Synergistic enhancement of nitrate selectivity in electrodialysis under high salinity via membrane hydrophobic modification and mass transfer optimization

High-salinity wastewater often contains NaCl, NaNO 3 , and Na 2 SO 4 , with mixed salts classified as hazardous waste in China, highlighting the urgent need for efficient separation and resource recovery. Electrodialysis (ED) demonstrates considerable potential due to its low energy consumption and controllable operational costs. However, commercial anion exchange membranes (AEMs) in the ED stack, while effective at separating ions with opposite charges, exhibit insufficient selectivity for ions of like charges, particularly for NO 3 − /SO 4 2− and NO 3 − /Cl − separations. This study aims to improve the selective separation of NO 3 − under high-salinity conditions. The commercial AGU-AEM was surface modified with polypyrrole (PPy) by in-situ oxidative polymerization to prepare a selective AEM (PPy-AEM), which was then applied to simulate high-salinity wastewater ED treatment. Scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and water contact angle analysis revealed that the dense PPy surface layer imparted high hydrophobicity, improving NO 3 − selectivity based on differences in ion dehydration energy. Furthermore, by optimizing flow rate and feed concentration to enhance mass transfer during PPy-AEM application in ED process, it was found that NO 3 − separation performance of the PPy-AEM could be further improved in high-salinity conditions. The synergistic strategy of membrane hydrophobic modification and enhanced mass transfer achieved NO 3 − /SO 4 2− and NO 3 − /Cl − selectivities of 7.90 and 3.63, representing increases of 8.23 and 3.74 times compared to the original membrane, mainly due to amplification of ion dehydration differences and mitigation of concentration polarization by the hydrophobic surface layer. This synergistic approach of membrane surface hydrophobic modification and mass transfer enhancement provides a new idea for efficient and selective NO 3 − separation from high-salinity wastewater, contributing to the development of effective processes and equipment for NO 3 − selective separation and promoting the sustainable utilization of saline wastewater or mixed salts.