Carbon tetrachloride transformation in a model iron-reducing culture: relative kinetics of biotic and abiotic reactions

The role of iron materials in the abiotic transformation and biotransformation of polybrominated diphenyl ethers (PBDEs): A review

Polybrominated diphenyl ethers (PBDEs), widely used as flame retardants, easily enter the environment, thus posing environmental and health risks. Iron materials play a key role during the migration and transformation of PBDEs. This article reviews the processes and mechanisms of adsorption, degradation, and biological uptake and transformation of PBDEs affected by iron materials in the environment. Iron materials can effectively adsorb PBDEs through hydrophobic interactions, π-π interactions, hydrogen/halogen bonds, electrostatic interactions, coordination interactions, and pore filling interactions. In addition, they are beneficial for the photodegradation, reduction debromination, and advanced oxidation degradation and debromination of PBDEs. The iron material-microorganism coupling technology affects the uptake and transformation of PBDEs. In addition, iron materials can reduce the uptake of PBDEs in plants, affecting their bioavailability. The species, concentration, and size of iron materials affect plant physiology. Overall, iron materials play a bidirectional role in the biological uptake and transformation of PBDEs. It is necessary to strengthen the positive role of iron materials in reducing the environmental and health risks caused by PBDEs. This article provides innovative ideas for the rational use of iron materials in controlling the migration and transformation of PBDEs in the environment.

Bisulfite reduction of soil iron for the reductive degradation of trichloroethylene

This study explored the potential reactivities of various reductants in inducing subsurface TCE degradation in natural soils. It was found that bisulfite (HSO₃^-) exhibited the ability to induce reduction in soil iron minerals, and increase the degradation of TCE in the soil slurry system; however, no TCE degradation occurred in the aqueous system. The role of TCE degradation by soil constituents, such as major soil mineral elements, Fe and humic acid (HA) on HSO₃^-, was examined in aqueous phase. It was seen that by themselves, the presence of Fe³⁺, HA, Fe₂O₃, FeOOH, and Fe₃O₄ did not result in substantial TCE removals. However, the presence of HSO₃^- can significantly induce iron reduction, producing a reducing condition that can result in complete TCE degradation. Furthermore, the reductive pathway was identified as the dominant degradation route via electron scavenging with periodate ion. To demonstrate the applicability of HSO₃^- reduction enhancement, a HSO₃^-/TCE mixed solution was flushed through a soil column, with gradually increased HSO₃^- concentrations, at a fixed flow rate, and also with varied flushing rates at a fixed HSO₃^- concentration. Based on our study, a 10 mM HSO₃^- solution may be effective for some environmental sites; however, each site requires specific evaluation based on contaminant concentrations and subsurface conditions.

Stimulation of pyrolytic carbon materials as electron shuttles on the anaerobic transformation of recalcitrant organic pollutants: A review

Pyrolytic carbon materials (PCMs) with various surface functionalities are widely used as environmentally friendly and cost-efficient adsorbents for the removal of organic and inorganic pollutants. Recent studies have illustrated that PCMs as electron shuttles (ESs) could also show excellent performances in promoting the anaerobic transformation of recalcitrant organic pollutants (ROPs). Numerous studies have demonstrated the excellent electron-shuttle capability (ESC) of PCMs to stimulate the anaerobic reductive transformation of ROPs. However, there is a lack of consistent understanding of the mechanism of ESC formation in PCMs and the stimulation mechanism for ROPs anaerobic transformation. To gain a more comprehensive understanding of the latest developments in the study of PCMs as ESs for ROPs anaerobic transformation, this review summarizes the formation mechanism, influencing factors, and stimulation mechanisms of ESC. ESC benefits from redox functional groups (quinone and phenol groups), persistent free radicals (PFRs), redox-active metal ions, conductive graphene phase, and porous nature of their surface. The factors influencing ESC include the highest treatment temperature (HTT), feedstocks, modification methods, and environmental conditions, of which, the HTT is the key factor. PCMs promote the reductive transformation of ROPs under anaerobic conditions via abiotic and biotic pathways. Eventually, the prospects for the ROPs anaerobic transformation enhanced by PCMs are proposed.

Fe(II) Redox Chemistry in the Environment

Iron (Fe) is the fourth most abundant element in the earth's crust and plays important roles in both biological and chemical processes. The redox reactivity of various Fe(II) forms has gained increasing attention over recent decades in the areas of (bio) geochemistry, environmental chemistry and engineering, and material sciences. The goal of this paper is to review these recent advances and the current state of knowledge of Fe(II) redox chemistry in the environment. Specifically, this comprehensive review focuses on the redox reactivity of four types of Fe(II) species including aqueous Fe(II), Fe(II) complexed with ligands, minerals bearing structural Fe(II), and sorbed Fe(II) on mineral oxide surfaces. The formation pathways, factors governing the reactivity, insights into potential mechanisms, reactivity comparison, and characterization techniques are discussed with reference to the most recent breakthroughs in this field where possible. We also cover the roles of these Fe(II) species in environmental applications of zerovalent iron, microbial processes, biogeochemical cycling of carbon and nutrients, and their abiotic oxidation related processes in natural and engineered systems.

Anaerobic reduction of high-polarity nitroaromatic compounds by electrochemically active bacteria: Roles of Mtr respiratory pathway, molecular polarity, mediator and membrane permeability

Electrochemically active bacteria (EAB) are effective for the bioreduction of nitroaromatic compounds (NACs), but the exact reduction mechanisms are unclear yet. Therefore, 3-nitrobenzenesulfonate (NBS) was used to explore the biodegradation mechanism of NACs by EAB. Results show that NBS could be anaerobically degraded by Shewanella oneidensis MR-1. The generation of aminoaromatic compounds was accompanied with the NBS reduction, indicating that NBS was biodegraded via reductive approach by S. oneidensis MR-1. The impacts of NBS concentration and cell density on the NBS reduction were evaluated. The removal of NBS depends mainly on the transmembrane electron transfer of S. oneidensis MR-1. Impairment of Mtr respiratory pathway was found to mitigate the reduction of NBS, suggesting that the anaerobic biodegradation of NBS occurred extracellularly. Knocking out cymA severely impaired the extracellular reduction ability of S. oneidensis MR-1. However, the phenotype of ΔcymA mutant could be compensated by the exogenous electron mediators, implying the trans-outer membrane diffusion of mediators into the periplasmic space. This work provides a new insight into the anaerobic reduction of aromatic contaminants by EAB.

Identification of Degradation Pathways of Chlorohydrocarbons in Saturated Low-Permeability Sediments Using Compound-Specific Isotope Analysis

This study aims to investigate whether compound-specific carbon isotope analysis (CSIA) can be used to differentiate the degradation pathways of chlorohydrocarbons in saturated low-permeability sediments. For that purpose, a site was selected, where a complex mixture of chlorohydrocarbons contaminated an aquifer-aquitard system. Almost 50 years after contaminant releases, high-resolution concentration, CSIA, and microbial profiles were determined. The CSIA profiles showed that in the aquitard cis-dichloroethene (cDCE), first considered as a degradation product of trichloroethene (TCE), is produced by the dichloroelimination of 1,1,2,2-tetrachloroethane (TeCA). In contrast, TeCA degrades to TCE via dehydrohalogenation in the aquifer, indicating that the aquifer-aquitard interface separates two different degradation pathways for TeCA. Moreover, the CSIA profiles showed that chloroform (CF) is degraded to dichloromethane (DCM) via hydrogenolysis in the aquitard and, to a minor degree, produced by the degradation of carbon tetrachloride (CT). Several microorganisms capable of degrading chlorohydrocarbons were detected in the aquitard, suggesting that aquitard degradation is microbially mediated. Furthermore, numerical simulations reproduced the aquitard concentration and CSIA profiles well, which allowed the determination of degradation rates for each transformation pathway. This improves the prediction of contaminant fate in the aquitard and potential magnitude of impacts on the adjacent aquifer due to back-diffusion.

Biochar accelerates microbial reductive debromination of 2,2',4,4'-tetrabromodiphenyl ether (BDE-47) in anaerobic mangrove sediments

A common congener of polybrominated diphenyl ethers, 2,2',4,4'-tetrabromodiphenyl ether (BDE-47), is a prevalent, persistent and toxic pollutant. It could be removed by reduction debromination by microorganisms but the rate is often slow. The study hypothesized that spent mushroom substrate derived biochar amendment could accelerate the microbial reductive debromination of BDE-47 in anaerobic mangrove sediment slurries and evaluated the mechanisms behind. At the end of 20-week experiment, percentages of residual BDE-47 in slurries amended with biochar were significantly lower but debromination products were higher than those without biochar. Such stimulatory effect on debromination was dosage-dependent, and debromination was coupled with iron (Fe) reduction. Biochar amendment significantly enhanced the Fe(II):Fe(III) ratio, Fe(III) reduction rate and the abundance of iron-reducing bacteria in genus Geobacter, thus promoting bacterial iron-reducing process. The abundances of dehalogenating bacteria in genera Dehalobacter, Dehalococcoides, Dehalogenimonas and Desulfitobacterium were also stimulated by biochar. Biochar as an electron shuttle might increase electron transfer from iron-reducing and dehalogenating bacteria to PBDEs for their reductive debromination. More, biochar shifted microbial community composition in sediment, particularly the enrichment of potential PBDE-degrading bacteria including organohalide-respiring and sulfate-reducing bacteria, which in turn facilitated the reductive debromination of BDE-47 in anaerobic mangrove sediment slurries.

Sulfidation of Iron-Based Materials: A Review of Processes and Implications for Water Treatment and Remediation

Iron-based materials used in water treatment and groundwater remediation-especially micro- and nanosized zerovalent iron (nZVI)-can be more effective when modified with lower-valent forms of sulfur (i.e., "sulfidated"). Controlled sulfidation for this purpose (using sulfide, dithionite, etc.) is the main topic of this review, but insights are derived by comparison with related and comparatively well-characterized processes such as corrosion of iron in sulfidic waters and abiotic natural attenuation by iron sulfide minerals. Material characterization shows that varying sulfidation protocols (e.g., concerted or sequential) and key operational variables (e.g., S/Fe ratio and sulfidation duration) result in materials with structures and morphologies ranging from core-shell to multiphase. A meta-analysis of available kinetic data for dechlorination under anoxic conditions, shows that sulfidation usually increases dechlorination rates, and simultaneously hydrogen production is suppressed. Therefore, sulfidation can greatly improve the efficiency of utilization of reducing equivalents for contaminant removal. This benefit is most likely due to inhibited corrosion as a result of sulfidation. Sulfidation may also favor desirable pathways of contaminant removal, such as (i) dechlorination by reductive elimination rather than hydrogenolysis and (ii) sequestration of metals as sulfides that could be resistant to reoxidation. Under oxic conditions, sulfidation is shown to enhance heterogeneous catalytic oxidation of contaminants. These net effects of sulfidation on contaminant removal by iron-based materials may substantially improve their practical utility for water treatment and remediation of contaminated groundwater.

Transformation of carbon tetrachloride and chloroform by trichloroethene respiring anaerobic mixed cultures and supernatant

Carbon tetrachloride (CT) and chloroform (CF) were transformed in batch reactor experiments conducted with anaerobic dechlorinating cultures and supernatant (ADC + S) harvested from continuous flow reactors. The Evanite (EV) and Victoria/Stanford (VS) cultures, capable of respiring trichloroethene (TCE), 1,2-cis-dichloroethene (cDCE), and vinyl chloride (VC) to ethene (ETH), were grown in continuous flow reactors receiving an influent feed of saturated TCE (10 mM; 60 mEq) and formate (45 mM; 90 mEq) but no CT or CF. Cells and supernatant were harvested from the chemostats and inoculated into batch reactors at the onset of each experiment. CT transformation was complete following first order kinetics with CF, DCM and CS₂ as the measurable transformation products, representing 20-40% of the original mass of CT, with CO₂ likely the unknown transformation product. CF was transformed to DCM and likely CO₂ at an order of magnitude rate lower than CT, while DCM was not further transformed. An analytical first order model including multiple key reactions effectively simulated CT transformation, product formation and transformation, and provided reasonable estimates of transformation rate coefficients. Biotic and abiotic treatments indicated that CT was mainly transformed via abiotic processes. However, the presence of live cells was associated with the transformation of CF to DCM. In biotic tests both TCE and CT were simultaneously transformed, with TCE transformed to ETH and approximately 15-53% less CF formed via CT transformation. A 14-day exposure to CF (CF_max = 1.4 μM) reduced all rates of chlorinated ethene respiration by a factor of 10 or greater.

Kinetics of nitrobenzene degradation coupled to indigenous microorganism dissimilatory iron reduction stimulated by emulsified vegetable oil

Widespread contamination by nitrobenzene (NB) in sediments and groundwater requires better understanding of the biogeochemical removal process of the pollutant. NB degradation, coupled with dissimilatory iron reduction, is one of the most efficient pollutant removal methods. However, research on NB degradation coupled to indigenous microorganism dissimilatory iron reduction stimulated by electron donors is still experimental. A model for remediation in an actual polluted site does not currently exist. Therefore, in this study, the dynamics was derived from the Michaelis-Menten model (when the mass ratio of emulsified vegetable oil and NB reached the critical value 91:1). The effect of SO₄^2-, NO₃^-, Ca²⁺/Mg²⁺, and the grain size of aquifer media on the dynamics were studied, and the NB degradation dynamic model was then modified based on the most significant factors. Utilizing the model, the remediation time could be calculated in a contaminated site.

Addition of iron oxides in sediments enhances 2,3,4,5-tetrachlorobiphenyl (PCB 61) dechlorination by low-voltage electric fields

The presence of iron oxides in sediments significantly improves anaerobic dechlorination of PCB (i.e., PCB 61) in bioelectrochemical reactors.

Simultaneous anaerobic transformation of tetrachloroethene and carbon tetrachloride in a continuous flow column

Tetrachloroethene (PCE) and carbon tetrachloride (CT) were simultaneously transformed in a packed column that was bioaugmented with the Evanite culture (EV). The data presented here have been obtained over a period of 1930days. Initially the column was continuously fed synthetic groundwater with PCE (0.1mM), sulfate (SO4(2-)) (0.2mM) and formate (2.1mM) or lactate (1.1mM), but not CT. In these early stages of the study the effluent H2 concentrations ranged from 7 to 19nM, and PCE was transformed to ethene (ETH) (81 to 85%) and vinyl chloride (VC) (11 to 17%), and SO4(2-) was completely reduced when using either lactate or formate as electron donors. SO4(2-) reduction occurred concurrently with cis-DCE and VC dehalogenation. Formate was a more effective substrate for promoting dehalogenation based on electron donor utilization efficiency. Simultaneous PCE and CT tests found CT (0.015mM) was completely transformed with 20% observed as chloroform (CF) and trace amounts of chloromethane (CM) and dichloromethane (DCM), but no methane (CH4) or carbon disulfide (CS2). PCE transformation to ETH improved with CT addition in response to increases in H2 concentrations to 160nM that resulted from acetate formation being inhibited by either CT or CF. Lactate fermentation was negatively impacted after CT transformation tests, with propionate accumulating, and H2 concentrations being reduced to below 1nM. Under these conditions both SO4(2-) reduction and dehalogenation were negatively impacted, with sulfate reduction not occurring and PCE being transformed to cis-dichloroethene (c-DCE) (52%) and VC (41%). Upon switching to formate, H2 concentrations increased to 40nM, and complete SO4(2-) reduction was achieved, while PCE was transformed to ETH (98%) and VC (1%), with no acetate detected. Throughout the study PCE dehalogenation to ethene was positively correlated with the effluent H2 concentrations.

The effect of magnetite on the start-up and N2O emission reduction of the anammox process

A reactor combined with magnetite could enhance the anammox performance and enrich morePlanctomycetesbacteria.

The interactive biotic and abiotic processes of DDT transformation under dissimilatory iron-reducing conditions

The objective of the study was to elucidate the biotic and abiotic processes under dissimilatory iron reducing conditions involved in reductive dechlorination and iron reduction. DDT transformation was investigated in cultures of Shewanella putrefaciens 200 with/without α-FeOOH. A modified first-order kinetics model was developed and described DDT transformation well. Both the α-FeOOH reduction rate and the dechlorination rate of DDT were positively correlated to the biomass. Addition of α-FeOOH enhanced reductive dechlorination of DDT by favoring the cell survival and generating Fe(II) which was absorbed on the surface of bacteria and iron oxide. 92% of the absorbed Fe(II) was Na-acetate (1M) extractable. However, α-FeOOH also played a negative role of competing for electrons as reflected by the dechlorination rate of DDT was inhibited when increasing the α-FeOOH from 1 g L(-1) to 5 g L(-1). DDT was measured to be toxic to S. putrefaciens 200. The metabolites DDD, DDE and DDMU were recalcitrant to S. putrefaciens 200. The results suggested that iron oxide was not the key factor to promote the dissipation of DDX (DDT and the metabolites), whereas the one-electron reduction potential (E1) of certain organochlorines is the main factor and that the E1 higher than the threshold of the reductive driving forces of DIRB probably ensures the occur of reductive dechlorination.

Degradation of carbon tetrachloride in aqueous solution in the thermally activated persulfate system

Thermal activation of persulfate (PS) has been identified to be effective in the destruction of organic pollutants. The feasibility of carbon tetrachloride (CT) degradation in the thermally activated PS system was evaluated. The experimental results showed that CT could be readily degraded at 50 °C with a PS concentration of 0.5M, and CT degradation and PS consumption followed the pseudo-first order kinetic model. Superoxide radical anion (O2(*-)) was the predominant radical species responsible for CT degradation and the split of CCl was proposed as the possible reaction pathways for CT degradation. The process of CT degradation was accelerated by higher PS dose and lower initial CT concentration. No obvious effect of the initial pH on the degradation of CT was observed in the thermally activated PS system. Cl(*-), HCO3(*-), and humic acid (HA) had negative effects on CT degradation. In addition, the degradation of CT in the thermally activated PS system could be significantly promoted by the solvents addition to the solution. In conclusion, the thermally activated PS process is a promising option in in-situ chemical oxidation/reduction remediation for degrading highly oxidized organic contaminants such as CT that is widely detected in contaminated sites.

Anaerobic degradation of polychlorinated biphenyls (PCBs) and polychlorinated biphenyls ethers (PBDEs), and microbial community dynamics of electronic waste-contaminated soil

Environmental contamination caused by electronic waste (e-waste) recycling is attracting increasing attention worldwide because of the threats posed to ecosystems and human safety. In the present study, we investigated the feasibility of in situ bioremediation of e-waste-contaminated soils. We found that, in the presence of lactate as an electron donor, higher halogenated congeners were converted to lower congeners via anaerobic halorespiration using ferrous ions in contaminated soil. The 16S rRNA gene sequences of terminal restriction fragments indicated that the three dominant strains were closely related to known dissimilatory iron-reducing bacteria (DIRB) and those able to perform dehalogenation upon respiration. The functional species performed the activities of ferrous oxidation to ferric ions and further ferrous reduction for dehalogenation. The present study links iron cycling to degradation of halogenated materials in natural e-waste-contaminated soil, and highlights the synergistic roles of soil bacteria and ferrous/ferric ion cycling in the dehalogenation of polychlorinated biphenyls (PCBs) and polybrominated biphenyl ethers (PBDEs).

Enhancing the effect of bisulfite on sequestration of selenite by zerovalent iron

The enhancing effect of HSO₃⁻ on Se(iv) sequestration varied with the headspace volume, HSO₃⁻ concentration and initial pH, respectively.

Tolerance of anaerobic bacteria to chlorinated solvents

The aim of this research was to evaluate the effects of four chlorinated aliphatic hydrocarbons (CAHs), perchloroethene (PCE), carbon tetrachloride (CT), chloroform (CF) and 1,2-dichloroethane (1,2-DCA), on the growth of eight anaerobic bacteria: four fermentative species (Escherichia coli, Klebsiella sp., Clostridium sp. and Paenibacillus sp.) and four respiring species (Pseudomonas aeruginosa, Geobacter sulfurreducens, Shewanella oneidensis and Desulfovibrio vulgaris). Effective concentrations of solvents which inhibited growth rates by 50% (EC50) were determined. The octanol-water partition coefficient or log Po/w of a CAH proved a generally satisfactory measure of its toxicity. Most species tolerated approximately 3-fold and 10-fold higher concentrations of the two relatively more polar CAHs CF and 1,2-DCA, respectively, than the two relatively less polar compounds PCE and CT. EC50 values correlated well with growth rates observed in solvent-free cultures, with fast-growing organisms displaying higher tolerance levels. Overall, fermentative bacteria were more tolerant to CAHs than respiring species, with iron- and sulfate-reducing bacteria in particular appearing highly sensitive to CAHs. These data extend the current understanding of the impact of CAHs on a range of anaerobic bacteria, which will benefit the field of bioremediation.

Identifying indicators of reactivity for chemical reductants in sediments

To conduct site-specific exposure assessments for contaminants containing reducible functional groups, it is imperative to know the identity and reactivity of chemical reductants in natural sediments and to associate their reactivity with easily measurable sediment properties. For this purpose the reactivity, as defined by pseudofirst order reduction rate constants for p-cyanonitrobenzene (pCNB), was measured in twenty-one natural sediments of different origins that were incubated to attain both anoxic (less reducing) and anaerobic (microbially reducing) conditions. The reactivity of the anoxic sediments increased with pH and an increasing amount of Fe(II) added. A good electron balance between pCNB reduction and Fe(II) consumption was observed for anaerobic sediments of high solids loading (50 g/L), but not when solids loading was 5 g/L. Based on cluster and regression analysis, pCNB reactivity in the anaerobic sediments correlates strongly with aqueous Fe(II) concentrations for sediments with low organic carbon (OC) content (<4.2%), but with dissolved OC concentrations (DOC) for the sediments with high OC content (>6.4%). These observations indicate surface-associated Fe(II) and reduced DOC are the predominant reductants in the anaerobic sediments, and that aqueous Fe(II) and DOC will serve as readily measurable indicators of pCNB reactivity in these systems.

Transformation impacts of dissolved and solid phase Fe(II) on trichloroethylene (TCE) reduction in an iron-reducing bacteria (IRB) mixed column system: a mathematical model

In this research, we conducted trichloroethylene (TCE) reduction in a column filled with iron and iron-reducing bacteria (IRB) and developed a mathematical model to investigate the critical reactions between active species in iron/IRB/contaminant systems. The formation of ferrous iron (Fe(II)) in this system with IRB and zero-valent iron (ZVI, Fe(0)) coated with a ferric iron (Fe(III)) crust significantly affected TCE reduction and IRB respiration in various ways. This study presents a new framework for transformation property and reducing ability of both dissolved (Fe(II)(dissolved)) and solid form ferrous iron (Fe(II)(solid)). Results showed that TCE reduction was strongly depressed by Fe(II)(solid) rather than by other inhibitors (e.g., Fe(III) and lactate), suggesting that Fe(II)(solid) might reduce IRB activation due to attachment to IRB cells. Newly exposed Fe(0) from the released Fe(II)(dissolved) was a strong contributor to TCE reduction compared to Fe(II)(solid). In addition, our research confirmed that less Fe(II)(solid) production strongly supported long-term TCE reduction because it may create an easier TCE approach to Fe(0) or increase IRB growth. Our findings will aid the understanding of the contributions of iron media (e.g., Fe(II)(solid), Fe(II)(dissolved), Fe(III), and Fe(0)) to IRB for decontamination in natural groundwater systems.

Large carbon isotope fractionation during biodegradation of chloroform by Dehalobacter cultures

Compound specific isotope analysis (CSIA) has been applied to monitor bioremediation of groundwater contaminants and provide insight into mechanisms of transformation of chlorinated ethanes. To date there is little information on its applicability for chlorinated methanes. Moreover, published enrichment factors (ε) observed during the biotic and abiotic degradation of chlorinated alkanes, such as carbon tetrachloride (CT); 1,1,1-trichloroethane (1,1,1-TCA); and 1,1-dichloroethane (1,1-DCA), range from -26.5‰ to -1.8‰ and illustrate a system where similar C-Cl bonds are cleaved but significantly different isotope enrichment factors are observed. In the current study, biotic degradation of chloroform (CF) to dichloromethane (DCM) was carried out by the Dehalobacter containing culture DHB-CF/MEL also shown to degrade 1,1,1-TCA and 1,1-DCA. The carbon isotope enrichment factor (ε) measured during biodegradation of CF was -27.5‰ ± 0.9‰, consistent with the theoretical maximum kinetic isotope effect for C-Cl bond cleavage. Unlike 1,1,1-TCA and 1,1-DCA, reductive dechlorination of CF by the Dehalobacter-containing culture shows no evidence of suppression of the intrinsic maximum kinetic isotope effect. Such a large fractionation effect, comparable to those published for cis-1,2-dichloroethene (cDCE) and vinyl chloride (VC) suggests CSIA has significant potential to identify and monitor biodegradation of CF, as well as important implications for recent efforts to fingerprint natural versus anthropogenic sources of CF in soils and groundwater.

Bio-current as an indicator for biogenic Fe(II) generation driven by dissimilatory iron reducing bacteria

Microbial reduction of insoluble iron minerals by dissimilatory iron reducing bacteria (DIRB) is an important environment process in the iron biogeochemical cycle. We reported that the bio-current generated from oxidation of organic matter by these bacteria in the presence of iron oxides can be used as an indicator for microbial dissolution of insoluble iron oxides. Bioelectrochemical experiments were conducted to investigate the effects of the specific bacteria and the phase identity of iron oxides on bio-current generation by recording the current response as a result of a poised constant potential. Experimental results indicated that the bio-current generation can be greatly enhanced by iron oxide addition under all the conditions varying in the type of pure culture or iron oxide. The increase in the bio-current was linearly correlated with the increased concentration of biogenic Fe(II) detected either by chemical analysis or cyclic voltammetry (CV) tests. This can be understood based on the proposed mechanism that the Fe(II)/Fe(III) couple functions as the electron mediator shuttling electrons from the microbes to the electrodes.

Successful microcosm demonstration of a strategy for biodegradation of a mixture of carbon tetrachloride and perchloroethene harnessing sulfate reducing and dehalorespiring bacteria

Carbon tetrachloride (CT) is known to inhibit the transformation of perchloroethene (PCE) to ethene by dehalorespiring bacteria, creating a challenge for the bioremediation of environments contaminated with both compounds. We report on the sequential use of sulfate reduction and dehalorespiration as a microbial strategy for the transformation of a mixture of CT (10 μM) and PCE (14 μM). Sulfide production in Desulfovibrio vulgaris cultures led to complete CT disappearance in as little as 12 days. The addition of amorphous ferric oxide decreased the proportion of chloroform (CF) produced from 65% to 30%. CT conversion rates were enhanced more than 13-fold where vitamin B(12) (5 μM) was added. In vitamin B(12)-containing D. vulgaris cultures, no chlorinated products were detected and carbon disulfide was the major product of CT transformation. PCE concentrations were not impacted upon by D. vulgaris activity. The subsequent inoculation of a PCE-respiring enrichment culture resulted in microbial PCE dechlorination to ethene.

Synergistic effect of microscale zerovalent iron particles combined with anaerobic sludges on the degradation of decabromodiphenyl ether

Polybrominated diphenyl ethers (PBDEs) are widely used flame retardants. Owing to their toxicity and increasing accumulation in the environment, the fate of PBDEs in nature is of serious concern. The combined effects of microscale zerovalent iron (MZVI) and anaerobic sludge in decabromodiphenyl ether (BDE-209) degradation were investigated. The co-incubation resulted in 63% and 29% enhancement of removal ability when compared to the single component conditions. By-products generated during the entire process followed a stepwise sequence with non-uniform accumulation rates. Microbes hindered the accessibility of MZVI to BDE-209 and reduced the removal ability in the initial stage (<12 h). According to the analysis of the microbial community change, co-incubation with MZVI leads to the enrichment of heterotrophic microbial populations bearing nitrate- or iron-reducing activities. The interaction between MZVI and microbes contributed to the synergistic effect. Our findings provide evidence for this synergistic effect and offer an alternative for developing better remediation strategies.

Abiotic reductive dechlorination of cis-dichloroethylene by Fe species formed during iron- or sulfate-reduction

This study investigated reductive dechlorination of cis-dichloroethylene (cis-DCE) by the reduced Fe phases obtained from in situ precipitation, which involved mixing of Fe(II), Fe(III), and S(-II) solutions. A range of redox conditions were simulated by varying the ratio of initial Fe(II) concentration ([Fe(II)](o)) to initial Fe(III) concentration ([Fe(III)](o)) for iron-reducing conditions (IRC) and the ratio of [Fe(II)](o) to initial sulfide concentration ([S(-II)](o)) for sulfate-reducing conditions (SRC). Significant dechlorination of cis-DCE occurred under highly reducing IRC and iron-rich SRC, suggesting that Fe (oxyhydr)oxides including green rusts are highly reactive with cis-DCE but that Fe sulfide as mackinawite (FeS) is nonreactive. Relative concentrations of sulfate to chloride were also varied to examine the anion impact on cis-DCE dechlorination. Generally, slower dechlorination occurred in the batches with higher sulfate concentrations. As indicated by higher dissolved Fe concentration, the slower dechlorination in the presence of sulfate was probably due to the decreased surface-complexed Fe(II). This study demonstrates that the chemical form of reduced Fe(II) is critical in determining the fate of cis-DCE under anoxic conditions.

Effects of Zwitterionic buffers on sorption of ferrous iron at goethite and its oxidation by CCl4

A major factor which controls sorption and oxidation of Fe(II) at the mineral-water interface is pH, hence buffers are commonly used to control pH in experimental studies. Here, we examined the effects of widely used organic buffers (3-morpholinopropane-1-sulfonic acid (MOPS) and 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES)) on Fe(II) uptake and oxidation by CCl(4) in aqueous suspensions of goethite. Significant sorption of these zwitterionic buffers occurred only at Fe(II)-loaded goethite but not at native goethite. The addition of MOPS and HEPES caused substantial release of Fe(II) from goethite, retarded the oxidation of surface-bound Fe(II) by CCl(4) and changed the reaction pathway as indicated by lower yields of CHCl(3). To explore electrostatic and steric contributions of MOPS and HEPES to the observed phenomena we studied sorption and competitive effects of model sorbates (Ca(2+), sulfonates) which suggest the formation of a complex between surface-bound Fe(II) and MOPS or HEPES. Our study shows for the first time that these frequently used zwitterionic organic buffers may interfere significantly with the surface chemistry and thus with redox reactions of Fe(II) at goethite. Hence, kinetic or mechanistic information obtained in such systems requires careful interpretation.

Geobacter: the microbe electric's physiology, ecology, and practical applications

Geobacter species specialize in making electrical contacts with extracellular electron acceptors and other organisms. This permits Geobacter species to fill important niches in a diversity of anaerobic environments. Geobacter species appear to be the primary agents for coupling the oxidation of organic compounds to the reduction of insoluble Fe(III) and Mn(IV) oxides in many soils and sediments, a process of global biogeochemical significance. Some Geobacter species can anaerobically oxidize aromatic hydrocarbons and play an important role in aromatic hydrocarbon removal from contaminated aquifers. The ability of Geobacter species to reductively precipitate uranium and related contaminants has led to the development of bioremediation strategies for contaminated environments. Geobacter species produce higher current densities than any other known organism in microbial fuel cells and are common colonizers of electrodes harvesting electricity from organic wastes and aquatic sediments. Direct interspecies electron exchange between Geobacter species and syntrophic partners appears to be an important process in anaerobic wastewater digesters. Functional and comparative genomic studies have begun to reveal important aspects of Geobacter physiology and regulation, but much remains unexplored. Quantifying key gene transcripts and proteins of subsurface Geobacter communities has proven to be a powerful approach to diagnose the in situ physiological status of Geobacter species during groundwater bioremediation. The growth and activity of Geobacter species in the subsurface and their biogeochemical impact under different environmental conditions can be predicted with a systems biology approach in which genome-scale metabolic models are coupled with appropriate physical/chemical models. The proficiency of Geobacter species in transferring electrons to insoluble minerals, electrodes, and possibly other microorganisms can be attributed to their unique "microbial nanowires," pili that conduct electrons along their length with metallic-like conductivity. Surprisingly, the abundant c-type cytochromes of Geobacter species do not contribute to this long-range electron transport, but cytochromes are important for making the terminal electrical connections with Fe(III) oxides and electrodes and also function as capacitors, storing charge to permit continued respiration when extracellular electron acceptors are temporarily unavailable. The high conductivity of Geobacter pili and biofilms and the ability of biofilms to function as supercapacitors are novel properties that might contribute to the field of bioelectronics. The study of Geobacter species has revealed a remarkable number of microbial physiological properties that had not previously been described in any microorganism. Further investigation of these environmentally relevant and physiologically unique organisms is warranted.

Effects of oxidation on the magnetization of nanoparticulate magnetite

Synthetic nanomagnetite has been suggested as a potential reactant for the in situ treatment of contaminated groundwater. Although the application of magnetite nanoparticles for environmental remediation is promising, a full understanding of particle reactivity has been deterred by the propensity of the nanoparticles to aggregate and become colloidally unstable. Attractive magnetic interactions between particles are partially responsible for their aggregation. In this study, we characterized the magnetic behavior of magnetite by determining the saturation magnetization, coercivity, remanent magnetization, susceptibility, and blocking temperature of synthetic magnetite using a superconducting quantum interference device (SQUID). We show how these properties vary in the presence of surface-associated solutes such as tetramethylammonium (TMA(+)) and ferrous (Fe(II)) cations. More importantly, because magnetite readily reacts with O(2) to produce maghemite, we analyzed the effect of oxidation on the magnetic properties of the particles. Because maghemite has a reported magnetic saturation that is less than that of magnetite, we hypothesized that oxidation would decrease the magnitude of the magnetic attractive force between adjacent particles. The presence of TMA(+) and Fe(II) caused a change in the magnetic properties of magnetite potentially because of alterations in its crystalline order. Magnetite oxidation caused a decrease in saturation magnetization, resulting in less significant magnetic interactions between particles. Oxidation, therefore, could lead to the decreased aggregation of magnetite nanoparticles and a potential enhancement of their colloidal stability.

Comparison of lactate, formate, and propionate as hydrogen donors for the reductive dehalogenation of trichloroethene in a continuous-flow column

A continuous-flow column study was conducted to analyze the reductive dehalogenation of trichloroethene (TCE) with aquifer material with high content of iron oxides. The column was bioaugmented with the Point Mugu (PM) culture, which is a mixed microbial enrichment culture capable of completely transforming TCE to ethene (ETH). We determined whether lactate, formate, or propionate fermentation resulted in more effective dehalogenation. Reductive dehalogenation, fermentation, and sulfate, Fe(III), and Mn(IV) reduction were all exhibited within the column. Different steady-states of dehalogenation were achieved based on the concentration of substrates added, with effective transformation to ETH obtained when ample electron donor equivalents were provided. Most of the metabolic reducing equivalents were channeled to sulfate, Fe(III), and Mn(IV) reduction. When similar electron reducing equivalents were added, the most effective dehalogenation was achieved with formate, with 14% of the electron equivalents going towards dehalogenation reactions, compared to 6.5% for lactate and 9.6% for propionate. Effective dehalogenation was maintained over 1000 days of column operation. Over 90% of electron equivalents added could be accounted for by the different electron accepting processes in the column, with 50% associated with soluble and precipitated Fe(II) and Mn(II). Bulk Fe(III) and Mn(IV) reduction was rather associated with lactate and propionate addition than formate addition. Sulfate reduction was a competing electron acceptor reaction with all three electron donors. DNA was extracted from solid coupon samples obtained during the course of the experiment and analyzed using 16S rRNA gene clone libraries and quantitative PCR. Lactate and propionate addition resulted in a significant increase in Geobacter, Spirochaetes, and Desulfitobacterium phylotypes relative to "Dehalococcoides" when compared to formate addition. Results from the molecular biological analyses support chemical observations that a greater percentage of the electron donor addition was channeled to Fe(III) reduction when lactate and propionate were added compared to formate, and formate was more effective than lactate in supporting dehalogenation. The results demonstrate the importance of electron donor selection and competing electron acceptor reactions when implementing reductive dehalogenation remediation technologies.

Redox behavior of magnetite: implications for contaminant reduction

The factors controlling rates of contaminant reduction by magnetite (Fe3O4) are poorly understood. Here, we measured the reduction rates of three ArNO2 compounds by magnetite particles ranging from highly oxidized (x = Fe2+/Fe3+ = 0.31) to fully stoichiometric (x = 0.50). Rates of ArNO2 reduction became almost 5 orders of magnitude faster as the particle stoichiometry increased from x = 0.31 to 0.50. To evaluate what was controlling the rate of ArNO2 reduction, we measured apparent 15N kinetic isotope effects ((15)N-AKIE) values for nitrobenzene and magnetite open-circuit potentials (E(OCP)). 15N-AKIE values were greater than unity for all magnetite stoichiometries investigated, indicating that mass transfer processes are not controlling the rate of ArNO2 reduction by magnetite. E(OCP) measurements showed that the E(OCP) for magnetite was linearly related to the stoichiometry, with more stoichiometric magnetite having a lower potential. Based on these results, we propose that conceptual models that incorporate both redox and Fe2+ diffusion processes, rather than those that rely solely on diffusion of Fe2+, are more appropriate for understanding contaminant reduction by magnetite. Our work indicates that particle stoichiometry should be considered when evaluating rates of contaminant reduction by magnetite.

Reductive dechlorination of carbon tetrachloride in acidic soil manipulated with iron(II) and bisulfide ion

Batch and column tests were conducted to investigate the effect of reductant concentration, reductant contact time, and suspension pH on reductive dechlorination of carbon tetrachloride (CT) by soil manipulated with Fe(II) and HS(-). Kinetic rate constants for the reductive dechlorination increased as the reductant concentrations increased. Fe(II) was more effective reductant than HS(-), resulting in higher rate constants. The contact time of 1 day for the soil with HS(-) and that of 4h with Fe(II) showed the highest reaction rates, respectively. The kinetic rate constants increased as the pH of soil suspensions with Fe(II) (5.2-8.0) and HS(-) (8.3-10.3) increased. Soil column with Fe(II) showed larger bed volumes (13.8) to reach a column breakthrough than that with HS(-) (4.0). Fe(II) treatment showed better removal of CT in the soil column with the addition of CaO than HS(-) treatment did. In contrast, HS(-) treatment not producing toxic products could be considered as an environmentally favorable reductant.

Transformation of carbon tetrachloride by biogenic iron species in the presence of Geobacter sulfurreducens and electron shuttles

The transformation of carbon tetrachloride (CT) by biogenic iron species produced from the bioreduction of various Fe(III) oxides in the presence of Geobacter sulfurreducens and electron shuttles were investigated. Cysteine and anthraquinone-2,6-disulfonate (AQDS) at concentrations of 0.5mM and 10microM, respectively, were added as the electron shuttles. Addition of electron shuttles enhanced the extent of reduction and rate of ferric oxide reduction. The bioreduction extents of ferric oxides by G. sulfurreducens in the presence of electron shuttles were 22.8-48.3% for ferrihydrite, 6.5-17.2% for hematite, and 3.0-11.3% for goethite. After normalization to the surface areas, a higher rate of CT reduction was observed per unit of adsorbed Fe(II) on crystalline oxides. The produced biogenic Fe(II) from crystalline iron oxides was 2.8-7.6 times lower than that obtained from ferrihydrite, while the surface area-normalized rate constant for iron-mediated CT transformation in the presence of goethite and hematite were, by factors of 2-21, higher than that obtained using ferrihydrite. These results clearly depict that G. sulfurreducens drove the reduction of CT primarily through the formation of biogenic iron species in the presence of electron shuttle under iron-reducing conditions and that it is a surface area dependent process.

Influence of magnetite stoichiometry on Fe(II) uptake and nitrobenzene reduction

Magnetite (Fe3O4) is a common biomineralization product of microbial iron respiration and is often found in subsurface anoxic environments, such as groundwater aquifers where aqueous Fe(II) is present We investigated the reaction between aqueous Fe(II) and magnetite using the isotopic selectivity of 57Fe Mössbauer spectroscopy and revisited the reduction of nitrobenzene by magnetite. Similar to our previous findings with Fe3+ oxides, we did not observe the formation of a stable sorbed Fe(II) species; instead, we observed oxidation of the Fe(II) to a partially oxidized magnetite phase. Oxidation of Fe(II) was accompanied by reduction of the octahedral Fe3+ atoms in the underlying magnetite to octahedral Fe2+ atoms. The lack of a stable, sorbed Fe(II) species on magnetite prompted us to reevaluate what is controlling the extent of Fe(II) uptake on magnetite, as well as contaminant reduction in the presence of magnetite and Fe(II). Uptake of Fe(II) by magnetite appears to be limited by the stoichiometry of the magnetite particles, rather than the surface area of the particles. More oxidized (or less stoichiometric) magnetite particles take up more Fe(II), with the formation of stoichiometric magnetite (Fe2+/Fe3+ = 0.5) limiting the extent of Fe(II) uptake. We also showthat stoichiometric magnetite, in the absence of aqueous Fe(II), can rapidly reduce nitrobenzene. Based on these results, we speculate that contaminant reduction that was previously attributed to Fe(II) sorbed on magnetite is due to a process similar to negative (n) doping of a solid, which increases the stoichiometry of the magnetite and alters the bulk redox properties of the particle to make reduction more favorable.

Carbon and hydrogen isotope fractionation during anaerobic toluene oxidation by Geobacter metallireducens with different Fe(III) phases as terminal electron acceptors

Microbial oxidation of BTEX compounds under iron-reducing conditions is an important attenuation process for fuel-contaminated sites. We evaluated the use of compound-specific isotope analysis for the identification and quantification of anaerobic toluene oxidation by Geobacter metallireducens. 13C and 2H enrichment of toluene was measured in laboratory batch systems and varied significantly for a solid vs a dissolved Fe(III) phase provided as terminal electron acceptor. 13C enrichment factors (epsilonC) in suspensions of a solid Fe(III) phase were between -1.0 and -1.3% per hundred, whereas epsilonC-values were significantly higher in solutions of Fe(lll) citrate (-2.9 to -3.6% per hundred. The same trend was observed for 2H fractionation. Solid phase reduction resulted in an epsilonH-value of -34.6 +/- 0.9% per hundred, compared to -98.4 +/- 3.0% per hundred for the reduction of dissolved Fe(lll). The linear correlation of delta(2)H vs. delta(13)C during toluene oxidation resulted in nearly identical slopes for both systems, confirming that the reaction mechanism, that is enzymatic methyl-group oxidation, was the same. We hypothesize that smaller 2H and 13C fractionation in suspensions is due to toluene transport limitations to cells of G. metallireducens at surfaces of solid Fe(III) phases. Enrichment factors determined in Fe(III) mineral suspensions should be more representative for anaerobic toluene degradation owing to the abundance of solid Fe(III) in soils and aquifers.

Effect of biogenic iron species and copper ions on the reduction of carbon tetrachloride under iron-reducing conditions

In this study, the cell-mediated and abiotic reduction of carbon tetrachloride (CCl(4)) by biogenic iron species produced from the reductive dissolution of ferrihydrite in the presence of Geobacter sulfurreducens and copper ions (Cu(II)) were investigated. 9,10-Anthraquinone-2,6-disulfonate (AQDS), serving as a surrogate of natural organic matters and an electron shuttling compound, was added to enhance the efficiency of biological reduction of the solid Fe(III) minerals. G. sulfurreducens drove the reduction of CCl(4), primarily through the formation of biogenic surface-bound iron species produced from the reductive dissolution of ferrihydrite, in the presence of 10microM AQDS. The pseudo-first-order rate constant (k(obsCT)) for CCl(4) transformation in the presence of ferrihydrite was 3.0 times higher than that resulting from the use of G. sulfurreducens alone. Addition of 0.5mM Cu(II) slightly inhibited both the growth of G. sulfurreducens and the production of biogenic Fe(II). However, the k(obsCT) values for CCl(4) transformation in ferrihydrite suspensions containing G. sulfurreducens and 0.3-0.5mM Cu(II) were 2.1-4.2 times higher than that observed in the absence of Cu(II). X-Ray powder diffraction analysis indicated that the added Cu(II) reacted with the biogenic Fe(II) ions to produce catalytic cuprous ions (Cu(I)) and secondary iron oxide minerals such as magnetite and goethite, resulting in accelerating the chemical transformation efficiency and rate of CCl(4) under iron-reducing conditions.

Iron-mediated microbial oxidation and abiotic reduction of organic contaminants under anoxic conditions

In anoxic environments, the oxidation of organic compounds, such as BTEX fuel components, by dissimilatory Fe(III) reduction can generate reactive mineral-bound Fe(II) species, which in turn are able to reduce other classes of organic and inorganic groundwater contaminants. In this study, we designed and evaluated an anaerobic batch reactor that mimicks iron-reducing conditions to investigate the factors that favor the coupling of microbial toluene oxidation and abiotic reduction of nitroaromatic contaminants. We investigated the influence of different Fe(III)-bearing minerals and combinations thereof on the coupling of these two processes. Results from laboratory model systems show that complete oxidation of toluene to CO2 by Geobacter metallireducens in the presence of Fe(III)-bearing minerals leads to the formation of mineral-bound Fe(II) species capable of the reduction of 4-nitroacetophenone. Whereas significant microbial toluene oxidation was only observed in the presence of amorphous Fe(III) phases, reduction of nitroaromatic compounds only proceeded with Fe(II) species bound to crystalline Fe(III) oxides. Our results suggest that in anoxic soils and sediments containing amorphous and crystalline iron phases simultaneously, coupling of microbial oxidation and abiotic reduction of organic compounds may allow for concurrent natural attenuation of different contaminant classes.

Assessing iron-mediated oxidation of toluene and reduction of nitroaromatic contaminants in anoxic environments using compound-specific isotope analysis

We evaluated compound-specific isotope analysis (CSIA) as a tool to assess the coupling of microbial toluene oxidation by Fe(III)-reducing bacteria and abiotic reduction of nitroaromatic contaminants by biogenic mineral-bound Fe(II) species. Examination of the two processes in isolated systems revealed a reproducible carbon isotope fractionation for toluene oxidation by Geobacter metal-lireducens with a solid Fe(III) phase as terminal electron acceptor. We found a carbon isotope enrichment factor, epsilonC, of -1.0 +/- 0.1 per thousand, which corresponds to an apparent kinetic isotope effect (AKIE(C)) of 1.0073 +/- 0.0009 for the oxidative cleavage of a C-H bond. Nitrogen isotope fractionation of the reduction of nitroaromatic compounds (NAC) by mineral-bound Fe(ll) species yielded a nitrogen isotope enrichment factor, epsilonN, of -39.7 +/- 3.4 per thousand for the reduction of an aromatic NO2-group (AKIE(N) = 1.0413 +/- 0.0037) that was constant for variable experimental conditions. Finally, AKIE values for C and N observed in coupled experiments, where reactive Fe(II) was generated through microbial activity, were identical to those obtained in the isolated experiments. This study provides new evidence on isotope fractionation behavior during contaminant transformation and promotes the use of CSIA for the elucidation of complex contaminant transformation pathways in the environment.

Particle size and aggregation effects on magnetite reactivity toward carbon tetrachloride

Nanoparticulate magnetite is found in many natural and engineered environments. This study characterized the reactivity of this material toward carbon tetrachloride (CCl4). Particle diameter plays an important role, with nominal (9 nm) magnetite suspensions exhibiting greater reactivity on both mass (k(m)) and surface area normalized (k(SA)) bases than (80 nm) magnetite suspensions. For the (9 nm) suspension, the aggregation state of the particles affects the measured km values. At 0.001 M ionic strength and pH 7, k(m) (=0.052-0.139 L g(-1) h(-1)) was as much as seven times larger than at 1 M (k(m) = 0.025-0.030 L g(-1) h(-1)). This decrease in reactivity with an increase in ionic strength is related to the measured diameter of the aggregates present in solution, thus implicating aggregate size as an important variable. This work is the first to indicate that both particle size and aggregation state must be considered when evaluating the reactivity of nanoparticle suspensions with groundwater contaminants.