Removal Of Fluoride From Groundwater By Induced Crystallization

Drinking water with high concentration of fluorine will do harm to the teeth andbones and lead to multiple organs lesions. On account of the problem on complexcourse and difficult operation management of removing high fluorine ground water，it isan important project to research and develop efficient removal process. A method whichadded phosp hate and calcium salt into the high fluorine content water was studied toremove the fluoride in groundwater. Further optimization of the precipitant dosage,stirring time, stirring rate and other parameters was investigated meanwhile the effectsand controlling parameters of self-made calcium fluorophosphate with high purity wasdiscussed, both of which were on basic of selecting the best seed fluorapatite. Finally,the mechanism of removing fluoride by induced crystallization was discussed. The mainconclusions are as follows:(1) The removal efficiency of fluoride ion reached93%under optimal inducedcrystallization condition: adding8g/L fluorapatite, and adding NaH2PO4and CaCl2, theratio of n(Ca): n(P): n(F)is10:5:1, stirring speed shoud be100r/min, the reaction timeshould be1hour. The fluoride ion concentration of effluent can be reduced to below1.0mg/L.(2) The removal rate of fluoride is as low as47.62%and the reaction solution wasacidic with pH4.15when selfmade calcium fluorophosphate was used as crystal alone.The removal efficiency of fluoride was up to92.8%and meets the standard of drinkingwater as the reaction conditions are as follows: the self-made calcium phosphate andcalcite were combined to perform as the crystal with the mass ratio of3:7and the mass concentration was6.328g/L. The addition of NaH2PO4and CaCl2should make the ratioof Ca, P and F is10:4:1. The stirring speed should be100r/min and the reaction timeshould be40min.(3) The mixed cystal was used repeatly to induce the crystallization for removingfluoride ions can keep the the removal efficiency of flouride between80.2%and99.5%.The content of fluoride in calcium phosphate crystal increased gradually from27.12%to61.85%. It was indicated that the increasing crystallization of fluoride in calciumhydrogen phosphate quantity consumes calcite.(4) After comparison of fluorapatite crystal on surface morphology, it was foundthat fluoride ion and precipitating agent generated crystallization, and grows in seed,forming floc structure special, which was good for removal of fluoride. Mechanismstudies showed that induction of seed worded obviously. When seed crystal andprecipitant were added at the same time, the removal efficiency of fluoride reach94.22%and residual fluoride concentration0.56mg/L. However, the removal efficiencyof fluoride was only25.92%without seed crystal. In addition, the removal efficiency offluoride above90%and meet the drinking water standard based on the precondition thateffluent’s at neutral pH using this method.