Indexed on: 21 Jun '16Published on: 20 Jun '16Published in: Water Research
Keggin-based aluminum nanoclusters have been shown to be efficient sorbents for the removal of arsenic from water. Obtaining a molecular-level understanding of the adsorption processes associated with these molecules is of fundamental importance, and could pave the way for rational design strategies for water treatment. Due to their size and the availability of experimental crystal structures, Al nanoclusters are computationally tractable at the density functional theory (DFT) level. Here, we compare the reactivity of three aluminum polycations: [Al13O4(OH)24(H2O)12]7+ (Al13), [Al30O8(OH)56(H2O)26]18+ (Al30), and [Al32O8(OH)60(H2O)30]20+ (Al32). We use DFT calculations to determine reactivity as a function of particle topography, using sulfate and chloride as adsorption probes. Our comparative modeling of outer-sphere adsorption of Cl− and SO42− on Al13, Al30, and A132 supports that the unique “hourglass” shape characteristic to Al30 gives rise to relatively strong adsorption in the molecular beltway, as well as a wide range of reaction energies as a function of particle topography. The DFT trends in outer-sphere adsorption energy stated in this paper suggest that the apparent shape-reactivity relationship supersedes reactivity predictions based on oxygen functional group type alone.