2000 GSA Annual Meeting -- Reno, Nevada
2000 GSA Annual Meeting -- Reno, Nevada
Author(s): GRIMES, David J., JOHNSON, Craig A., LEINZ, Reinhard W., and RYE, Robert O., US Geological Survey, Box 25046, MS 973, Denver, CO 80225, rleinz@usgs.gov
Keywords: cyanide, metals, photochemistry, mining, effluent
In assessing the environmental impact of cyanide (CN-) solution releases to surface waters, such as the widely publicized release that occurred at the Baia Mare gold mine, Romania in Jan. 2000, it is important to understand the processes that control cyanide stability. Cyanide is known to form stable complexes with many metals, which reduces its toxicity to aquatic life and produces species susceptible to precipitation and adsorption reactions. The cyanocomplexes of Fe and Co are particularly important because they are widely considered stable enough to be benign, and they are unregulated in many jurisdictions. We present data that reveal an important process affecting cyanocomplex stability in surface waters. In an effluent collection channel ~60 meters downstream from an ore heap undergoing rinsing, cyanide speciation and metal concentrations were found to display diel cycles. The cycles were caused by sunlight-induced dissociation of strong Fe- and Co-cyanocomplexes within the channel at rates that at midday generated tenths of mg of highly toxic free cyanide per liter in a matter of minutes. An estimate of the illumination time of the effluent allowed calculation of a midday half-life of the complexes of 21±9 min. The observed free cyanide levels, though ephemeral, could be injurious to sensitive fish species in receiving streams (eg. rainbow trout, bluegill sunfish) and would be non-compliant with effluent standards in many jurisdictions. The stability of strong cyanocomplexes in darkness led to lower free cyanide levels and enhanced mobility of Fe and Mn at night. Photochemical production of free cyanide in sunlight led to higher free cyanide levels and mobilization of Cu, Ag, and Au from the channel bed during the day, although the metals at no time reached concentrations of environmental concern. The evidence that Fe- and Co-cyanocomplexes can undergo rapid photodissociation under some circumstances giving rise to environmentally significant levels of free cyanide leads us to conclude that photodissociation of strong cyanocomplexes is a process that merits consideration in assessing risks to aquatic life wherever surface waters receive cyanide-bearing flow.
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