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 α-cyclodextrin. Three X-ray single-crystal
superstructuresKAu­(CN)2⊂​α-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)2⊂​α-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.