Mining electronics waste: Experimental and computational mechanistic insights into organic aqua regia as an extraction method

Abstract

In the previous decade, organic aqua regia (OAR) was discovered as a mixture of pyridine and thionyl chloride that has been observed to surprisingly yet notably selectively dissolve precious metals, dissolution‐resistant gold. Given the novel combination is two common organic solvents, it has immense potential to contribute to a circular economy of metal recycling, economically and environmentally. The underlying mechanisms behind its metal dissolution and interesting properties are not well‐understood. Using both the original mixture and derivatives, the role of each OAR reagent was analyzed in the dissolution of gold to begin probing and illuminating the novel mechanism. Three mechanisms were proposed: radical based, oxidant and ligand, and Lewis acid and base. Computational means were used to propose possible mechanisms. Experiments were used to explicate the possible mechanisms. Discoveries from the original publication were used to verify assessments and suggest future areas of study for further confirmation. Based on the finding that 2,2,6,6‐tetramethyl‐1‐piperidinyloxy (TEMPO) does not significantly affect the amount of gold dissolved, the radical mechanism was dismissed. Findings from experiments replacing pyridine with its derivatives disproved the likelihood of the oxidant and ligand mechanism. Finally, with the support of proton nuclear magnetic resonance (1H NMR) and other characterization techniques, the Lewis acid and base mechanism was determined to be the most probable mechanism of OAR. The mechanistic findings reported herein will pave the way for continued understanding of the mechanism to be exploited and optimized for other metals and organometallic chemistry in the recycling of metals.