Iron Monodentate Ligand Substituted Calcium Cross-Linking Gel Beads and Mechanism to Enhance Phosphate Adsorption

Modified adsorption of gel materials has received wide attention, but enhancing the efficiency and rate of phosphate adsorption by gel materials still needs further exploration. In this study, Fe(III) was doped into sodium alginate-based gel beads to analyze the principle of its action in enhancing the adsorption performance of the gel beads by revealing the role of Fe(III) cross-linking. Meanwhile, the adsorption mechanism of phosphate removal was explored. The addition of Fe(III) displaces part of the cross-linking between Ca(II) and sodium alginate, forming a solid monodentate covalent coordination structure with the carboxyl group of sodium alginate. 2% iron doping concentration forms better lattice support and pore structure so that its specific surface area increases and has abundant adsorption sites. Compared with the gel beads without iron, the gel beads with 2% iron realize a 4-fold increase in phosphate adsorption. With a high adsorption efficiency, the adsorption reaction can reach equilibrium in 2 h. The adsorption is mainly monolayer chemisorption, and the maximum adsorption capacity can reach 84.08 mgPO₄³⁻-P/g. Compared with calcium alginate, iron salts are preferentially and rapidly adsorbed to phosphate, followed by the formation of modified phosphates and iron hydrogen phosphate salts and the formation of the corresponding iron salts. The modified gel materials can remove phosphate efficiently in water treatment and provide a reference for the preparation of gel polymerization materials and phosphate adsorption studies.

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