A mechanistic insight into anionic phosphate adsorption on developed chitosan.ZnO@metakaolin biocomposite

Due to their high efficiency, chitosan (Cs)-based materials are increasingly utilized for wastewater treatment applications. In this study, a novel Cs-based biocomposite, Cs.ZnO@Mk was developed by blending Cs with zinc oxide (ZnO) and metakaolin (Mk), and was evaluated for its capability as a biosorbent to remove phosphate (PO₄^3-) ions from water. The biosorbent was characterized using FTIR, XRD, TGA, SEM, and XPS techniques. The determined optimal conditions for PO₄^3- removal were pH 4, initial concentration 100 mg/L, and contact time 120 min, achieving 98.9 % PO₄^3- removal efficiency. The adsorption isotherm and kinetic data were fitted well to Langmuir isotherm and pseudo-second-order kinetic models, while thermodynamic study affirmed that the adsorption process was exothermic. XPS analysis suggested that electrostatic attraction and ligand-exchange were the key mechanisms for PO₄^3- adsorption. Further mechanistic insights were gained through density functional theory (DFT) calculations and non-covalent interaction (NCI) analysis. Regeneration and counter-ions effect studies confirmed the effectiveness and stability of Cs.ZnO@Mk during PO₄^3- adsorption. This demonstrates well its potential as a robust biosorbent for effective PO₄^3- removal from industrial effluents.