[Comparison of Phosphate Adsorption onto Zirconium-Modified Bentonites with Different Zirconium Loading Levels]

In this study, zirconium-modified bentonites (ZrMBs) with different zirconium loading levels were prepared, and the adsorption behaviors of phosphate on these ZrMBs were comparatively investigated using batch experiments. The results showed that the kinetic process of phosphate on ZrMBs well followed the pseudo-second-second kinetic model. The kinetic process was divided into three stages, including a rapid external surface adsorption stage, a gradual adsorption stage where both the intra-particle diffusion and film diffusion were rate-controlled, and a final equilibrium adsorption stage. The equilibrium adsorption data of phosphate on ZrMBs could be well described by the Langmuir, Freundlich, Sips and Dubinin-Radushkevich isotherm models. Phosphate adsorption onto ZrMBs was more favorable under strongly acidic condition than under weakly acidic or neutral condition, while phosphate adsorption onto ZrMBs under weakly acidic or neutral condition was more favorable than that under alkaline condition. Coexistence of Na⁺ and K⁺ slightly enhanced phosphate adsorption onto ZrMBs, while coexisting Ca²⁺ greatly enhanced the phosphate adsorption. The presence of HCO₃^- or SO^2-₄ inhibited the adsorption of phosphate on ZrMBs. The mechanism for phosphate adsorption onto ZrMBs followed the ligand exchange and inner-sphere complexing mechanism. The phosphate adsorption capacity for ZrMB increased with increasing loading level of zirconium, while the amount of phosphate adsorbed on unit mass of ZrO₂ in ZrMB decreased with increasing loading amount of zirconium in ZrMB. When the loading amount of ZrO₂ in ZrMB increased from 3.61% to 13.15%, the maximum phosphate adsorption capacity (MPAC) for ZrMB increased from 3.83 to 9.03 mg·g^-1, while a further increase in the ZrO₂ loading amount to 19.63% resulted in a slight increase of MPAC to 9.66 mg·g^-1. However, an increase in the loading amount of ZrO₂ in ZrMB from 3.61% to 19.63% caused a decrease of the MPAC for the ZrO₂ located in ZrMB from 106 to 49.2 mg·g^-1. Considering both cost and adsorption capacity of adsorbent, the ZrMB with 13.15% of zirconium loading amount could be more suitably used as an adsorbent to remove phosphate from aqueous solution than the other ZrMBs.