## 3 Results
### Comparison of Metal Removal Efficiency
A consistent increasing concentration curve was observed for the majority of the metals studied (Figure S1 in Supplementary Materials); it agrees with the transport direction caused in metals by the electrokinetic fluxes, i.e., electroosmosis and electromigration [27]. Therefore, it can be concluded that the application of the electric field achieved the effective transport of the metals solubilized from the mine tailings to the cathode well, where they were removed; in  
Figure 2: Efficiencies (%) in metal solubilization from mine tailings using a soil slurry agitated system, using the BL medium (continuous lines) and an iron-free TK medium (dashed lines, reference tests). addition, the precipitation of metals in the area close to the cathode was avoided because the acidity was maintained through correct pH control. Figure 4 shows the final extraction efficiencies for the different metals after 15 days in all the treatments applied. It can be observed that using the BL medium led to higher extraction yields as compared to the reference tests. Most metals showed higher efficiencies in the EK-percolation treatments than those in the agitated slurry ones; only Zn had an opposite behavior, probably due to its high initial concentration and availability in the mine tailings. This result reinforces the interest to further explore the feasibility of EK-percolation through a more detailed analysis of the operating conditions in order to understand the advantages or disadvantages of the EK system compared to the agitated slurry one. For the analysis of the operating conditions (T, pH, or electroosmotic flow) in the EK-percolation treatments, we will use data previously reported in a previous work by our research team whose objective was to study the influence of the voltage gradient [15]. The pH was maintained at sufficiently acidic values in all the treatments, although it was easier for the slurry system (in which it was maintained at pH 2.1); for EK treatments, the pH ranged between 2.5 (at the beginning of the test) and approximately 0.5 (at the end). Regarding the temperature, it was maintained at 30 \({}^{\circ}\)C in the slurry system while in the EK treatments it rose to approximately 35 \({}^{\circ}\)C from day 5 to day 10, due to ohmic heating of the soil. This heating would be much greater if voltage gradients above 2 V cm\({}^{-1}\) (to accelerate the bioleaching) were used, which would be a clear disadvantage for the development of microorganisms that conduct conventional bioleaching; this was actually the reason why it was decided to use the BL medium for the EK-percolation tests. Similar redox potential values and an excess of Fe\({}^{3+}\) (the active species for metal solubilization) in both EK-percolation and agitated slurry treatments using the BL medium were observed. The availability of atmospheric oxygen was high in the agitated slurry system and therefore the acidophilic microorganisms grown in the BL medium could contribute, at least to a small extent, to the leaching effect; however, aeration of the in situ EK treatments was not  
Figure 3: Efficiencies (%) in metal solubilization from tailings using an EK-percolation system, using as an anolyte the BL medium (continuous lines) or an iron-free TK medium (dashed lines, reference tests). possible, which means that the only microbiological contribution to be considered in the bioleaching process was that carried out in the previous step of generating the BL medium. The two main differences between the two types of treatment were the ratio of leaching medium to solids (mL g\({}^{-1}\)) and the hydrodynamics of the systems. After 15 days, the slurry system used a total of 100 mL of BL medium per gram of tailings, while the EK treatment used only 0.44 mL per gram (the total volume of BL medium, 1300 mL, was added as an anolyte as the electroosmotic flow through the cathode well was withdrawn). These data would represent a large water saving and a significant cost reduction in the generation of the BL medium in the case of the EK system.