## 3 Results and Discussion
### Identification of Heavy-Metal-Containing Precipitates of Liquid Batch Test  
Figure 13 presents the XRD patterns of precipitates in GT-Zn0/0.5, GT-Ni0/0.5, GT-Cr0/0.5, and GT-multiple0/0.25/0.5. The representative peaks of 20.9\({}^{\circ}\), 35.9\({}^{\circ}\), 25.1\({}^{\circ}\)/32.5\({}^{\circ}\), 36.2\({}^{\circ}\), and 47.1\({}^{\circ}\) correspond to Zn(OH)\({}_{2}\), ZnO, ZnCO\({}_{3}\), Zn\({}_{5}\)(CO\({}_{3}\))\({}_{2}\)(OH)\({}_{6}\), and NiCO\({}_{3}\), respectively [51; 52; 53]. The presence of ZnCO\({}_{3}\) and NiCO\({}_{3}\) crystals in GT-Zn/Ni and GT-multiple confirms that the immobilization of Zn\({}^{2+}\) and Ni\({}^{2+}\) partially relies on the biomineralization induced by sword bean urease. Additionally, the XRD results indicate that organic molecules could promote ZnCO\({}_{3}\) and NiCO\({}_{3}\) nucleation and precipitation. In addition, XRD results confirmed the presence of Zn(OH)\({}_{2}\) and Zn\({}_{5}\)(CO\({}_{3}\))\({}_{2}\)(OH)\({}_{6}\), indicating that Zn\({}^{2+}\) immobilization also depended on the increase in pH during the EICP process. ZnO may come from the decomposition of Zn(OH)\({}_{2}\). In contrast, no Cr-containing minerals were detected in GT-Cr and GT-multiple. The XRD results of groups containing Cr(VI) reveal that the immobilization mechanism of Cr(VI) was quite different from that of Zn and Ni, which may be caused by the unique properties of Cr\({}_{2}\)O\({}^{7-}\). In an alkaline solution, the Cr\({}_{2}\)O\({}_{7}\)\({}^{2-}\) transforms into CrO\({}_{4}\)\({}^{2-}\)[54], but the CaCrO\({}_{4}\) precipitation cannot occur due to its low ion activity product (IAP). Hence, in groups without Ca\({}^{2+}\), Cr(VI) might be ab  
sorbed/complexed by organic molecules in crude urease extracts. In groups that added Ca\({}^{2+}\), the Cr(VI) could be immobilized through adsorption/complexation by CaCO\({}_{3}\) and organic molecules. The FTIR results for liquid batch tests are shown in Figure 14. In GT-Zn, Ni, Cr, and multiple0/0.25/0.5, the characteristic bands corresponding to amides and ester groups were found at 1450-1650 cm\({}^{-1}\) and 1740-1750 cm\({}^{-1}\), respectively. The absorption linked to the Amide I band results in stretching vibrations of the C=O bond within the amide, while absorption related to the Amide II band primarily induces bending vibrations of the N-H bond [55]. The C=O stretching vibration at 1740-1750 cm\({}^{-1}\) originated from ester groups [56]. Additionally, a broad adsorption band at around 3300 cm\({}^{-1}\) was attributed to \(-\)OH stretching [57]. In GT-multiple0.25/0.5, the bands for CaCO\({}_{3}\) were detected at 710, 877, and 1422 cm\({}^{-1}\). The presence of amides, ester, and hydroxyl functional groups further indicates that Zn\({}^{2+}\), Ni\({}^{2+}\), and Cr(VI) could be adsorbed or complexed by organic molecules. Figure 15 shows the SEM-EDS results of precipitates in liquid batch tests. The precipitates in GT-Zn0, GT-NiO, GT-Cr0, and GT-multiple0 have no evident crystal morphology, which may be caused by the coexistence of organic matter with heavy metal carbonate crystal. The EDS results reveal that these irregular precipitates contain C, O, Zn, Ni, and Cr elements. The proportion of C and O far exceeds the amount required for forming heavy metal carbonate precipitates, indicating that organic molecules could adsorb or complex with heavy metals except for forming carbonates and hydroxides. The EDS results show that the weight percentages (wt%) follow the order Zn > Ni > Cr, consistent with liquid batch test results, and indicate that the EICP method was most effective in immobilizing Zn\({}^{2+}\), followed by Ni\({}^{2+}\) and Cr(VI). In GT-multiple0.25/0.5, spherical crystal aggregates with porous structures on their surface were observed under 5000\(\times\) magnification. These porous structures may provide a large specific surface area for heavy metal adsorption.