## 3 Results and Discussion
### Removal Behavior of Cu\({}^{2+}\) by Acclimatized S. pasteurii  
Figure 6 shows the variation of the removal ratios of Cu\({}^{2+}\) with increasing the concentrations of Ca\({}^{2+}\) at different urea concentrations. As shown in Figure 6a, with the increase in Ca\({}^{2+}\) concentration, the removal ratio of Cu\({}^{2+}\) tended to gradually increase. When the concentration of Ca\({}^{2+}\) increased to 2.0 mol/L, the removal ratios of Cu\({}^{2+}\) had a slight decrease for the groups of initial concentrations of 400 and 800 mg/L, indicating that high concentrations of Ca\({}^{2+}\) and Cu\({}^{2+}\) inhibited the growth of bacteria, while the removal ratio increased slowly when the Cu\({}^{2+}\) concentration was relatively small (i.e., 50, 100 and 200 mg/L). Compared with Figure 6a, the removal ratio in Figure 6b was relatively higher, because with the increase in urea concentration, the CO\({}_{3}\)\({}^{2-}\) produced by urea hydrolysis continued to increase, providing more chances for precipitation. With the increase in Ca\({}^{2+}\) concentration, the removal ratio of Cu\({}^{2+}\) also increased, but when the Ca\({}^{2+}\) concentration increased to 2.0 mol/L, the removal ratios of Cu\({}^{2+}\) at concentrations of 200, 400 and 800 mg/L all decreased a little. It can be observed from Figure 6c that with further increasing the urea concentration, the corresponding Cu\({}^{2+}\) removal ratio had a small decrease compared with that in Figure 6b. The increase in Ca\({}^{2+}\) concentration could improve the removal efficiency of different concentrations of Cu\({}^{2+}\), because Ca\({}^{2+}\) concentration directly affects the formation of CaCO\({}_{3}\) precipitation and the efficiency of coprecipitation of heavy metals [47]. The removal ratio of Cu\({}^{2+}\) in Figure 6d was lower compared with that in Figure 6c, indicating that too much urea inhibited the biomineralization process. Figure 5: Removal ratio of Cd\({}^{2+}\) without Ca\({}^{2+}\). Figure 7 shows the removal ratio of Cu\({}^{2+}\) with increasing the concentration of urea from 0.5 to 2.0 mol/L without introducing a calcium ion; the removal ratio of Cu\({}^{2+}\) was in the range of 62.6-64.4% when the urea concentration was 0.5 mol/L. In the absence of Ca\({}^{2+}\), merely increasing the concentration of urea in the substrate of the medium did not significantly improve the removal efficiency of Cu\({}^{2+}\), suggesting that the concentration of urea had few effects on the removal of Cu\({}^{2+}\) when there was no calcium. It can be inferred that the mechanism of mineralization and immobilization of heavy metals by _S. pasteurii_ is closely related to the production and quantity of CaCO\({}_{3}\). Figure 6: Removal ratio of Cu\({}^{2+}\) with different urea concentrations ((**a**) \(C_{\text{urea}}\) = 0.5 mol/L, (**b**) \(C_{\text{urea}}\) = 1.0 mol/L; (**c**) \(C_{\text{urea}}\) = 1.5 mol/L; (**d**) \(C_{\text{urea}}\) = 2.0 mol/L). Under different conditions with varying the concentrations of urea and Ca\({}^{2+}\), the removal ratio of Cu\({}^{2+}\) could range from 62.1 to 84.2%. A higher urea concentration was not conducive to the removal of Cu\({}^{2+}\), while the increase in Ca\({}^{2+}\) concentration was conducive to the removal of Cu\({}^{2+}\). When the concentration in urea was greater than 1.0 mol/L, the existence of Ca\({}^{2+}\) could lead to a decreased removal ratio of Cu\({}^{2+}\), as the combination of Ca\({}^{2+}\) and CO\({}_{3}\)\({}^{2-}\) was the dominant reaction. In general, when the concentrations of urea and Ca\({}^{2+}\) were 1.0 and 1.5 mol/L, respectively, the best removal efficiency of Cu\({}^{2+}\) could be obtained, and the removal ratio of Cu\({}^{2+}\) could reach 78.1-82.1% when the initial concentration of Cu\({}^{2+}\) varied from 50 to 800 mg/L. Compared with the very low removal ratio of Cu\({}^{2+}\) from aqueous solutions by _S.