Reduction of NpO2+ and TcO4- at the FeII-montmorillonite-water interface

The fate of the long-lived, redox-sensitive radionuclides neptunium (237Np) and technetium (99Tc) in deep geological radioactive waste repositories is a major environmental issue. Both elements are highly soluble in their oxidized redox state, and are not (TcO4-) or only weakly sorbed (NpO2+) by negatively charged clay minerals. In the presence of adsorbed and/or structural FeII, however, clay minerals have been shown to reduce (co-)adsorbed contaminants such as U and Se [1, 2], thereby increasing the solid-water distribution coefficient (retention) by several orders of magnitude. In order to investigate if, to which extent, and under which conditions NpV and TcVII are reduced to their tetravalent oxidation states, we conducted Np and Tc batch adsorption experiments with iron-free montmorillonite and with DCB-reduced Wyoming (2.9 wt. % Fe) montmorillonite under different experimental conditions (i.e. anoxic, electrochemical reduction, in the absence and presence of dissolved FeII). The final oxidation state and the type of surface complex formed was elucidated by X-ray absorption spectroscopy at Np-L3, Tc-K and Fe-K edges. We show that both adsorbed and structural FeII are able to reduce NpV and TcVII to NpIV and TcIV, respectively, but the extent strongly depends on the available amount of FeII and on the experimental conditions. The reduced Np strongly complexes towards co-adsorbed Fe and no NpO2 formation is observed. In the case of Tc, mainly TcO2-like nanoparticles form. Surface complexation via Fe is only observed at low Tc surface loadings.