Reduction Pathway-Dependent Formation of Reactive Fe(II) Sites in Clay Minerals.

Structural Fe in clay minerals is an important, potentially renewable source of electron equivalents for contaminant reduction, yet our knowledge of how clay mineral Fe reduction pathways and Fe reduction extent affect clay mineral Fe(II) reactivity is limited. Here, we used a nitroaromatic compound (NAC) as a reactive probe molecule to assess the reactivity of chemically reduced (dithionite) and Fe(II)-reduced nontronite across a range of reduction extents. We observed biphasic transformation kinetics for all nontronite reduction extents of ≥5% Fe(II)/Fe(total) regardless of the reduction pathway, indicating that two Fe(II) sites of different reactivities form in nontronite at environmentally relevant reduction extents. At even lower reduction extents, Fe(II)-reduced nontronite completely reduced the NAC whereas dithionite-reduced nontronite could not. Our 57Fe Mössbauer spectroscopy, ultraviolet-visible spectroscopy, and kinetic modeling results suggest that the highly reactive Fe(II) entities likely comprise di/trioctahedral Fe(II) domains in the nontronite structure regardless of the reduction mechanism. However, the second Fe(II) species, of lower reactivity, varies and for Fe(II)-reacted NAu-1 likely comprises Fe(II) associated with an Fe-bearing precipitate formed during electron transfer from aqueous to nontronite Fe. Both our observation of biphasic reduction kinetics and the nonlinear relationship of rate constant and clay mineral reduction potential EH have major implications for contaminant fate and remediation.

Remarkable effect of Co substitution in magnetite on the reduction removal of Cr(VI) coupled with aqueous Fe(II): Improvement mechanism and Cr fate.

The interaction between magnetite and aqueous Fe(II) profoundly impacts the mineral recrystallization, trace-metal sequestration, and contaminant reduction. The iron ions in natural magnetite are extensively substituted by other cations. It is still unclear whether the substitution with thermodynamically favorable redox repairs (e.g., Co2+/Co3+) plays a vital role in the reducing capability of the coupled system. Herein, a series of Co-substituted magnetite samples (Fe3-xCoxO4, 0.00 ≤ x ≤ 1.00) were synthesized and tested for the reductive removal of Cr(VI) in the presence of Fe(II). Fe3-xCoxO4 had a spinel structure with the preferential occupancy of Co2+ on octahedral sites. No visible variation in the BET surface area was observed, whereas the surface site density increased gradually with Co substitution. Cr(VI) was found first adsorbed on the Fe3-xCoxO4 surface and then reduced to Cr(III) by the structural Fe2+ and the absorbed Fe(II), accompanied by the oxidation of bulk Fe2+ and surface Fe(II) in Fe3-xCoxO4 without phase transformation. The Cr(III) was precipitated on the Fe3-xCoxO4 surface with Fe(III), or substituted octahedral Fe in Fe3-xCoxO4. Both the reaction kinetics and the electron transfer efficiency revealed that Co substitution significantly improved the reactivity of Fe3-xCoxO4/Fe(II) towards Cr(VI) reduction. This was ascribed to the presence of the redox pairs Co2+/Co3+ and Fe2+/Fe3+ accelerating electron transfer from the Fe3-xCoxO4 interface to Cr(VI).

Impacts of detrital nano- and micro-scale particles (dNP) on contaminant dynamics in a coal mine AMD treatment system.

Pollutants in acid mine drainage (AMD) are usually sequestered in neoformed nano- and micro-scale particles (nNP) through precipitation, co-precipitation, and sorption. Subsequent biogeochemical processes may control nNP stability and thus long-term contaminant immobilization. Mineralogical, chemical, and microbiological data collected from sediments accumulated over a six-year period in a coal-mine AMD treatment system were used to identify the pathways of contaminant dynamics. We present evidence that detrital nano- and micron-scale particles (dNP), composed mostly of clay minerals originating from the partial weathering of coal-mine waste, mediated biogeochemical processes that catalyzed AMD contaminant (1) immobilization by facilitating heterogeneous nucleation and growth of nNP in oxic zones, and (2) remobilization by promoting phase transformation and reductive dissolution of nNP in anoxic zones. We found that dNP were relatively stable under acidic conditions and estimated a dNP content of ~0.1g/L in the influent AMD. In the AMD sediments, the initial nNP precipitates were schwertmannite and poorly crystalline goethite, which transformed to well-crystallized goethite, the primary nNP repository. Subsequent reductive dissolution of nNP resulted in the remobilization of up to 98% of S and 95% of Fe accompanied by the formation of a compact dNP layer. Effective treatment of pollutants could be enhanced by better understanding the complex, dynamic role dNP play in mediating biogeochemical processes and contaminant dynamics at coal-mine impacted sites.

Functional diversity and electron donor dependence of microbial populations capable of U(VI) reduction in radionuclide-contaminated subsurface sediments.

In order to elucidate the potential mechanisms of U(VI) reduction for the optimization of bioremediation strategies, the structure-function relationships of microbial communities were investigated in microcosms of subsurface materials cocontaminated with radionuclides and nitrate. A polyphasic approach was used to assess the functional diversity of microbial populations likely to catalyze electron flow under conditions proposed for in situ uranium bioremediation. The addition of ethanol and glucose as supplemental electron donors stimulated microbial nitrate and Fe(III) reduction as the predominant terminal electron-accepting processes (TEAPs). U(VI), Fe(III), and sulfate reduction overlapped in the glucose treatment, whereas U(VI) reduction was concurrent with sulfate reduction but preceded Fe(III) reduction in the ethanol treatments. Phyllosilicate clays were shown to be the major source of Fe(III) for microbial respiration by using variable-temperature Mössbauer spectroscopy. Nitrate- and Fe(III)-reducing bacteria (FeRB) were abundant throughout the shifts in TEAPs observed in biostimulated microcosms and were affiliated with the genera Geobacter, Tolumonas, Clostridium, Arthrobacter, Dechloromonas, and Pseudomonas. Up to two orders of magnitude higher counts of FeRB and enhanced U(VI) removal were observed in ethanol-amended treatments compared to the results in glucose-amended treatments. Quantification of citrate synthase (gltA) levels demonstrated a stimulation of Geobacteraceae activity during metal reduction in carbon-amended microcosms, with the highest expression observed in the glucose treatment. Phylogenetic analysis indicated that the active FeRB share high sequence identity with Geobacteraceae members cultivated from contaminated subsurface environments. Our results show that the functional diversity of populations capable of U(VI) reduction is dependent upon the choice of electron donor.

Arsenic speciation in arsenic-rich Brazilian soils from gold mining sites under anaerobic incubation.

BACKGROUND: Arsenic speciation in environmental samples is essential for studying toxicity, mobility and bio-transformation of As in aquatic and terrestrial environments. Although the inorganic species As(III) and As(V) have been considered dominant in soils and sediments, organisms are able to metabolize inorganic forms of arsenic into organo-arsenic compounds. Arsenosugars and methylated As compounds can be found in terrestrial organisms, but they generally occur only as minor constituents. We investigated the dynamics of arsenic species under anaerobic conditions in soils surrounding gold mining areas from Minas Gerais State, Brazil to elucidate the arsenic biogeochemical cycle and water contamination mechanisms. METHODS: Surface soil samples were collected at those sites, namely Paracatu Formation, Banded Iron Formation and Riacho dos Machados Sequence, and incubated in CaCl2 2.5 mmol L(-1) suspensions under anaerobic conditions for 1, 28, 56 and 112 days. After that, suspensions were centrifuged and supernatants analyzed for soluble As species by IC-ICPMS and HPLC-ICPMS. RESULTS: Easily exchangeable As was mainly arsenite, except when reducible manganese was present. Arsenate was mainly responsible for the increase in soluble arsenic due to the reductive dissolution of either iron or manganese in samples from the Paracatu Formation and Riacho dos Machados Sequence. On the other hand, organic species of As dominated in samples from the Banded Iron Formation during anaerobic incubation. DISCUSSION: Results are contrary to the expectation that, in anaerobic environments, As release due to the reductive dissolution of Fe is followed by As(V) reduction to As(III). The occurrence of organo-arsenic species was also found to be significant to the dynamics of soluble arsenic, mainly in soils from the Banded Iron Formation (BIF), under our experimental conditions. CONCLUSIONS: In general, As(V) and organic As were the dominant species in solution, which is surprising under anaerobic conditions in terrestrial environments. The unexpected occurrence of organic species of As was attributed to enrollment of ternary organic complexes or living organisms such as algae or cyanobacteria. PERSPECTIVES: These findings are believed to be useful for remediation strategies in mine-affected regions, as the organic As species are in general considered to be less toxic than inorganic ones and even As(V) is considered less mobile and toxic than As(III).

Uranium and technetium bio-immobilization in intermediate-scale physical models of an in situ bio-barrier.

We investigated the long-term effects of ethanol addition on U and Tc mobility in groundwater flowing through intermediate-scale columns packed with uncontaminated sediments. The columns were operated above-ground at a contaminated field site to serve as physical models of an in situ bio-barrierfor U and Tc removal from groundwater. Groundwater containing 4 microM U and 520 pM Tc was pumped through the columns for 20 months. One column received additions of ethanol to stimulate activity of indigenous microorganisms; a second column received no ethanol and served as a control. U(VI) and Tc(VII) removal was sustained for 20 months (approximately 189 pore volumes) in the stimulated column under sulfate- and Fe(III)-reducing conditions. Less apparent microbial activity and only minor removal of U(VI) and Tc(VII) were observed in the control. Sequential sediment extractions and XANES spectra confirmed that U(IV) was present in the stimulated column, although U(IV) was also detected in the control; extremely low concentrations precluded detection of Tc(IV) in any sample. These results provide additional evidence that bio-immobilization may be effective for removing U and Tc from groundwater. However, long-term effectiveness of bio-immobilization may be limited by hydraulic conductivity reductions or depletion of bioavailable Fe(III).

Hydration/expansion and cation charge compensation modulate the Brønsted basicity of distorted clay water.

This letter addresses how iron redox cycling and the hydration properties of the exchangeable cation influence the Brønsted basicity of adsorbed water in 2:1 phyllosilicates. The probe pentachloroethane undergoes facile dehydrochlorination to tetrachloroethene, attributed to increases in the Brønsted basicity of near-surface hydrating water molecules following the reduction of structural Fe(III) to Fe(II). This dehydrochlorination process is studied in the presence of Na(+)- or K(+)-saturated Upton montmorillonite [(Na0.82 (Si7.84 Al0.16)(Al3.10 Fe(3+)0.3 Mg0.66) O20 (OH)4] or ferruginous smectite [(Na0.87 Si7.38 Al0.62)(Al1.08) Fe(3+)2.67 Fe(2+)0.01 Mg0.23) O20 (OH)4]. The effect of iron redox cycling on pentachloroethane dehydrochlorination is studied using reduced or reduced and reoxidized smectite samples saturated with Na+ (fully expanded clay) or K+ (fully collapsed clay). Variations in the clay Brønsted basicity following Na+ -for- K+ exchange are explained by cationic charge compensation or interlayer hydration/expansion imposed by the nature of the exchangeable cation. Inverse relations between K+ fixation and clay water content as well as trends in pentachloroethane transformation indicate that increases in the Brønsted basicity result from increases in the clay hydrophilicity and shifts in the local activity of distorted clay water. Potassium fixation causes partially collapsed smectites bearing low amounts of structural Fe(II) to have a similar reactivity to that of fully expanded smectites (Na+ form) bearing higher amounts of structural Fe(II). In particular, the conversion of up to 80% of the pentachloroethane to tetrachloroethane by K+ -saturated, reoxidized Upton was explained because the fixation of K+ causes nonreversible expansion and incomplete reoxidation of structural Fe(II), which contributes to the stabilization of charge density near sites bearing Fe(II). Higher pentachloroethane conversions by Upton montmorillonite over ferruginous smectite, however, suggest that charge dispersion rather than site specificity contributes predominantly to clay reactivity. Thus, clay interlayer hydration/expansion imposed by the nature of the exchangeable cation alters water dissociation and proton exchange in Fe(II)-Fe(III) phyllosilicates susceptible to iron redox cycling.

Modulation of the genotoxicity of pesticides reacted with redox-modified smectite clay.

Pesticides are toxic agents intentionally released into the environment; their use raises public health and environmental concerns. In recent years there has been much attention to the biotic degradation of pesticides. Abiotic mechanisms in the soil can contribute to pesticide degradation yet the toxicological impact of such degradation is unclear. This study combines for the first time an investigation into abiotic mechanisms of degradation coupled with toxicological endpoints in mammalian cells. The genotoxicity of three commonly used agricultural pesticides was assessed before and after exposure to redox-modified clay minerals. The objectives of the study were to determine the genotoxicity of 2,4-dichlorophenoxy acetic acid (2,4-D), dicamba, and oxamyl, using single cell gel electrophoresis with Chinese hamster ovary (CHO) cells, and to determine the effect of the iron oxidation state in clay minerals (ferruginous smectite SWa-1) on the genotoxic potency of the pesticides. 2,4-D alone or following reaction with redox-modified clays did not induce DNA damage in CHO cells. Oxamyl alone induced a concentration-dependent increase in genomic DNA damage; however, its genotoxicity declined after reaction with reduced clay minerals. Dicamba was not genotoxic when directly analyzed. When dicamba was reacted with reduced clay, a concentration-dependent increase in genomic DNA damage was observed. This is the first reported case of a pesticide being converted into a genotoxin after exposure to redox-modified smectites. These data introduce a new paradigm on the interaction between redox-modified clays and pesticide-related environmental genotoxicity.

Alteration of mammalian-cell toxicity of pesticides by structural iron(II) in ferruginous smectite.

The ultimate concern over pesticides in the environment is their toxic impact on nontarget organisms, including humans. Soil clays are known to interact with pesticides in ways that decrease the concentration of the parent compound in the soil solution (adsorption, sequestration, degradation). These phenomena are generally regarded as beneficial, but toxicological verification is lacking. In this study, mammalian-cell cytotoxicity of four commonly used agricultural chemicals (2,4-D, alachlor, dicamba, and oxamyl) was assessed after exposure to either reduced or oxidized ferruginous smectite (SWa-1). Results revealed that treatment with reduced smectite produced differential effects on mammalian-cell viability, depending on the pesticide. Oxamyl and alachlor reacted with reduced SWa-1 showed a significant decrease in their overall cytotoxic potential. Dicamba reacted with the reduced-clay treatment and generated products that were more toxic than the parent pesticide. Finally, no differences were observed between redox treatments for 2,4-D. The significance of these results is that oxidized smectites have virtually no influence on the toxicity of pesticides, whereas reduced-Fe smectite plays an important role in altering the cytotoxic potential of agricultural pesticides. The Fe oxidation state of clay minerals should, therefore, be taken into account in pesticide management programs.

Dehydrochlorination of 1,1,1-trichloroethane and pentachloroethane by microbially reduced ferruginous smectite.

Reduction of structural Fe(III) in smectite clay minerals has been identified as a means to promote dechlorination of polychlorinated ethanes, but its environmental significance has yet to be fully assessed because Fe reduction has normally been achieved by agents uncommon in the environment (e.g., dithionite). This study reports the dehydrochlorination of pentachloroethane and 1,1,1-trichloroethane in the presence of ferruginous smectite reduced by two cultures of microorganisms, Shewanella oneidensis strain MR-1 (MR-R) and an enrichment culture from rice paddy soils (PS-R), in aqueous suspension under anoxic conditions. Microbially reduced ferruginous smectite facilitated dehydrochlorination of 1,1,1-trichloroethane to 1,1-dichloroethene with up to 60% conversion within 3 h of incubation time. In contrast, no formation of 1,1-dichloroethene was observed after incubation of 1,1,1-trichloroethane with chemically reduced ferruginous smectite for 24 h. Microbially reduced ferruginous smectite by MR-R and PS-R promoted the dehydrochlorination of pentachloroethane to tetrachloroethene by 80 and 15%, respectively, after 3 h of incubation time. The conversion of pentachloroethane to tetrachloroethene in the presence of chemically reduced ferruginous smectite after 24 h was 65%. These results indicate that structural Fe(II) in clay minerals has the potential to be an important reductant controlling the fate of organic chemicals in contaminated sediments.

Growth of iron(III)-reducing bacteria on clay minerals as the sole electron acceptor and comparison of growth yields on a variety of oxidized iron forms.

Smectite clay minerals are abundant in soils and sediments worldwide and are typically rich in Fe. While recent investigations have shown that the structural Fe(III) bound in clay minerals is reduced by microorganisms, previous studies have not tested growth with clay minerals as the sole electron acceptor. Here we have demonstrated that a pure culture of Shewanella oneidensis strain MR-1 as well as enrichment cultures of Fe(III)-reducing bacteria from rice paddy soil and subsurface sediments are capable of conserving energy for growth with the structural Fe(III) bound in smectite clay as the sole electron acceptor. Pure cultures of S. oneidensis were used for more detailed growth rate and yield experiments on various solid- and soluble-phase electron acceptors [smectite, Fe(III) oxyhydroxide FeOOH, Fe(III) citrate, and oxygen] in the same minimal medium. Growth was assessed as direct cell counts or as an increase in cell carbon (measured as particulate organic carbon). Cell counts showed that similar growth of S. oneidensis (10(8) cells ml(-1)) occurred with smectitic Fe(III) and on other Fe forms [amorphous Fe(III) oxyhydroxide, and Fe citrate] or oxygen as the electron acceptor. In contrast, cell yields of S. oneidensis measured as the increase in cell carbon were similar on all Fe forms tested while yields on oxygen were five times higher, in agreement with thermodynamic predictions. Over a range of particle loadings (0.5 to 4 g liter(-1)), the increase in cell number was highly correlated to the amount of structural Fe in smectite reduced. From phylogenetic analysis of the complete 16S rRNA gene sequences, a predominance of clones retrieved from the clay mineral-reducing enrichment cultures were most closely related to the low-G+C gram-positive members of the Bacteria (Clostridium and Desulfitobacterium) and the delta-Proteobacteria (members of the Geobacteraceae). Results indicate that growth with smectitic Fe(III) is similar in magnitude to that with Fe(III) oxide minerals and is dependent upon the mineral surface area available. Iron(III) bound in clay minerals should be considered an important electron acceptor supporting the growth of bacteria in soils or sedimentary environments.

Dechlorination of pentachloroethane by commercial Fe and ferruginous smectite.

Short-term experiments were conducted to investigate the effect of a commercial Fe and an iron-bearing clay mineral, ferruginuous smectite (SWa-1), on the degradation of pentachloroethane (PCA). After 3 h of contact time, SWa-1 catalyzed PCA dehydrochlorination to tetrachloroethene (PCE, 65% conversion), whereas commercial Fe promoted PCA stepwise dechlorination via dehydrochlorination (approximately 40% conversion) and subsequent PCE hydrogenolysis to trichloroethene (TCE). The addition of unaltered SWa-1 to commercial Fe led to a complete inhibition on TCE production, whereas the addition of reduced SWa-1 barely resulted in a 30% decrease.

Structural Perturbations in the Solid-Water Interface of Redox Transformed Nontronite.

Redox reactions of structural Fe affect many surface and colloidal properties of Fe-containing smectites in natural environments and many industrial systems, but few studies have examined the clay-water interface under oxidizing and reducing conditions. Infrared (FTIR) spectroscopy was used to investigate the effects of structural Fe oxidation state and hydration on layer Si-O stretching vibrations in Na-nontronite. Aqueous gels of unaltered, reduced, and reoxidized smectites were equilibrated at different swelling pressures, Pi, and water contents, m(w)/m(c), using a miniature pressure-membrane apparatus. One part of each gel was used for the gravimetric determination of m(w)/m(c); the other was transferred to an attenuated total reflectance cell in the FTIR spectrometer, where the spectrum of the gel was measured. The frequencies of four component peaks of Si-O stretching, nu(Si-O), in nontronite layers and of the H-O-H bending, nu(H-O-H), in the interlayer water were determined by using a curve-fitting technique. Reduction of structural Fe shifted the Si-O vibration to lower frequency and desensitized the Si-O vibration to the hydration state. A linear relation was found between nu(Si-O) and nu(H-O-H) for nontronite in each of its various oxidation states. These observations were interpreted to mean that structural Fe oxidation state has a significant impact on interfacial processes of the aqueous colloid system of Fe-rich phyllosilicates. Copyright 2000 Academic Press.