New Insights into the Photochemical Degradation of the Insensitive

Dec 15, 2017 - This study describes photolysis of the insensitive munition formulation IMX-101 [2,4-dinitroanisole (DNAN), NQ (nitroguanidine), and 3-...
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New insights into the photochemical degradation of the insensitive munition formulation IMX101 in water Annamaria Halasz, Jalal Hawari, and Nancy N. Perreault Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b04878 • Publication Date (Web): 15 Dec 2017 Downloaded from http://pubs.acs.org on December 16, 2017

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Environmental Science & Technology

New insights into the photochemical degradation of the insensitive munition formulation IMX-101 in water Manuscript ID: es-2017-04878w

Annamaria Halasz,1 Jalal Hawari,2 and Nancy N. Perreault1* 1

National Research Council Canada, 6100 Royalmount Ave, Montreal, Quebec, H4P 2R2, Canada 2

Ecole Polytechnique de Montréal, Department of Civil, Geological and Mining

Engineering, Montreal, Quebec, H3C 3A7, Canada ([email protected])

*Corresponding author: National Research Council Canada 6100 Royalmount Ave. Montreal, Quebec H4P 2R2, Canada E-mail: [email protected] Telephone: (514) 496-6279; Fax: (514) 496-6265

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ABSTRACT

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This study describes photolysis of the insensitive munition formulation IMX-101 [2,4-

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dinitroanisole (DNAN), NQ (nitroguanidine) and 3-nitro-1,2,4-triazol-5-one (NTO)] in

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aqueous solutions using a solar simulating photoreactor. Due to a large variance in the

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water solubility of the three constituents DNAN (276 mg L-1), NQ (5,000 mg L-1) and

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NTO (16,642 mg L-1), two solutions of IMX-101 were prepared: one with low

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concentration (109.3 mg L-1) and another with high concentration (2831 mg L-1). The

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degradation rate constants of DNAN, NQ and NTO (0.137, 0.075 and 0.202 d-1,

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respectively) in the low concentration solution were lower than those of the individually

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photolyzed components (0.262, 1.181 and 0.349 d-1, respectively). In the high

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concentration solution, the molar loss of NTO was 4.3 times higher than that of NQ after

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7 days of irradiation, although NQ was 2 times more concentrated and that NQ alone

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degraded faster than NTO. In addition to the known degradation products, DNAN

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removal in IMX-101 was accompanied by multiple productions of methoxy-

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dinitrophenols, which were not observed during photolysis of DNAN alone. One route

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for the formation of methoxy-dinitrophenols was suggested to involve photo-nitration of

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the DNAN photo-product methoxy-nitrophenol, during simultaneous photo-denitration of

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NQ and NTO in IMX-101. Indeed, when DNAN was photolyzed in the presence of

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15

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1 mass unit, indicating that denitration of DNAN and renitration of products

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simultaneously occurred. As the case with DNAN, we found that guanidine, a primary

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degradation product of NQ, also underwent renitration in the presence of NTO and the

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photocatalyst TiO2. We concluded that the three constituents of IMX-101 can be

NO2-labeled explosive CL-20, we detected methoxy-dinitrophenols with an increase of

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photodegraded in surface water and that fate and primary degradation products of IMX-

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101 can be influenced by the interactions between the formulation ingredients and their

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degradation products.

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Keywords: photolysis, IMX-101, 2,4-dinitroanisole, 3-nitro-1,2,4-triazol-5-one,

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nitroguanidine, insensitive munition

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INTRODUCTION

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The defense industry is currently seeking to replace traditional explosives by

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insensitive munitions (IM) that are less likely to detonate unintentionally. The U.S. Army

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already approved the IM formulation IMX-101 as a safer, and yet equally effective,

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replacement to 2,4,6-trinitrotoluene (TNT).1 2,4-Dinitroanisole (DNAN) and 3-nitro-1,2,4-

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triazol-5-one (NTO) are key components in emerging IM formulations such as IMX-101,

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IMX-104 and PAX-48. Additionally, IMX-101 contains nitroguanidine (NQ) (Table 1) and

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IMX-104 contains hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) commonly used in

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explosive compositions, e.g. Composition B (RDX, TNT and wax).2 While exhibiting

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comparable performance and processability, IMX-101 and IMX-104 are significantly less

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sensitive than TNT and Composition B, making the weapon systems they support safer

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to manufacture, transport and handle. Recent studies show that the insensitive nature of

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the explosive formulation leads to less efficient detonation in some detonation scenarios

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at the detonation point, which drives the need for better studying their fate in natural

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environment.3,4 The superior performance of IMX-101 can be explained by a series of

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complex chemical interactions between the ingredients when the formulation is

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subjected to a well-controlled thermal environment.5 The thermal stability of DNAN was

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found to be lower in IMX-101 than when examined alone, and this was attributed to

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interactions between the ingredients and their degradation products.6

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Table 1. Structure, distribution and water solubility of the IMX-101 constituents Name

Structure

Distribution

Water solubility*

% (w/w)

(mg L-1)

40-45

276

35-40

5,000

18-23

16,642

O NO 2

DNAN

NO 2 NH2

NQ

NTO

O2 N

N

NH 2 H N

O2N

N

O

NH

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*From references Hawari et al., 2015 (DNAN)7; Morris, 2002 (NQ)8; Dontsova et al.,

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2014 (NTO)9

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In nature, unexploded residues of munitions are bound to undergo some sort of biotic or

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abiotic degradation caused by soil microorganisms and plants or hydrolysis and

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photolysis. Biodegradation of IMX-101 in soil, compost and sludge10 and in surface

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soils11 was studied and phytoremediation to treat contaminated soil was reported.12

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Biodegradability of the IMX-101 constituents was also investigated under aerobic and

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anaerobic conditions.13-20 The main biodegradation products were dinitro-phenols15 and

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the amino- and diamino-derivatives from DNAN13,14 and 3-amino-1,2,4-triazol-5-one19

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from NTO.

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Photolysis is one of the major abiotic processes affecting the transformation of

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energetic compounds in waste streams and surface water bodies21 and near the soil

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surface.22 Photolytic transformation can affect the persistence and toxicity of

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contaminants. Photodegradation of individual munition constituents including NTO,

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DNAN, NQ and RDX was previously studied and showed that photodegradation is

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governed by the initial denitration of the nitro-compounds.23-28 On the other hand, little

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information is available on the photodegradation of explosive formulations in water,

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although explosives generally enter the environment as mixtures. Taylor et al.

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investigated photodegradation of DNAN as part of the IM formulations IMX-101, IMX-

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104 and PAX-21 during outdoor dissolution experiments.29 After the rapid dissolution of

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the most soluble NQ and NTO by precipitation, the remaining DNAN was

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photodegraded. The main DNAN degradation products identified in the effluent samples

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were methoxy-nitrophenols and methoxy-nitroanilines. An unknown compound, which

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also appeared during storage of samples in the dark, was suggested to be 2-nitroso-4-

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nitro-methoxybenzene.29 Photodegradation of the other two constituents NQ and NTO

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was not addressed based on the assumption that both can migrate rapidly through

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subsurface soil. A study of the aquatic toxicity of IMX-101 and its constituents, irradiated

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with 350-nm UV light, demonstrated that some photodegradation products were more

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toxic than the parent compounds, e.g., dinitrophenol (from DNAN), guanidine (from NQ)

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and 1,2,4-triazol-3,5-dione (from NTO).30

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While extensive work has been published on products distribution of DNAN under

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various conditions including photolysis, microbial degradation, treatment with ZVI, and

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alkaline hydrolysis, degradation products coming of IMX-101, where DNAN, NTO and

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NQ are present, have not been investigated to the authors’ knowledge. The aim of this

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work was to provide data on the photolysis of IMX-101 in water, at low and high

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concentrations, to address a range of possible environmental scenarios, using artificial

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sunlight generated from a SolSim solar simulating photoreactor. UV-B irradiation in a

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Rayonet photoreactor was also used to accelerate photodegradation and to facilitate

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products identification for determining the effect of interactions among the IMX-101

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ingredients and the fate of the formulation in the aqueous environment.

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MATERIALS AND METHODS

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Chemicals IMX-101, a yellow formulation composed of DNAN (40-45% w/w), NQ

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(35-40% w/w), and NTO (18-23% w/w) was provided by Defense Research and

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Development Canada (DRDC) - Valcartier. NTO (white powder) and DNAN (98.4%,

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yellow powder) were also provided by DRDC-Valcartier. NQ was purchased from

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Aldrich Chemical Co. (Milwaukee, WI) and contained 25% water as a stabilizer. NQ was

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air-dried for 72 h under fume hood prior to use. Stock solution of uniformly nitro-group-

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labeled

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99.4% purity), purchased from ATK Thiokol Propulsion (Brigham City, UT, USA), was

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prepared in acetonitrile to be used as a source of

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explosive degrades at 300 nm via N-NO2 cleavage to produce 4 moles of NO2¯ and

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NO3¯ for each mole of disappearing CL-20.31 Titanium(IV) oxide (rutile nanopowder;

[15NO2]CL-20

(2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane,

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NO2 during photolysis. This

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