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Munitions Constituents – Photolysis

Munitions compounds (MCs), including 2,4,6-trinitrotoluene (TNT), hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX), 2,4-dinitroanisole (DNAN), 3-nitro-1,2,4-triazol-5-one (NTO), and nitroguanidine (NQ), absorb light in the UV range and are therefore susceptible to photolysis on soil surfaces and in surface water. Photochemical reactions are important to consider when assessing the environmental impact of MCs since they can yield products that differ from their parent compounds in both toxicity and transport behavior. Quantum yield calculations can aid in predicting the photolysis rates and half-lives of MCs. The photolysis of MCs may be enhanced or inhibited in the presence of compounds that are also excited by UV irradiation. Munitions compounds (MCs), including 2,4,6-trinitrotoluene (TNT), hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX), 2,4-dinitroanisole (DNAN), 3-nitro-1,2,4-triazol-5-one (NTO), and nitroguanidine (NQ), absorb light in the UV range and are therefore susceptible to photolysis on soil surfaces and in surface water. Photochemical reactions are important to consider when assessing the environmental impact of MCs since they can yield products that differ from their parent compounds in both toxicity and transport behavior. Quantum yield calculations can aid in predicting the photolysis rates and half-lives of MCs. The photolysis of MCs may be enhanced or inhibited in the presence of compounds that are also excited by UV irradiation.


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Contributor(s): Dr. Warren Kadoya


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Introduction

Insensitive munitions, including IMX-101 and IMX-104, are replacing traditional explosives because they are less prone to accidental detonation and therefore safer for military personnel to handle. IMX-101, composed of 2,4-dinitroanisole (DNAN), 3-nitro-1,2,4-triazol-5-one (NTO), and nitroguanidine (NQ), will replace 2,4,6-trinitrotoluene (TNT) in artillery; IMX-104, composed of DNAN, NTO, and hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX), will replace Composition B (Comp B) in mortars[6]. As both traditional munitions compounds and these insensitive munitions compounds (collectively referred to as MCs) may be deposited onto firing ranges via incomplete detonation, understanding their environmental fate is of concern[7]. Phototransformation due to sunlight exposure is an important fate-controlling parameter for MCs and can occur on the surfaces of solid explosive particles, as shown in Figure 1, as well as in the aqueous phase following MC dissolution by rainwater. Furthermore, MC photolysis can be affected by the presence of natural organic matter and other compounds that are excited by sunlight.

Figure 1. Color change over time as particles of IMX-101 and IMX-104 were exposed to sunlight and rainfall, suggesting phototransformation. Image courtesy of the U.S. Army Engineer Research and Development Center[8].

Direct photolysis

Compounds that absorb ultraviolet (UV, 100-400 nm) and/or visible (Vis, 400-800 nm) light can undergo direct photolysis from sunlight. Light is absorbed in discrete units, or photons, and the energy of these photons is indirectly proportional to the wavelength of the light. When a chemical molecule absorbs a photon, its ground state electrons may become excited. As the excited electrons return to the ground state, they may undergo a chemical reaction that results in transformation. Figure 2 shows that the UV range is further divided into UV-A (315-400 nm), UV-B (280-315 nm), and UV-C (100-280 nm). Because most of UV-C is filtered out by the Earth’s atmosphere, direct photolysis of compounds at the surface primarily involves UV-A and UV-B[9].

Figure 2. The ultraviolet and visible spectrum of sunlight measured at noon in midsummer in Cleveland, Ohio in June 1986. Reproduced with permission from Q-Lab Corporation[9].

The MCs TNT, RDX, DNAN, NTO, and NQ may undergo direct photolysis since they all absorb light in the UV-Vis range (Figure 3). These graphs convey the probability that the compounds will absorb light at a given wavelength. The absorption maxima correspond to one or more electrons transitioning to an excited state.

Figure 3. UV-Vis absorbance spectra for munitions compounds. Spectra over the wavelength range of 200-800 nm were obtained using a Jasco V-630 UV-Vis Spectrophotometer and quartz cuvettes, UV transparent to 200 nm (YeHui Instruments). Adapted from Taylor et al. (2017)[10].

For a chemical bond to break via direct photolysis, a molecule must absorb a photon with higher energy than the energy of the bond. The energy of photons in the UV-Vis range is similar to the bond energies of several single covalent bonds found in organic molecules[1]. Therefore, many of the bonds in TNT, DNAN, NTO, and NQ are susceptible to photolysis from sunlight exposure (Figure 4).

Figure 4. Calculated bond energies (black) and corresponding photon wavelengths (blue, determined from Planck’s equation[1]) required to break the bonds in TNT, DNAN, NTO, and NQ (calculations by Dr. Diego Troya[11]). Energies for bonds in the aromatic and heterocyclic rings are not shown because they exceed the maximum energy of the photons in sunlight reaching the Earth’s surface (500 kJ/mol or 239 nm).

Photolysis products and pathways

The products of the direct photolysis of MCs have been studied in depth, both in photoreactors using UV bulbs emitting light at a given wavelength (Figure 5) or wavelength ranges (including simulated sunlight) and outdoors in natural sunlight. The photolysis of MCs can form mineral products as well as transformation products that may be more toxic than the original compounds.

Figure 5. Example of a UV photoreactor (A) with slots for UV bulbs (B) and a rotating carousel to hold samples (C). Reproduced with permission from The Southern New England Ultraviolet Company.

TNT

The formation of pink and red wastewater released by some ammunition plants led to investigations into the photolysis products of TNT starting in the 1970s. Laboratory experiments observed rapid phototransformation from sunlight in natural waters, some with half-lives less than an hour[12]. Numerous photolysis products of TNT have been reported and include, but are not limited to, 2-amino-4,6-dinitrobenzoic acid, 2,4,6-trinitrobenzaldehyde, 4,6-dinitroanthranil, 2,4,6-trinitrobenzonitrile, 2,4,6-trinitrobenzoic acid, 1,3,5-trinitrobenzene (TNB), 2,4,6-trinitrobenzyl alcohol, and an array of azo and azoxy compounds[2][13][14][15][16].

The mechanism of TNT photolysis is not completely understood as numerous products are formed, many of which are not readily synthesized or purchased[16]. However, evidence suggests that TNT is initially excited to a triplet state by UV light[12]. Figure 6 shows a proposed environmental reaction pathway for TNT that combines photo- and biological reactions based on studies in waste disposal lagoons[15].

Figure 6. Proposed environmental transformation pathway for TNT, including phototransformation and biotransformation reactions. Image courtesy of the U.S. Army Engineer Research and Development Center[15][17].

The aquatic toxicity of phototransformed TNT solutions and ammunition wastewaters was not found to be significantly different from that of untransformed TNT[18][19]. Although TNT phototransformation products tend to be more toxic than TNT, the relatively low product formation coupled with the disappearance of TNT may explain these results.

RDX

RDX photolysis products detected in aqueous systems include nitrite, nitrate, ammonia, formaldehyde, formic acid, formamide, nitrous oxide, and 4-nitro-2,4-diazabutanal[20][21][22][23]. Stable phototransformation products also detected include nitroso compounds MNX (hexahydro-1-nitroso-3,5-dinitro-1,3,5-triazine), DNX (hexahydro-1,3-dinitroso-5-nitro-1,3,5-triazine), and TNX (hexahydro-1,3,5-trinitroso-1,3,5-triazine), which contain one, two, and three nitroso groups each, respectively, in place of the nitro groups. The formation of unsaturated MUX (1,3-dinitro-1,2,3,4-tetrahydro-1,3,5-triazine) was also reported (Figure 7)[3].

Figure 7. Proposed phototransformation mechanism for RDX. Image courtesy of the U.S. Army Engineer Research and Development Center[3].

Peyton et al. (1999) compiled experimental results for RDX photolysis in a proposed reaction mechanism (Figure 7)[3]. The most common first step is the breaking of an N-N bond between a nitro group and a nitrogen atom in the heterocyclic ring, resulting in two radical species (structure (R) in the center and an NO2 radical). They hypothesize that the NO2 radical can become an NO radical that may then recombine with structure (R), giving nitroso product MNX. This process can be repeated to form products DNX and TNX (i.e., via cleavage of the other N-N bonds, as shown in the formation of structure [R2]). The formation of MUX, containing an unsaturation (C=N double bond in the ring) was likely due to the loss of HNO2 from RDX. Other products could include a combination of nitroso groups and unsaturations. The unsaturated compounds, including MUX, are expected to undergo hydrolysis, forming many smaller products (e.g., nitrite and formaldehyde)[3][23]. One of the major products of RDX photolysis is nitrate, a common groundwater pollutant of concern, despite being much less toxic than RDX[21]. The risk posed by the other toxic products should be evaluated, though they are formed in smaller quantities.

DNAN

The products formed from DNAN photolysis include nitrate, nitrite, 2,4-dinitrophenol (DNP), 2-methoxy-5-nitrophenol, 4-methoxy-3-nitrophenol, ammonium, formaldehyde, and formic acid[4][24][25].Rao et al. (2013) proposed a pathway for DNAN photolysis with photooxidation as the primary mechanism (Figure 8)[4]. According to the computational modeling of DNAN bond energies, the C-N bonds and the C-O bond of the methoxy group are most susceptible to photolysis (Figure 4)[26]. This is in agreement with the transformation products formed. Once DNAN is excited to a photo-activated triplet state, OH- in solution can displace either of its nitro groups via an SN2 reaction to form a methoxynitrophenol. This releases nitrite, which can then form nitrate through further photooxidation. DNP could form via O-demethylation of DNAN, which could occur by hydroxylation of the methyl group and the release of formaldehyde[27]. Formaldehyde or formic acid may react with NH2 groups in methoxynitroanilines or aminonitrophenols to produce formamide derivatives[25].

Figure 8. Proposed phototransformation pathways for DNAN. Redrawn from Rao et al. (2013).


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