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Membranes 2020, 10, 221 18 of 72 (2) energy consumption, (3) heat production by Joule effect and (4) impacts of water dissociation and vortices on the membrane integrity in the long term. Indeed, as demonstrated by Mikhaylin et al. [88], the energy cost for an electrodialytic process depends on the way the electricity is produced (nuclear plant, hydroelectric plant, coal plant, wind, etc.) and the source used (renewable and non-renewable). Concerning the energy consumption of the process in overlimiting condition and the production of heat, the studies are generally carried-out with controlling the temperature of the process, since the temperature of the solution increased rapidly due to the Joule effect and/or the continuous function of pumps. If the temperature of the solution has to be controlled, by the use of a heat-exchanger for example, this will lead to an additional energy consumption. However, it was demonstrated, that for a 62% demineralization of whey, by ED in ohmic conditions, the time required decreased by a factor of 2, the total mass flux increased 3 times whereas specific energy consumption was the same when the temperature was changed from 15 to 55 ◦C [89]. Consequently, such an induced increase of temperature in overlimiting condition could be a potential advantage. Indeed, as demonstrated very recently by Beaulieu et al. [90], it would improve the transfer of ions. However, according to these authors, the use of an overliming current condition coupled with the temperature increase induced did not improve the energy efficiency of the process. On the contrary, the application of overlimiting condition increased by 7–9 times the energy consumption compared to ohmic condition (for more information see Section 4.1). Santana Barros et al., recently evaluated the impact of underlimiting conditions and concluded that electrodialysis in overlimiting condition seems to be more advantageous than in underlimiting one [91]. However, the authors did not evaluate the energy consumption of both processes (for more information see Section 4.1). Many adverse effects of water splitting at the membrane interface on structural changes of IEM surface used in electrodialytic processes at limiting current density (LCD) condition has also been observed [46,92–94]. Enhanced dissociation of water molecules and changes in membrane physicochemical properties in used AEM was noticed, suggesting a gradual degradation of AEM during ED process which consequently increased the surface roughness. Indeed, many anion-exchange membranes intrinsically contain quaternary ammonium groups (–N(CH3)3+), in which the nitrogen atom does not carry any lone electron pair and consequently water splitting cannot occur (see Section 3.3.1, rate constant klim = 0 s−1). Nonetheless in overlimiting conditions, OH− electrogenerated by water dissociation can act upon these fixed quaternary ammonium groups to convert them into tertiary (–NH(CH3)2+) or secondary amine (–NH2(CH3)+) as described by Hofmann degradation (E2), elimination (E1) and nucleophilic substitution [93,95–97]. In addition, Choi and Moon [94] noticed this degradation after only one hour under a strong electric field. The generated tertiary amines can further induce the catalytic water splitting by reacting with pre-polarized water molecules by reversible protonation and deprotonation under strong electric field [92,94]. Such degradations of aliphatic quaternary ammonium group to tertiary amines were suggested at LCD and alkaline conditions [46,92,93,96]. In addition, according to Pismenskaya et al. [33] after 10 h of application in overlimiting current conditions and exposure to vortices, cavities would be formed on the surface of the ion-exchange membranes. However, these cavities would occupy only 0.4% of the membrane surface after 100 h of overlimitng current application and would even increase mass transfer. Thus, the effect of overlimiting current on the lifetime of membranes during extended treatment duration comparable at the ones in the industry (between 3000 and 20,000 h) is still unknown. 3.5. Pulsed Electric Field Application and Influences on Membrane Phenomena 3.5.1. Principle of Pulsed Electric Field The pulsed mode is a non-stationary regime applying a hashed current (Figure 12) or voltage: for a defined time, the system is under the influence of a constant current/voltage (Pulse lapse—Ton), then the current/voltage is turned off for a fixed time (Pause lapse—Toff). Pulsed electric field (PEF)PDF Image | Electrodialytic Processes
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