Water Molecules and Electroconvection on Salt Ion Transport: History
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Contributor:
Anna Kovalenko
,
Makhamet Urtenov
,
Vladimir Chekanov
,
Natalya Kandaurova
Electrodialysis has gained global recognition as a water purification method with the potential to enhance the overall efficiency of the purification process. The efficiency of electrodialysis depends strongly on the hydrodynamics of the process, as the advent of new high performance membranes on the world market removes the kinetic limitations associated with membranes and shifts the stage that determines the economic efficiency of desalination towards the liquid phase.
  • electrodialysis
  • desalting
  • electroconvection
  • dissociation/recombination of water molecules
  • quasi-stationary state

1. Introduction

Water stands out as humanity’s most vital resource and is crucial for all forms of life. However, at present, sustainable consumption is jeopardized in half of the world’s river basins [1]. The most precious form of water, clean drinking water, remains virtually inaccessible to around one billion people in developing countries.
The future scenario looks grim, not only due to climate change [2][3][4] but also because of the anticipated global population surge to 10 billion by 2050 and the rise in living standards accompanied by changing consumption patterns [5][6][7]. With a constrained supply of reliable drinking water, global water scarcity emerges as the foremost challenge for the world community [8]. The primary solution to address water scarcity involves economically and environmentally viable desalination methods [9][10].
Studies in recent years show that there are two approaches that reduce the mass transfer limitations on the electrolyte solution side [11][12]. The first is the use of spacers with which the flow of the solution can be controlled. Researchers have investigated this approach in researchers' study [13][14]. Secondly, the use of electroconvection under intense current modes. However, in this case, such destructive processes as the long-known dissociation reaction of water molecules and the recently discovered space charge breakdown [15][16] arise, which lead to the problem of a joint study of all these processes and their complex influence on salt ions.

2. Impact of the Dissociation/Recombination Reaction of Water Molecules on the Transport of Salt Ions

The papers [17][18] showed that the dissociation reaction of water molecules occurs intensively at current densities higher than the limiting one. Works by Sokirko and Harkatz [19][20] showed that the salt ion fluxes get exalted with the change of the electric field.
At the same time, in [21], it was first shown that the recombination reaction causes the appearance of a space charge region (SCR) in the central part of the desalination channel of the electrodialysis apparatus. The formation of SPZ in electrochemistry is associated with the presence of an interfacial boundary (solution/electrode or solution/ion exchange membrane). In [21], it was theoretically shown that H+ and OH− ions move from the near-membrane regions to the central part of the channel, where a recombination zone appears, in which an SCR is formed from the terminal rate of the recombination reaction. The positive spatial charge of this SCR is due to the local excess of H+ ions moving away from the anion exchange membrane, and the negative spatial charge is due to the local excess of OH− ions moving away from the anion exchange membrane and not having time to recombine due to the finiteness of the recombination reaction rate, although it is large. The dissociation reaction and formation of SCR in the recombination region affect the transport of salt ions as well as electroconvection. Moreover, a number of authors believe that the appearance of new charge carriers H+ and OH− can lead to a decrease, or even disappearance of the space charge, which is the basis for other transport mechanisms, such as electroconvection.
Thus, accounting for the effect of the dissociation/recombination reaction of water molecules is important for understanding transport processes in electromembrane systems.

3. Effects of Electroconvection on Salt Ion Transport

During studies of super-limit transport, several interesting phenomena were discovered. One of them is electroconvection, the existence of which was proved experimentally and theoretically by Rubinstein and colleagues [22]. Electroconvection makes it possible to deliver “fresh” solutions from the center of the desalting channel to the membrane surface and to withdraw the desalinated solution from the membrane. It keeps a relatively high concentration of electrolytes at the membrane surface, which restrains the development of the process of generation of H+ and OH− ions [23][24]. A smaller pH shift of the solution and its better mixing significantly reduce the rate of sludge formation in desalting and concentration chambers [25][26].
At present, electroconvection is the key mechanism of mass transfer intensification in EMS. Electroconvection also plays an important role in numerous microfluidic devices [27], such as electrokinetic micropumps [28], microconcentrators in analytical chemistry [29], shock electrodialysis [30][31], and others [32][33][34].
In the works of Rubinstein I. [35][36][37], Demekhin E.A., Kalaidin E.N. [38][39], the problems of occurrence and stability of electroconvection in micro- and nanofluidics in the absence of forced convection are investigated on the basis of mathematical modeling.
The works by Kwak R. [40], Pham V.S., Han J. [41], and others are devoted to the study of the patterns of transfer of salt ions taking into account electroconvection and the forced flow of solution. The peculiarity of electroconvection in the presence of forced convection is the presence of a different potential drop at which it occurs. In addition, electroconvection, which initially develops as a stable process, gradually becomes unstable with increasing potential drop, passing through a number of stages of bifurcations of electroconvective vortices and their interaction [42][43][44][45][46].
In [15][16], researchers first theoretically investigated the CVC characteristics for high current densities. The results show the presence of several modes of EMS operation associated with the development of electroconvective vortices arising near the cation exchange membrane (CEM) and subsequently near the anion exchange membrane (AEM). Then the vortices begin to actively interact, which leads to a breakdown of the space charge. Later vortices start to interact, which leads to space charge breakdown. This breakdown occurs when regions with positive and negative space charge detach from the membrane, move deep into the solution, meet, and their mutual neutralization occurs.
In [15][16], it was found that the effect of space charge breakdown restrains the development of electroconvective vortices, and leads to a decrease in the size and number of vortices in the breakdown region. This restrains a possible further increase in mass transfer due to electroconvection.
Thus, researchers have discovered a second limiting phenomenon at over limited current densities, which has never been analyzed before either experimentally or theoretically by simulations. The obtained results were published in [15][16].

4. Effects of Spacers on Salt Ion Transport

One of the effective methods to significantly increase the mass transport rate through membranes is the use of spacers, which allow the convective ion flux to be directed to the membrane surface [47][48][49].
Thus, the diffusion layer is significantly reduced in the channels with spacers and this leads to the current increase. This raises the question whether electroconvective vortices appear at high currents in such small diffusion layers.

This entry is adapted from the peer-reviewed paper 10.3390/membranes14010020

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