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

To understand the effect of precipitation pH and coexisting Mg on phosphate adsorption onto zirconium oxide (ZrO), ZrO particles precipitated at pH 5.3, 7.1 and 10.5, i.e., ZrO(5.3), ZrO(7.1) and ZrO(10.5), respectively were prepared and characterized, then their adsorption performance and mechanism in the absence and presence of Mg 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 Mg slightly inhibited the adsorption of phosphate on ZrO(5.3), including the adsorption capacity and rate, but coexisting Mg greatly increased the adsorption capacity and rate for ZrO(7.1) and ZrO(10.5). The enhanced adsorption of phosphate on ZrO(7.1) and ZrO(10.5) in the presence of Mg was mainly due to the formation of Mg-HPO ion pair (MgHPO) in the solution and then the adsorption of MgHPO on the adsorbent surface, forming the phosphate-bridged ternary complex Zr(OPOH)Mg. In the absence of Mg, the maximum phosphate adsorption capacity at pH 7 calculated from the Langmuir isotherm model decreased in the order of ZrO(7.1) (67.3 mg/g) > ZrO(5.3) (53.6 mg/g) ≈ ZrO(10.5) (53.1 mg/g), but it followed the order of ZrO(7.1) (97.0 mg/g) > ZrO(10.5) (79.7 mg/g) > ZrO(5.3) (51.3 mg/g) in the presence of Mg. The results of this work suggest that ZrO(7.1) is more suitable for use as an adsorbent for the effective removal of phosphate from municipal wastewater than ZrO(5.3) and ZrO(10.5), because Mg is generally present in this wastewater.