Andrea Geissler

Geomicrobiological Redox Cycling of the Transuranic Element Neptunium

Microbial processes can affect the environmental behavior of redox sensitive radionuclides, and understanding these reactions is essential for the safe management of radioactive wastes. Neptunium, an alpha-emitting transuranic element, is of particular importance because of its long half-life, high radiotoxicity, and relatively high solubility as Np(V)O(2)(+) under oxic conditions. Here, we describe experiments to explore the biogeochemistry of Np where Np(V) was added to oxic sediment microcosms with indigenous microorganisms and anaerobically incubated. Enhanced Np removal to sediments occurred during microbially mediated metal reduction, and X-ray absorption spectroscopy showed this was due to reduction to poorly soluble Np(IV) on solids. In subsequent reoxidation experiments, sediment-associated Np(IV) was somewhat resistant to oxidative remobilization. These results demonstrate the influence of microbial processes on Np solubility and highlight the critical importance of radionuclide biogeochemistry in nuclear legacy management.

Uranium redox cycling in sediment and biomineral systems

Under anaerobic conditions, uranium solubility is significantly controlled by the microbially mediated reduction of relatively soluble U(VI) to poorly soluble U(IV). However, the reaction mechanism(s) for bioreduction are complex with prior sorption of U(VI) to sediments significant in many systems, and both enzymatic and abiotic U(VI) reduction pathways potentially possible. Here, we describe results from sediment microcosm and Fe(II)-bearing biomineral experiments designed to assess the relative importance of enzymatic vs. abiotic U(VI) reduction mechanisms and the long-term fate of U(IV). In oxic sediments representative of the UK Sellafield reprocessing site, U(VI) was rapidly and significantly sorbed to surfaces and during microbially-mediated bioreduction, XAS analysis showed that sorbed U(VI) was reduced to U(IV) commensurate with Fe(III)-reduction. Additional control experiments with Fe(III)-reducing sediments that were sterilized after bioreduction and then exposed to U(VI), indicated that U(VI) reduction was inhibited, implying that enzymatic as opposed to abiotic mechanisms dominated in these systems. Further experiments with model Fe(II)-bearing biomineral phases (magnetite and vivianite) showed that significant U(VI) reduction occurred in co-precipitation systems, where U(VI) was spiked into the biomineral precursor phases prior to inoculation with Geobacter sulfurreducens. In contrast, when U(VI) was exposed to pre-formed, washed biominerals, XAS analysis indicated that U(VI) was recalcitrant to reduction. Reoxidation experiments examined the long-term fate of U(IV). In sediments, air exposure resulted in Fe(II) oxidation and significant U(IV) oxidative remobilization. By contrast, only partial oxidation of U(IV) and no remobilization to solution occurred with nitrate mediated bio-oxidation of sediments. Magnetite was resistant to biooxidation with nitrate. On exposure to air, magnetite changed from black to brown in colour, yet there was limited mobilization of uranium to solution and XAS confirmed that U(IV) remained dominant in the oxidized mineral phase. Overall these results highlight the complexity of uranium biogeochemistry and highlight the importance of mechanistic insights into these reactions if optimal management of the global nuclear legacy is to occur.

Microbial Communities Associated with the Oxidation of Iron and Technetium in Bioreduced Sediments

Anoxic Tc(IV)-containing sediments representative of the UK Sellafield reprocessing facility were exposed to either air or NO3 − to investigate redox cycling of technetium and iron. With air, oxidation of Fe(II) in the reduced sediments was accompanied by ∼75% mobilization of Tc to solution, as soluble Tc(VII). Nitrate additions resulted in the bio-oxidation of Fe(II), coupled to microbially mediated NO− 3 reduction but was accompanied by only very limited (<5%) mobilization of the reduced, sediment-bound Tc, which remained as Tc(IV). PCR-based 16S rRNA and narG gene analyzes were used to investigate changes in the microbial community during sediment oxidation by air and nitrate. Contrasting microbial communities developed in the different treatments and were dominated by Betaproteobacteria (including Herbaspirillum and Janthinobacterium spp.) in the presence of high NO− 3 concentrations. This suggests that the Betaproteobacteria are involved in the redox cycling of Fe and N in these systems, but are unable to mediate NO3 −-dependent Tc(IV) oxidation. These microorganisms may play a previously unrecognized yet pivotal role in influencing contaminant fate and transport in these environments which can have implications to the long-term stewardship of radionuclide-contaminated sediments.

The microbial ecology of land and water contaminated with radioactive waste: towards the development of bioremediation options for the nuclear industry.

The high financial and environmental costs associated with remediation of land contaminated through 60 years of global nuclear activity has underpinned the development of new passive in situ bioremediation processes for sites contaminated with nuclear waste. Many of these processes rely on successfully harnessing newly discovered natural biogeochemical cycles to manage contamination from key radionuclides. Of particular note are strategies that involve enzymatic and indirect redox transformations of actinides such as uranium, neptunium and plutonium and fission products such as technetium. This chapter will discuss the recent advances that have been made in understanding the microbial colonization of radioactive environments and the biological basis of microbial transformations of radioactive waste in these settings. In addition, the impact of co-contaminants such as nitrate on both the microbial ecology of sediments and radionuclide speciation will also be discussed.

Microbial transformations of actinides in the environment

The diversity of microorganisms is still far from understood, although many examples of the microbial biotransformation of stable, pollutant and radioactive elements, involving Bacteria, Archaea and Fungi, are known. In estuarine sediments from the Irish Sea basin, which have been labelled by low level effluent discharges, there is evidence of an annual cycle in Pu solubility, and microcosm experiments have demonstrated both shifts in the bacterial community and changes in Pu solubility as a result of changes in redox conditions. In the laboratory, redox transformation of both U and Pu by Geobacter sulfurreducens has been demonstrated and EXAFS spectroscopy has been used to understand the inability of G. sufurreducens to reduce Np(V). Fungi promote corrosion of metallic U alloy through production of a range of carboxylic acid metabolites, and are capable of translocating the dissolved U before precipitating it externally to the hyphae, as U(VI) phosphate phases. These examples illustrate the far-reaching but complex effects which microorganisms can have on actinide behaviour.

Biogeochemical changes induced by uranyl nitrate in a uranium waste pile

Treatments with uranyl nitrate induced strong changes in a subsurface bacterial community of a uranium mining waste pile. Most of the bacterial populations, stimulated at the initial stages of the treatment, were affiliated with species able to use the added nitrate for respiration. Mossbauer spectroscopic analysis showed that at the later incubation stages, when nitrate was reduced, reduction of Fe(III) to Fe(II) occurred. Time-resolved laser-induced fluorescence spectroscopic (TRLFS) analysis revealed that most of the added U(VI) was bound in organic and inorganic phosphate phases both of biotic origin.

Changes of bacterial community structure of a uranium mining waste pile sample induced by addition of U(VI)

Several portions of a uranium mining waste pile sample were supplemented with different amounts of U(VI) in form of uranyl nitrate to investigate the interactions between the indigenous bacterial populations and U(VI). Selective sequential extractions showed that most of the supplemented U(VI) remained predominantly in weak complexes and hence bioavailable. Analysis of 16S rDNA clone libraries from untreated and U(VI)-supplemented samples revealed changes in the structure of the bacterial community.