Abstract
Visualization of electroconvective (EC) vortices at the undulated surface of an AMX anion-exchange membrane (Astom, Osaka, Japan) was carried out in parallel with the measurement of chronopotentiograms. Weak polybasic acid salts, including 0.02 M solutions of tartaric (NaHT), phosphoric (NaH2PO4), and citric (NaH2Cit) acids salts, and NaCl were investigated. It was shown that, for a given current density normalized to the theoretical limiting current calculated by the Leveque equation (i/ilimtheor), EC vortex zone thickness, dEC, decreases in the order NaCl > NaHT > NaH2PO4 > NaH2Cit. This order is inverse to the increase in the intensity of proton generation in the membrane systems under study. The higher the intensity of proton generation, the lower the electroconvection. This is due to the fact that protons released into the depleted solution reduce the space charge density, which is the driver of EC. In all studied systems, a region in chronopotentiograms between the rapid growth of the potential drop and the attainment of its stationary values corresponds to the appearance of EC vortex clusters. The amplitude of the potential drop oscillations in the chronopotentiograms is proportional to the size of the observed vortex clusters.
Highlights
Two mechanisms for the generation of protons and hydroxyl ions are possible in membrane systems containing species of weak polybasic acids [12,23,26]
The first does not differ from water splitting (WS) [36,41,62,63,64], which is well known for the case of strong electrolytes (NaCl, KNO3, etc.)
Visualization of electroconvective vortices at the membrane surface and parallel measurements of the chronopotentiograms of this membrane are an informative way to study the mechanisms of mass transfer of species of polybasic weak acids
Summary
Electrodialysis, electrophoresis, and capacitive deionization with ion-exchange membranes are increasingly used in hybrid membrane technologies for the separation of anions from polybasic weak acids [1,2,3,4,5,6], amino acids [7], proteins [8,9], and natural dyes [10,11], as well as from biomass, communal and livestock effluents, and from food industry waste These methods are attractive due to the possibility of isolating these substances from strong electrolytes [12,13,14], concentration [15], fractionation [16], and the conversion from ionic into molecular forms and vice versa [5,17,18] without the use of additional chemical reagents. The protons are excluded from the tonation–deprotonation reactions [24,65,66]
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