## 3 Results and Discussion
### Heavy Metal immobilization Percentage  
The variation in heavy metal immobilization percentage over time for both individual and multiple groups is shown in Figure 6. In GT-Zn0, the immobilization percentage of Zn\({}^{2+}\) increased in the first 3 h, decreased during 3 to 12 h, and finally stabilized at 25.6% after 12 h. In GT-Ni0 and GT-Cr0, the immobilization percentage of Ni\({}^{2+}\) and Cr(VI) increased in the first 3 h and then stabilized at 10.6% and 8.7%, respectively. In GT-multiple0, the immobilization percentages of both Zn\({}^{2+}\) and Ni\({}^{2+}\) increased in the first 3 h, decreased during 3 to 12 h, and finally stabilized at 65.8% and 32.1%, respectively. In comparison, the immobilization percentage of Cr(VI) increased slowly and stabilized at 5.0% after 6 h.  
Figure 5: Variation in pH over time: (**a**) GT-Zn0/0.25/0.5; (**b**) GT-Ni0/0.25/0.5; (**c**) GT-Cr0/0.25/0.5; (**d**) GT-multiple0/0.25/0.5. The immobilization of Zn\({}^{2+}\), Ni\({}^{2+}\), and Cr(VI) in groups without Ca\({}^{2+}\) added was mainly dependent on (1) biomineralization and (2) adsorption or complexation by organic molecules. Specifically, biomineralization refers to the CO\({}_{3}\)\({}^{2-}\) or OH\({}^{-}\)produced during the EICP process combining with heavy metal ions to form heavy metal carbonate and hydroxide. In GT-Zn0 and GT-multiple0, the immobilization percentage of Zn\({}^{2+}\) and Ni\({}^{2+}\) increased first, then decreased, and finally stabilized. This is because Zn\({}^{2+}\) or Ni\({}^{2+}\) combine with CO\({}_{3}\)\({}^{2-}\) or OH\({}^{-}\) to form Zn(OH)\({}_{2}\), ZnCO\({}_{3}\), Ni(OH)\({}_{2}\), and NiCO\({}_{3}\) precipitates at first. As urea is further hydrolyzed, the pH and the concentration of ammonium ions in solutions increase. Consequently, some of the Zn(OH)\({}_{2}\), ZnCO\({}_{3}\), Ni(OH)\({}_{2}\), and NiCO\({}_{3}\) precipitates gradually dissolve and form [Zn(NH\({}_{3}\))\({}_{k}\)]\({}^{2+}\) or [Ni(NH\({}_{3}\))\({}_{k}\)]\({}^{2+}\) (where x ranges from1 to 6). In GT-Ni0, the trend of initial increase followed by decrease was not observed, which may be due to the urea hydrolysis completion efficiency exceeding 99% in the first 3 h, resulting in a stable state. In addition, heavy metals may also be immobilized through adsorption or complexation by organic molecules in solutions [37; 38; 39]. The crude sword bean urease solution is rich in organic molecules, such as amino acids, proteins, polysaccharides, and lipids. Studies have found that functional groups in organic molecules, such as carbonyl, carboxyl, amide, ester, and hydroxyl groups, play a crucial role in the complexation process with heavy metals. Proteins or polypeptides primarily interact with heavy metals through metallothioneein and phytochemicalin [40]. Other organic molecules containing amino and sulfhydryl groups tend to complex with heavy metals to form complex metal chelating rings with more stable structures [41]. In GT-Zn0, -Ni0, and -multiple0, Zn\({}^{2+}\) and Ni\({}^{2+}\) can be immobilized through both biomineralization and organic adsorption. However, in GT-Cr0 and GT-multiple0, Cr(VI) could not be immobilized only through biomineralization. Figure 6: Heavy metal immobilization percentage over time: (a) Zn\({}^{2+}\) in GT-Zn0/0.25/0.5 and GT-multiple0/0.25/0.5; (b) Ni\({}^{2+}\) in GT-Ni0/0.25/0.5 and GT-multiple0/0.25/0.5; (c) Cr in GT-Cr0/0.25/0.5 and GT-multiple0/0.25/0.5. Instead, it can be immobilized by adsorption or complexation with organic molecules in crude sword bean solutions. The difference in immobilization percentage among Zn, Ni, and Cr(VI) may also be due to various interactions between organic molecules and heavy metals. [PERSON] et al. (2013) [PERSON] et al. (2013) found the adsorption capacity of soy protein for heavy metal ions in solution was in the order of Zn > Ni > Cr.