The interactions of strontium and technetium with Fe(II) bearing biominerals: Implications for bioremediation of radioactively contaminated land

At nuclear contaminated sites, microbially-mediated Fe(III) reduction under alkaline conditions opens up the potential for co-treatment of the groundwater contaminants 99Tc, though reduction to less mobile Tc(IV) phases, and 90Sr, through increased sorption and/or precipitation promoted at higher pH. In the experiments described here, microbial enrichment cultures derived from representative Sellafield sediments were used to probe the effect of microbially-mediated Fe(III) reduction on the mobility of 99Tc and Sr (as stable Sr2+ at elevated concentrations and 90Sr2+ at ultra-trace concentrations) under both neutral and alkaline conditions. The reduction of Fe(III) in enrichment culture experiments at an initial pH of 7 or 9 resulted in the precipitation of an Fe(II) bearing biomineral comprised of siderite and vivianite. Results showed that TcO 4 - added at 1.6 × 10−6 M was removed (>80%) from solution concurrent with Fe(III) reduction at both pH 7 and pH 9. Furthermore, X-ray absorption spectroscopy of the reduced biominerals confirmed reduction of Tc(VII) to Tc(IV). To understand Sr behaviour in these systems, Sr2+ was added to enrichment cultures at ultra-trace concentrations (2.2 × 10−10 M (as 90Sr2+)) and at higher concentrations (1.15 × 10−3 M (as stable Sr2+)). In ultra-trace experiments at pH 7, microbially active systems showed enhanced removal of 90Sr compared to the sterile control. This was likely due to sorption of 90Sr2+ to the Fe(II)-bearing biominerals that formed in situ. By contrast, at pH 9, the sterile control showed comparable removal of 90Sr to the microbially active experiment even though the Fe-minerals formed were of very different character in the active (vivianite, siderite) versus sterile (an amorphous Fe(III)-phase) systems. Overall, 90Sr bioreduction experiments showed 60–70% removal of the added 90Sr across the different systems: this suggests that treatment strategies involving bioreduction and the promotion of Fe(III)-reducing conditions to scavenge Tc(IV) are not incompatible with treatment of groundwater 90Sr contamination. In systems with elevated Sr2+ concentrations and an initial pH of 7, microbially active systems showed <20% removal of added Sr2+ following Fe(III) reduction with little or no removal in sterile controls. At pH 9, significant Sr2+ was removed from solution in both sterile and microbially active experiments and was attributed to Sr-sorption to mineral phases both chemically precipitated in sterile controls, and biologically precipitated in the microbially active systems. These results confirm that in systems with an elevated natural or anthropogenic Sr2+ loading, bioreduction at modestly alkaline pH is compatible with co-treatment of both TcO 4 - and 90Sr2+. These data are discussed in terms of aqueous geochemistry trends, X-ray diffraction and morphological data, and thermodynamic modelling. The results demonstrate the potential for removal of trace levels of 99Tc and 90Sr2+ from groundwaters during stimulated bioreduction and highlight that in the presence of stable Sr2+, optimal removal for technetium and strontium is likely to occur under mildly alkaline, reducing conditions.