Dissertation
Redox chemistry of iron(II) minerals and their implications for contaminant fate in the environment
University of Iowa
Doctor of Philosophy (PhD), University of Iowa
Autumn 2024
DOI: 10.25820/etd.007547
Abstract
Iron (Fe) minerals are ubiquitous in sediments and soils and actively participate in important environmental processes such as microbial respiration, carbon cycling, geochemical weathering, nutrient cycling, and contaminant cleanup. Environmental contaminants, such as chlorinated solvents, heavy metals, and nitroaromatics, pose a threat to human and environmental health and have left little to no area on Earth unafflicted. To preserve and protect well-being of humans and the environment alike, I investigated the redox behavior of Fe minerals, which are naturally abundant and effective reducing agents and can play a critical role in contaminant reduction in the environment.
Some of the most commonly found groundwater contaminants in the United States are chlorinated solvents. To more sustainably cleanup legacy chlorinated solvent plumes, abiotic reduction by Fe minerals has been suggested as a method to naturally attenuate some sites. Here, I evaluated the reduction of cis-1,2-dichloroethene (cDCE) by Fe(II) minerals over a range of environmentally relevant conditions. I measured cDCE reduction by magnetite, chloride green rust, sulfate green rust, goethite, hematite, chukanovite, aluminum oxide, mackinawite, and clays with aqueous Fe(II) (Fe(II)aq). Despite the extensive range of conditions evaluated, I found that cDCE reduction occurred only (but not always) when conditions favored precipitation of ferrous (oxy)hydroxide (Fe(OH)2). I observed this with or without a primary mineral present, suggesting that reactive mineral intermediates are forming. I identified the reactive, metastable phases using Mössbauer spectroscopy and found that Fe(OH)2 formed in the presence of FeCl2 and FeSO4 and green rust-like precipitates formed in the presence of ferrous ammonium sulfate. My findings provide additional evidence that most stable Fe(II) minerals are unable to reduce cDCE at rates sufficiently fast for natural attenuation, if at all. However, transient and metastable Fe(II) intermediates appear to reduce cDCE and may play a role in chlorinated solvent plume degradation.
Magnetite, in the presence of Fe(II) at high pH, was able to partially reduce cDCE and has been shown to reduce other environmental contaminants such as higher chlorinated solvents, nitroaromatics, and metals. Magnetite, which contains both Fe(II) and Fe(III), can act as an environmental geobattery because it has the ability to accept, release, and store electrons during redox reactions, including contaminant reduction in the environment. Rates of contaminant reduction, however, decrease as magnetite stoichiometry (x = [Fe(II)]/[Fe(III)]) decreases. As stoichiometry varies, it becomes difficult to predict the redox behavior of magnetite and Fe(II) in fluctuating environmental conditions because the redox behavior and stoichiometry are both influenced by pH and aqueous Fe(II) conditions. In this work, I evaluated the open-circuit redox potentials of magnetite as a function of stoichiometry, pH, and aqueous Fe(II) concentration. In the presence of Fe(II)aq, I observed Nernstian behavior of the systems and found that redox potentials decrease linearly as stoichiometry, pH, and Fe(II)aq increase within the conditions evaluated (i.e., x = 0 to 0.50, pH 5.5 to 9.0, and 0 to 4.2 mM Fe(II)). I observed Fe(II) uptake by substoichiometric magnetite, which decreased potentials and increased theoretical stoichiometry. To further evaluate the recharge effect of Fe(II) on substoichiometric magnetite, I used Fe(II) sorption, acid dissolution, and Mössbauer spectroscopy to analyze and quantify reduction and transformation of maghemite to magnetite by Fe(II). I found that abiotic reduction and oxidation of magnetite (at near neutral pH) results in similar products independent of the initial solid. My findings suggest that the redox behavior of magnetite can be reasonably predicted by a Nernstian-like model as a function of stoichiometry, pH, and Fe(II)aq.
Details
- Title: Subtitle
- Redox chemistry of iron(II) minerals and their implications for contaminant fate in the environment
- Creators
- Caroline Chelsvig
- Contributors
- Michelle Scherer (Advisor)Drew Latta (Committee Member)David Cwiertny (Committee Member)Jessica Meyer (Committee Member)Johna Leddy (Committee Member)
- Resource Type
- Dissertation
- Degree Awarded
- Doctor of Philosophy (PhD), University of Iowa
- Degree in
- Civil and Environmental Engineering
- Date degree season
- Autumn 2024
- DOI
- 10.25820/etd.007547
- Publisher
- University of Iowa
- Number of pages
- xx, 152 pages
- Copyright
- Copyright 2024 Caroline Elizabeth Chelsvig
- Language
- English
- Date submitted
- 12/09/2024
- Description illustrations
- Illustrations, tables, graphs, charts
- Description bibliographic
- Includes bibliographical references (pages 99-115).
- Public Abstract (ETD)
Clean water is essential for life and much of our existing and potential water supply is contaminated by environmental pollutants that threaten human and environmental health. To protect our water, there is an urgent need to develop efficient, sustainable technologies for pollutant cleanup. Iron (Fe) minerals are abundant in soils and sediments and play major roles in cleaning water and chemical cycling. I studied the behavior of Fe minerals to understand the reactions Fe-minerals undergo in soils and to see if they could help clean large, contaminated groundwater sites.
An emerging sustainable technique for contaminant cleanup uses Fe-containing minerals to naturally break down pollutants. In this work, I tested whether chlorinated ethenes (CEs), a group of common groundwater pollutants, could be broken down by Fe minerals under conditions similar to those found in nature. My results showed that CEs were removed only when conditions favored the formation of ferrous hydroxide, a transient-phase Fe mineral. Using advanced solids characterization, I identified and studied these short-lived Fe phases. My findings indicate that most stable forms of Fe minerals are not effective at breaking down CEs on their own, however transient Fe minerals could help clean up CEs in groundwater.
I also studied magnetite, an Fe-mineral which acts like a battery in soils because it can charge and discharge current (i.e., electrons). The battery-like behavior of magnetite has been shown to remove some environmental pollutants, but little is known about the fundamental thermodynamic behavior of magnetite. To understand how magnetite functions as a geobattery, I measured voltages of magnetite for a range of soil conditions. I was able to describe and predict the recharge-discharge tendency of magnetite under a wide range of conditions using basic thermodynamic models, providing valuable insights into the behavior of magnetite as a geobattery in a variety of environments.
- Academic Unit
- Civil and Environmental Engineering
- Record Identifier
- 9984774959302771
Metrics
6 File views/ downloads
129 Record Views