# Optimization of Electric Field Assisted Mining Process Applied to Rare Earths in Soils
## 3 Results and Discussion
### Analysis of the Migration Performance of Y\({}^{3+}\)  
To obtain the Y\({}^{3+}\) migrational profile of the EMs, electrolyte aliquots were collected from the five sampling points of the electrokinetic reactor (Figure 1). From the discrete values of the Y\({}^{3+}\) concentration, response surfaces were constructed and fitted using the _Cubic_ interpolation command of Matlab(r) software. Figure 3 shows the Y\({}^{3+}\) migrational performance of EM-01 to EM-05.  
\begin{table}
\begin{tabular}{c c c c c} \hline
**Experiment** & **C (mol L\({}^{-1}\))** & \(\varepsilon\) **(V cm\({}^{-1}\))** & \(\overline{\text{C}}\) **(-)** & \(\overline{\tau}\) **(-)** \\ \hline EM-01 & 0.10 & 0.1818 & \(-1.00\) & \(-1.00\) \\ EM-02 & 1.00 & 0.1818 & \(1.00\) & \(-1.00\) \\ EM-03 & 0.10 & 0.5454 & \(-1.00\) & \(1.00\) \\ EM-04 & 1.00 & 0.5454 & \(1.00\) & \(1.00\) \\ EM-05 & 0.55 & 0.3636 & 0.00 & \(0.00\) \\ \hline \end{tabular}
\end{table}
Table 2: The physical and coded configurations of the electromining experiments in the synthetic soil.  
Figure 2: The profiles of current densities of the electromining experiments using the synthetic soil.  
According to Figure 3, at t = 0 h, there was the leaching of Y\({}^{3+}\) towards the anodic chamber due to the direction of the electrolyte pumping into the system at the beginning of the procedure. Subsequently, it can be observed that Y\({}^{3+}\) was mobilized towards the CC in all EMs. This mass transport was given by electromigration and diffusion. On the other hand, [PERSON] and [PERSON] [36], [PERSON] et al. [29], and [PERSON] et al. [19] claim that the migrational flow is the predominant mechanism to ion mass transport. Therefore, the migration was considered the only mechanism of mass transfer for all EMs, as proposed by [PERSON] et al. [19].  
Although there was migration of Y\({}^{3+}\) in all EMs, only EM-03 (Figure 3c) presented an inversion in the concentration profile of the species. EM-04 (Figure 3d) was conducted in the same electric field strength (0.5454 V cm\({}^{-1}\)) as EM-03, however this electromining was not considered satisfactory. This behavior can be justified by the use of the electrolyte in a higher concentration (1.0 mol L\({}^{-1}\)). The increase in this variable results in the increase of the number of dissociated species originated from the acetic acid. As the experiment was conducted for 72 h, the magnitude of the electric field in EM-04 was not sufficient to supply the increase in the number of species of this experiment. Therefore, for the mining of EM-04 to be considered satisfactory, it would require more procedure time; thus, from a qualitative analysis, the EM-03 was considered to achieve the best performance regarding the Y\({}^{3+}\) migration.  
Figure 3: The response surfaces to the Y\({}^{3+}\) migration behavior for the electromining experiments in the synthetic soil.  
Although EM-03 has shown the best migrational profile, the tendency to reduce the Y\({}^{3+}\) concentration in CC after 60 h of procedure was observed in this experiment. This behavior can be explained due to the increase in the concentration of Y\({}^{3+}\) in the cathodic region, which promotes the diffusion of the ion in the opposite direction to the CC.