# Organic Acid-Assisted Hydrothermal Leading of Silver from End-of-Life Photovoltaic Panels
## 1 Introduction  
The rapid global expansion of PV installations is raising concerns about the upcoming generation of PV waste. According to recent estimates, the amount of PV waste that will be generated by 2050 in the EU-27 alone is expected to reach 21-35 million tons (MT), 47-90% more than previously predicted, due to the recently increased EU targets for PV deployment [1, 2]. In recent decades, crystalline silicon (c-Si) modules have maintained 80-90% of the market share and, as such, dominate the PV waste stream. Polycrystalline silicon (p-Si) and monocrystalline silicon (m-Si) panels account for 51% and 41% of the market share of c-Si photovoltaic panels, respectively [3, 4]. By 2030, 86% of waste panels discarded in the EU-27 will be of this technology [1, 2]. Therefore, c-Si panels are the most valuable PV modules due to their large volume compared to other technologies. The main components of a c-Si module are glass, polymers, silicon cells, and the aluminum frame [5, 6]. Some elements such as silver, lead, tin, and other metals are also present in smaller quantities and must not be overlooked. Although Ag is only found in trace amounts in PV panels, its high economic value means that it can contribute significantly to the revenue from the recovery of panel materials [3]. Chemical leaching is the most effective and profitable method of metal recovery. Nitric acid (HNO\({}_{3}\)) is often used in silver leaching because of its strong reactivity and its affordable price [7]. Organic acids are considered mild acids that cause little or no pollution to the environment, unlike inorganic acids. Due to environmental regulations for waste disposal, it is appealing to create effective green leaching methods for metal extraction using organic acids [8; 9]. The leaching of Ag with organic acids is scarcely reported in the literature compared to inorganic acid leaching. [PERSON] et al. employed methanesulfonic acid (MSA) and hydrogen peroxide (H\({}_{2}\)O\({}_{2}\)) for 1-12 h at room temperature to teach silver from discarded Si panels [10]. An iodide solution at room temperature was used to treat waste Si panels [11]. [PERSON] et al. used citric acid with H\({}_{2}\)O\({}_{2}\) at 60 \({}^{\circ}\)C to teach 97% of silver [12]. Apart from these studies on silicon panels, [PERSON] used citric, maleic, and oxalic acid to leach silver at room temperature from CIGS solar cells; however, the results were not encouraging [13]. Conventional leaching methods use low temperatures (less than 90 \({}^{\circ}\)C) to extract metals from a variety of matrices. To achieve complete leaching, the low leaching temperature requires a long reaction time and a high acid concentration; this, however, raises costs and environmental impact [5; 14]. The hydrothermal leaching approach is a green method that employs lower acid concentrations, shorter reaction times and does not require extra oxidants or reductants, unlike conventional hydrometallurgy [15]. Hot pressurized water is used as the green reaction medium. In this technology, the use of high temperatures (\(\geq\)90 \({}^{\circ}\)C) accelerates the process, but increases energy consumption [16]. The method has been used to leach metals from lithium-ion batteries with organic acids [15], but to date, no hydrothermal leaching utilizing organic acids for PV cells has been reported. Therefore, this is the first attempt to leach silver from PV cells under these conditions. Our previous work reported hydrothermal silver leaching using mild inorganic acid; however, this is the first attempt to apply organic acid-assisted hydrothermal leaching for PV cells [2]. The temperature, time, and concentration of organic acids are optimized to improve the efficiency of the hydrothermal leaching of Ag. The response surface methodology (RSM) with a Box-Behnken design (BBD) is a statistical technique used to design the experiments [14]. Instead of varying one parameter at a time, this strategy modifies all the tested parameters simultaneously to examine how they affect silver leaching.