Preparation Of Functional Anion Exchange Membranes For Diffusion Dialysis And Electrodialysis

To date,one of tools which is inspiring industrial processes is the membrane technology as it fits great requirements such as high efficiency,low energy and cost effectiveness during separation processes.In membrane technology,diffusion dialysis and electrodialyis are attractive separation techniques in which researchers are putting tremendous efforts.Diffusion dialysis relies on the difference in chemical potential of species from one side to another while electrodialysis relies on electrical potential difference to separate ionic species from aqueous solutions and other uncharged components.In the current work,a series of anion exchange membranes were synthesized,electrochemically studied and applied for diffusion dialysis and electrodialysis.Firstly,imidazolium functionalized anion exchange membranes have been synthesized.Here,the preparation of anion exchange silica precursor(AESP)was based on alkylation of 3-chloropropyltriethoxysilane(CPTS)to 1-methyl imidazole and analogous anion exchange membranes were obtained through sol-gel process of AESP with polyvinyl alcohol(PVA)and tetraethoxysilane(TEOS)as a crosslinker.The obtained anion-exchange membranes showed ion-exchange capacities(IEC)of 1.34-1.86 mmol/g and water uptakes(WU)in the range of 44.11-118.86%.Also,the membranes exhibited moderate thermal stability and excellent mechanical properties.Diffusion dialysis tests showed H+ dialysis coefficients(UH)in the range of 1.87-4.83 x 10-2m/h and separation factors(S)in the range of 12.72-52.5.Secondly,pyridinium functionalized anion exchange membranes were prepared by etherification and sol-gel process between pyridinium salt and polyvinyl alcohol(PVA)in presence of TEOS.These obtained membranes demonstrated excellent thermal and chemical stabilities.When applied in diffusion dialysis,the membranes showed acid dialysis coefficients in the range of 1.74-2.48(10-2m/h)and separation factors in the range of 30.49-57.51.Alternatively,the covalently cross-linked pyridinium based AEMs were synthesized.Here,pyridinium anion exchange precursor was prepared by alkylation of 4-bromomethyl benzoic acid to pyridine and then the corresponding AEMs were obtained by esterification reaction through the prepared pyridinium salt and polyvinyl alcohol(PVA)in the presence of TEOS.The prepared AEMs showed Td values in the range of 322-335 ? and tensile strength(TS)in the range of 43.74-47.23 MPa while elongation at break(Eb)were in the range of 262.59-474.44%.Those membranes possessed acid dialysis coefficients(UH)in the range of 1.28-1.88 x 10-2 m/h and one of membrane which contained 40 wt%of PVA had the highest separation factor(S)of 71.35.Thirdly,another set of AEMs were prepared from double quatemization and sol-gel reaction of synthesized anion exchange precursor and PVA.The synthesis of anion exchange precursor resulted from alkylation reaction of 1,6-dibromohexane on 1,4-diazabicyclo[2.2.2]octane(DABCO)and ring opening reaction by 3-glycidoxypropyltrimethoxysilane.The prepared membranes had WU in the range of 82.6-148.5%and IECs in the range of 0.61-0.86 mmol/g.Those membranes benefited TS values in the range of 15.4-22.7 MPa and Eb values in the range of 441.2-541.0%.When applied for DD,acid dialysis coefficients(UH)between 30.0 and 44.9 m/h x 10-3 and separation factor(S)ranges of 20.9-32.3 were obtained.All diffusion dialysis results of synthesized membranes were greater than commercial membranes DF-120(UH=9 x m/h x 10-3 and S=18.5).Fourthly,BPPO-based anion exchange membranes with cationic head groups were fabricated,studied and applied for electrodialysis(ED).Here,3,5-pyridinedicarboxylic acid was firstly methylated and the obtained 3,5-pyridine dimethyl dicarboxylate was used as the loading material for the AEMs fabrication.The membranes show IECs in the range of 1.14-1.82 mmol/g and water uptakes in the range 18.41-43.67%.The calculated water molecules per each charged group were in the range of 8.98-13.32,LER between 7.61 and 16.19%and thickness between 68 and 102 ?m.Prior to the electrodialysis experiment,area resistance and transport number of the membranes were determined and the corresponding values were in the range of 4.33-2.01?.cm2 and 0.92-0.96 respectively.Compared with the commercial membranes Neosepta AMX,the fabricated BPPO-based membranes show better desalination performances during ED process.