Coagulation removal of fluoride by zirconium tetrachloride: Performance evaluation and mechanism analysis.

Fluoride (F-) pollution is a worldwide issue. Coagulation with aluminum (Al) salts is an efficient and economical method for the removal of F-. However, due to the strong complexation between Al3+ and F-, the residual F- and Al after coagulation usually exceed the limits. Zirconium (Zr) coagulants have drawn increasing attention due to their excellent flocculation ability for organic matter. In this work, the performance and mechanism of ZrCl4 coagulation for F- removal were investigated with the widely used Al2(SO4)3 as a reference. The optimum pH range is 4.0-6.0 for ZrCl4 and 8.0-10.0 for Al2(SO4)3. ZrCl4 was superior to Al2(SO4)3 for F- removal as the initial F- concentration was less than 30.0 mg L-1. Coexisting substances at environmental concentration levels showed negligible effects on F- removal by ZrCl4. Besides the better F- removal, another advantage of ZrCl4 over Al2(SO4)3 was the much lower residual metal concentration in the pH range of 4.0-11.0. The hydrolysis of Al2(SO4)3 was significantly inhibited due to the formation of Al-F complexes while the hydrolysis of ZrCl4 was not influenced even under strongly acidic conditions. Therefore, F- removal by Al2(SO4)3 was mainly achieved by preliminary complexation between Al3+ and F- and subsequent hydrolysis and polymerization of these complexes, while adsorption onto hydrolysates and ion exchange with surface hydroxyl groups were the main ways of F- removal by ZrCl4. The work here provides a new method for F- removal and may shed light on the application of Zr coagulants for other pollutants.