The selectivity of ion separation is desired when removing the entirety of ions from water is not the purpose and specific ions need to be separated because of their toxicity or economic value. For example, nitrate is a common pollutant in groundwater and poses a threat to water supply, particularly in small-scale drinking water systems. Ion exchange membranes are used in electrodialysis and membrane capacitive deionization (MCDI) to separate ions with opposite charge, but existing IEMs are not optimized for separating ions of like charge, such as between nitrate and chloride. Due to the different hydration energies of different anions, the relative transport number of anions between one other changes with increasing hydrophobicity of the membrane surface. Conductive polymers are potential candidates to modify the membrane surface to achieve tunable surface hydrophobicity. In this work, we modified a commercial anion exchange membrane (AEM) with polypyrrole (PPy) and systematically characterized the properties of PPy-modified IEMs, including water content, transport number, and electrical conductivity. We investigated the selectivity of ion diffusion through the membranes using a custom-made diffusion cell. Finally, we examined the ion separation performance of the modified membrane in MCDI. Our results showed that PPy-modified AEM exhibits enhanced selectivity of nitrate over chloride in both ion diffusion tests and MCDI experiments. Furthermore, the nitrate separation selectivity increases with increasing the hydrophobicity of the membrane surface.

Hydrophobic conductive polymer modified anion exchange membrane for selective nitrate separation in membrane capacitive deionization

ACS Fall 2021

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