Measurement and Modeling of U(IV) Adsorption to Metal Oxide Minerals

Chemical or biological reduction of U(VI) produces a variety of poorly soluble U(IV) species. In addition to uraninite (UO₂) and biomass-associated noncrystalline U(IV), recent research has found adsorbed U(IV) species on mineral surfaces. To build on these observations, we evaluated equilibrium adsorption of U(IV) to magnetite and rutile as a function of pH and total U(IV) loading. Surface complexation models that could successfully simulate the uptake of U(IV) by accounting for UO₂ precipitation and adsorption of U(IV) to both the minerals and the reactor surfaces were developed. Application of the models could determine the conditions under which adsorption as opposed to precipitation would dominate U(IV) uptake with solids. The model-predicted U(IV) surface coverages of the minerals were consistent with a recent spectroscopic study. Such models advance our ability to predict the equilibrium speciation of U(IV) in the subsurface.