From powder to bead: structure and phosphate capture in magnetic chitosan composites prepared via distinct synthetic pathways

Developing macroscopic, magnetically recoverable adsorbents can improve post-treatment handling, but changes in synthesis route and material form may also alter adsorption behavior. Here, magnetic chitosan powders (MCPs) prepared through a solvothermal route and magnetic chitosan beads (MCBs) formed by alkaline co-precipitation/gelation were compared in terms of structure, phosphate uptake, stability, and magnetic separation. Microscopy, N₂-BET, XPS, FTIR, magnetic measurements, and batch adsorption tests revealed distinct Fe–chitosan organizations associated with the different synthesis routes. Compared with the MCPs, the MCBs exhibited larger relative N 1s fitted contributions assigned to –NH₂–Fe coordination (41.39–50.61%). Although their dry-state N₂-BET surface areas were lower, the MCBs showed greater apparent titratable proton uptake under acidic aqueous conditions and higher phosphate adsorption capacities. Linearized Langmuir fitting yielded model-derived adsorption capacity of 62.11–65.35 mg g⁻¹ for the MCPs and 78.74–96.15 mg g⁻¹ for the MCBs. Phosphate uptake was better described by the pseudo-second-order model and involved both film diffusion and intraparticle diffusion. The combined pH-dependent adsorption, surface charge, FTIR, and XPS results suggest contributions from electrostatic attraction, Fe–phosphate ligand exchange, and hydrogen bonding. During two consecutive adsorption-drying cycles, dissolved Fe remained at or below 0.006 mg L − 1 , and the beads retained their visible form despite pH-dependent swelling. In municipal secondary effluent, the composites achieved phosphate removal ratios of 71–97% and were readily recovered magnetically. Overall, the MCBs combined higher phosphate adsorption capacities with a macroscopic form and convenient magnetic separation under the investigated conditions.