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Abstract: Background: Biogeochemical processing of metals including the fabrication of novel nanomaterials from metal contaminated waste streams by microbial cells is an area of intense interest in the environmental sciences. Results: Here we focus on the fate of Ce during the microbial reduction of a suite of Ce-bearing ferrihydrites with between 0.2 and 4.2 mol% Ce. Cerium K-edge X-ray absorption near edge structure (XANES) analyses showed that trivalent and tetravalent cerium co-existed, with a higher proportion of tetravalent cerium observed with increasing Ce-bearing of the ferrihydrite. The subsurface metal-reducing bacterium Geobacter sulfurreducens was used to bioreduce Ce-bearing ferrihydrite, and with 0.2 mol% and 0.5 mol% Ce, an Fe(II)-bearing mineral, magnetite (Fe(II)(III)2O4), formed alongside a small amount of goethite (FeOOH). At higher Ce-doping (1.4 mol% and 4.2 mol%) Fe(III) bioreduction was inhibited and goethite dominated the final products. During microbial Fe(III) reduction Ce was not released to solution, suggesting Ce remained associated with the Fe minerals during redox cycling, even at high Ce loadings. In addition, Fe L2,3 X-ray magnetic circular dichroism (XMCD) analyses suggested that Ce partially incorporated into the Fe(III) crystallographic sites in the magnetite. The use of Ce-bearing biomagnetite prepared in this study was tested for hydrogen fuel cell catalyst applications. Platinum/carbon black electrodes were fabricated, containing 10% biomagnetite with 0.2 mol% Ce in the catalyst. The addition of bioreduced Ce-magnetite improved the electrode durability when compared to a normal Pt/CB catalyst. Conclusion: Different concentrations of Ce can inhibit the bioreduction of Fe(III) minerals, resulting in the formation of different bioreduction products. Bioprocessing of Fe-minerals to form Ce-containing magnetite (potentially from waste sources) offers a sustainable route to the production of fuel cell catalysts with improved performance.

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Abstract: The past 60+ years of global nuclear activity has resulted a signifcant legacy of radioactive contaminated lands which have high economic costs associated with their remediation. Developing clean-up technologies that are environmentally friendly, economically viable and effective in the long-term is key, with in-situ remediation techniques as an important option. However, questions remain regarding the most favorable methods of remediation, and the long-term stability of any immobilised radionuclide(s). Here, we used sediment microcosms to assess the long-term (300 day) stability of immobilised U and Sr formed during anoxic microbial and chemical treatments, and assessed their stability during re-oxidation scenarios (with oxygen or nitrate additions, 100 days). We used six contrasting treatment approaches which resulted in 89 - >99%, and 65 – 95% removal efficiencies for U and Sr, respectively. These included two Zero Valent Iron (ZVI) based products (NANOFER 25S and Carbo-Iron); a slow-release electron donor (Metals Remediation Compound, MRC) to stimulate U(VI) bioreduction alongside a readily bioavailable electron donor control (lactate/acetate mix); electron donor (lactate/acetate) with elevated sulfate to stimulate metal and sulfate reduction; glycerol phosphate to promote both bioreduction of U(VI) and biomineralization of inorganic U/Sr phopshates; and finally a natural attenuation (no remediation agent added) control. X-ray Absorption Spectroscopy (XAS) revealed that whilst aqueous U was removed from solution via multiple mechanisms including sorption, reduction and incorporation, aqueous Sr was mostly removed via outer sphere complexation mechanisms. Re-oxidation with air led to increased U remobilisation (≤89%) compared to nitrate oxidation (≤73%), but neither oxygen or nitrate re-oxidation led to significant Sr remobilisation (≤38%), suggesting Sr speciation may be stable over extended timescales post remediation. Treatments amended with ZVI or glycerol phosphate not only removed the most U and Sr from solution (>99%) but they also retained the most U and Sr following re-oxidation (retaining ≥75% of the originally added U and Sr). XAS analyses suggests that enhanced immbilisation, as seen in the treatments amended with ZVI or glycerol phosphate, may be due to the U/Sr incorporation into mineral phases (i.e., iron oxyhydorxide and phospate pahses) This suggests that optimal (bio)remediation strategies should target both reduction and biomineralisation mechanisms to facilitate radionuclide-mineral incorporation, promoting longer-term stability.

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Abstract: Citrate is a key decontaminant used in the nuclear industry and here we explore its biogeochemical fate in the presence of Ni2+ and U(VI)O22+ under conditions relevant to low level radioactive waste (LLW) disposal. Anaerobic microcosm experiments were performed under nitrate- and sulfate-reducing conditions at between pH 9 and 10. Citrate (1 mM) was supplied as both an electron donor and a potential metal ion complexant. Incubation experiments with citrate, inoculated with nitrate- or sulfate-reducing microbial consortia, were challenged with three different concentrations of Ni: 0.01, 0.1 or 1 mM, or U: 0.005, 0.05, or 0.5 mM. The nitrate- and sulfate-reducing inocula were enriched from well characterised alkaline sediments obtained from high pH lime-workings. A multi-technique approach was adopted to characterise the aqueous geochemistry, solid phase mineralogy, and bacterial communities in each incubation system. In the 0.01 mM Ni systems citrate underwent full biodegradation under both nitrate and sulfate-reducing conditions in less than 15 days. In the sulfate-reducing experiments, 50% of the added 0.01 mM Ni(aq) was removed from solution and black solids formed; SEM and TEM analysis suggested that these were Ni-sulfides. For the higher Ni concentration incubations, no changes were observed in the nitrate-amended experiments. In the sulfate-amended experiments only citrate fermentation was observed, likely because elevated levels of Ni were toxic to nitrate- and sulfate-reducing bacteria in the inocula. Interestingly, although fermentative bacteria were key citrate degraders in the sulfate-amended experiments they did not dominate in the nitrate-amended experiments presumably due to competition from other microbes. In the U experiments, citrate degradation took place over 55 days in all systems except the 0.5 mM U/nitrate-amended incubations. In all U/sulfate-amended experiments, a dark-coloured precipitate formed and XAS analysis indicated that these solids contained reduced U(IV) with EXAFS suggesting that non-crystalline U(IV)–phosphate phases dominated. Microbial community analysis by 16S rRNA gene sequencing of endpoint samples identified fermenters and nitrate- and sulfate-reducing bacteria in the relevant incubations. Overall, findings suggest microbial degradation of citrate occurs under repository relevant conditions with Ni (at 0.01–0.1 mM) and U (at 0.005–0.5 mM) but with an inhibitory effect particularly at elevated Ni concentrations. Significantly, the work suggests that under anaerobic conditions relevant to LLW disposal, citrate undergoes biodegradation leading to the development of poorly soluble Ni sulfides and/or bioreduction of U(VI) to poorly soluble U(IV) phases. This suggests that both removal of citrate, and retention of Ni and U can occur in these environments and this information can be used to further inform development of safety cases for radioactive waste disposal.

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Abstract: The Piauí laterite (NE Brazil) was initially evaluated for Ni but also contains economic concentrations of Co. Our investigations aimed to characterise the Co enrichment within the deposit; by understanding the mineralogy we can better design mineral processing to target Co recovery. The laterite is heterogeneous on the mineralogical and lithological scale differing from the classic schematic profiles of nickel laterites, and while there is a clear transition from saprolite to more ferruginous units, the deposit also contains lateral and vertical variations that are associated with both the original intrusive complex and also the nature of fluid flow, redox cycling and fluctuating groundwater tables. The deposit is well described by the following six mineralogical and geochemical units: SAPFE, a clay bearing ferruginous saprolite; SAPSILFE, a silica dominated ferruginous saprolite; SAPMG, a green magnesium rich chlorite dominated saprolite; SAPAL, a white-green high aluminium, low magnesium saprolite; saprock, a serpentine and chlorite dominated saprolite and the serpentinite protolith. Not all of these units are ‘ore bearing’. Ni is concentrated in a range of nickeliferous phyllosilicates (0.1–25 wt%) including serpentines, talc and pimelite, goethite (up to 9 wt%), magnetite (2.8–14 wt%) and Mn oxy-hydroxides (0.35–19 wt%). Lower levels of Ni are present in ilmenites, chromites, chlorite and distinct small horizons of nickeliferous silica (up to 3 wt% Ni). With respect to Co, the only significant chemical correlation is with Mn, and Mn oxy-hydroxides contain up to 14 wt% Co. Cobalt is only present in goethite when Mn is also present, and these goethite grains contain an average of 0.19 wt% Co (up to a maximum of 0.65 wt%). The other main Co bearing minerals are magnetite (0.41–1.89 wt%), chlorite (up to 0.45 wt%) and ilmenite (up to 0.35 wt%). Chemically there are three types of Mn oxy-hydroxide, asbolane, asbolane-lithiophorite intermediates and romanechite. Spatially resolved X-ray absorption spectroscopy analysis suggests that the Co is present primarily as octahedrally bound Co3+ substituted directly into the MnO6 layers of the asbolane-lithiophorite intermediates. However significant levels of Co2+ are evident within the asbolane-lithiophorite intermediates, structurally bound along with Ni in the interlayer between successive MnO6 layers. The laterite microbial community contains prokaryotes and few fungi, with the highest abundance and diversity closest to ground level. Microorganisms capable of metal redox cycling were identified to be present, but microcosm experiments of different horizons within the deposit demonstrated that stimulated biogeochemical cycling did not contribute to Co mobilisation. Correlations between Co and Mn are likely to be a relic of parent rock weathering rather than due to biogeochemical processes; a conclusion that agrees well with the mineralogical associations.

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Abstract: Selenium (Se) is a toxic contaminant with multiple anthropogenic sources, including 79Se from nuclear fission. Se mobility in the geosphere is generally governed by its oxidation state, therefore understanding Se speciation under variable redox conditions is important for the safe management of Se contaminated sites. Here, we investigate Se behavior in sediment groundwater column systems. Experiments were conducted with environmentally relevant Se concentrations, using a range of groundwater compositions, and the impact of electron-donor (i.e., biostimulation) and groundwater sulfate addition was examined over a period of 170 days. X-Ray Absorption Spectroscopy and standard geochemical techniques were used to track changes in sediment associated Se concentration and speciation. Electron-donor amended systems with and without added sulfate retained up to 90% of added Se(VI)(aq), with sediment associated Se speciation dominated by trigonal Se(0) and possibly trace Se(-II); no Se colloid formation was observed. The remobilization potential of the sediment associated Se species was then tested in reoxidation and seawater intrusion perturbation experiments. In all treatments, sediment associated Se (i.e., trigonal Se(0)) was largely resistant to remobilization over the timescales of the experiments (170 days). However, in the perturbation experiments, less Se was remobilized from sulfidic sediments, suggesting that previous sulfate-reducing conditions may buffer Se against remobilization and migration.