Effective Fluoride Removal From Brackish Groundwaters By Flow-electrode Capacitive Deionization(FCDI)under A Continuous-flow Mode

Fresh water means an extremely precious natural resource for human beings,however groundwater was contaminated by factory wastewater as the result of coming industrial age.Among them,fluorine contaminated groundwater is especially harmful to the health of people.Admittedly,moderate fluorine intake is actually beneficial to human.However once fluorine intake of human body exceeds the standard,excessive fluorine will lead to human body suffering from fluorosis and other illness.Thus the treatment of fluoride-contaminated groundwater is of great importance.Flow-electrode capacitive deionization(FCDI)technology is an emerging environmentally friendly,low-energy consuming,and simple-to-operate desalination technology.Therefore,this paper demonstrated the feasibility o f utilizing FCDI technology for treatment of fluoride-contaminated brackish groundwater.First,the desalination performance of the FCDI system in short-circuit closed-cycle(SCC),single-cycle(SC),and independent closed-cycle(ICC)modes was compared under experimentally simulated fluoride-contaminated water samples.It was found that the FCDI system in SCC mode operated most outstandingly.In the SCC mode,the steady-state effluent conductivity was the lowest as well as the system energy consumption.In addition,the p H fluctuation of the FCDI flow electrode slurry in the SCC mode was small enough to neglect its effects on the experimental results.Then,we investigated how these experimental parameters effect the desalination performance of the FCDI system in the SCC mode.Experimental parameters including current density,hydraulic retention time(HRT),activated carbon loading and and feed concentration of coexisting Na Cl.The results indicated that the steady-state effluent chloride ion concentration decreased with elevated current density and dropped faster with increasing HRT or activated carbon loading.In contrast,the steady-state effluent fluoride ion concentration decreased rapidly to a value under low currents and then maintained stable in spite of the continuous increase in applied current as well as the hydraulic residence time and activated carbon content.Thus,a higher ion selectivity could be obtained at lower applied current and smaller HRT with the trade-off being that operation under these circumstances would generate outlet water with little change in conductivity compared t o the influent.On the other hand,the coexisting influent Na Cl concentration had a positive effect on the removal of both fluoride ions and chloride ions.As a result,the steady-state effluent fluoride ion and chloride ion concentrations are significantl y lower at higher influent Na Cl concentrations.Based on the above experimental simulation study of fluoride-contaminated water bodies,we subsequently used an experimental FCDI system to desalinate and treat water samples from subsurface fluoride-contaminated water.In this paper,the working performance of the FCDI system under various experimental parameters and the effect of fluoride removal are systematically studied,which aims to provide some operational experience for the subsequent FCDI system in water defluoridation.