## 3 Results and Discussion
### Variation in Urea Hydrolysis Completion Efficiency, Ca2+ Immobilization Percentage, and pH  
Figure 3 shows the variations in urea hydrolysis completion efficiency for the GT-Zn0/0.25/0.5, GT-NiO/0.25/0.5, GT-Cr0/0.25/0.5, and GT-multiple/0.25/0.5 after reacting for 3, 6, 9, 12, 24, 36, 48, 60, 72, 84 h. In the GT-ZnO, -NiO, and -Cr0, the initial urea completion efficiency was high, with over 90% urea being hydrolyzed in the first 3 h. Nearly 100% hydrolysis of urea was achieved within 9 to 12 h. In contrast, the final urea completion efficiency in the GT-multiple0 was only about 55%. The results indicated that individual heavy metal would not affect the catalytic capacity of crude urease. However, multiple heavy metals led to a 45% reduction in urea hydrolysis completion efficiency. With higher concentrations of added Ca2+, the urea hydrolysis completion efficiency for GT-Zn0.25/0.5, -NiO.25/0.5, and -Cr0.25/0.5 decreased to about 88 to 95%. This suggests that the addition of Ca2+ to these groups may slightly inhibit urea hydrolysis. In contrast to GT-Zn0.25/0.5, -NiO.25/0.5, and -Cr0.25/0.5, the urea hydrolysis completion efficiency of GT-multiple0.25 was much higher than that of GT-multiple0 and GT-multiple0.5, reaching 91% at the final stage. In comparison, the urea hydrolysis completion efficiency of GT-multiple0/0.5 was only about 55% and 4.9%, respectively. The results of GT-multiple0/0.25/0.5 suggest that a small amount of Ca2+ can enhance urea hydrolysis, whereas excessive Ca2+ significantly reduces urease activity, leading to a significant reduction in urea hydrolysis completion efficiency. The variation in urease activity with different Ca2+ concentrations is attributed to the interactions of Ca2+ with functional groups and the urease protein [34; 35]. These findings are consistent with [36], which reported that low concentrations of Ca2+ increase urease activity, while high concentrations of Ca2+ decrease it. The variation in Ca2+ immobilization percentage over time is shown in Figure 4. In GT-Zn0.25/0.5, GT-NiO.25/0.5, GT-Cr0.25/0.5, and GT-multiple0.25, the Ca2+ immobilization percentage increased during the first 6 to 9 h as urea hydrolysis progressed. After 9 to 12 h, nearly 100% of Ca2+ was immobilized due to the completion of urea hydrolysis. In contrast, the Ca2+ immobilization percentage in the GT-multiple0.5 was only about 7.5%, corresponding to the 4.9% of urea completion efficiency. The results suggest a direct relationship between Ca2+ immobilization and urea hydrolysis, indicating the biologically formed precipitation of Ca2+. Figure 5 shows the pH variation for GT-Zn0/0.25/0.5, GT-NiO/0.25/0.5, GT-Cr0/0.25/0.5, and GT-multiple0/0.25/0.5 over time at intervals of 3, 6, 9, 12, 24, 36, 48, 72, 84 h. It can be seen that the initial pH was up to 9.0-9.2 in GT-Zn0, -NiO, and -Cr0 after adding the EICP treatment solution. Then, the pH increased continuously and stabilized at approximately 9.4 after 6 h. In the GT-Zn0.25/0.5, -NiO.25/0.5, and -Cr0.25/0.5, the initial pH decreased with the higher concentration of added Ca2+, ranging from 7.6 to 8.0. Afterward, the pH increased and stabilized at 8.6 to 9.0 after approximately 9 to 12 h. Notably, the pH of GT-Zn0.5 decreased and then increased in the first 9 h, whereas a similar phenomenon wasn't observed in GT-Zn0.25, GT-NiO.25/0.5, or GT-Cr0.25/0.5, where a continuous increase in pH was observed instead. In the GT-multiple0/0.25/0.5, the initial pH decreased with higher concentrations of added Ca2+, ranging from 7.4 to 7.8. In GT-multiple0/0.25, the pH increased continuously and stabilized between 8.9 and 9.2 after 24 h. However, in the GT-multiple0.5, pH decreased in the first 3 h and then increased continuously, reaching a final pH of 7.6 after 24 h.  
Figure 4: Ca\({}^{2+}\) immobilization percentage over time: (**a**) GT-Zn0.25/0.5; (**b**) GT-Ni0.25/0.5; (**c**) GT-Cr0.25/0.5; (**d**) GT-multiple0.25/0.5.