Efficient removal of gold complexes from water by precipitation or liquid–liquid extraction using ionic liquids

Extraction of tetrachloroaurate or tetrabromoaurate anions has been carried out at acidic pHs (1 and 4) with several ionic liquids bearing halide or bis(trifluoromethanesulfonyl)imide NTf2− anions and cations 1-octyl-3-methylimidazolium, 1-octylpyridinium or 1-methyl-1-octylpyrrolidinium. The removal of gold anionic moieties from water was first studied by mixing aqueous solutions containing gold complexes and water soluble halide (chloride or bromide) ionic liquids. This lead to the formation of either a solid precipitate or a liquid phase corresponding to a hydrophobic ionic liquid based on a tetrahalogenoaurate anion. Values below 10−6 for the solubility products of gold-bearing ionic liquids were obtained. Liquid–liquid extraction of anionic gold complexes with hydrophobic ionic liquids was also carried out. Gold was successfully extracted from water whatever the pH and the ionic liquid used. Distribution coefficients ranged from 3 × 102 to 3 × 104. Investigation on the extraction mechanism revealed an anion exchange between one anionic gold complex and one NTf2− anion.