Hybridization-reactivity relationship in Pb(II) adsorption on α-Al2O3-water interfaces: A DFT Study

We report on a density functional theory study aimed at comparing the reactivity of differently structured α-Al2O3−water interfaces as probed through adsorption of the Pb(II) cation. We assign the Pb−O bonding in Pb(II)/Al2O3 geometries to in-plane and out-of-plane orbital contributions. From our analysis, the empirically known greater Pb(II) reactivity of α-Al2O3(11̅02) over α-Al2O3(0001) is ascribed to the ability of oxygen functional groups in the corrugated (11̅02) interface to hybridize more effectively with Pb(II) electronic states than oxygen functional groups in the topographically flat (0001) interface. The theoretical evidence of a Pb−O hybridization−reactivity relationship goes beyond bond-valence predictions that cite oxygen functional group coordination as a key predictor of mineral−water interface reactivity. We also report the details of adsorption-induced surface relaxations, including an example of surface hydrogen bond rearrangement, as well as evidence of long-range Pb−O interaction. To further assess the bonding saturation of lead in the optimized Pb(II)/Al2O3 structures, molecular H2O adsorption studies are carried out and shown to support that the cation coordination is largely satisfied through interactions with surface functional groups.