Water defluoridation by hydrotalcite and takovite and subsequent formation of new fluoride-bearing phases

Hydrotalcite, takovite and their calcination products were used to remove fluoride from water at various molar ratios of initial fluoride to solid (F(initial):hydrotalcite or F(initial):takovite) ranging from 0.1 to 2.0, and their theoretical fluoride uptake limit. X-Ray powder diffraction and 19F magic-angle spinning nuclear magnetic resonance spectra were used to characterize the solid samples and to investigate the fluoride removal mechanisms. Water defluoridation by uncalcined and calcined hydrotalcite attributes mainly to the intercalation of F- into their interlayers and adsorption of F- onto their external surfaces. The fluoride removal percent of calcined hydrotalcite are higher than those of uncalcined hydrotalcite at F(initial):hydrotalcite ratios varying between 0.1 and 1.5, whereas the situation is the reverse at a ratio of 2.0 in the 30 d sorption runs. It was induced by the precipitation of fluoride-bearing nordstrandite and sellaite during a long contact of high concentration fluoride solution with uncalcined hydrotalcite. In contrast, the sorption of fluoride by uncalcined and calcined takovite occurs predominantly on their external surfaces. Fluoride-bearing gibbsite or nordstrandite and NiF2 were formed as the fluoride solutions were treated by uncalcined takovite in the 30 d runs, which enhanced its defluoridation effect. The fluoride removal efficiency of calcined takovite is much lower than uncalcined takovite and calcined hydrotalcite, because the expected restoration of original layered structure of takovite did not happen during the reaction of calcined takovite with the fluoride solution.