Evaluation of Simple Amides in the Selective Recovery of Gold from Secondary Sources by Solvent Extraction

The recycling of metals from end-of-life secondary sources such as electronic waste remains a significant environmental and technological challenge currently detrimental to the development of circular economies. The complex nature of electronic waste, containing a myriad of different elemental metals, means that sophisticated yet simple separation methods need to be developed to recycle these valuable and often critical metal resources. In this work, simple primary, secondary, and tertiary amides are appraised as reagents that selectively transport gold from aqueous to organic phases in solvent extraction experiments. While the strength of extraction of gold from single-metal solutions is ordered 3° > 2° > 1°, the 3° and 2° amides are ineffective at gold transport from mixed-metal solutions of concentrations representative of smartphones due to the formation of a third phase. Increasing the polarity of the organic phase can negate third-phase formation but at the expense of selectivity. The identities of the species that reside in the organic and third phases have been studied by a combination of slope analysis, mass spectrometry, NMR spectroscopy, and computational methods. These techniques show that protonation of the amide L occurs at the oxygen atom, resulting in the protonated dimer HL₂ ⁺, which acts as a receptor for AuCl₄ ^- to form dynamic supramolecular aggregates in the organic phase. The characterization of a tin complex in the third phase by X-ray crystallography supports these conclusions and, furthermore, suggests the preference for the chelation of the proton by two amide molecules instead of the transport of hydronium into the organic phase and its subsequent use as a structural template.