# Organic Acid-Assisted Hydrothermal Leading of Silver from End-of-Life Photovoltaic Panels
## 3 Results and Discussion
### Hydrothermal Leading with Oxalic Acid  
In the Ag leaching experiments, a concentration of 1 M oxalic acid was optimal, while increasing the concentration above 1 M was inhibitory. The decrease in leaching yield with the increase in the concentration was attributed to the relatively low solubility of oxalic acid in water. Indeed, oxalic acid concentrations higher than 1 M produced saturated solutions, which were not conducive to acidosis. The acid concentration is already at its maximum in a saturated solution and additional acid additions do not actively dissolve in the solution. This can lead to a reduction in the effective concentration of acid available for the acidosis reaction [32]. The Pareto chart shows that the oxalic acid concentration was a significant parameter at the 95% confidence level, as shown in Figure 7. Oxalic acid has two carboxyl groups and dissociates in two steps, with the first step having pKa\({}_{1}\) = 1.2, indicating a relatively stronger acid than citric acid (pKa\({}_{1}\) = 2.79) in this step. However, at a higher temperature and pressure, the pKa values increase for both acids, but the relative difference may remain. The highest Ag concentrations were achieved at experimental conditions of 180 \({}^{\circ}\)C and 1 M for 60-150 min (1201-1376 ppb). The main effects plot also supports that the low concentration favored Ag leaching (Figure 8), while the other parameters were not so important. The surface plot of Ag versus the temperature and concentration indicated that the highest yields were obtained when the concentration was low and the temperature was high (Figure 9). The contour plot in Figure 10 of Ag versus the concentration and time shows that short times were enough for Ag leaching. The optimized conditions by the Minitab software for oxalic acid leaching were concentration 1 M, time 60 min, and temperature 210 \({}^{\circ}\)C, and were experimentally validated. The analysis of variance (Table S2, Supplementary Materials) indicates that only the concentration is a significant parameter (significance level \(\alpha\) = 0.05), while the other parameters and their interactions are not. Also, the lack-of-fit is not statistically significant as the \(p\)-value \(>\alpha\), meaning that the test detects no lack-of-fit. The plot of the residuals versus order (Figure S6, Supplementary Materials) verifies the assumption that the residuals are independent from one another, as they show no trends or patterns. As shown in Figure S6 (Supplementary Materials), the residuals on the plot fall randomly around the center line and, therefore, are not correlated. Additionally, the model summary estimates R-sq 85%, which is sufficient when considering the reduced performance of organic acids to leach Ag. To further examine the effect of concentration, experiments were performed at lower oxalic acid concentrations (0.6, 0.8, 1 M), as shown in Figure 11. For these experiments, the other conditions were kept at 180 \({}^{\circ}\)C and 150 min. As shown in the Pareto chart (Figure 7), the effect of the other parameters was insignificant;  
Figure 6: Pareto chart of the standardized effects for citric acid leaching, where response is Ag, A: concentration, B: time, and C: temperature. (**i**) a = 0.1; (**ii**) a = 0.2.  
so, they were kept constant, and only the concentration was varied to test whether lower concentrations of OA had a desirable effect on Ag leaching. It is obvious that 1 M oxalic acid yielded the best results (Figure 11). The Ag distribution in the cell grid is not uniform as there are denser and less dense lines. This uneven Ag coating is most likely the result of heat treatment during cell processing or cutting during the preparation of samples in the lab. This discrepancy in Ag content in the cell material could be the main cause of the error in the testing process, explaining the relatively large error bars in Figure 11[33]. Figure 8: Main effects plot for Ag leaching by oxalic acid (fitted means).