Efficient recovery of phosphate by Fe3O4/La-MOF: An insight of adsorption performance and mechanism from electrochemical properties

Phosphorus (P) is known as a valuable resource, and recycling it from wastewater can avoid environmental pollution and resource waste. Herein, a regenerable magnetic Fe3O4 and La-based metal–organic framework functional material (Fe3O4/La-MOF) was synthesized by solvent thermal method and its adsorption performances of phosphate were evaluated. The as-prepared magnetic Fe3O4/La-MOF exhibited magnetic separation efficiency of 97.9% and it could maintain the high adsorption capacity over a wide pH range from 3 to 10, as well as showed an excellent P adsorption selectivity in presence of Cl-, NO3–, HCO3–, CO32– and SO42-. The isotherm data were consistent with the Langmuir model, and the adsorption kinetic data conformed to the pseudo-second-order model, revealing that there was chemical adsorption between Fe3O4/La-MOF and phosphate. The electrochemical behavior of Fe3O4/La-MOF before and after phosphate adsorption showed that the band gap energy of LUMO (Lowest Unoccupied Molecular Orbital) and HOMO (Highest Occupied Molecular Orbital) of P adsorbed Fe3O4/La-MOF was lower than that of the material itself, which indicated the successful adsorption of phosphate on Fe3O4/La-MOF and the forming of a perfect hybrid structure. The characterization analysis of FT-IR and XPS illustrated that intra-sphere complex (LaOP) formed between Fe3O4/La-MOF and phosphate by ligand exchange, which might be the dominant mechanism, followed by electrostatic attraction and precipitation. This study highlighted the great promise of magnetic reusable Fe3O4/La-MOF composite for application in phosphate removal and recovery.