The use of hydrophobic deep eutectic solvents (HDES) for the extraction of valuable metals from aqueous electronic waste streams

Abstract

The exponential growth of electronic waste (e-waste) has become a critical environmental concern, requiring innovative approaches for resource recovery. Among the valuable metals contained within the e-waste, copper holds particular significance due to its abundance and widespread application in electronic devices. Traditional extraction methods often involve environmentally detrimental processes and become ineffective in complex metal mixtures, leading to the necessity of selective, greener methods. Deep eutectic solvents (DES) can be defined as a class of solvents formed by the combination of two or more components, which have been reported as alternative solvents for various applications, with hydrophobic deep eutectic solvent (HDES), those that are immiscible with water, being reported as an innovative alternative solvent for extractions from water. The primary aim of the project is to evaluate the effect of using HDESs for the extraction of copper from complex aqueous electronic waste streams and use this understanding to improve the extraction system. 24 HDESs were assessed for their ability to extract copper from both a pure copper sulfate solution and a mixed metal e-waste lixiviant. While high copper removal from the mixed metal solution could be obtained with lidocaine based HDES, these formed a precipitate which contained a complex mixture of metals, likely as a result of the formation of metal complexes with lidocaine and carboxylate ligands. The HDES methyltrioctylammonium chloride : decanoic acid (N8881Cl : C10A) was found to be the most effective at extracting copper from the aqueous electronic waste stream without forming a precipitate. The molar ratio of the N8881Cl : C10A HDES system had a surprisingly large impact on extraction efficiency, with the optimal hydrogen bond acceptor : hydrogen bond donor (HBA:HBD) molar ratio of 2:1. A model was proposed for the molar ratio dependence of the extraction. Further, attempts to optimize the extraction conditions using DOE were unsuccessful with the optimized conditions proposed by the model not leading to the highest extraction yield. An attempt at the electrodeposition of copper and the recycling of the HDES was made. Finally, further investigation is required to elaborate the mechanisms dominating the copper extraction and optimize extraction conditions for enhanced metal extraction efficiency. By approaching a sustainable solution to copper extraction and recovery from e-waste, this study contributes to advancing the field of resource recovery and environmental sustainability not only limited in metal but other material recycling.