Fluoride removal from water by meixnerite and its calcination product

Fluoride removal from water by a mechanochemically synthesized anion clay (meixnerite) and its calcination product was studied at initial fluoride:meixnerite molar ratios (FI:meix) of 0.1 to 2.0 — the theoretical fluoride uptake limit for meixnerite. Fluoride removal efficiency of calcined meixnerite was higher than uncalcined meixnerite at the same FI:meix, and the difference increased as the initial fluoride concentration increased. For the sorption runs performed at FI:meix = 2.0, 29% and 52% of the uptake capacity were attained for the uncalcined and calcined meixnerites, respectively. Analysis of sorption reaction rate data indicates that fluoride diffusion from solution to intraparticle active sites and its chemical sorption on active sites are important mechanisms in the uptake for both meixnerites, and the intraparticle fluoride diffusion in uncalcined meixnerite was slower than in calcined meixnerite. Moreover, XRD analyses indicate that secondary fluoride-containing phases (nordstrandite and sellaite) precipitated at high initial fluoride concentrations. When fluoride precipitates did not form at lower FI:meix (< 0.6), the higher fluoride uptake by calcined meixnerite is promoted by greater availability of F− ions to the meixnerite interlayers since the interlayers were generated during reaction of the F-containing solution with the calcined material. Thus some F− did not have to diffuse into the interlayers to replace existing OH− ions as it did for the uncalcined meixnerite. At FI:meix ≥ 0.6, precipitation of F-bearing nordstrandite also contributes to calcined meixneriteʹs improved ability to remove fluoride.