Synergistic adsorption mechanisms in bimetallic Zr/Fe-modified chitosan hydrogels for enhanced phosphate removal

This work presents a dual-adsorbent strategy for enhanced phosphate removal via chitosan-based materials with molecular-level mechanistic elucidation. Two complementary adsorbents are designed: spherical bimetallic beads (CSQ@Zr/Fe) and a porous hydrogel (CSQ@Zr). The CSQ@Zr/Fe beads demonstrate outstanding cyclic stability, retaining 84.2 % adsorption capacity after 5 regeneration cycles, while achieving a maximum adsorption capacity of 122.51 mg/g under optimized conditions. The CSQ@Zr hydrogel exhibits superior single-cycle performance with a capacity of 126.50 mg/g and rapid uptake kinetics, reaching adsorption equilibrium within 240 min. Both materials maintain high removal efficiency over a broad pH range (3-9) and show remarkable selectivity in complex ionic environments, with phosphate removal exceeding 90 % even in the presence of competing anions (Cl^-, NO₃^-, SO₄^2-). Comprehensive characterization and simulation studies reveal a synergistic adsorption mechanism dominated by electrostatic interactions (85-92 % contribution to binding energy), complemented by outer-sphere coordination (P-O-Zr: 2.25 Å; P-O-Fe: 1.85 Å) and hydrogen bonding networks. The hydrogel exhibits stronger phosphate confinement (diffusion coefficient: 0.544 × 10^-12 m²/s) compared to the beads (0.685 × 10^-12 m²/s), which correlates with its enhanced adsorption performance. This work provides not only high-performance adsorbents but also fundamental insights into the design of chitosan-based materials for sustainable phosphate removal and recovery applications.