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+-HPO42- 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.