Zirconia functionalized SBA-15 as effective adsorbent for phosphate removal

Phosphate adsorbents with molecular level dispersions of surface functionalities were prepared by covalent grafting of ZrO2 onto SBA-15. The synthetic adsorbents were characterized by X-ray diffraction, N2 adsorption/desorption, transmission electron microscope, UV–Vis diffuse reflectance spectroscopy, X-ray photoelectron spectroscopy and zeta-potential measurements in terms of surface chemistry and pore structure, and their adsorption properties toward phosphate were examined. All adsorption isotherms could be well described by the Freundlich model. Compared with SBA-15, markedly enhanced phosphate adsorption was observed on ZrO2 functionalized SBA-15, which was attributed to a combined effect of high surface exposure of the Zr–OH group and its strong inner-sphere complexing ability for phosphate. Phosphate adsorption was affected by the pH and ionic strength, wherein the adsorption capacity of the adsorbent increased at low pH and high ionic strength. Additionally, the adsorption process obeyed the pseudo-second-order kinetics and the rate constant decreased with initial phosphate concentration. Findings in this study highlight the potential of using functionalized SBA-15 with highly dispersed ZrO2 as an effective adsorbent for phosphate removal.

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► We prepared SBA-15-Zr with high surface exposure of ZrO2 moieties.
► Single-cycle functionalization leads to monolayer ZrO2 functionality.
► Two- or three-cycle functionalization results in multi-layer ZrO2 functionality.
► Monolayer ZrO2 exhibits higher normalized adsorption capacity for phosphate.