Lanthanum-doped silica xerogels for the removal of fluorides from waters.

The objective of this study was to determine the influence of different operational variables on fluoride (F-) removal from waters using lanthanum (La)-doped silica xerogels and the mechanisms involved in this process. Accordingly, four xerogels were synthesized, one acting as blank (X-B), two doped with LaCl3 and dried at different temperatures (X-LaCl and X-LaCl-M), and a fourth doped with La2O3 (X-LaO). The results show that fluorides are only removed when La-doped xerogels are utilized. In addition, X-LaCl yielded the highest adsorption capacity, removing 28.44% of the initial fluoride concentration at a solution pH of 7. Chemical characterization of materials confirmed that fluoride removal from waters is due to the precipitation of LaF3 on the surface of La-doped xerogels. The presence of dissolved organic matter on the aqueous solution also reduce the removal capacity of La xerogels. Finally, analysis of the influence of solution pH revealed that the adsorption capacity of all xerogels was highest at a solution pH of 7.