PHOTO-ENHANCED TOXICITY OF COBALT-CYANIDE TO AQUATIC LIFE

Cobalt-bound cyanide is a possible product resulting from the cyanide-treatment of ore for the extraction of gold. This strong metallocyanide complex is thought to be stable in the environment and of low hazard to aquatic life. However the potential exists for the complex to photo-dissociate in sunlight to give free cyanide, which is the most toxic of the cyanide species that occur in ore processing wastes. We found that cyanocobalt is more than 250 times more toxic to juvenile rainbow trout with the amount lethal to 50 % of the test population (LC50) of 0.4 mg/L at the low ultraviolet irradiance levels common in shallow aquatic habitats (4 microWatts/cm2 as UVB) compared to an LC50 of 113 mg/L in the absence of light. The toxicity was immediately apparent, occurring within hours of the onset of exposure, and was associated with increased concentrations of weak-acid dissociable cyanide. The organisms were unaffected by exposure to UV alone. The presence of dissolved organic carbon DOC in aqueous solutions significantly diminished the toxicity of cobalt-bound cyanide. Test organisms that vary in their tolerance to UV were affected similarly, an indication that mortality was a result of photoenhanced toxicity of the metallocyanide complex itself rather than oxidation of tissue-bound cyanide.