Thermodynamic controls on the microbial reduction of iron-bearing nontronite and uranium

Microbial removal of uranyl from aqueous solution by Leifsonia sp. in the presence of different forms of iron oxides

Immobilization of uranyl by indigenous microorganisms has been proposed as an economic and clean in-situ approach for removal of uranium, but the potential mechanisms of the process and the stability of precipitated uranium in the presence of widespread Fe(III) (hydr)oxides remain elusive. The potential of iron to serve as a reductant and/or an oxidant of uranium indicates that bioemediation strategies which mainly rely on the reduction of highly soluble U(VI) to poorly soluble U(IV) minerals to retard uranium transport in groundwater may be enhanced or hindered under different environmental conditions. This study purposes to determine the effect of ubiquitous Fe(III) (hydr)oxides (two-line ferrihydrite, hematite and goethite) on the removal of U(VI) by Leifsonia sp. isolated from an acidic tailings pond in China. The removal mechanism was elucidated via SEM-EDS, XPS and Mössbauer. The results show that the removal of U(VI) was retarded by Fe(III) (hydr)oxides when the initial concentration of U(VI) was 10 mg/L, pH was 6, temperature was 25 °C. Particularly, the retardatory effect of hematite on U(VI) removal was blindingly obvious. Also, it is worth noting that the U(VI) in the precipitate slow-released in the Fe(III) (hydrodr) oxide treatment groups, accompanied by an increase in Fe(II) concentration. SEM-EDS results demonstrated that the ferrihydrite converted to goethite may be the reason for U(VI) release in the process of 15 days culture. Mössbauer spectra fitting results further imply that the metastable iron oxides were transformed into stable Fe3O4 state. XPS measurements results showed that uranium product is most likely a mixture of Iron-U(IV) and Iron-U(VI), which indicated that the hexavalent uranium was converted into tetravalent uranium. These observations imply that the stability of the uranium in groundwater may be impacted on the prevailing environmental conditions, especially the solid-phase Fe(III) (hydr)oxide in groundwater or sediment.

Oxidation of Biogenic U(IV) in the Presence of Bioreduced Clay Minerals and Organic Ligands

Bioreduction of soluble U(VI) to sparingly soluble U(IV) is proposed as an effective approach to remediating uranium contamination. However, the stability of biogenic U(IV) in natural environments remains unclear. We conducted U(IV) reoxidation experiments following U(VI) bioreduction in the presence of ubiquitous clay minerals and organic ligands. Bioreduced Fe-rich nontronite (rNAu-2) and Fe-poor montmorillonite (rSWy-2) enhanced U(IV) oxidation through shuttling electrons between oxygen and U(IV). Ethylenediaminetetraacetic acid (EDTA), citrate, and siderophore desferrioxamine B (DFOB) promoted U(IV) oxidation via complexation with U(IV). In the presence of both rNAu-2 and EDTA, the rate of U(IV) oxidation was between those in the presence of rNAu-2 and EDTA, due to a clay/ligand-induced change of U(IV) speciation. However, the rate of U(IV) oxidation in other combinations of reduced clay and ligands was higher than their individual ones because both promoted U(IV) oxidation. Unexpectedly, the copresence of rNAu-2/rSWy-2 and DFOB inhibited U(IV) oxidation, possibly due to (1) blockage of the electron transport pathway by DFOB, (2) inability of DFOB-complexed Fe(III) to oxidize U(IV), and (3) stability of the U(IV)-DFOB complex in the clay interlayers. These findings provide novel insights into the stability of U(IV) in the environment and have important implications for the remediation of uranium contamination.

Thermodynamic considerations on the combined effect of electron shuttles and iron(III)-bearing clay mineral on Cr(VI) reduction by Shewanella oneidensis MR-1

Electron shuttles (ESs) and Fe-bearing clay minerals are commonly found in subsurface environments and have shown potential in enhancing the bioreduction of Cr(VI). However, the synergistic effect of ESs at different redox potentials and Fe-bearing clay minerals on Cr(VI) bioreduction, as well as the fundamental principles governing this process, remain unclear. In our study, we investigated the role of ESs and Fe(III) in Cr(VI) bioreduction. We found that the acceleration of ESs and Fe(III) are crucial factors in this process. Interestingly, the promotion of ESs on Cr(VI) and Fe(III) showed opposite trends. Electrochemical methods confirmed the limited steps are the extent of reduced ESs and the redox potential difference between ESs and Fe(III), separately. Furthermore, we investigated the combined effect of ESs and NAu-2 on Cr(VI) bioreduction. Our results revealed two segments: in the first segment, the ES (5-HNQ) and NAu-2 did not synergistically enhance Cr(VI) reduction. However, in the second segment, ESs and NAu-2 demonstrated a synergistic effect, significantly increasing Cr(VI) reduction by MR-1. These bioreduction processes all follow linear free energy relationships (LFERs). Overall, our study highlights the fundamental principles governing multivariate systems and presents a promising approach for the remediation of Cr(VI)-contaminated sites.

Disentangling the size-dependent redox reactivity of iron oxides using thermodynamic relationships

Nanoparticles often exhibit size-dependent redox reactivities, with smaller particles being more reactive in some cases, while less reactive in others. Predicting trends between redox reaction rates and particle sizes is often complicated because a particle's dimensions can simultaneously influence its aggregation state, reactive surface area, and thermodynamic properties. Here, we tested the hypothesis that interfacial redox reaction rates for nanoparticles with different sizes can be described with a single linear free-energy relationship (LFER) if size-dependent reactive surface areas and thermodynamic properties are properly considered. We tested this hypothesis using a well-known interfacial redox reaction: the reduction of nitrobenzene to aniline by iron-oxide-bound Fe²⁺. We measured the reduction potential (E_H) values of nano-particulate hematite suspensions containing aqueous Fe²⁺ using mediated potentiometry and characterized the size and aggregation states of hematite samples at circumneutral pH values. We used the measured E_H values to calculate surface energies and reactive surface areas using thermodynamic relationships. Nitrobenzene reduction rates were lower for smaller particles, despite their larger surface areas, due to their higher surface energies. When differences in surface areas and thermodynamic properties were considered, nitrobenzene reduction kinetics for all particle sizes was described with a LFER. Our results demonstrate that when Fe²⁺ serves as a reductant, an antagonistic effect exists, with smaller particles having larger reactive surface areas but also more positive reduction potentials. When Fe³⁺ serves as an oxidant, however, these two effects work in concert, which likely explains past discrepancies regarding how iron oxide particle sizes influence redox reaction rates.

Effect of Siderophore DFOB on U(VI) Adsorption to Clay Mineral and Its Subsequent Reduction by an Iron-Reducing Bacterium

Uranium mining and nuclear fuel production have led to significant U contamination. Past studies have focused on the bioreduction of soluble U(VI) to insoluble U(IV) as a remediation method. However, U(IV) is susceptible to reoxidation and remobilization when conditions change. Here, we demonstrate that a combination of adsorption and bioreduction of U(VI) in the presence of an organic ligand (siderophore desferrioxamine B, DFOB) and the Fe-rich clay mineral nontronite partially alleviated this problem. DFOB greatly facilitated U(VI) adsorption into the interlayer of nontronite as a stable U(VI)-DFOB complex. This complex was likely reduced by bioreduction intermediates such as the Fe(II)-DFOB complex and/or through electron transfer within a ternary Fe(II)-DFOB-U(VI) complex. Bioreduction with DFOB alone resulted in a mobile aqueous U(IV)-DFOB complex, but in the presence of both DFOB and nontronite U(IV) was sequestered into a solid. These results provide novel insights into the mechanisms of U(VI) bioreduction and the stability of U and have important implications for understanding U biogeochemistry in the environment and for developing a sustainable U remediation approach.

Synthesis of biochar@α-Fe2O3@Shewanella loihica complex for remediation of soil contaminated by hexavalent chromium: Optimization of conditions and mechanism

The reduction of hexavalent chromium combined with the process of dissimilatory iron reduction is an important strategy for microbial remediation of chromium-contaminated soil. However, its applicability is limited by the slow speed of bacterial bioreduction and the toxic effect of heavy metals on bacteria. Here, biochar (BC) was used as a substrate and was loaded with iron oxide in the form of hematite and Shewanella loihica to synthesize a BC@α-Fe₂O₃@S. loihica complex and thus achieve combined microbial-chemical remediation. After optimization by a Box-Behnken design, the optimal dosages of the complex, humic acid (as an electron shuttle), and sodium lactate (as an electron donor) were found to be 1.38 mL/g, 33.94 mg/g, and 12.95%, respectively. The Cr(VI) reduction rate in soil contaminated with 1000 mg/kg Cr(VI) reached 98.26%, and remediation could be achieved within 7 days. Characterization of the BC@α-Fe₂O₃@S. loihica complex before and after it was used for remediation by energy-dispersive X-ray spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and Fourier transform infrared spectroscopy proved that the oxygen-containing functional groups and aromatic compounds on the surface of the BC participated in the adsorption and reduction of Cr(VI) and that the loaded hematite particles were fully utilized by microorganisms. Therefore, the BC@α-Fe₂O₃@S. loihica complex has great potential for the remediation of Cr(VI)-contaminated soil.

Sustainable enhancement of Cr(VI) bioreduction by the isolated Cr(VI)-resistant bacteria

Bioreduction of mobile Cr(VI) to sparingly soluble Cr(III) is an effective strategy for in situ remediations of Cr contaminated sites. The key of this technology is to screen Cr(VI)-resistant bacteria and further explore the sustainable enhancement approaches towards their Cr(VI) reduction performance. In this study, a total of ten Cr(VI)-resistant bacteria were isolated from a Cr(VI) contaminated site. All of them could reduce Cr(VI), and the greatest extent of Cr(VI) reduction (98%) was obtained by the isolated CRB6 strain. The isolated CRB6 was able to reduce structural Fe(III) in Nontronite NAu-2 to structural Fe(II). Compared with the slow bioreduction process, the produced structural Fe(II) can rapidly enhance Cr(VI) reduction. The resist dissolution characteristics of NAu-2 in the redox cycling may provide sustainable enhancement of Cr(VI) reduction. However, no enhancement on Cr(VI) bioreduction by the isolated CRB6 was observed in the presence of NAu-2, which was attributed to the inhibition of Cr(VI) on the electron transfer between the isolated CRB6 and NAu-2. AQDS can accelerate the electron transfer between the isolated CRB6 and NAu-2 as an electron shuttle in the presence of Cr(VI). Therefore, the combination of NAu-2 and AQDS generated a synergistic enhancement on Cr(VI) bioreduction compared with the enhancement obtained by NAu-2 and AQDS individually. Our results highlight that structural Fe(III) and electron shuttle can provide a sustainable enhancement of Cr(VI) reduction by Cr(VI)-reducing bacteria, which has great potential for the effective Cr(VI) in-situ remediation.

Combined Effects of Fe(III)-Bearing Nontronite and Organic Ligands on Biogenic U(IV) Oxidation

Bioreduction of soluble U(VI) to sparingly soluble U(IV) solids was proposed as a remediation method for uranium contamination. Therefore, the stability and longevity of biogenic U(IV) are critical to the success of uranium remediation. However, co-occurrence of clay minerals and organic ligands could potentially reoxidize U(IV) to U(VI). Herein, we report a combined effect of Fe(III)-rich nontronite (NAu-2) and environmentally prevalent organic ligands on reoxidation of biogenic U(IV) at circumneutral pH. After 30 days of incubation, structural Fe(III) in NAu-2 oxidized 45.50% U(IV) with an initial rate of 2.7 × 10^-3 mol m^-2 d^-1. Addition of citrate and ethylenediaminetetraacetic acid (EDTA) greatly promoted the oxidative dissolution of U(IV) by structural Fe(III) in NAu-2, primarily through the formation of aqueous ligand-U(IV) complexes. In contrast, a model siderophore, desferrioxamine B (DFOB), partially inhibited U(IV) oxidation due to the formation of stable DFOB-Fe³⁺ complexes. The resulting U(VI) species intercalated into an NAu-2 interlayer or adsorbed onto an NAu-2 surface. Our results highlight the importance of organic ligands in oxidative dissolution of U(IV) minerals by Fe(III)-bearing clay minerals and have important implications for the design of nuclear waste storage and remediation strategies, especially in clay- and organic-rich environments.

Anaerobic oxidation of arsenite by bioreduced nontronite

The redox state of arsenic controls its toxicity and mobility in the subsurface environment. Understanding the redox reactions of arsenic is particularly important for addressing its environmental behavior. Clay minerals are commonly found in soils and sediments, which are an important host for arsenic. However, limited information is known about the redox reactions between arsenic and structural Fe in clay minerals. In this study, the redox reactions between As(III)/As(V) and structural Fe in nontronite NAu-2 were investigated in anaerobic batch experiments. No oxidation of As(III) was observed by the native Fe(III)-NAu-2. Interestingly, anaerobic oxidation of As(III) to As(V) occurred after Fe(III)-NAu-2 was bioreduced. Furthermore, anaerobic oxidization of As(III) by bioreduced NAu-2 was significantly promoted by increasing Fe(III)-NAu-2 reduction extent and initial As(III) concentrations. Bioreduction of Fe(III)-NAu-2 generated reactive Fe(III)-O-Fe(II) moieties at clay mineral edge sites. Anaerobic oxidation of As(III) was attributed to the strong oxidation activity of the structural Fe(III) within the Fe(III)-O-Fe(II) moieties. Our results provide a potential explanation for the presence of As(V) in the anaerobic subsurface environment. Our findings also highlight that clay minerals can play an important role in controlling the redox state of arsenic in the natural environment.

Combined Effects of Fe(III)-Bearing Clay Minerals and Organic Ligands on U(VI) Bioreduction and U(IV) Speciation

Reduction of U(VI) to U(IV) drastically reduces its solubility and has been proposed as a method for remediation of uranium contamination. However, much is still unknown about the kinetics, mechanisms, and products of U(VI) bioreduction in complex systems. In this study, U(VI) bioreduction experiments were conducted with Shewanella putrefaciens strain CN32 in the presence of clay minerals and two organic ligands: citrate and EDTA. In reactors with U and Fe(III)-clay minerals, the rate of U(VI) bioreduction was enhanced due to the presence of ligands, likely because soluble Fe³⁺- and Fe²⁺-ligand complexes served as electron shuttles. In the presence of citrate, bioreduced U(IV) formed a soluble U(IV)-citrate complex in experiments with either Fe-rich or Fe-poor clay mineral. In the presence of EDTA, U(IV) occurred as a soluble U(IV)-EDTA complex in Fe-poor montmorillonite experiments. However, U(IV) remained associated with the solid phase in Fe-rich nontronite experiments through the formation of a ternary U(IV)-EDTA-surface complex, as suggested by the EXAFS analysis. Our study indicates that organic ligands and Fe(III)-bearing clays can significantly affect the microbial reduction of U(VI) and the stability of the resulting U(IV) phase.

Microbial mobilization of arsenic from iron-bearing clay mineral through iron, arsenate, and simultaneous iron-arsenate reduction pathways

Clay minerals are an important host for arsenic in many arsenic-affected areas. The role of bioreduction of structural Fe(III) in clay minerals in the mobilization of arsenic from clay minerals, however, still remains unclear. In this study, Fe(III) reducing bacterium, As(V) reducing bacterium, and Fe(III)-As(V) reducing bacterium were employed to investigate the possible bioreduction pathways for arsenic release from Nontronite NAu-2. Results demonstrated that microbial reduction controlled arsenic mobilization from NAu-2 through Fe(III), As(V), and simultaneous Fe(III)-As(V) reduction pathways. Although the bioreduction of structural Fe(III) led to a negligible dissolution of NAu-2, it triggered a significant release of arsenic from NAu-2. The bioreduction of tetrahedral Fe(III) initiated the release of As(V), and the further bioreduction of octahedral Fe(III) induced the release of As(III) in NAu-2. In addition, bioreduction of As(V) resulted in the desorption and transformation of As(V) from NAu-2. Simultaneous bioreduction of Fe(III) and As(V) led to an almost complete release of As(V) from NAu-2. These findings suggest that simultaneous Fe(III)-As(V) reduction was the dominant pathway governing As(V) release from NAu-2, while structural Fe(III) reduction controlled As(III) release from NAu-2. Therefore, the bioreduction of iron-bearing clay minerals has a great potential for arsenic mobilization in the subsurface environment.

Iron(III) minerals and anthraquinone-2,6-disulfonate (AQDS) synergistically enhance bioreduction of hexavalent chromium by Shewanella oneidensis MR-1

Bioreduction of hexavalent chromium (Cr(VI)) to sparingly soluble trivalent chromium (Cr(III)) is a strategy for the remediation of Cr(VI) contaminated sites. However, its application is limited due to the slow bioreduction process. Here we explored the potential synergistic enhancement of iron(III) minerals (nontronite NAu-2, ferrihydrite, and goethite) and electron shuttle anthraquinone-2,6-disulfonate (AQDS) on the bioreduction of Cr(VI) by Shewanella oneidensis MR-1. AQDS alone increased the bioreduction rate of Cr(VI) by accelerating electron transfer from MR-1 to Cr(VI). Iron minerals alone did not increase the bioreduction rate of Cr(VI), where the electron transfer from MR-1 to Fe(III) minerals was inhibited due to the toxicity of Cr(VI) to MR-1. AQDS plus NAu-2 or ferrihydrite significantly enhanced the bioreduction rate of Cr(VI) as compared to AQDS or NAu-2/ferrihydrite alone, demonstrating that AQDS plus NAu-2/ferrihydrite had the synergistic effect on bioreduction of Cr(VI). Synergy factor (k_{cells+Fe+AQDS}/(k_cells+Fe + k_cells+AQDS)) was used to quantify the synergistic effect of AQDS and iron minerals on the bioreduction of Cr(VI). The synergy factors of AQDS plus NAu-2 were 2.09-4.63 (three Cr(VI) spikes), and the synergy factors of AQDS plus ferrihydrite were 1.89-4.61 (two Cr(VI) spikes). In the presence of Cr(VI), AQDS served as the electron shuttle between MR-1 and iron minerals, facilitating the reduction of Fe(III) minerals to Fe(II). The synergistic enhancement of AQDS and NAu-2/ferrihydrite was attributed to the generated Fe(II), which could quickly reduce Cr(VI) to Cr(III). Our results provide an attractive strategy to strengthen the bio-immobilization of Cr(VI) at iron-rich contaminated sites through the synergistic enhancement of iron(III) minerals and electron shuttle.

Fe(II) Interactions with Smectites: Temporal Changes in Redox Reactivity and the Formation of Green Rust

In this study, temporal changes in the redox properties of three 0.5 g/L smectite suspensions were investigated-a montmorillonite (MAu-1) and two nontronites (NAu-1 and NAu-2) in the presence of 1 mM aqueous Fe(II) at pH 7.8. X-ray absorption spectroscopy revealed that the amount of Fe(II) added quantitatively transformed into chloride-green rust (Cl-GR) within 5 min and persisted over 18 days. Over the same time, the reduction potential of all three suspensions increased by 50 to 150 mV to equilibrate at approximately -100 mV vs SHE. The reduction of a model organic contaminant, 4-chloronitrobenzene (4-CINB), also became increasingly slower over time with virtually no 4-CINB reduction being observed after 18 days. There was a strong correlation between reduction potential and the quantity of 4-ClNB reduced by MAu-1, although other factors were likely involved in the decreased redox reactivity observed in the nontronites. It is hypothesized that the temporal increase in reduction potential results from clay mineral dissolution resulting in increased Fe(III) contents in the Cl-GR. These results demonstrate that long-term studies are recommended to accurately predict contaminant management strategies.

Nanowires of Geobacter sulfurreducens Require Redox Cofactors to Reduce Metals in Pore Spaces Too Small for Cell Passage

Members of the Geobacteraceae family are ubiquitous metal reducers that utilize conductive "nanowires" to reduce Mn(IV) and Fe(III) oxides in anaerobic sediments. However, it is not currently known if and to what extent the Mn(IV) and Fe(III) oxides in soil grains and low permeability sediments that are sequestered in pore spaces too small for cell passage can be reduced by long-range extracellular electron transport via Geobacter nanowires, and what mechanisms control this reduction. We developed a microfluidic reactor that physically separates Geobacter sulfurreducens from the Mn(IV) mineral birnessite by a 1.4 μm thick wall containing <200 nm pores. Using optical microscopy and Raman spectroscopy, we show that birnessite can be reduced up to 15 μm away from cell bodies, similar to the reported length of Geobacter nanowires. Reduction across the nanoporous wall required reducing conditions, provided by Escherichia coli, and an exogenous supply of riboflavin. Our results discount electron shuttling by dissolved flavins, and instead support their role as bound redox cofactors in electron transport from nanowires to metal oxides. We also show that upon addition of a soluble electron shuttle (i.e., AQDS), reduction extends beyond the reported nanowire length up to 40 μm into a layer of birnessite.

Reductive dechlorination of carbon tetrachloride by bioreduction of nontronite

Reductive dechlorination of carbon tetrachloride (CT) was investigated during bioreduction of iron-containing clay mineral (i.e., nontronite) by iron-reducing bacteria (Shewanella putrefaciens CN32 (CN32)). In the absence of CT, the production of Fe(II) significantly increased in nontronite suspension with CN32 in 124 d (11.1% of Fe(III) reduction), resulting in formation of new secondary Fe(II) mineral phase (i.e., vivianite (Fe^II₃(PO₄)₂·8H₂O)). In the presence of CT, an acceleration of CT dechlorination was observed after 13 d and it reached almost 68% of removal efficiency at 32 d in nontronite suspension with CN32, which was 1.8 times higher than that by CN32 alone (37%). Significant amounts of formate (30.1%) and CO (2.4%) were measured during the CT dechlorination in the nontronite suspension with CN32. Results obtained from Fe(II) measurement and X-ray diffraction (XRD) showed the acceleration of Fe(II) production after 13 d and the formation of vivianite in the range of 13-25 d, suggesting that the biogenic vivianite enhanced the CT dechlorination in this study. Experimental results from batch kinetic tests, Fe(II) measurements, XRD analysis, and by-product study suggested that the formation of vivianite can play a crucial role for the enhanced reductive dechlorination of CT in phosphorous enriched subsurface environments with iron-containing clay minerals.

Degradation of 1, 4-dioxane by hydroxyl radicals produced from clay minerals

1,4-Dioxane is causing a general concern as an emerging contaminant in groundwater environment. Traditional remediation methods can be either inefficient or costly. In this study, we present a cost effective method for possible in situ remediation of 1,4-dioxane. Hydroxyl radicals (OH) produced from oxygenation of structural Fe(II) in ferruginous clay minerals significantly degraded high concentrations of 1,4-dioxane (up to 400μmol/L) within 120h under circumneutral pH and dark condition. The amount of 1,4-dioxane degradation was positively correlated with the amount of OH. The major degradation product of 1,4-dioxane was formic acid. Different clay mineral types, initial Fe(II) concentration, and buffer composition all affected OH production and 1,4-dioxane degradation efficiency. Nontronite, an iron-rich smectite, was a reusable and effective material for sustainable production of OH and 1,4-dioxane degradation, through regeneration of Fe(II) either biologically or chemically. The non-selectivity and strong oxidative power of OH make it a promising agent for remediating various kinds of organic contaminants in aqueous environment.

Uranium(IV) adsorption by natural organic matter in anoxic sediments

Uranium is an important carbon-free fuel source and environmental contaminant that accumulates in the tetravalent state, U(IV), in anoxic sediments, such as ore deposits, marine basins, and contaminated aquifers. However, little is known about the speciation of U(IV) in low-temperature geochemical environments, inhibiting the development of a conceptual model of U behavior. Until recently, U(IV) was assumed to exist predominantly as the sparingly soluble mineral uraninite (UO_2+x) in anoxic sediments; however, studies now show that this is not often the case. Yet a model of U(IV) speciation in the absence of mineral formation under field-relevant conditions has not yet been developed. Uranium(IV) speciation controls its reactivity, particularly its susceptibility to oxidative mobilization, impacting its distribution and toxicity. Here we show adsorption to organic carbon and organic carbon-coated clays dominate U(IV) speciation in an organic-rich natural substrate under field-relevant conditions. Whereas previous research assumed that U(IV) speciation is dictated by the mode of reduction (i.e., whether reduction is mediated by microbes or by inorganic reductants), our results demonstrate that mineral formation can be diminished in favor of adsorption, regardless of reduction pathway. Projections of U transport and bioavailability, and thus its threat to human and ecosystem health, must consider U(IV) adsorption to organic matter within the sediment environment.

Iron Isotope Fractionations Reveal a Finite Bioavailable Fe Pool for Structural Fe(III) Reduction in Nontronite

We report on stable Fe isotope fractionation during microbial and chemical reduction of structural Fe(III) in nontronite NAu-1. (56)Fe/(54)Fe fractionation factors between aqueous Fe(II) and structural Fe(III) ranged from -1.2 to +0.8‰. Microbial (Shewanella oneidensis and Geobacter sulfurreducens) and chemical (dithionite) reduction experiments revealed a two-stage process. Stage 1 was characterized by rapid reduction of a finite Fe(III) pool along the edges of the clay particles, accompanied by a limited release to solution of Fe(II), which partially adsorbed onto basal planes. Stable Fe isotope compositions revealed that electron transfer and atom exchange (ETAE) occurred between edge-bound Fe(II) and octahedral (structural) Fe(III) within the clay lattice, as well as between aqueous Fe(II) and structural Fe(III) via a transient sorbed phase. The isotopic fractionation factors decreased with increasing extent of reduction as a result of the depletion of the finite bioavailable Fe(III) pool. During stage 2, microbial reduction was inhibited while chemical reduction continued. However, further ETAE between aqueous Fe(II) and structural Fe(III) was not observed. Our results imply that the pool of bioavailable Fe(III) is restricted to structural Fe sites located near the edges of the clay particles. Blockage of ETAE distinguishes Fe(III) reduction of layered clay minerals from that of Fe oxyhydroxides, where accumulation of structural Fe(II) is much more limited.

Microbial Fe(III) oxide reduction potential in Chocolate Pots hot spring, Yellowstone National Park

Chocolate Pots hot springs (CP) is a unique, circumneutral pH, iron-rich, geothermal feature in Yellowstone National Park. Prior research at CP has focused on photosynthetically driven Fe(II) oxidation as a model for mineralization of microbial mats and deposition of Archean banded iron formations. However, geochemical and stable Fe isotopic data have suggested that dissimilatory microbial iron reduction (DIR) may be active within CP deposits. In this study, the potential for microbial reduction of native CP Fe(III) oxides was investigated, using a combination of cultivation dependent and independent approaches, to assess the potential involvement of DIR in Fe redox cycling and associated stable Fe isotope fractionation in the CP hot springs. Endogenous microbial communities were able to reduce native CP Fe(III) oxides, as documented by most probable number enumerations and enrichment culture studies. Enrichment cultures demonstrated sustained DIR driven by oxidation of acetate, lactate, and H2 . Inhibitor studies and molecular analyses indicate that sulfate reduction did not contribute to observed rates of DIR in the enrichment cultures through abiotic reaction pathways. Enrichment cultures produced isotopically light Fe(II) during DIR relative to the bulk solid-phase Fe(III) oxides. Pyrosequencing of 16S rRNA genes from enrichment cultures showed dominant sequences closely affiliated with Geobacter metallireducens, a mesophilic Fe(III) oxide reducer. Shotgun metagenomic analysis of enrichment cultures confirmed the presence of a dominant G. metallireducens-like population and other less dominant populations from the phylum Ignavibacteriae, which appear to be capable of DIR. Gene (protein) searches revealed the presence of heat-shock proteins that may be involved in increased thermotolerance in the organisms present in the enrichments as well as porin-cytochrome complexes previously shown to be involved in extracellular electron transport. This analysis offers the first detailed insight into how DIR may impact the Fe geochemistry and isotope composition of a Fe-rich, circumneutral pH geothermal environment.

Identification of Bacteria Synthesizing Ribosomal RNA in Response to Uranium Addition During Biostimulation at the Rifle, CO Integrated Field Research Site

Understanding which organisms are capable of reducing uranium at historically contaminated sites provides crucial information needed to evaluate treatment options and outcomes. One approach is determination of the bacteria which directly respond to uranium addition. In this study, uranium amendments were made to groundwater samples from a site of ongoing biostimulation with acetate. The active microbes in the planktonic phase were deduced by monitoring ribosomes production via RT-PCR. The results indicated several microorganisms were synthesizing ribosomes in proportion with uranium amendment up to 2 μM. Concentrations of U (VI) >2 μM were generally found to inhibit ribosome synthesis. Two active bacteria responding to uranium addition in the field were close relatives of Desulfobacter postgateii and Geobacter bemidjiensis. Since RNA content often increases with growth rate, our findings suggest it is possible to rapidly elucidate active bacteria responding to the addition of uranium in field samples and provides a more targeted approach to stimulate specific populations to enhance radionuclide reduction in contaminated sites.

Linear free energy relationships for the biotic and abiotic reduction of nitroaromatic compounds

Nitroaromatic compounds (NACs) are ubiquitous environmental contaminants that are susceptible to biological and abiotic reduction. Prior works have found that for the abiotic reduction of NACs, the logarithm of the NACs’ rate constants correlate with one-electron reduction potential values of the NACs (EH,NAC1) according to linear free energy relationships (LFERs). Here, we extend the application of LFERs to the bioreduction of NACs and to the abiotic reduction of NACs by bioreduced (and pasteurized) iron-bearing clay minerals. A linear correlation (R2=0.96) was found between the NACs’ bioreduction rate constants (kobs) and EH,NAC1 values. The LFER slope of log kobs versus EH,NAC1/(2.303RT/F) was close to one (0.97), which implied that the first electron transfer to the NAC was the rate-limiting step of bioreduction. LFERs were also established between NAC abiotic reduction rate constants by bioreduced iron-bearing clay minerals (montmorillonite SWy-2 and nontronite NAu-2). The second-order NAC reduction rate constants (k) by bioreduced SWy-2 and NAu-2 were well correlated to EH,NAC1 (R2=0.97 for both minerals), consistent with bioreduction results. However, the LFER slopes of log k versus EH,NAC1/(2.303RT/F) were significantly less than one (0.48–0.50) for both minerals, indicating that the first electron transfer to the NAC was not the rate-limiting step of abiotic reduction. Finally, we demonstrate that the rate of 4-acetylnitrobenzene reduction by bioreduced SWy-2 and NAu-2 correlated to the reduction potential of the clay (EH,clay, R2=0.95 for both minerals), indicating that the clay reduction potential also influences its reactivity.

Iron(III)-bearing clay minerals enhance bioreduction of nitrobenzene by Shewanella putrefaciens CN32

Iron-bearing clay minerals are ubiquitous in the environment, and the clay-Fe(II)/Fe(III) redox couple plays important roles in abiotic reduction of several classes of environmental contaminants. We investigated the role of Fe-bearing clay minerals on the bioreduction of nitrobenzene. In experiments with Shewanella putrefaciens CN32 and excess electron donor, we found that the Fe-bearing clay minerals montmorillonite SWy-2 and nontronite NAu-2 enhanced nitrobenzene bioreduction. On short time scales (<50 h), nitrobenzene reduction was primarily biologically driven, but at later time points, nitrobenzene reduction by biologically formed structural Fe(II) in the clay minerals became increasingly important. We found that chemically reduced (dithionite) iron-bearing clay minerals reduced nitrobenzene more rapidly than biologically reduced iron-bearing clay minerals despite the minerals having similar structural Fe(II) concentrations. We also found that chemically reduced NAu-2 reduced nitrobenzene faster as compared to chemically reduced SWy-2. The different reactivity of SWy-2 versus NAu-2 toward nitrobenzene was caused by different forms of structural clay-Fe(II) in the clay minerals and different reduction potentials (Eh) of the clay minerals. Because most contaminated aquifers become reduced via biological activity, the reactivity of biogenic clay-Fe(II) toward reducible contaminants is particularly important.