Bhoopesh Mishra

Sulfur-mediated electron shuttling during bacterial iron reduction

How bacteria manage to breathe on rust In the absence of oxygen, anaerobic bacteria turn to other chemical compounds during respiration. This can be helpful in detoxifying heavy-metal pollution. Flynn et al. (see the Perspective by Friedrich and Finster) found that alkaline conditions prevent a detoxifying bug—Shewanella oneidensis—from using enzymes to reduce rust-like minerals. Instead, the bacteria reduce elemental sulfur compounds, generating hydrogen sulfide that reduces the iron indirectly. This interplay between anoxic biogeochemical cycles may explain why some anaerobic bacteria contain the genetic machinery necessary to reduce multiple compounds besides oxygen. Science, this issue p. 1039; see also p. 974 Bacterial respiration of ferric iron involves sulfur intermediates in alkaline conditions [Also see Perspective by Friedrich and Finster] Microbial reduction of ferric iron [Fe(III)] is an important biogeochemical process in anoxic aquifers. Depending on groundwater pH, dissimilatory metal-reducing bacteria can also respire alternative electron acceptors to survive, including elemental sulfur (S0). To understand the interplay of Fe/S cycling under alkaline conditions, we combined thermodynamic geochemical modeling with bioreactor experiments using Shewanella oneidensis MR-1. Under these conditions, S. oneidensis can enzymatically reduce S0 but not goethite (α-FeOOH). The HS– produced subsequently reduces goethite abiotically. Because of the prevalence of alkaline conditions in many aquifers, Fe(III) reduction may thus proceed via S0-mediated electron-shuttling pathways.

Binding of HgII to High-Affinity Sites on Bacteria Inhibits Reduction to Hg0 by Mixed FeII/III Phases

Magnetite and green rust have been shown to reduce aqueous Hg(II) to Hg(0). In this study, we tested the ability of magnetite and green rust to reduce Hg(II) sorbed to 2 g · L(-1) of biomass (Bacillus subtilis), at high (50 μM) and low (5 μM) Hg loadings and at pH 6.5 and 5.0. At high Hg:biomass loading, where Hg(II) binding to biomass is predominantly through carboxyl functional groups, Hg L(III)-edge X-ray absorption spectroscopy showed reduction of Hg(II) to Hg(0) by magnetite. Reduction occurred within 2 h and 2 d at pH 6.5 and 5.0, respectively. At low Hg:biomass loading, where Hg(II) binds to biomass via sulfhydryl functional groups, Hg(II) was not reduced by magnetite at pH 6.5 or 5.0 after 2 months of reaction. Green rust, which is generally a stronger reductant than magnetite, reduced about 20% of the total Hg(II) bound to biomass via sulfhydryl groups to Hg(0) in 2 d. These results suggest that Hg(II) binding to carboxyl groups does not significantly inhibit the reduction of Hg(II) by magnetite. However, the binding of Hg(II) to biomass via sulfhydryl groups severely inhibits the ability of mixed Fe(II/III) phases like magnetite and green rust to reduce Hg(II) to Hg(0). The mobility of heavy metal contaminants in aquatic and terrestrial environments is greatly influenced by their speciation, especially their oxidation state. In the case of Hg, reduction of Hg(II) to Hg(0) can increase Hg mobility because of the volatility of Hg(0). Since Hg is typically present in aquatic and terrestrial systems at low concentrations, binding of Hg(II) to high-affinity sites on bacteria could have important implications for the potential reduction of Hg(II) to Hg(0) and the overall mobility of Hg in biostimulated subsurface environments.

Stable U(IV) Complexes Form at High-Affinity Mineral Surface Sites

Uranium (U) poses a significant contamination hazard to soils, sediments, and groundwater due to its extensive use for energy production. Despite advances in modeling the risks of this toxic and radioactive element, lack of information about the mechanisms controlling U transport hinders further improvements, particularly in reducing environments where U(IV) predominates. Here we establish that mineral surfaces can stabilize the majority of U as adsorbed U(IV) species following reduction of U(VI). Using X-ray absorption spectroscopy and electron imaging analysis, we find that at low surface loading, U(IV) forms inner-sphere complexes with two metal oxides, TiO2 (rutile) and Fe3O4 (magnetite) (at <1.3 U nm(-2) and <0.037 U nm(-2), respectively). The uraninite (UO2) form of U(IV) predominates only at higher surface loading. U(IV)-TiO2 complexes remain stable for at least 12 months, and U(IV)-Fe3O4 complexes remain stable for at least 4 months, under anoxic conditions. Adsorbed U(IV) results from U(VI) reduction by Fe(II) or by the reduced electron shuttle AH2QDS, suggesting that both abiotic and biotic reduction pathways can produce stable U(IV)-mineral complexes in the subsurface. The observed control of high-affinity mineral surface sites on U(IV) speciation helps explain the presence of nonuraninite U(IV) in sediments and has important implications for U transport modeling.

Influence of Chloride and Fe(II) Content on the Reduction of Hg(II) by Magnetite

Abiotic reduction of inorganic mercury by natural organic matter and native soils is well-known, and recently there is evidence that reduced iron (Fe) species, such as magnetite, green rust, and Fe sulfides, can also reduce Hg(II). Here, we evaluated the reduction of Hg(II) by magnetites with varying Fe(II) content in both the absence and presence of chloride. Specifically, we evaluated whether magnetite stoichiometry (x = Fe(II)/Fe(III)) influences the rate of Hg(II) reduction and formation of products. In the absence of chloride, reduction of Hg(II) to Hg(0) is observed over a range of magnetite stoichiometries (0.29 < x < 0.50) in purged headspace reactors and unpurged low headspace reactors, as evidenced by Hg recovery in a volatile product trap solution and Hg L(III)-edge X-ray absorption near edge spectroscopy (XANES). In the presence of chloride, however, XANES spectra indicate the formation of a metastable Hg(I) calomel species (Hg2Cl2) from the reduction of Hg(II). Interestingly, Hg(I) species are only observed for the more oxidized magnetite particles that contain lower Fe(II) content (x < 0.42). For the more reduced magnetite particles (x ≥ 0.42), Hg(II) is reduced to Hg(0) even in the presence of high chloride concentrations. As previously observed for nitroaromatic compounds and uranium, magnetite stoichiometry appears to influence the rate of Hg(II) reduction (both in the presence and absence of chloride) confirming that it is important to consider magnetite stoichiometry when assessing the fate of contaminants in Fe-rich subsurface environments.

Microscale geochemical gradients in Hanford 300 Area sediment biofilms and influence of uranium

The presence and importance of microenvironments in the subsurface at contaminated sites were suggested by previous geochemical studies. However, no direct quantitative characterization of the geochemical microenvironments had been reported. We quantitatively characterized microscale geochemical gradients (dissolved oxygen (DO), H(2), pH, and redox potential) in Hanford 300A subsurface sediment biofilms. Our results revealed significant differences in geochemical parameters across the sediment biofilm/water interface in the presence and absence of U(VI) under oxic and anoxic conditions. While the pH was relatively constant within the sediment biofilm, the redox potential and the DO and H(2) concentrations were heterogeneous at the microscale (<500-1000 μm). We found microenvironments with high DO levels (DO hotspots) when the sediment biofilm was exposed to U(VI). On the other hand, we found hotspots (high concentrations) of H(2) under anoxic conditions both in the presence and in the absence of U(VI). The presence of anoxic microenvironments inside the sediment biofilms suggests that U(VI) reduction proceeds under bulk oxic conditions. To test this, we operated our biofilm reactor under air-saturated conditions in the presence of U(VI) and characterized U speciation in the sediment biofilm. U L(III)-edge X-ray absorption spectroscopy (XANES and EXAFS) showed that 80-85% of the U was in the U(IV) valence state.

Immobilization of U(VI) from oxic groundwater by Hanford 300 Area sediments and effects of Columbia River water

Regions within the U.S. Department of Energy Hanford 300 Area (300 A) site experience periodic hydrologic influences from the nearby Columbia River as a result of changing river stage, which causes changes in groundwater elevation, flow direction and water chemistry. An important question is the extent to which the mixing of Columbia River water and groundwater impacts the speciation and mobility of uranium (U). In this study, we designed experiments to mimic interactions among U, oxic groundwater or Columbia River water, and 300 A sediments in the subsurface environment of Hanford 300 A. The goals were to investigate mechanisms of: 1) U immobilization in 300 A sediments under bulk oxic conditions and 2) U remobilization from U-immobilized 300 A sediments exposed to oxic Columbia River water. Initially, 300 A sediments in column reactors were fed with U(VI)-containing oxic 1) synthetic groundwater (SGW), 2) organic-amended SGW (OA-SGW), and 3) de-ionized (DI) water to investigate U immobilization processes. After that, the sediments were exposed to oxic Columbia River water for U remobilization studies. The results reveal that U was immobilized by 300 A sediments predominantly through reduction (80-85%) when the column reactor was fed with oxic OA-SGW. However, U was immobilized by 300 A sediments through adsorption (100%) when the column reactors were fed with oxic SGW or DI water. The reduced U in the 300 A sediments fed with OA-SGW was relatively resistant to remobilization by oxic Columbia River water. Oxic Columbia River water resulted in U remobilization (∼7%) through desorption, and most of the U that remained in the 300 A sediments fed with OA-SGW (∼93%) was in the form of uraninite nanoparticles. These results reveal that: 1) the reductive immobilization of U through OA-SGW stimulation of indigenous 300 A sediment microorganisms may be viable in the relatively oxic Hanford 300 A subsurface environments and 2) with the intrusion of Columbia River water, desorption may be the primary process resulting in U remobilization from OA-SGW-stimulated 300 A sediments at the subsurface of the Hanford 300 A site.

Redox Behavior of Uranium at the Nanoporous Aluminum Oxide-Water Interface: Implications for Uranium Remediation

Sorption-desorption experiments show that the majority (ca. 80-90%) of U(VI) presorbed to mesoporous and nanoporous alumina could not be released by extended (2 week) extraction with 50 mM NaHCO(3) in contrast with non-nanoporous α alumina. The extent of reduction of U(VI) presorbed to aluminum oxides was semiquantitatively estimated by comparing the percentages of uranium desorbed by anoxic sodium bicarbonate between AH(2)DS-reacted and unreacted control samples. X-ray absorption spectroscopy confirmed that U(VI) presorbed to non-nanoporous alumina was rapidly and completely reduced to nanoparticulate uraninite by AH(2)DS, whereas reduction of U(VI) presorbed to nanoporous alumina was slow and incomplete (<5% reduction after 1 week). The observed nanopore size-dependent redox behavior of U has important implications in developing efficient remediation techniques for the subsurface uranium contamination because the efficiency of in situ bioremediation depends on how effectively and rapidly U(VI) bound to sediment or soil can be converted to an immobile phase.

Spectroscopic and Computational Insights on Catalytic Synergy in Bimetallic Aluminophosphate Catalysts

A combined electronic structure computational and X-ray absorption spectroscopy study was used to investigate the nature of the active sites responsible for catalytic synergy in Co-Ti bimetallic nanoporous frameworks. Probing the nature of the molecular species at the atomic level has led to the identification of a unique Co-O-Ti bond, which serves as the loci for the superior performance of the bimetallic catalyst, when compared with its analogous monometallic counterpart. The structural and spectroscopic features associated with this active site have been characterized and contrasted, with a view to affording structure-property relationships, in the wider context of designing sustainable catalytic oxidations with porous solids.

Siderophore sorption to clays.

Siderophores are low molecular weight organic ligands exuded by some aerobic organisms and plants to acquire Fe under Fe-limited conditions. The hydroxamate siderophores may sorb to aluminosilicate clays through a variety of mechanisms depending upon the nature of the clay and of the siderophore along with solution conditions such as pH, ionic strength, and presence of metal cations. They may also affect metal binding to clays. Here, we review previous studies of siderophore sorption to aluminosilicate clays; briefly discuss how the techniques of X-ray diffractometry, Fourier-transform infrared spectroscopy, and X-ray absorption spectroscopy may be applied to such studies; review effects of siderophores on metal sorption to clays; and highlight some areas for future research.

Intracellular Hg(0) Oxidation in Desulfovibrio desulfuricans ND132

The disposal of elemental mercury (Hg(0)) wastes in mining and manufacturing areas has caused serious soil and groundwater contamination issues. Under anoxic conditions, certain anaerobic bacteria can oxidize dissolved elemental mercury and convert the oxidized Hg to neurotoxic methylmercury. In this study, we conducted experiments with the Hg-methylating bacterium Desulfovibrio desulfuricans ND132 to elucidate the role of cellular thiols in anaerobic Hg(0) oxidation. The concentrations of cell-surface and intracellular thiols were measured, and specific fractions of D. desulfuricans ND132 were examined for Hg(0) oxidation activity and analyzed with extended X-ray absorption fine structure (EXAFS) spectroscopy. The experimental data indicate that intracellular thiol concentrations are approximately six times higher than those of the cell wall. Cells reacted with a thiol-blocking reagent were severely impaired in Hg(0) oxidation activity. Spheroplasts lacking cell walls rapidly oxidized Hg(0) to Hg(II), while cell wall fragments exhibited low reactivity toward Hg(0). EXAFS analysis of spheroplast samples revealed that multiple different forms of Hg-thiols are produced by the Hg(0) oxidation reaction and that the local coordination environment of the oxidized Hg changes with reaction time. The results of this study indicate that Hg(0) oxidation in D. desulfuricans ND132 is an intracellular process that occurs by reaction with thiol-containing molecules.