Iron‑cerium doped chitosan beads for synchronous removal of phosphate and fluoride from aqueous solutions

Excessive phosphate and fluoride in water bodies pose great risks to ecological environments and human health. It was still a considerable challenge to efficiently address the combined pollution of phosphate and fluoride. In this study, iron‑cerium-doped chitosan beads (Fe/Ce-CB) were prepared using the coprecipitation method, and their synchronous removal performance for phosphate and fluoride was investigated in single and binary systems, including reaction time, adsorption isotherm, solution pH, and coexisting substances. The physicochemical properties of Fe/Ce-CB and the adsorption mechanisms of phosphate and fluoride were determined by various characterization techniques. In a single system, the maximum adsorption capacities of phosphate and fluoride were 75.3 and 40.8 mg g^-1, respectively, surpassing those of most reported chitosan-based adsorbents. In a binary system, phosphate and fluoride adsorption followed the extended Freundlich model, with phosphate adsorption dominating, achieving its maximum adsorption capacity at 65.6 mg g^-1. The phosphate and fluoride adsorption processes obeyed the pseudo-second-order (PSO) and pseudo-first-order (PFO) kinetic models, respectively, with their rate-limiting steps being intraparticle diffusion and external mass transfer, respectively. The adsorption of phosphate and fluoride was influenced by solution pH, and Ca²⁺ ions enhanced their adsorption capacities. Phosphate and fluoride preferentially occupied the adsorption sites with high energy on the Fe/Ce-CB surface during the adsorption processes, and their average site energies were 29.88 and 23.26 kJ mol^-1. Pore filling, electrostatic attraction, and ligand exchange were mainly responsible for phosphate and fluoride adsorption.