## 3 Results and Discussion
### Micromorphology of the Scoured Samples and CaCO\({}_{3}\) Crystals  
Figure 12 shows the micromorphology of the scoured samples. The hydrated phosphogypsum crystals of S1 had a uniform size distribution and overlapped each other. The microbially treated slopes had different degrees of cementation and different pore structures. S2 appeared to have a large area of cementation, and the generated CaCO\({}_{3}\) filled the macropores while the particles were tightly connected. Lots of macropores in S3 cementation caused some of the particles to be connected through the bridging. However, the single particles were apparent in S4, and the cementation was limited to a few particles with high bonding force. The size of macropores between the particles was larger than otherwise. Figure 11: The roughness and erosion depth of slopes. The micromorphology shows that the connection mode between particles had changed after microorganism modification, and the bond force was stronger. Previous studies show that bacteria secrete acidic soluble organic matter during the growth process, which will affect the nucleation of crystals and the connection of particles [56; 57]. The gypsum dihydrate particles formed by the hydration of gypsum hemihydrate are connected by intermolecular forces, which were easily destroyed under the continuous erosion of rainfall. The bacterial solution contains macromolecular organic matter, whose long chain could make the particles aggregate together. The functional groups in organic molecules such as -COOH and -OH will connect with phosphogypsum particles and calcium ions using hydrogen bonds or ionic bonds. This kind of connection needs a more destructive force to break. Figure 13 shows the CaCO\({}_{3}\) content of the scoured sample. The mass loss of the S1 sample after soaking in hydrochloric acid might be due to the dissolution of some impurities in phosphogypsum. The mass losses of S2, S3, and S4 far exceeded that of S1. The CaCO\({}_{3}\) content of S2, S3, S4 was 26.25%, 29.34%, 33.81%, respectively. It can be seen from the results that the increase in bacterial concentration induces more CaCO\({}_{3}\). The conclusion here is the same as the conclusions of [PERSON] et al. [58]. However, here, the distribution of CaCO\({}_{3}\) content in different parts became more uneven. Figure 12: Micromorphology of the scoured sample (**a**): S1; (**b**): S2; (**c**): S3; (**d**): S4). Figure 13: CaCO\({}_{3}\) content of the scoured samples. In order to explore the structure of CaCO\({}_{3}\) produced under different bacterial concentrations, the bacterial solutions of OD\({}_{600}\) = 0.1, OD\({}_{600}\) = 0.3, OD\({}_{600}\)= 0.5, and 0.5 M cementation solution were mixed to induce CaCO\({}_{3}\) in a pure liquid environment, and the crystals were dried in the oven and magnified 1000 times for observation. As shown in Figure 14, although it is not clear whether a single particle in Figure 14 is a single CaCO\({}_{3}\) crystal at this magnification, it can be analyzed as the smallest CaCO\({}_{3}\) unit. Figure 14a shows that the CaCO\({}_{3}\) units formed by OD\({}_{600}\) = 0.1 were dispersed. A few particles were connected in a linear arrangement. CaCO\({}_{3}\) units in Figure 14b were densely clustered with a bridge structure. In Figure 14c, the size of the units in OD\({}_{600}\) = 0.5 was similar to that in OD\({}_{600}\) = 0.3. The particles were first connected by linearly linked and agglomerated flocculation, finally forming an overhead structure. There were many structural pores in the accumulation, which caused the fine particles in the pores to be easily washed away. [PERSON] et al. reported that, compared with low-concentration bacterial liquids, bacteria in high-concentration bacterial liquids are more likely to aggregate and flocculate, which induces the formation of larger-sized CaCO\({}_{3}\) clusters [59]. [PERSON] et al. reported that the generation of a new crystal nucleus takes precedence over the growth of crystals under the same supersaturation in the solution [60]. [PERSON] et al.