LAUSR

Fluoride ion (F−) removal technologies from water (defluoridation) suffer from cost and/or efficiency drawbacks. The hugely available low-cost calcined gypsum (plaster of paris) represents a material of choice, especially in the developing countries. Partially soluble calcined gypsum introduces Ca2+ and SO42− into the water. In this work, the influence of tartaric acid, paracetamol, polyvinyl alcohol and Zn/Al layered double hydroxide additives (either retarders or accelerators) on defluoridation of water by calcined gypsum was studied. These additives act as retarders to the calcined gypsum setting and, hence, delay or disturb the crystallization of gypsum, offering the time for the crystallization of fluoro-compounds containing calcium, fluoride, phosphate and silicate. The more retarded the setting time, the more the Ca2+ is introduced in the solution, and hence, the more efficient the defluoridation process. The values of ΔG0 are negative for all additives, indicating the spontaneity of the fluoride removal process. The results of X-ray diffraction, Fourier transform infrared and scanning electron microscopy of the produced gypsum supported this interpretation.