Advancing the Electrosorption Selectivity of Nitrate Through Fine-Tuning Hydrophobic Ammonium Functional Groups in Anion Exchange Membranes for Membrane Capacitive Deionization

Selective NO3– separation is a crucial aspect in the valorization of nutrient resources from wastewater. This study aims to investigate the effect of membrane hydrophobicity on the selectivity of NO3– over monovalent anions of similar hydrated sizes, such as Cl–, in the single-pass membrane capacitive deionization (MCDI) process. Heterogeneous anion exchange membranes (AEMs) were fabricated using ion exchange resins with different ammonium functional groups, namely methyl, ethyl, and propyl chains. As demonstrated, ion exchange and electrodialysis experiments indicated a rising trend in NO3– selectivity over Cl– under both kinetic and equilibrium conditions as the length of the alkyl groups on the AEM increased. Single-pass MCDI experiments further showed that the NO3– selectivity increased with the longer alkyl chain length on ammonium groups. The enhanced hydrophobicity of the propyl-AEM resulted in a NO3– selectivity factor of up to 2.01, outperforming that of conventional MCDI (1.39). Meanwhile, the NO3– selectivity factor was maintained in the range of 2.01–2.26 at varied cell voltages, suggesting that the membrane hydrophobicity determines the selective ion transport process instead of the external electrical field. These findings offer a promising strategy of fine-tuning the membrane hydrophobicity to enhance the NO3– separation selectivity in MCDI, especially for applications in nutrient recovery from wastewater.