Interaction of Chromium(VI) with the α-Aluminum Oxide−Water Interface

Interfacial processes can control the transport, speciation, and ultimate fate of aqueous pollutants in groundwater. Here, we apply resonantly enhanced second harmonic generation as well as the χ (3) technique to study the interaction of chromium(VI) with the (11̄02) α-Al 2 O 3 −water interface. Adsorption isotherm measurements yield free energies of adsorption that are consistent with a hydrogen-bonding mechanism mediated through the outer-sphere solvation shell of chromium(VI). Results from measurements regarding the charge state of the α-Al 2 O 3 −water interface as well as the chromium(VI) saturation surface coverages and the pH-dependence of the chromium(VI) equilibrium binding constants are used to develop a thermodynamic and mass-balanced model that describes the interfacial interactions on the molecular level. Special attention is paid to the interfacial speciation state of chromium(VI) as a function of bulk solution pH. Scaling up, we estimate the mobility of chromium(VI) in alumina-rich soils by using the K d model. This work presents a significant advancement in our understanding of the molecular-level interactions between chromium(VI) and α-Al 2 O 3 and improves our ability to predict the environmental mobility, speciation, and ultimate fate of chromium(VI).