Surface modification and use of polymer complex agents to mitigate metal crossover of anion-exchange membranes

The facile surface modification of a commercial anion-exchange membrane (i.e., Neosepta-AFX, Astom Corp., Japan) was investigated with brominated poly(2,6-dimethyl-1,4-phenylene oxide) (BPPO), which had non-charged polar groups, and its quaternized form (QPPO) to improve the acid recovery efficiency in diffusion dialysis (DD). By coating a thin layer of BPPO on the membrane surface, the significant changes in the surface compactness and charge density were observed while the electrochemical properties were mostly maintained. From the DD experiments, it was revealed that the membrane modified with 1 wt% BPPO exhibited the moderate acid permeability as well as the highest acid selectivity (KAcid/KFe3+=48.81), which is more than double compared with that of the pristine membrane (KAcid/KFe3+=22.48) among the tested membranes. The electron-rich polar groups contained in BPPO are believed to provide moderate proton transport while the reduced swelling property of the membrane surface can effectively mitigate the crossover of metal cations. In addition, the acid selectivity (KAcid/KFe3+=30.69) was largely improved by using small molecular weight poly(ethyleneimine) (PEI, Mn=1800) with a small content (in the range of 1-5 wt%) as a solution additive for the growing size of metal species by the formation of polymer-metal complexes.