Redox properties of iron-organic matter phases: Influence of structure on biological Fe(III) reduction with implications for carbon cycling

• Applicant: Jeffrey Hudson, Ph.D.
• Subject Area: Mineralogy, Petrology and Geochemistry ; Soil Sciences ; Hydrogeology, Hydrology, Limnology, Urban Water Management, Water Chemistry, Integrated Water Resources Management
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 540763051

At redox interfaces in global aquatic environments, iron (Fe) can coprecipitate with diverse pools of natural organic matter (NOM) and other ions, forming poorly crystalline minerals that protect carbon against biologically mediated transformations to carbon dioxide (CO2). While this protection mechanism has large implications for climate change, the effects of organic matter (OM) on the redox properties of coprecipitated Fe phases are unknown. Poorly crystalline Fe (oxyhydr)oxides associated with NOM may have altered redox properties due to NOMs ability to influence Fe speciation and coordination environment. It remains unclear if a link exists between coprecipitate atomic-level properties and macro-scale processes (i.e., extents and rates of redox reactions). Because electron transfer to Fe minerals is thought to affect the formation of CO2 in anoxic environments, enhanced reductive dissolution rates of coprecipitates could counteract carbon protection. My central hypotheses are that 1a) NOM containing abundant carboxylate ligands will greatly decrease crystallinity of FeIII-OM coprecipitates compared to other metastable iron oxyhydroxides (i.e., ferrihydrite), 1b.) secondary ions associated with higher ionic strength environments will impede OM incorporation into the FeIII-OM coprecipitates, leading to crystalline and ordered phases compared to coprecipitates formed in low ionic-strength media, 2) FeIII-OM coprecipitates have enhanced redox properties (higher EH) and reducibility (both electrochemical and biotic) compared to “pure” iron (oxyhydr)oxide minerals (i.e., ferrihydrite), 3) abiotic and microbial FeIII-OM coprecipitate transformation pathways and products will differ from pure iron (oxyhydr)oxide minerals, and 4) microbial reduction of FeIII-OM coprecipitates will increase CO2 release from anoxic soils or aqueous suspensions relative to ferrihydrite in the presence of equivalent amounts of NOM or in NOM-free controls. I aim to use sophisticated solid-phase characterization techniques, including Mossbauer and synchrotron-based spectroscopy, to investigate and link bulk and atomic-scale structural properties of coprecipitates to their redox properties measured via electrochemical techniques. Electrochemical and biological reduction experiments will further be employed to link structural properties of coprecipitates to their abiotic and biological redox reactivities, respectively. Research will be conducted at the University of Tübingen in the Geomicrobiology Work Group under the guidance of Prof. Dr. Andreas Kappler as a supervisor due to Dr. Kappler’s expertise in Fe biogeochemistry and mineralogy, as well as the state-of-the-art laboratory facilities and professional opportunities provided at Tübingen. Additionally, expertise in environmental redox chemistry and electrochemistry will be provided under the guidance of junior group leader Dr. Prachi Joshi and in collaboration with Prof. Dr. Stefan Haderlein, both also at Tübingen.

• DFG Programme: WBP Position