Chitosan-CaFe Layered Double Hydroxide Hydrogel Beads for Phosphorus Recovery and Agricultural Reuse

Abstract

Phosphorus (P) loss to aquatic systems poses a dual challenge of eutrophication and depletion of finite phosphate-rock resources. This study developed chitosan (CS) hydrogel beads containing CaFe layered double hydroxide (LDH) for phosphate removal and potential nutrient reuse. Incorporation of CaFe-LDH increased the BET specific surface area from 6 to 28 m² g⁻¹ and enhanced phosphate adsorption. Among six formulations, CS/CaFe4 exhibited the highest phosphate adsorption capacity and was selected for further evaluation. Phosphate adsorption approached equilibrium within approximately 8 h. The maximum experimental adsorption capacity was approximately 42 mg P g⁻¹, and the Langmuir-estimated maximum capacity was 41.8 mg P g⁻¹, although the Freundlich model provided a slightly better fit than the Langmuir model. Phosphate adsorption remained relatively stable from pH 4 to 11 but declined sharply at pH 3. Cl⁻, NO₃⁻, and SO₄²⁻ had limited effects, whereas HCO₃⁻ reduced adsorption and Ca²⁺ and NH₄⁺ enhanced phosphate adsorption. Phosphate-loaded hydrogel released 0.10–0.14 mg P L⁻¹ in each of five sequential 24-h water-extraction cycles. In a 14-day wheat (Triticum aestivum) bioassay, observed seed germination was higher with phosphate-loaded hydrogel than in unamended soil; however, fresh hydrogel produced a similar response, whereas superphosphate produced the highest germination. Seedling length was similar among treatments. Collectively, these results identify CS/CaFe4 as a promising material for P capture and demonstrate that a measurable fraction of adsorbed P can be released from the phosphate-loaded hydrogel. Longer-term soil and plant studies are needed to determine P bioavailability, fertilizer value, and the feasibility of agricultural reuse.