THE ROLE OF THIOSULFATE OXIDIZING BACTERIA ON THE MOBILITY OF A GOLD THIOSULFATE COMPLEX

Thiosulfate oxidizing bacteria were enriched and isolated from water samples collected from the Driefontein mine in Witwatersrand Basin, South Africa. These bacteria were able to grow in the presence of up to 50 ppm gold and precipitate gold from a gold-thiosulfate complex. In bacterial culture systems, thiosulfate disproportionated to sulfur and other intermediate sulfur species that was enhanced by acid production. These sulfur species were then oxidized completely to sulfate. During bacterial growth, the pH of the medium solution decreased from 5.4 to 1.9 while the Eh increased from 0.3 to 0.5-0.6 Volts within a period of 75 days. The gold thiosulfate complex was stable in the bacterial systems until sulfur oxidation was complete, after which the bacteria began to precipitate gold from the gold thiosulfate complex. The bacterial systems, containing 5-50 ppm gold, precipitated 100 to 75% of the gold under diurnal light exposure while only 83 to 23 % of the gold was precipitated in the dark over a period of 75 days, respectfully. Under similar experimental conditions and duration, gold was not precipitated in chemical control experiments. The presence of gold in the bacterial systems reduced the bacteria growth rates and disrupted cell division producing elongated cells. The gold was precipitated as fine-grained colloids within the cells with approximate diameter of 5-10 nm, and precipitated gold was also formed outside the cells as amorphous solids with approximate sizes of 100 nm to 100 µm. Observations of ultra-thin sections using transmission electron microscopy revealed that the gold was concentrated in the cell envelope, especially along the cytoplasmic membrane, indicating that mobilization was likely enhanced via electron transport processes associated with energy generation.