Kinetics of Co-Mingled 99Tc and Cr Removal During Mineral Transformation of Ferrous Hydroxide

Four simulated waste streams relevant to the vitrification of Hanford nuclear waste were studied to evaluate the removal kinetics of technetium-99 (Tc) and co-mingled Cr(VI) during treatment with solid ferrous hydroxide (Fe(OH)2(s)). Simulants treated with Fe(OH)2(s) were reacted for 24 hours and sub-sampled periodically to monitor Tc and Cr removal. Solution sample analysis during the reaction was coupled with solid phase characterization, e.g. X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD), to establish influence of the solid phase product on Tc and Cr removal rates. Based on these results, the majority of Tc and Cr removal occurs within the first five minutes of simulant contact with Fe(OH)2(s). However, the order in which Tc and Cr are completely removed from each simulant depends on the simulant chemistry, the preferred reduction pathway, and the solid phase product, e.g. magnetite (Fe3O4) versus goethite (α-FeOOH). Low pH and low Cr concentrations favor rapid Tc removal, with XRD an...