## 3 Results and Discussion
### Biomineral Morphology  
The removal of Cd\({}^{2+}\), Cu\({}^{2+}\) and Pb\({}^{2+}\) from aqueous solutions via biomineralization driven by _S. pasteurii_ demonstrated very good efficiency. The removal ratio of Cd\({}^{2+}\) and Pb\({}^{2+}\) was better than that of Cu\({}^{2+}\), and nearly all the Cd\({}^{2+}\) and Pb\({}^{2+}\) ions in aqueous solutions could be removed. Figures 10 and 11 demonstrate the scanning electron microscope images and the X-ray diffraction patterns of the precipitates produced by the microbially induced carbonate precipitation. As can be observed from Figure 10a, the shape of the Ca\({}^{2+}\) precipitate seems irregularly spherical and blocky, the particle sizes are not uniform and some particles are combined together to form agglomerates. In addition, the surfaces of some particles are smooth, and some are covered by many fine crystals; therefore, there are abundant micropores with different sizes in the entire structure. Through XRD analysis in Figure 11a, it is found that the Ca\({}^{2+}\) precipitate is composed of two kinds of CaCO\({}_{3}\) minerals, namely calcite and vaterite, and the proportion of calcite and vaterite is, respectively, 75.0% and 25.0%. Figure 10: _Cont._  
Figure 9: Removal ratio of Pb\({}^{2+}\) without Ca\({}^{2+}\). As shown in Figures 10b and 11b, the overwhelming majority of Cd\({}^{2+}\)-Ca\({}^{2+}\) appreciipitates are in the form of calcite minerals, accounting for as high as 98.8%, and have a blocky appearance. There is also a very small amount of another mineral called Ca\({}_{0.25}\)Cd\({}_{0.75}\)O. The particles, which have different sizes, are relatively dispersed and abundant pores can be observed in the structure. The particle size of the Cu\({}^{2+}\)-Ca\({}^{2+}\) copreipitate is found to be relatively larger than the other precipitates and the color appears blue. In Figure 10c, we can observe that many particles of the Cu\({}^{2+}\)-Ca\({}^{2+}\) copreipitate aggregate and cement together to form clusters, and there seems to be no pore among the particles. Meanwhile, there are only large pores in this kind of structure, which is different from other precipitates. Some fine spherical particles can be observed on the surface of the clusters, and these are vaterite minerals, which can be confirmed by the XRD analysis in Figure 11c, demonstrating that the main mineral of Cu\({}^{2+}\)-Ca\({}^{2+}\) copreipitate is calcite (CaCO\({}_{3}\)), accounting for 95.3%. Figure 11: XRD spectra of precipitates ((a) Ca\({}^{2+}\) precipitate; (b) Cd\({}^{2+}\)-Ca\({}^{2+}\) copreipitate; (c) Cu\({}^{2+}\)-Ca\({}^{2+}\) copreipitate; (d) Pb\({}^{2+}\)-Ca\({}^{2+}\) copreipitate). Figure 10: SEM images of precipitates. In Figures 10d and 11d, the shape of most of Pb\({}^{2+}\)-Ca\({}^{2+}\) coprecipitates appears blocky, and the particles are much smaller and more dispersed than other precipitates. The main mineral is calcite, accounting for 96.5%, and the proportion of another mineral vaterite is only 3.5%. Although different types of crystals can be formed, vaterite is found to be in a very low percentage, similar with the reported studies [16]. As we observe from Figure 10, the shape and size of the Cu\({}^{2+}\)-Ca\({}^{2+}\) coprecipitate are quite different from others. The clusters without any pore formed by Cu\({}^{2+}\)-Ca\({}^{2+}\) coprecipitating enlarge the surface area of the particles, and inhibit more copper ions to enter into the structures, resulting in a much lower Cu\({}^{2+}\) removal ratio than Cd\({}^{2+}\) and Pb\({}^{2+}\). As for Cd\({}^{2+}\) and Pb\({}^{2+}\), the sizes of the coprecipitates are relatively small, and there are many small micropores between particles. These abundant pores and great specific surface areas are conducive to the adsorption of heavy metals, leading to the very high removal ratio of Cd\({}^{2+}\) and Pb\({}^{2+}\).