## 5 Conclusions  
In this paper, we used the method of cyclically spraying a bacteria solution with various concentrations (OD\({}_{600}\) = 0.1, OD\({}_{600}\) = 0.3, and OD\({}_{600}\) = 0.5) and a cementation solution (0.5 M) to modify the phosphogypsum material via MICP. We carried out a water shaking test and a permeability test to verify the water stability of the phosphogypsum material. The hydraulic erosion resistance of the modified phosphogypsum slope was studied through a rainfall scouring test, and the pH and heavy metal elements of the outflow were assessed. Three-dimensional laser scanning was used to analyze the micromorphology of the scoured slope. The following conclusions were obtained from the experimental results:  
1. The water stability and permeability of the sample treated by the microorganism are greatly improved. With the increase in the bacterial concentration, the difference between the content of CaCO\({}_{3}\) in the outermost layer and that in the inner layer gradually increases. The degree of permeability reduction is more uneven.
2. Phosphogypsum can be used as solidified material to simulate artificial slopes. The structure of the ordinary hydrated phosphogypsum slope was completely destroyed in the rainfall scouring test. When the slope is treated with OD\({}_{600}\) = 0.1, the cementation of particles is more uniform, with fewer and smaller macropores, and the slope is eroded by sheet erosion. With the increase in the concentration of the bacterial solution, the bacterial will agglomerate and flocculate, resulting in uneven cementation and more macropores, which lead to erosion pits and gullies under rainfall scouring. The OD\({}_{600}\) = 0.1 treated slope had the best erosion resistance and the minimum erosion loss.
3. Phosphogypsum slope treated by microbial treatment can greatly improve the water erosion resistance and reduce slope erosion. The erosion degree of the microbially treated slopes is much less than that of the untreated slope. When the content of calcium carbonate is high and unevenly distributed, it can lead to deeper erosion depth.
4. The structure of calcium carbonate induced by different concentrations of treatment solution is different. Under the same concentration of cementation solution, the increase in the bacterial concentration is associated with higher CaCO\({}_{3}\) content, and the CaCO\({}_{3}\) flocculate to form an overhead structure. The micropores in the structure easily cause the loss of fine particles.
5. The pH of the modified phosphogypsum outflow solution is neutral. The heavy metal elements meet the emission standards through microbial action and fixation after microbial treatment. This article proves the effectiveness of microbially modified phosphogypsum and provides a preliminary test basis for the utilization of phosphogypsum material in the reinforcement of slopes.  
**Author Contributions:** Conceptualization, Z.L. and Y.T.; data curation Y.T. and S.W.; formal analysis, Y.T.; investigation, Y.T., D.L., and L.F.; methodology, Y.T.; project administration, Z.L. and Y.Z.; validation, Z.L.; writing--original draft, Y.T.; writing and editing draft, Z.L. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This research is financially supported by the National Key Research and Development Plan of China (Grant No. 2019 YFC1509903), the National Natural Science Foundation of China (Grant No. 41790442, 42177146), the Second Tibetan Plateau Scientific Expedition and Research Program (STEP) (Grant No. 2019 QZKK0904) and the Key Research and Development Plan of Yunnan Province (Grant No. 202103 AA080013).  
**Conflicts of Interest:** The authors declare no conflict of interest.