The Effect Mechanism Of Water Chemistry Conditions On The Fluoride Removal By Induced Crystallization

Fluoride is a common element in groundwater.Moderate fluoride could confer a desirable benefit regarding dental caries,while long-term intake of excess fluoride would be harmful for human body.The objective of this study is the impact of hardness,pH value,and alkalinity of the groundwater on the fluoride removal by induced crystallization.And the fluoride removal in the high-alkaline groundwater was researched with the jar tests simulating the fluidized bed reactor.The effect mechanism of the fluoride removal under different water chemistry conditions was discussed by the characterization of the minerals and the fitting of MINTEQ.The main conclusions were as follows:?1?The presence of calcium(60.0?150.0 mg·L^-1)and magnesium(60.0?150.0mg·L^-1)could largely promote the removal of fluoride,and the residual fluoride could be less than 1.5 mg·L^-1.Meanwhile,the calcium and magnesium could be also removed effectively.The initial solution pH?5?10?and ion strength?0.001?0.1M?had little effect on the removal of fluoride,calcium and magnesium.The removal of fluoride by Magnesium potassium phosphate?MPP?could be recycled for eight times.The removal of fluoride was controlled by two routes:?i?induced crystallization?the formation of Ca₅?PO₄?₃F on the surface of MPP?,?ii?adsorption?the formation of?MgOH–F with the outer sphere surface complexations?.?2?When the initial solution pH was 4?10 and the contact time was 1 h,the residual fluoride could be satisfied with the WHO guideline.When the initial solution pH was 2and the contact time prolonged to 144 h,the residual fluoride would be less than 1.5mg·L^-1.When the initial solution pH was 12 and the contact time was 144 h,the residual fluoride was 3.47 mg·L^-1.When the initial solution alkalinity was 0?1.95 mM and the contact time was 1 h,the fluoride could be removed effectively.When the initial solution alkalinity was 9.92 mM and the contact time prolonged to 144 h,the residual fluoride could be less than 1.5 mg·L^-1.When the initial solution alkalinity was 17.12 mM and the contact time was 144 h,the residual fluoride was 6.26 mg·L^-1.The fluoride would be removed by the induced crystallization of Ca₅?PO₄?₃F when the initial solution pH?10or initial alkalinity?9.92 mM.And the fluoride would be partially removed by the formation of FCO₃-apatite when the initial solution pH was 12 or initial solution alkalinity was 17.12 mM.?3?The FBC run 2791 bed volume?BV?totally.The residual fluoride was less than1.5 mg·L^-1 during the whole FBC performance.The turbidity in the effluent of FBC was1.5?5.4 NTU during the performance of 0?920 BV.The effluent turbidity in FBC was less than 1.0 NTU during the performance of 921?2791 BV.The fine particles associated with turbidity would be intercepted by the clarify layer so that the turbidity in the FBC effluent could be less 1.0 NTU.The fluoride could be removed by the formation of Ca₅?PO₄?₃F both by the homogeneous nucleation and the heterogeneous nucleation.