The Removal of Phosphate from Aqueous Solutions by Sepiolite/ZrO₂ Composites: Adsorption Behavior and Mechanism

Abstract

The sepiolite/ZrO2 composites were prepared by sepiolite (Sep) modification with zirconium propoxide in toluene, at room temperature for 24 h (Sep –ZrI) or 95 °C for 4h (sample Sep –ZrII). The efficiency of the obtained adsorbents for the removal of phosphate from aqueous solutions at initial pH = 4 and pH = 8, was investigated. Characterization of the samples showed that higher temperature of synthesis for shorter time provided slightly higher content of amorphous Zr-phase, which was deposited on sepiolite fibers as a thin layer and agglomerated nanoparticles. Compared to Sep, the composites had lower point of zero charge and higher specific surface area and pore volume. The adsorption kinetics follows the pseudo second-order model. The adsorption capacities of the composites were approximately the same at both initial pH and higher at initial pH = 4 than at pH = 8. The XPS and ATR-FTIR of Sep-ZrI before and after adsorption identified the formation of inner-sphere complexes as the mechanism of phosphate adsorption. The slow release during desorption with NaOH solution confirmed strong bonds of phosphates with the surface of the composites.