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

The development of new materials for environmental remediation is a topic of high priority due to the increasing contamination of water. Although phosphate is not toxic, it has been recognized as one the main species responsible for eutrophication of fresh water bodies, and thus remediation techniques are continuously investigated to remove it from aqueous media. A Mg-Al layered double hydroxide (Mg-Al LDH) was synthesized by the 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 electrophoretic mobilities. The solid was stable at pH>5 in NaCl aqueous solutions. Lower pH resulted in a fast dissolution, and at pH(3) dissolution was complete after 100 min. Phosphate adsorption kinetics, isotherms and electrophoretic mobility 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 was 2.25 mmol g(-1). This is the highest adsorption value when compared with the performance of other phosphate sorbents in the literature. The synthesized LDH, therefore, is a promising environmentally friendly solid to be used in wastewater treatment systems or to remove phosphate from aquatic environments.