Removal of phosphate from aqueous solution by a novel Mg(OH)2/ZrO2 composite: Adsorption behavior and mechanism

Abstract

In this work, Mg(OH)2 was incorporated into ZrO2 to obtain a novel Mg(OH)2/ZrO2 composite (MZ) by a simple co-precipitation method for phosphate adsorption. For comparison purpose, pure ZrO2 and Mg(OH)2 were also fabricated. The as-prepared MZ, ZrO2 and Mg(OH)2 were characterized by SEM, EDS, N2 adsorption/desorption and pHPZC, and their phosphate adsorption properties were comparatively investigated in batch mode. The MZ samples before and after phosphate adsorption were characterized by XPS, and the phosphate-adsorbed MZ was characterized by 31P NMR. The results showed that MZ was mainly composed of ZrO2 and Mg(OH)2. In addition, MZ had a larger specific surface area than single-component ZrO2 and Mg(OH)2. MZ exhibited a much higher affinity towards phosphate in aqueous solution than pure ZrO2 and Mg(OH)2. The Langmuir maximum phosphate adsorption capacity of MZ was 87.2 mg PO4/g at initial pH 7, which was 84% and 4.3 times higher than those of single-component ZrO2 and Mg(OH)2, respectively. Phosphate was adsorbed onto the MZ surface mainly via a ligand exchange between the ZrO2 phase and phosphate forming inner-sphere phosphate complexes as well as the reaction between the Mg(OH)2 phase and phosphate forming Mg-P compounds such as MgHPO4 and Mg3(PO4)2. The enhanced phosphate adsorption by the incorporation of Mg(OH)2 into ZrO2 could be attributed to: (i) the increase in the specific surface area of the adsorbent material; and (ii) the enhanced adsorption of phosphate on the ZrO2 component by the presence of Mg2+ that was dissolved from the Mg(OH)2 component. Results of this work indicate that MZ is a more promising adsorbent for the removal of phosphate from wastewater than Mg(OH)2 and ZrO2.