The Performance Enhancement Of Monovalent Selective Cation Exchange Membrane Based On Surface Modification

With the progress of industrialization,the increasing difficulty on water treatment and the improved requirements of aquatic products,the expectation on electrodialysis technology has been raising gradually.Monovalent selective cation exchange membranes,as the core components of selective electrodialysis,have received particular attention from various fields such as selective removal of salts from water to produce industrial soft water and potable water,production of sodium chloride from electrodialysis concentration of seawater,lithium extraction from brine,and acid recovery or specific ions removal in liquid waste and so on.However,the low membrane perm-selectivity and the trade-off effect between monovalent selectivity and ion flux become a bottleneck restricting the further development of monovalent cation separation.Therefore,how to effectively enhance the membrane perm-selectivity while alleviating the ion flux loss in the secondary limiting process of ion channels has become the key to monovalent selective cation exchange membranes.In this paper,the research progress of“membrane perm-selectivity improving”and“the trade-off effects between ion flux and perm-selectivity relieving”was systematically reviewed.Based on the two mechanisms of surface energy barriers and micro-structure design,different functionalized modified layers were prepared to enhance membrane perm-selectivity and alleviate the trade-off effect.Based on the idea of surface electrostatic energy barrier,a series of PANi-Qn membranes were prepared by in-situ polymerization-deposited polyaniline on polyphenyl sulfone?SPPSU?followed by quaternizing with methyl iodide.The positive charge density of modification layer could be adjusted by controlling the degree of quaternization,which strengthen the electrostatic force repulsion effect,thereby achieving the purpose of restricting multivalent cation penetration and regulating monovalent cation transport.The optimal quaternized polyaniline membrane?PANi-Q24?exhibited a higher perm-selectivity(P_Mg^Na=4.1,P_Mg^Li=1.75)than commercial monovalent-selective cations exchange membrane CIMSBased on the idea of surface hydration energy barrier,a series of PPy-I-n C membranes were prepared by in-situ polymerization-deposited polypyrrole on polyphenyl sulfone?SPPSU?followed by functionalizing with hydrophobic alkyl chains.The hydrophobicity of modification layer could be tuned by the carbon chain length of quaternizing reagent,which improve the surface hydration energy barrier,thereby accomplishing the target of limiting multivalent cation with higher hydration energy transport.The optimal membranes showed better perm-selectivity betweenmonovalent cation perm-selective membrane CIMS(P_Mg^Na=3.56,P_Mg^Li=1.11).Based on the idea of surface micro-structure regulation and inspired by the fold structure of the small intestinal epithelium,a series of 2L-PPy-Qn membranes were prepared by in-situ multiple rapid pyrrole polymerization on polyphenyl sulfone?SPPSU?followed by quaternizing with methyl iodide.The polypyrrole layers rich in ridge-and-valley structures boost up the surface area thus enhancing the cations permeation.Moreover,the free tunable charge density on polypyrrole layers improved the dynamic effect of electrostatic energy barrier,which limits the permeation of multivalent cations and regulates the transport of monovalent cations.The optimal modified polypyrrole layer showed better perm-selectivity between monovalent and divalent cations(P_Mg^Na=27.89)than commercial monovalent cation perm-selective membrane CIMS(P_Mg^Na=3.56).