# A Comparative Study on Bieleaching Properties of Various
## 3 Results and Discussion
### Comparison of pH Change during Boleaching of Mineral Samples  
Figure 4 shows the change in the pH of the leachate during the chemical and biological oxidation of the mineral samples. During the 21 d leaching experiment, the pH decreased for the abiotic control case because of the consumption of H\({}^{+}\) ions in the growth culture medium by the inorganic oxidation of sulfide minerals (shown as -C in Figure 4). For the bacterial samples (shown as -B in Figure 4), the change in the pH of the leachate was lower than that in the abiotic control samples for all sample conditions. The pH decreased rapidly at the beginning of the experiment and then decreased slowly after 7 d. The pH was lower than 3 in all samples after 3 d of the boleaching experiment. This is because metal sulfides oxidatively solubilize sulfur and metals due to boleaching processes, which reduce the pH and enhance the solubilization of additional metal compounds [21]. In addition, as the pH in the leachate decreased from 5.11 to less than 3, the bacterial samples exhibited a pH below the point of zero charge (PZC) [22; 23; 24]. Therefore, the surface charge of the mineral sample changed from negative to positive, and the negatively charged bacteria in the \begin{table}
\begin{tabular}{c c c} \hline \hline
**Samples** & **Matched Phases** &
\begin{tabular}{c} **Quantity** \\ **(wt \%)** \\ \end{tabular} \\ \hline SP & PbS & 21 \\  & ZnS & 78 \\  & PbS & 57 \\  & ZnS & 42 \\  & PbS & 67 \\  & ZnS & 32 \\  & FeS\({}_{2}\) & \textgreater{}95 \\  & CCP & CuFeS\({}_{2}\) & \textgreater{}95 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Match! phase analysis results of mineral samples used in boleaching experiments. Figure 3: Reflecting microscope image of mineral samples used in boleaching experiments (Sp: sphalerite, Gr: galena, Py: pyrite, Ccp: chalcopyrite, Qtz: quartz). leachate came into contact with the surface of the mineral sample due to electrical attraction. This suggests the possibility of direct biological oxidation. The PZC of the mineral samples used in this study was confirmed in other studies [25]. The PZCs of sphalerite, galena, and pyrite were 4-5 in a study on sulfide mineral flotation using recycled water. [PERSON] et al. [26] showed that the PZC of chalcopyrite in a bioleaching experiment using _Sulfobous metallicus_ YN24 was 3.80, and the PZC of pyrite was 2.7 [27]. The decrease in pH (\(\Delta\)pH) for the bacterial samples compared to abiotic samples is summarized in Table 2. This estimation was based on the measured data for the change in the leachate pH through the chemical and biological oxidation of various mineral samples. The maximum decreasing pH (\(\Delta\)pH) was estimated to be approximately 2.62-3.01, and there was no difference in \(\Delta\)pH that correlated to the characteristics of the mineral samples. However, the pH decrease rate was highest in the SP sample (sphalerite content: 78%) and lowest in the CCP sample (chalcopyrite content: >95%). Similarly, [PERSON] [28] reported that the efficiency of heap/dump leaching is highly dependent on the constituent minerals of the ore. The leaching of ores takes several months for chalcocite (Cu\({}_{2}\)S) and covellite (CuS), whereas it takes years for CCP. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Sample** & **\(\Delta\)pH** & **Elements** & \begin{tabular}{c} **Leading Concentration** \\ **(mg/L)** \\ \end{tabular} \\ \hline SP & 2.62 & \begin{tabular}{c} Pb \\ Zn \\ Pb \\ \end{tabular} & 17.90 \\ GN1 & 2.49 & \begin{tabular}{c} Zn \\ Pb \\ \end{tabular} & 103.40 \\ GN2 & 2.55 & \begin{tabular}{c} Pb \\ Zn \\ \end{tabular} & 8.72 \\ PY & 2.55 & \
\begin{tabular}{c} Fe \\ Cu \\ \end{tabular} & 0.59 \\ CCP & 3.01 &
\begin{tabular}{c} Fe \\ \end{tabular} & 8.93 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: pH reduction and leaching metal concentrations derived from bioleaching experiments using an indigenous acidophilic bacterium (_Acidiphilium cryptum_).