Pyrrhotite, a non-stoichiometric iron sulfide with the composition Fe1-xS (0 ≤ x ≤ 0.125), is commonly associated with metal ores and is a major component of mine tailings. Understanding the dissolution of such sulfide minerals is critical to maintaining water quality standards in regions with active and historical mines. The dissolution of pyrite (FeS2) is well characterized, especially in acidic systems; however, the mechanisms and kinetics of pyrrhotite oxidation require further study. The presence of iron- and sulfur-oxidizing bacteria is known to increase the rate of pyrrhotite dissolution in extremely acidic systems, but their impact is not well understood in less acidic or near-neutral systems (pH 4-7) such as those that would likely be produced by mining of Minnesota’s Duluth Complex. The goal of this study is to compare dissolution rates of pyrrhotite in sterile and microbially inoculated solutions under moderately acidic to near-neutral conditions (pH 4-7). To ensure that the dissolution experiments are performed with environmentally-relevant microorganisms, we enriched and isolated bacteria from weathered Duluth Complex waste rock. Based on rRNA gene sequencing, enrichments in which solid pyrrhotite was the only available electron donor are populated by a consortium of Sulfuriferula spp., Thiomonas spp., and rare organisms. Two different kinetic experiments are currently underway. In one experiment, we are comparing pyrrhotite dissolution in test tube bioreactors, and using magnetic susceptibility measurements as a means of quantifying the dissolution rate without disturbing the chemical system. These magnetic measurements are reinforced with less frequent sulfate and pH measurements. In a second experiment, we are using electron backscatter diffraction to explore whether pyrrhotite displays any crystallographically-controlled dissolution anisotropy.

Microbial contributions to pyrrhotite oxidation

253rd National Meeting (2017)

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