Rapid and Heterogeneous Oxidation of Pyrite Surfaces by Thiobacillus ferrooxidans and T. thiooxidan

B. C. Bostick,¹ K. Lester,² C. Doyle,² T. Kendelewicz,² G. E. Brown, Jr.,² S. Fendorf,² and A. M. Spormann²

¹Department of Earth Sciences, Dartmouth College, Hanover, NH 03755 USA
²Department of Environmental and Geological Sciences, Stanford University, Stanford, CA 94305 USA

Pyrite oxidation is the critical step in the formation of acid mine drainage, which acidifies the environment, and releases associated toxic metals. Certain microbes accelerate pyrite oxidation, and, as such, a more detailed understanding of their role in the production of acid mine drainage is needed. This study is one of the first to use x-ray photoelectron spectroscopy to probe bacterial pyrite oxidation, providing insights into the fundamental processes by which oxidation occurs. Freshly fractured pyrite surfaces were oxidized by Thiobacillus ferrooxidans over a range of solution pH and were compared with chemical oxidation of similar surfaces under the same conditions. Microbial oxidation resulted in the formation of distinct oxidized surface species distributed non-uniformly over the pyrite surface. In contrast, chemical oxidation proceeds through a similar mechanism but without the build-up of passivating oxidation products. Both mechanisms occurred at isolated regions of the surface that were not correlated with step edges or other topographical features. These results demonstrate that microbial oxidation occurs through a mechanism similar to chemical oxidation, but that the increased rate of biological oxidation results the formation of distinct products on the pyrite surface. This build-up of surface oxidation products may impact the long-term reactivity of pyrite surfaces.