Selective separation of nitrate from chloride using PVDF-based anion-exchange membranes

In this study we investigated the selective separation of nitrate from chloride using newly designed PVDF-based anion-exchange membranes. Five membranes were successfully manufactured by casting, each containing a different PVDF concentration (from 0 to 50 % w/w). A polyaromatic anion-exchange polymer was used as a binder. Experimental data shows that the binder had no impact on the nitrate selectivity while increasing the PVDF concentration enhanced nitrate transport. The membrane with a 50 % w/w PVDF content (PVDF-50), exhibited a nearly double selectivity than a commercial membrane (Neosepta AMX). However, we also found that the membrane electrical resistance increased with the PVDF concentration. As the nitrate selectivity was found to be independent of the membrane thickness, selective boundary layer effects were ruled out, suggesting that the ion selectivity was mainly driven by the increased affinity between the anion and the membrane, with less hydrated ions more favourably transported. To confirm this hypothesis, PVDF-50 was tested using a multiion solution, including bromide and fluoride. This equimolar solution experiment indicated a direct correlation between ion selectivity and hydration energy of the ion species. Lastly, we investigated the transport of divalent ions through the hybrid membrane, showing a 10 % transport for sulfate.