Mechanism And Application Of Flow Electrode Capacitive Deionization For Efficient Removal Of Aqueous Copper/Arsenic

With the global industrial development,the problem of aqueous heavy metal pollution is becoming increasingly serious,which threatens the ecological environment and human health.Most conventional water treatment processes can effectively remove cationic and anionic heavy metal pollutants.However,the widespread presence of organic chelating agents in industrial production and natural water can lead to the formation of stable organic chelated heavy metal ions,making them hard to remove by traditional water treatment processes.Moreover,some aqueous heavy metal pollutants exist in stable electrically neutral forms（e.g.H₃As O₃）,resulting the difficulty of removing them utilizing ion adsorption/migration-based removal methods.Flow-electrode capacitive deionization（FCDI）is a newly emerging electrochemical water treatment process in recent years,which has the characteristics of low energy consumption,high efficiency,and excellent stability in continuous operation,and has shown outstanding performance in the removal of heavy metal cationic and anionic pollutants,but research on removal of organic complexed heavy metals and H₃As O₃ has not been reported.In this study,organic chelated Cu and electrically neutral As（Ⅲ）were selected as typical representative pollutants,and the structure and traditional FCDI operation mode were adjusted to treat organic chelated Cu and electrically neutral As（Ⅲ）,and the influencing factors and mechanisms during the removal process were studied.This work provides a new pathway for the removal of aqueous organic chelated and electroneutral heavy metals.The main research results and contents are as follows:1.A flow-electrode capacitive deionization self-enhanced oxidation（FCDI-SEO）system was applied to realize the removal effect and mechanism of Cu（II）in FCDI system under two typical organic complexes ethylenediaminetetraacetic acid（EDTA）and citric acid（CA）,and the effects of operation mode,p H and coexisting ions were investigated.The results showed that FCDI-SEO could effectively remove organic complexed Cu in water bodies.In the FCDI-SEO system,most of the EDTA-Cu（97%）migrated to the anode chamber with negative charge complexed Cu ions,only a small part of EDTA-Cu was dissociated in the desalination chamber to form free Cu migrating to the cathode;while CA-Cu was easily dissociated,about 71%of Cu migrated to the cathode chamber as free Cu²⁺,and 29%of Cu migrated to the anode as complex form.At the initial p H 3.5 and constant current 10 m A,the removal rate of Cu in EDTA-Cu and CA-Cu solutions under the isolation closed-circuit（ICC）mode was as high as 99.3%and 100%,respectively,and the corresponding removal efficiency of EDTA and CA was 99.5%and 95.5%,respectively.After entering the anode chamber,the degradation efficiency of EDTA and CA was 66.1%and 60.2%,respectively.The difference of Cu removal efficiency between ICC and short-circuit closed-circuit（SCC）mode was not large,but the power consumption of SCC mode was slightly lower than that of ICC mode.With the increase of p H from 3.5 to 5.5,the removal efficiency of Cu and organic ligands in EDTA-Cu system changed little,while the removal rate of Cu and organic ligands in EDTA system decreased slightly.When the concentration of sodium ion was 180 mg L^-1（Na₂SO₄ and Na Cl）,the removal efficiency of Cu in EDTA-Cu system was about 80%,and the removal efficiency of Cu in CA-Cu system was about 90%.After operation for five cycles and regeneration of electrode,FCDI-SEO could still achieve stable removal efficiency.2.An asymmetric flow electrode capacitive deionization（AFCDI）system was constructed using a fixed graphite plate as the cathode,which enabled the efficient,continuous and low energy consumption removal of As（Ⅲ）.At the constant voltage of 1.2V,the initial concentration of AFCDI system was 150.0μg L^-1 within 90 min,and the As（Ⅲ）solution with an initial p H of 7.5 was treated to a total As（T-As）concentration of 8.9μg L^-1,with energy consumption of only 0.04 k Wh m^-3,T-As/Cl^-selectivity of 29.8,and the maximum concentration of H₂O₂ of 393.6μmol L^-1.The cathodic reduction of dissolved oxygen to generate H₂O₂ and the electro-adsorption of H⁺in the desalination chamber jointly led to the high degree of self-alkalization of the system（from 7.50 to 10.90 at 1.2V）,which promoted the dissociation and oxidation of As（Ⅲ）.By detecting the valence state of As in the desalination chamber and anode chamber,it was found that As was mainly removed in the form of As（V）.In the range of initial p H 5.5-9.5,the performance of AFCDI treating As（Ⅲ）was almost unaffected,and the T-As removal efficiency was between90.8%-94.1%.Addition of different kinds of support electrolytes had no significant effect on the removal rate of As（Ⅲ）,and the presence of Ca²⁺was found to promote the removal of T-As.In the 20 cycles,the T-As removal efficiency of AFCDI system was 89.3%–94.3%.AFCDI system was able to treat 164.3μg L^-1 As（Ⅲ）in real groundwaters to a T-As concentration of 9.3μg L^-1 in 120 min,with energy consumption of only 0.07 k Wh m^-3.After treatment,the p H value of the alkalized solution could be reduced to close to the initial state through open-circuit operation or aeration.