Rapid phosphate removal by La-Cit hybridized with Fe3O4: Performance and mechanism analysis

The efficient removal of phosphate from aqueous systems is critical for mitigating eutrophication and enhancing aquatic ecological sustainability. In this study, a regenerable magnetic composite material, designated as MLCit, was synthesized via an in-situ embedding method. This composite integrates a novel phosphate-binding lanthanum citrate complex (La-Cit) with Fe 3 O 4 nanoparticles to achieve efficient phosphate removal. Comparative analysis revealed that MLCit-1 (La/Fe molar ratio = 1:1) exhibits enhanced adsorption performance relative to four other magnetic lanthanum-based composites (La 3+ /Fe 3 O 4 , La 2 (CO 3 ) 3 /Fe 3 O 4 , La(OH) 3 /Fe 3 O 4 , and La 2 O 3 · x H 2 O/Fe 3 O 4 ). Specifically, MLCit-1 demonstrated a phosphate adsorption capacity 1.5–9 times higher than these counterparts, with an initial adsorption rate 1.46–2.22 times greater. The phosphate adsorption capacity increased significantly with more lanthanum proportion in MLCit. Structural characterization confirmed that MLCit-4 (La/Fe molar ratio = 4:1) adopts a core–shell structure, comprising Fe 3 O 4 nanoparticles encapsulated by amorphous La-Cit. The maximum adsorption capacity of MLCit-4 reached 77.85 mg P/g, with equilibrium achieved within 3 h at a dosage of 0.5 g/L. MLCit-4 exhibited broad pH applicability (4–11), exceptional anti-interference ability, and achieved residual phosphate concentrations below 0.05–0.1 mg P/L in four environmental water samples (TP < 5 mg P/L). The exhausted MLCit-4 could be easily regenerated by 3 h treatment of the 2.5 M NaOH and 1.875 M NaCl mixture. Remarkably, MLCit-4 retained its phosphate removal efficacy over 20 consecutive adsorption–desorption cycles, while simultaneously enabling the enrichment of phosphate in the desorbent. Mechanistic studies identified ligand exchange between P and citrate, forming inner-sphere complexes, with supplementary contributions from electrostatic interactions.