## 3 Results and Discussion
### Geochemical Modeling Results
In GS-Zn0.25/0.5, GS-Ni0.25/0.5, and GS-multiple0.25, as the urea hydrolysis completion efficiency increased, the immobilization percentage of Zn\({}^{2+}\) and Ni\({}^{2+}\) (characterized by the precipitation percentage of ZnCO\({}_{3}\) and NiCO\({}_{3}\)) increased at 99.9% and then decreased rapidly until the final immobilization percentage was close to 0%. The precipitates in these groups were mainly composed of ZnCO\({}_{3}\) and NiCO\({}_{3}\). In contrast, a similar trend was not observed in GT-Zn0.25/0.5, GT-Ni0.25/0.5, and GT-multiple0.25. Instead, in the liquid batch tests, as the urea hydrolysis completion efficiency improved, the immobilization percentage of Zn\({}^{2+}\) and Ni\({}^{2+}\) increased. The final immobilization percentages of Zn\({}^{2+}\) and Ni\({}^{2+}\) were 99.9%/99.9%, 43.24%/64.2%, and 99.9%/50.18%, respectively. The difference between the simulation results and the liquid batch test results can be attributed to the combined effects of organic molecules and calcium carbonate. These factors influence the immobilization of Zn\({}^{2+}\) and Ni\({}^{2+}\) through adsorption, complexation, and the promotion of nucleation. In GS-multiple0.5, the simulation results showed that as urea hydrolysis progressed, the immobilization percentage of Zn\({}^{2+}\) and Ni\({}^{2+}\) gradually increased and stabilized at around 45% and 52%, respectively. Zn\({}^{2+}\) and Ni\({}^{2+}\) combined with CO\({}_{3}\)\({}^{2-}\) to form carbonate precipitates due to only 4.9% of urea hydrolysis. A similar trend was observed in GT-multiple0.5, where the immobilization percentage of Zn\({}^{2+}\) and Ni\({}^{2+}\) gradually increased and stabilized at 61.2% and 56.4%, respectively. The final immobilization percentages in the liquid batch tests were higher than those in the simulation results due to the additional adsorption/complexation effects of calcium carbonate and organic molecules in tests. In GS-Cr0/0.25/0.5 and GS-multiple0/0.25/0.5, Cr(VI) was scarcely immobilized, with immobilization percentages ranging from 5.0 to 17.48% in GT-Cr0/0.25/0.5 and GT-multiple0/0.25/0.5, indicating significant disparities between the simulation and liquid batch test results. In GS-Cr0, the Cr0\({}_{4}\)\({}^{2-}\) accounted for over 99% of the total Cr(VI), and precipitates were hardly formed throughout the process. Both simulation and liquid batch test results suggested that Cr(VI) in GT-Cr0 and GT-multiple0 was immobilized mainly through the adsorption/complexation effects of organic molecules. In GS-Cr0.25/0.5, as the urea hydrolysis completion efficiency increased, the concentration of CaCr0\({}_{4}\)(aq) increased and accounted for 99.9% of the Cr(VI) in the solution. The simulation results proved that the ion activity product (IAP) of Ca\({}^{2+}\) and Cr0\({}_{4}\)\({}^{2-}\) was less than the solubility product constant (Ksp) of CaCr0\({}_{4}\)(s), preventing the chemical precipitation of Cr(VI). Therefore, the immobilization of Cr(VI) in GT-Cr0.25/0.5 and GT-multiple0.25/0.5 may be by incorporation into the calcite surface or interaction with organic molecules to form a complex, leading to immobilization [49; 50]. This could not be simulated by Visual MINTEQ.