Sustainable and effective gold(I) separation from aurocyanides wastewater and its mechanism using guanidinium ionic liquids

Abstract

Efficient recovery of gold(I) from aurocyanide wastewater offers a dual opportunity for secondary resource utilization and environmental harm reduction. Herein, four environmentally friendly hexaalkylguanidium ionic liquids (HGILs) were synthesized and for the first time used to extract gold(I) from aurocyanide wastewater. Critical parameters, including HGILs and modifier species, dosage, pH, initial gold(I) concentration, and temperature, were optimized. The HGIL systems exhibited excellent gold(I) extraction performance with over 99.0% of gold(I) being extracted under optimal conditions. The composition ratio of the extracted complex was determined by Job's method and slope analysis. Spectral characterization included X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy and nuclear magnetic resonance spectroscopy and density functional theory calculations confirmed that gold(I) extraction followed an anion exchange mechanism. A dicyanoaurate Au(CN)2− anion and two n-octanol molecules produced the supermolecule anion [Au(CN)2−·2 n-octanol] via hydrogen bonding, which combined with a guanidium cation to generate the stable extraction complex [HGIL+·Au(CN)2−·2 n-octanol] through electrostatic attraction. The gold(I) loaded in the ionic liquid phase was effectively stripped with potassium thiocyanate solution. The regenerated HGIL system maintained a high gold(I) extraction efficiency of over 90.0% after five extraction–stripping cycles. In addition, a two-step stripping strategy was developed for the selective separation of gold(I) and other impurity metal ions from real aurocyanide wastewater. This study proposes an efficient green process for recovering gold(I) from mine wastewater, which has potential practical application prospects.