The effect of iron-oxidising bacteria on the stability of gold (I) thiosulphate complex

An acidophilic, iron-oxidising bacterial consortium was collected from Rio Tinto near Berrocal, Spain. This primary enriched culture was used to examine the effect of acidophilic iron-oxidising bacteria on the stability of soluble gold (I) thiosulphate. Stationary phase cultures and separate components of the cultures (i.e., aqueous ferric iron, iron oxyhydroxide precipitates and non-mineralised bacterial cells) were exposed to gold (I) thiosulphate solutions forming different experimental-gold systems. These experimental systems rapidly removed gold from solutions containing 0.002 mM – 20 mM gold thiosulfate. Scanning and transmission electron microscopy demonstrated that the different culture fractions immobilised gold differently: the entire bacterial culturegold systems precipitated 100 nm-size gold colloids; aqueous ferric iron-gold systems precipitated colloidal gold sulphide that ranged in diameter from 200 nm to 2 m; iron oxyhydroxide-gold systems precipitated 5 nm-size gold sulphide colloids; and the bacteria-gold systems precipitated gold colloids ~2 nm in size along the bacterial cell envelope. Aqueous and solid ferric iron was critical in the destabilisation of the gold (I) thiosulphate complex. Analysis of the entire bacterial culture-, aqueous ferric ironand iron oxyhydroxide-gold systems exposed to 2 mM gold using X-ray absorption near edge spectroscopy demonstrated that Au + was immobilised from solution as gold sulphide (Au2S). The reaction between iron-oxidising bacteria and their ferric iron by-products with gold (I) thiosulphate demonstrated that thiosulphate ions would be an unstable gold complexing ligand in nature. Gold (I) thiosulphate is intuitively transformed into nanometer-scale gold sulphide or elemental gold within natural, acidic weathering AC C EP TE D M AN U SC R IP T ACCEPTED MANUSCRIPT Iron-oxidising bacteria affect gold (I) thiosulphate stability 3 environments with the potential to precipitate gold in jarosite that can subsequently be preserved in gossans over geological time.