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Membranes 2020, 10, 221 20 of 72 times in comparison with continuous current (CC), depending on the pulse-pause characteristics [54]. Regarding fouling, recent studies reported the prevention of fouling by application of PEF. Indeed, due to the disappearance of the boundary layer and the regular re-homogenization of the solution around the membranes, the PEFs have proven to be an effective solution to mitigate fouling caused by humate [39,102], proteins [103,104], peptides/amino acids [105], lignin [106], natural organic matter [107], partially hydrolyzed polyacrylamid [84] and mineral scaling [64,65,83–85,104,108–113] during electrodialytic processes (Table 1). Deep investigations have been carried out to reveal the mechanisms of scaling prevention under the action of PEF [104,109,110,114] as well as fouling mitigation by natural organic matter [107,115–117] and lignin [106]. Regarding specific migration of mineral ions, the difference in monovalent and divalent ion demineralization caused by PEF could be explained by several points reported in the literature. First of all, it has been shown that low current density favor divalent ion migration due to their stronger interaction with the membrane’s functional groups [118,119]. Inversely, a higher current density increases the concentration polarization and favors the separation of monovalent ions due to their higher diffusivity in the membrane’s boundary layer. During pause duration, the diffusion layer is rapidly enriched in ions and the concentration polarization is reduced, but a potential difference is preserved and a slight electro-osmotic movement towards and through the membranes persists in the solution [54]. Hence, divalent ions such as calcium and magnesium will preferentially interact with the membrane’s ion-exchange groups due to their higher sorption coefficient [120] (for more explanations see Section 4.2.1). Finally, the equipment needed for the generation of pulse-pause from a generator is quite simple, via the use of a pulse generator coupled to the power supply or the automatic pulse generation via a computer controlling the power supply. Consequently, the simplicity of such a piece of equipment and their low cost should favor the introduction of PEF at large scale. Despite its simplicity and flexibility, and since no special anode is required in comparison with EDR, PEF is not as intensive for self-cleaning. Moreover, the total working time of the ED system is increased by pausing the system. The only way to have a similar working time as for CC to run a specific process is to increase the membrane area and number of cells needed. Such a drawback has not been considered yet in the literature, which is a critical factor in desalination or other ED applications such as EDBM. Indeed, the operation time directly affects the economy of the processes at industrial scale. However, very recently, Lemay et al. [65] demonstrated for the first time that a high frequency PEF condition of 5 Hz required almost the same time, including the PEF pause duration, than CC condition to reach the same final demineralization rate and that with a lower energy consumption (for more precisions see Section 4.2.1). In addition to the points highlighted before, to fully evaluate the potential of PEF application, it will be important to study the membrane integrity after long use periods of PEF, following electroconvective appearance and water dissociation phenomena as mentioned previously in Section 3.4.3 for overlimiting current conditions. Finally, it is worth to mention that the application of PEF, at a larger scale, will depend on the availability of power sources designed to be continuously switched on and off.PDF Image | Electrodialytic Processes
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