Preparation And Characterization Of Monovalent Cation Perm-selective Membranes

Electrodialysis is an electro-driven membrane separation technology,which has good technical compatibility,simple operation,low energy consumption and the easy replacement of membrane unit etc.It has been widely used in many areas,such as seawater desalination,wastewater utilization and chemical production.However,with the development of electrodialysis technology and the improved demand of industry,the conventional electrodialysis which can only separate cations and anion in high efficiency is becoming insufficient.A new-type membrane,called monovalent cation perm-selective membrane which can separation monovalent cations and multivalent cations during electrodialysis,has shown great potential in areas like lithium extraction from seawater,the purification of sodium chloride and the recovery of acid or base from wastewater.The separation performance of monovalent cation perm-selective membrane determines the separation efficiency of electrodialysis.The separation mechanisms include electrostatic barrier effect,pore size sieving effect and the ion-dipole interaction and so on.Based on these mechanism,different kind of membranes have been fabricated and we tried to figure out how these mechanisms affected the separation performance.The works concerned are as follows:1)Nanofibrous composite membranes with different degrees of quaternization have been fabricated by immersing BPPO electrospun nanofibrous mats into SPPO solution followed by quaternization.The separation process happens during the whole transmission process in the membrane,which can lower the area resistance and improve the ion flux.Sulfonic groups of SPPO act as continuous carriers for cations transmission while the quaternary ammonium groups play an important role in separating monovalent cations from divalent cations based on Coulomb's law.The post-quaternization reaction was used because the solubility of nanofibers and the matrix polymer should be quite different,otherwise the nanofiber will dissolve into the doping solution and it can't be well dispersed in the whole membrane.Superior monovalent permselectivity can be achieved after the quaternization reaction.Among all the nanofibrous composite membranes,the membrane NQS1 exhibits the highest permselectivity and good ion flux,and it is better than those of commercial CSO membrane.Furthermore,NQS1 has high limiting current density,low swelling degree and excellent thermal stability and durability.Since the electrospinning can easy control the composition of nanofibrous mat,our work offers good references for preparation of other types of monovalent cations perm-selective membrane.2)We developed an effective approach for fabricating electro-nanofiltration membranes with different compactness,by interfacial polymerization of different amines with 1,3,5-Benzenetricarboxylic acid chloride on the negatively charged porous substrate.The polyamide layers synthesized using different amines exhibits distinctive physicochemical properties.When using amines with small molecular weights,they can penetrate through the nascent polyamide layer to continue the reaction,resulting in high crosslink degree(demonstrated by XPS)and more compact surface(demonstrated by PEG rejection).Meanwhile,the unreacted amines and acyl chlorides on the surface are few,forming more neutral surface(demonstrated by zeta potential).In addition,the effect of surface properties on the final separation performance has been investigated.E-HPAN exhibits the most promising Na+/Mg2+separation performances including the superior perm-selectivity and high Na+flux at a current density of 14 mA cm-2,which can be attributed to the compact polyamide layer with the low charge density.Moreover,the prepared electro-nanofiltration membranes show high limiting current density and excellent stability.Especially,these high-performance electro-nanofiltration membranes can be produced by making only small changes to the existing commercial RO membrane manufacturing line.Therefore,it is desired to scale up in practical industrial manufacture.3)The soluble porous organic cage CC3 crystal having the narrow pore size distribution was synthesized and characterized.The XRD pattern of CC3 crystals do not change before and after soaking in the salt solution,which indicates that CC3 crystal has high stability in salt solution.The CC3 crystal was dissolved in a 98%dichloromethane/2%methanol solvent and then spin-coated on the AAO template to obtain a CC3 composite film which had a CC3 dense layer on the surface.A series of electrodialysis and diffusion dialysis experiments for Na+/Mg2+ separation and Li+/Mg2+separation were studied,in order to get the monovalent permselectivities of CC3 composite membrane.The result shows that the membrane has pretty good Na+/Mg2+ permselectivity and Li+/Mg2+ permselectivity in electrodialysis and diffusion dialysis.What's more,the Na+/Mg2+separation efficiency is higher than the Li+/Mg2+separation efficiency,for both membrane-based separation process.Since the whole CC3 composite membrane doesn't have any functional group,the only separation mechanism could be size sieving effect.So,we demonstrate that it is the hydration radius not the ionic radius determines its permselectivity.4)A monovalent cation perm-selective memrbane containing sulfonic acid groups in the side chain and crown ether in the main chain is synthesized via one step by the polyacylation reaction.Dibenzo-14-crown-4 is selected because it has been demonstrated to have a specific ion-dipole interaction with Li+.The specific ion-dipole interaction might improve the permselectivity of the membrane.The content of crown ether in the membrane can be accurately tuned by changing the ratio of crown ether in raw materials.The results of electrodialysis shows that the transport of Li+is slowed down due to the ion-dipole interaction,but the difference between the crown ether-based membrane and the common cation exchange membrane is trivial.Even after reducing the current density and adding acid in concentrated chamber to accelerate the desorption between the metal and crown ether,the permselectivity is not satisfying.In summary,the sulfone group mainly determines the permselectivity of the crown ether-based membrane.