Process Analysis And Optimization Of Selective Separation Of Mon-/di-valent Cations In Electrodialysis Under The Influence Of Current Density And Ion Concentration

Human beings are faced with global water problems,and the treatment of ion solutes in water is one of the important measures to deal with water problems.It is still difficult to efficiently separate the same electrical ions in water.Electrodialysis(ED)based on ion exchange membrane(IEM)has good ion selective separation potential,and in the face of fluctuating water quality conditions,how to efficiently and selectively separate monoand di-valent ions is of great significance.The influence of current density,varying total concentration,equivalent concentration ratio between the di-and mono-valent ion as well as different types of cation exchange membranes(CEM)on their permselectivity in ED were discussed in this study.The key control factors and internal mechanism were analyzed from the aspects of electrostatic adsorption and Donnan effect,and the separation effect was optimized and adjusted.The results of separation between mono-and di-valent cations based on standard CEM showed that,increasing the current density from 25%of the limiting current density to 37.5%and 50%,the permselectivity of Mg2+ over Na+(PNa+Mg2+)was reduced by 20%and 15%,respectively.This agrees with the presence of the diffusion boundary layer where decreasing transport of Mg2+ can be ascribed to its lower diffusivity in solution.Increasing the total concentration from 0.1 eq/L to 0.2 and 0.4,the charge screening effect under high salt conditions leads to the decrease of the permselectivity of Mg2+ over Na+(PNa+Mg2+)from 2 to 0.6.According to the solution-diffusion model,the decrease partition coefficient of Mg2+ over Na+in the membrane further verified the effect of total concentration on the permselectivity.In addition,when the fraction of Mg2+ increased from 0.25 to 0.5 and 0.74,it was also found that the permselectivity of Mg2+ over Na+decreased.This is due to the neutralization caused by the stronger binding affinity of divalent cation to the charged site of CEM,which also agrees with decreased partition of Mg2+.The results of selective membrane CSO and standard membrane CMV separation of mono-and di-valent cations showed that,the permselectivity of selective membrane system was less than that of standard membrane under the same experimental conditions.Electrochemical impedance spectroscopy(EIS)results showed that there was an additional modified layer on the surface of the selective membrane compared with the standard membrane,which made it have a huge modified layer resistance in the divalent ion solution(RF=430 KΩ·cm2).Meanwhile,since the electrostatic-repulsive-force between the modified layer and the high valence ions is stronger than that between the monovalent ions,the resistance of the double layer and the diffusion boundary layer was 3-40 times larger than that of the standard membrane,thus affecting the transfer of divalent ions.On the other hand,the calculation results of electrochemical methods showed that the time of different ions passing through the modified layer was significantly different(τNa+=0.0006 s,τMg2+=1.59 s),which causing the selective separation between mono-and di-valent cations(τMg2+/τNa+=2650).Decreasing the current density in two kinds of membrane systems can increase the permselectivity of Mg2+ over Na+.The theory that the presence of the diffusion boundary layer where decreasing transport of Mg2+ can be ascribed to its lower diffusivity in solution is applicable to the selective and standard CEMs.In addition,the effect of total concentration and equivalent concentration ratio between the di-and monovalent ion on the permselectivity showed the opposite changes under different membrane systems.the charge screening effect under high salt conditions and electrostatic-repulsive-force between the modified functional layer and high valence ions probably explain the permselectivity variation.In summary,the influence of current density,varying total concentration,equivalent concentration ratio between the di-and monovalent ion as well as different types of cation exchange membranes(CEM)on their permselectivity in ED were systematically illustrated.The results show that the electrodialysis process can obtain better selective permeability by selecting appropriate current density or adjusting the concentration or proportion of ion components in the solution to be treated.In addition,standard or selective CEMs can be combined to adj ust the order or develop better membrane materials considering the actual requirements,in order to achieve satisfactory permselectivity between mono-and di-valent cations in ED.