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Iodine-oxidizing bacteria could leach gold underground

By Tetsuo Satoh |

Gold is usually leached from ore using hazardous substances, such as cyanide, mercury, aqua regia and others. Although there has been some progress in developing bioleaching technology, such methods still require mining and processing the ore before micro-organisms can go to work. Now, Yuichi Sugai and colleagues at Kyushu University (http://reps.mine.kyushu-u.ac.jp/reps) have demonstrated the ability to leach gold directly from ore — without mining operations — using bacteria isolated from a natural gas field.

The underground brine water in natural gas fields in Japan contains high concentrations of iodide (120 parts per million, or 2,000 times higher than the concentration in seawater), as well as iodide-oxidizing bacteria (IOB), which oxidize I into iodine (I2). Because gold can be dissolved in I/I2 mixture, Sugai came up with the idea to use the IOB for gold leaching. Eight strains of IOB were isolated from a natural gas field and incubated in the culture medium containing nutrients, iodide and gold ore (gold grade: 0.26 wt.%, pulp density: 3.3 wt./vol.%) at 30ºC. The strains oxidized I into I2 to generate triiodide, (I3), which releases gold from the ore into solution as gold diiodide, (AuI2). Three IOB strains successfully dissolved gold from ore completely within 30 days — the best within 5 days. This proof-of-concept demonstrates the possibility for in situ gold leaching, which has the potential to be more economical and environmentally sustainable than traditional techniques. “IOB are bad bacteria in the gas field, because they cause corrosion of wells and pipes,” says Sugai, but adds that “they can become good” because of this special ability.

The study was funded by a university grant, and the results published in the May 2 issue of Minerals.

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