(360aj) Determination of Electronic Driving Factor for Selective Adsorption of Arsenic over Phosphorous Oxoanions By Fe(III)-Crosslinked Chitosan Using DFT

Adsorption-based water remediation is a promising “under-the-sink” technology for removing various contaminants from drinking water. Unfortunately, the co-occurrence of target contaminants and their less-toxic, iso-structural counterparts creates significant competition for the often-limited binding sites on conventional adsorbents, thus degrading removal efficiencies. In this work, we performed Density Functional Theory (DFT) calculations to understand the key factors that control the selectivity of arsenic oxoanions, H₂AsO₄^- and H₃AsO₃, and phosphorous oxoanions, H₂PO₄^-­and H₃PO₃, by transition-metal crosslinked chitosan. We found that pure chitosan does not preferentially bind As(V) or P(V) since there was only a marginal difference of ~0.01eV in binding energies of both species. However, crosslinking chitosan with Fe (III) facilitates coordination-complex formation with the oxoanions. Analysis of the Crystal Orbital Hamilton Populations (COHP) revealed that the selectivity of As over P is controlled by the difference in the filling of antibonding molecular orbitals in the metal-ligand bond. Differences in localization of electron density in the HOMO resulted in stronger binding of the charged species, As(V) and P(V), over As(III) and P(III). These results show that slight variations in O partial charges of the oxoanions can provide a valuable tool to distinguish between nearly identical ions and, therefore, preferentially extract one over the other.