Electro-transport of protons, alkali metal cations and solvent molecules through a commercial cation-exchange membrane conditioned with aqueous and organic solutions

Abstract

Transport competition between the protons and the alkyl metal cations Li +, Na +, K + and Cs + across a cation-exchange membrane (CEM), called CMH, was studied in terms of an electrodialysis process with three different configurations: water/water (W/W), solvent/water (S/W) and solvent/solvent (S/S). Under an electrical field, the cations migrated from the feed compartment towards the receiving compartment. The transport number of the metallic cation was generally found to be smaller than that of the proton in the W/W configuration, explaining the proton jumps in the presence of water. However, in the S/W configuration, the transport number of the metallic cation increased significantly, showing the effect of the solvent on the cation. Unfortunately, the results of the S/S configuration were not meaningful since an acid-base titration of the proton was not reliable in the receiving compartment. In this case we limited ourselves to the values of the metallic cations obtained from atomic absorption spectroscopy. We correlated the transport number with W/W and S/W configurations, which was inversely proportional with the variation of the tension during the tests; but this was not the case with the S/S configuration. With the last configuration, we observed that the transport number was a function of salt solubility and the size of cation. Moreover, the variation of voltage was inversely proportional to salt solubility. The electro-transport of solvent molecules through the membrane in the S/W configuration was achieved by Raman spectroscopy. The results showed that the lowest flux was obtained with acetonitril, whilst for the remaining configurations methanol/water and N-methyl-formamid (NMF)/water, the values were almost equal. Nevertheless, the ratio of the number of moles between the solvent and the metallic cations transferred through the membrane pointed out the solvation effect. The highest value obtained was with the lithium cation.