Solubilization of plutonium hydrous oxide by iron-reducing bacteria

A critical review on radioactive waste management through biological techniques

Our world is subject to various kinds of pollution and contamination due to rapid growth and development of industrialization. Though, industries are helping to improve the human life style in many ways in day to day life such as power generation to treatment of diseases. At the same time, industries emit the waste which causes major environmental pollution and leads to harmful for all living organism. As the renewable energy sources are depleting, energy/power generation become a major research around the world. Nuclear energy is one of the promising energy to sort out the energy demand, but the problem associated with the nuclear energy is the management and treatment of radioactive waste/emission/effluent since which is more dangerous to all living organism. There is a large scale contamination of radioactive waste associated for the past 60 years of global nuclear activity. It is necessary to pay special attention to the management of radioactive wastes in order to approach pollution-free environment and avoid diseases to living organism through various clean-up strategies. In this review, we discussed the wide ranges of strategies available for radioactive waste management such as physical, chemical, and biological methods. Bioremediation may be the powerful tool for treatment of radioactive wastes. Additionally, discussed on recent advancement have been made in treatment of radioactive waste through microbial transformation as well as phytoremediation which play a major role in disposal of radioactive waste.

Plutonium Oxidation States in the Waste Isolation Pilot Plant Repository

The Waste Isolation Pilot Plant (WIPP), a deep geologic repository located 660 meters underground in bedded salt, is designed to isolate U.S. defense-related transuranic waste from the accessible environment. Plutonium isotopes are the most important radionuclides in WIPP waste. Plutonium solubility in WIPP brines (ionic strengths from 5.3 to 7.4) is strongly dependent on its oxidation state, with much lower solubilities associated with Pu(III) and Pu(IV) than with the higher Pu(V) and Pu(VI) oxidation states. The large quantity of metallic iron in WIPP waste and waste containers is expected to undergo anoxic corrosion, producing strongly reducing conditions and high hydrogen gas pressures after repository closure and brine intrusion. Because reducing conditions will prevail in the WIPP repository, the most important long-term oxidation states will be Pu(III) and Pu(IV). We performed a literature review to evaluate the effects of WIPP chemical and physical processes (not colloidal) on plutonium oxidation states that included reactions with reducing agents such as iron solids and aqueous species and radiolysis of solids and aqueous species. The results of this review indicate that equilibrium between Pu(III) solids and aqueous species will control dissolved plutonium concentrations in WIPP brines. We also performed geochemical modeling calculations using the ThermoChimie database to support this assessment of plutonium oxidation states in the long-term WIPP repository. Control of plutonium solubilities by Pu(III) solid instead of Pu(IV) solid may lead to higher predicted plutonium concentrations in brines potentially released to the ground surface by an inadvertent drilling intrusion into the long-term WIPP repository. The results of this study demonstrate that Pu(III) solid solubilities provide a reasonable upper bound for dissolved plutonium concentrations in WIPP brines.

Hydrolysis of trivalent plutonium and solubility of Pu(OH)3 (am) under electrolytic reducing conditions

The aim of this work is to determine the solubility product of plutonium hydroxide under reducing conditions and to ascertain the stability of Pu(OH)₃ (am) in water. Hydrolysis of Pu(iii) and solubility of Pu(OH)₃ (am) were investigated at a constant ionic strength of 0.1 M NaClO₄. Coulometric titration was adopted to adjust the pH of plutonium solutions, during which the electrolytic reducing conditions maintained the oxidation state of Pu(iii). Chemical speciation for dissolved plutonium was investigated using sensitive spectrophotometry coupled with a liquid waveguide capillary cell. The spectroscopic investigations indicated that dissolved Pu(iv), Pu(v), and Pu(vi) species were ignorable under these experimental conditions. The absorbance of Pu³⁺ ions decreased due to hydrolysis of Pu(iii) but the absorbance of Pu(iii) hydrolysis species was not distinguishable. The formation constant for the first hydrolysis species (log ^*β'₁) determined in the present study is -6.62 ± 0.25. The non-crystalline structure of the plutonium precipitate was observed through X-ray diffraction. The solubility product of Pu(OH)₃ (am), log ^*K'_s,0 is determined to be 15.23 ± 0.50. These results indicate a stronger tendency for the hydrolysis of Pu(iii) and higher stability (lower solubility) of Pu(OH)₃ (am) compared to Am(iii).

Interactions of Plutonium with Pseudomonas sp. Strain EPS-1W and Its Extracellular Polymeric Substances

Safe and effective nuclear waste disposal, as well as accidental radionuclide releases, necessitates our understanding of the fate of radionuclides in the environment, including their interaction with microorganisms. We examined the sorption of Pu(IV) and Pu(V) to Pseudomonas sp. strain EPS-1W, an aerobic bacterium isolated from plutonium (Pu)-contaminated groundwater collected in the United States at the Nevada National Security Site (NNSS) in Nevada. We compared Pu sorption to cells with and without bound extracellular polymeric substances (EPS). Wild-type cells with intact EPS sorbed Pu(V) more effectively than cells with EPS removed. In contrast, cells with and without EPS showed the same sorption affinity for Pu(IV). In vitro experiments with extracted EPS revealed rapid reduction of Pu(V) to Pu(IV). Transmission electron microscopy indicated that 2- to 3-nm nanocrystalline Pu(IV)O₂ formed on cells equilibrated with high concentrations of Pu(IV) but not Pu(V). Thus, EPS, while facilitating Pu(V) reduction, inhibit the formation of nanocrystalline Pu(IV) precipitates.

IMPORTANCE

Our results indicate that EPS are an effective reductant for Pu(V) and sorbent for Pu(IV) and may impact Pu redox cycling and mobility in the environment. Additionally, the resulting Pu morphology associated with EPS will depend on the concentration and initial Pu oxidation state. While our results are not directly applicable to the Pu transport situation at the NNSS, the results suggest that, in general, stationary microorganisms and biofilms will tend to limit the migration of Pu and provide an important Pu retardation mechanism in the environment. In a broader sense, our results, along with a growing body of literature, highlight the important role of microorganisms as producers of redox-active organic ligands and therefore as modulators of radionuclide redox transformations and complexation in the subsurface.

Microbial mobilization of plutonium and other actinides from contaminated soil

We examined the dissolution of Pu, U, and Am in contaminated soil from the Nevada Test Site (NTS) due to indigenous microbial activity. Scanning transmission x-ray microscopy (STXM) analysis of the soil showed that Pu was present in its polymeric form and associated with Fe- and Mn- oxides and aluminosilicates. Uranium analysis by x-ray diffraction (μ-XRD) revealed discrete U-containing mineral phases, viz., schoepite, sharpite, and liebigite; synchrotron x-ray fluorescence (μ-XRF) mapping showed its association with Fe- and Ca-phases; and μ-x-ray absorption near edge structure (μ-XANES) confirmed U(IV) and U(VI) oxidation states. Addition of citric acid or glucose to the soil and incubated under aerobic or anaerobic conditions enhanced indigenous microbial activity and the dissolution of Pu. Detectable amount of Am and no U was observed in solution. In the citric acid-amended sample, Pu concentration increased with time and decreased to below detection levels when the citric acid was completely consumed. In contrast, with glucose amendment, Pu remained in solution. Pu speciation studies suggest that it exists in mixed oxidation states (III/IV) in a polymeric form as colloids. Although Pu(IV) is the most prevalent and generally considered to be more stable chemical form in the environment, our findings suggest that under the appropriate conditions, microbial activity could affect its solubility and long-term stability in contaminated environments.

Formation and release behavior of iron corrosion products under the influence of bacterial communities in a simulated water distribution system

To understand the formation and release behavior of iron corrosion products in a drinking water distribution system, annular reactors (ARs) were used to investigate the development processes of corrosion products and biofilm community as well as the concomitant iron release behavior. Results showed that the formation and transformation of corrosion products and bacterial community are closely related to each other. The presence of sulfate-reducing bacteria (SRB, e.g. Desulfovibrio and Desulfotomaculum), sulfur-oxidizing bacteria (SOB, e.g. Sulfuricella), and iron-oxidizing bacteria (IOB, e.g. Acidovorax, Gallionella, Leptothrix, and Sphaerotilus) in biofilms could speed up iron corrosion; however, iron-reducing bacteria (IRB, e.g. Bacillus, Clostridium, and Pseudomonas) could inhibit iron corrosion and iron release. Corrosion scales on iron coupons could develop into a two-layered structure (top layer and inner layer) with time. The relatively stable constituents such as goethite (α-FeOOH) and magnetite (Fe3O4) mainly existed in the top layers, while green rust (Fe6(OH)12CO3) mainly existed in the inner layers. The IOB (especially Acidovorax) contributed to the formation of α-FeOOH, while IRB and the anaerobic conditions could facilitate the formation of Fe3O4. Compared with the AR test without biofilms, the iron corrosion rate with biofilms was relatively higher (p < 0.05) during the whole experimental period, but the iron release with biofilms was obviously lower both at the initial stage and after 3 months. Biofilm and corrosion scale samples formed under different water supply conditions in an actual drinking water distribution system verified the relationships between the bacterial community and corrosion products.

Bioremediation: a genuine technology to remediate radionuclides from the environment

Radionuclides in the environment are a major human and environmental health concern. Like the Chernobyl disaster of 1986, the Fukushima Daiichi nuclear disaster in 2011 is once again causing damage to the environment: a large quantity of radioactive waste is being generated and dumped into the environment, and if the general population is exposed to it, may cause serious life-threatening disorders. Bioremediation has been viewed as the ecologically responsible alternative to environmentally destructive physical remediation. Microorganisms carry endogenous genetic, biochemical and physiological properties that make them ideal agents for pollutant remediation in soil and groundwater. Attempts have been made to develop native or genetically engineered (GE) microbes for the remediation of environmental contaminants including radionuclides. Microorganism-mediated bioremediation can affect the solubility, bioavailability and mobility of radionuclides. Therefore, we aim to unveil the microbial-mediated mechanisms for biotransformation of radionuclides under various environmental conditions as developing strategies for waste management of radionuclides. A discussion follows of '-omics'-integrated genomics and proteomics technologies, which can be used to trace the genes and proteins of interest in a given microorganism towards a cell-free bioremediation strategy.

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.

Adhesion of Shewanella oneidensis MR-1 to iron (Oxy)(Hydr)oxides: microcolony formation and isotherm

The adhesion of dissimilatory metal reducing bacteria (DMRB) to iron (oxy)(hydr)oxides may play an important role in their respiration on ferric iron-containing minerals, but few quantitative surface cell density measurements have been made thus far. We used confocal microscopy to examine the adhesion of a common DMRB species, Shewanella oneidensis MR-1, onto iron (oxy)(hydr)oxide particulate-coated glass slides across a broad range of bulk (i.e., solution phase) cell densities from 10(5) cells/mL to 2 x 10(9) cells/mL. At bulk cell densities less than 1 x 10(7) cells/mL, cells adhered to the slide surface formed an evenly distributed, homogeneous monolayer, while at the bulk cell densities higher than 2 x 10(8) cells/mL the adhered cells formed distinct microcolonies. As a result of this complex adhesion behavior, simple Langmuir or Freundlich adsorption isotherms do not capture the relationship between the surface cell density and the bulk cell density over the entire range of bulk cell densities. Thus a new, two-step isotherm was developed that incorporated both isolated attached cells at low cell densities as well as microcolonies at higher cell densities.

Interaction of aerobic soil bacteria with plutonium(VI)

We studied the interaction of Pu(VI) withPseudomonas stutzeriATCC 17588 andBacillus sphaericusATCC 14577, representatives of the main aerobic groups of soil bacteria present in the upper soil layers. The biosorption studies have shown that these soil bacteria accumulate high amounts of Pu(VI). The relative sorption efficiency toward Pu(VI) related to the amount of biomass used decreased with increasing biomass concentration due to increased agglomeration of the bacteria resulting in a decrease of the number of available complexing groups. Spores ofBacillus sphaericusshowed a higher biosorption than the vegetative cells at low biomass concentration which decreased significantly with increasing biomass concentration. At higher biomass concentrations (>0.7 g/L), the vegetative cells of both strains and the spores ofB. sphaericusshowed comparable sorption efficiencies. Investigations on the pH dependency of the biosorption and extraction studies with 0.01 M EDTA solution have shown that the biosorption of plutonium is a reversible process and the plutonium is bound by surface complexation. Optical absorption spectroscopy showed that one third of the initially present Pu(VI) was reduced to Pu(V) after 24 hours. Kinetic studies and solvent extraction to separate different oxidation states of Pu after contact with the biomass provided further information on the yield and the kinetics of the bacteria-mediated reduction. Long-term studies showed that also 16% of Pu(IV) was formed after one month. The slow kinetics of this process indicate that under our experimental conditions the Pu(IV) was not a produced by microbial reduction but seemed to be rather the result of the disproportionation of the formed Pu(V) or autoreduction of Pu(VI).

Plutonium speciation affected by environmental bacteria

Plutonium has no known biological utility, yet it has the potential to interact with bacterial cellular and extracellular structures that contain metal-binding groups, to interfere with the uptake and utilization of essential elements, and to alter cell metabolism. These interactions can transform plutonium from its most common forms, solid, mineral-adsorbed, or colloidal Pu(IV), to a variety of biogeochemical species that have much different physico-chemical properties. Organic acids that are extruded products of cell metabolism can solubilize plutonium and then enhance its environmental mobility, or in some cases facilitate plutonium transfer into cells. Phosphate- and carboxylate-rich polymers associated with cell walls can bind plutonium to form mobile biocolloids or Pu-laden biofilm/mineral solids. Bacterial membranes, proteins or redox agents can produce strongly reducing electrochemical zones and generate molecular Pu(III/IV) species or oxide particles. Alternatively, they can oxidize plutonium to form soluble Pu(V) or Pu(VI) complexes. This paper reviews research on plutonium-bacteria interactions and closely related studies on the biotransformation of uranium and other metals.

Plutonium(V/VI) Reduction by the Metal-Reducing Bacteria Geobacter metallireducens GS-15 and Shewanella oneidensis MR-1

We examined the ability of the metal-reducing bacteria Geobacter metallireducens GS-15 and Shewanella oneidensis MR-1 to reduce Pu(VI) and Pu(V). Cell suspensions of both bacteria reduced oxidized Pu [a mixture of Pu(VI) and Pu(V)] to Pu(IV). The rate of plutonium reduction was similar to the rate of U(VI) reduction obtained under similar conditions for each bacteria. The rates of Pu(VI) and U(VI) reduction by cell suspensions of S. oneidensis were slightly higher than the rates observed with G. metallireducens. The reduced form of Pu was characterized as aggregates of nanoparticulates of Pu(IV). Transmission electron microscopy images of the solids obtained from the cultures after the reduction of Pu(VI) and Pu(V) by S. oneidensis show that the Pu precipitates have a crystalline structure. The nanoparticulates of Pu(IV) were precipitated on the surface of or within the cell walls of the bacteria. The production of Pu(III) was not observed, which indicates that Pu(IV) was the stable form of reduced Pu under these experimental conditions. Experiments examining the ability of these bacteria to use Pu(VI) as a terminal electron acceptor for growth were inconclusive. A slight increase in cell density was observed for both G. metallireducens and S. oneidensis when Pu(VI) was provided as the sole electron acceptor; however, Pu(VI) concentrations decreased similarly in both the experimental and control cultures.

Reductive dissolution of Pu(IV) by Clostridium sp. under anaerobic conditions

An anaerobic, gram positive, spore-forming bacterium Clostridium sp., common in soils and wastes, capable of reduction of Fe(III) to Fe(II), Mn(IV) to Mn(II), Tc(VII) to Tc(IV), and U(VI) to U(IV), reduced Pu(IV) to Pu(III). Addition of 242Pu (IV)-nitrate to the bacterial growth medium at pH 6.4 resulted in the precipitation of Pu as amorphous Pu(OH)4 due to hydrolysis and polymerization reactions. The Pu (1 x 10(-5) M) had no effect upon growth of the bacterium as evidenced by glucose consumption; carbon dioxide and hydrogen production; a decrease in pH of the medium from 6.4 to 3.0 due to production of acetic and butyric acids from glucose fermentation; and a change in the Eh of the culture medium from +50 to -180 mV. Commensurate with bacterial growth, Pu was rapidly solubilized as evidenced by an increase in Pu concentration in solution which passed through a 0.03 microm filtration. Selective solvent extraction of the culture by thenoyltrifluoroacetone (TTA) indicated the presence of a reduced Pu species in the soluble fraction. X-ray absorption near edge spectroscopic (XANES) analysis of Pu in the culture sample at the Pu LIII absorption edge (18.054 keV) showed a shift of -3 eV compared to a Pu(IV) standard indicating reduction of Pu(IV) to Pu(III). These results suggestthat, although Pu generally exists as insoluble Pu(IV) in the environment, under appropriate conditions, anaerobic microbial activity could affect the long-term stability and mobility of Pu by its reductive dissolution.

Influence of sources on plutonium mobility and oxidation state transformations in vadose zone sediments

Well-defined solid sources of Pu(III) (PuCl3), Pu(IV) (Pu (NO3)4 and Pu (C2O4)2), and Pu(VI) (Pu02(NO3)2) were placed in lysimeters containing vadose zone sediments and exposed to natural weather conditions for 2 or 11 years. The objective of this study was to measure the release rate of Pu and the changes in the Pu oxidation states from these Pu sources with the intent to develop a reactive transport model source-term. Pu(III) and Pu(IV) sources had identical Pu concentration depth profiles and similar Pu release rates. Source release data indicate that PuIV(C2O4)2 was the least mobile, whereas Pu(VI)O2(NO3)2 was the most mobile. Synchrotron X-ray fluorescence (SXRF) revealed that Pu was very unevenly distributed on the sediment and Mn concentrations were too low (630 mg kg(-1)) and perhaps of the wrong mineralogy to influence Pu distribution. The high stability of sorbed Pu(IV) is proposed to be due to the formation of a stable hydrolyzed Pu(IV) surface species. Plutonium X-ray absorption near-edge spectroscopy (XANES) analysis conducted on sediment recovered at the end of the studyfrom the Pu(IV)(NO3)4- and Pu(III)(III)Cl3-amended lysimeters contained essentially identical Pu distributions: approximately 37% Pu(III), 67% Pu(IV), 0% Pu(V), and 0% Pu(VI). These results were similar to those using a wet chemistry Pu oxidation state assay, except the latter method did not detect any Pu(III) present on the sediment but instead indicated that 93-98% of the Pu existed as Pu(IV). This discrepancy was likely attributable to incomplete extraction of sediment Pu(III) by the wet chemistry method. Although Pu has been known to exist in the +3 oxidation state under microbially induced reducing conditions for decades, to our knowledge, this is the first observation of steady-state Pu(III) in association with natural sediments. On the basis of thermodynamic considerations, Pu(III) has a wide potential distribution, especially in acidic environments, and as such may warrant further investigation.

Plutonium(IV) reduction by the metal-reducing bacteria Geobacter metallireducens GS15 and Shewanella oneidensis MR1

The bacterial reduction of actinides has been suggested as a possible remedial strategy for actinide-contaminated environments, and the bacterial reduction of Pu(VI/V) has the potential to produce highly insoluble Pu(IV) solid phases. However, the behavior of plutonium with regard to bacterial reduction is more complex than for other actinides because it is possible for Pu(IV) to be further reduced to Pu(III), which is relatively more soluble than Pu(IV). This work investigates the ability of the metal-reducing bacteria Geobacter metallireducens GS15 and Shewanella oneidensis MR1 to enzymatically reduce freshly precipitated amorphous Pu(IV) (OH)(4) [Pu(IV)(OH)(4(am))] and soluble Pu(IV)(EDTA). In cell suspensions without added complexing ligands, minor Pu(III) production was observed in cultures containing S. oneidensis, but little or no Pu(III) production was observed in cultures containing G. metallireducens. In the presence of EDTA, most of the Pu(IV)(OH)(4(am)) present was reduced to Pu(III) and remained soluble in cell suspensions of both S. oneidensis and G. metallireducens. When soluble Pu(IV)(EDTA) was provided as the terminal electron acceptor, cell suspensions of both S. oneidensis and G. metallireducens rapidly reduced Pu(IV)(EDTA) to Pu(III)(EDTA) with nearly complete reduction within 20 to 40 min, depending on the initial concentration. Neither bacterium was able to use Pu(IV) (in any of the forms used) as a terminal electron acceptor to support growth. These results have significant implications for the potential remediation of plutonium and suggest that strongly reducing environments where complexing ligands are present may produce soluble forms of reduced Pu species.

c-Type cytochrome-dependent formation of U(IV) nanoparticles by Shewanella oneidensis

Modern approaches for bioremediation of radionuclide contaminated environments are based on the ability of microorganisms to effectively catalyze changes in the oxidation states of metals that in turn influence their solubility. Although microbial metal reduction has been identified as an effective means for immobilizing highly-soluble uranium(VI) complexes in situ, the biomolecular mechanisms of U(VI) reduction are not well understood. Here, we show that c-type cytochromes of a dissimilatory metal-reducing bacterium, Shewanella oneidensis MR-1, are essential for the reduction of U(VI) and formation of extracellular UO(2) nanoparticles. In particular, the outer membrane (OM) decaheme cytochrome MtrC (metal reduction), previously implicated in Mn(IV) and Fe(III) reduction, directly transferred electrons to U(VI). Additionally, deletions of mtrC and/or omcA significantly affected the in vivo U(VI) reduction rate relative to wild-type MR-1. Similar to the wild-type, the mutants accumulated UO(2) nanoparticles extracellularly to high densities in association with an extracellular polymeric substance (EPS). In wild-type cells, this UO(2)-EPS matrix exhibited glycocalyx-like properties and contained multiple elements of the OM, polysaccharide, and heme-containing proteins. Using a novel combination of methods including synchrotron-based X-ray fluorescence microscopy and high-resolution immune-electron microscopy, we demonstrate a close association of the extracellular UO(2) nanoparticles with MtrC and OmcA (outer membrane cytochrome). This is the first study to our knowledge to directly localize the OM-associated cytochromes with EPS, which contains biogenic UO(2) nanoparticles. In the environment, such association of UO(2) nanoparticles with biopolymers may exert a strong influence on subsequent behavior including susceptibility to oxidation by O(2) or transport in soils and sediments.

Microbial reduction of metals and radionuclides

The microbial reduction of metals has attracted recent interest as these transformations can play crucial roles in the cycling of both inorganic and organic species in a range of environments and, if harnessed, may offer the basis for a wide range of innovative biotechnological processes. Under certain conditions, however, microbial metal reduction can also mobilise toxic metals with potentially calamitous effects on human health. This review focuses on recent research on the reduction of a wide range of metals including Fe(III), Mn(IV) and other more toxic metals such as Cr(VI), Hg(II), Co(III), Pd(II), Au(III), Ag(I), Mo(VI) and V(V). The reduction of metalloids including As(V) and Se(VI) and radionuclides including U(VI), Np(V) and Tc(VII) is also reviewed. Rapid advances over the last decade have resulted in a detailed understanding of some of these transformations at a molecular level. Where known, the mechanisms of metal reduction are discussed, alongside the environmental impact of such transformations and possible biotechnological applications that could utilise these activities.

Reductive capacity of natural reductants

Reductive capacities of soil minerals and soil for Cr(VI) and chlorinated ethylenes were measured and characterized to provide basic knowledge for in-situ and ex-situ treatment using these natural reductants. The reductive capacities of iron-bearing sulfide (pyrite), hydroxide (green rust; GR(SO4)), and oxide (magnetite) minerals for Cr(VI) and tetrachloroethylene (PCE) were 1-3 orders of magnitude greater than those of iron-bearing phyllosilicates (biotite, vermiculite, and montmorillonite). The reductive capacities of surface soil collected from the plains of central Texas were similar and slightly greater than those of iron-bearing phyllosilicates. The reductive capacity of iron-bearing soil minerals for Cr(VI) was roughly 3-16 times greater than that for PCE, implying that Cr(VI) is more susceptible to being reduced by soil minerals than is PCE. GR(SO4) has the greatest reductive capacity for both Cr(VI) and PCE followed by magnetite, pyrite, biotite, montmorillonite, and vermiculite. This order was the same for both target compounds, which indicates that the relative reductive capacities of soil minerals are consistent. The reductive capacities of pyrite and GR(SO4) for chlorinated ethylenes decreased in the order: trichloroethylene (TCE) > PCE > cis-dichloroethylene (c-DCE) > vinyl chloride (VC). Fe(II) content in soil minerals was directly proportional to the reductive capacity of soil minerals for Cr(VI) and PCE, suggesting that Fe(II) content is an important factor that significantly affects reductive transformations of target contaminants in natural systems.

Reduction of Fe(III), Cr(VI), U(VI), and Tc(VII) by Deinococcus radiodurans R1

Deinococcus radiodurans is an exceptionally radiation-resistant microorganism capable of surviving acute exposures to ionizing radiation doses of 15,000 Gy and previously described as having a strictly aerobic respiratory metabolism. Under strict anaerobic conditions, D. radiodurans R1 reduced Fe(III)-nitrilotriacetic acid coupled to the oxidation of lactate to CO(2) and acetate but was unable to link this process to growth. D. radiodurans reduced the humic acid analog anthraquinone-2,6-disulfonate (AQDS) to its dihydroquinone form, AH(2)DS, which subsequently transferred electrons to the Fe(III) oxides hydrous ferric oxide and goethite via a previously described electron shuttle mechanism. D. radiodurans reduced the solid-phase Fe(III) oxides in the presence of either 0.1 mM AQDS or leonardite humic acids (2 mg ml(-1)) but not in their absence. D. radiodurans also reduced U(VI) and Tc(VII) in the presence of AQDS. In contrast, Cr(VI) was directly reduced in anaerobic cultures with lactate although the rate of reduction was higher in the presence of AQDS. The results are the first evidence that D. radiodurans can reduce Fe(III) coupled to the oxidation of lactate or other organic compounds. Also, D. radiodurans, in combination with humic acids or synthetic electron shuttle agents, can reduce U and Tc and thus has potential applications for remediation of metal- and radionuclide-contaminated sites where ionizing radiation or other DNA-damaging agents may restrict the activity of more sensitive organisms.

Health impacts of large releases of radionuclides. Roles of micro-organisms in the environmental fate of radionuclides

Micro-organisms play important roles in the environmental fate of radionuclides in both aquatic and terrestrial ecosystems, with a multiplicity of physico-chemical and biological mechanisms effecting changes in mobility and speciation. Physico-chemical mechanisms of removal, which may be encompassed by the general term 'biosorption', include adsorption, ion exchange and entrapment. These are features of living and dead organisms as well as their derived products. In living cells biosorptive processes can be directly and indirectly influenced by metabolism, and may be reversible and affected by changing environmental conditions. Metabolism-dependent mechanisms of radionuclide immobilization include metal precipitation as sulfides, sequestration by metal-binding proteins and peptides, and transport and intracellular compartmentation. Chemical transformations of radionuclide species, particularly by reduction, can result in immobilization. Microbial processes involved in solubilization include autotrophic and heterotrophic leaching, complexation by siderophores and other metabolites, and chemical transformations. Such mechanisms are important components of natural biogeochemical cycles for radionuclides and should be considered in any analyses of environmental radionuclide contamination. Several micro-organism-based biotechnologies, e.g. those based on biosorption or precipitation, are of potential use for the treatment of radionuclide contamination.

Bioremediation of metal contamination

Recent studies have demonstrated that microbes might be used to remediate metal contamination by removing metals from contaminated water or waste streams, sequestering metals in soils and sediments or solubilizing metals to aid in their extraction. This is primarily accomplished either by biosorption of metals or enzymatically catalyzed changes in the metal redox state. Bioremediation of metals is still primarily a research problem with little large-scale application of this technology.

Environmental processes mediated by iron-reducing bacteria

Considerable progress has been made towards enhancing our understanding of the phylogeny, ecology and biogeochemical role of dissimilatory iron-reducing bacteria. The known phylogenetic range of iron-reducing bacteria has expanded considerably, as has the known range of iron minerals that serve as a source of Fe(III) for anaerobic respiration. In addition, the number of biotechnological applications of iron-reducing bacteria, including remediation of soils and sediments contaminated with metals, radionuclides and organics, is rapidly expanding.

Influence of microorganisms on the environmental fate of radionuclides

Microorganisms have a significant influence on the environmental fate of radionuclides in aquatic and terrestrial ecosystems with a multiplicity of physico-chemical and biological mechanisms effecting changes in mobility and speciation. Physico-chemical mechanisms of removal include association with extracellular materials, metabolites and cell walls which are features of living and dead organisms. In living cells, some physico-chemical processes are reversible, influenced by metabolism and changing environmental conditions. Metabolism-dependent mechanisms of radionuclide immobilization include sulphide precipitation, transport and intracellular compartmentation and/or sequestration by proteins and peptides. In addition, chemical reduction to less soluble forms can result in immobilization. Microbial processes involved in radionuclide solubilization include autotrophic and heterotrophic leaching, and complexation by siderophores and other metabolites. Such mechanisms are important components of biogeochemical cycles for radionuclides and should be considered in any analyses of environmental radionuclide contamination. In addition, several microorganism-based biotechnologies are receiving interest as potential treatment methods.