(538i) Understanding That the H-Bonding Network Controls Se Oxoanion Adsorption on Metal Oxides for Drinking Water Remediation – a Density Functional Theory Study

Oxo-anions such as selenium and arsenic are highly soluble in water and toxic to human and aquatic life at very low concentrations. Adsorption is a promising remediation strategy, particularly in low population densities area and communities with self-run wells, because of its potentially lower cost than the construction of chemical treatment plants. The adsorption of oxo-anions on existing sorbents, such as hematite and alumina, is however, not selective and thus drives up water treatment costs. This problem is exacerbated by the fact that selenium and arsenic oxoanions occur at much lower concentrations than similar S and P ions. This leads to increased financial burden for water treatment. Recent experiments have found that absorptivity and adsorption motif depends on the surface facet exposed. Here we use first principal calculations to understand the facet dependent behavior of hematite based adsorption. We built a hybrid solvent model, including implicit and explicit solvation and van der Waals forces, to compare adsorption energies in inner vs. outer sphere configuration. We correlate the adsorption energies with physical quantities including surface configuration, pH, solvation, bond distances, etc. We find the changes in surface water network has the largest correlation, and thus control, over the adsorption behavior; this effect is more pronounced than the correlation between bare surface-oxyanion adsorption energy. The controlling effect has critical impact because this explains the criticality of pH on adsorption behavior, because pH controls the number of protons on the surface and the oxoanion which control the surface water network, and thus adsorption energy. In addition to shedding light on selenate adsorption on hematite and alumina surfaces, this work highlights the criticality of correctly considering solvation, and outer sphere adsorption, in first principles analysis of oxoanion adsorption.