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Membranes 2021, 11, 575 14 of 29 3.2.3. pH Variation During experiments, it was observed that the LiCl solution’s measured pH increased in all cases from pH 7–8 to a basic pH between 12–13. This increase can be attributed to the undesired migration of OH− anions across the cation-exchange membrane due to the membrane not being 100% permselective, and also to the high OH− ion mobility associated with the Grotthuss mechanism, involving proton hopping by ionization of water molecules. It has been reported that OH− ions’ migration through the cation-exchange membranes reduces current efficiency in membrane processes for sodium hydroxide production [40], which in bipolar membrane electrodialysis can be manifested by an increase in the feed salt solution’s pH [53]. In this work, a higher pH increase was observed in the LiCl feed solution when working with more concentrated LiOH solutions. Hence, the lithium transport number decrease can be mainly attributed to electrical current transport by OH− leakage through the cation-exchange membrane. 3.2.4. Influence of LiCl Concentration To obtain a better understanding of the effects of LiCl concentration and concentration difference between electrolytes on each side of the membrane, additional experiments were performed for 0.5 wt% and 5.0 wt% LiOH at 25 wt% LiCl (Tests 5 and 6 in Table 3); The corresponding results are presented in Figure 6. When comparing the effects of LiCl concentration (see Figures 5 and 6), results show that for the CMX membrane at 0.5 wt% LiOH concentration, the transport number decreased with LiCl concentration. Thus, when using a LiCl solution at 14 wt%, a value of 0.72 was obtained, while with a LiCl solution at 25 wt%, the transport number was 0.66. The same behavior was not obtained for the CMB membrane, where the transport number was 0.63–0.65 for the same comparison. This can be attributed to the fact that the CMB membrane has an average thickness 18.9% higher compared to the CMX membrane, with 50–60% higher exchange capacity and 13.3–37.5% higher water uptake (see Tables 1, 6 and 7). In addition, the higher electrical resistance of the membrane would be associated with a denser polymeric structure [27]. These characteristics would allow the reduction of the influence of high LiCl concentrations on the lithium transport number. At higher LiOH concentrations, for the CMX membrane it was measured that at a concentration of 5.0 wt% LiOH, the lithium transport number Membranes 2021, 11, x FOR PEER REVIEW 15 of 30 decreased from 0.42 to 0.36 when using LiCl solutions of 14 wt% and 25 wt%, respectively. In the case of the CMB membrane, the lithium transport number was 0.37 and 0.44 for the same comparison. Figure 6. Lithium transport number in cation membranes (CMX and CMB) according to LiOH Figure 6. Lithium transport number in cation membranes (CMX and CMB) according to LiOH concentration and current density. LiCl 25 wt%. concentration and current density. LiCl 25 wt%. In the particular case of this work, the transport number decrease can be attributed to the fact that high concentrations of LiCl and LiOH promote higher concentration of Li+ in the membrane as a counterion. This, along with the principle of electroneutrality and Donnan exclusion, causes increase in the concentration of Cl− and OH− co-ions in the mem-PDF Image | Bipolar Membrane Electrodialysis for LiOH Production
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