CHARACTERIZATION OF PB(II) ADSORPTION ON HYDRATED MINERAL SURFACES THROUGH ELECTRONIC STRUCTURE CALCULATIONS

Theoretical geochemistry is emerging as an insightful and predictive research field. In this work, we apply state-of-the-art density functional theory electronic structure calculations and analysis methods to the adsorption of lead on hydrated alumina and hematite surfaces. The complex nature of environmental interfaces mandates a thoughtful and layered approach to modeling: A prerequisite step is to employ ab initio thermodynamics to solve for the lowest-energy hydrated surface structures under relevant conditions. Subsequent study of Pb(II) adsorption requires consideration of both surface adsorption sites and proton displacement patterns. Finally, delineation of structure-property relationships is achieved by systematic comparison of adsorption energies, bonding geometries, and electronic structure analysis.

We model inner-sphere Pb(II) adsorption on c-cut and r-cut alumina and hematite surfaces and report the relative energies and details of bonding geometries. Our theoretical adsorption energies reproduce the experimentally observed trend that the surface reactivity towards Pb(II) is ranked as Fe₂O₃(0001) > Al₂O₃(1-102) ~ Fe₂O₃(1-102) >> Al₂O₃(0001), and we present our analysis of the factors governing this order. In addition to a common bulk structure, many of the stable hydrated phases of these oxides are also isostructural. We exploit this outcome by dividing the large pool of c-cut and r-cut hydrated surfaces into subspaces that allow for the isolation of particular parameters. This enables us to identify and decipher the roles of reactivity factors such as oxide composition, surface structure, exposed oxygen functional groups, surface hydrogen bonding, directional Pb-O overlap, local and long-range adsorption-induced surface relaxations, Pb-cation repulsion, and the role of the partially filled hematite d-band. We discuss implications of our results for more complicated adsorption scenarios such as binuclear Pb(II) adsorption and offer predictions about other contaminant/oxide adsorption systems.