Construction Of Asymmetric Capacitive Desalination System And Its Enhancement On Ion Removal

Efficient,energy-saving and environmentally friendly desalination technology is the most effective way to alleviate the global water crisis.Compared with other technologies,capacitive deionization(CDI)is more energy-efficient which relies on the storage of ions in electrode materials to reduce salinity.Enhancement of ion removal in CDI to improve desalination capacity is an important development direction.Many researches have been carried out from the aspects of loading anion and cation exchange membranes,modification of materials and development of composite electrodes.However,there are some shortcomings,such as high cost,limited enhancement rate of desalination and complex material preparation.Here,without modification of materials and development of composite electrodes,the asymmetrical configuration is constructed for enhancement of ion removal from the perspective of packing only one ion exchange membrane and utilizing two different kinds of electrode materials.The main research contents and conclusions are as follows:(1)An asymmetric membrane capacitive deionization device with anion exchange membrane packed alone(AMCDI-AEM)was constructed.A series of anion exchange membrane was lab-synthesized from poly(2,6-dimethyl-1,4-phenylene oxide).The effect of the main membrane properties,such as ion exchange capacity,water uptake and membrane resistance on the desalination performance of AMCDI-AEM was systematically discussed.The results indicate that anion exchange membrane is necessary in membrane-assisted CDI.Compared with the conventional CDI,AMCDI-AEM enhances ion removal,with both the salt adsorption capacity and charge efficiency increased and the long-time stability of the electrodes improved.An anion exchange membrane with high ion exchange capacity,low membrane resistance and low water uptake is beneficial for AMCDI-AEM.In comparison with the asymmetric membrane capacitive desalination device,the symmetric membrane capacitive device integrated with both anion and cation exchange membrane)has higher charge efficiency(95.0 vs.54.7%),but the desalination capacity is almost unchanged(7.2 vs.7.4 mg g-1)(2)The asymmetric capacitive desalination device(CDI-Bi-AC)with bismuth as the anode material and activated carbon as the cathode material is assembled.The removal performance and mechanism of chloride ions were mainly studied.It is found that under the same conditions,the chloride removal capacity of the asymmetric CDI-Bi-AC device is 2?3 times that of the symmetric capacitive desalination device(CDI-AC-AC,with activated carbon as both the anode and cathode materials.Moreover,the pH value of the treated solution can be controlled by changing the applied voltage and the charging time.However,the cycle stability of the asymmetric device needs to be further improved.It is confirmed that the removal of chloride ions is achieved by its oxidation reaction with Bi electrode,which is converted to bismuth oxychloride(BiOCl).(3)Based on the superiority of the asymmetric device for chloride ions removal and the electrochemical inertness of the Bi electrode in single sodium sulfate(Na2SO4)solution,CDI-Bi-AC was further used for selective removal of chloride ions from mixed NaCl and Na2SO4 solution.Results show that although the ability of bismuth electrode to capture chloride ions is suppressed by sulfate ions,bismuth electrode still shows selectivity for chloride ions.The mole ratio of chloride ions to sulfate ions is the key factor affecting selectivity for chloride ions.The selectivity coefficient increases when the mole ratio rises.The highest selectivity coefficient of 4.5 is achieved in mixed solution with the mole ratio of 8 at voltage of 1.6 and 2.0 V.Based on the results,conversion-reaction selective mechanism is proposed for selective removal of cations,that is,the target ions are removed by conversion reaction with electrode material,and most of other ions remain in solution(4)An asymmetric flowable electrode pair configuration with AC as anode and intercalation material(copper hexacyanoferrate,CuHCF)as cathode was constructed and applied in flow-electrode capacitive desalination cell.An asymmetric flow-electrode capacitive desalination system(FCDI-AC-CuHCF)was also contrived,which contained a desalination cell and a concentration cell.A forward voltage and a reversed voltage were respectively applied and desalinated solution and concentrated solution were simultaneously obtained.And semi continuous operation was realized.Results show an appropriate flow rate of the electrode fluid is the basis of the efficient and stable FCDI-AC-CuHCF operation.High voltage favors desalination.Salt removal rate increases from 0.01 to 0.11 mg cm-2 min-1 and salt removal efficiency increases from 11 to 91%respectively when the voltage rises from 1.2 to 2.8 V,and the charge efficiency remains above 90%.Compared with the conventional symmetric system with activated carbon electrode pair(FCDI-AC-AC),the novel asymmetric FCDI-AC-CuHCF system shows superiority at high voltage(2.0-2.8 V)in terms of higher salt removal rate,salt removal efficiency and charge efficiency.