Alkylation as a strategy for optimizing water uptake and enhancing selectivity in polyethyleneimine-based anion-exchange membranes for brine mining via electrodialysis

Brine treatment poses a significant challenge for the growing desalination industry, yet it also holds valuable elements and a substantial amount of water. To efficiently extract these elements and increase water recovery, the development of advanced, highly selective separation technologies is urgently needed. This study addresses this challenge by optimizing polyethyleneimine (PEI)-based anion exchange membranes (AEMs) through an alkylation strategy to enhance water uptake control and ion selectivity. The aim is to achieve the high separation efficiency required for effective reverse osmosis (RO) brine mining via electrodialysis. The careful design of functional amine groups with a mixed composition of alkyl substituents enabled the development of membranes with reduced water uptake and high charge density, providing the best conductivity/selectivity ratio, enhanced ion selectivity, and decreased water-splitting activity. The unmodified PEI-membrane already demonstrated a competitive performance compared to common commercial AEMs membranes used in electrodialysis, such as FujiFilm® AEM Type 1 and 2, Ralex® AM-PP, and Neosepta® AMX. However, the alkylation further improved the performance significantly. Among modified membranes, PEI alkylated with propyl followed by methyl (PEI-Pr-Me) achieved the highest current efficiency of 93 %, while PEI alkylated with a mixture of four (C_1[sbnd]C₄)n-alkyl groups had the highest Cl⁻/SO₄²⁻-selectivity coefficients of up to 8.7 and the lowest water transfer across the membrane. This tailored functionalization approach presents a promising pathway for improving AEMs' performance in desalination brine treatment, enabling more efficient water and mineral recovery.