Nafion-117 behavior during cation separation from spent chromium plating solutions

This study investigated the behavior of Nafion-117 ion-exchange membranes that are commonly used as the separator in electrolytic regeneration of spent chromium plating solutions. In the co-presence of Cu 2+, Ni 2+, and Fe 3+ impurities (̃0.01-0.1 M) in ̃0.5-2.0 M H 2CrO 4, the exchange equivalents of Ni 2+ by Nafion-117 was double that of Cu 2+ (0.02 mmol of H+/(g of Nafion)) but smaller than that (̃0.03-0.15 mmol of H+/(g of Nafion)) of Fe 3+, exhibiting the order Fe 3+ ≥ Ni 2+ ≥ Cu 2+ in Nafion affinity. The water content (̃41%) of a Nafion-117 membrane soaked in 2 M H 2CrO 4 was greater than those (̃35-38%) in the contaminated 2 M chromic acid solutions, whereas the porosity (0.24) of the former was smaller than those (0.29-0.37) of the latter. For the Nafion-117 in 2 M chromic acid solutions with/without cation impurities, (1) the tortuosity coefficient of Fe 3+ was about 1.8 times that of Cu 2+ or Ni 2+, (2) the calculated pore radius was 3.2 nm for a multiimpurity case (0.1 M Cu(II), Fe(III), Ni(II), and Cr(III) each) based on the Gierke's model, and (3) the cation impurities did not significantly influence the numbers of water molecules in Nafion clusters. The Nafion-117 withstood the pH < 0 environment with only minor rotation in the fluorocarbon backbone due to accommodation of anionic Cr(VI) within clusters. © 2009 American Chemical Society.