Abstract
Although Nafion-117 is a cation-exchange membrane, anions can readily diffuse through it when it is in contact with a high ionic strength or low pH solution. In this study, diffusion coefficients of anions (bisulfate and Cr(VI)) through a Nafion membrane were measured under these two conditions using a two-compartment cell. In addition, the partition coefficients of these anions in the membrane were measured. For sulfate concentrations of about 0.1 M, the average bisulfate partition coefficient was about 0.23 (mg/cm 3 water)/(mg/cm 3 membrane). The adsorption isotherm for Cr(VI) in Nafion membranes equilibrated with 0.1 M NiSO 4 + CrO 3 solution was linear (C m =0.079C e +12.93, C e as mg/cm 3 CrO 3 , r 2 >0.99), indicating that the Cr(VI) partition coefficient decreased with increasing chromic acid concentration (1-4 M range). The apparent diffusivities of HSO 4 - and Cr(VI) anions in the Nafion membrane were 1.08x10 -6 and 8.06x10 -7 cm 2 /s, respectively. The bisulfate concentration variation resulting from diffusion in the system was well-simulated by a numerical program based on Fick's second law. An alumina ceramic diaphragm was also tested for comparison. The bisulfate partition and diffusion coefficients in the ceramic were 0.4 and 2.17x10 -6 cm 2 /s, respectively.
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