NaLa(CO₃)₂ hybridized with Fe₃O₄ for efficient phosphate removal: Synthesis and adsorption mechanistic study

Effectively eutrophication control and phosphate recovery have received increasing attention in recent years. In this study, a regenerable magnetic NaLa(CO₃)₂/Fe₃O₄ composites (MLC) which includes a novel phosphate-binding lanthanum species NaLa(CO₃)₂ hybridized with Fe₃O₄ nanoparticle was developed through a modified solvothermal method for phosphate removal from contaminated water. Based upon preliminary screening of synthesized MLC with different La-to-Fe molar ratios in terms of phosphate adsorption capacity and synthetic product yield, a MLC composite with a La-to-Fe molar ratio of 2:1 (MLC-21) was selected for further characterization and evaluation. MLC-21 exhibits a high magnetic separation efficiency of 97%, high phosphate adsorption capacity of 77.85 mg P/g, wide applicable scope of pH ranging from 4 to 11, excellent selectivity for phosphate in the presence of competing ions (Cl^-, NO₃^-, HCO₃^-, SO₄^2-, Ca²⁺, and Mg²⁺), good reusability with above 98% desorption efficiency using NaOHNaCl mixture and 83% adsorption capacity remained during five recycles. Furthermore, a real effluent wastewater with phosphate concentration of 1.96 mg P/L was used to verify the performance of MLC-21 through a magnetic separation integrated system (AMSS). By using the response surface methodology (RSM), the optimum parameters were determined to be 0.26 g/L of adsorbent dosage, 26.28 h of adsorption time and 24.12 min of magnetic separation time for meeting the phosphate emission standard of 0.5 mg P/L. The phosphorus in three representative eutrophic water bodies can be efficiently reduced to below 0.1 mg P/L by MLC-21 adsorption at different dosages. Electrostatic attraction and the inner-sphere complexation between La(HCO₃)₂⁺/La(CO₃)₂^- and P via ligand exchange forming LaPO₄ were responsible for the phosphate adsorption mechanisms of MLC.