During firing, propellant residues are scattered onto the soil surface where their energetic compounds can be dissolved by precipitation. The residues, like the unfired propellants, are composed of nitrocellulose imbibed with either 2,4-DNT (single-base), nitroglycerin (NG) (double-base), or NG and nitroguanidine (NQ) (triple-base). Although nitrocellulose is insoluble, 2,4-DNT, NG, and NQ are soluble; 2,4-DNT and NG also are toxic. Consequently, data on how quickly 2,4-DNT, NG, and NQ are dissolved from propellant residues are needed to determine the flux of these compounds to soil. Once in soil solution, the partition coefficient, Kd, and degradation rate, k values, are needed to predict the transport of energetics through the vadose zone and to groundwater.
The objective of this project was to characterize nitrocellulose-based propellant residues that result from firing commonly used military munitions and quantify how quickly 2,4-DNT in single-base propellants, NG in double-base propellants, and NQ in triple-base propellants leached from their nitrocellulose matrices.
The dissolution rates for 2,4-DNT, NG, and NQ were measured for different propellants using laboratory batch and drip tests where no soil was present and soil column studies, which used similar propellant and residues as source terms, to determine partition coefficients and degradation rates. Because the surfaces of propellants and residues may play an important role in dissolution of the energetic constituents, these were studied using both light and electron microscopy.
2,4-DNT was found to be well bound to NC and dissolved out slowly, but both NG and NQ have fast initial dissolution followed by slower mass loss. The amount of NG dissolved is a function of the NG/NC ratio in the propellant, and both the mass loss data and the microscopy results suggested that NG exists as fine liquid droplets within an NC matrix rather than as dispersed molecules. NG droplets near the grain surface are quickly dissolved, and once this layer of liquid NG is depleted, NG diffuses through the NC matrix slowly (~10-14 cm2 s−1). The NQ also dissolves rapidly initially, but quickly mass loss for the NQ becomes smaller than that for NG, despite higher NQ concentrations in the studied triple-base propellants. NQ is added as a crystal during manufacturing and was observed to remain solid in the propellant, so dissolution of the NQ crystal would have to precede its removal by water. Both 2,4-DNT and NG are added as liquids and cannot be distinguished from the NC matrix. Therefore, their distribution and movement within the nitrocellulose matrix is poorly understood, hampering the ability to derive a physically based dissolution model that can predict energetic losses from a variety of propellant types.
Different interactions between 2,4-DNT, NG, NQ, and the soils were seen in both the soil batch and column studies. The 2,4-DNT interacted strongly with soils and had the highest adsorption and transformation rates measured. As a result, no 2,4-DNT was detected in column outflow. NG experienced both adsorption and transformation in the soils, resulting in retardation of the breakthrough curve and decreased concentrations in the outflow. The short half-life of NG in most soils suggests that it should rarely reach groundwater. NQ, on the other hand, does not readily adsorb to soil, and does not degrade or transform. The investigators expect that NQ dissolved from propellants would reach groundwater.
This project filled a critical data gap needed for understanding the fate and transport of propellant energetics. Knowing how propellant residues release their NG, 2,4-DNT, and NQ allows the load of dissolved energetic onto soils to be predicted if the number of rounds fired and amount of residue deposited per round are known.