# A Comparative Study on Bieleaching Properties of Various
## 3 Results and Discussion
### Metal Extraction in Bioleaching and the Boleaching Mechanism Based on Suffide Minerals  
Figure 5 shows the results for the metal concentration leached over time in the leaching experiment. Although the metal extraction in bioleaching acts on the mineral surface and results in low metal concentrations in the leachate, those with bacteria showed a higher leaching concentration than the abiotic control case for all mineral samples. This is due to biological leaching by biological oxidation, including biological direct contact oxidation and chemical oxidation of the mineral samples. In order to confirm the direct oxidation in the bioleaching experiment, SEM-EDS analysis was performed on the leaching residue after the experiment was completed (Figure 6). As a result, bacteria were attached to the surface of the residue under all mineral sample conditions. Corrosion action (direct oxidation) was observed on the surface of bacterial residues in SP and GN1 samples. In the case of the SP and PY samples, a large number of bacteria were attached to the imperfections, crevasses, and pores of the residue surface. Meanwhile, trace elements that may be present in the mineral samples may inhibit microbial biogeochemical activity [29], but their effects were not considered in this study. Figure 5: Changes in metal concentration in leachate under various mineral sample conditions. (**(a,b)** SP, **(c,d)** GN1, **(e,f)** GN2, **(g)** PY, and **(h,i)** CCP). The metal extraction in bioleaching differed depending on the mineral content of the samples (Table 2). The leaching concentration was higher for Zn (103.40 mg/L) than for Pb (17.90 mg/L) in SP with a high ZnS content (78%). In GN1 and GN2 with high PbS content (57% and 67%), Pb (6.58 mg/L and 8.72 mg/L) showed higher concentrations than Zn (4.00 mg/L and 2.90 mg/L). These results confirm that as the composition ratio of PbS increased in GN2, the metal extraction in bioleaching of Pb also increased compared to GN1, and the opposite result was shown in the case of Zn. Nonetheless, the leaching rate of Zn was faster than that of Pb (Table 3), and this trend was also observed in previous studies. According to literature studies of heavy metal leaching on mine tailings using indigenous sulfur-oxidizing bacteria, the leaching rate of Zn was about 2.2 times higher than that of Pb during the same leaching time [30,31]. The leaching concentration of Fe in PY was minimal (0.59 mg/L) because the surface accumulation of Fe\({}^{2+}\) could result in inhibition of the oxidizing capacity of microorganisms [32]. In the results of biological leaching experiments on low grade sulfide minerals by [PERSON] et al. [33], the leaching concentrations over the same leaching time were in the order of Ni \(>\) Zn \(>\) Fe \(>\) Cu = Co. In a biological leaching study on lead zinc mine tailings consisting of complex minerals such as sphalerite, pyrite, calcite, and quartz, Zn leaching rates were 2.74 times (10% pulp density) and 9.25 times (20% pulp density) higher than the Fe leaching rate [34]. Meanwhile, some precipitates, such as anglesite, may be formed during the bioleaching process [35,36], but they were not considered in this study. Based on the above results, the bioleaching mechanism based on sulfide minerals was able to be categorized into the following reactions: indirect biological oxidation (Equation (8)) by increased bacterial activity, chemical oxidation derived from reduced solution pH (Figures 4 and 5), and consequently direct bacterial oxidation (Equations (2)-(7)) under low PZC (Figure 6) [37, 38, 39]. Therefore, the rate of pH decrease is faster than that of metal leaching because of the reaction time required to increase the bacterial activity in the leachate and the direct oxidation by bacteria at low PZC.