Compound Specific Isotope Analysis of Mineral-Mediated Abiotic Reduction of Nitro Compounds

Dr. William Arnold | University of Minnesota

ER-2618

Objective

While it is well established that abiotic processes lead to the degradation of various nitro compounds, including energetic and insensitive munitions, verifying that such processes occur under field conditions remains a challenge. Confirmation of abiotic attenuation requires a method that does not rely on changes in contaminant concentration or detection of reaction products. A method that allows proof of abiotic degradation reactions is compound specific isotope analysis (CSIA). CSIA relies on measuring the changes in the carbon, nitrogen, and hydrogen isotope ratios of the target contaminants along the groundwater (or soil/sediment) gradient. The changes in the isotope ratio are indicative of the transformation process(es) occurring.

The overall objective of this project is to quantify the nitrogen, carbon, and hydrogen isotope fractionation factors of nitro munitions compounds during their abiotic reactions with iron bearing minerals. The project will also test the effectiveness of reaction rate enhancements and their effect on the isotope fractionation.

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Technical Approach

In this project, the team will measure the nitrogen, carbon, and hydrogen isotope fractionation factors that result from the abiotic attenuation of munitions compounds. In this fundamental research project, work will begin by establishing reaction kinetics with well-characterized, synthetic minerals in batch reactors. The role of initial concentration, the presence of organic matter, and multiple cycles of contaminant exposure will be explored. During these experiments, samples will be taken for CSIA analysis. While CSIA methods are available for aromatic compounds, new methods will need to be developed for nitramine munitions. Experiments will then be conducted with natural materials collected from three Department of Defense (DoD) sites where it is known or suspected that abiotic attenuation is occurring. The isotope fractionation measured with the natural materials will be compared to that in the well-characterized systems. The final tasks are to add amendments to restore/enhance reactivity by adding reducing equivalents that regenerate ferrous iron and/or form new mineral phases and to test the CSIA method using groundwater samples from field sites.

ER-2618_project_graphic

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Benefits

If monitored natural attenuation is to be part of a remediation strategy at DoD sites with groundwater contaminated by munitions, stakeholders will demand verification that degradation, and not just sorption or dilution, of contaminants is occurring along groundwater flow paths. Successful completion of this project will provide the DoD, regulatory agency personnel responsible for monitoring remedial efforts, engineering consultants, and the greater scientific community a methodology that demonstrates when abiotic degradation of munitions compounds is occurring in groundwater systems. This analytical method will lead to robust inclusion of engineered or fortuitous abiotic attenuation in conceptual site models. In addition, the work will allow the effectiveness of enhancements to induce, accelerate, or maintain abiotic attenuation to be evaluated. (Anticipated Project Completion - 2021)

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Publications

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.

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.

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.

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.

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Points of Contact

Principal Investigator

Dr. William Arnold

University of Minnesota

Phone: 612-625-8582

Program Manager

Environmental Restoration

SERDP and ESTCP

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