Department Environmental Chemistry

Assessing explosives degradation by compound-specific isotope analysis

Fig.: Thomas Hofstetter
Explosives and their numerous transformation pathways

Contamination of soils and sediments with explosives such as trinitrotoluene (TNT), and cyclic nitramines (RDX) are widespread problems at military training installations, abandoned production facilities, and munition disposal sites. These compounds are not only toxic and potentially carcinogenic but are also transformed to products of even higher toxicity. Assessing the routes of transformation of such explosives is, however, particularly challenging. Explosives were often spilled in large quantities and they are present in different phases as solid residues, sorbed to soil constituents, and in dissolved form. Moreover, NAC transformation can occur by different, often competing, reaction pathways over years and decades.

To provide estimates of explosives degradation through natural attenuation and through remediation by in and ex situ treatment, we have developed methodologies on the basis of compound-specific stable isotope analysis (CSIA). With this approach, we quantify biological and mineral-catalyzed transformation of explosives as well as various structurally related nitroaromatic compounds and selected nitramines through changes of their stable isotope composition. This so-called isotope fractionation allows for studying combinations of reactive processes that occur over long timescales and regardless of whether contaminant concentrations are affected by dilution and sorption. Applications of CSIA from specific studies of microbial explosives degradation to evaluation of (enhanced) natural attenuation is document in our publications. 

Publications

Assessment of 2,4-dinitroanisole transformation using compound-specific isotope analysis after in situ chemical reduction of iron oxides

Ferrous iron-bearing minerals are important reductants in the contaminated subsurface, but their availability for the reduction of anthropogenic pollutants is often limited by competition with other electron acceptors including microorganisms and poor accessibility to Fe(II) in complex hydrogeologic settings. The supply of external electron donors through in situ chemical reduction (ISCR) has been proposed as one remediation approach, but the quantification of pollutant transformation is complicated by the perturbations introduced to the subsurface by ISCR. Here, we evaluate the application of compound specific isotope analysis (CSIA) for monitoring the reduction of 2,4-dinitroanisole (DNAN), a component of insensitive munitions formulations, by mineral-bound Fe(II) generated through ISCR of subsurface material from two field sites. Electron balances from laboratory experiments in batch and column reactors showed that 3.6% to 11% of the total Fe in the sediments was available for the reduction of DNAN and its partially reduced intermediates after dithionite treatment. The extent of DNAN reduction was successfully quantified from its N isotope fractionation measured in the column effluent based on the derivation of a N isotope enrichment factor, ϵN, derived from a comprehensive series of isotope fractionation experiments with numerous Fe(II)-bearing minerals as well as dithionite-reduced subsurface materials. Our observations illustrate the utility of CSIA as a robust approach to evaluate the success of in situ remediation through abiotic contaminant reduction.
Berens, M. J.; Hofstetter, T. B.; Bolotin, J.; Arnold, W. A. (2020) Assessment of 2,4-dinitroanisole transformation using compound-specific isotope analysis after in situ chemical reduction of iron oxides, Environmental Science and Technology, 54(9), 5520-5531, doi:10.1021/acs.est.9b07616, Institutional Repository

Exploring the utility of compound-specific isotope analysis for assessing ferrous iron-mediated reduction of RDX in the subsurface

Subsurface contamination with the explosive hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) at ordnance production and testing sites is a problem because of the persistence, mobility, and toxicity of RDX and the formation of toxic products under anoxic conditions. While the utility of compound-specific isotope analysis for inferring natural attenuation pathways from stable isotope ratios has been demonstrated, the stable isotope fractionation for RDX reduction by iron-bearing minerals remains unknown. Here, we evaluated N and C isotope fractionation of RDX during reduction by Fe(II) associated with Fe minerals and natural sediments and applied N isotope ratios to the assessment of mineral-catalyzed RDX reduction in a contaminant plume and in sediment columns treated by in situ chemical reduction. Laboratory studies revealed that RDX was reduced to nitroso compounds without denitration and the concomitant ring cleavage. Fe(II)/iron oxide mineral-catalyzed reactions exhibited N isotope enrichment factors, ϵN, between -6.3±0.3‰ and -8.2±0.2‰, corresponding to an apparent 15N kinetic isotope effect of 1.04-1.05. The observed variations of the δ15N of ∼15‰ in RDX from groundwater samples suggested an extent of reductive transformation of 85% at an ammunition plant. Conversely, we observed masking of N isotope fractionation after RDX reduction in laboratory flow-through systems, which was presumably due to limited accessibility to reactive Fe(II).
Tong, Y.; Berens, M. J.; Ulrich, B. A.; Bolotin, J.; Strehlau, J. H.; Hofstetter, T. B.; Arnold, W. A. (2021) Exploring the utility of compound-specific isotope analysis for assessing ferrous iron-mediated reduction of RDX in the subsurface, Environmental Science and Technology, 55(10), 6752-6763, doi:10.1021/acs.est.0c08420, Institutional Repository

Different mechanisms of alkaline and enzymatic hydrolysis of the insensitive munition component 2,4-dinitroanisole lead to identical products

The emerging use of 2,4-dinitroanisole (DNAN) in insensitive munitions formulations has caused concern about future contamination of subsurface environments, generating significant interest in understanding and identifying its transformation processes. Here we characterized the C and N isotope fractionation associated with abiotic and biological DNAN hydrolysis through alkaline hydrolysis at high pH as well as enzymatic hydrolysis by Nocardioides sp. JS1661 and partially purified DNAN O-demethylase. Whereas both reactions generated 2,4-dinitrophenol (DNP), compound-specific isotope analysis (CSIA) of DNAN and DNP revealed that these reactions occur by different mechanisms. Alkaline hydrolysis was associated with apparent 13C and 15N kinetic isotope effects (13C-AKIE and 15N-AKIE) of 1.044 ± 0.003 and 1.0027 ± 0.0004, respectively, reflecting the previously postulated nucleophilic aromatic substitution mechanism. Conversely, enzyme-catalyzed DNAN hydrolysis exhibited a 13C-AKIE of 1.027 ± 0.005 and a 15N-AKIE of 1.0032 ± 0.0003. On the basis of these AKIE values and the C and N isotope fractionation of DNP, our results imply that enzymatic O-demethylation of DNAN occurs through a nucleophilic substitution reaction at the aliphatic C of the methoxy group. This work provides a basis for the assessment of DNAN transformation by CSIA, as the C and N isotope fractionation patterns observed in this work are distinct from other hypothesized degradation pathways.
Ulrich, B. A.; Palatucci, M.; Bolotin, J.; Spain, J. C.; Hofstetter, T. B. (2018) Different mechanisms of alkaline and enzymatic hydrolysis of the insensitive munition component 2,4-dinitroanisole lead to identical products, Environmental Science and Technology Letters, 5(7), 456-461, doi:10.1021/acs.estlett.8b00258, Institutional Repository