Preparation And Performance Characterization Of Monovalent Selective Anion Exchange Membranes Based On Three Different Modification Methods

In recent years,electrodialysis technology has been developed quickly.It plays a more and more important role in desalination of brackish water,and also in the purification and the treatment of effluents,and waste water.However,one successful technology is impossible to solve all problems.We need to adjust the electrodialysis to meet the different problems and conditions in practical application.Examples include the removal of F^-or NO₃^-from groundwater and the separation of SO₄^2-for avoiding scale formation in water treatment processes.If we want to solve above mentioned problems in the examples,a monovalent selective anion exchange membrane with excellent performance is urgently needed.In order to prepare the monovalent selective anion exchange membranes with perfect performance,the content of this paper is based on surface modification and the internal modification method to improve the stability of modified layer and prepare monovalent selective anion exchange membranes by one pot method,respectively.Moreover,the porous and charged nonionic exchange membranes inspired by the structure and character of nanofiltration membranes are prepared used as monovalent selective anion exchange membranes for fractionation of monovalent anions in electrodialysis,which also discusses in the paper.The main research contents are as follows:?1?Based on the surface modification,the problem of the modified layer exfoliates from the substrate membrane could be solved by constructing a covalently crosslinked interface on the surface of modified membranes.The polyethyleneimine?PEI?was quickly and uniformly deposited onto the surface of an anion exchange membrane(partly quaternized poly?phenylene oxide??QPPO?by the method of electro-deposition.By taking advantage of the chemical reaction between the amine groups of PEI and the residual active benzyl bromide groups of QPPO to construct a covalently crosslinked interface,the modified membranes were prepared.A series of electrodialysis experiments in which Cl^-/SO₄^2-separation and long-term stability were investigated to demonstrate the monovalent anion selectivity and stability of the membranes.The results show the covalently crosslinked interface on the surface of modified membranes is the key to solve the un-stability of the modified layer by the traditional electrostatic deposition method and the method in our study is easy to be industrialized.?2?Based on the internal modification,the monovalent selective anion selective membranes were prepared by one pot method.As we all know,the process of traditional surface modification was complicated and the thickness of the modified layer is difficult to control.So this method in our study well solved the problems.The internal modified membranes were prepared by simply adding different amounts of sulfamerazine?SF?to partially-quaternized chloromethylated polysulfone?QPSF?.The dense and linked structure was formed by the reaction between primary amine of SF and the chloromethyl groups of QPSF.At the same time,the modified membranes had different monovalent selectivity under different pH conditions because of the de-protonation or protonation of SF.A series o f electrodialysis experiments in which Cl^-/SO₄^2-separation was investigated to demonstrate the different monovalent anion selectivity under different SF adding amounts and pH conditions.This study demonstrates a promising strategy for preparing monovalent anions selective membranes by internal modification.?3?No matter surface modification or internal modification,the modified membranes were all dense.Whether it could be used as a monovalent selective anion exchange membrane for electrdialysis or not,when we changed its dense structure into porous structure.So inspired by the structure and character of nanofiltration membranes,we prepared a series of the porous and charged nonionic exchange membranes.These membranes were modified by polydopamine to control the compactness and charged property and further improved by crossing.A series of electrodialysis experiments in which Cl^-/SO₄^2-separation was investigated to demonstrate anion selectivity.This study gives us more inspiration to prepare porous monovalent selective anion exchange membranes in the future and indicates the synergistic interaction between the sieving mechanism and electrostatic repulsion is a key parameter for the selective transport of monovalent/multivalent anions.