A study of chemolithoautotrophic bacterial rhodanese, and its potential contribution to cyanide wastage during cyanidation of biooxidized concentrates

Refractory gold-bearing metal-sulfide ores and concentrates can be successfully and economically treated by biooxidation prior to cyanidation. However, one of the major drawbacks of the biooxidation process is the excessive consumption of cyanide during the gold dissolution process. The largest proportion of cyanide wastage is attributed to thiocyanate formation. Thiocyanate can be formed by spontaneous chemical reactions between reactive sulfur species and cyanide. The enzyme rhodanese (thiosulfate: cyanide sulfurtransferase EC 2.8.1.1) is also able to catalyze the formation of thiocyanate using thiosulfate and cyanide as substrates. Therefore, the most relevant members of the microbial consortium responsible for biooxidation of gold-bearing ores or concentrates were investigated to determine whether they were able to contribute to thiocyanate formation by means of a enzyme reaction mechanism indicative of rhodanese. Together with a Thiobacillus caldus strain (T. caldus MNG), isolated from a biooxidation pilot plant, the sulfur-oxidizers Thiobacillus ferrooxidans ATCC 33020 (also able to oxidize iron) and Thiobacillus thiooxidans ATCC 19377 demonstrated rhodanese activity. However, the obligate iron-oxidizer Leptospirillum ferrooxidans DSM 2705 had no detectable rhodanese activity. High levels of rhodanese activity were detected from the mixed microbial population of a biooxidation pilot plant, which appeared to be dominated by T. caldus MNG. This T. caldus strain was initially identified in the biooxidation pilot plant using the PCR based 16S rDNA profiling technique of Rawlings (1995), and the identification confirmed by 16S rDNA sequencing. Therefore, T. caldus MNG is considered the major contributor to the rhodanese activity of the biooxidation pilot plant mixed culture. Within the range of sulfur substrates tested, the rhodanese enzyme of T. caldus MNG behaved exclusively as a thiosulfate: cyanide sulfurtransferase, and the rhodanese activity of T. caldus MNG appeared to be dependent on the physiological state of the cell during batch growth. An attempt to isolate a rhodanese gene from T. caldus MNG by Southern hybridization, using the Azotobacter vinelandii rhdA gene as a probe, was unsuccessful. On balance of the information available from this study, and reported elsewhere, rhodanese activity probably does not contribute as much to thiocyanate formation in the cyanidation plant as does the spontaneous chemical formation of thiocyanate.

Grantor dc:publisher.institution

Department of Molecular and Cell Biology

Year dc:date.issued

1998

Author dc:creator

• Gardner, Murray Newell

Language dc:language.iso

eng

Handle dc:identifier.uri

http://hdl.handle.net/11427/9686

OAI identifier oai:identifier

oai:open.uct.ac.za:11427/9686