Combined Geometric and Electronic Structure Effects Control the Selective Adsorption of Arsenic over Phosphate Using Transition-Metal Cross-Linked Chitosan: A DFT Study

We present a comprehensive electronic structure-guided investigation of selective oxyanion binding in transition metal-cross-linked chitosan (TMC) sorbents. Using density functional theory (DFT), we evaluated the competitive binding of arsenate As(V), arsenite As(III), and phosphate P(V) across eight first-row transition metals (V-Zn) in +2 and +3 oxidation states and in tri- and tetra-coordinated environments. Statistical correlation and regression modeling revealed that binding strength and selectivity are governed by electronic structure properties, including the energy spread of the TM d-states and their spin-state adaptability upon binding. As(V) selectivity correlates strongly with broader d-orbital energy distribution and greater spin-state reorganization, both of which enhance the metal center's capacity to reorganize electronically in response to oxyanion coordination. These insights go beyond conventional descriptors such as charge or bond strength, emphasizing the dynamic electronic flexibility of the metal center as the key to selective As(V) capture. Among tricoordinated systems, Fe²⁺-CS showed the strongest (-1.25 eV) and most As(V)-selective binding (ΔΔ<i>E</i>_bind = - 0.13 eV vs P(V)), driven by high spin and delocalized d-states. In tetra-coordinated systems, Ni²⁺-CS achieved As(V) selectivity (ΔΔ<i>E</i>_bind = - 0.12 eV vs P(V)) through a cooperative geometry and spin-state transition to higher spin. Oxidation enhanced As(V) binding, with Co³⁺-CS achieving the highest selectivity (ΔΔ<i>E</i>_bind = - 0.23 eV vs P(V)) via a monodentate binding mode and binding-induced spin transition. These findings improve our fundamental understanding of the electronic origins of oxyanion selectivity and establish a tunable design framework for next-generation sorbents targeting arsenic in complex water matrices.