Contaminant Adsorption on Nanoscale Particles: Structural and Theoretical Characterization of Cu2+ Bonding on the Surface of Keggin-Type Polyaluminum (Al30) Molecular Species

The adsorption of contaminants onto metal oxide surfaces with nanoscale Keggin-type structural topologies has been well established, but identification of the reactive sites and the exact binding mechanism are lacking. Polyaluminum species can be utilized as geochemical model compounds to provide molecular level details of the adsorption process. An Al-30 Keggin-type species with two surface-bound Cu2+ cations (Cu2Al30-S) has been crystallized in the presence of disulfonate anions and structurally characterized by single-crystal X-ray diffraction. Density functional theory (DFT) calculations of aqueous molecular analogues for Cu2Al30-S suggest that the reactivity of Al-30 toward Cu2+ and SO42- shows opposite trends in preferred adsorption site as a function of particle topology, with anions preferring the beltway and cations preferring the caps. The bonding competition was modeled using two stepwise reaction schemes that consider Cu2Al30-S formation through initial Cu2+ or SO42- adsorption. The associated DFT energetics and charge density analyses suggest that strong electrostatic interactions between SO42- and the beltway of Al-30 play a vital role in governing where Cu2+ binds. The calculated electrostatic potential of Al-30 provides a theoretical interpretation of the topology-dependent reactivity that is consistent with the present study as well as other results in the literature.