Defluoridation Of FAP-coated Calcite By Induced Crystallization

Fluorine is one of the important constituent elements in the human body.Long-term drinking appropriate concentration of fluoride water is beneficial to human health,while long-term intake of excess fluoride would be harmful for human body such as fluorosis and skeletal fluorosis.This study designed a new seed named FAP-coated calcite for fluoride removal by induced crystallization.The effect of coating percentage,dosage of precipitants,phosphate resource and reacting time on defluoridation was investigated.Finally,the regeneration and reused ability of FAP-coated calcite was discussed.The main conclusions were as follows:（1）The calcite was partially dissolved under the action of phosphate and formed a needle-like structure on the surface,at the same time,Ca²⁺was released into the solution.Then the calcium fluorophosphate crystals generated in the solution were stably attached to the needle-like structure,thus calcium fluorophosphate coating appealed.The morphology and area of the calcium fluorophosphate coating on the surface of calcite changed with the coating time.When the coating time was less than 6 h,the crystal form of calcium fluorophosphate was spherical;when the coating time was more than 6 h,it was rose petal-shaped.Different coating areas were achieved by controlling the coating time.0,20-25%,70-75%,80-85%,90-95%,100%coating areas were in lined with 1 h,2h,4 h,6 h,12 h,24 h.（2）The FAP-coated-calcite with 80～85%of the surface area of the coating performed better on fluoride removal.When used 100%coating percentage calcite as seed crystals,the effluent fluoride concentration was less than 1.5 mg·L^-1 just during the performance of 1-102 BV while only phosphate precipitant was added（phosphorus/fluorine mole ratio was 3:1）.When used 90-95%coating percentage calcite as seed crystals,optimizing the operating conditions of the reactor（extending the hydraulic retention time and adding appropriate amount of phosphoric acid at the same time）could make the effluent fluoride concentration slightly stable below 1.5 mg·L^-1.When used 80-85%coating percentage calcite as seed crystals,during the 0-144 BV run by the reactor,the fluoride concentration in the effluent was always stably below 0.5 mg/L,and the fluorine removal rate was up to 99.80%.（3）During the 696 BV operated by the reactor,the fluoride concentration in the effluent was less than 1.5 mg/L and the p H was always in the neutral range,which met the WHO drinking water standard.The operating conditions were:the height of seed crystal was 30 cm,the molar ratio of PO₄^3-to F^-was 3:1,the phosphate was a mixed solution of Na H₂PO₄ and H₃PO₄（the molar ratio was 4:1）,the contact time in the reactor was 16 min,and the intermittent operation was at room temperature.（4）When fluoride removal was over,the calcium fluorophosphate coating on the surface of the seed crystals with saturated fluoride removal capacity could be separated from the body by ultrasonic cleaning after high-temperature baking.The degree of separation was controlled by controlling the baking or cleaning time,so that the coated calcite was reused for defluoridation.For example,the reactor was re-run with calcite regenerated by ultrasound for 12 hours as a seed crystal,and 279 BV was run.The fluoride concentration in the effluent was stably between 1.19-2.26 mg/L,and the fluorine removal rate was 76.2-87.5%.Performance has been restored to some extent.