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Membranes 2021, 11, 575 13 of 29 3.2.2. Influence of Initial LiOH Concentration The lithium transport number decreases with LiOH concentration. For the CMX and CMB membranes, at 0.5 wt% LiOH concentration, the lithium transport number was in the range of 0.83–0.72 and 0.71–0.63, respectively. That is, more than 60% of the electric current was used for lithium transport through the cationic membranes. For the CMX membrane, when concentrating the LiOH solution from 0.5 wt% to 2.5, 5.0, and 8.0 wt%, the average decrease in the lithium transport number was 35%, 47%, and 64%, respectively. Meanwhile, in the CMB membrane, for the same concentrations, the average decrease was 29%, 40%, and 75%, respectively. The best result was 0.83 with the CMX membrane at an initial LiOH concentration of 0.5 wt%. One of the possible causes of the decrease in the lithium transport number with LiOH concentration is the decrease in water content in the cationic membrane [37]. Due to the increase in concentration, membranes tend to lose water; this can be attributed to the accumulation of osmotic pressure within them [37,46]. According to the work varied out by Izquierdo-Gil et al. [47], comparing water content and salt transport in cationic Nafion membranes of different thickness, it was reported that water uptake increases with membrane thickness and decreases with cation size. In the present work, although the CMB membrane was thicker and showed higher water uptake than the CMX membrane, it had a lower lithium transport number—a condition that can be attributed to other characteristics of the CMB membrane, such as fixed charge density and polymeric matrix cross-linking [48]. Water uptake into the membrane allows ionic binding between two sides of the membrane, making electrical conductivity possible [47,49]. Increasing the concentration of electrolytes in contact with the membrane causes an increase in the concentration of counterions in the membrane, which in turn generates osmotic deswelling, further increasing its ion concentration [46]. Eventually, water loss and high counterion concentration can reduce the membrane’s fixed charge density. This has been explained thermodynamically by Kamcev et al. [46,50] by means of the counterion condensation model. Such a situation occurs when counterions close to fixed charges at a separation of less than the Bjerrum length do not have sufficient thermal energy to free themselves from the interaction of electrostatic forces, and tend to remain in this region, generating a shielding in fixed charges. Counterions trapped in this region are considered “condensed” and, therefore, immobile under a concentration gradient. However, counterions, under an electric field, exhibit higher diffusion coefficients than those that are not condensed, due to the shorter distance they must move in the membrane structure [51,52]. Although counterions’ condensation favors their transport, high concentration of these counterions implies a reduction in Donnan potential, and causes an increase in co-ion sorption in the membrane [52]. For the application of membranes in LiOH production with concentrated solutions, it is highly probable that counterion condensation occurs, which under an electric field would favor lithium migration through the cation-exchange membrane. However, this may increase OH− co-ion concentration, which, due to its high mobility (the highest among all anions) promotes undesired OH− transport, resulting in a decrease in the lithium transport number. The use of membranes with higher fixed charge density would present higher ionic conductivity, facilitating the transport of lithium ions across the membrane [51], as long as OH− ion leakage is reduced. In this work, membranes’ water uptake tended to decrease with LiOH concentration. However, this variation does not capture the behavior of the lithium transport number for all cases. This lithium transport number reduction is better explained by the presence of OH−, and its interaction with the cation-exchange membrane. On the other hand, the decrease in the observed lithium transport number can be explained by the fact that the salt transport rate across the membrane decreases with decreasing concentration difference on both sides of the membrane, causing the permeation rate to decrease [47]—as occurs in the case of this work, where the LiOH concentration increased and the concentration difference with the LiCl solution decreased.PDF Image | Bipolar Membrane Electrodialysis for LiOH Production
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