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 Al 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 ZrCl coagulation for F removal were investigated with the widely used Al(SO) as a reference. The optimum pH range is 4.0-6.0 for ZrCl and 8.0-10.0 for Al(SO). ZrCl was superior to Al(SO) for F removal as the initial F concentration was less than 30.0 mg L. Coexisting substances at environmental concentration levels showed negligible effects on F removal by ZrCl. Besides the better F removal, another advantage of ZrCl over Al(SO) was the much lower residual metal concentration in the pH range of 4.0-11.0. The hydrolysis of Al(SO) was significantly inhibited due to the formation of Al-F complexes while the hydrolysis of ZrCl was not influenced even under strongly acidic conditions. Therefore, F removal by Al(SO) was mainly achieved by preliminary complexation between Al 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 ZrCl. The work here provides a new method for F removal and may shed light on the application of Zr coagulants for other pollutants.