## 3 Results and Discussion
### ED with Four-Compartment Cell
When the Na\({}^{+}\) concentration becomes significantly higher than that of Li\({}^{+}\) in the feed, we notice a stabilization of the Na\({}^{+}\) recovery rate, a slight decrease in the Li\({}^{+}\) recovery rate and especially a reduction of membrane selectivity, which nevertheless remains at relatively high levels (almost 100). A competition between Li\({}^{+}\) and Na\({}^{+}\) passages takes place and  
Figure 7: Li\({}^{+}\) and Na\({}^{+}\) recovery rates as a function of imposed current density using a four-compartment cell and for [Li\({}^{+}\)]\({}_{\rm F}\) = [Na\({}^{+}\)]\({}_{\rm F}\) = 0.05 M.  
seems to provide an optimum of selectivity at [Na\({}^{+}\)]\({}_{\rm F}\)/[Li\({}^{+}\)]\({}_{\rm F}\) ratio of around 10. Beyond this ratio and at Na\({}^{+}\) concentrations of about 2000 mg\(\cdot\)L\({}^{-1}\), the membrane tends to lose its significant selectivity due to an important leakage of Na\({}^{+}\) through the charged polymer and, to a lesser extent, through the LICGC particles. Here, Li\({}^{+}\) transport through LICGC percolation pathways is more significant than Na\({}^{+}\) transfer mainly through the charged polymer, although its transfer through LICGC channels should not be neglected. Furthermore, we note that the Na\({}^{+}\) recovery rate decreases as a function of its concentration in the feeding compartment. This fact proves that LCM remains a sufficiently impermeable barrier to Na\({}^{+}\) co-ion passage. We also compared the selectivity values obtained using ED cell at 0.5 mA\(\cdot\)cm\({}^{-2}\) (this study) and Cross-Ionic Dialysis operation (CID) (previous work [48]). In both cases, we used an HCl solution in the receiver, a ratio [Na\({}^{+}\)]\({}_{\rm F}\)/[Li\({}^{+}\)]\({}_{\rm F}\) of 20 and a 4 h treatment. Therefore by CID, we recovered 3.19 mg\(\cdot\)L\({}^{-1}\) of Li\({}^{+}\) and 0.15 mg\(\cdot\)L\({}^{-1}\) of Na\({}^{+}\) with S(Li/Na) = 421, while by ED we recovered 2.8-times more of Li\({}^{+}\) (8.86 mg\(\cdot\)L\({}^{-1}\)) and 15.9-times more of Na\({}^{+}\) (2.38 mg\(\cdot\)L\({}^{-1}\)) but with a 5.7-times smaller selectivity coefficient (S(Li/Na) = 74.5). Through this comparison, we can see that the imposed current has a more pronounced effect on the passage of Na\({}^{+}\) than that of Li\({}^{+}\) given its previously presented properties (R\({}_{\rm h}\), u and D). Thus, we can deduce that ionic transport through LCM membrane by ED is due, in our case, to the combined effect between the low density of the imposed current and the ionic concentration gradient. All this allows us to explain the difference in the selectivity of LCM towards Li\({}^{+}\) between DIC and ED, when comparing the findings of this paper with our previous one. For Li\({}^{+}\) recovery, especially using membrane technologies, the trade-off or the compromise between the permeability, extraction rate and membrane selectivity towards Li\({}^{+}\) remains a challenge. A further crucial parameter for membrane technology and its applicability concerns energy consumption. Indeed, this latter needs to be as low as possible to achieve the best cost-effectiveness of the proposed process. Thus, the main goal of worldwide research in this field is finding the most appropriate solution/membrane that  
Figure 8: Recovered Li\({}^{+}\) and Na\({}^{+}\) concentrations, their recovery rates and LCM selectivity coefficient vs. the [Na\({}^{+}\)]\({}_{\rm F}\)/[Li\({}^{+}\)]\({}_{\rm F}\) ratio, using log–log coordinates. simultaneously ensures an optimum recovery rate with a maximum Li-selectivity and minimum energy consumption to achieve the highest efficiency and the purest product quality. Table 4 presents a comparison of selectivity coefficients and recovery rates between our LCM membrane and others membranes cited in the literature and used in the ED process. This table highlights the difficulty of finding a suitable membrane, especially at a higher Na concentration in the feed compartment, which provides both a good Li-recovery rate and high selectivity.