Effect of polymeric matrix in anion-exchange membranes on nitrate-chloride separations

Summary

Selective separation of monovalent ions such as nitrate from chloride using Anion-Exchange Membranes (AEMs) is challenging. Previously, we showed that an increased polyvinylidene fluoride (PVDF) concentration in AEMs manufactured with an anion-exchange ionomer solution (Fumion FAS-24) increased nitrate over chloride selectivity. The membrane containing 50 wt% of PVDF showed higher selectivity compared to two commercial membranes (AMX and ACS from Neosepta) when tested in electrodialysis. This improved selectivity was associated with increased hydrophobicity of the membrane, facilitating the permeation of less hydrated ions such as nitrate. However, due to concerns regarding per- and polyfluoroalkyl substances (PFAS), there is a quest for substitutes for fluoropolymers. In this study, we investigated whether using alternative polymers to PVDF influences nitrate/chloride separation performance. Polyvinyl chloride (PVC) and polyacrylonitrile (PAN) were blended with Fumion FAS-24 to manufacture new AEMs. The nitrate/chloride selective separation performance of these membranes was tested in electrodialysis and compared with the recently introduced PVDF-based AEM. Results show that although the PVDF-based membrane presents higher hydrophobicity, the PAN-based membrane possesses slightly lower selectivity, while the PVC-based membrane exhibits higher nitrate selectivity. This study proves that increasing the membrane hydrophobicity is a valid strategy to increase selectivity toward nitrate. However, it also suggests that other parameters, such as fixed charge concentration, can play a role. Therefore, balancing properties such as hydrophobicity and fixed charge concentration is imperative to achieving optimal selectivity and performance when developing ion-selective membranes.