Reduction kinetics of nitroaromatic and munitions compounds with hydroquinones and humic acids
Date
2021
Authors
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Publisher
University of Delaware
Abstract
In the USA, there are more than 126,000 sites of the Department of Defense (DoD), Department of Energy (DoE), and other agencies that are highly contaminated with explosives and chlorinated solvents, with associated remediation costs ranging from $110 billion to $127 billion. Costs are enormous because expensive and inefficient conventional cleanup techniques are used. Since explosives are a major environmental concern threatening access to clean soil and water, it is desirable to find a more cost-effective approach to remediate contaminated sites. ☐ Explosives, also referred to as munitions compounds (MCs), can be transformed through biological and abiotic redox reactions (i.e., electron transfer). While seemingly diverse and complex, most of the redox-active components in soils are iron- and carbon-based. If the role of these soil constituents can be delineated, it might be possible to understand and predict the reduction of MCs in soils, and therefore, harness these naturally occurring reactions as a viable alternative to conventional remediation methods. ☐ This dissertation, as part of a larger project aiming at predicting whole-soil reactivity, investigates the abiotic reduction of nitroaromatics (NACs)/MCs by humic acids (HAs) – a representative class of compounds in dissolved organic matter (DOM) – and employs hydroquinones as model reductants to better understand the factors that control NACs/MCs reactivity. ☐ The reduction of NACs was first investigated to validate hydrogen atom transfer (HAT) Gibbs free energy as an alternative predictor of reduction rate constants. Through this work, linear free energy relationships (LFERs) for six hydroquinone species were constructed, and the accuracy of HAT energy-based models for predicting NAC reduction rate constants was demonstrated. These results represent an advancement in the field as HAT energies can be calculated in silico more accurately than previously proposed predictors. Moreover, they free us from the need for experimentally determined first electron reduction potentials, which are limited. ☐ Rates of reduction of MCs by hydroquinones were then obtained, and electron affinity (EA)- and HAT-based LFERs were established. The results show that despite the structural differences of MCs, their reduction rate constants can be predicted through LFERs using appropriate thermodynamic descriptors. Since the utility of LFERs depends on either measured degradation rates or known thermodynamic descriptors, and both were unknown for some of the MCs investigated, these findings advance our predictive capabilities. ☐ Finally, it was demonstrated that HAs can reductively transform MCs, but the electron utilization was not complete. This was likely due to physical barriers associated with HA conformation that limited MC access to reactive sites. When reduced to the same extent, HAs exhibited comparable reduction potential and similar reactivity toward MCs. These results imply that, despite the structural complexity of HAs, there exist measurable properties that can partially explain the observed reactivity. This is a significant stride towards predicting the role of carbonaceous materials in whole soil reactivity. ☐ For future work, the findings from this dissertation on 1) the reducibility of the MCs as confirmed and predicted through LFERs, and 2) the factors that control DOM reactivity must be evaluated and incorporated into whole soil reactivity models.
Description
Keywords
Dissolved organic matter, Electron affinity, Hydrogen atom transfer, Munitions compounds, Reduction kinetics