Transfer of electrolyte ions and water dissociation in anion-exchange membranes under intense current conditions

Abstract

Polarization characteristics of electromembrane systems (EMS) based on the Russian commer cial heterogeneous membranes MA 40 and MA 41, the anion exchange heterogeneous membrane AMH (Mega, Czech Republic), and the modified membrane MA 40M are studied by the method of rotating mem brane disk in dilute sodium chloride solutions. The effective transport numbers of ions are found; the partial voltammetric characteristics (VAC) with respect to chloride and hydroxyl ions are measured; the limiting cur rent densities are calculated as a function of the membrane disk rotation rate. In terms of the theory of the overlimiting state of EMS, based on experimental VAC and the dependences of the effective transport num bers on the current density, the following internal parameters of systems under study are calculated: the space charge and electric field strength distribution over the diffusion layer and the membrane. It is shown that water dissociation can be virtually completely eliminated by substituting chemically stable quaternary ammo nium groups inert with respect to water dissociation in the surface layer of a heterogeneous anion exchange membrane MA 40 for the active ternary and secondary functional amino groups. The maximum electric field strength values at the membrane/solution interface, which were found in the framework of the theory of over limiting state, turned out to be close for all anion exchange membranes studied, namely, (7-9) × 10 6 V/cm. This suggests that it is the nature of ionogenic groups in the surface layer rather than the field effect that plays the decisive role in the membrane ability to accelerate the water dissociation reaction. It is proved experimen tally that in highly intense current modes of the electrodialysis process, the thermal hydrolysis of quaternary ammonium bases occurs in strongly basic MA 41 and AMH membranes by the Hofmann reaction to form ternary amino groups catalytically active in water dissociation reaction. Based on the concept on the catalytic mechanism of water dissociation, the fraction of ternary amino groups formed by thermal hydrolysis in the surface layer (the space charge region) of monopolar anion exchange membranes MA 41 and AMH is assessed quantitatively as 0.7 and 6.5%, respectively.

Partial Fluxes of Phosphoric Acid Anions through Anion-Exchange Membranes in the Course of NaH2PO4 Solution Electrodialysis

“…In the case of NaCl (Figure 1), the shapes of the total and partial CVCs were close to those described in the literature [45,46] for strong electrolytes, which do not participate in the proton-exchange reactions. The experimental limiting current, which is determined by the tangent intersection method (Figure 1), was close to ilimLev.…”

Positive Impact of Pulsed Electric Field on Lactic Acid Removal, Demineralization and Membrane Scaling during Acid Whey Electrodialysis

“…pH decreases in the AWComp during acid whey deacidification were reported, in previous studies in DC current conditions [10,11], to be caused by the dissociation of lactic acid following lactate migration, and by protons generated from water splitting phenomenon appearing mainly, after a certain time, on the AEM diluate side, as observed by other authors on model salt solutions [22,23]. However, the results of the current study demonstrate that the application of an adjusted PEF ratio was efficient in the reduction of such pH variations.…”

Transport properties of amino acid ions at isoelectric point in electrodialysis

“…The relevant data were shown in the left part of Table 4 (the simulated ED process). Table 4 indicated that both of glutamic acid and lysine were inclined to be adsorbed by cation-exchange membranes in the simulated ED process, which was reasonable concerning the fact that cation-exchange membranes usually have larger exchange capacities and higher stability than anion-exchange membranes [26]. However, the adsorption characteristics changed in the actual ED process.…”