Proton-coupled electron transfer (PCET) in thionocarbamate adsorption

Abstract

Meeting the world's energy and climate goals significantly increases demand for specific elements, including copper (Cu) and silver (Ag). Thionocarbamates, exemplified by O-isopropyl N-ethyl thionocarbamate (IPETC), are known for their high selectivity for Cu sulfides in froth flotation. However, the origins of the ligand's selectivity at the molecular level, its interaction with the important Cu sulfide minerals, and the role of pH in such interactions are still not well understood. Although Cu and Ag belong to the same group (IB) in the periodic table, in plant practice, IPETC is typically not known or used for Ag recovery from polymetallic ores. The reasons for such empirical differences are not known. To develop a better understanding of molecular level processes, we studied the interaction of IPETC with Cu and Ag metals as well as with covellite (CuS), chalcocite (Cu₂S), and acanthite (Ag₂S) at pH 5 and pH 9 using single-mineral micro-flotation, adsorption, X-ray photoelectron spectroscopy (XPS), electrochemistry, and density functional theory (DFT). Depending on the substrate, its oxidation state, and pH, IPETC can be adsorbed in protonated and/or deprotonated forms. The protonation state of IPETC impacts its adsorption density at the solid-liquid interface and mineral floatability. A mixture of protonated and deprotonated IPETC packs more tightly due to reduced electrostatic repulsion, suggesting a beneficial role of mixed adsorption for floatability. Deprotonation is explained by an electrochemical mechanism that includes proton-coupled electron transfer (PCET). The poorer flotation performance of IPETC on Ag₂S compared to Cu sulfides is explained by the weaker coordination of Ag(I) with IPETC compared to Cu(I), combined with the greater chemical nobility (oxidation resistance) of Ag and Ag sulfides compared to Cu and Cu sulfides. These results shed light on the adsorption mechanism of IPETC and provide a foundation for applications of thionocarbamates in flotation, the development of more effective functional ligands, and the advancement of more sustainable separation techniques for metal sulfides in wastewater treatment and environmental remediation.