Redox chemistry of iron(II) minerals and their implications for contaminant fate in the environment
Creators: name: Chelsvig, Caroline
Publisher: name: University of Iowa
Year: 2026
Subjects: Environmental Science

description: 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.
description: 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.
description: 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.