[Adsorption of Phosphate from Aqueous Solutions on Sediments Amended with Magnetite-Modified Zeolite]

Understanding the characteristics of phosphate adsorption onto magnetite-modified zeolite (MZ)-amended sediment is helpful for knowing the exchange behavior and process of phosphorus at the interface between the overlying water and MZ-amended sediment. Furthermore, it is helpful for the application of MZ as an amendment to control phosphorus release from sediment. To achieve this goal, the adsorption of phosphate on unamended and MZ-amended sediments was comparatively investigated using a series of batch experiments, and the fractionation of phosphorus in the phosphate-adsorbed MZ was studied using a sequential extraction process. The kinetic data of phosphate adsorption onto unamended and MZ-amended sediments were more suitably fitted to the Elovich model than to the pseudo-first-order and pseudo-second-order models. The equilibrium adsorption data of phosphate adsorption onto the unamended and MZ-amended sediments were well described by the Langmuir, Freundlich, and Dubinin-Radushkevic isotherm model. The phosphate adsorption performance of the unamended and MZ-amended sediments decreased with increasing solution pH from 4 to 11. The presence of cations, such as K⁺, Mg²⁺, and Ca²⁺, enhanced the adsorption of phosphate on the unamended and MZ-amended sediments, and the promoting effect decreased in the order of Ca²⁺ > Mg²⁺ > K⁺, whereas the presence of HCO₃^- inhibited the adsorption of phosphate. The mechanisms for phosphate adsorption onto the unamended and MZ-amended sediments involved electrostatic attraction and ligand exchange, while the mechanism for the adsorption of phosphate on MZ in the amended sediment involved ligand exchange. The sequential extraction analysis of phosphate-adsorbed MZ showed that 49.4% of phosphorus in MZ existed in the mobile form (NH₄Cl-P, BD-P, and NaOH-nrP), which could be easily released from MZ. Therefore, the used MZ should be recovered from sediment using external magnetic fields after its application. The results of this study indicated that MZ is a promising sediment amendment for the control of internal loading in rivers.