Fabrication Of The Electro-Nanofiltration Membranes For Anion Separation

Electro-nanofiltration technology is a kind of technology with high-effeciency mono-and di-valent ion separation performance.The monovalent ion selective separation membranes in electro-nanofiltration process are named electronanofiltration membranes(ENMs)due to their nanofiltration membrane structure.Compared with the dense polymer membranes applied in selectrodialysis process,the ENMs have better ion separation ability and higher limiting current density.Herein,the thesis fabricated a series of ENMs with polyamide structure to achieve excellent monoand di-valent anion separation performance and high limiting current density through the membrane structure design method.Moreover,the influence of the physical properties of the functional layer including surface charge and hydrophilicity/hydrophobicity to the anion separation performance was also investigated systematically.The main research is as follows:1)The p-phenylenediamine ENMs were fabricated via the reaction between pphenylenediamine/2,5-diaminobenzenesulfonic acid and trimesoyl chloride(TMC)while the sulfonic acid group density and ion separation performance of the ENMs were adjusted easily by changing the ratio of p-phenylenediamine to 2,5diaminobenzenesulfonic acid.Due to the suitable pore structure of the polyamide crosslinked network and high negative surface charge density,the ENMs show outstanding anion separation performance.The Cl-/SO42-selectivity of the ENMs is 23 times higher than that of the commercial ACS membrane at the same measurement condition.Moreover,the ENMs also show excellcent properties such as low area resistance and high limiting current density compared with the ACS membrane while the Cl-flux is lower.2)The ethylenediamine ENMs were fabricated via the reaction between the ethylenediamine and trimesoyl chloride and the the influence of the monomer concentration and reaction time was also investigated systematically.Compared with the p-phenylenediamine monomer,the two advantages of ethylenediamine monomer is smaller molecular size and the lack of the benzene structure.Compared with the ethylenediamine ENMs,the Cl-flux increases by 148%while the selectivity maintains a high level.3)The surface hydrophobicity of the ethylenediamine ENMs was enhanced by the perfluorooctyl chloride modification method.The more hydrophobic surface causes the stronger repulsion effect between the high valent ions and the membrane surface and the Cl-/SO42-separation performance is further improved.Compared with the original ethylenediamine ENMs,the Cl-flux of the ethylenediamine ENMs with enhanced surface hydrophobicity decreases by 21%while the Cl-/SO42-selectivity increases by 93%,which indicated the effectiveness of the hydrophobic modification method.4)the simple method for enchancing the pore size uniformity and decreasing the height of the functional layer was adding inorganic salts in the amine solution.The enhanced pore size uniformity enhances the size sieving effect of the membrane so that the Cl-/SO42-separation performance is further improved.Compared with the original ethylenediamine ENMs,the Cl-flux of the ethylenediamine ENMs with enhanced functional layer compactness decreases by 5%while the Cl-/SO42-selectivity nearly increases by 200%,which indicated the effectiveness of the method of adding inorganic salts.5)The MOFs/PEI ENMs were fabricated via the spraying-deposition method in which the MOFs/polyethyleneimine(PEI)depersion and TMC solution were sprayed on the substrate and reacted with each other on the substrate.In the membrane structure,the role of the MOF particles is to provide the pore channals with excellent ion separation ability while the role of the polyamide network is to fix the MOF particles in the functional layer.However,the MOFs layer is not dense so that the anions can go across the ENMs through both the gaps between the MOFs paticles and the pore channals,which leads to the poor anion separation performance of MOFs/PEI ENMs.