Effect of calcium ion on phosphate adsorption onto hydrous zirconium oxide

In this study, the behaviors and mechanisms of phosphate adsorption onto hydrous zirconium oxide (HZO) in the absence and presence of Ca2+ were comparatively studied to determine the effect of Ca2+ on the adsorption of phosphate HZO. The adsorption data in the absence and presence of Ca2+ satisfactorily fitted to the Langmuir isotherm and pseudo-second-order kinetic models. The phosphate adsorption in the absence of Ca2+ showed little pH dependence in the range of 4–6, but it decreased sharply with the pH increase at pH above 6. The phosphate adsorption in the presence of Ca2+ showed little pH dependence in the range of 4–9. The mechanism for phosphate adsorption onto HZO mainly followed the inner-sphere complexing mechanism, and the surface chloride and hydroxyl groups played the key role in the phosphate adsorption. The presence of Ca2+ significantly enhanced the adsorption of phosphate on HZO over a pH range of 6–9, and the formation of Ca2+-phosphate species in the phosphate/calcium solution such as CaHPO40 and the formation of phosphate-bridged ternary complexes (Zr-P-Ca) on the HZO surface played the key role in the enhancement of phosphate adsorption by coexisting Ca2+. Results of this work demonstrated the potential use of the HZO for phosphate removal from phosphate/calcium-coexisted wastewater.