Historical PerspectiveNovel ionic separation mechanisms in electrically driven membrane processes
Graphical abstract
Introduction
Electromembrane processes couple ionic separation and transport through an ion selective membrane with a potential difference as the driving force [1]. ED is an example of an electromembrane process which has been applied for over 50 years for the desalination of brackish water [2]. Now, new ED-based processes have been developed and a number of different applications have been achieved, including desalination, pure water production, chemical synthesis, wastewater treatment, and energy conversion and storage [[3], [4], [5], [6]].
Ion concentration polarization is an important electrokinetic phenomenon found in electrochemical systems such as ED and other electromembrane methods, and it occurs at the interface between two channels of sufficiently different size (e.g., a microchannel and a nanochannel) [7]. In electromembrane processes, the nanochannel serves as the ion exchange membrane. Ion concentration polarization occurs due to counterion permeation and coion exclusion by the membrane which creates imbalances in concentration at the membrane surface and in the bulk solution. This phenomenon causes a dramatic increase in cell resistance due to the vanished ion concentration at the membrane surface. While ion concentration polarization can be used for desalination of seawater in a microfluidic device [8], this phenomenon results in operational problems in electromembrane processes. Increasing the potential difference at this point has no meaningful effect on ion transport, and this special regime is known as the diffusion limited current (DLC). As a consequence, higher energy consumption is needed to maintain the desired current or ionic flux. In the case of energy storage, the increase of cell resistance is undesirable because it slows down electrochemical reactions and reduces the ionic current [9]. The increasing cell resistance also results in intensive water dissociation, which typically corresponds to lower current efficiency. In addition, generated hydroxide ions from the dissociation reaction may cause severe scaling on anion exchange membranes due to precipitation of multivalent salts [[9], [10], [11], [12]]. Another important issue regarding the lower ionic separation rate is the capital cost of the electromembrane process. A lower ionic separation rate requires a larger system or plant for a similar output. In addition, there is a trade-off between energy efficiency and desalination rate [13]. Operating at high energy consumption could lead to a higher desalination rate. On the other hand, it results in a high operating cost due to the high energy consumption and low energy efficiency. If current transfer can be improved, the energy consumption and the system size may be decreased simultaneously.
An electromembrane process is usually operated below the DLC to avoid unused electrical power. Experimental studies have shown, however, that operating at current densities in excess of the DLC (i.e., at overlimiting current, or OLC) improves ion transport [14]. This improvement in turns allows for reduction of the area of ion exchange membranes and the corresponding capital costs of an electromembrane process for a given throughput [15]. These interesting results have driven numerous investigations to determine better strategies to achieve OLC [16]. Progress on the theory of OLC also includes the finding of novel ionic separation mechanisms that can result in OLC, such as ion concentration polarization at micro/nanochannel interfaces [17], shock deionization [9], and deep deionization by using conductive media (e.g., ionic bridges) in electromembrane cells [18]. In nanofluidic systems, charged nanochannels or nanopores are constructed for ion transport. The charged nanopores can attract counterions and exclude coions so that they can replace the role of traditional ion exchange membranes [19]. Applying a potential difference in these channels can induce electroosmotic streams leading to OLC [20]. In deionization shock waves, the presence of charged porous media in an electromembrane cells can increase the ion transport due to the presence of surface conduction by electromigration and surface convection by electroosmotic vortices [21]. As a result, OLC is obtained wherein the ion concentration polarization can propagated through the pores as a depletion (or deionization) shock wave. The shock waves enable extreme gradients in concentration, which is normally not possible in conventional ED systems. Surface conduction and electroosmosis—which complement deionization shocks—also enable desalination at very low concentrations, such that the feed could be dilute but still contaminated with trace amounts of contaminant. Bridging ion transport from the bulk solution to ion exchange membranes is another way to surpass the DLC. By introducing conductive media inside an electromembrane cell, it is possible to sustain continuous deionization and ion removal in a dilute solution with high electrical resistance. This technique has been adopted in modified ED and is known as electrodeionization (EDI), which has been successfully commercialized for the production of ultrapure water [18,[22], [23], [24], [25]].
This review focuses on novel ionic separation mechanisms that can enhance the ion transport of electromembrane processes via OLC. First, progress on the theoretical and practical quantification of DLC is introduced, followed by a brief discussion on enhanced ion transport in the OLC regime. This paper then highlights ion concentration polarization at micro/nanochannel interfaces, deionization shock waves, and ionic bridge mechanisms. The final part of this paper discusses how these novel ionic separation mechanisms are applied in electromembrane processes for water treatment.
Section snippets
Theoretical and practical estimation of the diffusion limited current (DLC)
A key parameter for designing and operating electromembrane processes is DLC. During the operation of electromembrane processes, ion concentration polarization occurs due to asymmetry in the transference of oppositely charged ions in solution (or electrolyte) across an ion selective element [2]. In the bulk solution, the transport of cations and anions is usually similar. At ion exchange membrane surfaces of in diluate compartments, the concentration of ions decreases due to the permeation of
Ion transport in micro/nanofluidic systems
Nanofluidics refer to a study of fluid motion in a nanosized structure (0–100 nm) [20]. In micro/nanochannels, wall properties are comparatively important to the ionic transport. The thickness of Debye layer is comparable to the size of the channels resulting in unique permselectivity of ions [73]. This effect is associated with sidewall-ion interactions. Applying electrical fields in nanochannels can result in the movement of liquid in the channel due to the motion of counterions. The fluid
Deionization shock waves
In microstructures with charged surfaces, surface conduction and electroosmosis play important roles in ion transport especially at high voltage where nonlinear dynamics arise from the competing transport of ions both in the bulk solution and at the solid. The nonlinear dynamics result in the depletion of ions due to concentration polarization at OLC [16,80]. The basic principle of this ion transport mechanism is known as a “deionization shock wave” and has been discussed in detail in ref. [9].
Shock electrodialysis (shock ED)
As previously discussed, one of the possible ways to operate an electromembrane process at OLC is to use the novel mechanism of deionization shocks. Shock ED refers to a separations process that is based on deionization shocks in porous media. In shock ED, an electrolyte is transported into a weakly charged porous slab with micrometer-sized pores that is placed between ion exchange membranes and electrodes [86]. Meanwhile, anolyte and catholyte solutions are circulated through the anode and
“Ionic bridges” for deep deionization in electrodeionization (EDI) systems
Ion concentration polarization limits the use of conventional ED systems in ionic separation of dilute solutions. When the salt concentration on the membrane surface approaches zero at the DLC, increasing the potential difference leads to no further ion removal. Therefore, conventional ED cannot achieve deep deionization and is not capable of producing high purity water. Later on, a modified ED process was introduced which combines conventional ED with ion exchange processes in a single unit
Conclusion and future outlook
Experimental evidence shows that the use of OLC can enhance current transfer. By utilizing OLC, it is possible to reduce the required ion exchange membrane area and the corresponding investment cost of an electromembrane plant. Therefore, numerous investigations have been made to gain a better understanding of mechanisms and find strategies to achieve OLC.
Overlimiting current transfer has been observed in various electromembrane systems. In nanofluidic systems, charged nanochannels or nanopores
Declaration of Competing Interest
None.
Acknowledgement
This research was funded by the Indonesian Ministry of Research, Technology and Higher Education under WCU Program managed by Institut Teknologi Bandung.
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