Experimental Study On Plasma Modification Of Cation Exchange Membrane To Enhance Ion Selectivity

With the rapid development of electrodialysis technology,ordinary ion exchange membrane can no longer meet the needs of special production such as impurity salt purification,hard water softening,salt lake lithium extraction and so on.Ion exchange membrane with high selective separation performance came into being.As the core component of electrodialysis technology,the preparation of ion exchange membrane with high selective separation performance is particularly important.Among many preparation methods,low temperature plasma,as a highly efficient and energy-saving modification technology,has the advantages of simple operation,controllable conditions,and no need for specific functional groups in the reaction monomer,which makes it have great application value in the preparation of univalent selective ion exchange.In this study,in order to achieve the selective separation of Na⁺/Ca²⁺ions by cation exchange membrane in electrodialysis process,a low temperature plasma grafting polymerization modification method was adopted.Based on two separation mechanisms,electrostatic repulsion effect and hydration effect,respectively,the positive charge density and hydrophobicity of the surface of the membrane were regulated by controlling the reaction time of the quarter-ammonium reaction and the carbon chain length of the iodinated straight chain alkane.Univalent selective cation exchange membranes with different separation properties were prepared,and the changes of water content,ion exchange capacity,membrane resistance,Zeta potential,ion flux,selective permeability coefficient and microscopic properties of the ion-exchange membranes before and after modification were compared and then the effects of modification methods based on two separation mechanisms on the selective permeability of ion-exchange membranes were explored.Firstly,commercial CJMC-3 membrane was used as the base membrane,aniline was selected as the modified monomer,and the polyaniline modified layer was formed on the surface of the original membrane by using the low temperature plasma grafting polymerization method.The influences of five factors including the graft monomer concentration,oxidant concentration,discharge current,discharge time and polymerization time on the Na⁺and Ca²⁺fluxes during the modification process were studied.Aiming at increasing flux,the optimal modification conditions were optimized as follows:aniline monomer concentration of 6%,Fe Cl₃ concentration of 1.2 mol/L,discharge current of 5 A,discharge time of 5 min and polymerization time of 12 h.At this time,the Na⁺and Ca²⁺fluxes of PANI membranes were3.93×10^-4mol/（m²·s）and 3.02×10^-4mol/（m²·s）,respectively.Secondly,based on the electrostatic repulsion effect,PANI membrane prepared under the optimal condition of low temperature plasma grafting polymerization was used as the base membrane.The quaternary ammonium type polyaniline modified layer was generated on the surface of the ion exchange membrane by quaternization of CH₃I.The positive charge density on the surface of the membrane was regulated by controlling the quaternization reaction time.The electrostatic repulsion of Ca²⁺on ion exchange membrane was stronger than that on Na⁺,which realizes the selective separation of Na⁺/Ca²⁺in the electrodialysis process.When the quaternization time was 24 h,the selective separation coefficient of PANI-Q₂₄ membrane reached 4.10.Then,the influence of ion ratio on ion flux and separation effect was investigated.The results showed that as the concentration ratio of Na Cl and Ca Cl₂ in the mixed solution increased from 1:1 to 10:1,the proportion of Na⁺ions increased from 50%to 90.9%,while the proportion of Ca²⁺ions decreased from 50%to 9.09%.When Ca²⁺migrated to the surface of PANI-Q₂₄ membrane,it was not only subject to electrostatic repulsion stronger than Na⁺,but also surrounded by a large number of Na⁺,which makes it more difficult to approach and migrate across the membrane.The Na⁺flux increased,and Ca²⁺flux decreased.Accordingly,the selective permeability coefficient of PANI-Q₂₄ membrane increased from 4.10 to 4.88.Finally,based on the hydration energy effect,PANI membrane prepared under the optimal condition of low temperature plasma grafting polymerization was used as the base membrane,and the quaternization time remained unchanged for 24h.The PANI membrane was quaternized with iodine-straight chain alkanes with different carbon chain lengths,and the hydrophobicity of the membrane surface was adjusted by changing the carbon chain length of iodine-alkanes.The hydrophobic modified membrane restricted the dehydration of polyvalent Ca²⁺with high hydration energy into the modified layer,which realized the selective separation of Na⁺/Ca²⁺in the electrodialysis process.When the length of carbon chain was 5,the selective separation coefficient of ion exchange membrane reached 6.99.Then,the influence of ion ratio on ion flux and separation effect was investigated.The results showed that as the concentration ratio of Na Cl and Ca Cl₂ mixed solution increased from 1:1 to 10:1,the proportion of Na⁺doubled,and the proportion of Ca²⁺decreased to 9.09%.Ca²⁺was not only subject to stronger electrostatic repulsion than Na⁺,but also surrounded by a large number of Na⁺,which made it more difficult to approach and migrate across the membrane.Accordingly,the selective permeability coefficient of PANI-C₅ membrane increased from 6.99 to 10.60.Research has shown that using low-temperature plasma graft polymerization can give ion exchange membranes selective separation of different valence ions.Modified membranes prepared based on the hydration energy effect have better separation effects than modified membranes prepared based on the electrostatic repulsion effect.They can effectively achieve special production needs such as salt purification and hard water softening,and have enormous technical and economic value.