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. (C) 2016 Elsevier B.V. All rights reserved.