GOLD SPECIATION IN NATURAL WATERS: I. SOLUBILITY AND HYDROLYSIS REACTIONS OF GOLD IN AQUEOUS SOLUTION

Abstract

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 H 2 O AuO 2 H 4- n 1- n + nH + + e - in order to identify the stoichiometry and stability of the hydrolyzed species formed. The monohydroxide, AuOH(H 2 O) 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 logK 1 = -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(H 2 O) 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, S 2 O 3 2- 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 S activities typical of both fresh and sea water show that AuOH(H 2 O) 0 is probably the dominant dissolved Au species in these environments, with the exception of anoxic ocean and lake waters.