GOLD SPECIATION IN NATURAL-WATERS .1. SOLUBILITY AND HYDROLYSIS REACTIONS OF GOLD IN AQUEOUS-SOLUTION

Solubility measurements of Au in dilute to concentrated aqueous NaOH solutions at 25°C have been carried out. The data were fitted to the general half reaction Au(c) + 2 H2O → AuO2H4-n1-n + nH+ + e- in order to identify the stoichiometry and stability of the hydrolyzed species formed. The monohydroxide, AuOH(H2O)0, is found to be the most stable species up to pH ~ 12. The equilibrium constant for the formation of this species (i.e., n = 1) is logK1 = -22.57 ± 0.44. Consideration of competitive complexation of Au by a number of inorganic ligands which are encountered in natural waters indicates that AuOH(H2O)0 is the most stable inorganic Au species over a wide range of Eh, pH, and ligand activities. The only inorganic ligands which may occur in natural waters at concentrations high enough to stabilize Au(I) include HS- under reducing conditions, S2O32- under alkaline oxidizing conditions, Cl- in very acidic, oxidizing brines, and possibly CN- locally, in environments where there is biogenic and/or anthropogenic production of cyanide. Calculated equilibrium pe-pH diagrams for Cl- and ΣS activities typical of both fresh and sea water show that AuOH(H2O)0 is probably the dominant dissolved Au species in these environments, with the exception of anoxic ocean and lake waters. © 1990.