Gold sorption onto pyrite and goethite: A radiotracer study

The sorption of gold onto pyrite and goethite in deionized water and seawater containing 510 pg L^-1 Au has been determined using radiotracer techniques. Most of the gold is removed from solution in the first two days. Gold colloids, present in solution along with dissolved gold [predominantly Au(OH)H₂O⁰], are rapidly removed upon addition of the substrates. The removal of gold colloids is due either to direct uptake of the Au colloids or to dissolution and subsequent adsorption. Dissolution of gold colloids is to be expected if equilibrium is maintained between colloids and aqueous gold species. Pyrite is a much more efficient scavenger of gold than is goethite. The surface-normalized partition coefficients, K_sd's, for pyrite range from 2.9-5.6 · 10² [Lm^-2] in deionized water to 0.7-1.1 · 10² [Lm^-2] in seawater. The K_sd for goethite in deionized water is 5.1-0.53 [L m^-2]; its K_sd in seawater is >3.7 · 10^-2 [L m^-2], but probably lower than in deionized water. The difference in sorption efficiency between the pyrite surface and goethite surface reflects either: 1. (1) the lack of oxidizable groups on the goethite surface which could reduce dissolved Au(I) species to elemental gold; or 2. (2) the stability of the Au-S bond formed on the sulfide surface compared to the Au-O bond formed on the oxide surface. Based on the results of this study, it is predicted that fine-grained pyrite particles may reach ore-grade Au concentrations (i.e., 1-10 μg g^-1 Au) if they are continuously exposed to a gold-bearing solution containing at least 2 ng L^-1 gold. Growth of pyrite during gold sorption may explain the common occurrence of invisible gold in pyrite. Goethite, on the other hand, cannot reach ore-grade Au concentrations unless the gold concentration in the gold-bearing solution is higher than 2 μg L^-1, which is higher than gold concentrations in natural solutions.