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Abstract: Technetium is a problematic contaminant at nuclear sites and little is known about how repeated microbiologically-mediated redox cycling impacts its fate in the environment. We explore this question in sediments representative of the Sellafield Ltd. site, UK, over multiple reduction and oxidation cycles spanning ~ 1.5 years. We found the amount of Tc remobilised from the sediment into solution significantly decreased after repeated redox cycles. X-ray Absorption Spectroscopy (XAS) confirmed that sediment bound Tc was present as hydrous TcO2-like chains throughout experimentation and that Tc’s increased resistance to remobilisation (via reoxidation to soluble TcO4-) resulted from both shortening of TcO2 chains during redox cycling and association of Tc(IV) with Fe phases in the sediment. We also observed that Tc(IV) remaining in solution during bioreduction was likely associated with colloidal magnetite nanoparticles. These findings highlight crucial links between Tc and Fe biogeochemical cycles that have significant implications for Tc’s long-term environmental mobility, especially under ephemeral redox conditions.

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Abstract: Calcite formation via an amorphous calcium carbonate (ACC) precursor phase potentially offers a method for enhanced incorporation of incompatible trace metals, including Sr2+. In batch crystallisation experiments where CaCl2 was rapidly mixed with Na2CO3 solutions the Sr2+ : Me2+ ratio was varied from 0.001 to 0.1; and, the pathway of calcite precipitation was directed by either the presence or absence of high Mg2+ concentrations (i.e. using a Mg2+ : total Me2+ ratio of 0.1). In the Mg-free experiments crystallisation proceeded via ACC → vaterite → calcite and average Kd Sr values were between 0.44-0.74. At low Sr2+ concentrations (Sr2+ : Me2+ ratio ≤ 0.01) EXAFS analysis revealed that the Sr2+ was incorporated into calcite in the 6 fold coordinate Ca2+ site. However, at higher Sr2+ concentrations (Sr2+ : Me2+ ratio = 0.1), Sr2+ was incorporated into calcite in a 9-fold site with a local coordination similar to Ca2+ in aragonite, but calcite-like at longer distances (i.e. > 3.5 Å). In the high-Mg experiments the reaction proceeded via an ACC → calcite pathway with higher Kd Sr of 0.90-0.97 due to the presence of Mg2+ stabilising the ACC phase and promoting rapid calcite nucleation in conjunction with higher Sr2+ incorporation. Increased Sr2+ concentrations also coincided with higher Mg2+ uptake in these experiments. Sr2+ was incorporated into calcite in a 9-fold coordinate site in all the high-Mg experiments regardless of initial Sr2+ concentrations, likely as a result of very rapid crystallisation kinetics and the presence of smaller Mg2+ ions compensating for the addition of larger Sr2+ ions in the calcite lattice. These experiments show that the enhanced uptake of Sr2+ ions can be achieved by calcite precipitation via ACC, and may offer a rapid, low temperature, low-cost, method for removal of several incompatible Me2+ ions (e.g. Pb2+, Ba2+, Sr2+) during effluent treatment.

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Abstract: The speciation and fate of neptunium as Np(V)O2+ during the crystallization of ferrihydrite to hematite and goethite was explored in a range of systems. Adsorption of NpO2+ to iron(III) (oxyhydr)oxide phases was reversible and for ferrihydrite, occurred through the formation of mononuclear bidentate surface complexes. By contrast, chemical extractions and X-ray absorption spectroscopy (XAS) analyses showed the incorporation of Np(V) into the structure of hematite during its crystallization from ferrihydrite (pH 10.5). This occurred through direct replacement of octahedrally coordinated Fe(III) by Np(V) in neptunate-like coordination. Subsequent XAS analyses on mixed goethite and hematite crystallization products (pH 9.5 and 11) showed that Np(V) was incorporated during crystallization. Conversely, there was limited evidence for Np(V) incorporation during goethite crystallization at the extreme pH of 13.3. This is likely due to the formation of a Np(V) hydroxide precipitate preventing incorporation into the goethite particles. Overall these data highlight the complex behaviour of Np(V) during the crystallization of iron(III) (oxyhydr)oxides, and demonstrate clear evidence for neptunium incorporation into environmentally important mineral phases. This extends our knowledge of the range of geochemical conditions under which there is potential for long term immobilisation of radiotoxic Np in natural and engineered environments.

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Abstract: Technetium, uranium and neptunium are contaminants that cause concern at nuclear facilities due to their long half8life, environmental mobility and radiotoxicity. Here we investigate the impact of microbial reduction of Fe(III) in biotite and chlorite, and the role that this has in enhancing mineral reactivity towards soluble TcO4, UO2[2+] and NpO[2+]. When reacted with unaltered biotite and chlorite, significant sorption of U(VI) occurred in low carbonate (0.2 mM) buffer whilst U(VI), Tc(VII) and Np(V) showed l ow reactivity in high carbonate (30 mM) buffer. On reaction with the microbially reduced minerals, all radionuclides were removed from solution with U(VI) reactivity influenced by carbonate. Analysis by X-ray absorption spectroscopy (XAS) confirmed reductive precipitation to poorly soluble U(IV) in low carbonate conditions: both Tc(VII) and Np(V) in high carbonate buffer were also fully reduced to poorly soluble Tc(IV) and Np(IV) phases. U(VI) reduction was inhibited under high carbonate conditions. Furthermore, EXAFS analy sis suggested that in the reaction products, Tc(IV) was associated with Fe, Np(IV) formed nano8particulate NpO2 , and U(IV) formed nanoparticulate UO2 in chlorite and was associated with silica in biotite. Overall, microbial reduction of the Fe(III) associated with biotite and chlorite primed the minerals for reductive scavenging of radionuclides: this has clear implications for the fate of radionuclides in the environment.

Open Access

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Abstract: The behaviour of U(VI) in hyperalkaline fluid / calcite systems was studied over a range of U(VI) concentrations (5.27 x 10-5 microM to 42.0 microM) and in two high pH systems, young and old synthetic cement leachate in batch sorption experiments. These systems were selected to be representative of young- (pH 13.3) and old-stage (pH 10.5) leachate evolution within a cementitious geological disposal facility. Batch sorption experiments, modelling, extended X-ray absorption fine structure spectroscopy, electron microscopy, small angle X-ray scattering and luminescence spectroscopy were used to define the speciation of U(VI) across the systems of study. At the lowest concentrations (5.27 x 10-5 microM 232U(VI)) significant U removal was observed for both old and young cement leachates, and this was successfully modelled using a first order kinetic adsorption modelling approach. At higher concentrations (> 4.20 microM) in the young cement leachate, U(VI) showed no interaction with the calcite surface over an 18 month period. Small angle X-ray scattering techniques indicated that at high U concentrations (42.0 microM) and after 18 months, the U(VI) was present in a colloidal form which had little interaction with the calcite surface and consisted of both primary and aggregated particles with a radius of 7.6 ± 1.1 and 217 ± 24 Å, respectively. In the old cement leachate, luminescence spectroscopy identified two surface binding sites for U(VI) on calcite: in the system with 0.21 microM U(VI), a liebigite-like Ca2UO2(CO3)3 surface complex was identified; at higher U(VI) concentrations (0.42 microM), a second binding site of undetermined coordination was identified. At elevated U(VI) concentrations (> 2.10 microM) in old cement leachate, both geochemical data and luminescence spectroscopy suggested that surface mediated precipitation was controlling U(VI) behaviour. A focused ion beam mill was used to create a section across the U(VI) precipitate - calcite interface. Transmission electron microscope images of the section revealed that the calcite surface was coated with a nano crystalline, U containing phase. Selected area electron diffraction images of the U precipitate which was formed at a U(VI) concentration of 4.20 microM were consistent with the formation of calcium uranate. XAS spectroscopy at higher concentrations (> 21.0 microM) suggested the formation of a second U(VI) phase, possibly a uranyl oxyhydroxide phase. These results indicated that in the young cement leachate, U(VI) did not react with the calcite surface unless U(VI) concentrations were very low (5.27 x 10-5 microM). At higher concentrations, speciation calculations suggested that U(VI) was significantly oversaturated and experimental observations confirmed it existed in a colloidal form that interacted with the mineral surface only weakly. In the old cement leachate systems at low concentrations batch sorption and luminescence data suggested that U(VI) removal was being driven by a surface complexation mechanism. However, at higher concentrations, spectroscopic methods suggest a combination of both surface complexation and surface mediated precipitation was responsible for the observed removal. Overall, U(VI) behaviour in hyperalkaline calcite systems is distinct from that at circumneutral pH conditions: at high pH and anything but low U(VI) concentrations, a surface mediated precipitation mechanism occurs; this is in contrast to circumneutral pH conditions where U(VI) surface complexation reactions tend to dominate.