Interfacial adsorption of gold nanoparticles on arsenian pyrite: New insights for the transport and deposition of gold nanoparticles

Gold nanoparticles (AuNPs) are found extensively in the near-surface Earth system. The transport, aggregation, and deposition of AuNPs have been potentially recognized as an important process and even a fundamental intermediary step both in formation of high-grade gold deposits and in the dispersion of gold into the Earth's surface environment. The interfacial adsorption between AuNPs and sulfide minerals is a long-recognized process regulating transport and deposition of gold in hydrothermal fluids, and the occurance of Au and As in pyrite are often observed in various hydrothermal Au deposits. However, the adsorption behavior of AuNPs on sulfide minerals and the fundamental chemical mechanism for the AuAs relationship remain enigmatic. In this work, we selected citrate as a surrogate for typical natural organic compounds abundantly found in lower-temperature hydrothermal systems and Earth's surface environment, and experimentally investigated the adsorption of negatively charged AuNPs on pyrite and arsenian pyrite under anaerobic conditions. We demonstrate that pyrite and nanoparticle surface properties, and solution chemistry, control the adsorption process. Negligible adsorption of AuNPs on pyrite (absent of As) was verified under all experimental conditions likely due to high electrostatic repulsion. However, we observed AuNPs adsorption on arsenian pyrite and suggest that this interaction is due to negative charge-assisted hydrogen bonds ((−)CAHBs) between hydroxyl/oxyl groups on the surface of arsenian pyrite and carboxyl groups on the citrate capping agent of AuNPs. Also, the lower negative charge density of arsenian pyrite surfaces significantly reduces electrostatic repulsion between both negatively charged AuNPs and arsenian pyrite. The solution pH, coexisting organics (citrate) concentration, and ionic strength are demonstrated to play substantial roles in the adsorption of AuNPs on arsenian pyrite. However, the solution pH appears to be an overriding factor controlling the adsorption of AuNPs which decreases with the increase of pH. With respect to citrate concentration, the adsorption rates of AuNPs onto arsenian pyrite initially decreases, but only slightly, and then increases. Increased ionic strength also promotes the adsorption of AuNPs. Our findings highlight a potentially important role of AuNPs in transport and deposition processes of gold in aqueous mediums, and provide new insights on the geochemical behavior of metal nanoparticles in the hydrothermal systems and Earth's surface environment.