###### Abstract  
The aim of this study was to investigate the hydrothermal leaching of silver and aluminum from waste monocrystalline silicon (m-Si) and polycrystalline silicon (p-Si) photovoltaic panels (PV) from both cells and metal ribbons using mild HNO\({}_{3}\) solutions. Prior to leaching, pretreatment was applied to remove the fluoropolymer backsheet and thermally degrade the ethyl vinyl acetate (EVA) polymer. Several hydrothermal parameters were investigated, such as the liquid-to-solid (L/S) ratio, HNO\({}_{3}\) concentration (N), time (t) and temperature (T). Based on preliminary tests, the HNO\({}_{3}\) concentration was set in the range of 1-2 N to reduce hazardous waste effluents. The response surface methodology (RSM) was applied to optimize the hydrothermal leaching parameters. It was found that processing time was the most important factor for Ag leaching, followed by HNO\({}_{3}\) concentration and L/S ratio, while the processing temperature (100-140 \({}^{\circ}\)C) was not a statistically significant factor. Aluminum leaching was efficient under most hydrothermal conditions. For comparison, leaching was also applied at lower temperatures of 25-45 \({}^{\circ}\)C for prolonged times; however, lower efficiencies were observed. Under the optimal hydrothermal conditions, Ag can be completely leached, while Al dissolution was favored at hydrothermal conditions compared with lower temperature leaching. Silver leaching efficiency was 100% under hydrothermal conditions; however, under conventional lower temperature conditions, it was 80.7-85.3% for m-Si and p-Si waste panels. Under conventional lower temperature conditions, Al leaching efficiency was 56.6-61.3% for p-Si and m-Si waste panels. crystalline silicon PV panels; silver; recycling; e-waste; response surface methodology +
Footnote †: journal: Applied  
next 50 years, while other metals will last for another 50-500 years [2; 6]. Although Ag is present in tiny amounts in PV panels, it contributes significantly to the revenue from recycled panel materials due to its high economic value [2]. According to [PERSON] et al. [7], Ag represented 35.2% of the economic value derived from material recovery from a Si module in 2019. In the EU-27, the economic value of Ag will represent 17.6% of the total revenue from recycled materials by 2040, although its mass share in the accumulated PV waste (considering all PV technology waste) will be only 0.05% [3]. The average quantity of Ag in a PV panel is estimated at approximately 10 g per square meter. According to US resources, the Ag content should surpass 700 g/t when mined to be profitable as a primary product. As a result, Ag recovery from PV panel waste is becoming increasingly important [8; 9; 10]. Initially, research on PV panel recycling focused on silicon, glass and aluminum recovery [11; 12]; however, more recent studies also aim at silver leaching and recovery [5; 13]. In order to maximize Ag leaching efficiency, the operating parameters of temperature, time, L/S ratio and HNO\({}_{3}\) concentration should be optimized. This can be accomplished with appropriate statistical techniques, such as the response surface methodology (RSM) with a Box-Behnken design (BBD) [14]. This design investigates the effects of the parameters on silver leaching by simultaneously differentiating all parameters under consideration, instead of differentiating one parameter at a time. Thus, possible interactions between factors can be identified. The Box-Behnken design effectively avoids the realization of the complete experimental design, which would otherwise make the process more demanding in terms of time and resources, without losing any information on the effects of the parameters on the process. In this design, three levels of each factor are defined: low, middle and high [14; 15]. Among the response surface designs, BBD is slightly more efficient compared to the central composite design, and is much more efficient than the three-level full factorial design [16].