Selective phosphate removal from water and wastewater using metal-based nanoparticles : process fundamentals and removal mechanisms

Abstract

Eutrophication of water bodies is a serious and widespread environmental problem. Achieving low levels of phosphate concentration to prevent eutrophication is one of the important goals of the wastewater engineering and surface water management. Meeting the increasingly stringent standards is feasible in using a phosphate-selective sorption system. In this study, zirconium-, lanthanum-, and cerium-based sorbents were developed for selective phosphate removal from water and wastewater. Two zirconium oxide-based sorbents (i.e. ZrO₂@SiO₂@Fe₃O₄ and ZrO₂@Fe₃O₄) were developed and compared for phosphate removal. The direct coating of ZrO₂ on Fe₃O₄ to form the core/shell ZrO₂@Fe₃O₄ leaded to a significantly ehanced phosphate sorption kinetics and sorption capacity in comparison to ZrO₂@SiO₂@Fe₃O₄. The developed ZrO₂@Fe₃O₄ sorbent showed a high selectivity towards phosphate in the presence of competitive anions, and a high phosphate recovery rate. To further design phosphate-selective sorbents with higher sorption capacity, La(OH)₃/Fe₃O₄ nanocomposites with varied La-to-Fe mass ratios were synthesized through a precipitation and hydrothermal method. The optimized La(OH)₃/Fe₃O₄ (4:1) nanocomposite had fast sorption kinetics, high sorption capacity, and strong selectivity for phosphate in presence of competing ions. Sorption-desorption cyclic experiments demonstrated the good reusability of the La(OH)₃/Fe₃O₄ (4:1) nanocomposite. Mechanism studies indicated that both the ZrO₂@Fe₃O₄ and La(OH)₃/Fe₃O₄ (4:1) removed phosphate by forming the inner-sphere complexation through the ligand exchange process, i.e., hydroxyl groups on the surface of the Zr- and La-based sorbents were replaced by the phosphate. Therefore, increasing the amount of the surface hydroxyl groups of metal-based sorbents can greatly enhance the phosphate sorption performance. Consequently, a straightforward, effective method was proposed for increasing the amount of the surface hydroxyl groups of CeO₂ particles. By simply reducing the volume of water used during the ethylene glycol-mediated solvothermal synthesis of CeO₂ particles, the amount of surface hydroxyl groups was increased, and the phosphate sorption performance was greatly improved.

Date of Award	2019
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology