## 3 Results and Discussion
### Removal Behavior of \(Cd^{2+}\) by Acclimatized _S. pasteurii_  
Figure 4 shows the variation of removal ratios of Cd\({}^{2+}\) with increasing the concentrations of Ca\({}^{2+}\) at different urea concentrations. It can be observed from Figure 4a that when the Ca\({}^{2+}\) concentration was 0, the removal ratio of Cd\({}^{2+}\) was the lowest. When the Ca\({}^{2+}\) concentration was 0.5 mol/L, the removal ratio of Cd\({}^{2+}\) was the highest and the removal ratios of five groups with different concentrations of Cd\({}^{2+}\) were almost the same at about 97.9-98.8%, implying that the Cd\({}^{2+}\) ions in the solution were almost completely removed. With the increase in Ca\({}^{2+}\) concentration, the removal ratio of Cd\({}^{2+}\) started to decrease gradually, and the final removal ratios remained at 76.3-86.4%. The increase in Ca\({}^{2+}\) led to the acceleration of the formation rate of CaCO\({}_{3}\) precipitation, and the CaCO\({}_{3}\) precipitation might adhere to the surface of bacteria, hindering the further combination of CO\({}_{3}\)\({}^{2-}\) with Cd\({}^{2+}\) and Ca\({}^{2+}\), resulting in the decrease in MICP rate [20]. Comparing Figure 4a with Figure 4b, the removal behavior of Cd\({}^{2+}\) had little change for the groups with 0 and 0.5 mol/L Ca\({}^{2+}\) concentrations when raising the urea concentration from 0.5 to 1.0 mol/L. The removal ratios of Cd\({}^{2+}\) were as high as 98.0-99.0% when the concentration of urea and Ca\({}^{2+}\) was 1.0 and 0.5 mol/L, respectively. As observed in Figure 4b, when the Ca\({}^{2+}\) concentration was increased from 0.5 to 1.0, 1.5 and 2.0 mol/L, the removal ratio of Cd\({}^{2+}\) remained almost unchanged, demonstrating that a further increase in Ca\({}^{2+}\) concentration would not be beneficial to the cadmium biomineralization process. The removal behaviors of Cd\({}^{2+}\) in Figure 4c,d were very different from those in Figure 4a,b, and when the urea concentration was further increased to 1.5 and 2.0 mol/L, the best removal efficiency of Cd\({}^{2+}\) happened when there was no Ca\({}^{2+}\). After adding Ca\({}^{2+}\) into the solutions, the Cd\({}^{2+}\) removal efficiency began to decrease, and the removal ratio decreased with the increase in Ca\({}^{2+}\) concentration, which seems different with the result published by [PERSON] et al. [35], who found that the Ca\({}^{2+}\) supplement improved the biomineralization of Cd\({}^{2+}\) by _S. pasteurii_. In that study, the Ca\({}^{2+}\) concentration was in the range of 5-250 mmol/L, and urea concentration was 20 mmol/L, both smaller than the lowest concentration of Ca\({}^{2+}\) (0.5 mol/L) and urea (0.5 mol/L) in our study. From Figure 4a,b, we can see when the \(C_{\text{urea}}\leq 1.0\) mol/L and Ca\({}^{2+}\leq 0.5\) mol/L, Ca\({}^{2+}\) supplement can facilitate Cd\({}^{2+}\) biomineralization, and as a matter of fact, [PERSON] et al. 's result is coincident with our results. It can be observed from Figure 5 that when there was no Ca\({}^{2+}\) in the solutions, the removal ratio of Cd\({}^{2+}\) increased with the increase in urea concentration. Especially when the urea concentration increased to 1.5 mol/L, the removal ratio of Cd\({}^{2+}\) increased significantly. When the urea concentration was raised to 2.0 mol/L, the removal ratio of Cd\({}^{2+}\) could be in the range of 89.9 to 99.7%. From the above results, we can infer that the increase in urea concentration was favorable for the removal of Cd\({}^{2+}\), while the increase in Ca\({}^{2+}\) concentration was not good for the removal of Cd\({}^{2+}\). Both urea and Ca\({}^{2+}\) had a significant influence on the removal behavior of Cd\({}^{2+}\). The optimal removal efficiency of Cd\({}^{2+}\) could be achieved when the urea and Ca\({}^{2+}\) concentration was 1.0 and 0.5 mol/L, respectively. The acclimatized _S. pasteurii_ presented a high affinity for a wide range concentration of cadmium ions and demonstrated an obvious advantage in removing Cd\({}^{2+}\), in contrast to other published studies [20, 41, 44].