Monovalent cation exchange membranes prepared by Fe3+-assisted coupled surface modification of PEI-PPy for potassium extraction from simulated brine

Separation of mono- and multivalent cations is of great importance in the recovery of potassium resource. This study employed a Fe3+-assisted two-step modification process to prepare monovalent cation exchange membranes (MCEMs) with high perm-selectivity and performance stability for the separation of K+/Mg2+. Initially, a positively charged polyethyleneimine (PEI) layer was electrodeposited onto the commercial cation exchange membrane (CEM). Subsequently, hydrophobic pyrrole molecules were polymerized onto the first PEI layer to produce the final MCEM. The electrodeposition and interfacial polymerization were efficiently coupled together by the incorporation of Fe3+ on the internal PEI layer which provides sufficient growth sites for the external polypyrrole (PPy) layer. The positive charge density and the hydrophobicity of the membrane surface were simultaneously increased. Additionally, this two-step modification method can also prevent the blocking of the basement membrane channels caused by the diffusion of pyrrole monomers. As a result, the prepared membranes achieve a perm-selectivity of 14.1 for the separation of K+/Mg2+, along with a relatively high K+ ion exchange flux (0.28 mmol/(m2·s)). The MCEMs reported in this study exhibit good performance in potassium extraction from simulated brine, which has a promising application prospect. [Display omitted] •Fe3+ successfully coupled PEI and PPy layers on the surface of CEM.•PEI intensifies the separation of K+/Mg2+ through electrostatic interaction.•PPy increases the perm-selectivity owing to its dense and hydrophobic structure.•PPy improves the stability of the modified layers.