Abstract
The interplay between the ion exchange capacity, water content and concentration dependences of conductivity, diffusion permeability, and counterion transport numbers (counterion permselectivity) of CJMA-3, CJMA-6 and CJMA-7 (Hefei Chemjoy Polymer Materials Co. Ltd., China) anion-exchange membranes (AEMs) is analyzed using the application of the microheterogeneous model to experimental data. The structure–properties relationship for these membranes is examined when they are bathed by NaCl and Na2SO4 solutions. These results are compared with the characteristics of the well-studied homogenous Neosepta AMX (ASTOM Corporation, Japan) and heterogeneous AMH-PES (Mega a.s., Czech Republic) anion-exchange membranes. It is found that the CJMA-6 membrane has the highest counterion permselectivity (chlorides, sulfates) among the CJMAED series membranes, very close to that of the AMX membrane. The CJMA-3 membrane has the transport characteristics close to the AMH-PES membrane. The CJMA-7 membrane has the lowest exchange capacity and the highest volume fraction of the intergel spaces filled with an equilibrium electroneutral solution. These properties predetermine the lowest counterion transport number in CJMA-7 among other investigated AEMs, which nevertheless does not fall below 0.87 even in 1.0 eq L−1 solutions of NaCl or Na2SO4. One of the reasons for the decrease in the permselectivity of CJMAED membranes is the extended macropores, which are localized at the ion-exchange material/reinforcing cloth boundaries. In relatively concentrated solutions, the electric current prefers to pass through these well-conductive but nonselective macropores rather than the highly selective but low-conductive elements of the gel phase. It is shown that the counterion permselectivity of the CJMA-7 membrane can be significantly improved by coating its surface with a dense homogeneous ion-exchange film.
Highlights
Over the last 10 years, the number of publications aimed at developing new and improving existing anion exchange membranes (AEMs) has doubled and reached more than 650 articles per year (Scopus)
The difference in the sequences found for CX and Q is caused by a twofold increase in the water content in the AMH-PES and CJMA-7 membranes compared to the other membranes under study
In the case of CJMA-6 and CJMA-7 membranes, which are characterized by the highest f 2app values, the ratio D SO2− /DCl − increases. This means that the structural features of the ion-exchange matrix of these membranes make it possible to at least partially remove the restrictions on the transport of sulfate ions in comparison with chloride ions. This property of the new CJMA-6 and CJMA-7 membranes is very attractive for their use in the electrodialysis processing of sulfate and other solutions containing large anions
Summary
Over the last 10 years, the number of publications aimed at developing new and improving existing anion exchange membranes (AEMs) has doubled and reached more than 650 articles per year (Scopus). AEMs with new fixed groups [24], ion-exchange matrices [25], cross-linking agents [26], inert fillers and reinforcing materials [27], as well as methods of membrane manufacturing [28,29] and modifications of their surfaces [6,18,19] are actively developing. These new approaches are summarized in reviews [6,30]. The latter reduces current efficiency of electrodialysis and does not allow obtaining brines of high concentration reducing current efficiency of electrodialysis
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