Harnessing pH-Dependent Pathways in Magnetic Ca/Fe Hydroxide Composites: A Multi-mechanistic Approach to Phosphate Removal and Resource Circularity

Excessive phosphate discharge into aquatic systems drives eutrophication while depleting finite phosphorus resources. To address this dual challenge, a magnetic calcium/ferric hydroxide composite was developed via co-precipitation for efficient phosphate recovery. The optimized magnetic calcium/ferric hydroxide composite (Ca:Fe = 3:1) integrates CaFe-layered double hydroxide and Ca(OH) 2 , achieving a phosphate uptake capacity of 224.2   mg/g. Pseudo-second-order kinetics confirmed the chemisorption mechanism, and the Freundlich isotherm revealed a multilayer adsorption behavior. Thermodynamic analysis confirmed the exothermic nature of the process. The composite performed robustly across pH 2 - 12, in the presence of competing anions, and in real water matrices. Mechanism studies identified pH-dependent pathways: electrostatic attraction and ligand exchange on amorphous akageneite under pH<6; hydroxyapatite precipitation and ligand exchange with ferrihydrite at pH≥6. LDH facilitated uptake via ion exchange and self-assembly under high phosphate concentrations (≥600   mg/L) and strongly alkaline conditions (pH≥12). The material maintained its magnetic properties, enabling easy separation and reuse over five cycles with >90% performance retention. This work presents a high-capacity, recyclable adsorbent that supports both eutrophication control and resource recovery, while offering mechanistic insights for designing multifunctional metal-based adsorbents.