A spectrophotometric study of aqueous Au(III) halide–hydroxide complexes at 25–80 °C

The mobility and transport of gold in low-temperature waters and brines is affected by the aqueous speciation of gold, which is sensitive in particular to pH, oxidation and halide concentrations. In this study, we use UV–Vis spectrophotometry to identify and measure the thermodynamic properties of Au(III) aqueous complexes with chloride, bromide and hydroxide. Au(III) forms stable square planar complexes with hydroxide and halide ligands. Based on systematic changes in the absorption spectra of solutions in three binary systems NaCl–NaBr, NaCl–NaOH and NaBr–NaOH at 25 °C, we derived log dissociation constants for the following mixed and end-member halide and hydroxide complexes: [AuCl3Br]−, [AuCl2Br2]−, [AuBr3Cl]− and [AuBr4]−; [AuCl3(OH)]−, [AuCl2(OH)2]−, [AuCl(OH)3]− and [Au(OH)4]−; and [AuBr3(OH)]−, [AuBr2(OH)2]− and [AuBr(OH)3]−. These are the first reported results for the mixed chloride–bromide complexes. Increasing temperature to 80 °C resulted in an increase in the stability of the mixed chloride–bromide complexes, relative to the end-member chloride and bromide complexes. For the [AuCl(4−n)(OH)n]− series of complexes (n = 0–4), there is an excellent agreement between our spectrophotometric results and previous electrochemical results of Chateau et al. [Chateau et al. (1966)]. In other experiments, the iodide ion (I−) was found to be unstable in the presence of Au(III), oxidizing rapidly to I2(g) and causing Au to precipitate. Predicted Au(III) speciation indicates that Au(III) chloride–bromide complexes can be important in transporting gold in brines with high bromide–chloride ratios (e.g., >0.05), under oxidizing (atmospheric), acidic (pH < 5) conditions. Native gold solubility under atmospheric oxygen conditions is predicted to increase with decreasing pH in acidic conditions, increasing pH in alkaline conditions, increasing chloride, especially at acid pH, and increasing bromide for bromide/chloride ratios greater than 0.05. The results of our study increase the understanding of gold aqueous geochemistry, with the potential to lead to new methods for mineral exploration, hydrometallurgy and medicine.