A new gold rush is taking place in the American Southwest, due in large part to improvements in the processes that use cyanide to extract gold from low-grade ore deposits. Along with increased gold production, however, has come an increase in cyanide contamination of surface and ground water in areas affected by gold mining. The objective of this study was to determine the potential for bioremediation of total (free and complexed) cyanide in heap-leached ore.

Two methods, including the addition of a carbon amendment and/or cyanide degrading microorganisms to cyanide heap-leached ore, were evaluated with regard to their influence on loss of total cyanide in microcosm studies. Heap-leached ore, containing approximately 400 mg cyanide/kg ore, was obtained from the Summitville Mine in southern Colorado. The presence of a cyanide degrading microbial assemblage in Summitville ore was verified by isolating microorganisms on plates supplied with free (HCN(g)) as the sole source of nitrogen. However, stimulation of the indigenous community did not occur with respect to loss of total cyanide in ore microcosms using sucrose as a carbon amendment. Cyanide degrading inocula, consisting of Pseudomonas fluorescens, an Industrial (from Pintail Systems, Inc.) microbial consortium and a native Summitville species, also had no effect on the rate of loss of total cyanide compared to uninoculated controls. Loss of total cyanide in biological and control treatments and was relatively linear over time at a rate of 4.9 (+0.7) mg CN-/kg ore/day. While bioremediation may not an be an effective treatment for remediating complexed cyanide, there exists the potential for biological degradation of free cyanide generated from the dissociation of metal complexed cyanide in heap leached ore.