Production of rhodanese by bacteria present in bio-oxidation plants used to recover gold from arsenopyrite concentrates

Considerably larger quantities of cyanide are required to solubilize gold f ollowing the bio-oxidation of gold-bearing ores compared with oxidation by physical-chemical processes. A possible cause of this excessive cyanide con sumption is the presence of the enzyme rhodanese. Rhodanese activities were determined for the bacteria most commonly encountered in bio-oxidation tan ks. Activities of between 6.4 and 8.2 mu mol SCN- min(-1) mg protein(-1) we re obtained for crude enzyme extracts of Thiobacillus ferrooxidans. Thiobac illus thiooxidans and Thiobacillus caldus, but no rhodanese activity was de tected in Leptospirillum ferrooxidans. Rhodanese activities 2-2.5-fold high er were found in the total mixed cell mass from a bio oxidation plant. T. f errooxidans synthesized rhodanese irrespective of whether it was grown on i ron or sulphur. With a PCR-based detection technique, only L. ferrooxidans and T. caldus cells were detected in the bio-oxidation tanks. As no rhodane se activity was associated with L. ferrooxidans, it was concluded that T. c aldus was responsible for all of the rhodanese activity. Production of rhod anese by T. caldus in batch culture was growth phase-dependent and highest during early stationary phase. Although the sulphur-oxidizing bacteria were clearly able to convert cyanide to thiocyanate, it is unlikely that this r hodanese activity is responsible for the excessive cyanide wastage at the h igh pH values associated with the gold solubilization process.