Pyrite formation at ambient temperature – resolving the interplay between microbes, iron minerals, sulfur supplies and associated organics

• Applicants: Professor Dr. Andreas Kappler; Dr. Muammar Mansor, Ph.D.; Professor Dr. Martin Obst; Professor Dr. Stefan Peiffer
• Subject Area: Mineralogy, Petrology and Geochemistry
• Term: since 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 503493769

Pyrite (FeS2) is a key mineral in the biogeochemical cycling of iron (Fe), sulfur (S), oxygen, carbon and associated heavy metals on the Earth’s (sub)surface. Despite its importance, laboratory experiments at ambient temperatures have so far failed to form pyrite at comparable precipitation rates and morphology (i.e., framboids) to those observed in nature, thus preventing confident extrapolation of insights from laboratory system to the environment. To overcome this problem, we have developed 4 work packages (WPs) focused on experimental systems that takes advantage of the collaboration between geomicrobiologists from the Tübingen Geomicrobiology Group and geochemists from the University of Bayreuth. The focus of this work is on pyrite formation via the recently described ferric hydroxide surface (FHS) pathway, involving the reaction network initiated by the reaction of Fe(III) minerals with sulfide (direct from H2S source or indirectly via microbial reduction of elemental sulfur). In WP1, we will utilize batch reactors to compare pyrite formation by three microbial species with different elemental sulfur (S0) and Fe(III) reduction mechanisms – Sulfospirillum deleyianum (S0 reducer), Geobacter sulfurreducens (Fe(III) and S0 reducer) and Desulfocapsa sulfoexigens (S0 disproportionater). In WP2, batch reactors will be similarly applied to determine how different initial substrates (S0 and Fe(III) minerals with varying crystallinity and associated organics) affect pyrite formation in parallel microbial and abiotic experiments. In comparison, in WP3 we will apply flow-through reactors with different modes of sulfide supply – single pulse, steady state, or multiple slow pulses with ripening period in between – to determine how sulfide supply rates affect pyrite formation. In all cases, the rate of pyrite formation and the morphology will be determined, together with the dynamics of aqueous and solid-phase Fe-S species (e.g., FeSaq, surface-bound polysulfides, greigite). A combination of sequential extraction, XRD, Raman, Mössbauer, magnetic susceptibility, voltammetry, high performance liquid chromatography, X-ray absorption spectromicroscopy and electron microscopy will be utilized, which will be jointly available through our collaboration. Finally, in WP4, a novel micro flow-through reactor design will be optimized that will allow unprecedented in-situ analysis and high temporal resolution into pyrite formation and Fe-S species dynamics via Raman microspectroscopy. The WP4 setup allows for varying the microbial presence (WP1), the initial Fe(III) minerals (WP2) and the mode of sulfide supply (WP3), thus complementing the overall project. Two PhD students will work closely together throughout this project, combining the interdisciplinary field of microbiology and inorganic chemistry in order to gain new insights into the formation of pyrite – an important but still poorly understood mineral.

• DFG Programme: Research Grants
• Co-Investigator: Privatdozentin Dr. Catherine McCammon