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Membranes 2020, 10, 221 30 of 72 studies showed also that electroconvective vortices minimized fouling/scaling at both membranes. According to the authors, small intermembrane distances are recommended, since the intermembrane distance plays an important role in ionic transfer when water dissociation is dominant. They concluded also that electrodialysis in an overlimiting condition seems to be more advantageous than in an underlimiting one, but they did not evaluate the energy consumption of both processes [91]. In addition, on acid whey demineralization and lactate recovery, Beaulieu et al. [90] studied the impact of electroconvective vortices on ED process efficiency. They compared the ED process both in underlimiting and overlimiting current conditions in terms of demineralization, lactate recovery, ion migration, temperature increase, overall system resistance and energy consumption. The results showed that overlimiting current conditions led to a higher demineralization (87% based on the total cation concentration) in half the time (30 min at 3 A vs. 60 min at 0.7 A) while the migration of lactate (around 33%) was not enhanced or improved by the overlimiting condition and the formation of ECVs. ECVs are known to shrink the laminar boundary layers as well as to reduce or eliminate the concentration polarization at the membrane interface, leading to the release of current carriers and a better mass transfer of ionic species [21,54,66], however, all the main studies reported in the literature were carried out on model solutions, and solutions containing organic acids had never been tested. Thus, it appeared from these results that the mass transfer of organic acid seemed to be not improved by electroconvective vortices in contrary of mineral species. Indeed, the production of H+, in the whey compartment at the interface of the AEM after the LCD was reached would have impacted the migration of the lactate due to its pKa of 3.86 to produce non-charged lactic acid, decreasing its potential to migrate. Concerning ion migration, whatever the condition of current applied, the potassium was the ion having the highest migration rate related to its high electrophoretic mobility and concentration. Furthermore, as already observed in the literature, a higher current density favors the concentration polarization and increases the migration of monovalent ions, which is related to their higher diffusivity in the membrane’s boundary layer [83,118,119]. This explains why calcium and magnesium closely followed potassium with similar migration rates in the overlimiting condition. However, in the underlimiting condition, magnesium had a smaller migration rate (41.4% vs. 51.2% and more for other cations) that can be explained by its slower mass transfer [65,130] and the absence of ECVs that can enhance its migration and all other cations [21,64–66]. Finally, concerning the energy consumption, it was reported that the application of overlimiting condition increased the REC, compared to ohmic condition, during lactic acid recovery and ion removal by electrodialysis: the increase in current was 4.3-fold, but the energy consumption increased respectively by 8–11 folds. 4.2. Application of PEF during ED 4.2.1. ED-PEF The application of PEF on complex solutions (Ion-exchange brine solution, wastewater, food by-products, beverages, etc.) more close to real industrial applications is recent and mainly benefits from the advantages of energy consumption and improved transfers. When natural organic matter (NOM) is removed by ion-exchange (IX) via anionic IX resin from potable water for aesthetic, operational and indirect health concerns, it generates spent brine. To overpass this main drawback related to the strict discharge regulations and limited and costly brine management options, ED-PEF desalination of the IX brine was tested recently [107]. The highest demineralization rate (95.5%) was achieved when the IX spent brine was treated under the PEF regime with conventional permselective AEMs instead of monovalent permselective AEMs (AMX vs. ACS) and the PEF regime was efficient in intensifying the desalination process only when the conventional AEMs were used. However, desalinating the IX spent brine with AMX membranes and under PEF regime resulted in a higher sulfate content in the produced NaCl solution. By drawing a correlation between the AEMs characterization data and evolution of the electrodialytic parameters, the authors deducted that fouling and/or adsorption of low molecular weight acids and low molecular weight neutrals fractionsPDF Image | Electrodialytic Processes
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