Evaluation of phosphate adsorption on zirconium/magnesium-modified bentonite

Abstract

ABSTRACTIn this work, a zirconium/magnesium-modified bentonite (ZrMgBT) was prepared and characterized by SEM, EDS, XRD and pHPZC. The performance and mechanism of phosphate adsorption onto ZrMgBT was evaluated in detail using batch experiments and 31P NMR. Results showed the adsorption isotherm data were well described by the Langmuir, Freundlich and Dubinin-Radushkevich models, and the kinetic data fitted better to the pseudo-second-order kinetic model than the pseudo-first-order kinetic model. The phosphate adsorption capacity of ZrMgBT was slightly affected by the presence of Na+, K+, Cl−, and , but it was enhanced by coexisting Mg2+ and . The mechanism for phosphate adsorption onto ZrMgBT at pH 7 was mainly the complexation reaction between phosphate and zirconium. In addition, ZrMgBT exhibited more excellent adherence to phosphate than zirconium-modified bentonite (ZrBT). Especially, the maximum monolayer phosphate adsorption capacity for ZrMgBT at pH 7 and 0.5 g/L of adsorbent dosage calculated based on the Langmuir isotherm model (13.0 mg P/g) was 67.5% higher than that for ZrBT. The higher phosphate adsorption capacity for ZrMgBT than ZrBT could be attributed to the higher specific surface area as well as higher Mg2+ releasing ability of the former. The enhancement of phosphate adsorption by the release of Mg2+ from ZrMgBT could be mainly due to the formation of in the solution firstly and then the adsorption of on ZrMgBT forming ≡Zr(OPO3H)Mg on the ZrMgBT surface. In general, we conclude that ZrMgBT is a more promising adsorbent for phosphate removal from aqueous solution than ZrBT.