Electrodepositing polydopamine and iron (III) complexed polyacrylic acid for monovalent selective anion exchange membrane fabrication

Monovalent selective anion exchange membranes were efficiently prepared by two-steps electrodeposition. Firstly, a polydopamine (PDA) interlayer was formed onto the surface of a commercial anion exchange membrane (AEM), followed by the deposition of a top layer of Fe3+ complexed polyacrylic acid (Fe-PAA). The PDA interlayer formation was investigated by reversing the polarity of the electric field applied. In particular, modified membrane prepared with PDA layer facing the cathode (labeled as FC-PDA-M) exhibited lower negative charge surface density compared to the membrane prepared with DA solution close to the anode (labeled as FA-PDA-M). This effect resulted in an increased deposition of PAA, due to the diminished electrostatic repulsion, when a solution of Fe3+ complexed polyacrylic acid with a Fe/PAA ratio of 1/4 was electrodeposited on the PDA layer. After eluting the Fe3+ with EDTA, the two manufacture membranes, labeled as FC-PDA/(Fe)PAA-M and FA-PDA/(Fe)PAA-M, were characterized. The former exhibited a higher Cl−/SO42− permselectivity than the latter, although the latter demonstrated better performance compared to both the pristine AEM and a PAA-PDA-M obtained without applying any current during the deposition process. The effect the Fe3+ addition and varied the Fe/PAA ratio was also investigated. The membranes obtained using the Fe/PAA ratio of 1/4 exhibited best separation performance. Finally, the stability of this membrane was assessed by conducting ten consecutive electrodialysis (ED) experiments, revealing consistent Cl−/SO42− permselectivity values within the 5–6 range.