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ERIC Number: ED526742
Record Type: Non-Journal
Publication Date: 2009
Pages: 195
Abstractor: As Provided
Reference Count: 0
ISBN: ISBN-978-1-1095-7265-0
ISSN: N/A
Influence of Iron Speciation on Redox Cycling and Reactivity with Persistent Organic Contaminants
Kim, Dongwook
ProQuest LLC, Ph.D. Dissertation, University of Illinois at Urbana-Champaign
Although a number of past studies have been aimed at characterizing iron's redox properties in aqueous systems and its contribution to natural attenuation processes of groundwater contaminants, many questions remain. It is especially important to understand the molecular properties that control the reactivity of both Fe[superscript II] and Fe[superscript III] with oxidizing and reducing agents, respectively. Unfortunately, most studies to date have focused on iron reactions in heterogeneous systems where molecular-level understanding of the reacting Fe species is limited. In this study, Fe[superscript II]/Fe[superscript III] complexes with low molecular weight organic ligands were used as surrogate models for studying iron-mediated redox processes. Initially, the redox reactivity of Fe[superscript II]-organic ligand complexes with nitro-organic explosives was investigated. Ligand-screening experiments demonstrate that organic ligands containing catechol, thiol, and hydroxamate functional groups form Fe[superscript II]-complexes capable of rapidly reducing both N-heterocyclic nitramine explosives and monosubstituted nitrobenzenes. Detailed kinetic investigations show that the reactivity of Fe[superscript II]-organic complexes is significantly dependent on solution conditions (e.g., pH and [Fe[superscript II]]/[organic ligand] ratios). Observed reaction rate constants for contaminant reduction measured in batch reactions (k[subscript obs], s[superscript -1]) increase with ligand concentration and pH when Fe[superscript II] concentration is fixed. Correlation analysis reveals that a single Fe[superscript II] species typically dominates overall Fe[superscript II] reactivity with target compounds (FeL[subscript 2] [superscript 6-] FeHL[superscript 0], and FeL[subscript 3]- for tiron, DFOB, and acetohydroxamic acid, respectively; L = ligand). These species share a common characteristic in that they possess the lowest standard reduction potentials [E[subscript H][superscript 0](Fe[superscript III]/Fe[superscript II])] among possible Fe[superscript II] complexes with each ligand. For nitroaromatic contaminants, linear free energy relationships (LFERs) are observed between species-specific second-order rate constants (k[subscript i]; M[superscript -1] s[superscript -1]) and reduction potentials of the Fe[superscript III]/Fe[superscript II] redox couple, E[subscript H][superscript 0](Fe[superscript III]/Fe[superscript II]), and the nitroaromatic compound, E[subscript H][superscript 1'] (ArNO[subscript 2]). Kinetic studies indicate that some Fe[superscript II]-organic complexes lose their apparent reactivity with contaminants over time, For Fe[superscript II]-hydroxamate complexes, loss of Fe[superscript II] reactivity results from FeII oxidation coupled with reduction of hydroxamate Lewis base groups. The reduction and redox cycling of aqueous Fe[superscript III] complexes with the model siderophore ligand DFOB (desferrioxamine B) in solutions containing a biogenic reducing agent, flavin mononucleotide (FMN), was also investigated. Results of kinetic studies show that Fe[superscript III]-DFOB complexes are reduced to the corresponding Fe[superscript II] complexes by the fully reduced hydroquinone form of FMN (FMN[subscript HQ]) over a wide pH range. Reaction rates are strongly dependent on pH and FMN[subscript HQ] concentration. The observed rate constants for the forward Fe[superscript III] reduction rate (k[subscript f,obs], min[superscript -1]) increase with increasing FMN[subscript HQ] concentration and decreasing pH, the latter trend being opposite to the trend for Fe[superscript II]-DFOB reacting with nitroaromatic contaminants. At higher pH conditions, incomplete Fe[superscript III] reduction is also observed because two reverse processes re-oxidize Fe[superscript II] in the experimental system, autodecomposition of Fe[superscript II]-DFOB complexes (Fe[superscript II] oxidation coupled with hydroxamate ligand reduction) and reaction of Fe[superscript II]-DFOB with the fully oxidized flavin mononucleotide product (FMN[subscript OX]). Although no significant net reduction of Fe[superscript III]-DFOB can be measured at pH 7, formation of ligand autodecomposition products is observed, indirectly indicating that Fe[superscript III]-DFOB reduction is occurring followed by autodecomposition of some portion of the resulting Fe[superscript II]-DFOB complexes. Steady state [Fe[superscript II]]/[Fe[superscript III]] ratios observed at different pH conditions are consistent with both kinetic and equilibrium models developed in this study. Quantitative comparison between kinetic trends and changing Fe speciation reveals that reduced and oxidized FMN species react predominantly with diprotonated Fe[superscript III]- and Fe[superscript II]-DFOB complexes, respectively, where one of the hydroxamate groups is protonated and open coordination positions are available on the central Fe ions. This finding suggests that formation of ternary complex (FMN-Fe-DFOB) formation may be facilitating inner-sphere electron transfers between the flavin and metal center. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com/en-US/products/dissertations/individuals.shtml.]
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Publication Type: Dissertations/Theses - Doctoral Dissertations
Education Level: N/A
Audience: N/A
Language: English
Sponsor: N/A
Authoring Institution: N/A