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Membranes 2020, 10, 221 37 of 72 Tanaka et al. [187] studied the performance of EED for HI concentration using a CEM made from poly (ethylene-co-tetrafluoroethylene)-styrene grafted polymer (ETFE-St) and a commercially available Nafion 212 CEM. They found that the H+ selectivity was mostly influenced by the back diffusion of I− which depended on the affinity of the membrane material with I2. An acid and oxidative resistant sulphonated copolymer (PVSU) synthesized by chemical grafting of 2-methyl-2-(N-(3-sulfopropyl) acrylamido) propane-1-sulfonic acid (MSAPS) with dehydrofluorinated poly (vinylidene fluoride-cohexafluoropropylene) (PVDF-co-HFP) was also recently studied as CEM material [188]. While PVSU CEMs allowed a near 100%-current efficiency, it was shown that the high concentration of fixed charges in the membrane matrices significantly reduced the energy consumption required to produce 1 mol H2. The stability of AEMs, mainly used for organic and inorganic acid purification, is a significantly higher hurdle compared to CEMs [182]. The reasons for it are the chloromethylation and quaternization steps during AEM production process which remain technically challenging [186]. As such Duan et al. [186,189] proposed an alternative way to synthesize new polysulfone-based membrane materials: quaternary ammonium polysulfone (QAPSU) and comb-shaped quaternized ammonium polysulfone (Cx-QAPSU). Using chlorotrimethylsilane as a non-toxic chloromethylation reagent and trimethylamine ethanol solution as the quaternization reagent allowed for an efficient synthesis. QAPSU and Cx-QAPSU used in an EED stack led to 65% and up to 85% metal ion removal, respectively, as well as higher current efficiency and lower energy consumption for phosphoric acid purification than commercial LE 1201 membrane [186,189]. Other applications of EED were recently developed to answer current environmental issues regarding water contamination. Wu et al. [190] studied the feasibility of EED for phenol removal from salty wastewater. The process is based on hydroxide generation at the cathode, where phenol could be converted in phenoxide ions which, after transport through the AEM, could be converted back as phenol molecules in the anode compartment. Although 90% removal of phenol was achieved, it was found that hydroxide and sulfate anions are competing with phenoxide anions leading to an increased energy consumption. Ideal conditions were obtained at high phenol and low salt concentrations in the feed [190]. Similarly as SED, EED was implemented with bipolar membranes (EEDBM) for lithium recovery from artificial lake brines [191]. In comparison to conventional EED or EDBM it allowed to produce lithium hydroxide both in the base and cathode compartments leading to an increased current efficiency. 4.7. Membrane Capacitive Deionization (MCDI) Capacitive Deionization (CDI) is a non-membrane technique first described in the 1960s and since then mainly used for water desalination. It differs from conventional ED as it is based on electrode reactions to remove and concentrate ions. A phenomenon called electrosorption consists in the electrostatic attraction and adsorption of ions at the interface between the solution and the porous electrodes while an electric potential is applied. Then, ions can be desorbed, thus regenerating the electrodes [4,192,193]. The adsorption capacity of the electrodes is considered to be the most important factor in terms of performances [193]. A more recent technology derived from CDI by adding IEMs in order to improve its results was incepted in the mid-2000s [194]: membrane capacitive deionization (MCDI). Membranes may seem of secondary importance compared to electrodes; however, they have the key role of improving the charge efficiency by avoiding the transport of co-ions of similar charge as the electrodes. Furthermore, MVC or MVA can be used to optimize experiments with complex multi-ion solutions. It is also possible to coat the membrane directly on the electrode, allowing a thinner layer than a separate membrane, however long-term performances are still to be evaluated [195]. Flow-electrode capacitive deionization (FCDI) is the most recent and promising MCDI-derivated technologies (Figure 19). The use of membranes in CDI helped develop the concept of flow-electrode, where active carbon particles circulate in an electrode compartment separated from the feed stream by IEMs [195,196]. Contrary to stationary electrodes which require a distinct regeneration/desorption step, having a flow electrode enables a continuous operating mode as carbon particles are constantly recycledPDF Image | Electrodialytic Processes
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