## 3 Results and Discussion
### ED with Four-Compartment Cell  
This cell allows us to isolate the anodic and cathode compartments and to avoid any contact of the membrane with the products of redox reactions. The isolation of these compartments is done by placing two anion exchange membranes as shown in Figure 5. ED experiments are carried out for 4 h using a feeding solution at [Li\({}^{+}\)]\({}_{\rm F}\) = [Na\({}^{+}\)]\({}_{\rm F}\) = 0.05 M. The  
This decrease of H\({}^{+}\) transport through the LCM leads to an increase in the total number of passed cations (Li\({}^{+}\)+Na\({}^{+}\)) by passing to a four-compartment one. Additionally, as it was shown earlier [48], the presence of protons affects the membrane performance on dialysis processes (DD and ICD) because of their high mobility, their high diffusion and their easy passage through the different components of the membrane matrix (the charged polymer and the LICGC particles). The recovery rate evolution of the two cations as a function of the imposed current density using a four-compartment cell is illustrated in Figure 7. Using this configuration, we were able to extract 14.85% Li\({}^{+}\) and only 0.98% Na\({}^{+}\) with S(Li/Na) = 15.1 by imposing a current density of 12 mA\(\cdot\)cm\({}^{-2}\) for 4 h, whereas only 1.26% Li\({}^{+}\) and 0.03% Na\({}^{+}\) could be extracted with higher selectivity of the order of 43.6 by imposing a low current density of 0.5 mA\(\cdot\)cm\({}^{-2}\). We followed the pH variations in the feed and receiver compartments for the two current densities 0.5 and 7.2 mA\(\cdot\)cm\({}^{-2}\). We observed that the increase of the current density significantly reduced the pH from 5.3 at the initial time to 2.2 after 4 h of ED for the feed and from 0.9 to 0.6 for the receiver. This expected acidification of the solutions can be attributed to the protons transport through the AMX protection membranes and the LCM selective membrane. Indeed, protons move in successive jumps from one water molecule to another, following the Grothus diffusion mechanism from the anode compartment to the feed compartment through the AMX [59]. Figure 5: Schematic illustration and principle of four-compartment ED cell. Figure 6: Effects- of the imposed current density on the Li\({}^{+}\)/Na\({}^{+}\) separation performances of LCM using a four-compartment cell and for [Li\({}^{+}\)]\({}_{\rm F}\) = [Na\({}^{+}\)]\({}_{\rm F}\) = 0.05 M.  
As mentioned earlier, once in the feed compartment, these protons intensively cross the LCM membrane and thus inhibit its performance. Despite the improvement in LCM separation performance from a two-compartment ED cell to a four-compartment ED cell, the Li\({}^{+}\)/Na\({}^{+}\) separation efficiency remains relatively limited due to the intensive passage of H\({}^{+}\) through the AMX and LCM. These limitations could be overcome by using anion exchange membranes with low proton leakage. In order to test the LCM separation performances as a function of the [Na\({}^{+}\)]\({}_{\rm F}\)/[Li\({}^{+}\)]\({}_{\rm F}\) concentration ratio, ED experiments were performed using the four-compartment cell and applying a current density of 0.5 mA\(\cdot\)cm\({}^{-2}\) for 4 h. We chose different concentrations and concentration ratios in the attempt to be comparable to those encountered in lithium brines. The results are reported in Table 3. Figure 8 gives the variations of recovered lithium and sodium concentrations, their recovery rates as well as the LCM selectivity coefficient vs. the [Na\({}^{+}\)]\({}_{\rm F}\)/[Li\({}^{+}\)]\({}_{\rm F}\) concentration ratio, using log-log coordinates. Figure 8 reveals that the recovered Li\({}^{+}\) and Na\({}^{+}\) concentrations decreased continuously when their concentrations also declined in the feed compartment. This was expected since both diffusion and electro-migration phenomena are proceeding in the same direction. Membrane selectivity remains important in favor of Li\({}^{+}\) even if the Na\({}^{+}\) concentration sometimes exceeds the Li\({}^{+}\) one.