Dissolution Rate, Weathering Mechanics, and Friability of TNT, Comp B, and Octol

ER-1482

Objective

ER-1482 Project Graphic

Schematic showing how this study (shown in pink) fits into the bigger picture of mass loading of HE onto range soils and processes that affect the fate of HE.

At military testing and training ranges, particulate explosives commonly are found on and in soils. A critical problem facing range managers is to determine whether live-fire training is likely to contaminate groundwater under their ranges. One important but poorly understood factor needed for this assessment is the dissolution rates of each high explosive (HE). Dissolution is the rate-limiting step preceding biodegradation or aqueous transport. Understanding how explosive residues weather and break apart when subjected to mechanical force is also critical. Recent studies indicate that the TNT and RDX in Comp B do not dissolve independently, rather the slower dissolution of RDX controls the dissolution of Comp B. These data were successfully modeled, and the dissolution rate now can be predicted given the size of a Comp B particle. The modeling results, however, need to be validated and extended to other explosive compounds.

The objective of this project was to obtain scientific data required to characterize explosive residues that result from commonly used military munitions including TNT, Comp B, and Octol. Comp B and Octol are heterogeneous mixtures of HE. Specifically, this study focused on the dissolution of explosive compounds as a function of particle size, rainfall rate, and temperature. Because particle size strongly influences dissolution rate, weathering effects and environmental factors influencing how energetic compounds break apart were also documented.

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Technical Approach

Well-controlled laboratory experiments and outdoor tests were used to obtain additional dissolution data on Comp B and comparable data on TNT and Octol. The experimental design mimicked field conditions on training ranges, where spatially isolated particles of explosives on the soil surface are dissolved by rainfall. The dissolution data was used to validate a ‘drop impingement’ dissolution model. To study how explosive compounds weather and to measure their friability, a large HE mass was disaggregated. Disaggregation increases the surface area and substantially increases the dissolution rate. The project team measured the change in physical appearance and crushing strength of pieces of explosives by low-order detonations in a semi-arid area and a salt marsh and applied forces were estimated from common processes such as rain impingement, wind transport, and nearby detonations.

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Results

In laboratory tests, water was dripped on mm-sized pieces of TNT, Comp B, Tritonal, and Octol to obtain mass loss versus time curves. The particles took several months to dissolve and a good mass balance (greater than 96%) for the Comp B, TNT, and Tritonal particles was obtained. During the outdoor tests, 3-year dissolution records for TNT, Tritonal, Comp B, and C4 were obtained by collecting and analyzing the precipitation, provided through the use of a rain gauge, interacting with 34 cm-sized HE chunks.

A drop-impingement dissolution model was developed based on laboratory tests and validated using the outdoor test data. The model assumes that raindrops intercepted by HE particles drip off fully saturated in HE. Particle size, HE type, annual rainfall, and average temperature are the key input parameters. A good agreement (root mean square prediction errors of 12-13%) between the measured and predicted 3-year dissolution records was obtained. Given these parameters, the model offers a simple and accurate method to predict aqueous-phase HE influx into range soils.

For a known starting population of HE particles, two processes increase the uncertainty of HE influx estimates: fracture of the HE pieces to create additional surface area and photo-transformation of the HE into compounds not quantified by Method 8330B. Of the 34 chunks placed outside, nine cracked, fourteen spilled mm-sized pieces, and four split; these processes are probably common during the decades-long lifespans of gram chunks of HE. The model accounts for mass loss with time from a chunk, but not the dramatic increase resulting if the chunk splits. The outdoor tests revealed that dissolved TNT mass was only about one-third of the mass lost from the TNT and that Tritonal chunks and dissolved RDX mass was about one-half of the RDX mass lost from Comp B chunks. The formation, dissolution, and transport of phototransformation products were processes inherent to outdoor exposure of explosives. The influx of these products into range soils can exceed that for the explosive itself and thus warrants more attention. Based on this research, 1 g chunks of TNT or Tritonal will require approximately 100 years to dissolve at annual rainfall of 100 cm/yr and 200 years at 50 cm/yr.

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Benefits

Accurate energetic dissolution rates are needed to support the range assessments planned by the military Services, whether they entail modeling or actual sampling combined with modeling. In the absence of such rates, gross estimates based on previously published work will be used. This project’s results will aid efforts to estimate the expected lifespan of large HE masses and their influence on range loads and can support the development of guidance for periodic range clearing. They fill a critical data gap in identifying the parameters necessary for fate and transport analyses.

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Points of Contact

Principal Investigator

Dr. Bonnie Packer

U.S. Army Environmental Command (USAEC)

Phone: 703-607-7977

Program Manager

Environmental Restoration

SERDP and ESTCP

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