Dual-phase chitosan incorporation in alginate/Fe(III) hydrogel beads: Multi-scale spectroscopic evidence for gradient-confined phosphate precipitation and implications for adsorbent design.

Phosphorus overloading drives eutrophication in aquatic environments, and reducing phosphate to low concentrations remains a central challenge for water treatment. The rational design of high-performance biopolymer-based adsorbents requires a mechanistic understanding of how hydrogel microstructure governs ion immobilisation, yet such understanding remains incomplete for chitosan (CS)/sodium alginate (SA) systems, where phosphate capture has largely been attributed to generic electrostatic and ligand-exchange models. Here, we propose a dual-phase chitosan incorporation strategy, in which CS is distributed in both the internal alginate matrix and the external cross-linking phase under sustained acidic conditions, and combine it with multi-scale characterisation-including SEM-EDS, XPS, XRD, TEM/SAED, and synchrotron Fe K-edge XANES/EXAFS-to establish how this design governs the phosphate sequestration pathway in CS/SA-Fe/Ca/CS hydrogel beads. The results are consistent with a proposed gradient-confined precipitation pathway: interfacial enrichment by protonated amino groups, inner-sphere Fe-O-P coordination accompanied by displacement of Fe-N and Fe-O-COO- bonds, and spatially constrained crystallization into short-range ordered FePO4-type nanoclusters distributed from the bead surface toward the core. Chitosan amino groups are proposed to act as dynamic regulators through sequential protonation, ligand displacement, and re-protonation, sustaining phosphate capture across a broad pH range. The dual-phase design reconciles the mechanical stability-mass transfer trade-off, and the optimized material delivers a Sips model-derived capacity of 109 mg P g-1 (dry-bead basis), remains recyclable over five adsorption-desorption-re-crosslinking cycles, and lowers phosphate to below 0.5 mg P L-1 in domestic and synthetic wastewater and below 0.3 mg P L-1 in phosphogypsum leachate. These findings link hydrogel microstructure to phosphate immobilisation performance and provide a mechanistic and experimental foundation for the rational design of high-performance polysaccharide-based adsorbents.