## 3 Results and Discussion
### Geochemical Modeling Results
In GS-Zn0, GS-Ni0, and GS-multiple0, as the urea hydrolysis completion efficiency increased, the immobilization percentage of Zn\({}^{2+}\) and Ni\({}^{2+}\) increased at 99.9% and then decreased rapidly until the final immobilization percentage was close to 0%. During this stage, the ZnCO\({}_{3}\) and NiCO\({}_{3}\) precipitate continuously formed, reaching a peak precipitation percentage of 99.9%. A small amount of hydrozinc precipitates was produced when trace urea was hydrolyzed, but they rapidly dissolved. As the hydrolysis of urea progressed, the overall alkalinity of the solution and the concentration of NH\({}_{4}\)\({}^{+}\) increased. This caused the ZnCO\({}_{3}\) and NiCO\({}_{3}\) precipitates to gradually dissolve into Zn\({}^{2+}\) and Ni\({}^{2+}\). Subsequently, the Zn\({}^{2+}\) and Ni\({}^{2+}\) combined with NH\({}_{3}\) to form [Zn(NH\({}_{3}\))\({}_{\rm x}\)]\({}^{2+}\) and [Ni(NH\({}_{3}\))\({}_{\rm x}\)]\({}^{2+}\) (where x ranges from 1 to 6). Ultimately, upon completion of urea hydrolysis, no ZnCO\({}_{3}\) or NiCO\({}_{3}\) precipitate remained in the simulations, with all precipitates converted to [Zn(NH\({}_{3}\))\({}_{\rm x}\)]\({}^{2+}\) and [Ni(NH\({}_{3}\))\({}_{\rm x}\)]\({}^{2+}\). In the liquid batch tests, similar trends of first an increase followed by a decrease in immobilization percentage were observed in GT-Zn0 and GT-multiple0-Zn/Ni. However, unlike the simulated results, the final immobilization percentages of Zn\({}^{2+}\)/Ni\({}^{2+}\) in GT-Zn0 and GT-multiple0 were 25.6% and 65.8%/32.1%, respectively, rather than 0%. This indicates that not all Zn\({}^{2+}\)/Ni\({}^{2+}\) were converted into [Zn(NH\({}_{3}\))\({}_{\rm x}\)]\({}^{2+}\) and [Ni(NH\({}_{3}\))\({}_{\rm x}\)]\({}^{2+}\). In GT-Ni0, the expected trend of first an increase followed by a decrease was not observed; instead, the immobilization percentage of Ni\({}^{2+}\) increased in the first 3 h and then stabilized at 10%, indicating that not all Ni\({}^{2+}\) was converted into [Ni(NH\({}_{3}\))\({}_{\rm x}\)]\({}^{2+}\). The difference between the liquid batch tests and simulated results for the final immobilization percentage of Zn\({}^{2+}\) and Ni\({}^{2+}\) may be due to the role of organic molecules. The only difference between the Visual MINTEQ simulation and the liquid batch test was that Visual MINTEQ could not account for organic molecules. Studies have shown that the organic molecules can immobilize heavy metals by adsorbing/complexing with heavy metal ions, as mentioned in Section 3.3. In addition, the organic molecules in crude urease sword bean extracts may provide more nucleation sites for forming metal carbonates or metal hydroxide precipitates and stabilize the precipitates, preventing them from being dissolved into [Zn(NH\({}_{3}\))\({}_{\rm x}\)]\({}^{2+}\) or [Ni(NH\({}_{3}\))\({}_{\rm x}\)]\({}^{2+}\). Similar results were reported by [48], who utilized the Geochemists' Workbench (GWB) to simulate the precipitation process of Ni\({}^{2+}\) induced by MICP and compared these results with experimental results. The simulations also showed that NiCO\({}_{3}\) eventually dissolved and formed [Ni(NH\({}_{3}\))\({}_{\rm x}\)]\({}^{2+}\). However, the experimental results showed the presence of NiCO\({}_{3}\). They proposed that the difference between experimental and simulation results may be due to the conformational changes in extracellular proteins during biomineralization. Specifically, the \(\beta\)-sheet structures of proteins increased while the \(\alpha\)-helices and other disordered structures decreased. These changes in protein structure might provide more nucleation sites to promote the nucleation and crystallization of NiCO\({}_{3}\).