Supramolecular Gold Stripping from Activated Carbon Using α-Cyclodextrin

Abstract

We report the molecular recognition of the Au(CN)2- anion, a crucial intermediate in today's gold mining industry, by α-cyclodextrin. Three X-ray single-crystal superstructures - KAu(CN)2α-cyclodextrin, KAu(CN)2(α-cyclodextrin)2, and KAg(CN)2(α-cyclodextrin)2 - demonstrate that the binding cavity of α-cyclodextrin is a good fit for metal-coordination complexes, such as Au(CN)2- and Ag(CN)2- with linear geometries, while the K+ ions fulfill the role of linking α-cyclodextrin tori together as a result of [K+···O] ion-dipole interactions. A 1:1 binding stoichiometry between Au(CN)2- and α-cyclodextrin in aqueous solution, revealed by 1H NMR titrations, has produced binding constants in the order of 104 M-1. Isothermal calorimetry titrations indicate that this molecular recognition is driven by a favorable enthalpy change overcoming a small entropic penalty. The adduct formation of KAu(CN)2α-cyclodextrin in aqueous solution is sustained by multiple [C-H···π] and [C-H···anion] interactions in addition to hydrophobic effects. The molecular recognition has also been investigated by DFT calculations, which suggest that the 2:1 binding stoichiometry between α-cyclodextrin and Au(CN)2- is favored in the presence of ethanol. We have demonstrated that this molecular recognition process between α-cyclodextrin and KAu(CN)2 can be applied to the stripping of gold from the surface of activated carbon at room temperature. Moreover, this stripping process is selective for Au(CN)2- in the presence of Ag(CN)2-, which has a lower binding affinity toward α-cyclodextrin. This molecular recognition process could, in principle, be integrated into commercial gold-mining protocols and lead to significantly reduced costs, energy consumption, and environmental impact.