Polyelectrolyte multilayers coating of aliphatic polyamide anion-exchange membranes to increase monovalent/divalent anions selectivity in electrodialysis

Layer-by-layer (LBL) modification of aliphatic polyamide anion-exchange membranes (AEMs) with poly(4-styrenesulfonate) (PSS) and protonated poly(allylamine) (PAH) coatings increases monovalent/divalent anion transport selectivities. Remarkably, in electrodialysis (ED) Cl−/SO42− selectivities increase from 1.3 to ∼180. In addition, current efficiencies reach ∼90 %. Selectivity, however, decreased to ∼50 when the Cl−/SO42− source-phase ion concentrations increased from 10 to 100 mM and from ∼180 to ∼10 when the current density went up from 0.63 to 2.23 mA·cm−2. The current efficiency also went down to ∼50 % at 2.23 mA cm−2 which indicates the introduction of some parasitic phenomenon at higher current densities. Moreover, for a coated membrane, the polyelectrolyte multilayer (PEM) coating facing the source phase is comparatively more selective in ED whereas the PEM facing the receiving phase is relatively more selective in diffusion dialysis (DD). Also, the (PSS/PAH)11–films (11 bilayers of PSS and PAH polyelectrolytes) which are terminated in polycations show much lower Cl−/SO42− selectivity and current efficiency as compared to the (PSS/PAH)10PSS-films (10 bilayers of PSS and PAH polyelectrolytes capped with a layer of PSS polyelectrolyte) which are terminated in polyanions. These relatively inexpensive but highly selective and current efficient AEMs can be used in ED for the purification of salts containing divalent and/or multivalent anions.