Experimental study on fluoride removal from mine water by hydroxylapatite

Excessive fluorine ions in water can cause harm to human body. Hydroxylapatite(HAP)is an ideal new en⁃ vironmental function for removal fluorine. In view of the problem of excessive fluorine content in the mine water， using HAP filter adsorption process，this paper had investigated the influence of empty bed contact time(EBCT)，inlet water filtration speed，pH value and inlet fluorine ion concentration by the adsorption column dynamic experiment. The adsorption mechanism of fluoride on HAP was discussed by means of SEM，EDS and FT⁃IR. The results show that the fluorination removal of HAP is due to the adsorption and replacement reaction between the hydroxyl crystal surface of the filter material and the fluorine ions in water，and there is also a certain degree of HAP decomposition reaction. After the reaction，the HAP filter material surface is Ca ∶ P ∶ F≈5 ∶ 3 ∶ 1，and the hydroxyl apatite has become flu⁃ orapatite. The longer the empty bed contact time，the better fluoride removal，but after increasing EBCT to 80 minutes， it is difficult to reduce the fluoride content in the water. The higher the inlet water filtration rate，the worse the wa⁃ ter treatment effect，when it is more than 0.37 m / h，the contact time between the water fluorine ion and the filter mate⁃ rial is relatively reduced. However，the migration of the fluorine ion to the internal filter material crystal structure is slow，making the filter material surface in a state of “saturated adsorption”，while the internal lattice hydroxyl group is not fully utilized. The concentration of fluoride ions in outgoing water increases with the concentration of fluoride ions in incoming water. When the concentration of fluoride ions become high，the migration of the fluoride ions on the surface of the filter crystal to the lattice is a control step and the rate is slow. Excessive acid and alkali have negative effects on fluoride effect. In the acidic environment， F- is difficult to ionize in aqueous solution， and OH- and F- have competitive adsorption under alkaline conditions.The optimal process conditions for fluoride removal are as fol⁃ lows:EBCT is 80 min，inlet filter rate is 0.37 m / h，and inlet pH is about 7.5. Under this condition，when the incoming water fluoride ion is about 6.0 mg/L，the outgoing water fluoride ion concentration can operate stably at less than 1.0 mg / L. At the inlet water filtration rate of 0.21 m / h，the treatment water can reach 309 Bed volume，and the fluo⁃ rine ion removal rate in the stable operation stage can be above 97.5%. The hydroxyl apatite can be used as an excel⁃ lent adsorbent for the treatment of groundwater containing fluorine in mining areas.