## 3 Results and Discussion
### Geochemical Modeling Results  
The geochemical modeling results of variations in Ca\({}^{2+}\) immobilization percentage, pH, and heavy metal immobilization percentage with urea hydrolysis are shown in Figures 7-12. Figure 7 illustrates that the simulated Ca\({}^{2+}\) immobilization percentage increased continuously with urea hydrolysis before stabilizing. The final Ca\({}^{2+}\) immobilization percentages of GS-ZnO.25/0.5, GS-Ni0.25/0.5, GS-Cr0.25/0.5, and GS-multiple0.25/0.5 were 99.9%, 99.9%, 99.9%, 99.9%/4.52%, which were consistent with liquid batch results, indicating that the Ca\({}^{2+}\) immobilization efficiency is directly determined by the urea hydrolysis completion efficiency. Figure 8: Simulation results of pH: (**a**) GS-Zn0/0.25/0.5; (**b**) GS-Ni0/0.25/0.5; (**c**) GS-Cr0/0.25/0.5; (**d**) GS-multiple0/0.25/0.5. Figure 7: Simulation results of Ca\({}^{2+}\) immobilization percentages: (**a**) GS-Zn0/0.25/0.5; (**b**) GS-Ni0/0.25/0.5; (**c**) GS-Cr0/0.25/0.5; (**d**) GS-multiple0/0.25/0.5. Figure 10: Simulation results of GS-Ni0/0.25/0.5: (**a**) Ni\({}^{2+}\) immobilization percentage in GS-Ni0; (**b**) Ni\({}^{2+}\) immobilization percentage in GS-Ni0.25; (**c**) Ni\({}^{2+}\) immobilization percentage in GS-Ni0.5. Figure 9: Simulation results of GS-Zn0/0.25/0.5: (**a**) Zn\({}^{2+}\) immobilization percentage in GS-Zn0; (**b**) Zn\({}^{2+}\) immobilization percentage in GS-Zn0.25; (**c**) Zn\({}^{2+}\) immobilization percentage in GS-Zn0.5. Figure 11: Simulation results of GS-Cr0/0.25/0.5: (**a**) Cr(VI) immobilization percentage in GS-Cr(VI)0; (**b**) Cr(VI) immobilization percentage in GS-Cr(VI)0.25; (**c**) Cr(VI) immobilization percentage in GS-Cr(VI)0.5. Figure 12: Simulation results of GS-multiple0/0.25/0.5: (**a**) Zn\({}^{2+}\), Ni\({}^{2+}\), and Cr(VI) immobilization percentage in GS-multiple0.25; (**b**) Zn\({}^{2+}\), Ni\({}^{2+}\), and Cr(VI) immobilization percentage in GS-multiple0.5. Figure 8 depicts the trend of pH variation. In GS-Zn0, -Ni0, -Cr0, and -multiple0, the pH increased rapidly at first and then increased slowly with continued urea hydrolysis, finally stabilizing around 9.2-9.3, which was consistent with the liquid batch results. In contrast, in GS-Zn0.25/0.5, GS-Ni0.25/0.5, GS-Cr0.25/0.5, and GS-multiple0.25/0.5, the precipitation of calcium carbonate led to a decrease in pH, as described in Section 3.2. When the urea hydrolysis completion efficiency was less than a certain threshold (referred to as the turning point, e.g., in GS-Zn0.25, the turning point was 23.9%), the pH increase from urea hydrolysis was lower than the pH reduction caused by calcium carbonate precipitation, resulting in a decrease in the overall pH of the solution at this stage. When the urea hydrolysis completion efficiency surpassed the turning point, the increase in pH caused by hydrolysis outweighed the decrease caused by calcium carbonate precipitation, resulting in a rapid increase in the overall pH, which then stabilized at around 8.6 to 9.0. The turning point for groups with 0.25 M Ca\({}^{2+}\) added was about 24%, while for those with 0.5 M Ca\({}^{2+}\), it was approximately 40%. This indicates that the addition of Ca\({}^{2+}\) would lead to a decrease in pH. In the liquid batch test results, the initial sampling was performed at 3 h. Since most groups had exceeded the turning point in the first 3 h, the trend of pH first decreasing followed by increasing was not observed across all experimental groups; such a similar trend was only observed in GT-Zn0.5 and GT-multiple0.5. In Figures 9-12, the precipitation percentages of ZnCO\({}_{3}\) (the amount of Zn in ZnCO\({}_{3}\) divided by the initial amount of total Zn\({}^{2+}\)), hydrozinc, and NiCO\({}_{3}\) were used to characterize the immobilization percentage of Zn\({}^{2+}\) and Ni\({}^{2+}\).