Zirconium-loaded zeolite composites for selective sorption of 1-hydroxyethylidene-1,1-diphosphonic acid (HEDP) from water: Performance and mechanism

Abstract

Three zirconium-loaded zeolite (Zr-Z) composites—namely, Zr4+-Z, ZrO2-Z, and ZrMo2O8-Z—were prepared by co-precipitation of Zr4+, ZrO2, and ZrMo2O8, respectively, with artificial zeolite, and these composites were used for the efficient removal of a representative phosphonate, i.e., 1-hydroxyethylidene-1,1-diphosphonic acid (HEDP). The adsorption efficiency of the three composites for HEDP at an initial concentration of 2 mg/L was more than double that of artificial zeolite. Pseudo-second-order kinetics could fit the adsorption process of the three composites well. The heterogeneous adsorption by Zr4+-Z, ZrO2-Z, and ZrMo2O8-Z was determined by using the Sips model. And the theoretical maximum HEDP-adsorption capacities of Zr4+-Z, ZrO2-Z and ZrMo2O8-Z were 3.52 mg P/g, 3.38 mg P/g and 3.77 mg P/g, respectively, at 308 K. At pH 4–9, the three Zr-Z composites could achieve efficient and stable HEDP adsorption. Among Co-existing ions (Cl−, HCO3−, NO3−, SO42−, Ca2+, Mg2+), 50 mM HCO3− hindered adsorption due to its competition with HEDP. The characterization results show that the adsorption mechanism involved hydrogen bonds, electrostatic attraction, and ligand exchange. In addition, the reusability and column adsorption performance of the Zr-Z composites were explored for the efficient removal of the organic phosphonate HEDP from wastewater in practical applications.