Effect of Particle Size and Pyrite Oxidation on the Sorption of Gold Nanoparticles on the Surface of Pyrite

The sorption of gold nanopartilces (AuNPs) with diameters of 16 or 39 nm on the surface of pyrite (80–100 or 140–160 mesh) was experimentally studied by systematically evaluating the effects of atmosphere, pH, reaction time and particle sizes on the sorption behavior. We have found that oxidation of pyrite plays a critical role in the sorption process, which increases the pH_iep of pyrite to 4–5 and decreases the pH of the suspension. The presence of citrate and AuNPs does not seem to significantly change the pyrite oxidation pathway. A smaller particle size of pyrite and aerobic atmosphere accelerates the overall oxidation rate. Under suitable conditions, the negatively charged AuNPs can be adsorbed onto positively charged surface of oxidized pyrite through electrostatic attraction. A complete sorption of AuNPs was observed at pH < 3 after two days of pyrite oxidation. The initially alkaline conditions (pH 10) appear to promote the pyrite oxidation, and the resulting oxidized colloidal particles (iron oxide or hydroxide) could adsorb or destabilize AuNPs due to heteroaggregation. Our experimental findings suggest that surface charge property and reaction conditions are crucial in determining the migration and distribution of nanoparticles in the natural environment, which may improve our understanding of mineralization mechanisms of “invisible gold” and the potential fate of AuNPs in aquatic environments.