High sorption of phosphate on Mg-Al layered double hydroxides: Kinetics and equilibrium

Abstract

The development of new materials for environmental remediation is a topic of high priority due to the increasingcontamination of water. Although phosphate is not toxic, it has been recognized as one the main species responsiblefor eutrophication of fresh water bodies, and thus remediation techniques are continuously investigatedto remove it from aqueous media. A Mg-Al layered double hydroxide (Mg-Al LDH) was synthesized bythe co-precipitation method at constant pH 9. It was characterized by XRD, FTIR, TG-DSC, SEM and TEM,together with dissolution kinetics, phosphate adsorption kinetics, adsorption isotherms and electrophoreticmobilities. The solid was stable at pH>5 in NaCl aqueous solutions. Lower pH resulted in a fast dissolution, andat pH 3 dissolution was complete after 100 min. Phosphate adsorption kinetics, isotherms and electrophoreticmobility enabled to establish that phosphate adsorbs via three different adsorption modes: anion exchange,electrostatic attraction and surface complexation. The phosphate adsorption capacity of the LDH at pH 5 was2.25 mmol g−1. This is the highest adsorption value when compared with the performance of other phosphatesorbents in the literature. The synthesized LDH, therefore, is a promising environmentally friendly solid to beused in wastewater treatment systems or to remove phosphate from aquatic environments.