Mechanistic Insights into the Selectivity for Arsenic over Phosphate Adsorption by Fe³⁺-Cross-Linked Chitosan Using DFT

Fe³⁺-cross-linked chitosan exhibits the potential for selectively adsorbing arsenic (As) over competing species, such as phosphate, for water remediation. However, the effective binding mechanisms, bond nature, and controlling factor(s) of the selectivity are poorly understood. This study employs ab initio calculations to examine the competitive binding of As(V), P(V), and As(III) to neat chitosan and Fe³⁺-chitosan. Neat chitosan fails to selectively bind As oxyanions, as all three oxyanions bind similarly via weak hydrogen bonds with preferences of P(V) = As(V) > As(III). Conversely, Fe³⁺-chitosan selectively binds As(V) over As(III) and P(V) with binding energies of −1.9, −1, and −1.8 eV for As(V), As(III), and P(V), respectively. The preferences are due to varying Fe³⁺-oxyanion donor-acceptor characteristics, forming covalent bonds with distinct strengths (Fe-O bond ICOHP values: - 4.9 eV/bond for As(V), - 4.7 eV/bond for P(V), and −3.5 eV/bond for As(III)). Differences in pK_a between As(V)/P(V) and As(III) preclude any preference for As(III) under typical environmental pH conditions. Furthermore, our calculations suggest that the binding selectivity of Fe³⁺-chitosan exhibits a pH dependence. These findings enhance our understanding of the Fe³⁺-oxyanion interaction crucial for preferential oxyanion binding using Fe³⁺-chitosan and provide a lens for further exploration into alternative transition-metal-chitosan combinations and coordination chemistries for applications in selective separations.