Preparation Of MnLa-double Hydroxides And Research Of Defluoridation Capacity And Mechanism In Flow-electrode Capacitive Deionization

China is located in the world’s high fluorine zone,there is widespread fluoride contamination in groundwater.With the development of social economy,industrial production will produce a large number of fluoride wastewater,which will pose a serious threat for ecological environment and human health.Capacitive deionization（CDI）has advantage of no secondary pollution,low energy consumption and easy operation compared to other water treatment technologies and has been widely used in the field of seawater desalination.Flow-electrode capacitive deionization（FCDI）,as a new type of electro-adsorption desalination technology emerging in the last decade,has been widely studied for its advantages such as simple electrode preparation,continuous desalting,and large adsorption capacity.The development of FCDI electrode material is the core research content in this field.At present,layered double hydroxides（LDHs）has been widely used in CDI technology due to their advantages such as simple preparation,good chemical stability,interlayer anion regulation,and pseudo capacitance.However,there are few researches on LDHs in the field of FCDI.In this study,a new MnLa-LDHs adsorption material is prepared by coprecipitation and applied in FCDI to study the removal efficiency of the system for F^-in water.Meanwhile,combined with material characterization and electrochemical test results,we try to reveal its defluorination mechanism.Specific research contents are as follows:1)MnLa-LDHs is synthesized by coprecipitation method at room temperature,and the adsorption performance of the material on F^-in water was investigated.When the initial concentration is 100 mg/L,the adsorption capacity can reach 29.44 mg/g.When the initial concentration is 10 mg/L,the F^-removal efficiency is as high as 99.5%.The adsorption capacity decreases with the increase of the p H of raw water.When the p H of raw water is 2,the maximum adsorption capacity can reach 32.48 mg/g.When the p H of raw water is 12,the adsorption capacity is only 4.58 mg/g.The order of influence of coexisting anions on adsorption capacity is as follows:CO₃^2->C₂O₄^2->H₂PO₄^->C₆H₅O₇^3->SO₄^2->NO₃^-≈Cl^-.Anion with a low valence state has little effect on the adsorption capacity of MnLa-LDHs.The adsorption process accords with Langmuir adsorption isothermal model and pseudo-second-order kinetic equation.In thermodynamic simulation,ΔG^θ<0、ΔH^θ>0、ΔS^θ>0。2)With MnLa-LDHs/carbon black as anode and activated carbon/carbon black as cathode materials,Hybrid FCDI is constructed for desalting test.When the applied voltage is 2 v,the distance between plates is 1.5 mm,the slurry flow rate of electrode is 40 m L/min,the influent flow rate is 20 m L/min,the mass of MnLa-LDHs is 5 g,the mass of carbon black is 0.5 g,and the mass of activated carbon is 6 g,the FCDI has the highest desalting efficiency.3)When the p H of raw water is neutral,the system has the highest average salt adsorption rate,adsorption capacity and removal efficiency.In the comparison test of activated carbon anode,it is proved that the introduction of MnLa-LDHs can effectively enhance the defluorination ability of FCDI.Under the influent concentration of 1000 mg/L,the maximum adsorption capacity of F^-is 84.02 mg/g.Under the condition of 10 mg/L influent concentration and 180 min operation,the F^-concentration in water can be effectively reduced to less than 1mg/L.Meanwhile,we evaluate the energy consumption and charge efficiency of the FCDI,in which FCDI unit is 11.6 k W·h/kg-F^-,peristaltic pump unit is 12.5 k W·h/m³,Charge efficiency is 10.99%.After 4 cycles,the salt adsorption capacity of the system is only 59.37 mg/g,which decreases to 47.1%of the initial value.4)The defluorination mechanism of MnLa-LDHs in FCDI system is mainly ion exchange,hydrogen bonding,electrostatic attraction and lanthanum complexation.