Deciphering abiotic and biotic transformation mechanisms of insensitive munitions DNAN and NTO via stable isotope analysis and models
Date
2022
Authors
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Publisher
University of Delaware
Abstract
Deciphering the fates of organic contaminants in natural environments has continuously been advanced by the development, validation, and application of new analytical and modeling approaches. Compound-specific stable isotope analysis (CSIA) and models (e.g., density functional theory (DFT)), beyond their origin in geochemistry and quantum mechanics, have been germinated as one of the most powerful tools in diverse branches of science, with the analytical and modeling results broadly impacting common social issues such as drinking water quality, food authenticity, and forensics. In this dissertation study, I systematically established the methodologies, including a low-temperature derivatization/gas chromatography-isotope ratio mass spectrometry (derivatization/GC-IRMS) for 13C- and 15N-CSIA, position-specific stable isotope evolution (PSIE) by a combined method of CSIA, position-specific stable isotope analysis (PSIA) and DFT, and Bayesian learning for stable isotope fractionating evolution (SIFE), which contributes to not only understanding the abiotic and biotic transformation mechanisms of two insensitive munition compounds 2,4-dinitroanisole (DNAN) and 3-nitro-1,2,4-triazol-5-one (NTO), but also guiding the source identification and in situ remediation strategies of a broad list of organic compounds/contaminants in concern. ☐ A brief background – regarding the insensitive munitions, as well as the development and applications of CSIA, together with my research motivations are presented in Chapter 1. ☐ In Chapter 2, Appendix A and Appendix B, I introduce the development and validation of the modified solid phase extraction/liquid-liquid ultrasound assistant extraction (SPE/LLUAE)/GC-IRMS method for 13C- and 15N-CSIA of DNAN, as well as a low-temperature derivatization/GC-IRMS method for 13C- and 15N-CSIA of NTO. The 13C- and 15N-CSIA methods developed in my dissertation study achieved the nanogram-level method detect limits (MDLs) for both DNAN and NTO, yielding a precision, for both δ13C and δ15N values, of ± 0.2‰ in DNAN and of ± 0.3‰ in NTO, respectively. Given DNAN as a typical nitroaromatic compound and NTO as a surrogate for other heterocyclic compounds, these methods may help to expand the list of environmental contaminants for which multi-element CSIA can be accomplished. ☐ With the question of “which carbon site in DNAN is really attacked during alkaline hydrolysis?”, I firstly carried out the 18O-tracer experiments with bulk 18O isotope analyzed by high temperature conversion/elemental analyzer-isotope ratio mass spectrometry (TC/EA-IRMS) to verify a SN2Ar nucleophilic substitution mechanism for DNAN during alkaline hydrolysis (Chapter 3 and Appendix C). Secondly, with the verified reaction mechanism and three DNAN in-house reference materials – with identical bulk and position specific δ13C values, I proposed a novel combined methodology of CSIA, PSIA by 13C nuclear magnetic resonance (NMR) spectroscopy, and kinetic isotope effect (KIE) modeling by DFT, to track the real-time PSIE in a model compound – in this case, DNAN – at nanogram level in situ (Chapter 4). ☐ Photochemical transformation of nitroaromatic compounds putatively triggers by the excitation at the nitro- (-NO2) group(s), breaking a N-bond – in this case, C-N bond or N-H bond in DNAN and NTO – with the generation of radical(s) at N atom(s) and/or C atom(s). Attracted by these photon-induced femtosecond reactions, I investigate the photo-chemical transformation mechanisms by examining the C and N isotope fractionations in DNAN (Chapter 5) and NTO (Chapter 6) under UV-C (~ 254 nm) and UV-A (~ 350 nm) irradiations, as well as demonstrate the importance of chloride, as reactive halogen species (RHSs) actively interacting with reactive nitrogen and oxygen species (RNSs and ROSs), in the photolysis of NTO (Chapter 6). For DNAN (Chapter 5), I reported the contrasting C and N isotope effects under UV-A (~350 nm) vs. UV-C (~254 nm), which lead to a conclusion that DNAN may act as a photosensitizer and follow product-to-parent reversion. For NTO (Chapter 6), I propose a competitive reattachment of chlorine radical (Cl•) to 1,2,4-triazole-5-one radical (TO•) and the aggressive consumption of nitrite radical (NO2•), blocking the re-nitration pathway that shifts C isotope fractionations in NTO under UV-C (~254 nm) irradiation. These derived C and N isotope enrichment factors and the correlations of C and N isotope fractionations in DNAN and NTO associated with photochemical transformations provide new insights for in situ remediation. ☐ In Chapter 7, an identical and global N/C isotope fractionation correlation value of 10.39 ± 0.13 is discovered during biotic reduction of DNAN by various bacteria strains and consortia, which well differentiates the observed value of 46.66 ± 9.97 associated with the chemical reduction (i.e., natural organic matter (NOM) AQS reduction) of DNAN (Appendix D). The consensus of C and N isotope fractionations indicate that DNAN transformation is being driven by a similar enzymatic pathway for each of the bacterial strains investigated, regardless of redox conditions, indicating the involvement of either an oxygen-insensitive nitro-reductase or possibly the XenB reductase that has recently been shown to reduce trinitrotoluene (TNT) in both the presence and absence of oxygen. ☐ Lastly, as a summary of this dissertation research (Chapter 8), I establish a Standard Operating Procedure (SOP) for DNAN in-house reference materials, as well as the source identification maps for fingerprinting untransformed DNAN in the field. To illustrate the possibility of evaluating the fate of DNAN in natural environments using the deciphered transformation mechanisms, data collected from an outdoor dissolution-photodegradation experiment of IMX101 and IMX104, and an in situ chemical reduction experiment of DNAN in soil columns are plotted with the trajectories of the correlations of carbon and nitrogen dual-element isotopic fractionations (Chapter 8). Bayesian learning theory is further adapted by using the Stable Isotope Mixing Models (SIMMs) in R with simmr to further examine the mixing patterns of identical transformation mechanisms on the individual or grouped individual field samples. The trajectory map and the attempt on developing an isotope fractionation mixing model is a breakthrough step to enable the documentation, and potentially source prediction, of organic compounds in the environment. ☐ This dissertation strives (1) to enrich the isotope database; (2) to enlighten the way of deciphering transformation mechanisms via stable isotope analysis and models; and (3) to provide a new diagnostic measurement tool that may allow the transformation mechanisms, fates, and source identifications of DNAN and NTO, as well as a broad list of organic compounds/contaminants in concern, to be more clearly understood in natural environments.
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Keywords
Biodegradation, Compound-specific stable isotope analysis, Nucleophilic substitution, Organic contaminants, Photolysis, Chemical reduction