## 4 Results and Discussion
### Hydrothermal Leading of c-Si Panels  
The hydrothermal leaching of Si panels revealed that the reaction time was the most important factor for Ag leaching, followed by HNO\({}_{3}\) concentration (within the range of the tested parameters). Pareto charts of significant factor terms and their interactions in response to leaching efficiency are shown in Figure 3. These charts show the absolute values of the standardized effects from largest to smallest. A reference line (dashed) is drawn to show which effects are statistically significant. The Pareto chart is used for decision-making as it shows the effects of independent variables on leaching efficiency. The basic principle of Pareto analysis is based on the 80/20 rule, meaning that 80% of the results come from 20% of the means or causes; thus, few factors (20%) are vital, and many factors (80%) are insignificant [35; 36]. Significant terms are above the dashed red line, while non-significant terms are below the red line. The L/S ratio was the least important factor for both m-Si and p-Si panels. In addition, the temperature was not a statistically significant factor for either sample (m-Si and p-Si) and there were no significant interactions between the parameters. The analysis of variance of the model is presented in Table S1 (Supplementary Materials).  
Figure 2: The cylindrical steel reactor (Technistro) internally coated with PTFE.  
Figure 4 presents the interaction between time and HNO\({}_{3}\) concentration regarding Ag leaching. Ag leaching efficiency increased significantly with increasing leaching time using 2 N HNO\({}_{3}\) for p-Si and m-Si. Figure 5 shows other factor interactions. Increasing the HNO\({}_{3}\) concentration and temperature also increased leaching efficiency; however, their effect was less significant than increasing the hydrothermal processing time. Reducing the L/S ratio increased the leaching yield, i.e., the concentration of Ag was higher. The main effects plots of temperature, time, L/S ratio and HNO\({}_{3}\) concentration on Ag leaching are depicted in Figure S1 (Supplementary Materials). The combination of temperatures, times, HNO\({}_{3}\) concentrations and L/S ratio in Ag leaching experiments resulted in leaching yields ranging from 0.12% to 74% for p-Si, while for m-Si, the efficiency of Ag leaching ranged between 5.7% and 66.2%. The highest Ag leaching yield was achieved at 2 N HNO\({}_{3}\), L/S = 10, 120 \({}^{\circ}\)C and 75 min. The lowest yield value was achieved at 1.5 N HNO\({}_{3}\), L/S = 15, 100 \({}^{\circ}\)C and 30 min. Based on the optimization obtained using mathematical equations defining the BBD/RSM, the optimized conditions to maximize Ag leaching (i.e., a 100% yield) through hydrothermal treatment are 2 N HNO\({}_{3}\), L/S = 10, 140 \({}^{\circ}\)C and 2 h leaching, which was experimentally validated.  
Aluminum leaching was also investigated under the same conditions. The main effects plot for the parameters (HNO\({}_{3}\), time, temperature, and L/S ratio) revealed that Al leaching increased favorably with processing time and L/S ratio (Figure S2 Supplementary Materials).  
Figure 3: Pareto chart of standardized effects of the factors and interactions for Ag leaching from m-Si and p-Si panels.  
Materials), while the other two factors were less significant. Efficient Al leaching was observed using 1 N HNO\({}_{3}\) under hydrothermal conditions. For m-Si and p-Si samples, the efficiency of Al leaching ranged from 23% to 88% and from 29% to 85%, respectively. The optimized conditions for maximum Al leaching were 2 N HNO\({}_{3}\), L/S = 10, 140 \({}^{\circ}\)C and 2 h leaching. In conclusion, Ag is the most valuable metal in PV panel waste; however, favorable Al leaching can be accomplished under the same conditions.