Adsorption of U(VI) on Stoichiometric and Oxidised Mackinawite; a DFT Study

Naturally occurring minerals, such as the iron sulfide mackinawite, play a key role in the remediation of uranium from groundwater systems. Here, density functional theory (DFT) is used to investigate the interaction of uranium with the most stable surface of stoichiometric mackinawite, {001}-S. The high reactivity of the mineral towards oxygen may affect its ability to sequester uranium, therefore two models of oxidised mackinawite are also used to study the effect of surface oxidation on adsorption. Weak adsorption of mononuclear uranyl(VI) complexes is found on stoichiometric mackinawite; however, equivalent adsorption modes on the oxidised mackinawite models generally exhibit stronger adsorption. Some of the most energetically stable DFT structures are found to match well with experimental extended X-ray absorption fine structure (EXAFS) data. The implications for the proposed use of mackinawite as a scavenger material for uranium in groundwater systems are discussed.