Mechanism of Bacterial Pyrite Oxidation

Abstract

The oxidation by Ferrobacillus ferrooxidans of untreated pyrite (FeS 2 ) as well as HCl-pretreated pyrite (from which most of the acid-soluble iron species were removed) was studied manometrically. Oxygen uptake was linear during bacterial oxidation of untreated pyrite, whereas with HCl-pretreated pyrite both a decrease in oxygen uptake at 2 hr and nonlinear oxygen consumption were observed. Ferric sulfate added to HCl-pretreated pyrite restored approximately two-thirds of the decrease in total bacterial oxygen uptake and caused oxygen uptake to revert to nearly linear kinetics. Ferric sulfate also oxidized pyrite in the absence of bacteria and O 2 ; recovery of ferric and ferrous ions was in excellent agreement with the reaction Fe 2 (SO 4 ) 3 + FeS 2 = 3FeSO 4 + 2S, but the elemental sulfur produced was negligible. Neither H 2 S nor S 2 O 3 2− was a product of the reaction. It is probable that two mechanisms of bacterial pyrite oxidation operate concurrently: the direct contact mechanism which requires physical contact between bacteria and pyrite particles for biological pyrite oxidation, and the indirect contact mechanism according to which the bacteria oxidize ferrous ions to the ferric state, thereby regenerating the ferric ions required for chemical oxidation of pyrite.