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Membranes 2020, 10, 221 43 of 72 4.10.2. Electrodialysis with Nanofiltration Membrane (EDNF) The use of charged filtration membranes stacked in an ED cell was demonstrated to be effective for demineralization of solutions. Hence, in 2011, Bazinet and Moalic were the first to use a nanofiltration membrane (NF) in an ED stack for the demineralization of sea water [249]. They demonstrated that the use of NF membrane can also allow the selective separation of cations. More recently, Ge et al. exemplified such a selective cation fractionation of EDNF with H+/Zn2+ and Na+/Mg2+ systems and demonstrated that an NF membrane can increase the limited current density in ED [250]. In 2020, EDNF was tested for acid whey demineralization and simultaneous lactate recovery in under- and over-limiting conditions [90]. The integration of a nanofiltration (NF) membrane in an ED conventional stack led to a similar lactate recovery rate than conventional ED (around 29% in 60 min) with respective demineralization rates of 20.3% and 50.8%. In overlimiting conditions, around 36% lactate recovery were obtained for both configurations, in 30 min, but with demineralization rates of 77.2 and 27.5% for conventional ED and EDNF respectively. However, for the EDNF configuration, the recovery of lactic acid was almost doubled in the overlimiting condition. Different hypotheses were proposed to explain these results; (1) the pore size of the NF membrane since NF membranes have a high permeability for monovalent cations and organic acid anions with low molecular weight, but they limit the passage of organic compounds with a molecular mass that exceed 300 Da (ex. lactose) [251,252], (2) the temperature evolution during the process because when the system reaches higher temperature, there is expansion of the polymer included in their structure [253] and (3) the fact that no LCD was obtained for NF membrane, since H+ produced, in the whey compartment at the interface of the AEM after the LCD was reached would have reacted with some lactate anions (pKa of 3.86) to produce non-charged lactic acid, decreasing their potential to migrate. Concerning the relative energy consumption (REC), in underlimiting conditions no difference in REC, for demineralization (around 20 Wh/g K+) and lactate recovery (around 8.6 Wh/g lactate) steps, for both membrane configurations was reported. However, when the underlimiting current condition was applied similar REC were calculated for the demineralization step (around 163 Wh/g K+) but around 68% more energy was needed for conventional ED (109.5 vs. 65.1 Wh/g lactate): the overlimiting condition increased by 7–9 times the energy consumption compared to the underlimiting condition [90]. Recently, Ye et al. [254] used EDNF for textile wastewaters treatment. Their novel loose NF-based ED process, exhibited high permeation of salt (98.9% desalination) while retaining dyes (99.4% recovery) with low fouling showing promises as a sustainable depollution process. 4.11. Reverse Electrodialytic Technologies In a context of depletion of natural resources, reverse technologies are promising alternatives for clean energy production. Reverse electrodialysis (RED) is based on salinity gradient power (SGP) also called “Blue Energy” which is thought to represent 2.4 TW when considering estuaries worldwide, i.e., 80% of 2020′s global electricity generation [255]. By using ionic concentration gradients, RED promotes the flow of ions in a non-powered ED stack in order to generate electric energy. As displayed in Figure 23, high-salinity stream (HSS) and low-salinity stream (LSS) are used as feed solutions. Their adjacent compartments are separated by IEMs, according to a conventional ED configuration. The concentration gradient between the two compartments induces anion and cation migration from the HSS towards the LSS selectively across the AEM and CEM, respectively. Electrode solution consists of a redox rinse, usually FeCl3 and FeCl2. As such, the transport of Cl− to the electrolyte at the anode triggers an oxidation of the Fe2+/Fe3+ couple, while the transport of Cl− from the cathodic electrolyte leads to a reduction, the resulting exchange of electrons generate electric power [256,257].PDF Image | Electrodialytic Processes
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