Interfacial mechanisms of phosphate adsorption on calcium-functionalized carboxymethyl chitosan: From macroscopic kinetic-isotherm modeling to molecular simulations.

The targeted recovery of phosphate from complex wastewater matrices remains a critical environmental challenge. Herein, we engineered a mesoporous, calcium-crosslinked carboxymethyl chitosan (CaCs) matrix as a highly selective, high-capacity interfacial adsorbent. Macroscopic solid-liquid studies demonstrated an exceptional maximum adsorption capacity of 358.25 mg P/g at a practical, unadjusted pH of 5.25, outperforming contemporary rare-earth and transition-metal doped composites without requiring hazardous acidification. Rigorous non-linear modeling indicated pseudo-first-order kinetics and heterogeneous Koble-Corrigan isotherm behavior. To elucidate the molecular capture pathways, FTIR spectroscopy was triangulated with Density Functional Theory (DFT) and Metropolis Monte Carlo simulations. These computational models established a synergistic dual-mode mechanism: oxyanions are primarily sequestered via inner-sphere bidentate complexation within localized Ca2+ dimer nodes (Eads = -1.69 eV), heavily stabilized by secondary hydrogen bonding from the biopolymer backbone. Operationally, the CaCs matrix exhibited intense selectivity for phosphate against competing background anions and humic acid. Thermodynamic regeneration via 5.0 M NaCl preserved the fundamental crosslinking architecture, maintaining >94% removal efficiency over five consecutive cycles. Crucially, economic feasibility analysis revealed an exceptionally low Cost Efficiency Index (CEI) of $0.0032 USD/mg P, underscoring its commercial viability. By bridging macroscopic mass-transfer dynamics with quantum-level thermodynamic proof and scalable economics, this study establishes CaCs as a uniquely superior, environmentally benign interface for sustainable nutrient management.