Selective Separation Of Ca²⁺/Na⁺ By Modified Cation Exchange Membrane In Electrodialysis

ion exchange membrane technology is widely used in a variety of fields including lithium cell membrane,hydrogen,oxygen fuel cell,EDI process and electrodialysis water purification.In recent years,even though the rapid development of common ion exchange membrane technique has basically satisfied the needs of various fields,this technique is needed to be improved to have the ability of monovalent selectivity of ions when it comes to the application involving the separation of ions with different valence states,such as the extraction of sodium chloride by electrodialysis concentration,the production of drinking water by seawater desalination,and the recovery and utilization of waste acid,etc.Therefore,the improvement of the monovalent selectivity of ion exchange membrane has become an urgent problem in the field of ion exchange membrane.This experiment developed an ion exchange membrane with univalent selectivity by coating polyethylene imide on the CMV cation exchange membrane of Asahi Glass Co.,LTD.In this experiment,epoxy groups were pre-fixed by grafting methacrylate anhydride oil?GMA?and then modified by coating polyethylene imide.A series of parameters such as CMV film thickness,water content,ion-exchange capacity,swelling degree,contact Angle,ion migration number,film resistance,Zeta potential and other physicochemical properties before and after modification were tested and EIS was used to conduct impedance analysis.Experimental data showed that the modified CMV film had greater impedance and was applicable to Maxwell-Wagner model,which simulated equivalent circuit for the CMV film before and after modification and verified the existence of the modified polyethylene imide layer.Based on EIS simulation calculation,the time constant of different ions passing through polyethylene imide modification layer can be calculated,which can be used to quantitatively calculate the time of Ca²⁺and Na⁺passing through ion exchange membrane.Finally,by applying the CMV cation exchange membrane before and after modification in electrodialysis simulation experiment,the monovalent selectivity of the membrane was compared by measuring Ca²⁺and Na⁺migration Numbers and ion fluxes as functions of current density.The performance of CMV film before and after modification was evaluated by comparing the parameters of desalting efficiency,energy consumption,salinity removal and electrodialysis.The experimental results show that:?1?The existence of polyethylene imide coating was proved by subblue dye reagent in CMV film whose ATR-FTIR spectrum before and after coating with polyethylene imide showed tensile vibration at?1450cm^-1.The reduction in stretching vibration of S=O/C-O at?1180 cm^-1 is due to sulfonic acid functional groups coated on the membrane surface of polyethylene imide?e.g.S=O?while the decrease of c-O bond is caused by the amine group of polyethylenimine reacting with epoxy group.?2?The moisture content of CMV film before and after modification was 25 and 26%.The ion exchange capacity was 2.0 and 2.1 MEq/g dry film.Thickness:0.092 and 0.102mm;The degree of swelling is 38 and 42%;Contact angles are 32 and 37°;The ion migration Numbers were 0.97 and 1.12.All the performance parameters of CMV membrane were increased after modification.This can be attributed to the larger ion exchange capacity in membrane which increased the water content of the ion exchange membrane leading to higher electrical conductivity and ion selective permeability.After modification,the amino group was introduced into the CMV film to form a stable amide bond with the epoxy group in GMA.The surface of CMV was coated with polyethylenimine,which resulted in the increase of film thickness and swelling degree.The increase of ion migration number also indicates the increase of ion migration rate per unit time.Due to the relative hydrophobicity of polyethylenimine,the contact Angle of CMV increased slightly after modification.?3?As a result,Zeta potential measurements showed that the CMV membrane surface was positively charged after the formation of a polyethylene imide layer proving the existence of the polyethylenimine layer and explaining the reason of the change of the film surface layer with increasing monovalent selectivity.?4?EIS measurements provided quantitative evidence for Ca²⁺and Na⁺selective separation by measuring that difference of properties of CMV films before and after modification.The EIS electrochemical impedance spectroscopy was used to define time constant,which quantitatively estimated the transmission time needed for the different ions to pass through the modified layer.When ions pass through the modified layer,an additional characteristic frequency impedance peak F_max is generated.As shown in the Bode diagram,f_maxax is 6 Hz for 0.05m NaCl solution and 0.03m for CaCl₂ solution which indicates that the passing time of Na⁺and Ca²⁺in modified CMV membrane of are 0.03s and 5s respectively,and Na⁺takes priority over Ca²⁺to pass through the ion exchange membrane,thus verifying the monovalent ion selectivity of the modified CMV membrane.For NaCl and CaCl₂ mixed solutions,due to the competition between Ca²⁺and Na⁺,the fmax value of the mixed solution is low.?5?CMV film was applied to electrodialysis simulation experiment before and after modification.Based on limiting current density,ion flux,desalting performance,energy consumption,salinity removal and RTN value were measured.The results show that the desalting efficiency of CMV film is similar before and after modification at the same current density.In the experiment,the change of conductivity was used as a function of energy consumption.When the change of conductivity was 0?20%,the energy consumption tended to be stable.When the change of conductivity was higher than 20%,the energy consumption of the modified CMV membrane electrodialysis system was gradually higher than that of the CMV membrane with the decrease of conductivity.Since current density is inversely proportional to salinity,salinity is used as a function of current density to compare CMV films before and after modification.At low current density,the salinity difference is large.Until the current density increases to a certain range,the salt removal remains the same.?6?In a small scale electrodialysis system,the monovalent selectivity of Na⁺was studied and the accuracy of EIS simulation experiment was verified.RTN showed that the polyethylene imide coating had no effect on the selectivity of the same valence ions,but it could improve the selectivity of different valence ions.At low current density of 0.6-1.3 mA/cm₂,the maximum Na⁺ion selectivity of CMV membrane was improved by 8 times and the maximum Na⁺ion flux of CMV membrane was increased by 2 times.With the increase of current density below2.5 mA/cm₂,the modified ion selection further increases while the ion flux decreases.?7?At last,the four parameters of CMV film before and after modification were compared,and it was found that the electromigration coefficient,concentration diffusion coefficient,water electrical permeability coefficient and concentration permeability coefficient of CMV film after modification were all superior to CMV film.