PNNL

Abstract

Four simulated waste streams relevant to the vitrification of Hanford nuclear waste were studied to the evaluate the removal kinetics of technetium-99 (Tc) and co-mingled Cr(VI) during treatment with solid ferrous hydroxide (Fe(OH)2(s)). Simulants were: (i) 1 M NaOH (control system), (ii) 1M NaOH solution with 1570 ppm Cr(VI), (iii) secondary off-gas (OG) waste stream simulant, and (iv) 5M Na LAW (low activity waste). Simulants treated with Fe(OH)2(s) were reacted for 24 hours and sub-sampled periodically to monitor Tc and Cr(VI) 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(VI) removal rates. Based on these results, the majority of Tc and Cr(VI) removal occurs within the first five minutes of simulant contact with Fe(OH)2(s). However, the order in which Tc and Cr(VI) are completely removed from each simulant depends on the simulant chemistry, the preferred reduction pathway, and the nature of the solid phase product, e.g. magnetite (Fe3O4) versus goethite (a-FeOOH). Low pH and low Cr(VI) concentrations favor rapid Tc removal, with XRD and XAS confirming that Fe3O4 readily incorporates reduced Tc(IV) into its structure. High pH, high Cr(VI) concentrations, and the presence of other co-mingled constituents, favor heterogenous Tc removal early in the reaction (1 hour, Tc removal slows as homogenous reduction and removal of Cr(VI) starts to dominate, concurrent with an increase in the formation of FeOOH as the solid phase reaction product.