Preparation Of Mono-valent Permselectivity Heterogeneous Cation Exchange Membrane

With the development of electrodialysis technology, conventional ion exchange membrane has been unable to meet the needs of industry. Some ion-exchange membranes with special properties, eg:mono-valent permselectivity show good prospect in industrial applications, so the researches in this regard are very common. During the last thirty years, a surface modification route with the aim of contributing to improve the mono-valent permselectivity of ion exchange membrane has been tackled by many works. In order to endow long-term stability to the resulted membrane, studies on the stable covalently grafting of a polyelectrolyte layer on the surface of membrane have been proved to be feasible and effective. Furthermore, layer by layer self-assembly (LbL) surface modification as a simple multi-functional method developed in the 1990’s has already obtained the widespread application. So in this paper, polyethylenimine is selected as the modification material to improve heterogeneous cation exchange membrane transport properties through two modified membrane methods aforementioned.Covalently grafting:in order to graft polyethylenimine on the surface of membrane, in this paper, azide acid is grafted on the surface of membrane through insert reaction; the other method can be described as a spontaneous grafting based on the 4-aminophenylacetic acid chemistry using additional reducing agents without any electrochemical assistance. Once the membrane is carboxylated, polyethyleneimine can be easily grafted on the surface of membrane through amidation reaction.Chemical reaction LbL grafting:owning to glutaraldehyde can react with the primary amine to forming Schiff base compounds, so the membrane could be immersed into glutaraldehyde solution for polyethylenimine self-crosslinking. Furthermore, the residual aldehyde of glutaraldehyde can react with polyethylenimine again. When immersing the membrane into the solution of glutaraldehyde and polyethylenimine, respectively, the resulted membrane can be coated by the layer-by-layer assembly of glutaraldehyde/polyethylenimine multilayers.When the transport properties of modified membranes are tested, it finds that the leakage ratio of Zn2+ and Mg2+ decreased both more than 60% under the dynamic electrodialysis experiments of H+/Zn2+, Na+/Mg2+ binary system, so it is confirmed that the modification methods are effective and the endowment mono-valent permselectivity to cation exchange membrane is successful. Besides, I-V curves show that membrane resistance increases and limiting current density also increases a bit, which means the homogeneity of the membrane increases; chronopotentiometry experiments verify the conductive area of membrane surface increases, so the films become more homogeneous; the hydrophilicity of the membrane surface decreases through NaCl diffusion experiments.