Removal Of Cr(Ⅵ) From Brackish Wastewater By Flow-electrode Capacitive Deionization

The heavy metal chromium(Cr)released into the water environment is posing a great threat to human health and ecosystem.Cr mainly exists in nature as trivalent chromium(Cr(III))and hexavalent chromium(Cr(VI)).Cr(VI)has carcinogenic and mutagenic effects,and has stronger toxicity and fluidity than Cr(III).Cr(VI)is mainly introduced into the water environment from effluents of smelting,painting,dyeing,electroplating and other industries.According to China’s comprehensive wastewater discharge standard,the maximum allowable concentration of Cr(VI)that can enter the environment is 0.5 mg/L.In order to reduce the concentration of Cr(VI)below the strictly allowed lower limit,e-ffluents containing Cr(VI)must be treated with effective techniques.Capacitive deionization(CDI),has become a promising technology,which can easily remove charged ion species from aqueous solutions.In previous studies,the feasibility of removing Cr(VI)from aqueous solution was studied by conventional CDI,and the parameters were optimized.However,for conventional CDI,its ability to remove ions is limited due to the static characteristics of the electrode.In addition,the preparation process of the fixed electrode is complicated.In addition,after ion electro-adsorption,the electrode needs to be regenerated for a period of time by short circuit or polarity reversal,which makes the production of purified water discontinuous.Compared with the traditional CDI,the flowing electrode capacitive deionization(FCDI)using flowing carbon suspension has great advantages because of its simple electrode preparation,high adsorption capacity,continuous ion removal and high water recovery rate.Common patterns in experiments is the short-circuited closed cycle(SCC)operation,which realizes continuous deionization in FCDI through the off-site regeneration of the flowing electrode without additional desorption steps.In SCC operation mode,the charged anode and cathode carbon suspensions flowing out of FCDI battery are continuously mixed in a container(charge neutralization occurs in the container,and adsorbed ions are released,so that carbon particles can be regenerated),and then recycled to FCDI battery.Considering the advantages and characteristics of FCDI technology,FCDI technology is expected to provide a new solution and an advantageous method for the removal of Cr(VI)in aqueous solution.Objective of this study is to investigate the suitability and effectiveness of FCDI in treating salty wastewater containing Cr(VI).The circulation of flow electrode slurry adopts SCC operation mode,and the brackish water containing Cr(VI)adopts single-pass operation mode.The effects of applied current density,influent concentration of coexisting Cl~-,hydraulic retention time(HRT),dosage of activated carbon,initial p H of influent and other common coexisting anions on the removal efficiency were systematically investigated.The results show that,compared with Cl~-,Cr(VI)is preferentially removed and has higher ion selectivity.In many cases,the removal behaviors of Cr(VI)and Cl~-are obviously different.Cl~-concentration in steady-state effluent decreases almost linearly with the increase of current density,while Cr(VI)concentration in steady-state effluent decreases to a certain value with the increase of current density,and the decrease is not significant.Large HRT or activated carbon load is beneficial to reduce the concentration of Cr(VI)and Cl~-in steady effluent.When the initial p H of influent is about 5～6,it is most beneficial to remove Cr(VI)without adjusting the p H value.Different coexisting anions have different degrees of inhibition on the removal of Cr(VI),among which carbonate has the greatest inhibition.This study has certain reference value for popularizing FCDI as a feasible technology for treating bitter and salty wastewater containing Cr(VI).