Fe3+ ions induced rapid co-deposition of polydopamine-polyethyleneimine for monovalent selective cation exchange membrane fabrication

Selective monovalent cation transport is crucial for the concentration of target ion(s) using electro-membrane processes prior to precipitation. Co-deposition of biomimetic adhesive polydopamine (PDA) and the positively charged polyethyleneimine (PEI) has emerged as an effective approach for building monovalent perm-selectivity functional coatings for cation exchange membranes (CEMs). Here we report a facile strategy to fabricate monovalent selective cation exchange membranes through quick co-deposition of PDA-PEI induced by ferric (Fe3+) ions. With CEM substrate ion exchanged with Fe3+ ions, the polymerization of DA on the membrane surface was accelerated. The mechanism of the promoting effect of Fe3+ ions on the PDA-PEI co-deposition was explored by UV spectroscopy, SEM, AFM, zeta potential and area resistance measurements. In this approach, the characteristics of the membrane surface were easily manipulated by varying the Fe3+ concentration. The selective CEM fabricated through 0.0001 M FeCl3 pretreatment and only 30-min PDA-PEI deposition showed remarkably enhanced Na+/Mg2+ selectivity of 8.3, low electrical resistance and excellent stability. This study provides a simple and scalable technique for time-saving and environmentally friendly construction of PDA based coating for membrane modifications.