Gibbsite (100) and Kaolinite (100) Sorption of Cadmium(II): A Density Functional Theory and XANES Study of Structures and Energies.

Due to the potential toxicity of cadmium (Cd2+) and its presence in various waste products found in the environment, it is necessary to develop methods to attenuate and remediate Cd2+ waste. Sorption of Cd2+ to mineral surfaces is a potential route to accomplish this goal. This work focused on improving our molecular-scale understanding of the chemistry of Cd2+ interactions with gibbsite and kaolinite mineral surfaces. Plane-wave density functional theory (DFT) energy minimization calculations and molecular dynamics simulations were used to study the adsorption energies and the nature of the bonds between Cd2+ and the mineral surfaces for possible inner- and outer-sphere surface complexes. Models resulting from the DFT calculations were used to calculate theoretical XANES spectra that were compared with experimental Cd LIII XANES of aqueous Cd2+ as a proxy for outer-sphere Cd2+ hydrated complexes associated with the mineral surfaces. These studies suggest that Cd2+ would favorably bond to the (100) surfaces of both kaolinite and gibbsite through a bidentate mononuclear interaction. However, the results indicate that mixtures of surface complexes on these minerals are likely.