Dissolution, sorption, and phytoremediation of IMX-101 explosive formulation constituents: 2,4-dinitroanisole (DNAN), 3-nitro-1,2,4-triazol-5-one (NTO), and nitroguanidine
Introduction
The US military is developing safer, insensitive munitions (IM) designed to be less sensitive to shock and sympathetic detonations [1], [2] while remaining as effective as past explosives. The recently approved IMX-101, an IM formulation, will replace TNT in some munitions [3]. IMX-101 contains 2,4-dinitroanisole (DNAN, C7H6N2O5, CAS No. 119-27-7), 3-nitro-1,2,4-trizole-5-one (NTO, C2H2N4O3, CAS No. 932-64-9), and 1-nitroguanidine (NQ, CH4N4O2, CAS No. 556-88-7) [4]. Live fire testing and training activities have resulted in unexploded ordinance (UXO), low-order detonation, and high explosive residues on training ranges [5]. Off-base migration of explosive contaminants can result in base closures or restricted training activities due to federal regulatory actions. Given the estimated future use of IMX-101 in munitions, an understanding of IMX-101 constituent attenuation on testing and training ranges will aid both in determining their expected fate and transport and for the development of remediation strategies. While NQ's environmental fate and transport has been the subject of past investigations, due to its usage in triple-based propellants [6], DNAN and NTO have not been widely utilized, and less is understood with respect to potential environmental attenuation mechanisms.
After release onto testing and training range soils, the IMX-101 constituents may undergo attenuation through processes such as dissolution into water, transport through soil pore water, adsorption onto soil, and natural transformation processes (i.e. photodegradation, plant uptake or transformation, and microbial biodegradation). Chemical and physical properties of compounds can be used to evaluate the potential environmental fate and transport of explosives. Based on the solubility of DNAN, NTO, and NQ (0.28, 49, and 5 g L−1, respectively), only NTO and NQ will readily dissolve into water under environmental conditions [7], [8], [9]. Adsorption of NQ to soils and sediments has been reported [9], [10]. DNAN adsorption has been studied using granular activated carbon [8] and alkali and organosolv lignins [11]; however, no studies have evaluated the potential of DNAN adsorption to soils. Henry's constants (atm m3 mol−1 at 25 °C) of 5.5 × 10−4 and 7 × 10−6 have been reported for DNAN and NQ, respectively [8], [9], suggesting these IMX-101 compounds are non-volatile. Photodegradation of the IMX-101 constituents DNAN and NQ has been reported [12], [13]; however, both resulted in more toxic by-products (2,4-dinitrophenol, guanidine, urea, cyanoguanidine, and nitrite). UV photodegradation of NTO was observed to occur only with the inclusion of TiO2 as a catalyst [14]. After dissolution into water and transport through soil pore water, biological degradation of the IMX-101 constituents may occur. Microbiological degradation has been previously reported for each IMX-101 constituent individually under both oxic and anoxic conditions from a variety of inocula [14], [15], [16], [17], [18].
Low volatility explosives that do not bind irreversibly to soils and are not rapidly degraded by microorganisms may undergo transformation by plants. Phytoremediation, or the use of green plants for in situ treatment of explosives, occurs via four mechanisms. First, phytodegradation occurs when the plants extract the explosives from the soil or groundwater and metabolize these compounds to less toxic products. Second, phytoextraction is the uptake of contaminants into the plant biomass without explosive transformation. Next, rhizodegradation is the breakdown of explosives through microbial activity in soil around the plant roots that is enhanced by root exudates. Last, phytovolatilization is the uptake and transpiration to the atmosphere of contaminants in a modified form. Many types of plants have been investigated for the phytoremediation of explosives including grasses (e.g. fescue, switchgrass, bromegrass), water plants (e.g. parrot feather), and trees (e.g. poplars) [19]. Perennial grass species that are native to testing and training ranges of the temperate regions of the USA are promising candidates for phytoremediation as they are low-growing, fire-resistant and recover rapidly from disruption by heavy equipment. Very limited information is available on the ability of plants to aid in the removal of the IMX-101 constituents from the environment; however, NQ has been shown to be translocated to the leaves of both tall fescue (Festuca arundinacea) and soybean (Glycine max) [20]. It is therefore expected that some level of biological removal of IMX-101 compounds may occur from soils.
The overall goal of this study was to evaluate the potential fate and transformation mechanisms of IMX-101 in the environment. Specifically, studies were conducted to (1) determine the rate of IMX-101 fragment dissolution during simulated rainfall, (2) determine DNAN and NTO soil sorption coefficients, (3) assess the ability of grasses, common to testing and training ranges, to germinate in and phytoremediate IMX-101 contaminated soil, and (4) evaluate the effect of the addition of IMX-101 degrading enrichment cultures on phytoremediation rates.
Section snippets
Chemicals
The IMX-101 formulation containing 40–45% DNAN, 18–23% NTO, and 35–40% NQ was provided by Picatinny Arsenal. Analytical standards for DNAN, NTO, and NQ were provided by BAE Holston.
Particle dissolution experiments
Gravity-fed deionized (DI) water was dropped onto a small fragment (150 mg, approximately 0.85 × 0.8 cm2) of IMX-101 at a height of 15 cm above the particle and an average flow rate of 12 mL h−1. An IMX-101 particle was suspended on a screen mesh above a 4-L flask used to collect the water containing the dissolved IMX-101
Dissolution study
Dissolution of an IMX-101 particle was conducted to evaluate the rate at which the IMX-101 constituents dissolve from UXO and low-order detonations on testing and training ranges during rainfall events. In this study rainfall was simulated over 22 days by drop impingement of 8 L of DI water onto an IMX-101 particle approximately 0.85 × 0.8 cm2 as shown in Fig. 1. Corresponding images of the particle initially and on days 11 and 22 suggest variable dissolution rates of the three compounds in the
Acknowledgements
The authors thank Philip Samuels of Picatinny Arsenal and BAE systems for providing IMX-101, DNAN, NTO, and NQ. The authors also thank Dr. Sierra Talbot, Dr. Brian Popp, and Dr. Karen Buzby of WVU for their help in sample analysis. We acknowledge the use of the WVU Shared Research Facilities.
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