## 3 Results and Discussion
### Extraction with ED  
Figure 4 shows the results from the combined hydrometallurgical-electrodialytic experiment. The figure shows the time-transient percentage of dissolved metal (Co\({}^{2+}\) and Li\({}^{+}\)) in the central and the catholyte compartments, indicating the initial amount in the solid particles. It can be observed that the total amount of Co\({}^{2+}\) and Li\({}^{+}\) in the liquid phase increased during the first 2 days of treatment up to 25.53% \(\pm\) 0.89% and 56.36% \(\pm\) 0.96%, respectively. After that time, the concentration of aqueous Li\({}^{+}\) remained constant, indicating that a point was reached from where it was not possible to extract more lithium nor dissolve more particles, as observed before in the extraction experiments. Despite there being no more extraction after 2 days, the dissolved Li\({}^{+}\) was transported from the central compartment to the catholyte in the electrodialytic cell. In the case of cobalt, the total concentration in the liquid phase decreased with the treatment as it got electrodeposited at the surface of the cathode. Electrodeposited Co\({}^{0}\) was dissolved in HNO\({}_{3}\) after the experiment, and it corresponded to 11% of the total Co in the system (initially contained in the particles), and to 80% of the extracted cobalt. Therefore, when a certain concentration of Co\({}^{2+}\) is reached at the catholyte, there may be competition between the electrolytic reduction of water to produce hydrogen gas and the reduction of Co\({}^{2+}\) into metallic Co\({}^{0}\) shown in Equation (8). Although the hydrogen gas evolution seems thermodynamically more favorable at the working pH values, overpotentials may promote the electrodeposition of Co\({}^{2+}\) into Co\({}^{0}\). The pH values at the central and the cathode compartments were both approximately constant at pH \(\approx\) 1.2 during the experiments. The average value of the electric potential drop measured at the electrodes of each cell increased from 2.53 to 2.94 V.  
The results suggest a switch between the hydrometallurgical and the electrodialytic mechanisms in controlling the recuperation rate of the target metals. In order to make the technique energetically efficient, it would be important to optimize the applied electric current to produce the ED separation at the same rate as the extraction of metals from the solid particles. In the experiments presented here, the applied electric current was set from the dissolution rate observed in the linear part at the beginning of the extraction experiments shown in Section 3.1. Figure 4: Time-transient values for percentage of extracted metal in the different compartments with respect to the amount in the initial solid. —: Cobalt;...: Lithium. Error bars represent two times the standard deviation of the duplicated texts. Figure 5 shows the percentage of Co and Li (referred to the total amount of these chemical species in the initial solid samples) in the different components of the experimental set-up at the end of the experiment. It can be observed that after 6 days of treatment, 33% of Co and 62% of Li was extracted. This indicates again that in the absence of reducing agents, the dissolution of the LiCoO2 (s) particles is limited by certain competitive mechanisms, such as the precipitation of Co3O4 (s). Figure 5 also shows that 16% of Co and 30% of Li was transported to the catholyte compartment. Furthermore, approximately 68% of the cobalt in the catholyte compartment was found electrodeposited on the cathode surface. Figure 5 shows that small fractions of Co (0.5%) and Li (3.9%) were found in the anolyte compartment, as CEMs do not hinder diffusion of the cations. It can also be observed certain amount of Co (4.4%) and Li (0.9%) accumulated within the structure of the CEMs. Cobalt had a bigger tendency to accumulate within the membranes, as expected from divalent cations. No metal was found on the anode electrode.