Cation exchange membrane is a membrane that has selective effect on cations, usuallysulfonic acidType with fixingGroupAnd dissociatable ions, such as sodium sulfonic acid type fixed group is sulfonic acid radical, and dissociated ion is sodium ion.Cation exchange membrane can be regarded as a kind of polymerelectrolyteBecause the positive film is negatively charged, although the original dissociationPositive ionIt is dissociated into water by the action of water molecules, but is energized outside the membrane throughelectric fieldAs a result, positively charged cations can pass through the cation membrane, while anions cannot because of homophobic repulsion, so they have selective permeability.
Preparation method
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The preparation of cation exchange membrane generally adopts two methods: the first is the modification of the membrane body, which is mainly to produce a suitable monopolar membrane by adjusting the process parameters (especially the degree of crosslinking). When contacting with the mixed solution of acid and metal divalent salts, the proton flux is higher than the metal ion flux;The second is membrane surface treatment, including increasing the density of membrane surface and depositing a thin anion exchange layer on the surface of the positive membrane to generate positive charges, thus creating an electric repulsive barrier to limit the relative penetration of other valence cations.If the crosslinking agent is slightly increased, the selectivity of divalent ions to monovalent ions will be reduced.[1]
Development history
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Due to the need of electrodialysis to produce salt from concentrated seawater, Japan successfully developed a univalent selective ion exchange membrane in 1960, and Asahi Kasei successfully introduced membrane technology into the salt industry in the same year.In 1961, Walton and Jordan began to explore the exchange equilibrium of divalent ions in the presence of cation exchange membrane, which provided a practical reference for the theory of ion migration and equilibrium on the membrane surface.Sata tried it in 1976PolyethyleneimineModified ion exchange membraneThe unit price of ion exchange membrane was successfully selected.In 1979, Sata used the impregnation method to modify the membrane surface.In 1989, Sata proposed to sulfonate the membrane surface with chlorosulfonic acid, and then aminate it, so that a layer of sulfa bond functional group is formed on the membrane surface.This method is to graft the modified substance onto the membrane surface in the form of chemical bond, thus avoiding the falling off of the modified substance from the membrane surface and extending the service life of the membrane.
Takata and others improved this method by usingSulfoxide chlorideThe sulfonic acid radical is converted into sulfonyl chloride, and then aminated to form sulfonamide.In the same year, Sata et al. used polypyrrole and cationic membrane to prepare composite membrane.Inspired by this, in 2006,Gohil used polypyrrole for membrane modification, and successfully prepared monovalent selective membrane. In 2002, Amara used polyethyleneimine as the modified material, and compared the two methods by impregnation and electrodeposition respectively. It was concluded that the membrane modified by impregnation at low concentration had better selectivity, while at high concentration, the membrane modified by electrodeposition had higher separation efficiency.In 2005, Chamoulaud used isopentamide as amination reagent to aminate to form sulfonamide, which reduced the passing rate of Zn2+ions by 20 times.
China started to try to study ion exchange membrane in 1958. Although it started later than Japan, Europe and the United States, it also made some progress.In 1977, China realized the application of electrodialysis technology in salt industry.In 1981, Tan Qi and others used cheap and less toxic organic amine as a treatment agent to apply it to electrodialysis desalination, and found that the selective permeability of the membrane was significantly increased. Although there was no large-scale production of monovalent and bivalent selective ion exchange membranes in China, this kind of ion exchange membrane with special performance has been fully valued by domestic scholars,And made some breakthroughs in theory.[2]
Modification method
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Doping modification
Doping modification is to improve the selective permeability of the membrane by virtue of some specific advantages of additives.Although polyelectrolyte can block the membrane channel, ions with larger hydration radius can pass through the membrane, which also indicates that different repulsive forces will have a significant impact on the migration of ions.The composite membrane prepared with polyether ether ketone, polyether sulfone, etc. has low resistance and good unit price selectivity.The modified organic-inorganic composite membrane was prepared by using polyaniline as a functional material, and was applied to the selective separation of monovalent ions.The results show that the mixed membrane has no obvious effect on the migration of Na+, but greatly improves the ion rejection rate.The composite membrane was prepared by chemical polymerization of anilineIon exchange capacityAnd selectivity. It is concluded that polyaniline will degrade under the condition of long-term polymerization. Only when the modified layer on the membrane surface is thin enough and the surface is flat enough, the separation efficiency will be improved.
Surface modification
Surface modification refers to the process of particle bombardment or radiation on the ion exchange membrane to form a modified layer on the membrane surface or increase the functional groups on the membrane surface to improve the selective permeability of the membrane.In recent years, membrane surface modification includes: electrodeposition surface modification, photochemical reaction surface modification, impregnation surface modification.These methods are based on the existing base membrane, and modify the physical and chemical properties of the membrane surface.[3]
Surface modification analysis
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Surface modification of plasma coating
With the help ofPlasma technologyTo modify the membrane, the commonly used modified material is metal nanoparticles because of its superior electrical, adsorption and stability properties.Nano metal particle coating can increase the charge density on the membrane surface, reduce or shrink the pores on the membrane surface and inside, thereby improving the membrane potential and ion selective permeability, and reducing the membrane resistance.Too thick particle coating will play an opposite role in improving the performance of the membrane, because too many nanoparticles will block the channel of ion migration, and will surround the ion exchange groups on the surface and inside of the membrane, thus impeding the exchange and migration of ions.With the help of plasma technologypolyvinyl chloride/Styrene butadiene rubber cation exchange membrane was modified.In addition to the commonly used metal nanoparticles, we can also use organic materials to modify the membrane with the help of plasma technology.By adjusting the monomer flow rate, power and other parameters to control the deposition rate, an ultra-thin anion exchange layer containing pyridine ring substances was deposited on the membrane surface.The modified membrane has better selective permeability of high valence cations.
Surface modification by radiation
With the help of radiation, the concentration of fixed active groups on the membrane surface can be increased, and the free space inside the matrix can be reduced, thus improving the selective permeability of the membrane to ions. However, radiation modification will slightly reduce the conductivity of the membrane, because the reduction of free space inside the membrane often has a greater impact on the conductivity than the increase of the concentration of surface active groups.The cation exchange membrane was modified by radiation.The membrane used is naturalPolymer fiberAs the matrix, the element has a large swelling degree due to the negative electricity of its surface.In the experiment, three radiation intensities were used to treat the membrane to determine the effect of radiation dose on the membrane performance, and then the permeability, resistanceIon migration number, ion selective permeability, and the membrane potential was measured through different concentrations of sodium chloride and magnesium chloride solutions.
Surface modification of coating by immersion method
The matrix material of cation exchange membrane is mostly hydrophobicPolymerIn order to improve the ion exchange capacity and other properties of the membrane, it is necessary to improve its hydrophilicity. In addition to the methods described above, organic polymer coating on the membrane can also achieve good results, and the coating polymer and the matrix polymer are firmly combined through chemical reaction.Conductive polymers have good prospects in improving the hydrophilicity of membranes. They have attracted much attention due to their stable chemical properties and potential economic value.Polyvinylidene fluoride (PVDF) can be prepared by impregnation with polyaniline and its derivatives
Modification of membrane.First, polyaniline, polyIV ethylaniline and polyIV are prepared by chemical polymerization-ToluidineHejuO-AminoanisoleAdd a certain amount of sulfonic acid to prepare a methanol solution containing polyaniline or its derivatives, conduct ultrasonic treatment at room temperature for 5 min, and then place the PVDF membrane in the above polymer solution for 24 h to realize membrane modification.After treatment, a dense, hydrophilic and ion selective polymer adhesive layer is formed on the surface of the membrane.This modification method.Simple operation, low cost, good reliability and stability in removing calcium and magnesium ions from water, which can be used in water softening process.[4]
