# Organic Acid-Assisted Hydrothermal Leading of Silver from End-of-Life Photovoltaic Panels
## 3 Results and Discussion
### Hydrothermal Leading with Citric Acid  
For citric acid, the experiments with the highest yield were obtained for 105 min, at temperatures of 180-210 \({}^{\circ}\)C and concentrations of 1.5-2 M. When the experimental time increased to 150 min, the yield decreased (other conditions 1-2 M, 150-180 \({}^{\circ}\)C), indicating that very long treatment times are not desirable. This could be attributed to the effect of pressure increasing with time, as the decomposition constants of weak acids rise with higher applied pressure, making the acids weaker [31]. The shorter time of 60 min did not favor Ag leaching, probably because the reactor requires some time to reach the desired temperature. Once the hydrothermal reactor was placed in the hot furnace, the time was recorded, and this time was longer than the actual time at the desired temperature, as it included the preheating period. This is clearly illustrated in Figure 4, showing the contour plot of Ag versus temperature and time. In Figure 4, the concentration of Ag increased when the time was around 105 min and the temperature was 210 \({}^{\circ}\)C. These conditions yielded the highest results when the concentration of citric acid was 2 M (413 ppb). The interaction diagram of the concentration-time and concentration-temperature parameters depicted in Figure S1 (Supplementary Materials) clearly confirms these conditions. Indeed, metal-organic acid complexes become more soluble at higher temperatures, which improves the solubility of metals [16]. The effects of temperature-time and concentration-time on Ag leaching are also depicted in the surface plots illustrated in Figures S2 and S3 (Supplementary Materials), confirming these observations. The effect of the main parameters on the Ag leaching yield is displayed in Figure 5. As can be seen, the optimal time was about 100 min and temperature was 210 \({}^{\circ}\)C, while the effect of concentration was less pronounced. The interaction plot for the parameters is displayed in Figure S3 (Supplementary Materials), revealing that no significant interactions occurred between concentration-temperature and time-temperature, while slight interactions existed only for concentration-time. When the treatment time was 60 min, the yield slightly increased with the increase in the acid concentration; however, when the time was 150 min, the yield decreased with the increase in the concentration, as prolonged times may lead to increased pressure affecting the dissociation constants. Considering the Pareto chart in Figure 6, it is shown that all parameters were not statistically significant at the \(\alpha\) = 0.1 level. However, at the \(\alpha\) = 0.2 level, the temperature became statistically significant, while the other factors did not. According to the RSM design optimization, the optimum conditions for citric acid leaching were 210 \({}^{\circ}\)C, 2 M, and a time of 95 min, which were verified experimentally. The analysis of variance (Table S1, Supplementary Materials) suggests that only the temperature parameter is significant for CA leaching (significance level \(\alpha\) = 0.2). The test does not detect lack-of-fit because the P-value for lack-of-fit is apparently greater than \(\alpha\), suggesting that lack-of-fit