Rapid synthesis of lanthanum metal-organic frameworks for efficient phosphate removal from water: Efficiency, stability, environmental safety, and mechanism

In this study, a flower-like, multistage-structured, aminated lanthanum metal-organic framework, NH2-La-MOF-n, in which n designates the molar ratio of lanthanum chloride to 5-aminoisophthalic acid in the precursor solution, was synthesized using a microwave method at atmospheric pressure for phosphorus recovery from water. The phosphorus recovery process by NH2-La-MOF-n conformed to the Langmuir adsorption isotherm and pseudo-second-order kinetic models, and NH2-La-MOF-4 exhibited the maximum phosphorus adsorption capacity (134.10 mg P/g). Applied in dynamic adsorption mode, NH2-La-MOF-4 efficiently recover phosphate, and the dynamic phosphorus recovery process was consistent with the Clark model (R-2 > 0.96). This MOF was found suitable for phosphorus recovery over a wide pH range from 5.0 to 11.0. The synthesized NH2-La-MOF-4 has high selectivity for phosphorus, even when applied phosphorus recovery from natural water. The efficiency of NH2-La-MOF-4 in recovering phosphate was still above 80 % after five cycles of adsorption-desorption. The risk of phosphorus adsorbed by NH2-La-MOF-4 being re-released into the overlying water is low, and the growth of aquatic plants is not only not affected, but also promoted when the dosage of phosphorus-loaded NH2-La-MOF-4 was smaller than 0.5 g/L. The mechanism of phosphorus recovery by NH2-La-MOF-4 includes electrostatic attraction, p-pi conjugation, complexation, and ligand exchange. This study provides a new way to recover phosphate from water.