Adsorption and subsequent reduction of U(VI) onto clay minerals containing Fe(II) can control the mobility of uranium in subsurface environments. Clays such as montmorillonite provide substantial amounts of the surface area for adsorption of metal contaminants in many subsurface environments, and montmorillonite-containing materials are also proposed to be used as buffer materials in the storage of spent nuclear fuel. Montmorillonite clay was reduced by dithionite to generate reduced montmorillonite comprising of structural Fe(II). We investigated the extent of reduction of U(VI) adsorbed to reduced montmorillonite at different pH and sodium concentrations using X-ray absorption spectroscopy and chemical extractions. More than 99% of U(VI) was reduced to U(IV) at low sodium concentration at both low pH and near-neutral pH. The extent of U(VI) reduction at high sodium concentration and near-neutral pH was only 70%. As assessed through bicarbonate extraction tests, surface bound U(VI) on unreduced montmorillonite was much more labile than surface bound U(IV) on the reduced montmorillonite surface. Uranium adsorbed to the clay at cation exchange sites is more resistant to bicarbonate-induced mobilization than uranium bound to the montmorillonite edge sites. We also developed a non-electrostatic surface complexation model to interpret the speciation of U(IV) adsorbed to reduced montmorillonite as a function of pH and sodium concentration. Our findings established the potential importance of structural Fe(II) in low iron content smectites in controlling uranium mobility in subsurface environments.

U(VI) reduction by structural iron(II) present in montmorillonite

ACS Spring Meeting 2021

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