CHEMICAL AND BIOLOGICAL PATHWAYS IN THE BACTERIAL OXIDATION OF ARSENOPYRITE

A moderately thermophilic mixed culture of bacteria catalysed the oxidative solubilization of arsenopyrite to give Fe(III), S(VI) and As(V). Toxic effects were observed in a few experiments due to the build-up of As(III) (up to 90 mM), The bacterial oxidation of arsenopyrite involved direct attack of the bacteria on the mineral to give As(III). Subsequent oxidation of As(III) to As(V) occurred through reaction with Fe(III), but only in the presence of pyrite, which provided a catalytic surface. Arsenopyrite was unable to act as a catalyst. The pyrite-catalysed oxidation of As(III) to As(V) by Fe(III) usually only went to completion in the presence of bacteria, possibly due to their role in the provision of clean catalytic surfaces. Thus, toxic concentrations of As(III) may accumulate in reactors during the bacterial oxidation of arsenopyrite due to the absence of pyrite or a clean pyrite surface or to low concentrations of the effective oxidizing agent, Fe(III).