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Membranes 2020, 10, 221 31 of 72 of NOM deteriorated AMX membranes. However, during the three consecutive runs the characteristics of the IEMs and applied electric field (CC vs. PEF) had no significant impact on the relative energy consumption values (≤2.1 Wh/g NaCl). Polymer-flooding produced water is also an abundant by-product from the oil and gas industry, with potential reuse after partial demineralization [131,132]. To avoid concentration polarization and fouling which hampers the desalination of polymer-flooding produced water (PFPW) by electrodialysis, Sosa-Fernandez et al. carried out an extended study on the application of eight different PEF conditions [84] (Table 1). They demonstrated that the application of PEF improved the ED performance in terms of demineralization (around 9% for PEF 1 s/1 s in comparison with CC mode of current) and energy consumption (36% reduction for PEF 1 s/3 s). When comparing the performance of the 1 s/1 s and the 1 s/3 s regimes, the gain in energy consumption for 1 s/3 s was demonstrated higher than the gain in demineralization for 1 s/1 s. However, the regime 1 s/3 s also implies that the membrane stack is effectively in use only 25% of the time. Considering all these factors, they concluded that the most promising of the regimes would be 1 s/1 s. They also claimed that in general, the shorter the pulses, the higher the demineralization rate and the lower the energy consumption. The only exception was for the runs with 0.1 s pulses, which rendered low energy consumption but also lower demineralization percentages, possibly due to the high frequency that could cause a closest packing of the partially hydrolyzed polyacrylamide [39]. Two by-products produced in very large quantities, by the dairy industry, all around the world are sweet and acid wheys. Lemay et al. [64] reported that a sweet whey demineralization treatment with 2 h of applied current led to an increased mass transfer efficiency with the frequency of the pulse/pause combination; 5 Hz (0.1 s/0.1 s) being the best condition. For acid whey demineralization (3 h treatments), the best PEF conditions providing the highest cumulative removal of Ca2+, Mg2+ and Na+ were 5 s/5 s and 15 s/15 s [83]. In addition, the migration of lactic acid from acid whey was accelerated by 16% and the energy consumption decreased by 33% for PEF condition of 25 s/25 s [113]. The higher migration rates of Ca2+ and Mg2+ observed, during whey treatment by electrodialysis for PEF conditions with longer pause lapse, was explained by the partial restoration of (1) the concentration profiles in the solutions adjacent to the membrane surface and (2) the membrane control over the kinetics of ion separation [65]. Indeed, high electrostatic interaction of the divalent ions with the fixed ions essentially reduces their mobility in the membrane. However, at low current densities, the factor of higher affinity of calcium (and magnesium) is the dominant one, and sulfo-cation-exchange membranes are preferably permeable for the doubly charged cations [133–135]. Nevertheless, this specific permselectivity towards doubly charged counter ions is lost with increasing current density [133,134,136]. In PEF, a partial restoration of the specific permselectivity is observed. The reason of partial restoration of the specific permselectivity is that, during the pause lapse, there is a partial reconstitution of the concentration profiles in the solution adjacent to the membrane surface [65,99,101,121]. That is, the concentration profiles near the membrane interface tend to restore their shape corresponding to zero current density. With this, the membrane also partially restores the control over the kinetics of ion separation. Furthermore, the remnant vortices during the pause lapse would contribute in mixing the near-surface solution, hence, in a quicker restoration of the concentration profiles. In addition, the ECVs developed near the membrane during the pulse of current should mix the near-surface solution and to maintain a higher solution concentration at the surface. To confirm this hypothesis of selective transport of divalent ions under PEF, a model based on the non-stationary 1D Nernst–Planck and Poisson equations was proposed by Lemay et al. [65]. The computation using the model gives the following values of the effective transport numbers for Ca2+ and Na+ at j = 0.6 jlim: 0.52 and 0.48 in the continuous current mode, and 0.68 and 0.32 (respectively, for Ca2+ and Na+) in the PEF mode at frequency equal to 0.5 Hz. In addition, in 2020, Lemay et al. [65] demonstrated for the first time, during demineralization of sweet whey, that a high frequency PEF condition of 0.1 s/0.1 s (5 Hz) required almost the same time, including the PEF pause duration, than continuous current condition to reach a final demineralization rate of 70% and that with lower number of charges transported, energy consumption and pH variationsPDF Image | Electrodialytic Processes
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