Modified Polyimide Porous Anion Exchange Membranes For Bipolar Membrane Electrodialysis Applications

The bipolar membrane electrodialysis?BMED?process has the characteristics and advantages of high efficiency and environmental friendliness,and hence has a good application potential in the production of organic acids and the treatment/recovery of industrial wastewater.At present,the BMED process has been reported in the production of organic acids such as acetic acid and citric acid.However,further research is needed on how to improve the structure and properties of the membranes so as to reduce ion migration resistance and increase production efficiency.In particular,for organic acids having relatively high molecular weights and low migration activities,such as lactobionic acid and N-2-hydroxyethylpiperazine-N-2- ethanesulfonic acid?HEPES?,the production efficiency is low and the energy consumption is high when conventional dense anion exchange membranes are utilized.In this thesis,a series of P84 co-polyimide porous anion exchange membranes are prepared and applied to the process of producing HEPES or removing boron from aqueous solution by BMED.The main contents of the different chapters are as follows:The first part is to prepare a series of P84 co-polyimide membranes with porous structure by phase inversion method.The pore size of the membrane is adjusted by changing the concentration of the coating solution for the phase inversion process.Polyethyleneimine?PEI?having a polyamine structure is selected to crosslink the P84base membrane,followed by quaternization through reaction with bromoethane to obtain porous anion exchange membranes.After characterization of the properties including Fourier transform infrared?FTIR?,membrane morphology,mechanical strength and thermal stability,the membranes are used in the BMED process to produce HEPES.The results show that the porous P84 co-polyimide membranes have high permeability to macromolecular organic acid and can effectively realize the production of such acid,achieving higher recovery than traditional dense membranes.After 6 hours of operation at 40 V,the recovery of HEPES is in the range of52.3-61.6%,and the recovery rate of the commercial membrane CJMA-3 as a comparison is 46.5%.Meanwhile,the value of the dense P84 co-polyimide membrane M-D is only about 7.4%.The second part is to prepare the P84 co-polyimide anion exchange membranes of different structures by changing the non-solvent in the phase inversion process;and to prepare QGO-P84 composite anion exchange membranes by introducing quaternized graphene oxide?QGO?into P84 matrix.The area resistance,water uptake,ion exchange capacity and morphology are characterized.The results show that the non-solvent species has a significant effect on the pore structure of the membrane.The composite porous membranes have better physico-chemical properties with area resistance of 1.6-1.9?cm²,water uptake of 63-80%,ion exchange capacity of 1.23-1.65 mmol/g.The above membranes are then used in BMED process to remove boron from the aqueous solution.The separation efficiency is 76.6%after running 3 h under 30V by using the optimal composite membrane M₅₀-QGO1 which contains 1wt%QGO,and the value of commercial membrane CJMA-3 is 51.6%.The composite membranes also show superiority in the aspect of energy consumption and current efficiency.For instance,the current efficiency is 94.9%and the energy consumption is26.16 kW h/kg by using the composite membrane M₅₀-QGO1 under 20V,which are better than those of commercial membrane CJMA-3?81.2%and 30.56 kWh/kg?.The final chapter is a summary of the full text.By utilizing different series of porous polyimide membranes in the above two application fields,it can be found that the porous structure of the membranes is beneficial to the ions migration during BMED process,thereby enabling higher HEPES yield and higher boron separation efficiency.