Abstract
In electromembrane systems, the theoretical study of salt ion transport usually uses mathematical models of salt ion transport in the depleted diffusion layer of ion-exchange membranes. This study uses a one-dimensional mathematical model of salt ion transport in a cross-section of a desalination channel formed by anion-exchange and cation-exchange membranes, taking into account an effect of a dissociation/recombination reaction of water molecules. The reaction on the one hand leads to an overlimiting mass transfer due to the effect of exaltation of the limiting current. On the other hand, an appearance of new electric charge carriers (hydrogen and hydroxyl ions) can reduce the space charge that occurs in membranes and suppress an electroconvective mechanism of overlimiting transport. Thus, there is a problem of studying these phenomena together, taking into account their mutual influence, and this article is devoted to the solution of this problem. Theoretically, using a method of mathematical modeling and numerical research, main regularities are established; in particular, it is shown that the dissociation/recombination reaction of water molecules does not lead to the destruction of the double electric layer at the membranes, but also creates a new double electric layer in the middle of the desalination channel. Thus, the space charge and the dissociation/recombination reaction significantly affect each other and simultaneously the transport of salt ions.
Highlights
Electrodialysis is widely used in wastewater treatment processes, including for toxic heavy metal salts [1,2,3,4]
In a number of works [17,18], we have shown that, in the expanded region of the space charge, the electric field intensity reaches such high values that it pulls apart the hydrogen and hydroxyl ions in this region and a non-catalytic reaction of dissociation of water molecules occurs at the maximum possible speed
Starting with the potential jump 0.3 V, the concentration of hydroxyl ions and salt ions becomes of the same order and, it is necessary to take into account the dissociation/recombination reaction, this potential jump corresponds to a prelimiting current
Summary
Electrodialysis is widely used in wastewater treatment processes, including for toxic heavy metal salts [1,2,3,4]. The appearance of an extended space charge region and its accompanying electroconvection and the effect of the dissociation/recombination reaction of water molecules on the transport of salt ions in these articles are theoretically studied separately. Modification of the membrane surface reduces the catalytic dissociation of water molecules to almost complete disappearance [20,21] This leaves two inextricably linked phenomena, namely, the non-catalytic dissociation/recombination reaction of water molecules and the appearance and development of an extended space charge region and their effect on salt ion transport, which must be studied together. The article shows theoretically that the dissociation/recombination reaction of water molecules does not lead to the destruction of the double electric layer in ion-exchange membranes, and creates a new double electric layer in the middle of the desalination channel both at prelimiting currents and at overlimiting currents. To find out what happens in the internal cross-section points of the desalination channel
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