The Role of Polyethylenimines in PEI-Based Anion Exchange Membranes: Enhancing Phosphate Recovery in Electrodialysis

The dual role of phosphorus as an essential life-sustaining element and a potential environmental pollutant has made it a key focus for many industries seeking effective strategies for its separation and potential recovery. This study proposes a solution for this challenge through developing and structural tuning of polyethylenimine (PEI) based anion-exchange membranes (AEMs) for efficient phosphate electrodialysis separation, achieved by tailoring membrane composition through variation in amine structure, Amine : Bisphenol A diglycidyl ether (BADGE) ratio, and methylation degree. These strategies enabled control over membrane water uptake (9–100%) and ion-exchange capacity (0.3–2.4 mmol/g). A clear relationship was established between lower water uptake, higher charge density, and enhanced selectivity toward monovalent ions over multivalent ones, attributed to sub-nanometer channel formation and a size-sieving separation mechanism. Methylated membranes demonstrated the highest phosphate recovery and reduced water splitting, while non-methylated weak-base membranes, despite their lower performance, offer a more environmentally friendly synthesis route. In a tandem-AEM electrodialysis configuration, the optimized membranes with improved selectivity enriched phosphate from 3.0 to 9.2 mmol/L, which directly enhanced the struvite precipitation efficiency from 85% in the initial solution to 99%. These findings underline the potential of molecularly engineered PEI-based AEMs developed in this study for efficient phosphate recovery from waste streams and offer new insight into the largely unexplored nature of membrane selectivity toward phosphate anions.