Methods are needed to verify that abiotic attenuation of energetic compounds, such as trinitrotoluene and hexahydro-1,3,5-trinitro-1,3,5-triazine, in groundwater is occurring, and it must also be possible to verify that strategies to enhance abiotic processes are having the desired effects. The overall objective of the project was to quantify the isotope fractionation factors of nitro compounds, including new components in insensitive munitions, during their abiotic reactions with iron bearing minerals. The central hypothesis is that abiotic attenuation processes will lead to specific fractionation of carbon (C) and nitrogen (N) contained with the pollutants. Specific objectives were to measure isotope enrichment factors during the abiotic reduction of nitroaromatic and nitramine explosives and assess how solution conditions and multiple reduction cycles influenced these values.

Testing of the overall objective was met by accomplishing the following overall tasks:

1. Mineral synthesis and characterization
2. Kinetic studies with synthetic minerals
3. Compound xpecific isotope analysis (CSIA) method development
4. Kinetic studies with natural materials
5. Testing of regeneration/enhancement of reactivity

CSIA was performed on samples collected from batch and column reactors in Tasks 2, 4, and 5.

Reaction conditions (mineral identity, pH, presence of natural organic matter) influence reaction kinetics, but the isotope fractionation of N and C is unaffected by reaction conditions. Additionally, the fractionation measured with natural materials is similar to that with synthetic materials, and regeneration of reactivity with dithionite also leads to consistent isotope fractionation. Experiments in column reactors and samples from a field site reveal that transport processes make the interpretation of isotope fractionation more difficult, but ancillary data (including the presence of reaction products) allow assessment as to whether reduction of nitroaromatic and nitramine explosives is occurring.

This project has developed the tools necessary to assess if abiotic reduction is occurring in groundwater in situations where natural attenuation or an active remediation technology is being applied. While ancillary data is helpful in making the assessment, CSIA measurements are able to provide information as to 1) whether degradation is occurring, 2) the process responsible for the degradation, and 3) the extent of degradation versus dilution/non-degradative processes leading to concentration decreases. The methodologies will allow more robust information to be collected for evaluation of remediation success by responsible parties, practitioners, and stakeholders. (Project Completion - 2021)

Berens, M.J., B.A. Ulrich, J.H. Strehlau, T.B. Hofstetter, and W.A. Arnold. 2019. Mineral Identity, Natural Organic Matter, and Repeated Contaminant Exposures Do Not Affect the Carbon and Nitrogen Isotope Fractionation of 2,4-Dinitroanisole during Abiotic Reduction. Environmental Science: Process & Impacts, 21:51-62. doi.org/10.1039/C8EM00381E

Berens, M.J., T.B. Hofstetter, T. Bolotin, and W.A. Arnold. 2020. Assessment of 2,4-Dinitroanisole Transformation Using Compound-Specific Isotope Analysis after In Situ Chemical Reduction of Iron Oxides. Environmental Science & Technology, 54:5520-5531. doi.org/10.1021/acs.est.9b07616

Strehlau, J.H., M.J. Berens, and W.A. Arnold. 2018. Mineralogy and Buffer Identity Effects on RDX Kinetics and Intermediates during Reaction with Natural and Synthetic Magnetite. Chemosphere, 213:602–609. doi.org/10.1016/j.chemosphere.2018.09.139

Ulrich, B.A., M. Palatucci, J. Bolotin, J.C. Spain, and T.B. Hofstetter. 2018. Different Mechanisms of Alkaline and Enzymatic Hydrolysis of the Insensitive Munition Component 2,4-Dinitroanisole Lead to Identical Products. Environmental Science & Technology, 5:456-471. doi.org/10.1021/acs.estlett.8b00258