The Fate of Nitroaromatic Contaminants in Anaerobic Environments: Formation of Coupling Products between Reduced Nitroaromatic Intermediates and Covalent Bonding of Aromatic Amines to Humus Model Compounds
Author
Kadoya, WarrenIssue Date
2020Advisor
Sierra-Alvarez, Maria ReyesField, James A.
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The University of Arizona.Rights
Copyright © is held by the author. Digital access to this material is made possible by the University Libraries, University of Arizona. Further transmission, reproduction, presentation (such as public display or performance) of protected items is prohibited except with permission of the author.Embargo
Release after 06/17/2022Abstract
Nitroaromatic compounds are a class of toxic, synthetic chemicals used in a variety of industries, including explosives, pharmaceuticals, and pesticides. They may enter the environment through wastewater discharge or deposition onto soil surfaces, i.e. on firing ranges. Rainwater can dissolve nitroaromatics and transport them into the subsurface, where they may encounter anaerobic conditions. There, soil microbes and/or reduced minerals may catalyze the reduction of nitroaromatics to aromatic amines via three, two-electron transfers per nitro group. This study builds on previous work on the fate of 2,4-dinitroanisole (DNAN), an insensitive munitions compound that is replacing trinitrotoluene (TNT) in explosives formulations to reduce the risk of accidental detonations. It was found that DNAN formed azo dimers and trimers, which can be more toxic than nitroaromatic compounds, when incubated in anaerobic sludge and soil. Furthermore, 14C-radiolabeled DNAN became immobilized to the insoluble fraction of soil organic matter (humus) in soil incubations, which was enhanced under reducing conditions. This could be used as a strategy to “remove” nitroaromatics from the subsurface environment. The objective of this work is to understand the mechanisms that caused DNAN to form azo compounds and become incorporated into humus in anaerobic incubations. The hypothesis was that once DNAN became reduced biologically (catalyzed by microbes), abiotic nucleophilic substitution reactions occurred, either between reduced intermediates of DNAN to form azo compounds or between humic moieties, such as quinones, and DNAN-derived aromatic amines to form “bound residues.” These reactions were originally thought to take place only under aerobic conditions, with aromatic amines forming free radical species. We conducted biological incubations of 4-nitroanisole in anaerobic granular sludge and chemical pairing experiments between reduced 4-nitroanisole intermediates, 4-nitrosoanisole and 4-aminoanisole, and between aromatic amines, including those resulting from DNAN reduction, and model quinone compounds. Using ultra-high performance liquid chromatography, UV-Vis spectroscopy, and mass spectrometry, we studied reactant disappearance and product formation. In anaerobic sludge incubations of 4-nitroanisole, azo dimer 4,4ʹ-dimethoxyazobenzene formed but was subsequently reductively cleaved. Hypothesizing that the formation of this azo dimer was due to the chemical coupling of 4-nitroanisole reduced intermediates 4-nitrosoanisole and 4-aminoanisole, we incubated these compounds in abiotic, anoxic conditions. Although 4,4ʹ-dimethoxyazobenzene formed, the major product was 4-methoxy-4ʹ-nitrosodiphenylamine, another coupling product. We studied the toxicity of these two products to Aliivibrio fischeri and found that they were orders of magnitude more toxic than reactants 4-nitrosoanisole and 4-aminoanisole. In studies chemically pairing aromatic amines, which would result from nitroaromatic reduction, with quinone compounds that model humus, we detected the formation and accumulation of covalently-bonded Michael adducts and imines resulting from nucleophilic addition pathways. These results provide insight into the mechanisms through which DNAN molecules both bind to each other and to quinone moieties present in humus as they are sequentially reduced to aromatic amines. To prevent the accumulation of toxic coupling products and enhance immobilization to humus on sites contaminated with nitroaromatics, anaerobic conditions should be created, along with the addition of electron donor and organic carbon amendments to promote reducing conditions, which both cleave coupling products and generate aromatic amines, and increase humus content, respectively.Type
textElectronic Dissertation
Degree Name
Ph.D.Degree Level
doctoralDegree Program
Graduate CollegeEnvironmental Engineering