###### Abstract
This approach (RDM-BBD) is currently missing from the literature to optimize Ag leaching from waste PV panels. The traditional acid-leaching process involves leaching metals from various matrices in non-pressurized hot water using mainly inorganic acids. In this process, the reaction temperature should be low in order to avoid water evaporation (<80-90 \({}^{\circ}\)C). The low leaching temperature requires a long reaction time, as well as a high acid concentration, to ensure complete leaching; however, this increases costs and environmental load [17; 18; 19]. Nonetheless, conventional acid leaching has been extensively applied to leach metals from waste PV panels [8; 13]. Some studies also considered the leaching of Al along with Ag from PV cells [20; 21]. Due to the high value of Ag, the main focus should be on this metal. The literature on the recycling of PV panels focuses on Ag recovery from cells, ignoring the ribbons [5; 22]. However, significant amounts of Ag are adhered to the metal ribbons, and should be recovered and not lost. Currently, there is little research on Ag recovery from metal ribbons [21]; thus, this research gap should be filled. In previous studies, the most used method for Ag recovery from solar cells was etching or leaching in an acidic solution based on nitric acid (HNO\({}_{3}\)) at concentrations of 2.3-14 N and temperatures of 25-80 \({}^{\circ}\)C. These experiments were performed in various vessels, with or without agitation [9; 22]. Other studies used electrospinning Ag with methanesulfonic acid solution as the electrolyte [23]. In contrast with traditional leaching methods, the hydrothermal leaching process is not limited by the boiling point of water at 100 \({}^{\circ}\)C. Using pressurized hot water as the solvent and with elevated temperatures, the hydrothermal method can efficiently leach metal ions from various matrixes. The high temperature accelerates the process, shortens the reaction time, and decreases the required acid concentration [24; 25]. In this way, hydrothermal leaching is more efficient and environmentally friendly than traditional leaching methods [25; 26]. Although hydrothermal leaching is a potential leaching method, so far, the process has not been utilized for PV waste, leaving a significant research gap to be filled. Hydrothermal metal leaching has been efficiently utilized for other types of waste, such as lithium-ion battery cathode materials [25], electroplating sludge [26]and copper anode slime [27]. The efficient recovery of precious metals from waste PV panels under mild conditions is necessary to reduce the environmental impact and achieve financial benefits. The present study fills this research gap using weak leaching agents to avoid the generation of hazardous waste effluents. Several hydrothermal experiments were performed, investigating the influence of four parameters on leaching efficiency: liquid to solid ratio (L/S), HNO\({}_{3}\) concentration (N), time (t) and temperature (T). To efficiently plan the experiments, the response surface methodology (RSM) with a Box-Behnken design was used. The objectives of this study were: (i) to leach Ag and Al from both PV cells and ribbons, (ii) to investigate the influence of four parameters (L/S, HNO\({}_{3}\), t and T) on the hydrothermal leaching of Ag and Al, (iii) to compare hydrothermal with conventional leaching and (iv) to investigate the influence of milling and agitation on leaching efficiency.