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Membranes 2020, 10, 221 39 of 72 of the resin can gather 1000–100,000 times more ions than found in solution. Therefore, as transport rate is also determined by concentration (in addition to ion mobility), mass transport by successive collections/discharges in the IX resin can be 50–5000 times faster than in solution [203]. Figure 20. (a) Principle of continuous EDI (CEDI) and (b) current regimes in CEDI and ED (adapted from Hakim et al. [205]). CEM: cation-exchange membrane; AEM: anion-exchange membrane; IX: ion-exchange; A−: anions; C+: cations. The ingenuity of CEDI is that contrary to batch mode EDI, the IX resin can be regenerated continuously in situ in overlimiting conditions due to water splitting in H+ and OH− at the interface of anionic and cationic exchange materials (beads, membranes) (Figure 20a) [203]. This feature is one of the main evolution compared to traditional EDI and more advanced forms of IX such as mixed-bed ion-exchange (MBIX) [209], also known as mixed-bead Deionization (MBDI) [192], which uses strong mineral acids and bases for the regeneration of the resin, greatly lowering environmental-friendly aspects. This continuous feature as well as the high conductivity of the cell allow for a deep deionization compared to ED and IX taken individually. CEDI is then sometimes referred to as hybrid ion exchange/electrodialysis (IXED) [207]. The first commercial inception of CEDI was in 1987 and about three decades later, thousands of commercial CEDI systems produce high purity deionized water with 0.1–1500 m3/h operating capacities [204,206]. Electrical consumption for water treatment by CEDI is usually 0.05–0.8 kWh/m3 depending on effluent and purity targeted. The main parameters responsible for its efficiency are the current strength, the flow velocity, the temperature and the total dissolved solids in the compartments [206]. Recent advances based on a better understanding of kinetics mechanisms allowed to explore other utilizations such as fractional electrodeionization (FEDI®), recently implemented at large scale. Bolstered by a 60-stack plant with a 105-m3/h capacity, this water-purification technology is already well-established and can produce highly purified water [210,211]. It is operated following two distinct stages of EDI. During the first stage, a low voltage is applied to the system in order to remove strongly charged species. As hydrogen cations are more mobile than hydroxide anions, they are preferentially transported towards the concentrate compartment where pH 4.5–5 can be obtained, thus limiting scaling caused by multivalent cations. The second stage at higher voltage is taking place at a higher pH (9–9.5) due to the excess of hydroxide anions in the feed compartment which allows a deeper deionization of weakly charged silica ions. Using two stages improves the flexibility of the technology, allowing a larger range of hardness in the feed [205,211]. Other technological achievements include electrodeionization reversal (EDIR) with polarity inversion to reduce fouling [212,213] or electrodeionization with bipolar membranes (EDI-BM or BMEDI) in order to regenerate the IX resin with H+ and OH− ions produced by the bipolar membrane [207]. Furthermore, the development of high-performance materials fostered wafer enhanced electrodeionization (WE-EDI) or resin wafer EDIPDF Image | Electrodialytic Processes
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