Layer-by-layer modification of aliphatic polyamide anion-exchange membranes to increase Cl-/SO42- selectivity

Development of anion-exchange membranes (AEMs) with selectivities among anions may expand the applications of these membranes in salt separations. This work shows that layer-by-layer coating of aliphatic polyamide AEMs with poly(4-styrenesulfonate) (PSS)/protonated poly(allylamine) (PAH) films enhances Cl-/SO42- dialysis selectivities, and the extent of this enhancement depends on the source-phase salt concentrations. For a source phase containing 0.01 M NaCl and 0.01 M Na2SO4 and a receiving phase of 0.01 M Na2CO3, a (PSS/PAH)SPSS coating (5 PSS/PAH "bilayers" capped with a layer of PSS) increases the Cl-/SO42- selectivity from 1.7 to 5.3 in diffusion dialysis and from 1.3 to 7.4 in electrodialysis. Remarkably, the diffusion dialysis selectivity of a coated membrane increases to 140 when the salt concentrations in the source phase are 0.1 M. Even with bare membranes, selectivity increases to 13 with 0.1 M source-phase salt concentrations. Moreover, AEMs coated only on the receiving side show much higher selectivities but lower fluxes than those coated only on the source side. Partitioning experiments and modelling suggest that the increased Cl-/SO42- selectivities at high source phase salt concentrations stem from enhanced Cl- partitioning and electromigration that disproportionately decreases SO42- flux in the AEM.