Munitions Constituents – Photolysis - Revision history

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 *[[Munitions Constituents - Deposition]]
 *[[Munitions Constituents - Dissolution]]
 *[[Munitions Constituents - Sorption]]
 *[[Munitions Constituents - IM Toxicology]]

← Older revision		Revision as of 21:56, 21 February 2025
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−	Munitions compounds (MCs), including [[wikipedia:TNT\|2,4,6-trinitrotoluene]] (TNT), [[wikipedia:RDX\|hexahydro-1,3,5-trinitro-1,3,5-triazine]] (RDX), [[wikipedia:2,4-Dinitroanisole\|2,4-dinitroanisole]] (DNAN), 3-nitro-1,2,4-triazol-5-one (NTO), and [[wikipedia:Nitroguanidine\|nitroguanidine]] (NQ), absorb light in the [[wikipedia:Ultraviolet\|ultraviolet]] (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.<div style="float:right;margin:0 0 2em 2em;">__TOC__</div>	+	[[Munitions Constituents\|Munitions compounds]] (MCs), including [[wikipedia:TNT\|2,4,6-trinitrotoluene]] (TNT), [[wikipedia:RDX\|hexahydro-1,3,5-trinitro-1,3,5-triazine]] (RDX), [[wikipedia:2,4-Dinitroanisole\|2,4-dinitroanisole]] (DNAN), 3-nitro-1,2,4-triazol-5-one (NTO), and [[wikipedia:Nitroguanidine\|nitroguanidine]] (NQ), absorb light in the [[wikipedia:Ultraviolet\|ultraviolet]] (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.<div style="float:right;margin:0 0 2em 2em;">__TOC__</div>

	'''Related Article(s):'''		'''Related Article(s):'''

@@ Line 61: / Line 61: @@
 <br />''NTO''
-Aqueous NTO formed ammonium, nitrate, nitrite, and a urazole intermediate when irradiated with UV light<ref name=":12" /><ref name=":27" /><ref name=":14" /><ref>Schroer, H.W., Londono, E., Li,  X., Lehmler H-J., Arnold, W., and Just, C.L., 2023. Photolysis of 3-Nitro-1,2,4-triazol-5-one: Mechanisms and Products. ACS ES&T Water, 3(3) pp. 783-792.  [https://pubs.acs.org/doi/full/10.1021/acsestwater.2c00567 doi: 10.1021/acsestwater.2c00567][[Special:FilePath/Schroer2023.pdf| Article pdf]]. </ref>. A significant amount of the original NTO mass was not recovered. This may be due, in part, to the volatilization of gaseous species since bubbles formed in the irradiated solutions. A proposed pathway for NTO photolysis is shown in Figure 9<ref name=":12" />. The first step is the breaking of the C-NO<sub>2</sub> bond, which has the lowest bond energy (Figure 4). This forms a nitro radical and a triazolone radical. The nitro radical can then form nitrite and nitrate, and the triazolone radical can form urazole upon reaction with water. Urazole and/or NTO may undergo hydrolysis that breaks open the heterocyclic ring and ultimately forms degradation products such as ammonium and carbon dioxide gas. In aquatic toxicity studies, UV-irradiated NTO was found to be up to 100 times more toxic than non-irradiated NTO<ref name=":13" /><ref name=":14">Moores, L.C., Kennedy, A.J., May, L., Jordan, S.M., Bednar A. J., Jones S.J., Henderson D.L., Gurtowski L., and Gust, K.A., 2020. Identifying degradation products responsible for increased toxicity of UV-degraded insensitive munitions. Chemosphere, 240, 124958. [https://doi.org/10.1016/j.chemosphere.2019.124958. doi: 10.1016/j.chemosphere.2019.124958]</ref>. The specific photolysis products responsible for this increase in toxicity are still being investigated.
+Aqueous NTO formed ammonium, nitrate, nitrite, and a urazole intermediate when irradiated with UV light<ref name=":12" /><ref name=":27" /><ref name=":14" /><ref name=":29">Schroer, H.W., Londono, E., Li,  X., Lehmler H-J., Arnold, W., and Just, C.L., 2023. Photolysis of 3-Nitro-1,2,4-triazol-5-one: Mechanisms and Products. ACS ES&T Water, 3(3), pp. 783-792.  [https://pubs.acs.org/doi/full/10.1021/acsestwater.2c00567 doi: 10.1021/acsestwater.2c00567][[Special:FilePath/Schroer2023.pdf| Article pdf]]. </ref>. A significant amount of the original NTO mass was not recovered. This may be due, in part, to the volatilization of gaseous species since bubbles formed in the irradiated solutions. A proposed pathway for NTO photolysis is shown in Figure 9<ref name=":12" />. The first step is the breaking of the C-NO<sub>2</sub> bond, which has the lowest bond energy (Figure 4). This forms a nitro radical and a triazolone radical. The nitro radical can then form nitrite and nitrate, and the triazolone radical can form urazole upon reaction with water. Urazole and/or NTO may undergo hydrolysis that breaks open the heterocyclic ring and ultimately forms degradation products such as ammonium and carbon dioxide gas. In aquatic toxicity studies, UV-irradiated NTO was found to be up to 100 times more toxic than non-irradiated NTO<ref name=":13" /><ref name=":14">Moores, L.C., Kennedy, A.J., May, L., Jordan, S.M., Bednar A. J., Jones S.J., Henderson D.L., Gurtowski L., and Gust, K.A., 2020. Identifying degradation products responsible for increased toxicity of UV-degraded insensitive munitions. Chemosphere, 240, 124958. [https://doi.org/10.1016/j.chemosphere.2019.124958. doi: 10.1016/j.chemosphere.2019.124958]</ref>. The specific photolysis products responsible for this increase in toxicity are still being investigated.
 ''NQ''
@@ Line 160: / Line 160: @@
 When some compounds absorb photons that promote their electrons to an excited state, they can transfer this energy to nearby molecules instead of being chemically transformed themselves. These compounds are called photosensitizers. MCs can undergo transformation via indirect photolysis in the presence of photosensitizers such as natural organic matter, which contains light-absorbing [[wikipedia:Chromophore|chromophores]] and is found in soil and surface water<ref name=":21">MacCarthy, P., and Suffet, I.H., 1988.  PREFACE, Introduction to Aquatic Humic Substances and Their Influence on the Fate and Treatment of Pollutants. <u>In</u>: I.H. Suffet and P. MacCarthy (eds). Aquatic Humic Substances and Their Influence on the Fate and Treatment of Pollutants. American Chemical Society, Washington, D.C., pp. xiii -xxx. ISBN: 9780841214286/eISBN: 9780841224018 [https://pubs.acs.org/doi/abs/10.1021/ba-1988-0219.pr001 doi: 10.1021/ba-1988-0219.pr001] [[Special:FilePath/MacCarthy1988.pdf| Chapter pdf]]</ref><ref>Schwarzenbach, R.P., Gschwend. P.M., and Imboden, D.M.,2003. Chapter 16 - Indirect Photolysis: Reactions with Photooxidants in Natural Waters and in the Atmosphere. <u>In</u>: R.P. Schwarzenbach, P.M. Gschwend, and D.M. Imboden (eds). Environmental organic chemistry. 2nd ed. John Wiley & Sons, Inc., Hoboken, NJ, pp. 655-686. ISBN: 9780471350538/eISBN: 9780471649649 [https://doi.org/10.1002/0471649643 doi:10.1002/0471649643.ch16]</ref>. This transformation occurs if the photosensitizer reacts directly with the MC or if it forms reactive species, such as [[wikipedia:Hydroxyl_radical|hydroxyl radicals]] and singlet oxygen, that then (in most cases) oxidize the MC. Nitrate, nitrite, and iron-containing complexes are also important photosensitizers in natural waters that generate reactive species<ref>Blough, N.V., and Zepp, R.G., 1995. Reactive Oxygen Species in Natural Waters. <u>In</u>: C.S. Foote, J.S. Valentine, A. Greenberg, J.F. Liebman (eds).  Active Oxygen in Chemistry. Structure Energetics and Reactivity in Chemistry Series (SEARCH Series), Vol 2. Springer, Dordrecht, Netherlands, pp. 280-333. ISBN: 9780751403718/eISBN: 9789400708747 [https://doi.org/10.1007/978-94-007-0874-7_8 doi: 10.1007/978-94-007-0874-7_8]</ref>.
-Environmentally relevant concentrations of [[wikipedia:Humic_substance|humic]] and [[wikipedia:Fulvic_acid|fulvic]] acids either inhibited or had no effect on insensitive MC photolysis rates<ref name=":9" /><ref name=":20" /><ref name=":21" />. However, humic substances were suspected of acting as triplet sensitizers of TNT upon UV irradiation and increased phototransformation rates<ref name=":3" /><ref name=":5" />. Several studies demonstrated that the photolysis of MCs was enhanced in the presence of TiO<sub>2</sub> nanoparticles and [[wikipedia:Fenton's_reagent|Fenton’s reagent]], likely due to the production of hydroxyl radicals<ref name=":9" /><ref>Le Campion, L., Giannotti, C., Ouazzani, J., 1999.  Photocatalytic degradation of 5-nitro-1,2,4-triazol-3-one NTO in aqueous suspention of TiO2. Comparison with fenton oxidation. Chemosphere, 38(7), pp. 1561-1570. [https://doi.org/10.1016/S0045-6535(98)00376-2 doi: 10.1016/S0045-6535(98)00376-2]</ref><ref name=":22">Halasz, A., Hawari, J., and Perreault, N.N., 2018. New Insights into the Photochemical Degradation of the Insensitive Munition Formulation IMX-101 in Water. Environmental Science and Technology, 52(2), pp. 589-596. [https://doi.org/10.1021/acs.est.7b04878 doi: 10.1021/acs.est.7b04878]</ref><ref>Liu, Z., He, Y., Li, F., Liu, Y., 2006. Photocatalytic Treatment of RDX Wastewater with Nano-Sized Titanium Dioxide (5 pp). Environmental Science and Pollution Research, 13(5), pp. 328-332. [https://doi.org/10.1065/espr2006.08.328 doi: 10.1065/espr2006.08.328]</ref><ref>Schmelling, D.C., and Gray, K.A., 1995.  Photocatalytic transformation and mineralization of 2,4,6-trinitrotoluene (TNT) in TiO2 slurries. Water Research, 29(12), pp. 2651-2662. [https://doi.org/10.1016/0043-1354(95)00136-9 doi: 10.1016/0043-1354(95)00136-9]</ref>.
+Environmentally relevant concentrations of [[wikipedia:Humic_substance|humic]] and [[wikipedia:Fulvic_acid|fulvic]] acids either inhibited or had no effect on insensitive MC photolysis rates<ref name=":9" /><ref name=":27" /><ref name=":29" /><ref name=":20" /><ref name=":21" />. However, humic substances were suspected of acting as triplet sensitizers of TNT upon UV irradiation and increased phototransformation rates<ref name=":3" /><ref name=":5" />. Several studies demonstrated that the photolysis of MCs was enhanced in the presence of TiO<sub>2</sub> nanoparticles and [[wikipedia:Fenton's_reagent|Fenton’s reagent]], likely due to the production of hydroxyl radicals<ref name=":9" /><ref>Le Campion, L., Giannotti, C., Ouazzani, J., 1999.  Photocatalytic degradation of 5-nitro-1,2,4-triazol-3-one NTO in aqueous suspention of TiO2. Comparison with fenton oxidation. Chemosphere, 38(7), pp. 1561-1570. [https://doi.org/10.1016/S0045-6535(98)00376-2 doi: 10.1016/S0045-6535(98)00376-2]</ref><ref name=":22">Halasz, A., Hawari, J., and Perreault, N.N., 2018. New Insights into the Photochemical Degradation of the Insensitive Munition Formulation IMX-101 in Water. Environmental Science and Technology, 52(2), pp. 589-596. [https://doi.org/10.1021/acs.est.7b04878 doi: 10.1021/acs.est.7b04878]</ref><ref>Liu, Z., He, Y., Li, F., Liu, Y., 2006. Photocatalytic Treatment of RDX Wastewater with Nano-Sized Titanium Dioxide (5 pp). Environmental Science and Pollution Research, 13(5), pp. 328-332. [https://doi.org/10.1065/espr2006.08.328 doi: 10.1065/espr2006.08.328]</ref><ref>Schmelling, D.C., and Gray, K.A., 1995.  Photocatalytic transformation and mineralization of 2,4,6-trinitrotoluene (TNT) in TiO2 slurries. Water Research, 29(12), pp. 2651-2662. [https://doi.org/10.1016/0043-1354(95)00136-9 doi: 10.1016/0043-1354(95)00136-9]</ref>.
-Co-contaminants can hinder the photolysis of a compound of interest by absorbing some of the incident light. They can also serve as photosensitizers that further transform the compound, in some cases producing additional products. Since MCs are usually released into the environment in formulations containing TNT, RDX, DNAN, NTO, and/or NQ, it is important to determine if there are any photochemical interactions between these compounds. Halasz et al. (2018) reported that DNAN, NTO, and NQ in irradiated IMX-101 all phototransformed at slower rates than the individual irradiated compounds<ref name=":22" /> In addition, methoxydinitrophenols formed when IMX-101 and IMX-104 were irradiated, which were not detected when DNAN was irradiated alone<ref name=":11" /><ref name=":22" />. This may have occurred by the simultaneous photonitration of methoxynitrophenols (formed from DNAN photolysis) and photodenitration of NQ and NTO.
+Co-contaminants can hinder the photolysis of a compound of interest by absorbing some of the incident light. They can also serve as photosensitizers that further transform the compound, in some cases producing additional products. Since MCs are usually released into the environment in formulations containing TNT, RDX, DNAN, NTO, and/or NQ, it is important to determine if there are any photochemical interactions between these compounds. Halasz et al. (2018) reported that DNAN, NTO, and NQ in irradiated IMX-101 all phototransformed at slower rates than the individual irradiated compounds<ref name=":22" /> In addition, methoxydinitrophenols formed when IMX-101 and IMX-104 were irradiated, which were not detected when DNAN was irradiated alone<ref name=":11" /><ref name=":22" />. This may have occurred by the simultaneous photonitration of methoxynitrophenols (formed from DNAN photolysis) and photodenitration of NQ and NTO.When DNAN was irradiated with either NTO or NQ, DNAN phototransformed faster and both NTO and NQ phototransformed slower than when irradiated individually<ref name=":27" />. When NTO and NQ were irradiated together, NTO phototransformed faster and NQ phototransformed slower than when irradiated individually<ref name=":27" />.
 ==Outdoor Phototransformation==

@@ Line 145: / Line 145: @@
 |NQ
 |Simulated sunlight
-|8.8 × 10<sup>–3</sup> <ref>Schroer, H.W., 2018.  Biotransformation and Photolysis of 2,4-Dinitroanisole, 3-Nitro-1,2,4-Triazol-5-One, and Nitroguanidine. Dissertation, Doctor of Philosophy, Civil and Environmental Engineering, The University of Iowa. [https://iro.uiowa.edu/esploro/outputs/9983776982702771?institution=01IOWA_INST&skipUsageReporting=true&recordUsage=false doi:10.17077/etd.rn3l166x]. Dissertation pdf. </ref>
+|8.8 × 10<sup>–3</sup> <ref>Schroer, H.W., 2018.  Biotransformation and Photolysis of 2,4-Dinitroanisole, 3-Nitro-1,2,4-Triazol-5-One, and Nitroguanidine. Dissertation, Doctor of Philosophy, Civil and Environmental Engineering, The University of Iowa. [https://iro.uiowa.edu/esploro/outputs/9983776982702771?institution=01IOWA_INST&skipUsageReporting=true&recordUsage=false doi:10.17077/etd.rn3l166x]. [[Special:FilePath/Schroer2018.pdf| Dissertation pdf]] </ref>
 |-
 |NQ

@@ Line 72: / Line 72: @@
 [[File: PhotolysisFig10.png | thumb |445x445px| left | Figure 10. Proposed phototransformation pathways for NQ. Redrawn from Becher et al. (2019)<ref name=":12" />.]]
 ==Photolysis Kinetics==
-In dilute solutions, the direct photolysis of a compound may be described as a pseudo first-order process, in which the log of the compound concentration decreases linearly with time. In contrast, when the compound concentration is high enough that it absorbs most of the incident light, the photolysis rate no longer depends on the compound concentration, and the process may be described as zero order<ref name=":1" /><ref name=":17">Logan, S.R., 1997. Does a photochemical reaction have a reaction order? Journal of Chemical Education, 74(11), pp.1303. [https://doi.org/10.1021/ed074p1303 doi: 10.1021/ed074p1303]</ref>. First-order kinetics were reported in studies irradiating MCs at 1 mg L<sup>-1</sup>, whereas zero-order kinetics were reported for MCs irradiated at higher initial concentrations<ref name=":9" /><ref name=":15" /><ref name=":3" /><ref name=":16" /><ref name=":20">Moores, L.C., Jones, S.J., George, G.W., Henderson, D.L., and Schutt, T.C., 2020. Photo degradation kinetics of insensitive munitions constituents nitroguanidine, nitrotriazolone, and dinitroanisole in natural waters. Journal of Photochemistry and Photobiology A: Chemistry, 386, 112094. [https://doi.org/10.1016/j.jphotochem.2019.112094 doi: 10.1016/j.jphotochem.2019.112094]</ref>. The phase of an MC can impact its photolysis rate, as aqueous RDX was found to transform significantly faster than solid RDX<ref name=":8" />. In addition, the half-lives of UV-irradiated compounds are highly dependent on sunlight exposure and intensity, which vary with time (of day and year) and location (latitude and altitude). Therefore, it is more useful to describe photolysis in terms of quantum yields rather than reaction rates or half-lives, since they account for differences in experimental setup and irradiance and are more readily comparable<ref name=":17" />.
+In dilute solutions, the direct photolysis of a compound may be described as a pseudo first-order process, in which the log of the compound concentration decreases linearly with time. In contrast, when the compound concentration is high enough that it absorbs most of the incident light, the photolysis rate no longer depends on the compound concentration, and the process may be described as zero order<ref name=":1" /><ref name=":17">Logan, S.R., 1997. Does a photochemical reaction have a reaction order? Journal of Chemical Education, 74(11), pp.1303. [https://doi.org/10.1021/ed074p1303 doi: 10.1021/ed074p1303]</ref>. First-order kinetics were reported in studies irradiating MCs at 1 mg L<sup>-1</sup>, whereas zero-order kinetics were reported for MCs irradiated at higher initial concentrations<ref name=":9" /><ref name=":15" /><ref name=":3" /><ref name=":16" /><ref name=":20">Moores, L.C., Jones, S.J., George, G.W., Henderson, D.L., and Schutt, T.C., 2020. Photo degradation kinetics of insensitive munitions constituents nitroguanidine, nitrotriazolone, and dinitroanisole in natural waters. Journal of Photochemistry and Photobiology A: Chemistry, 386, 112094. [https://doi.org/10.1016/j.jphotochem.2019.112094 doi: 10.1016/j.jphotochem.2019.112094]</ref>. The physical form of an MC can impact its photolysis rate, as aqueous RDX, DNAN, NTO, and NQ were found to transform significantly faster than the corresponding solid MCs<ref name=":8" /><ref name=":27" />. In addition, the half-lives of UV-irradiated compounds are highly dependent on sunlight exposure and intensity, which vary with time (of day and year) and location (latitude and altitude). Therefore, it is more useful to describe photolysis in terms of quantum yields rather than reaction rates or half-lives, since they account for differences in experimental setup and irradiance and are more readily comparable<ref name=":17" />.
 ==Quantum Yields==
@@ Line 92: / Line 92: @@
 |-
 |TNT
-|Simulated  sunlight
+|Simulated sunlight
 |2.6 x 10<sup>-3</sup> <ref name=":5" />
 |-
@@ Line 102: / Line 102: @@
 |312 nm max  (280-315 nm)
 |3.7 x 10<sup>-4</sup> <ref name=":9" />
 |-
 |DNAN
@@ Line 108: / Line 112: @@
 |-
 |DNAN
-|Simulated  sunlight
+|350 nm max  (305-420 nm)
 |1.1 x 10<sup>-4</sup> <ref name=":9" />
 |-
@@ Line 114: / Line 122: @@
 |Sunlight
 |3.2 x 10<sup>-3</sup> <ref name=":20" />
 |-
 |NTO
 |Sunlight
 |5.9 x 10<sup>-5</sup> <ref name=":20" />
 |-
 |NQ

@@ Line 68: / Line 68: @@
 UV-irradiated NQ was found to be orders of magnitude more toxic to aquatic organisms than NQ that was not irradiated<ref name=":6" /><ref name=":13" /><ref name=":14" /><ref name=":23">
-Gust, K.A., Stanley, J.K., Wilbanks, M.S., Mayo, M.L., Chappell P., Jordan, S.M., Moores, L.C., Kennedy, A.J., and Barker, N.D., 2017. The increased toxicity of UV-degraded nitroguanidine and IMX-101 to zebrafish larvae: Evidence implicating oxidative stress. Aquatic Toxicology, 190, pp. 228-245. [https://doi.org/10.1016/j.aquatox.2017.07.004 doi: 10.1016/j.aquatox.2017.07.004]</ref><ref>van der Schalie, W.H.,1985.  The toxicity of nitroguanidine and photolyzed nitroguanidine to freshwater aquatic organisms. U S Army Medical Bioengineering Research and Development Laboratory. [https://apps.dtic.mil/sti/citations/ADA153045 ADA153045] [[Special:FilePath/ADA1985.pdf|Report pdf]]</ref><ref>Moores, L.C., Kennedy, A.J, Rabalais, L., Jones, S.J., George G.W., Zetterholm, S.G., Acrey, B., Amar, S.K., Gust, K.A., 2022. Effect of UV-light exposure duration, light source, and aging on nitroguanidine (NQ) degradation product profile and toxicity. Science of The Total Environment'','' 823, pp.153554, [https://doi.org/10.1016/j.scitotenv.2022.153554 doi:10.1016/j.scitotenv.2022.153554].</ref>. Furthermore, it was responsible for most of the toxicity of irradiated IMX-101. Moores et al. (2020) found that guanidine, nitrite, ammonia, nitrosoguanidine, and cyanide produced from NQ photolysis were each more toxic to ''[[wikipedia:Daphnia_pulex|Daphnia pulex]]'' than NQ, with nitrite and cyanide contributing the most to the toxicity<ref name=":14" />. When adding up the individual toxicities caused by these photolysis products, only 25% of the overall toxicity caused by exposure to irradiated NQ was accounted for. This implied that additional, unidentified products with greater toxicity to ''D. pulex'' formed and/or that exposure to all the products at once created a synergistic toxic effect. Moores et al. (2022) suspect that the NQ photolysis products with the highest toxicity to ''D. pulex'' include nitrosoguanidine, nitrosourea, and hydroxylamine, though further testing is required.
+Gust, K.A., Stanley, J.K., Wilbanks, M.S., Mayo, M.L., Chappell P., Jordan, S.M., Moores, L.C., Kennedy, A.J., and Barker, N.D., 2017. The increased toxicity of UV-degraded nitroguanidine and IMX-101 to zebrafish larvae: Evidence implicating oxidative stress. Aquatic Toxicology, 190, pp. 228-245. [https://doi.org/10.1016/j.aquatox.2017.07.004 doi: 10.1016/j.aquatox.2017.07.004]</ref><ref>van der Schalie, W.H.,1985.  The toxicity of nitroguanidine and photolyzed nitroguanidine to freshwater aquatic organisms. U S Army Medical Bioengineering Research and Development Laboratory. [https://apps.dtic.mil/sti/citations/ADA153045 ADA153045] [[Special:FilePath/ADA1985.pdf|Report pdf]]</ref><ref name=":28">Moores, L.C., Kennedy, A.J, Rabalais, L., Jones, S.J., George G.W., Zetterholm, S.G., Acrey, B., Amar, S.K., Gust, K.A., 2022. Effect of UV-light exposure duration, light source, and aging on nitroguanidine (NQ) degradation product profile and toxicity. Science of The Total Environment'','' 823, pp.153554, [https://doi.org/10.1016/j.scitotenv.2022.153554 doi:10.1016/j.scitotenv.2022.153554].</ref>. Furthermore, it was responsible for most of the toxicity of irradiated IMX-101. Moores et al. (2020) found that guanidine, nitrite, ammonia, nitrosoguanidine, and cyanide produced from NQ photolysis were each more toxic to ''[[wikipedia:Daphnia_pulex|Daphnia pulex]]'' than NQ, with nitrite and cyanide contributing the most to the toxicity<ref name=":14" />. When adding up the individual toxicities caused by these photolysis products, only 25% of the overall toxicity caused by exposure to irradiated NQ was accounted for. This implied that additional, unidentified products with greater toxicity to ''D. pulex'' formed and/or that exposure to all the products at once created a synergistic toxic effect. Moores et al. (2022) suspect that the NQ photolysis products with the highest toxicity to ''D. pulex'' include nitrosoguanidine, nitrosourea, and hydroxylamine, though further testing is required<ref name=":28" />.
 [[File: PhotolysisFig9.png | thumb | 650px | Figure 9. Proposed phototransformation pathways for NTO. Redrawn from Becher et al. (2019)<ref name=":12" />.]]
 [[File: PhotolysisFig10.png | thumb |445x445px| left | Figure 10. Proposed phototransformation pathways for NQ. Redrawn from Becher et al. (2019)<ref name=":12" />.]]

@@ Line 28: / Line 28: @@
 </ref>.]][[wikipedia:Insensitive_munition|Insensitive munitions]], including [[wikipedia:IMX-101|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 [[wikipedia:2,4-Dinitroanisole|2,4-dinitroanisole]] (DNAN), 3-nitro-1,2,4-triazol-5-one (NTO), and [[wikipedia:Nitroguanidine|nitroguanidine]] (NQ), will replace [[wikipedia:TNT|2,4,6-trinitrotoluene]] (TNT) in artillery; IMX-104, composed of DNAN, NTO, and [[wikipedia:RDX|hexahydro-1,3,5-trinitro-1,3,5-triazine]] (RDX), will replace [[wikipedia:Composition_B|Composition B]] (Comp B) in mortars<ref>BAE Systems, 2021. [https://www.baesystems.com/en-us/feature/making-explosives-safer Making explosives safer]</ref>. 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<ref>Pennington, J.C., Silverblatt, B., Poe, K., Hayes, C.A., and Yost, S, 2008. Explosive residues from low-order detonations of heavy artillery and mortar rounds. Soil and Sediment Contamination: An International Journal, 17(5), pp. 533-546. [https://doi.org/10.1080/15320380802306669 doi: 10.1080/15320380802306669]</ref>. 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.
 ==Direct Photolysis==
-[[File: PhotolysisFig3.png | thumb |530x530px| 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)<ref>Taylor, S., Becher, J., Beal, S., Ringelberg, D., Spanggord, R., and Dontsova, K., 2017. Photo-transformation of explosives and their constituents. <u>In</u>: The Environmental Aspects of Munitions Workshop, Joint Army-Navy-NASA-Air Force (JANNAF), Kansas City, MO, May 22, 2017.</ref>.]][[File: PhotolysisFig2.png | thumb |377x377px| Figure 2. The ultraviolet and visible spectrum of sunlight measured at noon in midsummer in Cleveland, Ohio in June 1986. Reproduced with permission from [https://www.q-lab.com/ Q-Lab Corporation]<ref name=":0" />.|left]] Compounds that absorb [[wikipedia:Ultraviolet|ultraviolet]] (UV, 100-400 nm) and/or [[wikipedia:Visible_spectrum|visible]] (Vis, 400-800 nm) light can undergo direct photolysis from sunlight. Light is absorbed in discrete units, or [[wikipedia:Photon|photons]], and the energy of these photons is inversely 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 on the ground primarily involves UV-A and UV-B<ref name=":0">Brennan, P., and Fedor, C., 1994. Sunlight, UV, & accelerated weathering. [https://www.q-lab.com/ Q-Lab Corporation], Technical Bulletin LU-0822. [[Special:FilePath/Brennan1994.pdf| Paper pdf]]</ref>.
+[[File: PhotolysisFig3.png | thumb |530x530px| 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)<ref>Taylor, S., Becher, J., Beal, S., Ringelberg, D., Spanggord, R., and Dontsova, K., 2017. Photo-transformation of explosives and their constituents. <u>In</u>: The Environmental Aspects of Munitions Workshop, Joint Army-Navy-NASA-Air Force (JANNAF), Kansas City, MO, May 22, 2017.</ref>.]][[File: PhotolysisFig2.png | thumb |377x377px| Figure 2. The ultraviolet and visible spectrum of sunlight measured at noon in midsummer in Cleveland, Ohio in June 1986. Reproduced with permission from [https://www.q-lab.com/ Q-Lab Corporation]<ref name=":0" />.|left]] Compounds that absorb [[wikipedia:Ultraviolet|ultraviolet]] (UV, 100-400 nm) and/or [[wikipedia:Visible_spectrum|visible]] (Vis, 400-800 nm) light can undergo direct photolysis from sunlight. Light is absorbed in discrete units, or [[wikipedia:Photon|photons]], and the energy of these photons is inversely 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 on the ground primarily involves UV-A and UV-B<ref name=":0">Brennan, P., and Fedor, C., 1994. Sunlight, UV, & accelerated weathering. [https://www.q-lab.com/ Q-Lab Corporation], Technical Bulletin LU-0822. [[Special:FilePath/Brennan1994.pdf|Article pdf]]</ref>.
 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.
 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 [[wikipedia:Covalent_bond|covalent bonds]] found in organic molecules<ref name=":1" />. Therefore, many of the bonds in TNT, DNAN, NTO, and NQ are susceptible to photolysis from sunlight exposure (Figure 4).

@@ Line 53: / Line 53: @@
 ''DNAN''
-The products formed from DNAN photolysis include nitrate, nitrite, [[wikipedia:2,4-Dinitrophenol|2,4-dinitrophenol]] (DNP), 2-methoxy-5-nitrophenol, 4-methoxy-3-nitrophenol, [[wikipedia:Ammonium|ammonium]], formaldehyde, and formic acid<ref name=":1" /><ref name=":9" /><ref>
+The products formed from DNAN photolysis include nitrate, nitrite, [[wikipedia:2,4-Dinitrophenol|2,4-dinitrophenol]] (DNP), 2-methoxy-5-nitrophenol, 4-methoxy-3-nitrophenol, [[wikipedia:Ammonium|ammonium]], formaldehyde, and formic acid<ref name=":9" /><ref>
-Taylor, S., Walsh, M.E., Becher, J.B., Ringelberg, D.B., Mannes, P.Z., and Gribble, G.W., 2016. Photo-degradation of 2,4-dinitroanisole (DNAN): An emerging munitions compound. Chemosphere, 167, pp.193-203. [https://doi.org/10.1016/j.chemosphere.2016.09.142 doi:10.1016/j.chemosphere.2016.09.142]</ref><ref name=":10">Hawari, J., Monteil-Rivera, F., Perreault, N., Halasz, A., Paquet, L., Radovic-Hrapovic, Z., Deschamps, S., Thiboutot, S., Ampleman, G., 2015.  Environmental fate of 2, 4-dinitroanisole (DNAN) and its reduced products. Chemosphere, 119, pp.16-23. [https://doi.org/10.1016/j.chemosphere.2014.05.047 doi: 10.1016/j.chemosphere.2014.05.047]</ref><ref>Kadoya, W.M., Beal, S.A., Taylor, S., and Dontsova, K., 2024 The effects of physical form, moisture, humic acids, and mixtures on the photolysis of insensitive munitions compounds. Science of The Total Environment, 955, pp. 177255.  [https://doi.org/10.1016/j.scitotenv.2024.177255 doi: 10.1016/j.scitotenv.2024.177255].</ref>.Rao et al. (2013) proposed a pathway for DNAN photolysis with photooxidation as the primary mechanism (Figure 8)<ref name=":9" />. According to the computational modeling of DNAN bond energies, the C-N bonds and the C-O bond of the [[wikipedia:Methoxy_group|methoxy group]] are most susceptible to photolysis (Figure 4)<ref name=":11">Qin, C., Abrell, L., Troya, D., Hunt, E., Taylor, S., and Dontsova, K., 2021. Outdoor dissolution and photodegradation of insensitive munitions formulations IMX-101 and IMX-104: Photolytic transformation pathway and mechanism study. Chemosphere, 280, 130672. [https://doi.org/10.1016/j.chemosphere.2021.130672 doi: 10.1016/j.chemosphere.2021.130672]</ref>. This is in agreement with the transformation products formed. Once DNAN is excited to a photo-activated triplet state, OH<sup>-</sup> in solution can displace either of its nitro groups via an [[wikipedia:SN2_reaction|S<sub>N</sub>2 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 [[wikipedia:Hydroxylation|hydroxylation]] of the [[wikipedia:Methyl_group|methyl group]] and the release of formaldehyde<ref>Studziński. W., Gackowska, A., Przybyłek, M., Gaca, J., 2017. Studies on the formation of formaldehyde during 2-ethylhexyl 4-(dimethylamino)benzoate demethylation in the presence of reactive oxygen and chlorine species. Environmental Science and Pollution Research, 24(9), pp. 8049-8061. [https://doi.org/10.1007/s11356-017-8477-8 doi:10.1007/s11356-017-8477-8] [[Special:FilePath/Studzinski2017.pdf| Article pdf]]</ref>. Formaldehyde or formic acid may react with NH<sub>2</sub> groups in methoxynitroanilines or aminonitrophenols to produce formamide derivatives<ref name=":10" />.
+Taylor, S., Walsh, M.E., Becher, J.B., Ringelberg, D.B., Mannes, P.Z., and Gribble, G.W., 2016. Photo-degradation of 2,4-dinitroanisole (DNAN): An emerging munitions compound. Chemosphere, 167, pp.193-203. [https://doi.org/10.1016/j.chemosphere.2016.09.142 doi:10.1016/j.chemosphere.2016.09.142]</ref><ref name=":10">Hawari, J., Monteil-Rivera, F., Perreault, N., Halasz, A., Paquet, L., Radovic-Hrapovic, Z., Deschamps, S., Thiboutot, S., Ampleman, G., 2015.  Environmental fate of 2, 4-dinitroanisole (DNAN) and its reduced products. Chemosphere, 119, pp.16-23. [https://doi.org/10.1016/j.chemosphere.2014.05.047 doi: 10.1016/j.chemosphere.2014.05.047]</ref><ref name=":27">Kadoya, W.M., Beal, S.A., Taylor, S., and Dontsova, K., 2024 The effects of physical form, moisture, humic acids, and mixtures on the photolysis of insensitive munitions compounds. Science of The Total Environment, 955, pp. 177255.  [https://doi.org/10.1016/j.scitotenv.2024.177255 doi: 10.1016/j.scitotenv.2024.177255].</ref>.Rao et al. (2013) proposed a pathway for DNAN photolysis with photooxidation as the primary mechanism (Figure 8)<ref name=":9" />. According to the computational modeling of DNAN bond energies, the C-N bonds and the C-O bond of the [[wikipedia:Methoxy_group|methoxy group]] are most susceptible to photolysis (Figure 4)<ref name=":11">Qin, C., Abrell, L., Troya, D., Hunt, E., Taylor, S., and Dontsova, K., 2021. Outdoor dissolution and photodegradation of insensitive munitions formulations IMX-101 and IMX-104: Photolytic transformation pathway and mechanism study. Chemosphere, 280, 130672. [https://doi.org/10.1016/j.chemosphere.2021.130672 doi: 10.1016/j.chemosphere.2021.130672]</ref>. This is in agreement with the transformation products formed. Once DNAN is excited to a photo-activated triplet state, OH<sup>-</sup> in solution can displace either of its nitro groups via an [[wikipedia:SN2_reaction|S<sub>N</sub>2 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 [[wikipedia:Hydroxylation|hydroxylation]] of the [[wikipedia:Methyl_group|methyl group]] and the release of formaldehyde<ref>Studziński. W., Gackowska, A., Przybyłek, M., Gaca, J., 2017. Studies on the formation of formaldehyde during 2-ethylhexyl 4-(dimethylamino)benzoate demethylation in the presence of reactive oxygen and chlorine species. Environmental Science and Pollution Research, 24(9), pp. 8049-8061. [https://doi.org/10.1007/s11356-017-8477-8 doi:10.1007/s11356-017-8477-8] [[Special:FilePath/Studzinski2017.pdf| Article pdf]]</ref>. Formaldehyde or formic acid may react with NH<sub>2</sub> groups in methoxynitroanilines or aminonitrophenols to produce formamide derivatives<ref name=":10" />.
 [[File: PhotolysisFig8.png | thumb |440x440px| left | Figure 8. Proposed phototransformation pathways for DNAN. Redrawn from Rao et al. (2013)<ref name=":9" />.]]
@@ Line 61: / Line 61: @@
 <br />''NTO''
-Aqueous NTO formed ammonium, nitrate, nitrite, and a urazole intermediate when irradiated with UV light<ref name=":1" /><ref name=":2" /><ref name=":12" /><ref name=":14" />. A significant amount of the original NTO mass was not recovered. This may be due, in part, to the volatilization of gaseous species since bubbles formed in the irradiated solutions. A proposed pathway for NTO photolysis is shown in Figure 9<ref name=":12" />. The first step is the breaking of the C-NO<sub>2</sub> bond, which has the lowest bond energy (Figure 4). This forms a nitro radical and a triazolone radical. The nitro radical can then form nitrite and nitrate, and the triazolone radical can form urazole upon reaction with water. Urazole and/or NTO may undergo hydrolysis that breaks open the heterocyclic ring and ultimately forms degradation products such as ammonium and carbon dioxide gas. In aquatic toxicity studies, UV-irradiated NTO was found to be up to 100 times more toxic than non-irradiated NTO<ref name=":13" /><ref name=":14">Moores, L.C., Kennedy, A.J., May, L., Jordan, S.M., Bednar A. J., Jones S.J., Henderson D.L., Gurtowski L., and Gust, K.A., 2020. Identifying degradation products responsible for increased toxicity of UV-degraded insensitive munitions. Chemosphere, 240, 124958. [https://doi.org/10.1016/j.chemosphere.2019.124958. doi: 10.1016/j.chemosphere.2019.124958]</ref>. The specific photolysis products responsible for this increase in toxicity are still being investigated.
+Aqueous NTO formed ammonium, nitrate, nitrite, and a urazole intermediate when irradiated with UV light<ref name=":12" /><ref name=":27" /><ref name=":14" /><ref>Schroer, H.W., Londono, E., Li,  X., Lehmler H-J., Arnold, W., and Just, C.L., 2023. Photolysis of 3-Nitro-1,2,4-triazol-5-one: Mechanisms and Products. ACS ES&T Water, 3(3) pp. 783-792.  [https://pubs.acs.org/doi/full/10.1021/acsestwater.2c00567 doi: 10.1021/acsestwater.2c00567]. Article pdf. </ref>. A significant amount of the original NTO mass was not recovered. This may be due, in part, to the volatilization of gaseous species since bubbles formed in the irradiated solutions. A proposed pathway for NTO photolysis is shown in Figure 9<ref name=":12" />. The first step is the breaking of the C-NO<sub>2</sub> bond, which has the lowest bond energy (Figure 4). This forms a nitro radical and a triazolone radical. The nitro radical can then form nitrite and nitrate, and the triazolone radical can form urazole upon reaction with water. Urazole and/or NTO may undergo hydrolysis that breaks open the heterocyclic ring and ultimately forms degradation products such as ammonium and carbon dioxide gas. In aquatic toxicity studies, UV-irradiated NTO was found to be up to 100 times more toxic than non-irradiated NTO<ref name=":13" /><ref name=":14">Moores, L.C., Kennedy, A.J., May, L., Jordan, S.M., Bednar A. J., Jones S.J., Henderson D.L., Gurtowski L., and Gust, K.A., 2020. Identifying degradation products responsible for increased toxicity of UV-degraded insensitive munitions. Chemosphere, 240, 124958. [https://doi.org/10.1016/j.chemosphere.2019.124958. doi: 10.1016/j.chemosphere.2019.124958]</ref>. The specific photolysis products responsible for this increase in toxicity are still being investigated.
 ''NQ''