## 4 Results and Discussion
### Conventional Leading of Ag at Lower Temperatures  
Since the elevated temperature of the hydrothermal process was not a statistically significant parameter for Ag leaching, further experiments were conducted at lower temperatures for comparison. In addition, as time was found to be an important factor, the effect of extending the leaching time was further investigated. The experiments were conducted at 25 \({}^{\circ}\)C and 45 \({}^{\circ}\)C, keeping the HNO\({}_{3}\) concentration at 2 N and the L/S at 10. The effect of stirring was further investigated.  
Figure 4: Fitted contour plots of Ag concentration vs. time and HNO\({}_{3}\) concentration for m-Si and p-Si panels.  
Figure 5: Fitted contour plots for m-Si and p-Si panels: (**a**) Ag concentration vs. temperature and HNO3; (**b**) Ag concentration vs. L/S and HNO3; (**c**) Ag concentration vs. time and temperature.  
The effect of stirring on Ag leaching was significant, with leaching efficiency increasing by 40% when stirring at 200 rpm (2 N HNO\({}_{3}\), L/S = 10, 45 \({}^{\circ}\)C, 2 h); thus, stirring was applied to all subsequent experiments at 25 \({}^{\circ}\)C and 45 \({}^{\circ}\)C (Figure S3 Supplementary Materials). The positive effect of stirring was ascribed to the access of metals to the leaching agent [33]. The effect of temperature is illustrated in Figure 6a for m-Si. It was observed that leaching with 2 N HNO\({}_{3}\) for 6 h at 45 \({}^{\circ}\)C had 80% efficiency, compared with 63.3% efficient at 25 \({}^{\circ}\)C; thus, the temperature of 45 \({}^{\circ}\)C was used in subsequent experiments. Next, leaching was performed at 45 \({}^{\circ}\)C for powder and flaked m-Si and p-Si panels. For m-Si panels, the maximum Ag leaching efficiency from flakes was 76.5% at 6 h; meanwhile, for p-Si panels, the leaching efficiency was 78% at 6 h (Figure 6b). Thus, the use of the powdered sample increased the leaching efficiency by 4.7-5.4%. Complete Ag leaching could not be achieved under low temperature conditions, even when the reaction time was increased to 24 h (2 N HNO\({}_{3}\), L/S = 10). For 24 h leaching, the Ag efficiency was 80.7% and 85.3% from m-Si and p-Si waste panels, respectively. However, [PERSON] et al. [20] observed almost total leaching (100%) from PV flakes for 5 h leaching at 25 \({}^{\circ}\)C but with a 3 N HNO\({}_{3}\) concentration, while for PV powder, 100% leaching efficiency was achieved at 25 \({}^{\circ}\)C, 3 N HNO\({}_{3}\), for 2 h. Furthermore, [PERSON] et al. [37] reported 99.4% Ag leaching efficiency at 80 \({}^{\circ}\)C with 5 N HNO\({}_{3}\) for 1 h.  
For Al, the leaching efficiency from the m-Si panel was 61.7% at 45 \({}^{\circ}\)C for 6 h (vs. 56.6% from p-Si panel) which compares to 44.7% efficiency in the m-Si panel at 25 \({}^{\circ}\)C for 6 h. However, for 24 h leaching, the efficiency was 61% at both 25 \({}^{\circ}\)C and 45 \({}^{\circ}\)C in the m-Si panel (Figure S4 Supplementary Materials). For the unpulverized sample, the m-Si panel leaching efficiency was 49% at 45 \({}^{\circ}\)C for 8 h (Figure S5 Supplementary Materials). Complete Al leaching was not achieved under low temperature conditions; however, it was favored under hydrothermal conditions. In the first case, the dissolution of Al may be partially inhibited by the formation of an aluminum oxide (Al\({}_{2}\)O\({}_{3}\)) layer on the surface and Al-Si compounds formed during acid treatment [38]. After acid leaching, Al can only be fully dissolved with NaOH [33]. [PERSON] et al. [37] reported 76% Al leaching efficiency at 80 \({}^{\circ}\)C with 5 N HNO\({}_{3}\) for 1 h; however, [PERSON] et al. [20] observed a 100% Al leaching yield after 2 h leaching at 25 \({}^{\circ}\)C with 3N HNO\({}_{3}\).