## 3 Results  
### Comparison of Metal Removal Efficiency  
Figure 2 shows the percentages of metal leached from tailings corresponding to the agitated soil slurry system for the two treatments applied, i.e., with use of the BL medium and without it (using iron-free TK medium, which acts as the reference test). Throughout the experiment, the temperature and pH were maintained at 30 \({}^{\circ}\)C and 2.1 \(\pm\) 0.1, respectively, to prevent the precipitation of solubilized metals. The concentration of Fe\({}^{3+}\) varied between 300 and 450 mg L\({}^{-1}\), while the redox potential ranged from 500 to 600 mV. After 15 days of bioleaching, a clear enhancement in metal solubilization when the BL medium was used, as compared to the reference tests, can be seen in Figure 2 for Zn, Mn, Co, Ni, and Cr, while that effect was practically negligible for Cd. It can be also observed in Figure 2 that most of the leached metals were extracted within the first five days of the experiment and a plateau was reached from day 8 onwards, indicating that leaching nearly stopped. Our earlier research on the bioleaching of the same mine tailings used in the present work demonstrated that the presence of microorganisms in the tailings was poor. Therefore, the bioleaching rate is quite limited until the activation of the native microorganisms is accomplished [21]. Thus, it can be said that the metal solubilization observed in the reference test was mainly caused by chemical leaching because of the acidity of the TK medium; the fact that the plateau after day 5 has a similar shape in the two treatments clearly shows the low degree of contribution of native microorganisms to metal leaching. Furthermore, the differences observed between the two treatments for each metal were due solely to the leaching effect caused by the BL medium; the Fe\({}^{3+}\) concentration coming from the BL medium was high enough throughout the experiment to be considered not to be a limiting reagent. A previous study by [PERSON] et al. [23] investigated metal leaching from solid wastes, such as printed circuit boards, and compared the effectiveness of chemically versus biologically produced ferric sulfate; the results showed no significant differences between the two methods. This implies that the biologically produced liviviant agent, mainly composed of sulfuric acid and ferric iron, can be used as an environmentally friendly leaching agent. Furthermore, [PERSON] et al. [24] studied the production and regeneration of ferric iron by acidophilic microbes for the processing of sulfidic ones with the aim of achieving an economical and sustainable two-step leaching process. They found that the bioregeneration of ferric iron was rapid and efficient and allowed for cost savings. This would reinforce our strategy of using the BL medium. The leaching patterns corresponding to the EK-percolation treatment are shown in Figure 3. It can be seen that Mn, Cd, Cr, and Zn exhibited similar patterns to those found for the slurry agitated system: that is, a maximum metal leaching rate was achieved in the first 5 days until reaching a plateau on day 8, when leaching practically stopped. On the contrary, the behavior of Ni and Co is different, with leaching increasing even after 15 days. Although not studied, it can be suggested that those differences may be due to the different standard reduction potentials, geochemical partitioning in mine tailings, and electrochemical properties of the metal sulfides. The dissolution of each metal occurs due to redox reactions, and metal sulfides with lower reduction potentials would dissolve better [25; 26]. To determine the total metal concentration in the soil, once the experiment was concluded the soil was separated into five portions to examine the metal concentration along the soil and to confirm the metal transport from the anode to the cathode.