they enter natural cycles and how they impact both the physical and chemical behaviour of soil. Future research should look at a) increasing Fe bioavailability and its constitutive ions (Kovács et al., 2013;Perminova, 2017;Cieschi et al., 2017); b) improving the physicochemical properties of soils (Medina et al., 2021); c) improving the formation of organo-mineral complexes that could provide soils with a high buffering capacity, potentially increasing the cation exchange capacity and promoting microbial activities for remediation purposes (Lu et al., 2020;Wang et al., 2020;Manzoor et al., 2021); d) improving organic carbon stabilization (Colombo et al., 2017;Durn et al., 2019;Lu et al., 2020;Moens et al., 2021;Medina et al., 2021); e) correcting iron deficiency with Fe-HS complexes (Zimbovskaya et al., 2020;Soliemenzadeh and Fekri, 2021), and f) reducing the environmental impacts of modern agriculture on greenhouse gas (N 2 O) emission by accelerating abiotic oxidation (Demangeat et al., 2018). ...
Goethite, hematite, ferrihydrite, and other iron oxides bind through various sorption reactions with humic substances (HS) in soils creating nano-, micro-, and macro-aggregates with a specific nature and stability. Long residence times of soil organic matter (SOM) have been attributed to iron-humic substance (Fe-HS) complexes due to physical protection and chemical stabilization at the organic–mineral interface. Humic acids (HA) and fulvic acids (FA) contain many acidic functional groups that interact with Fe oxides through different mechanisms. Due to the numerous interactions between mineral Fe and natural SOM, much research has led into a better identification and definition of HS. In this review, we first focus on the surface colloidal properties of Fe oxides and their reactivity toward HS. These minerals can be efficiently identified by usual techniques, such as XRD, FTIR spectroscopy, XAS, Mössbauer, diffuse reflectance spectroscopies (DRS), HRTEM, ATM, NanoSIMS. Second, we present the recent state of art regarding the adsorption/precipitation of HS onto iron mineral surfaces and their effects on binding metalloid and trace elements. Finally, we consider future research directions based on recent scientific literature, with particular focus on the ability of Fe nano-particles to increase Fe bioavailability, improve carbon sequestration, reduce greenhouse gas emissions, and decrease the impact of persistent organic and inorganic pollutants. The methodology in this field has rapidly developed over the last decade. However, new procedures to estimate the nature of Fe-HA bonds will be important contributions in clarifying the role of natural iron oxides in soil for carbon stabilization.
With the increasing application of nanoscale zero-valent iron (nZVI) in environmental remediation, exploring the reciprocal effects of heavy metals (HMs) and dissolved organic matter (DOM) on their immobilizations by nZVI is crucial for predicting carbon dynamics and contaminant behavior in the environment. By using manifold analyses, the reciprocal effects of HMs (Cr(VI) and Cd(II)) and humic acid (HA) on their immobilizations by modulating the interfacial interactions with nZVI were systematically investigated. It was found that the previous interaction of HA especially its contained nonfluorescent substances (i.e., lipids, polysaccharides, and lignin) with nZVI improved the immobilization of HMs on nZVI; the previous interaction of Cd(II) with HA or nZVI promoted the sequestration of organic carbon (up to 1.8-fold) especially the aromatic carbon and humic-like fluorophores of HA by nZVI, but Cr(VI) had no such effect. The oxidized iron complexes on nZVI dominated the ultimate immobilizations of HA and HMs. The increased immobilizations of unsaturated hydrocarbons and quinone ketones by the formed nZVI-HM complexes decreased the reduction capability but enhanced the photochemical stability of the residual aqueous HA. These findings are of significance to understanding nZVI interactions with DOM and HMs and the effects on the carbon cycle and HM immobilization in the environment.
Phytoremediation is a cost-effective and environmentally sustainable remediation technology for many types of contaminants, but has low efficacy and applicability for persistent organic pollutant- (POP-) contaminated soils. Reactive nanoparticulate zero-valent iron (nZVI) can enhance plant growth and synergistically promote the remediation of POP-contaminated soil. Here, we investigated the efficacy and mechanisms by which nZVI (0, 10, 100, and 1000 mg·kg-1) and alfalfa (Medicago sativa) synergistically remediate polychlorinated biphenyl- (PCB-) contaminated agricultural soil for 120 days. The results show that the alfalfa remediation efficiencies of PCB28- and PCB180-contaminated soil are significantly elevated from 66.7% and 38.5% to 93.1% and 52.3% with the increase of nZVI from 0 to 1000 mg·kg-1, respectively. These values are significantly greater than those of nZVI and phytoremediation strategies alone, and the synergistic effect is induced by the alterations of microecological environment of the alfalfa rhizosphere. Plant root metabolomic analysis and rhizosphere microbial community profiling reveal that nZVI influences amino acid metabolism and this change in metabolites could be an energy source for reshaping the rhizosphere microbial community towards PCB degradation. The capabilities of the microbiota to utilize carbon sources are facilitated in PCB-contaminated soil with nZVI and alfalfa co-treatment. In addition to significant microbial degradation, root exudates are shown to promote the production of hydroxyl radicals in the simulated nZVI-alfalfa system. This work provides a new strategy using nanomaterial-facilitated phytoremediation to promote the restoration of POP-contaminated soils.
Knowledges of nanoparticulate zero-valent iron (nZVI) transformation in soils and its relationship with the potential impacts on soil properties are crucial to evaluate the environmental implication and application of nZVI. This study investigated nZVI transformation and the effects on soil properties in eight soils with various ageing time and soil moisture content (SMC). Spherical nZVI was gradually oxidized, collapsed, and adhered to clay minerals, and crystalline maghemite and magnetite were the primary oxidation products. Compared with the flooded condition, nZVI oxidation was accelerated under 70% SMC but was limited under 30% SMC. Acidic soil with lower content of dissolved aromatic carbon was advantage to nZVI oxidation under the flooded condition, while carboxymethylcellulose coating and iron oxides on nZVI surface limited nZVI oxidation. The aged nZVI existed mainly in the form of association with soil mineral or organic matter rather than in ion-exchangeable or carbonate form. nZVI treatment promoted soil aromatic carbon sequestration and decreased soil redox potential, and the impacts of nZVI on soil pH, electrical conductivity, ζ-potential, dissolved organic carbon, and catalase and urease activities were dependent on soil type and SMC. The findings are of significance for the evaluation of the environmental risk and proper application of nZVI.
With the fast development of nanotechnology, reactive engineered nanomaterials (ENMs) are increasingly discharged into the environment, where they interact with environmental components and organisms and thus pose potential risks. The interactions‐derived formation of nano‐environmental and nano‐bio interfaces determines environmental behaviors and biological effects of ENMs, and ubiquitous dissolved organic matter (DOM) is bound to impact the interfacial interactions and the resulted environmental risks. Herein, we systematically investigated adsorptive interactions between ENMs and various DOM representatives, and thereby demonstrated the effects of DOM on the aqueous suspension/aggregation, mobility in porous media, adsorption of contaminants, transformation, and biological accumulation and toxicity of ENMs. Overall, we conclude that natural DOM can in general expand environmental distribution of ENMs while limit their toxicity to organisms.
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Sequestration of organic carbon (OC) in environmental systems is critical to mitigating climate change. Organo-mineral associations, especially those with iron (Fe) oxides, drive the chemistry of OC sequestration and stability in soils. Short-range-ordered Fe oxides, such as ferrihydrite, demonstrate a high affinity for OC in binary systems. Calcium commonly co-associates with OC and Fe oxides in soils, though the bonding mechanism (e.g., cation bridging) and implications of the co-association for OC sequestration remain unresolved. We explored the effect of calcium (Ca2+) on the sorption of dissolved OC to 2-line ferrihydrite. Sorption experiments were conducted between leaf litter-extractable OC and ferrihydrite at pH 4 to 9 with different initial C/Fe molar ratios and Ca2+concentrations. The extent of OC sorption to ferrihydrite in the presence of Ca2+increased across all tested pH values, especially at pH ≥ 7. Sorbed OC concentration at pH 9 increased from 8.72 ± 0.16 to 13.3 ± 0.20 mmol OC g-1ferrihydrite between treatments of no added Ca2+and 30 mM Ca2+addition. Batch experiments were paired with spectroscopic studies to probe the speciation of sorbed OC and elucidate the sorption mechanism. ATR-FTIR spectroscopy analysis revealed that carboxylic functional moieties were the primary sorbed OC species that were preferentially bound to ferrihydrite and suggested an increase in Fe-carboxylate ligand exchange in the presence of Ca at pH 9. Results from batch to spectroscopic experiments provide significant evidence for the enhancement of dissolved OC sequestration to 2-line ferrihydrite and suggest the formation of Fe-Ca-OC ternary complexes. Findings of this research will inform modeling of environmental C cycling and have the potential to influence strategies for managing land to minimize OM stabilization.
Synthesis of Fe3O4/a-SiO2 nanoparticle has been conducted based on natural materials by using the co-precipitation method. The used silica was a-SiO2 collected from silica sand from Bancar Tuban, Indonesia. The Fe3O4 collected from iron sand's Lumajang, Indonesia. The samples was set for Fe3O4 : a-SiO2: PEG composition of 1 : 2 : 1. The process was maintained by melting the PEG 4000 and following by adding a-SiO2 and Fe3O4. The samples were characterized by using X-Ray Diffractometry (XRD), Transmission Electron Microscopy (TEM), and Fourier Transform Infra-Red (FTIR) spectroscopy. The analysis of the XRD data showed that additional silica into Fe3O4 tended to change the diffraction pattern that represents the existent of both Fe3O4 and silica. The SEM images presented that the samples formed in nanometric size with core-shell structure. Furthermore, the FTIR spectra showed that Si-O-Fe bound appeared at 555-568 cm⁻¹. Thus, this work was able to produce the Fe3O4/a-SiO2 nanocomposites by using co-precipitation method in a core-shell structure.
A considerable number of studies have been conducted to investigate the effect of physical and chemical variables on the transport of nanoscale zerovalent iron (nZVI) in granular media. However, the role of soil grain size as a crucial factor in nanoparticle mobility is less understood. The present research work sought to examine the simultaneous effects of soil grain size and particle concentration on the transport of nZVI coated with carboxymethyl cellulose (CMC-nZVI), using saturated sand packed column experiments. To this end, a total of 12 tests were conducted by combining four different particle concentrations (C = 10, 200, 3000, 10,000 mg/l) and three grain sizes (dc = 0.297–0.5 mm, 0.5–1 mm, 1–2 mm). The effluent nZVI concentration and water pressure drop along the column were measured. The results showed that during the injection time, decreasing the grain size and increasing the particle concentration reduces the mobility of CMC-nZVI due to ripening phenomena, while during the flushing time (introducing deionized water), such changes in grain size and particle concentration increase the mobility of CMC-nZVI due to a release from the secondary energy minimum well (in the DLVO theory).
Hematite (alpha-Fe2O3) particles are prepared and synchronously deposited on the surface of polyamide (PA) fabric using ferric sulfate as the precursor, sodium hydroxide as the precipitant, and sodium dodecyl benzene sulfonate as the dispersant in a low temperature hydrothermal process. The Fe2O3 coated PA fabric is then modified with silane coupling agent Z-6040. The Fe2O3 coated PA fabric and remaining particles are systematically characterized by different techniques, such as small-spot micro X-ray fluorescence (μ-XRF), field-emission scanning electron microscopy (FESEM), energy dispersive X-ray spectroscopy (EDX), transmission electron microscopy (TEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), thermal gravimetric analysis (TGA), differential scanning calorimetry (DSC), diffuse reflectance spectrum (DRS), and vibrating sample magnetometer (VSM). The properties of tensile, durable washing and photocatalytic activity are investigated. The experimental results show that Fe2O3 particles composed of nanoparticles having the average crystallite size of 37.8 nm are grafted onto PA fabric and enhanced by coupling agent via the C-Fe, O-Fe and Si-O-Fe bonds. It is found that, after treatments, the thermal stability of PA fabric hardly changes; the visible light absorption capability and magnetism are gained; and the tensile property decreases slightly. It is also confirmed that the Fe2O3 coated PA fabric can withstand the repeated washings up to 20 times and photodegrade the adsorbed methyl orange (MO) exposed to ultraviolet (UV) irradiation. Therefore, the present method provides a new strategy for the production of durable magnetic fabric.
Eleven well characterized humic substances (HSs) were adsorbed from aqueous solution onto a Na-kaolin clay. The adsorption affinity (K-L), maximum adsorption capacity (b), a coefficient of desorption hysteresis (H), and the concentration of irreversibly adsorbed HS (IHS) were derived from adsorption-desorption isotherms. These parameters were correlated with structural features of the HS. The adsorption affinity was shown to correlate directly with the aromaticity of the HS and inversely with their polarity, expressed as the O/C atomic ratio. A dependency between polarity and maximum adsorption capacity was not confirmed. The parameters b, H, and IHS expose close correlation with the molecular weight (MW) and the partial negative charge of HS (Z) at the operating pH value. The following quantitative relationship was obtained: b = 715 - 0.06 x MW - 529 x Z (r = 0.92). It allows a selection of HS with respect to the largest content of organic matter in HS-kaolin clay complexes. Among the HS studied the high molecular weight materials enriched with C- and H-substituted aromatics, such as coal and peat humic acids (HAs), are shown to be the most preferential materials for preparing stable HS-clay complexes.
Purpose
The fractionation of soil humic acids (HAs) according to their hydrophobicity is a common procedure in the study of this polydispersed complex natural mixture, so that reversed-phase high-performance liquid chromatography (RP-HPLC) is used resulting in humic components of differing hydrophobic/hydrophilic properties. However, a comparative study of the hydrophobicity of fractions isolated from different soil HAs have not been addressed so far.
Materials and methods
The RP-HPLC with online absorbance detection was used for analysis of International Humic Substances Society soil standard HAs, chernozem soil HAs, and their electrophoretic fractions A, B, and C + D, obtained by tandem size exclusion chromatography–polyacrylamide gel electrophoresis. The strong relationship between hydrophobicity, electrophoretic mobility (EM), molecular size (MS), specific absorbance at 280 nm and aromaticity of HAs fractions was found.
Results and discussion
Independently of soil HAs genesis fraction A with lowest EM and highest MS is essentially more hydrophobic (60–73 % of the fraction amount remained adsorbed on the RP column) than medium EM and MS fraction B (33–47 % of the fraction amount remained adsorbed on the RP column). The lowest hydrophobicity belongs to fraction C + D with highest EM and lowest MS.
Conclusions
The most hydrophilic aromatic fraction C + D seems to have been bound with other mostly aliphatic hydrophobic fractions A and B through non covalent (possibly hydrogen) bonds. These data could be relevant to better understanding the overall makeup of soil HAs and their structural organization.
A large source of uncertainty in present understanding of the global carbon cycle is the distribution and dynamics of the soil organic carbon reservoir. Most of the organic carbon in soils is degraded to inorganic forms slowly, on timescales from centuries to millennia1. Soil minerals are known to play a stabilizing role, but how spatial and temporal variation in soil mineralogy controls the quantity and turnover of long-residence-time organic carbon is not well known2. Here we use radiocarbon analyses to explore interactions between soil mineralogy and soil organic carbon along two natural gradients—of soil-age and of climate—in volcanic soil environments. During the first 150,000 years of soil development, the volcanic parent material weathered to metastable, non-crystalline minerals. Thereafter, the amount of non-crystalline minerals declined, and more stable crystalline minerals accumulated. Soil organic carbon content followed a similar trend, accumulating to a maximum after 150,000 years, and then decreasing by 50% over the next four million years. A positive relationship between non-crystalline minerals and organic carbon was also observed in soils through the climate gradient, indicating that the accumulation and subsequent loss of organic matter were largely driven by changes in the millennial scale cycling of mineral-stabilized carbon, rather than by changes in the amount of fast-cycling organic matter or in net primary productivity. Soil mineralogy is therefore important in determining the quantity of organic carbon stored in soil, its turnover time, and atmosphere–ecosystem carbon fluxes during long-term soil development; this conclusion should be generalizable at least to other humid environments.
Stabilization of organic matter (OM) by sorption to minerals is thought to be due to (i) sorption into small pores (Ø < 50 nm) that prevents hydrolytic enzymes approaching and decomposing the organic substrate or (ii) reduced availability of organic molecules after formation of strong multiple bonds by complexation of organic ligands at mineral surfaces. We tested these two potential mechanisms by studying the binding of dissolved OM to microporous goethite (α-FeOOH). The size of organic molecules dissolved prior to and after equilibration with goethite was determined using atomic force microscopy (AFM). The goethite–OM complexes were analysed for bulk and surface elemental composition (by X-ray photoelectron spectroscopy, XPS), specific surface area (SSA) and mesopore and micropore volumes (by N2 adsorption/desorption), by scanning electron microscopy (SEM), and by diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy. The absolute density of goethite–OM complexes was determined by gas pycnometry and the sorbed OM’s apparent density was calculated by assuming no major changes in the volumes of the goethite upon sorption of OM. The stability of the OM–mineral interactions was tested in desorption experiments and by treatment with NaOCl. Surface accumulation of OM by sorption decreased the N2-accessible SSA of the goethite, mostly because micropores (Ø < 2 nm) were rendered inaccessible to N2. The decrease in accessibility of micropores was most pronounced at small surface OM concentrations. The majority of dissolved organic molecules detected with AFM prior to interaction with goethite were globular with a diameter of 4–10 nm, the rest were mainly linear, 20–100 nm long and 4–8 nm thick. After contact with goethite, the latter type of molecules dominated, which suggests preferential sorption of globular molecules. Their size exceeded or equalled the size of micropores and small mesopores (Ø < 10 nm) and so sorption therein is unlikely. Also, the changes in volumes of pores with a size of 2–50 nm were smaller than the estimated volume of the OM sorbed. The apparent density of sorbed OM always exceeded that of the freeze-dried OM and was largest at small surface concentrations. DRIFT spectroscopy showed that most carboxyl groups at the goethite surface were in their complexed form. The proportion of complexed carboxyl groups dropped at larger surface concentrations, parallel to the decrease in micropore volume. Thus, micropores seem to favour the formation of multiple complex bonds per molecule. Scanning electron microscopy showed that at small surface concentrations, OM coated the goethite crystals and crystallites tightly, while at larger surface concentrations bulky accumulations of OM were more abundant. Even strongly desorbing reagents such as NaOH and Na pyrophosphate released only part of the sorbed OM. Treatment with NaOCl removed mainly bulky accumulations of OM; the OM tightly bound to goethite crystals was hardly affected by NaOCl. We conclude that molecules tightly bound via multiple complex bonds, probably at the mouths of small pores, are barely desorbable and resist the attack of chemical reagents and probably also of enzymes.
Over the last decade, nanoparticles have been used more frequently in industrial applications and in consumer and medical products, and these applications of nanoparticles will likely continue to increase. Concerns about the environmental fate and effects of these materials have stimulated studies to predict environmental concentrations in air, water, and soils and to determine threshold concentrations for their ecotoxicological effects on aquatic or terrestrial biota. Nanoparticles can be added to soils directly in fertilizers orplant protection products or indirectly through application to land or wastewater treatment products such as sludges or biosolids. Nanoparticles may enter aquatic systems directly through industrial discharges or from disposal of wastewater treatment effluents or indirectly through surface runoff from soils. Researchers have used laboratory experiments to begin to understand the effects of nanoparticles on waters and soils, and this Account reviews that research and the translation of those results to natural conditions. In the environment, nanoparticles can undergo a number of potential transformations that depend on the properties both of the nanoparticle and of the receiving medium. These transformations largely involve chemical and physical processes, but they can involve biodegradation of surface coatings used to stabilize many nanomaterial formulations. The toxicity of nanomaterials to algae involves adsorption to cell surfaces and disruption to membrane transport. Higher organisms can directly ingest nanoparticles, and within the food web, both aquatic and terrestrial organisms can accumulate nanoparticles. The dissolution of nanoparticles may release potentially toxic components into the environment. Aggregation with other nanoparticles (homoaggregation) or with natural mineral and organic colloids (heteroaggregation) will dramatically change their fate and potential toxicity in the environment. Soluble natural organic matter may interact with nanoparticles to change surface charge and mobility and affect the interactions of those nanoparticles with biota. Ultimately, aquatic nanomaterials accumulate in bottom sediments, facilitated in natural systems by heteroaggregation. Homoaggregates of nanoparticles sediment more slowly. Nanomaterials from urban, medical, and industrial sources may undergo significant transformations during wastewater treatment processes. For example, sulfidation of silver nanoparticles in wastewater treatment systems converts most of the nanoparticles to silver sulfides (Ag(2)S). Aggregation of the nanomaterials with other mineral and organic components of the wastewater often results in most of the nanomaterial being associated with other solids rather than remaining as dispersed nanosized suspensions. Risk assessments for nanomaterial releases to the environment are still in their infancy, and reliable measurements of nanomaterials at environmental concentrations remain challenging. Predicted environmental concentrations based on current usage are low but are expected to increase as use increases. At this early stage, comparisons of estimated exposure data with known toxicity data indicate that the predicted environmental concentrations are orders of magnitude below those known to have environmental effects on biota. As more toxicity data are generated under environmentally-relevant conditions, risk assessments for nanomaterials will improve to produce accurate assessments that assure environmental safety.
Since the 1900s, the link between soil biotic activity, soil organic matter (SOM) decomposition and stabilization, and soil aggregate dynamics has been recognized and intensively been studied. By 1950, many studies had, mostly qualitatively, investigated the influence of the five major factors (i.e. soil fauna, microorganisms, roots, inorganics and physical processes) on this link. After 1950, four theoretical mile-stones related to this subject were realized. The first one was when Emerson [Nature 183 (1959) 538] proposed a model of a soil crumb consisting of domains of oriented clay and quartz particles. Next, Edwards and Bremner [J. Soil Sci. 18 (1967) 64] formulated a theory in which the solid-phase reaction between clay minerals, polyvalent cations and SOM is the main process leading to microaggregate formation. Based on this concept, Tisdall and Oades [J. Soil Sci. 62 (1982) 141] coined the aggregate hierarchy concept describing a spatial scale dependence of mechanisms involved in micro- and macroaggregate formation. Oades [Plant Soil 76 (1984) 319] suggested a small, but very important, modification to the aggregate hierarchy concept by theorizing the formation of microaggregates within macroaggregates. Recent research on aggregate formation and SOM stabilization extensively corroborate this modification and use it as the base for furthering the understanding of SOM dynamics. The major outcomes of adopting this modification are: (1) microaggregates, rather than macroaggregates protect SOM in the long term; and (2) macroaggregate turnover is a crucial process influencing the stabilization of SOM. Reviewing the progress made over the last 50 years in this area of research reveals that still very few studies are quantitative and/or consider interactive effects between the five factors. The quantification of these relationships is clearly needed to improve our ability to predict changes in soil ecosystems due to management and global change. This quantification can greatly benefit from viewing aggregates as dynamic rather than static entities and relating aggregate measurements with 2D and 3D quantitative spatial information.
Although there is an increasing use of nanoparticulate zero-valent iron (nZVI) in the remediation of contaminated soil and water environments, there is still little understanding of the mechanisms of interaction between nZVI and dissolved organic matter (DOM), including the resulting effects on the fate and functions of both nZVI and DOM. Using a manifold analytical design, the adsorption of DOM (humic acid and fulvic acid) by nZVI (two naked nZVIs of different sizes and one carboxymethyl cellulose (CMC)-coated nZVI) was investigated. In addition, the effect of that sorption on DOM chemical stability and on nZVI transformation were systematically investigated. The CMC coating on nZVI limited the adsorption of DOM. DOM fractions of moderate and high molecular weight (MW, >3 kDa) had a relatively high affinity to bare nZVI surfaces, whereas low MW fractions such as quinone-like and protein-like fluorophores preferentially remained in aqueous phase. Interaction with DOM accelerated oxidation of the core Fe0 of nZVI due to strong iron chelation of aqueous DOM and potential electron transfer mediator action of the limitedly adsorbed quinone-like DOM fractions. Compared with pristine DOM, the DOM fraction with nZVI was much more stable, while the residual DOM fraction in aqueous phase after the nZVI adsorption was of lower photochemical/chemical stability as indicated by greater reduction capability, mineralization percentage, and photodegradation percentage. These findings increase our understanding of the effects of nZVI interaction with DOM and have implications on the carbon cycle and on the large scale use of nZVI.
Copper oxide nanoparticles (CuO NPs) is one of the most commonly used metal oxide nanoparticles for commercial and industrial products. An increase in the manufacturing and use of the CuO NPs based products has increased the likelihood of their release into the aquatic environment. This has attracted major attention among researchers to explore their impact in human as well as environmental systems. CuO NPs, once released into the environment interact with the biotic and abiotic constituents of the ecosystem. Hence the objective of the study was to provide a holistic understanding of the effect of abiotic factors on the stability and aggregation of CuO NPs and its correlation with their effect on the development of zebrafish embryo. It has been observed that the bioavailability of CuO NPs decrease in presence of humic acid (HA) and heteroagglomeration of CuO NPs occurs with clay minerals. CuO NPs, CuO NPs + HA and CuO NPs + Clay significantly altered the expression of genes involved in development of dorsoventral axis and neural network of zebrafish embryos. However, the presence of HA with clay showed protective effect on zebrafish embryo development. These findings provide new insights into the interaction of NPs with abiotic factors and combined effects of such complexes on developing zebrafish embryos genetic markers.
Heteroaggregation with clay mineral particles (CMPs) is significant to the environmental application and fate of increasingly produced nanoparticulate zero-valent iron (nZVI). Co-settling, kinetic aggregation, calculation of the classical Derjaguin-Landau-Verwey-Overbeek interaction energy, and electron microscopic observation were carried out to investigate the interaction between nZVIs (three naked nZVIs of different sizes and one carboxymethyl cellulose (CMC) coated nZVI) and CMPs (kaolinite and montmorillonite). Under pH 6.5 and 9.5 conditions, Lewis acid-base interaction contributed to the attachment between nZVIs and CMPs, while electrostatic attraction was involved in the nZVIs-CMPs attachment under pH 3.5. Compared with heteroaggregates formed by nZVIs attaching to CMPs edges and faces under pH 6.5 and 3.5 conditions, the heteroaggregates were smaller with nZVIs mainly connecting to CMPs edges under pH 9.5. Small nZVI homoaggregates were bound to CMP edges at low nZVI concentrations (nZVI/CMPs mass ratio at 0.015) with CMPs concentrations at 330 mg/L and large nZVIs-CMPs heteroaggregates formed by nZVI bridging with increasing nZVI concentrations. The smallest nZVI exhibited the strongest heteroaggregation with CMPs; CMC coating inhibited the interfacial interaction and heteroaggregation between nZVIs and CMPs; kaolinite had higher potential to interact with nZVIs at neutral condition. These findings are helpful for understanding the interaction between nZVIs and minerals and of significance to the environmental remediation using nZVIs.
Granular mixtures composed of zero valent iron (ZVI) and lapillus at two different weight ratios (i.e. 30:70 and 50:50) were tested through column experiments for the simultaneous removal of Cu ²⁺ , Ni ²⁺ and Zn ²⁺ present in aqueous solutions at high concentrations. The results were used to evaluate the feasibility of the above-mentioned granular mixtures as reactive media in permeable reactive barriers (PRB) for the remediation of groundwater polluted by metals. Test results showed that the two granular reactive media efficiently removed the three heavy metals under study according to the following removal sequence Cu > Zn > Ni. The granular mixture with the higher iron content showed a proportionally higher removal rate but also a higher reduction of hydraulic conductivity over time. Different removal mechanisms occurred for the three contaminants in question. Considering that for Ni and Zn the main removal mechanism was probably adsorption, we used different mathematical models, in order to predict the breakthrough curves for the adsorption mechanisms. The Adams-Bohart model showed the best fit with the experimental data and it was thus used to predict the zinc removal front within the barrier thickness. Finally, we showed that the mathematical approach may be used for the design of PRBs for the reactive media and contaminants used in this research.
Sulfidated nanoscale zerovalent iron (S-NZVI) is a new remediation material with higher reactivity and greater selectivity for chlorinated organic contaminants such as trichloroethene (TCE) than NZVI. The properties of S-NZVI and the effects of groundwater constituents like natural organic matter (NOM) on its reactivity are less well-characterized than for NZVI. In this study, S-NZVI (Fe/S mole ratio = 15) was synthesized by sonicating NZVI in a Na 2 S solution, yielding particles with greater surface charge, less aggregation, and higher reactivity with TCE compared to NZVI. The cytotoxicity of S-NZVI was not mitigated effectively due to the smaller size. The addition of Suwannee River humic acid (SRHA) increased the negative surface charge magnitude and dispersion stability and reduced the toxicity of both NZVI and S-NZVI significantly, but also enhanced the corrosion of particles and the formation of non-conductive film. The degradation rate constant (k sa ) of both NZVI and S-NZVI was thus reduced with the increasing concentration of SRHA, which decreased by 78% and 60% to be 0.0004 and 0.0053 L m ⁻² h ⁻¹ , respectively, with 200 mg C/L SRHA. Additionally, the performance of S-NZVI in field was evaluated to be depressed in simulated groundwater and the negative effect was exacerbated with increased concentration of SRHA. Hydro-chemical conditions like dissolved oxygen (DO), pH, and temperature also influenced the reactivity of S-NZVI. Hence, S-NZVI was a preferred candidate for in-situ remediation of TCE than NZVI. Nevertheless, the integrity of the FeS shell on S-NZVI influenced by NOM need to be considered during the long-term use of S-NZVI in groundwater remediation.
As a natural organic carbon skeleton, humic acid (HA) was loaded with nanoscale zero-valent iron (nZVI) Particles to remove chloramphenicol (CAP) from aqueous solution. The pore morphology and structure, the type, the distribution and valence state of element, and the class of functional groups on the surface of the material were shown by SEM/EDS, XPS, BET and FTIR. When the load ratio of nZVI on HA was 1:30, the iron content in the material was minimized, the specific gravity of the economic material-HA was increased, and the removal efficiency of CAP was 80.0% or higher. In addition, the mass ratio of nZVI on HA, the dosage of nZVI/HA-30, the initial pH and CAP concentration of the solution, these four general factors, played an important role in the efficiency and equilibrium time of the CAP removal. The removal efficiency of CAP by nZVI/HA-30 was 84.2% when the dosage was 1.0 g (100 mL) ⁻¹ , the initial concentration of CAP was 30 mg L ⁻¹ and the pH was 3. The reaction pathway and removal mechanism of ZVI/HA-30 were studied by the concentration of total and ferrous iron ions in the solution, UV–Vis and MS. The CAP was continuously denitrified and dechlorinated, decomposed into easily degradable substances by nZVI particles supported on HA, which was consistent with the first-order kinetic model within 5 min. This newly synthesized material was economical and efficient, easy to store, effectively prevented agglomeration and passivation of nZVI, and had a good application prospect for removing contaminants from water.
The interactions between nanoparticles and humic acid (HA) are critical to understanding the environmental risks and applications of nanoparticles. However, the interactions between HA fractions and graphene oxide (GO, a popular carbon nanosheet) at the molecular level remain largely unclear. Four HA fractions with molecular weights ranging from 4.6 to 23.8 kDa were separated, and the large HA fractions presented low oxygen contents and many aromatic structures. The binding constants of the large HA fractions on GO were 2.6- to 3551-fold higher than those of the small HA fractions, while the maximum adsorption capacities of the larger HA fractions onto GO were much higher. Atomic force microscopy (AFM) found that the small and large HA fractions were spread over the center and the edge of the GO nanosheets, respectively. Density functional theory (DFT) simulation and nuclear magnetic resonance spectroscopy confirmed the above phenomena (three adsorption patterns, ‘vs’, ‘ps’ and ‘pea’) and revealed that HA bonded to the GO nanosheets mainly through van der Waals force and π−π interactions. The integrating analysis of binding affinity, AFM and DFT provides new insights into the environmental behavior of GO and the applications of GO in pollutant removal under the exposure of HA.
Nanoscale zerovalent iron (NZVI) is one of the most extensively studied nanomaterials in the fields of wastewater treatment and remediation of soil and groundwater. However, rapid oxidative transformations of NZVI can result in reduced NZVI reactivity. Indeed, the surface passivation of NZVI is considered one of the most challenging aspects in successfully applying NZVI to contaminant degradation. The oxidation of NZVI can lead to the formation of FeII-bearing phases (e.g., FeIIO, FeII(OH)2, FeIIFeIII2O4) on the NZVI surface or complete oxidation to ferric (oxyhydr)oxides (e.g., FeIIIOOH). This corrosion phenomenon is dependent upon various factors including the composition of NZVI itself, the type and concentration of aqueous species, reaction time and oxic/anoxic environments. As such, the co-existence of different Fe oxidation states on NZVI surfaces may also, in some instances, provide a unique reactive microenvironment to promote the adsorption of contaminants and their subsequent transformation via redox reactions. Thus, an understanding of passivation chemistry, and its related mechanisms, is essential not only for effective NZVI application but also for accurately assessing the positive and negative effects of NZVI surface passivation. The aim of this review is to discuss the nature of the passivation processes that occur and the passivation byproducts that form in various environments. In particular, the review presents: i) the strengths and limitations of state-of-the-art techniques (e.g., electron microscopies and X-ray based spectroscopies) to identify passivation byproducts; ii) the passivation mechanisms proposed to occur in anoxic and oxic environments; and iii) the effects arising from synthesis procedures and the presence of inorganics/organics on the nature of the passivation byproducts that form. In addition, several depassivation strategies that may assist in increasing and/or maintaining the reactivity of NZVI are considered, thereby enhancing the effectiveness of NZVI in contaminant degradation.
Knowledge of the dynamic changes in molecular size of natural colloidal organic matter (COM) along the aquatic continuum is of vital importance for a better understanding of the environmental fate and ecological role of dissolved organic matter and associated contaminants in aquatic systems. We report here the pH- and cation-dependent size variations of COMs with different sources (river and lake) quantified using flow field-flow fractionation (FIFFF), fluorescence spectroscopy and parallel factor analysis (PARAFAC), attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy, and zeta potential analysis. Increasing pH caused a decline in molecular sizes and an obvious size transformation from the >10 kDa to 5-10 kDa and further to 1-5 kDa size fraction, whereas the opposite trend was observed for increasing cation (e.g., Ca2+ and Cu2+) abundance. Compared with lakewater COM, the riverwater COM exhibited a greater pH-dependent dispersion but less extent in cation-induced aggregation, demonstrating that the dispersion and aggregation dynamics were highly dependent on COM source and solution chemistry (e.g., pH and cations). Based on ATR-FTIR analysis, the extensive dissolution of C=O and C-O functional groups resulted in a greater pH-dependent dispersion for river COM. Fluorescence titration revealed that, despite their similar cation-induced aggregation behavior, the binding constants of all the PARAFAC-derived components for Cu2+ were 1-2 orders of magnitude higher than those for Ca2+ (logKM: 4.54-5.45 vs. 3.35-3.70), indicating a heterogeneous nature in cation-DOM interactions. The greater extent of decline in zeta potential for lake COM suggested a Ca-induced charge neutralization and aggregation mechanism. However, for Cu-induced aggregation, chemical complexation was the predominant pathway for the river COM, with higher binding constants, while charge neutralization and chemical complexation co-induced the aggregation of lake COM. Thus, natural COMs may have different environmental behavior along the aquatic continuum and further affect the fate and transport of contaminants in aquatic environments.
Nano zero-valent iron (nZVI), possessing excellent reductive activity and adsorption performance, has been and will be widely applied in the remediation of contaminated soil and groundwater. However, superior reactive nZVI can also interact with soil components, which may not only affect soil properties and its ecological functions but also impact on the existing form and remediation function of nZVI.Interactions between nZVI and soil air, pore water, clay minerals, organic matter, and microorganism are addressed, and the influences on the soil composition and property and on the transformation and function of nZVI are discussed. Under the influence of soil components, nZVI may be transformed to various iron-based chemicals, such as Fe3O4, γ-Fe2O3, α-Fe2O3, α-FeOOH, and γ-FeOOH. Meanwhile, nZVI and its derivates would affect the soil environment by altering soil air composition, pore water pH, and/or physicochemical properties of soil clay minerals and organic matter. Furthermore, nZVI can also affect the soil microbial community, facilitating or inhibiting microbial growth and reproduction. The microbe initiating transformation can regulate the remediation function and fate of nZVI in the soil environment. At the end of the text, future research directions are put forward. This review is believed to boost scientific research and technology advance in environmental applications of nZVI. © 2017, Editorial Office of Progress in Chemistry. All right reserved.
We investigated concurrent effects between nano-sized zero-valent iron (NZVI) and dissolved organic matter (DOM). Specific UV absorbance of DOM revealed that aromatic/hydrophobic moieties of DOM were bounded to NZVI surfaces. The DOM fluorescence emission peak shifted toward lower wavelength after NZVI exposure, which indicated removal of aromatic DOM fractions. This blue shift of the emission peak also attributes to the reduction of electron acceptors through NZVI-DOM charge transfer complexes. High molecular weight (10³–10⁴ Da) DOM fractions, which are suspected to be both aromatic and hydrophobic, were removed. X-ray absorption spectroscopy (XAS) elucidated that Fe(0) content in the 30-d aged NZVI in the presence of DOM (61.6%) was substantially higher than that in the absence of DOM (25.0%). Corrosion and oxidation of NZVI were mitigated due to interruption of electron transfer by surface bounded DOM and stabilization of Fe(II) by Fe-DOM complexes. The XAS also revealed that the evolution of the iron (oxyhydr)oxide shell of NZVI was significantly altered by complexed aromatic DOM.
Heterogeneous distributions of proton binding sites within sub-fractions of fulvic acid (FA3-FA13) were investigated by use of synchronous fluorescence spectra (SFS), combined with principle component analysis (PCA) and two-dimensional correlation spectroscopy (2D-COS). Tryptophan-like, fulvic-like and humic-like materials were observed in SFS. Tyrosine-like materials were identified by use of SFS-PCA analysis. Combined information from synchronous-asynchronous maps and dissociation constants (pKa) was used to describe heterogeneity of binding sites for protons within each sub-fraction. Heterogeneous distributions of proton binding sites were observed in fulvic-like, humic-like, tryptophan-like, and tyrosine-like materials of five sub-fractions and even in the single fulvic-like materials in FA3 and tryptophan-like materials in FA9 and FA13. Values of pKa of sub-fractions ranged from 2.20 to 5.29, depending on associated wavelengths in synchronous-asynchronous maps and use of the modified Stern-Volmer equation. The larger values of pKa (4.17-5.29) were established for protein-like materials (including tryptophan-like and tyrosine-like materials) in comparison to those (2.20-3.38) for humic-like and fulvic-like materials in sub-fractions. Sequential variations of 274nm (pKa 4.15-5.29)→360-460nm (pKa 2.78-2.39) for FA5-FA13 revealed that binding of protons to tryptophan-like materials appeared prior to humic-like/fulvic-like materials. In FA9, protons were preferentially binding to tryptophan-like materials than tyrosine-like materials. In FA3, protons were preferentially binding to humic-like materials than fulvic-like materials. Relative differences of values of pKa for fluorescent materials within each sub-fraction were consistent with sequential orders derived from asynchronous maps. Such an integrated approach, SFS-PCA/2D-COS, has superior potential for further applications in exploring complex interactions between dissolved organic matter and contaminants in engineered and natural environments.
The increasing application of iron-based nanoparticles (NPs), especially high concentrations of zero-valent iron nanoparticles (nZVI), has raised concerns regarding their environmental behavior and potential ecological effects. In the environment, iron-based NPs undergo physical, chemical, and/or biological transformations as influenced by environmental factors such as pH, ions, dissolved oxygen, natural organic matter (NOM), and biotas. This review presents recent research advances on environmental transformations of iron-based NPs, and articulates their relationships with the observed toxicities. The type and extent of physical, chemical, and biological transformations, including aggregation, oxidation, and bio-reduction, depend on the properties of NPs and the receiving environment. Toxicities of iron-based NPs to bacteria, algae, fish, and plants are increasingly observed, which are evaluated with a particular focus on the underlying mechanisms. The toxicity of iron-based NPs is a function of their properties, tolerance of test organisms, and environmental conditions. Oxidative stress induced by reactive oxygen species is considered as the primary toxic mechanism of iron-based NPs. Factors influencing the toxicity of iron-based NPs are addressed and environmental transformations play a significant role, for example, surface oxidation or coating by NOM generally lowers the toxicity of nZVI. Research gaps and future directions are suggested with an aim to boost concerted research efforts on environmental transformations and toxicity of iron-based NPs, e.g., toxicity studies of transformed NPs in field, expansion of toxicity endpoints, and roles of laden contaminants and surface coating. This review will enhance our understanding of potential risks of iron-based NPs and proper uses of environmentally benign NPs.
To explore the adsorption mechanisms of a soil humic acid (HA) on purified kaolinite and montmorillonite, a combination of adsorption measurements, attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy and isotherm titration calorimetry (ITC) was employed at pH 4.0, 6.0 and 8.0. The adsorption affinities and plateaus of HA on the two clays increased with decreasing pH, indicating the importance of electrostatic interaction. The effects were more significant for kaolinite than for montmorillonite. The substantial adsorption at pH 8.0 indicated hydrophobic interaction and/or H-bonding also played a role. The ATR-FTIR results at pH 8.0 showed that the Si-O groups located at basal-faces of the two clays were involved in the adsorption process. For kaolinite, at pH 4.0 and 6.0, HA adsorption occurred via OH groups on the edge faces and basal octahedral face (both positively charged), plus some adsorption at Si-O group. The exothermic molar adsorption enthalpy decreased relatively dramatically with adsorption up to adsorption values of 0.7 μmol/g on montmorillonite and 1.0 μmol/g on kaolinite, but the decrease was attenuated at higher adsorption. The high exothermic molar enthalpy of HA binding to the clays was ascribed to ligand exchange and electrostatic binding, which are enthalpy-driven. At high adsorption values, JGHA adsorption by hydrophobic attraction and H-bonding also occurs.
With the increasing environmental application and discharge of iron-based nanoparticles (NPs), a comprehensive understanding of their fate and ecotoxicological effect in the aquatic environment is very urgent. In this study, toxicities of 4 zero-valent iron NPs (nZVI) of different sizes, 2 Fe2O3 NPs of different crystal phases, and 1 type of Fe3O4 NPs to a green alga (Chlorella pyrenoidosa) were investigated, with a focus on the effects of particle size, crystal phase, oxidation state, and environmental aging. Results show that the algal growth inhibition of nZVI increased significantly with decreasing particle size; with similar particle sizes (20–30 nm), the algal growth inhibition decreased with oxidation of the NPs with an order of nZVI > Fe3O4 NPs > Fe2O3 NPs, and α-Fe2O3 NPs presented significantly higher toxicity than γ-Fe2O3 NPs. The NP-induced oxidative stress was the main toxic mechanism, which could explain the difference in algal toxicity of the NPs. The NP-cell heteroagglomeration and physical interactions also contributed to the nanotoxicity, whereas the effect of NP dissolution was negligible. The aging in distilled water and 3 surface water samples for 3 months increased surface oxidation of the iron-based NPs especially nZVI, which decreased the toxicity to algae. These findings will be helpful for the understanding of the fate and toxicity of iron-based NPs in the aquatic environment.
This work is focused on the use cassava peel as a natural fiber for thermoplastic starch (TPS) based
on the cassava starch. Starch was extracted from the cassava tuber and the peel was used as a film
fiber in order to obtain fully biodegradable composites. The composite films were prepared using
casting technique. The addition of peel results in an increase in the thickness, water content and
water absorption of the films while decreasing the density and water solubility. Moreover, no
significant effect was noticed on the thermal properties of the composite films. Scanning electron
microscopy showed that the films containing a small size of peel had a better compact structure and
a homogeneous surface without pores. The addition of 6% peel increased the elastic modulus and
tensile stress up to 449.74 and 9.62 MPa, respectively, this being the most efficient reinforcing
agent. Also the temperature variation of the dynamic-mechanical parameters of cassava starch/peel
composites was investigated using DMA test. It was observed that the incorporation of peel
increased the tensile strength and modulus, In conclusion, of all-plant composites can be prepared
using cassava as both the matrix and the reinforcement, adding value to the residue of starch
extraction. Based on its excellent properties, cassava starch/peel composite films are suitable for
various purposes such as packaging, automotive and agro-industrial application, at lower cost.
This study was conducted to discuss fluorescence spectroscopic properties of dissolved fulvic acids (FA) isolated from salined flavo-aquic soils around Wuliangsuhai Lake in Hetao Irrigation District, and to evaluate humification degree of FA and soil salinization processes. Composite soil samples of different depths (0-20, 20-40, 40-60, and 60-80 cm) were collected from four different halophyte communities along a saline-impact gradient around Wuliangsuhai Lake, namely, Comm. (Community) Salicornia europaea (CSE), Comm. Suaeda glauca (CSG), Comm. Kalidium foliatum (CKF), and Comm. Sophora alopecuroides (CSA). Ten humification indices (HIX) (A(4)/A(1), A(465), I(383)/I(330), I(460)/I(355), I(460)/I(380), S(355-460), S(380-460), AF(3)/AF(1), AF(3)/AF(2), and r), deduced from fluorescence spectra of FA, were used to assessing humification degree. Aromatic C structure of FA was the most complex in the CSA soils, and humificaiton degree was the highest too, followed by CKF, CSG, and CSA. There were significant relationships among the 10 HIX (P < 0.01), and A(465), S(355-460), and S(380-460) can more indistinctly differentiate humification degree than the other seven HIX. The 10 HIX exhibited good correlations with exchangeable sodium percentage (ESP), and humification degree rose with the decreasing ESP. The HIX not only indicate humification degree, but character soil salinization processes. Therefore HIX may be used as a surrogate for ESP, and may be indicative of soil salinization processes. This study can provide a theoretical basis for the prevention of desertification and saline soil remediation.
The biochemical composition of dissolved organic matter (DOM) strongly influences its biogeochemical role in freshwater ecosystems, yet DOM composition measurements are not routinely incorporated into ecological studies. To date, the majority of studies of freshwater ecosystems have relied on bulk analyses of dissolved organic carbon and nitrogen to obtain information about DOM cycling. The problem with this approach is that the biogeochemical significance of DOM can only partially be elucidated using bulk analyses alone because bulk measures cannot detect most carbon and nitrogen transformations. Advances in fluorescence spectroscopy provide an alternative to traditional approaches for characterizing aquatic DOM, and allow for the rapid and precise characterization of DOM necessary to more comprehensively trace DOM dynamics. It is within this context that we discuss the use of fluorescence spectroscopy to provide a novel approach to tackling a long-standing problem: understanding the dynamics and biogeochemical role of DOM. We highlight the utility of fluorescence characterization of DOM and provide examples of the potential range of applications for incorporating DOM fluorescence into ecological studies in the hope that this rapidly evolving technique will further our understanding of the biogeochemical role of DOM in freshwater ecosystems.
The association of organic matter (OM) with minerals is recognized as the most important stabilization mechanism for soil organic matter. This study compared the properties of Fe-OM complexes formed from adsorption (reaction of OM to post-synthesis ferrihydrite) versus coprecipitation (formation of Fe solids in presence of OM). Coprecipitates and adsorption complexes were synthesized using dissolved organic matter (DOM) extracts from a forest little layer at varying molar C/Fe ratios of 0.3-25.0. Sample properties were studied by N2 gas adsorption, XRD, FTIR, Fe EXAFS and STXM-NEXAFS techniques. Coprecipitation resulted in much higher maximum C contents (~130 mg g-1 C difference) in the solid products than adsorption, which may be related to the formation of precipitated insoluble Fe(III)-organic complexes at high C/Fe ratios in the coprecipitates as revealed by Fe EXAFS analysis. Coprecipitation led to a complete blockage of mineral surface sites and pores with >= 177 mg g-1 C and molar C/Fe ratios >= 2.8 in the solid products. FTIR and STXM-NEXAFS showed that the coprecipitated OM was similar in composition to the adsorbed OM. An enrichment of aromatic C was observed at low C/Fe ratios. Association of carboxyl functional groups with Fe was shown with FTIR and STXM-NEXAFS analysis. STXM-NEXAFS analysis showed a continuous C distribution on minerals. Desorption of the coprecipitated OM was less than that of the adsorbed OM at comparable C/Fe ratios. These results are helpful to understand C and Fe cycling in the natural environments with periodically fluctuating redox conditions, where coprecipitation can occur.
The production and use of nanoparticles leads to the emission of manufactured or engineered nanoparticles into the environment. Those particles undergo many possible reactions and interactions in the environment they are exposed to. These reactions and the resulting behavior and fate of nanoparticles in the environment have been studied for decades through naturally occurring nanoparticulate (1–100 nm) and colloidal (1–1000 nm) substances. The knowledge gained from these investigations is nowhere near sufficiently complete to create a detailed model of the behavior and fate of engineered nanoparticles in the environment, but is a valuable starting point for the risk assessment of these novel materials. It is the aim of this Review to critically compare naturally observed processes with those found for engineered systems to identify the “nanospecific” properties of manufactured particles and describe critical knowledge gaps relevant for the risk assessment of manufactured nanomaterials in the environment.
Ferric oxyhydroxides play an important role in controlling the bioavailability of oxyanions such as arsenate and phosphate in soil. Despite this, little is known about the properties and reactivity of Fe(III)-organic matter phases derived from adsorption (reaction of organic matter (OM) to post-synthesis Fe oxide) versus coprecipitation (formation of Fe oxides in presence of OM). Coprecipitates and adsorption complexes were synthesized at pH 4 using two natural organic matter (NOM) types extracted from forest floor layers (Oi and Oa horizon) of a Haplic Podzol. Iron(III) coprecipitates were formed at initial molar metal-to-carbon (M/C) ratios of 1.0 and 0.1 and an aluminum (Al)-to-Fe(III) ratio of 0.2. Sample properties were studied by X-ray diffraction, X-ray photoelectron spectroscopy (XPS), N2 gas adsorption, dynamic light scattering, and electrophoretic mobility measurements. Arsenic [As(V)] adsorption to Fe-OM phases was studied in batch experiments (168 h, pH 4, 100 μM As). The organic carbon (OC) contents of the coprecipitates (82–339 mg g−1) were higher than those of adsorption complexes (31 and 36 mg g−1), leading to pronounced variations in specific surface area (9–300 m2 g−1), average pore radii (1–9 nm), and total pore volumes (11–374 mm3 g−1) but being independent of the NOM type or the presence of Al. The occlusion of Fe solids by OM (XPS surface concentrations: 60–82 atom% C) caused comparable pHPZC (1.5–2) of adsorption complexes and coprecipitates. The synthesis conditions resulted in different Fe-OM association modes: Fe oxide particles in ‘M/C 0.1’ coprecipitates covered to a larger extent the outermost aggregate surfaces, for some ‘M/C 1.0’ coprecipitates OM effectively enveloped the Fe oxides, while OM in the adsorption complexes primarily covered the outer aggregate surfaces. Despite of their larger OC contents, adsorption of As(V) was fastest to coprecipitates formed at low Fe availability (M/C 0.1) and facilitated by desorption of weakly bonded OC and disaggregation. In contrast, ‘M/C 1.0’ coprecipitates showed a comparable rate of As uptake as the adsorption complexes. While small mesopores (2–10 nm) promoted the fast As uptake particularly to ‘M/C 0.1’ coprecipitates, the presence of micropores (<2 nm) appeared to impair As desorption. This study shows that the environmental reactivity of poorly crystalline Fe(III) oxides in terrestrial and aquatic systems can largely vary depending on the formation conditions. Carbon-rich Fe phases precipitated at low M/C ratios may play a more important role in oxyanion immobilization and Fe and C cycling than phases formed at higher M/C ratios or respective adsorption complexes.
Batch experiments were carried out to investigate fractionation and kinetics of humic acid (HA) during adsorption onto hematite and the effect of phosphate. The concentrations and weight-average molecular weight (Mw) of HA in solution were determined by total organic carbon (TOC) analyzer and high-performance size exclusion chromatography (HPSEC). Addition of phosphate (simultaneously or beforehand) decreased HA adsorption due to competition. Fractions of HA with Mw values larger than 4000 Da were preferably adsorbed. The adsorption kinetics can be described with Pseudo-second order model or Elovich kinetic model. Fractions of HA with relatively low Mw (3000–4000 Da) were quickly adsorbed in the first hour, and then were replaced slowly by larger ones (>5000 Da). This phenomenon could be explained by fast diffusion of relatively small HA particles from solution to mineral surface, but the overall binding affinity is higher for the bigger particles as a result of more reactive groups present on bigger particles compared to smaller particles. At last, molecular weight distribution results in adsorption isotherms and kinetics indicated that the replacement between relatively small and larger fractions of HA could last more than 24 h.
Sonolysis, reduction by elemental iron (Fe{sup 0}), and a combination of the two processes were used to facilitate the degradation of nitrobenzene (NB) and aniline (AN) in water. The rates of reduction of nitrobenzene by Fe{sup 0} are enhanced in the presence of ultrasound. The first-order rate constant, K{sub US}, for nitrobenzene degradation by ultrasound is 1.8 x 10{sup {minus}3} min{sup {minus}1}, while in the presence of Fe{sup 0}, the rate was found to be substantially faster. The observation of similar degradation rates for aniline in each system suggests that the sonication process was not affected by the presence of Fe{sup 0}. The observed rate enhancements for the degradation of nitrobenzene can be attributed primarily to the continuous cleaning and chemical activation of the Fe{sup 0} surfaces by acoustic cavitation and to accelerated mass transport rates of reactants, intermediates, and products between the solution phase and the Fe{sup 0} surface. The relative concentrations of nitrosobenzene and aniline, the principal reaction intermediates generated by Fe{sup 0} reduction, are altered substantially in the presence of ultrasound.
Soil quality concepts are commonly used to evaluate sustainable land management in agroecosystems. The objectives of this review were to trace the importance of soil organic matter (SOM) in Canadian sustainable land management studies and illustrate the role of SOM and aggregation in sustaining soil functions. Canadian studies on soil quality were initiated in the early 1980s and showed that loss of SOM and soil aggregate stability were standard features of nonsustainable land use. Subsequent studies have evaluated SOM quality using the following logical sequence: soil purpose and function, processes, properties and indicators, and methodology. Limiting steps in this soil quality framework are the questions of critical limits and standardization for soil properties. At present, critical limits for SOM are selected using a commonly accepted reference value or based on empirically derived relations between SOM and a specific soil process or function (e.g., soil fertility, productivity, or erodibility). Organic matter fractions (e.g., macro-organic matter, light fraction, microbial biomass, and mineralizable C) describe the quality of SOM. These fractions have biological significance for several soil functions and processes and are sensitive indicators of changes in total SOM. Total SOM influences soil compactibility, friability, and soil water-holding capacity while aggregated SOM has major implications for the functioning of soil in regulating air and water infiltration, conserving nutrients, and influencing soil permeability and erodibility. Overall, organic matter inputs, the dynamics of the sand-sized macro-organic matter, and the soil aggregation process are important factors in maintaining and regulating organic matter functioning in soil.
Adsorption behaviors of fulvic acid (FA) and humic acid (HA) on kaolinite, smectite and vermiculite were investigated. To explore the adsorption mechanism, characterization of both the adsorption FA/HA-clay complexes and suspensions was conducted by utilizing multiple analytical techniques including liquid-state 1H nuclear magnetic resonance spectroscopy, high performance size exclusion chromatography, UV–vis spectroscopy, Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. Both FA and HA underwent fractionation during the adsorption due to different affinity for the functional moieties of FA/HA on the minerals. More HA was adsorbed than FA on kaolinite and smectite mainly via hydrophobic interaction. Electron transfer from aromatic units of FA to iron cation induced more FA than HA adsorbed onto vermiculite at higher FA/HA concentrations (>20 mg C/L). Specific surface area and pore volume analyses indicated HA with larger particle size was prone to accumulate on the external mineral surfaces, while FA was easier than HA to block pores of the minerals. The increased pH in clay suspensions after FA/HA adsorption suggested that ligand exchange occurred and FA/HA-clay complexes formed, particularly for the 2:1 type minerals of smectite and vermiculite with the increase of pH at 0.41 and 0.62 units, respectively. Furthermore, the increase of the equilibrium pH or the decrease of the ionic strength led to the reduction of FA/HA adsorption on all the three minerals. Due to rich in iron cation, more carboxyl and hydroxyl functional groups were facilitated for the ligand exchange and cation-bridging on vermiculite, and thus improved the adsorption capacity. The results of this study will improve our understanding of the roles of mineral interfacial properties, characteristics of FA and HA in the adsorption of FA/HA on clay minerals.
Information on the interaction of nanoparticles with natural organic matter (NOM) is essential for understanding their environmental impacts. In this study the adsorption and desorption of humic acids (HA) and fulvic acids (FA) on SiO2 particles in size of 20, 100 and 500 nm were investigated. The adsorption of HA and FA on 20 nm SiO2 was much stronger compared with their adsorption on 100 and 500 nm SiO2, probably due to the specific surface properties of the nanoparticles. The adsorption of HA and FA was pH-dependent, particularly for HA adsorption. The adsorption of HA showed apparent difference between the ionic strength of 0.01 and 0.1 mol/L NaNO3, but no obvious difference was observed for FA adsorption. Desorption results presented obviously hysteresis with more obvious for HA desorption. The results of this study demonstrated evidently that 20 nm SiO2 particles had much higher affinity to HA and FA than did 100 and 500 nm particles, which may have significant effects on their behaviors in the environment.Graphical abstractAdsorption isotherms of humic acids (HA) and fulvic acids (FA) on SiO2 particles at pH 4.0 and Langmuir model (real lines) and Freundlich model (dotted lines) fittings.Highlights► Adsorption of humic and fulvic acids on SiO2 heavily depends on particle sizes. ► Humic and fulvic acids interact differently with the nanoparticles. ► Cation bridge interaction contributes importantly to the humic substance adsorption.
Molecular weight (MW) of humic materials is a key factor controlling proton and metal binding and organic pollutant partitioning. Several studies have suggested preferential adsorption of higher MW, more aromatic moieties to mineral surfaces; quantification of such processes is fundamental to development of predictive models. We used high pressure size exclusion chromatography (HPSEC) to quantify MW changes upon adsorption of a muck fulvic acid (MFA) extracted from a peat deposit to kaolinite and goethite, at pH 3.7, 6, and 8 at 22 °C, I = 0.01 (NaCl), 24-h reaction time. MFA adsorption affinity was greater for goethite than for kaolinite. At concentrations less than the adsorption maximum (Amax) for both adsorbents, the weight-average MW (Mw) of MFA remaining in solution decreased by as much as several hundred Daltons relative to control samples, indicating preferential adsorption of the higher MW components. At concentrations more than Amax, Mw of MFA in solution did not change appreciably. Although total adsorption decreased significantly as pH increased, fractionation as measured by change in Mw remained similar, perhaps indicating greater selectivity for higher MW components at higher pH.
Absorptivities at λ = 280 nm normalized to mg C L-1 (ϵ) suggested preferential adsorption of more aromatic moieties to kaolinite. ϵ could not be used for goethite-reacted samples because high Fe concentrations in the aqueous phase brought about by goethite dissolution interfered with the spectroscopic analysis. Preliminary kinetic experiments suggested that smaller molecules adsorbed first and were replaced by larger molecules whose adsorption was thermodynamically favored.
Aggregation and dispersion of mineral particles spontaneously take place under changing environmental in natural systems. The structure of particle network in soils, the retardation or release of colloidal particles, and their mobility and transport are inherently influenced by natural organic matter bound to the mineral matrix. Since the surface properties of clay mineral and metal oxide particles, and the electrified mineral–water interfaces play a major role in formation, structure and strength of aggregates, any surface modification, especially by polyanionic organic complexants such as humic substances, has a significant affect on particle interaction in a mineral assemblage. The permanently and/or conditionally charged clay minerals (montmorillonite and kaolinite) and iron oxides (hematite and magnetite), as known major mineral components in natural waters, were selected for studying their surface charge characteristics and pH dependent interactions. We discuss how the surface charge correlates with particle aggregation
through some characteristic examples for homo and heterocoagulation of similar and dissimilar mineral particles under acidic condition (at pH �4) in the dilute and concentrated systems studied by means of light scattering and rheology, respectively. The adsorption of a brown coal derived humic acid, and its influence on the surface charge character and particle aggregation of clay and iron oxide particles were also studied in dilute and concentrated suspensions. Humic acids can be bound to the most reactive surface sites of clay and oxide particles, i.e. to Al-OH mainly at the edges of clay lamellae, and to Fe-OH on iron oxides, in surface complexation reaction, therefore their role in particle aggregation is specific. Relations between surface complexation, surface charge modification, and particle aggregation in pure and mixed montmorillonite–iron oxide systems are explained.
The photochemical reduction of Cr(VI) by iron and aquatic dissolved organic matter (DOM) was investigated. DOM sampled from a number of surface waters (a eutrophic wetland, a blackwater stream, and river water from a mix-use watershed) was used in this study. Moreover, a fulvic acid from Lake Fryxell, Antarctica, was also used to represent a DOM derived from a strictly autochthonous source. Cr(VI) reduction to Cr(III) at pH 5.5 was observed for all target DOMs used in this study, but rates varied widely. In general, photoreduction rates increased with increasing iron concentrations, but the type of DOM appeared to influence the kinetics to a larger degree. The rate of reduction was significantly greater for DOM derived from terrestrial systems than from predominantly autochthonous materials even if additional iron was added to the later. A positive correlation was observed between rates of Cr(VI) photoreduction and properties of the isolated DOM samples whereby faster reduction was observed for larger more aromatic substrates. On the basis of the fast rates reported for the dark reduction of Cr(VI) to Cr(III) by Fe(II)-organic ligands, we hypothesize that the rate-limiting step in these reactions is the photoreduction of Fe(III) to Fe(II) by a ligand-to-metal charge-transfer pathway after absorption of light by Fe(III)-DOM complexes or by reduction of Fe(III) by superoxide or other intermediates formed after light absorption by DOM. Thus, the rate of Cr(VI) photoreduction to Cr(III) in natural sunlit waters is dependent upon both the amount of iron present and the nature of the dissolved organic matter substrate.
An analogous study to 2:1 type montmorillonite [Tombácz, E., Szekeres, M., 2004. Colloidal behavior of aqueous montmorillonite suspensions: the specific role of pH in the presence of indifferent electrolytes. Appl. Clay Sci. 27, 75–94.] was performed on 1:1 type kaolinite obtained from Zettlitz kaolin. Clay minerals are built up from silica tetrahedral (T) and alumina octahedral (O) layers. These lamellar particles have patch-wise surface heterogeneity, since different sites are localized on definite parts of particle surface. pH-dependent charges develop on the surface hydroxyls mainly at edges besides the permanent negative charges on silica basal plane due to isomorphic substitutions. Electric double layers (edl) with either constant charge density on T faces (silica basal planes) or constant potential at constant pH on edges and O faces (hydroxyl-terminated planes) form on patches. The local electrostatic field is determined by the crystal structure of clay particles, and influenced by the pH and dissolved electrolytes. The acid–base titration of Na-kaolinite suspensions showed analogous feature to montmorillonite. The initial pH of suspensions and the net proton surface excess vs. pH functions shifted to the lower pH with increasing ionic strength indicating the presence of permanent charges in both cases, but these shifts were smaller for kaolinite in accordance with its much lower layer charge density. The pH-dependent charge formation was similar, positive charges in the protonation reaction of (Si–O)Al–OH sites formed only at pHs below ∼ 6–6.5, considered as point of zero net proton charge (PZNPC) of kaolinite particles. So, oppositely charged surface parts on both clay particles are only below this pH, therefore patch-wise charge heterogeneity exists under acidic conditions. Electrophoretic mobility measurements, however, showed negative values for both clays over the whole range of pH showing the dominance of permanent charges, and only certain decrease in absolute values, much larger for kaolinite was observed with decreasing pH below pH ∼ 6. The charge heterogeneity was supported by the pH-dependent properties of dilute and dense clay suspensions with different NaCl concentrations. Huge aggregates were able to form only below pH ∼ 7 in kaolinite suspensions. Coagulation kinetics measurements at different pHs provided undisputable proofs for heterocoagulation of kaolinite particles. Similarly to montmorillonite, heterocoagulation at pH ∼ 4 occurs only above a threshold electrolyte concentration, which was much smaller, only ∼ 1 mmol l− 1 NaCl for kaolinite, than that for montmorillonite due to the substantial difference in particle geometry. The electrolyte tolerance of both clay suspensions increased with increasing pH, pH ∼ 6–6.5 range was sensitive, and even a sudden change occurred above pH ∼ 6 in kaolinite. There was practically no difference in the critical coagulation concentration of kaolinite and montmorillonite (c.c.c.∼ 100 mmol l− 1 NaCl) measured in alkaline region, where homocoagulation of negatively charged lamellae takes place. Rheological measurements showed shear thinning flow character and small thixotropy of suspensions at and above pH ∼ 6.7 proving the existence of repulsive interaction between uniformly charged particles in 0.01 M NaCl for both clays. The appearance of antithixotropy, the sudden increase in yield values, and also the formation of viscoelastic systems only at and below pH ∼ 6 verify the network formation due to attraction between oppositely charged parts of kaolinite particles. Under similar conditions the montmorillonite gels were thixotropic with significant elastic response.
SEM observations of low solid content vitrified clay suspensions reveal that clay platelets build porous three-dimensional networks with platelets contacting each other mostly by their edges. To explain this behaviour, which must require long range edge-to-edge (EE) attractive forces, a hydrophobic-like interaction has been proposed. This interaction may be induced by the presence of nano-bubbles existing on the edges of clay crystals which may cause clay particles to flocculate. The following indirect evidence for such hydrophobic behaviour is presented. First, a clay platelet is shown attached to an oil drop by its edge; second, clay flocs were attracted by a vertically placed Teflon strip but not to the hydrophilic mica basal surface; third, a much thicker porous sediment occurred in CO2-saturated water solution compared with vacuum degassed water.
Mineral-bound humic substances modify the surfaces of clay minerals, changing the nature and number of adsorption sites for contaminants. Due to their effect on the surface charge of colloidal particles they can also change the particles mobility and thus their transport behavior. In this paper the influence of natural organic matter on the zetapotential of kaolinite and montmorillonite is shown. Adsorption experiments with kaolinite and montmorillonite show that due to adsorption the humic substances are fractionated. The equilibrium pH value does not influence the fractionation due to adsorption. Large hydrophobic molecules showed the strongest affinity towards the clay surfaces. Substances with a small apparent molecular size and a high content of carboxylic functional groups made up the non-adsorbing fraction of the NOM. Ca2+ had no measurable effect on the adsorption. The low influence of the adsorbed NOM on the zetapotential of the clay minerals suggests physical adsorption as the predominant adsorption mechanism.
The effects of the ionic strength (maintained by LiCl, NaCl or KCl) and Ca2+ and Mg2+ concentration on the coagulation of purified humic acids (HA) was studied. Solutions of known ionic strengths, pcH, Ca2+ and Mg2+ concentrations were prepared with HA and filtered to obtain the fraction with a size smaller than 100 kD. After a 50 day storage, samples of these solutions were filtered again (100 kD) and the total organic C (TOC) of the filtered solutions measured. The HA coagulation increased with salt concentration, with the cationic charge, and for cations of the same charge, with the cationic charge density. The coagulation decreased for pcH values of 4 to 7–8 in the absence of and presence of Mg2+ and Ca2+. In the absence of the divalent cations, the coagulation has a constant value for pcH>8, but, in the presence of Mg2+ and Ca2+, increases at pcH values greater than 9. The coagulation of humic materials occurs whether the samples are exposed to light or kept in the dark, although the coagulation kinetics are slower for the samples kept in the dark. The size distribution of size-fractionated humic solutions changes over time to a size distribution similar to that of the original humic solution before it was size-fractionated. The results are explained by the DLVO theory.
The sorption of organic matter from four species of phytoplankton to clean mineral surfaces was studied using sequential adsorption experiments. The soluble intracellular components from four phytoplankton species, Phaeocystis globosa, Gymnodinium sanguineum, Scrippsiella trochoidea, and Ditylum brightwellii, were repeatedly exposed to three minerals; montmorillonite, kaolinite, and chlorite. The surface-reactive fraction of organic matter in phytoplankton dissolved organic carbon (DOC) solutions ranged from 47% to 85% of the total. Adsorption partition coefficients (Kd) ranged from 53 to 175 l kg−1. These partition coefficients are significantly higher than the partition coefficients commonly observed for sedimentary porewater organic matter (≤30 l kg−1), and suggest that phytoplankton exudates contain a component of organic matter with considerable surface reactivity. S. trochoidea and D. brightwellii consistently had the highest Kd values and the greatest reactive organic matter component relative to P. globosa and G. sanguineum. On average, the highest Kd values were associated with chlorite (average Kd of 119 l kg−1) and montmorillonite (average of 105 l kg−1) rather than kaolinite (average Kd of 89 l kg−1). Organic matter from all four phytoplankton species interacted most with montmorillonite (average=82% reactive), followed by chlorite (72% reactive) and kaolinite (63% reactive). Significant variation in both the extent of surface-reactive material and the partition coefficients of the reactive material using this matrix of source organisms and mineral surfaces suggest that both the mineral and organic matter sources are influential in the sorption of organic matter to sediment surfaces.
Natural organic matter (NOM) is a complex mixture of different organic components (or fractions), yet few studies have examined the fractional adsorption of NOM on mineral surfaces. In this study, we fractionated NOM into hydrophobic (HbA) and hydrophilic (HL) subcomponents and two size fractions (with nominal molecular weights cut off at 3000 (3 K) dalton in an attempt to elucidate the adsorption and desorption mechanisms of NOM on iron oxide surfaces. Results indicated that, on a C weight basis, larger size HbA fraction was preferentially adsorbed (with a higher adsorption affinity and capacity) over smaller size HL fraction. However, on an O weight basis, less HbA fraction was adsorbed relative to the HL fraction, because HbA contained about 1.34 times more C but 0.82 times less O than the HL. These observations are consistent with results which indicate that only limited adsorption sites are available on the iron oxide surfaces and that the mechanism of HbA and HL adsorption was dominated by surface complexation-ligand exchange. FTIR and NMR spectroscopy and studies with several substituted benzoic acids/phenols further indicated that carboxyl and hydroxyl functional groups of these NOM fractions were actively involved in the reactions, and the steric arrangement of these functional groups may have played an important role in determining the adsorption of NOM fractions. Desorption studies indicated that the adsorbed NOM macromolecules on iron oxide surfaces were strongly bound at a given pH and ionic composition, resulting in a strong adsorption-desorption hysteresis. One possible explanation for the observed hysteresis is that the solution composition and equilibria are not identical between adsorption and desorption phases of the experiment because of preferential or selective adsorption of certain NOM fractions. This study implies that, due to the polydispersity of NOM, the competitive and fractional adsorption-desorption of NOM subcomponents must be considered in order to better predict NOM partitioning between the solution and solid phases and, therefore, the transport behavior of NOM in the subsurface soil environment.