Heterogeneous reduction of U 6+ by structural Fe 2+ from theory and experiment

Abstract

Computational and experimental studies were performed to explore heterogeneous reduction of U 6+ by structural Fe 2+ at magnetite (Fe 3O 4) surfaces. Molecular Fe–Fe–U models representing a uranyl species adsorbed in a biatomic bidentate fashion to an iron surface group were constructed. Various possible charge distributions in this model surface complex were evaluated in terms of their relative stabilities and electron exchange rates using ab initio molecular orbital methods. Freshly-cleaved, single crystals of magnetite with different initial Fe 2+/Fe 3+ ratios were exposed to uranyl-nitrate solution (pH ∼ 4) for 90 h. X-ray photoelectron spectroscopy and electron microscopy indicated the presence of a mixed U 6+/U 5+ precipitate heterogeneously nucleated and grown on stoichiometric magnetite surfaces, but only the presence of sorbed U 6+ and no precipitate on sub-stoichiometric magnetite surfaces. Calculated electron transfer rates indicate that sequential multi-electron uranium reduction is not kinetically limited by conductive electron resupply to the adsorption site. Both theory and experiment point to structural Fe 2+ density, taken as a measure of thermodynamic reducing potential, and sterically accessible uranium coordination environments as key controls on uranium reduction extent and rate. Uranium incorporation in solid phases where its coordination is constrained to the uranate type should widen the stability field of U 5+ relative to U 6+. If uranium cannot acquire 8-fold coordination then reduction may proceed to U 5+ but not necessarily U 4+.