Effect of precipitation pH and coexisting magnesium ion on phosphate adsorption onto hydrous zirconium oxide

To understand the effect of precipitation pH and coexisting Mg²⁺ on phosphate adsorption onto zirconium oxide (ZrO₂), ZrO₂ particles precipitated at pH 5.3, 7.1 and 10.5, i.e., ZrO₂(5.3), ZrO₂(7.1) and ZrO₂(10.5), respectively were prepared and characterized, then their adsorption performance and mechanism in the absence and presence of Mg²⁺ were comparatively investigated in this study. The results showed that the Elovich, pseudo-second-order and Langmuir isotherm models correlated with the experimental data well. The adsorption mechanism involved the complexation between phosphate and zirconium. Coexisting Mg²⁺ slightly inhibited the adsorption of phosphate on ZrO₂(5.3), including the adsorption capacity and rate, but coexisting Mg²⁺ greatly increased the adsorption capacity and rate for ZrO₂(7.1) and ZrO₂(10.5). The enhanced adsorption of phosphate on ZrO₂(7.1) and ZrO₂(10.5) in the presence of Mg²⁺ was mainly due to the formation of Mg²⁺-HPO₄^2- ion pair (MgHPO₄⁰) in the solution and then the adsorption of MgHPO₄⁰ on the adsorbent surface, forming the phosphate-bridged ternary complex Zr(OPO₃H)Mg. In the absence of Mg²⁺, the maximum phosphate adsorption capacity at pH 7 calculated from the Langmuir isotherm model decreased in the order of ZrO₂(7.1) (67.3 mg/g) > ZrO₂(5.3) (53.6 mg/g) ≈ ZrO₂(10.5) (53.1 mg/g), but it followed the order of ZrO₂(7.1) (97.0 mg/g) > ZrO₂(10.5) (79.7 mg/g) > ZrO₂(5.3) (51.3 mg/g) in the presence of Mg²⁺. The results of this work suggest that ZrO₂(7.1) is more suitable for use as an adsorbent for the effective removal of phosphate from municipal wastewater than ZrO₂(5.3) and ZrO₂(10.5), because Mg²⁺ is generally present in this wastewater.