Capacitive Deionization Of Hexavalent Chromium From Water By Nickel–Ferric-Layered Double Hydroxide Composite Materials

The toxic heavy metal chromium（Cr（Ⅵ））from industrial activities are discharged into water bodies,which is a serious problem of environmental safety,even harm to human health.In order to purify the wastewater containing high concentration of Cr（Ⅵ）,capacitive deionization（CDI）technology has attracted wide attention due to the advantages of simple operation,low energy consumption,and no secondary pollution among various treatment methods.Moreover,finding electrode materials with high capacitance,excellent electrochemical conductivity,and good stability plays an important role in CDI system.As the layered double hydroxide（LDH）is widely and deeply studied in the field of removing heavy metal ions.It is especially important to combine LDH with the new technology of CDI for removing heavy metal ions.In order to improve the electrode performance of LDH in CDI technology,two kinds of NiFe-LDH-based electrode materials were synthesized by hydrothermal method and in situ polymerization method,and used for Cr（Ⅵ）removal by CDI.The deionization properties and mechanisms of NiFe-LDH-based composites were studied by various characterizations and CDI cyclic experiments.The specific research contents and results are as follows:（1）The composite of NiFe-LDH and molybdenum disulfide（NiFe/MoS₂）was synthesized by hydrothermal method.X-ray diffraction（XRD）spectroscopy,Fourier transform infrared（FTIR）spectroscopy,scanning electron microscopy（SEM）,energy dispersive spectroscopy（EDS）,transmission electron microscopy（TEM）,Brunauer-Emmett-Teller（BET）surface area,X-ray photoelectron spectroscopy（XPS）,cyclic voltammetry（CV）,electrochemical impedance spectroscopy（EIS）and galvanostatic charge/discharge（GCD）were used to characterize NiFe/MoS₂.The characterization results show that NiFe-LDH uniformly distributed in the flower cluster frame of MoS₂.The addition of MoS₂ improves the conductivity,capacitance reversibility,charging efficiency,coulomb efficiency,and stability of NiFe-LDH.Using NiFe/MoS₂ as the anode and activated carbon（AC）as the cathode,the deionization capacity and removal rate of NiFe/MoS₂ for Cr（Ⅵ）（100 mg/L,100 m L）were 49.7 mg/g and 99.4%,respectively,with CDI cyclic experiment conditions of 1.2 V,20 m L/min,and 240 min.The pseudo-second-order kinetic equation well fit the deionization data,and the Langmuir isotherm model described the maximum theoretical electrosorption capacity of Cr（Ⅵ）by NiFe/MoS₂ as106 mg/g.The analysis of characterization and experimental data show that the mechanisms of deionization process included electrostatic attraction,surface complexation,and reduction.（2）The NiFe-LDH and polypyrrole composite material（NiFe/PPy）was synthesized by in-situ polymerization method.The morphology,functional groups,and electrochemical properties of NiFe/PPy were analyzed by a series of characterization methods（XRD,FTIR,SEM,EDS,TEM,BET,XPS,CV,EIS,and GCD）.The results showed that the catenarian PPy perfectly assembled on NiFe-LDH,and significantly improved the electrochemical performance of NiFe-LDH.The deionization properties of aqueous Cr（Ⅵ）by NiFe/PPy was studied with batch CDI cyclic experiments.The deionization capacity of NiFe/PPy for 100 m L100 mg/L Cr（Ⅵ）was 47.9 mg/g,and the maximum theoretical electrosorption capacity was111 mg/g.The experimental data of deionization fit the pseudo-second-order kinetic model,Langmuir and Freundlich isotherm model,respectively.The removal mechanisms included electrostatic attraction,surface complexation,and reduction to low toxic Cr（III）.Therefore,NiFe/PPy with CDI has potential applications in the removal of actual wastewater containing Cr（Ⅵ）.The two NiFe-LDH-based electrode materials were prepared in this dissertation,which has the advantages of straightforward synthesis methods,simple operation,and no secondary pollution.The composites with CDI can effectively remove high concentrations of aqueous Cr（Ⅵ）and the mechanisms involved electrostatic attraction,surface complexation,and reduction.This can offer theoretical support for the removal of Cr（Ⅵ）with LDHs composites by CDI.