Multicomponent absorption of anions in commercial anion-exchange membranes

Abstract

The equilibrium uptake of aqueous NaCl–NaBr and NaCl–NaNO 3 mixtures into Neosepta ® AMX and Selemion ® AMV anion-exchange membranes was measured experimentally and modeled theoretically, where the total external salt concentration was fixed at 0.1 M. The model, which considered the membrane microstructure as an array of cylindrical pores of identical radius and included electrostatic interactions and ion hydration effects, accurately predicted anion uptake in the membranes. During competitive uptake, the anion with the larger hard-sphere radius was preferentially absorbed, with an observed and computed selectivity trend of NO 3 − > Br − > Cl −. By matching experimental data to the model, the average radius of pores in water-equilibrated membranes was found to be 3.3 nm (AMX) and 2.4 nm (AMV), indicating that an AMV membrane (1.98 mmol/g IEC) contains a greater number of smaller pores as compared to AMX (1.35 mmol/g IEC). The calculated pore-wall charge density in the two membranes was nearly the same. The NO 3 −/Cl − uptake selectivity was greater than that for Br −/Cl − in both membranes, but the selectivity for a given anion pair was effectively the same for the two membranes at a given external solution composition. This result was explained by model calculations which showed that anion uptake selectivity was essentially independent of pore radius (for pores in the 1.5–4.0 nm range) and a strong function of the fixed-charge site concentration on the pore wall.