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

Transport competition between the protons and the alkyl metal cations Li+, Na+, K+ and Cs+ across a cationexchange 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.

Fig. 7. Evolution of cell voltage during the ED of the different solutions of NaI and NaCl. in hydro-organic or organic solutions. It can be explained by the difference of conductivity of the two couples of salts: NaCl or NaI with Na2SO4 during the test. The nature of the solvent influenced the transport of metallic ions passing through a cationexchange membrane in an ED process. The MeOH/W configuration was the most favourable to the transfer of the cations because of the difference of salt solubility in the two media and of the variation of cation sizes during passage through the CMH membrane. This latter seems to be more permeable to acetonitril than methanol and N-methyl-formamid in the presence of sodium iodide. However, the amount of the acetonitril decreased drastically with potassium iodide. This result seems to be plausible since the ratio calculated with this salt is in good agreement with the value of the potassium solvation number. In the presence of the chloride halides, the solvation of the cations was deduced from the MeOH/W configuration, following that observed in water. The voltage of the ED cell depended on both the salt and solvent. We observed that the transport number was inversely proportional to the variation of voltage during the tests with the W/W and MeOH/W configurations, but with the MeOH/MeOH configuration, the transport number was function of cation size and its solubility in solvent. The variation of tension with this configuration was inversely proportional to the salts solubility. The co-ion was not seen in the transport number but played a role in the conductivity of solution and in the variation of voltage at the end of experiment. The feasibility of this process in both hydroorganic and organic media leads us to undertake future research by studying the transport of polyammonium cations, which are the main components of anion-exchange membranes.