# Yeast Extract Affecting the Transformation of Biogenic Tooeleite and Its Stability
## 3 Results and Discussion
### Biogenic Tooeleite Formation
#### 3.1.1 Effect of the Initial pH  
pH is one of the most important factors for the removal of As and the formation of biogenic tooeleite because it influences iron species and bacterial growth. Furthermore, the effect of the initial pH on the precipitation process was studied in the range of 1.0-6.0.  
The chemical equation of Fe(II) oxidation and Fe(III) hydrolysis can be expressed as follows:  
\[\mathrm{Fe^{2+}}\,+\,\mathrm{O_{2}}\,+\,\mathrm{H^{+}}\xrightarrow{A_{f}} \mathrm{Fe^{3+}}\,+\,\mathrm{H_{2}O} \tag{1}\]  
\[\mathrm{Fe^{3+}}\,+\,\mathrm{H_{2}O}\,\,\rightarrow\,\mathrm{FeOH^{2+}}\,+\, \mathrm{H^{+}} \tag{2}\]  
Iron oxidation consumes H\({}^{+}\) and increases pH while the hydrolysis of Fe(III) counteracts the increase in pH of the solution. Figure 1a shows that the concentrations of Fe(II) all decreased and Fe(II) was completely oxidized on the second day at pH = 2.0\(\sim\)6.0. The abiotic oxidation of Fe(II) was very slow at pH < 3, but Fe(II) oxidation could be accelerated by _A. ferrooxidans_[35]. Fe(II) oxidation rate was relatively slow during 4 d at initial pH = 1.0 as bacterial activity was reduced under such low-pH conditions. In contrast, the Fe(II) oxidation rate was significantly higher when the initial pH was 6.0, which was the dominant oxidation of Fe(II) by oxygen in the air [36]. At the same time, the pH decreased to 3.5 after 1 d indicating that the hydrolysis of iron was far stronger than the oxidation of Fe(II) (Figure 1b). The pH of the system remained stable at approximately 2.0 after 2 d when the initial pH 2.0-4.0. Hence, it can be concluded that a suitable initial pH range (2.0-4.0) could slow down the local pH mutation, providing a suitable micro-environment for the normal growth of bacteria and the stability of system supersaturation [37].  
The removal efficiency of As and the concentration of As leaching are illustrated in Figure 2a. The As removal efficiency apparently increased with increasing pH from 1.0 to 1.8 while it decreased slightly at pH 2-6. The XRD patterns (Figure 2b) confirmed that tooeleite was the only detectable mineral consistent with PDF 44-1468. This proved the formation of tooeleite in the pH range of 1.8-4.0, which is wider than that in a previous report (2.0-3.5) [9]. The analysis of XRD patterns corresponding to pH revealed that acidic conditions lead to better crystallization and the optimal crystallized tooeleite was obtained at pH = 2.0. Meanwhile, the concentration of As leaching was as low as approximately 13 mg/L. The above observations showed that a suitable initial pH range provided suitable ore-forming conditions for tooeleite. Therefore, pH 2.0 was chosen for capably treating acid waste-water and synthesizing relatively stable tooeleite.  
Figure 1: Concentrations of dissolved Fe(II) (**a**) and pH change (**b**) in the biogenic tooeleite process under different initial pH conditions.  
Figure 2: As removal efficiency, As leaching concentration (**a**) and XRD patterns of the precipitates (**b**) obtained at different initial pH values.