Reaction of Cyanide with Hg⁰-Contaminated Gold Mining Tailings Produces Soluble Mercuric Cyanide Complexes

Elemental mercury (Hg⁰) contamination in artisanal and small-scale gold mining (ASGM) communities is widespread, and Hg⁰-contaminated tailings are often reprocessed with cyanide (^-CN) to extract residual gold remaining after amalgamation. Hg⁰ reacts with ^-CN under aerobic conditions to produce Hg(CN)₄^2- and other Hg(CN) complexes. The production of solvated Hg(CN) complexes increases upon agitation in the presence of synthetic and authentic Hg⁰-contaminated tailings that aid in dispersing the Hg⁰, increasing its reactive surface area. Adult rats were exposed to various concentrations of Hg(CN)₂, and accumulation in organs and tissues was quantified using direct mercury analysis. The primary site of Hg(CN)₂ accumulation was the kidney, although accumulation was also detected in the liver, spleen, and blood. Little accumulation was observed in the brain, suggesting that Hg(CN)₂ complexes do not cross the blood-brain barrier. Renal tissue was particularly sensitive to the effects of Hg(CN)₂, with pathological changes observed at low concentrations. Hg(CN)₂ complexes are handled by mammalian systems in a manner similar to other inorganic species of Hg, yet appear to be more toxic to organ systems. The findings from this study are the first to show that Hg(CN)₂ complexes are highly stable complexes that can lead to cellular injury and death in mammalian organ systems.