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Dissertation
Redox behavior of magnetite in the environment: moving towards a semiconductor model
University of Iowa
Doctor of Philosophy (PhD), University of Iowa
Autumn 2009
DOI: 10.17077/etd.on05jbdv
Abstract
<p>Magnetite (Fe<sub>3</sub>O<sub>4</sub>) is a commonly found in the environment and can form via several pathways, including biotic and abiotic reduction of Fe<sup>3+</sup> oxides and the oxidation of Fe<sup>2+</sup> and Fe<sup>0</sup>. Despite extensive research, the redox behavior of magnetite is poorly understood. In previous work, the extent and kinetics of contaminant reduction by magnetite varied by several orders of magnitude between studies, two fundamentally different models are used to explain magnetite oxidation (i.e., core-shell diffusion and redox-driven), and reported reduction potentials vary by almost 1 V. In other fields of science (e.g., physics), magnetite stoichiometry (<em>x</em> = Fe<sup>2+</sup>/Fe<sup>3+</sup>) is a commonly measured property, however, in environmental studies, the stoichiometry is rarely measured.</p>
<p>The stoichiometry of magnetite can range from 0.5 (stoichiometric) to 0 (completely oxidized), with intermediate values (0 < <em>x</em> < 0.5) referred to as nonstoichiometric or partially oxidized magnetite. To determine the relationship between magnetite stoichiometry and contaminant fate, the reduction rates of three substituted nitrobenzenes (ArNO<sub>2</sub>) were measured. The kinetic rates varied over five orders of magnitude as the particle stoichiometry increased from <em>x</em> = 0.31 to 0.50. Apparent <sup>15</sup>N kinetic isotope effects (<sup>15</sup>N-AKIE) values for ArNO<sub>2</sub> were greater than unity for all magnetite stoichiometries investigated, and indicated that mass transfer processes are not controlling the reaction rate. To determine if the reaction kinetics were redox-driven, magnetite open circuit potentials (E<sub>OCP</sub>) were measured. E<sub>OCP</sub> values were linearly related to the stoichiometry, with more stoichiometric magnetite having a lower potential, in good agreement with redox-driven models.</p>
<p>The reaction of aqueous Fe<sup>2+</sup> and magnetite was investigated. Similar to previous findings for other Fe<sup>3+</sup> oxides, the formation of a stable sorbed Fe<sup>2+</sup> species was not observed; instead, the sorbed Fe<sup>2+</sup> underwent interfacial electron transfer to form a partially oxidized magnetite phase, which was accompanied by reduction of the underlying magnetite. The lack of a stable sorbed Fe<sup>2+</sup> species on magnetite indicated that the traditional surface complexation model was incorrect; instead, the uptake of Fe<sup>2+</sup> by magnetite appeared to be limited by the whole particle (i.e., the sorbed and underlying phases combined) reaching a stoichiometry of 0.5.</p>
Details
- Title: Subtitle
- Redox behavior of magnetite in the environment: moving towards a semiconductor model
- Creators
- Christopher Aaron Gorski - University of Iowa
- Contributors
- Michelle M. Scherer (Advisor)Edward G. Gillan (Committee Member)Richard L. Valentine (Committee Member)Timothy E. Mattes (Committee Member)Kevin M. Rosso (Committee Member)
- Resource Type
- Dissertation
- Degree Awarded
- Doctor of Philosophy (PhD), University of Iowa
- Degree in
- Civil and Environmental Engineering
- Date degree season
- Autumn 2009
- Publisher
- University of Iowa
- DOI
- 10.17077/etd.on05jbdv
- Number of pages
- xviii, 215 pages
- Copyright
- Copyright © 2009 Christopher Aaron Gorski
- Comment
This thesis has been optimized for improved web viewing. If you require the original version, contact the University Archives at the University of Iowa: https://www.lib.uiowa.edu/sc/contact/.
- Language
- English
- Description bibliographic
- Includes bibliographical references (pages 198-215).
- Academic Unit
- Civil and Environmental Engineering
- Record Identifier
- 9983776952702771
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