Isotope fractionation during oxidation of tetravalent uranium by dissolved oxygen

We conducted laboratory experiments to investigate isotopic fractionations during oxidation of tetravalent uranium, U(IV), by dissolved oxygen. In hydrochloric acid media with the U(IV) dissolved, the δ²³⁸U value of the remaining U(IV) increased as the extent of oxidation increased. The δ²³⁸U value of the product U(VI) paralleled, but was offset to 1.1±0.2‰ lower than the remaining U(IV). In contrast, oxidation of solid U(IV) by dissolved oxygen in 20mM NaHCO₃ solution at pH=9.4 caused only a weak fractionation (~0.1‰ to 0.3‰), with δ²³⁸U being higher in the dissolved U(VI) relative to the solid U(IV). We suggest that isotope fractionation during oxidation of solid U(IV) is inhibited by a "rind effect", where the surface layer of the solid U(IV) must be completely oxidized before the next layer is exposed to oxidant. The necessity of complete conversion of each layer results in minimal isotopic effect. The weak shift in δ²³⁸U of U(VI) is attributed to adsorption of part of the product U(VI) to the solid U(IV) surfaces.