Preparation And Characterization Of Durable Mono-valent Perm-selectivity Cation Exchange Membrane

To prepare the mono-valent perm-selective cation exchange membrane （CEM）,one of the most important ways is to modify the surface of the CEM. In practicalapplications, the detachment of the thin cationic charged layer on the CEM seems tobe unavoidable because its existence on the base CEM only relies on the weakelectrostatic interaction. There are many studies on the way to enhance the interactionbetween the modified layer and the membrane surface to prolong the service life ofmodified membranes. Due to its the environmental kindness this study choose thechitosan to endow the mono-valent ion selectivity to the ordinary CEM by coatingmethod with two different interaction between the modified layer and the membranesurface, the first is strengthening physical method, the second is chemical bondingmethod.In the physical method, coating method was used to introduce the chitosan intothe surface of the CEM, in order to enhance the durablity of the modified CEMs. Theinfluences of preparing condition on properties of modified membrane areinvestigated, including different concentration of chitosan solution and preparingstages. On the dynamic electrodialysis experiments of H⁺/Zn²⁺and Na⁺/Mg²⁺binarysystem. It is found that when the concentration of the chitosan is2%（w/v）, the valueof the transport number of Zn²⁺and Mg²⁺is minimum can be0.003708,0.05238,respectively, compared with bare CEM has fallen by87%,77%, respectively. For thedifferent preparation stages for the H⁺/Zn²⁺, the CEM modified by cross-linkedquaternized chitosan has the lowest value of transport number of Zn²⁺can be0.003708, compared with bare CEM has fallen by87%. For the Na⁺/Mg²⁺system, theCEM modified by cross-linked chitosan has the minimum value of transport numberof Mg²⁺can be0.01664, compared with bare CEM has fallen by93%. The experimentresults indicated that the prepared mono-valent perm-selectivity CEM for themultivalent cation permeability significantly reduced. The SEM is used for membranemorphology characterization. The results indicate that membrane surface after modification appears more tight and smooth. The experiment of the modified CEMs’working life indicates that the modified CEM after dealing with strict experimentalcondition as washing thoroughly with2%（v/v） acetic acid for different time showsgood durability and higher prem-selectivity for multivalent cations.For the chemical method, using coating method and UV radiation to introducethe chitosan derivent to the surface of the CEM to form chemical bonding enhancingthe durability of the modified CEM. The influences of preparing condition onproperties of modified membrane are investigated, including different preparingstages. On the dynamic electrodialysis experiments of H⁺/Zn²⁺binary system, it isfound that the CEM modified by AZ-CS and cross-linked quarternized1%CS, thevalue of transport number of Zn²⁺is minimum can be0.008756, a77%reductioncompared to the unmodified CEM. The CEM modified by AZ-CS and cross-linked1%chitosan for the Na⁺/Mg²⁺system has the minimum value of0.02864of transportnumber of Mg²⁺, an87%reduction compared to the unmodified CEM. Theexperiment results indicated that the prepared mono-valent perm-selectivity CEM forthe multivalent cation permeability significantly reduced. The AFM are used formembrane morphology characterization. The results indicate that membrane surfaceafter modification appears more dence, the roughness reduced from0.00871μ m to0.0067μm. The experiment of the modified CEMs’ working life indicates that themodified CEM after dealing with strict experimental condition as ultrasonic washingthoroughly with2%（v/v） acetic acid for different time shows stable perm-selectivityand good durability.In this work, the two different modified methods of preparing CEMs withselective separation performance are investigated in detail. The experimental resultsindicate that it is feasible to use that two different modified to prepare cationexchange membrane with selective separation performance between monovalent andmultivalent ions meanwhile the modified CEMs has relatively long working life andgood durability.