Mechanism of interaction between Acidithiobacillus ferrooxidans and arsenopyrite

A strain of Acidithiobacillus ferrooxidans was used to study the bio-oxidation mechanism of arsenopyrite(FeAsS).A series of experiments were designed to compare differences between chemical oxidation and bio-oxidation of arsenopyrite.Changes of solution chemistry,solid-phase products,surface structure and the chemical states of surface elements were examined over a period of 35 days.The results show that the bio-oxidation of arsenopyrite consisted of three stages with colonization and growth stages of A.ferrooxidans:(1)Arsenopyrite was oxidized in the acidified solution within 7 days and there was no interaction between mineral and bacteria;(2)A.ferrooxidans came into the lag phase and arsenopyrite began to interact with bacteria from the eighth to the twenty-first day of oxidation;(3)Ferric ion was regenerated quickly by bacteria and consequently chemically oxidized arsenopyrite as A.ferrooxidans reached the exponential phase after 21 days.Changes in ion concentrations during the experiments showed that the rate of chemical oxidation of arsenopyrite was higher than that of bio-oxidation in the first and the second stages,while the rate of bio-oxidation was higher than that of chemical oxidation after 21 days.Ferric ions and arsenates were deposited as insoluble iron arsenates by bacteria to reduce the toxic effects of arsenic ions on the growth of A.ferrooxidans.Oxidized species proportions of the three essential constituents of arsenopyrite were enhanced with bacterial population growing and Fe3+ concentration increasing.When the bacteria reached the exponential phase in the third stage,more oxidized species were formed on the mineral surface exposed to A.ferrooxidans than on the surface exposed to Fe3+.Overlayers were formed on mineral surfaces by the accumulation of intermediate products,elemental sulphur(S0)and orpiment(As2S3),in both bacterial and chemical oxidation process.In contrary to the surface reacted abiotically with Fe3+,the intermediate products were oxidized in bio-oxidation and arsenopyrite was indirectly oxidized by A.ferrooxidans by biologic consumption of Fe2+ and regeneration of Fe3+.