『Abstract
An Acidithiobacillus thiooxidans spp., isolated from the
Driefontein Consolidated Gold Mine, Witwatersrand Basin, Republic
of South Africa was able to precipitate gold from Au(S2O3)23- in the presence
of up to 0.26 mM gold. In chemical control experiments and with
the presence of dead bacteria, gold was not precipitated under
similar experimental conditions and duration. During growth, the
pH of the culture medium decreased from pH 5.4 to 1.9, while the
Eh increased from 0.3 to between 0.5 to 0.6 V within a period
of 75 days. In the active (live) bacterial culture systems, acid
production enhanced thiosulfate disproportionation, after which
the elemental sulfur and any other intermediate sulfur species
were oxidized completely to sulfate. The gold, Au(S2O3)23-, was stable
in the bacterial systems until sulfur oxidation was complete;
then the bacteria precipitated gold from Au(S2O3)23-. The bacterial
systems (0.02-0.26 mM gold) precipitated 87 to 100% of the gold
under diurnal light exposure, while only 11 to 69% of the gold
was precipitated in the dark. The presence of gold (≧0.08 mM)
reduced bacterial growth, disrupted cell division causing cell
elongation, and was ultimately toxic to this bacterium, killing
the cultures. The gold was precipitated inside the bacterial cells
as fine-grained colloids ranging between 5 and 10 nm in diameter
and in the bulk fluid phase as crystalline micrometer-scale gold.
Observations using transmission electron microscopy revealed that
the gold was deposited throughout the cell; however, it was concentrated
in the cell envelope, especially along the cytoplasmic membrane,
suggesting that gold precipitation was likely enhanced via electron
transport processes associated with energy generation. Seven months
after population growth had stopped, the gold had formed coiled
or wire gold, irregular and rounded structures with an approximate
size ranging from 0.5 to 5μm, and crystalline octahedral gold.』
1. Introduction
2. Materials and experimental procedures
2.1. Materials
2.2. Bacterial enrichment and isolation
2.3. Bacterial experiments
2.3.1. Type 1 bacterial experiments
2.3.2. Type 2 bacterial experiments
2.3.3. Type 3 bacterial experiments
2.4. Abiotic experiments
2.5. Bacterial viability and total bacterial counts
2.6. Chemical analyses
2.7. Transmission electron microscopy
2.8. Scanning electron microscope and X-ray diffraction
3. Results
3.1. Bacterial experiments
3.2. Abiotic experiments
3.3. Bacterial viability and total bacterial counts
3.4. TEM, SEM, and XRD
4. Discussion
4.1. Comparison of bacterial experimental systems and abiotic
experimental systems
4.2. Effects of chloride and sulfur species in solutions on the
speciation of gold
4.3. Effects of gold on A. Thiooxidans
4.4. Comparison of gold precipitation by A. thiooxidans in the
dark and under light exposure
4.5. Possible mechanisms of gold precipitation by A. thiooxidans
4.6. Implications for the formation of supergene gold
Acknowledgments
References