# Yeast Extract Affecting the Transformation of Biogenic Tooeleite and Its Stability
## 3 Results and Discussion
### The Role of Yeast Extract during the Formation of Tooleite and As Stabilization Mechanism  
#### 3.3.1 Effect of Yeast Extract on Tooleite Transformation  
The XRD patterns of the precipitate obtained at different times without (a) or with (b) yeast extract are shown in Figure 6. This result revealed that the precipitation formed in the initial stage of the reaction was an amorphous phase. Figure 6 shows that typical tooleite peaks were detected at 6 h in absence of yeast extract, but a peak obtained at 6 h exhibits a weak band at 27\({}^{\circ}\) in the presence of yeast extract. The latter XRD patterns display intensity of tooleite peaks increased over time, and no other crystals occurred during the reaction. As observed above, it is clear that the incorporation of yeast extract delays tooleite formation. Figure 7 shows the SEM images of tooleite particles prepared without or with yeast extract. It can be seen from Figure 7(a-1,a-5,a-10) that the form of this precipitate is the flower structure of platy particles aggregating together, which is consistent with previous research [43]. However, the SEM results of tooleite synthesized in the presence of yeast extract showed that the platy structure of the final product was thicker and clearer (Figure 7(b-5,b-10)). It has been reported that different transformation pathways affect the final mineral morphology [44,45]. Furthermore, yeast extract may play an important role in the mineral transformation process so that tooleite particles with such a specific morphology could be formed. Figure 5: As removal efficiency, As leaching concentration (**a**) and XRD patterns of the precipitates (**b**) under different yeast extract concentrations. The FTIR spectra of samples collected at different times without (Figure 8a,b) and with (Figure 8c,d) yeast extract revealed changes in the functional groups of precipitates during mineral transformation. As shown in Figure 8a,c, the FTIR spectra of precipitate shows gradually enhanced absorption peaks at 750-950 cm\({}^{-1}\), 686-692 cm\({}^{-1}\), and 511-513 cm\({}^{-1}\), which belong to the stretching vibration peak of As-O [46], Fe-O, and Fe-O-As [47]. The above bands ascribed to tooeleite and their intensities increased gradually during 24 h [48]. Figure 6: XRD patterns of the precipitates obtained under 0.00% (**a**) and 0.10% (**b**) yeast extract concentrations at different reaction times. Figure 7: SEM images of precipitates under optimum conditions (hollow structure marked by a red box), (a-1), (a-5), and (a-10) at magnifications of 10,000, 50,000, and 100,000 without yeast extract; (b-1), (b-5), and (b-10) at corresponding magnifications with yeast extract. Furthermore, the XRD patterns of the precipitates obtained at 3 h with yeast extract does not look much different than that without yeast extract, both precipitates were amorphous phase (Figure 6). So it was important to verify the functional groups of precipitates formed at 3 h. In Figure 8b, a sharp band at 1127 cm\({}^{-1}\) and two shoulders at 1042 cm\({}^{-1}\) and 1195 cm\({}^{-1}\) can be attributed to the \(v_{3}\) triply degenerate antisymmetric stretching vibration of SO\({}_{4}^{2-}\) in schwertmannite [49]. The removal of As by schwertmannite takes place only at pH values from 2 to 7 [50], which further proved that the precursor was schwertmannite adsorbing As. In Figure 8d, two small bands (1124 cm\({}^{-1}\) and 1040 cm\({}^{-1}\)) appeared which corresponded to the substances produced by the interaction of SO\({}_{4}^{2-}\), Fe\({}^{3+}\), and OH\({}^{-}\) (1058 and 1112 cm\({}^{-1}\)) [51]. It was demonstrated that the amorphous material was most likely basic ferric sulfate (MFex(SO4)\({}_{y}\)(OH)\({}_{\rm z}\), M\({}^{+}\) = K\({}^{+}\), Na\({}^{+}\), NH\({}_{4}\)\({}^{+}\)) [51; 52]. Although, in the system without yeast extract, the bands at 1195 cm\({}^{-1}\), 1127 cm\({}^{-1}\), and 1042 cm\({}^{-1}\) gradually disappeared, the band at 1104 cm\({}^{-1}\) remained over time (Figure 8a).