Dissertation
Goethite biogeochemistry: role of electron transfer and mineral defects
University of Iowa
Doctor of Philosophy (PhD), University of Iowa
Autumn 2019
DOI: 10.17077/etd.005201
Abstract
Iron oxides are ubiquitous in Earth’s crust. Its redox reactions in the environment not only control the cycling and availability of important elements such as C, N, S, but also influence the speciation and mobility of contaminants. During the last few decades, significant advances have been made in understanding the reaction of Fe(II) with Fe(III) oxides. However, there are knowledge gaps in the mechanistic understanding, specifically on what drives Fe(II)-Fe(III) electron transfer. Despite substantial experimental evidence for Fe(II)-Fe(III) electron transfer, recent studies in computational chemistry calculations suggest that oxidation of sorbed Fe(II) by goethite is kinetically inhibited, unless surface defects are present. In this thesis, we explored the connections between surface defects and Fe redox chemistry. More specifically, we investigated how the presence of surface defects influence (i) Fe(II)-goethite electron transfer, (ii) microbial respiration of goethite, and (iii) the stability and reversibility of Fe(II)-goethite electron transfer. Isotope labeled 56Fe goethite was synthesized by hydrolysis and further hydrothermally treated to remove surface defects such as Fe vacancies. Goethites with more and fewer defects were reacted with 57Fe(II) and 57Fe Mössbauer spectroscopy and revealed that Fe(II)-goethite electron transfer is inhibited in goethite with fewer defects, demonstrating that surface defects play an important role in enabling electron transfer. We further used isotope-labeled (natural abundant or 56Fe) goethite to track the origin of the Fe(II) reduced by Geobacter sulfurreducens when goethite with more and fewer defects were both present. The isotopic composition of the microbially reduced atoms revealed that most of the reduced atoms arise from goethite containing more defects and demonstrate that surface defects facilitate the respiration of Fe(III) reducing bacteria. Finally, we used selective isotope labeling (56goethite + 56Fe(II) + 57Fe(II)) to investigate the fate of new Fe(II) that sorbed onto goethite that has been previously exposed to Fe(II). The longer goethite is pre-exposed to Fe(II), the more Fe(II)-goethite electron transfer was exposed, and we demonstrated that a passivation layer of sorbed Fe(II) contributed to the observed inhibition. Importantly, however, electron transfer was partially restored upon removal of the layer of Fe(II) by extraction or oxidation.
Our results help resolve the disagreement between the computational chemical calculations and experimental observations. We further demonstrated that even small changes at the surface of iron minerals might change their bioavailability and determine which minerals will be reduced. Finally, in this work, we showed that electron transfer is a process that can be self-inhibitory unless the minerals are exposed to geochemical fluctuations, such as changes in pH and water table fluctuations.
Details
- Title: Subtitle
- Goethite biogeochemistry: role of electron transfer and mineral defects
- Creators
- Luiza Notini de Andrade
- Contributors
- Michelle M Scherer (Advisor)Drew E Latta (Committee Member)Andreas Kappler (Committee Member)David M Cwiertny (Committee Member)Gregory H LeFevre (Committee Member) - University of Iowa, Civil and Environmental EngineeringSyed Mubeen (Committee Member)
- Resource Type
- Dissertation
- Degree Awarded
- Doctor of Philosophy (PhD), University of Iowa
- Degree in
- Civil and Environmental Engineering
- Date degree season
- Autumn 2019
- DOI
- 10.17077/etd.005201
- Publisher
- University of Iowa
- Number of pages
- xv, 158 pages
- Copyright
- Copyright 2019 Luiza Notini de Andrade
- Language
- English
- Description illustrations
- color illustrations
- Description bibliographic
- Includes bibliographical references (pages 144-158).
- Public Abstract (ETD)
Iron (Fe) minerals are present in soils and water and are responsible for much of the vibrant orange, red, and black we see in soils. Reactions between different forms of Fe control the cycling and availability of important elements such as carbon, nitrogen, and sulfur, and influence the mobility of environmental pollutants present in soil and groundwater.
In this thesis, we explored how defects on Fe minerals influence the behavior of Fe minerals in water. We used stable Fe isotopes and instruments able to detect different isotopes in both the water and the mineral. We investigated how defects would affect the mixing of Fe atoms and how microbes use Fe minerals to gain energy (i.e., respire). We found iron atoms mix less when defects are not present. We also found that the presence of defects makes it easier for Fe reducing bacteria to respire on Fe minerals. Finally, we discovered that the mixing of the atoms slows down as Fe minerals contact Fe in water. Importantly, however, the mixing can be partially restored when surface Fe is removed from the mineral.
Our results help resolve the disagreement between the computational chemical calculations and experimental observations. We further demonstrated that even small changes at the surface of iron minerals might change their bioavailability and determine which minerals will be used in microbial respiration. Finally, in this work, we showed that mixing of Fe atoms is a process that inhibits itself unless the minerals are in regions of constant changes.
- Academic Unit
- Civil and Environmental Engineering
- Record Identifier
- 9983779999202771
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