Study On Removal Of Fluoride From Wastewater By Limestone/READ-F Reactor

With the development of modern industry, the application of fluoride compounds is widely used in the industries. At the same time, much high concentration wastewater containing fluoride, which poses threat to environment and people's health, is from the industries following exposition and processing ore containing fluoride, steel and iron, electronic, metal smelting, Al-electrolysis, glass, chemical fertilizer. Water resources are increasingly scarcity in today, the pollution of fluoride is being taken serious by people. Hence, it is important to purify the wastewater containing fluoride to realize the sustainable development of our society.Traditional precipitation methods for fluoride removal generate huge amounts of sludge to be dewatered and the concentration of fluoride in the purified wastewater is far above the national discharge criteria. Limestone is a kind of environmentally- friendly material, which is widely used in pollution remediation, water resource utilization and water treatment. It is an important channel to improve the solubility of limestone to increase the fluoride removal reactivity.A limestone reactor for fluoride removal was designed based on the mechanism of accelerating the dissolution of limestone in acidic solution with CO₂. The reactor functions by equilibrating the fluoride solution with CO₂ gas to increase carbonic acid concentration and forcing limestone to dissolve and CaF₂ to precipitate. Due to the high efficiency of selective adsorption for F^- , the deep purification of wastewater containing fluoride was carried out with READ-F resin. In order to ensure the stability of effluent and expand the application of limestone reactor, the limestone/READ-F reactor was designed to treat the high concentration fluoride wastewater.Simulated wastewater with sodium fluoride and industrial fluoride wastewater were purified with limestone/READ-F reactor. Factors associated with the purification efficiency including initial pH value, retention time, CO₂ flow rate, and feed fluoride concentration, were investigated. Results indicated that limestone/READ-F reactor has a good defluoridation effect, defluoridation ratio of low concentration (≤50mg/L) fluoride wastewater is up to 85% and 98% respectively by limestone reactor and READ-F resin, and the residual fluoride concentration of high fluoride wastewater can meet the national first-grade discharge standard by limestone/READ-F reactor under the optimal conditions. Moreover, the dynamic behavior and defluoridation mechanism of limestone/READ-F reactor were also discussed. It showed that CaF2 was heterogeneous primary nucleation as advantage nucleation and the electric double layer of limestone induced CaF2 crystallization in limestone reactor. The reaction kinetic equation of CaF2 precipitation was rn=3.9771[Ca²⁺]_n[F^-]_n²-2.4353 and the precipitation generated mainly in step, kink and hole of limestone. The ion exchange adsorption of F^- on READ-F was entropy driven occurring mainly in the external surface and macrospore region. It was fitted to pseudo-second-order reaction kinetics and Frendlich adsorption isotherm equation well.