Accumulation and Toxicity of Mercury-Cyanide Complexes

Abstract

Mercury is a heavy metal toxicant found in numerous occupational and environmental settings. A major source of mercury pollution in environmental settings comes from the occupational use of mercury in artisanal and small-scale gold mining (ASGM). ASGM utilizes elemental mercury (Hg0) for extraction of gold from ore, which leads to Hg0-contaminated tailings. These tailings are often reprocessed with cyanide (‾CN) to extract residual gold. Hg0 reacts with ‾CN to form mercury-cyanide (Hg(CN)) complexes, which are released into the environment with discarded tailings. These complexes create numerous environmental and health problems. To assess the disposition of Hg(CN)2 complexes in mammalian systems, wistar rats were injected with 0.5 mol/kg mercury chloride (HgCl2) or Hg(CN)2. Each injection solution contained radioactive mercury ([203Hg2+]) and/or radioactive cyanide ([14C]-NaCN). After 24 h, animals were euthanized and organ samples were collected for determination of Hg(CN)2 content and for histological analyses. To assess the disposition of Hg(CN)2 complexes in an aquatic organism, zebrafish (Danio rerio) were exposed to various concentrations of Hg(CN)2 and accumulation in both whole fish and specific organs was measured. The current data show that the disposition of Hg(CN)2 in zebrafish is similar to that when rats were exposed to HgCl2. Interestingly, when rats were exposed to NaCN alone, the uptake was significantly less than that of rats exposed to Hg(CN)2. Experiments using adult zebrafish showed that Hg(CN)2 accumulates readily in fish. Zebrafish embryos exposed to Hg(CN)2 experienced alterations in developmental processes. The current data provide important information about the handling of Hg(CN)2 complexes in mammalian systems and aquatic organisms.