Supramolecular Gold Stripping from Activated Carbon Using α-Cyclodextrin

We report the molecular recognition of the Au(CN)(2)(-) anion, a crucial intermediate in today's gold mining industry, by alpha-cyclodextrin. Three X-ray single-crystal superstructures-KAu(CN)(2)subset of alpha-cyclodextrin, KAu(CN)(2)subset of(alpha-cyclodextrin)(2), and KAg(CN)(2)subset of(alpha-cyclodextrin)(2)-demonstrate that the binding cavity of alpha-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 alpha-cyclodextrin tori together as a result of [K+center dot center dot center dot O] ion-dipole interactions. A 1:1 binding stoichiometry between Au(CN)(2)(-) and alpha-cyclodextrin in aqueous solution, revealed by H-1 NMR titrations, has produced binding constants in the order of 10(4) 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)(2)subset of alpha-cyclodextrin in aqueous solution is sustained by multiple [C-H center dot center dot center dot pi] and [C-H center dot center dot center dot 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 alpha-cyclodextrin and Au(CN)(2-) is favored in the presence of ethanol. We have demonstrated that this molecular recognition process between alpha-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 alpha-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.

Automated summary

The molecular recognition of Au(CN)(2)(-) anions by alpha-cyclodextrin has been successfully demonstrated, showing a strong binding affinity with a 1:1 stoichiometry and favorable enthalpy changes. This process can be applied to the efficient stripping of gold from surfaces and selective separation of different metal ions, potentially leading to reduced costs, energy consumption, and environmental impact in gold mining protocols.