Deep eutectic solvents in E-waste recycling: preparation, properties, and hydrometallurgical metal recovery

Deep eutectic solvents (DESs) are promising green solvents for hydrometallurgical metal recovery, formed by heating and mixing hydrogen bond donors and acceptors in specific molar ratios, where hydrogen-bond network formation rather than chemical reactions depresses the melting point and yields a stable, tuneable liquid phase. Owing to their low volatility, high polarity, low toxicity, and ability to complex metal ions, DESs provide an attractive alternative to conventional mineral acids for leaching metals from printed circuit boards (PCBs). This review critically examines preparation routes, key physicochemical properties relevant to metal dissolution, and their mechanistic role in enhancing leaching efficiency of DESs. A structured literature-selection approach was adopted by screening publications between 2003-2024 using keywords related to preparation of DESs, metal recovery, and PCBs, focusing on studies reporting extraction efficiencies (85 to 100%), leaching mechanisms, recyclability, and solvent degradation. Across the surveyed literature, DESs such as choline chloride:ethylene glycol, choline chloride:urea, and choline chloride:oxalic acid consistently exhibit high efficiencies for copper (100%), nickel (100%), gold (≥95%), and silver (100%) recovery under mild operating conditions (40-100 °C). Emerging trends include oxidation-assisted DES leaching and integration with electrochemical recovery. However, challenges such as high viscosity, mass transfer limitations, incomplete recyclability, and potential long-term degradation remain under-reported. Overall, this review signifies the current advancements, limitations, and knowledge gaps in DES-based hydrometallurgy and highlights future directions required for scalable, circular-economy-aligned recovery of valuable metals from E-waste.