Alkaline Hydrolysis of the Cyclic Nitramine Explosives RDX, HMX, and CL-20:  New Insights into Degradation Pathways Obtained by the Observation of Novel Intermediates

• Published in: Environmental science & technology Vol. 37; no. 9; pp. 1838 - 1843
• Main Authors: Balakrishnan, Vimal K, Halasz, Annamaria, Hawari, Jalal
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 01.05.2003
• Subjects: Applied sciences; Azocines - chemistry; Biological and physicochemical phenomena; Biological and physicochemical properties of pollutants. Interaction in the soil; Bridged-Ring Compounds - chemistry; Contamination; Earth sciences; Earth, ocean, space; Engineering and environment geology. Geothermics; Environmental cleanup; Environmental Pollutants; Exact sciences and technology; Explosives; Heterocyclic Compounds, 1-Ring - chemistry; Hydrogen-Ion Concentration; Hydrolysis; Natural water pollution; Nitro Compounds - chemistry; Pollution; Pollution, environment geology; Soil and sediments pollution; Soils; Toxicity; Triazines - chemistry; Water pollution; Water treatment and pollution; Reaction intermediate; Nitro compound; Nitroso compound; Pollutant behavior; Nitrogen heterocycle; Chemical degradation; Soil pollution; Ring cleavage; Hydrolysis; Octogen; Hexogen; Reaction mechanism; Explosives; Water pollution; Ground water
• ISSN: 0013-936X; 1520-5851
• DOI: 10.1021/es020959h

Hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX, I) and octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) hydrolyze at pH > 10 to form end products including NO2 -, HCHO, HCOOH, NH3, and N2O, but little information is available on intermediates, apart from the tentatively identified pentahydro-3,5-dinitro-1,3,5-triazacyclohex-1-ene (II). Despite suggestions that RDX and HMX contaminated groundwater could be economically treated via alkaline hydrolysis, the optimization of such a process requires more detailed knowledge of intermediates and degradation pathways. In this study, we hydrolyzed the monocyclic nitramines RDX, MNX (hexahydro-1-nitroso-3,5-dinitro-1,3,5-triazine), and HMX in aqueous solution (pH 10−12.3) and found that nitramine removal was accompanied by formation of 1 molar equiv of nitrite and the accumulation of the key ring cleavage product 4-nitro-2,4-diazabutanal (4-NDAB, O2NNHCH2NHCHO). Most of the remaining C and N content of RDX, MNX, and HMX was found in HCHO, N2O, HCOOH, and NH3. Consequently, we selected RDX as a model compound and hydrolyzed it in aqueous acetonitrile solutions (pH 12.3) in the presence and absence of hydroxypropyl-β-cyclodextrin (HP-β-CD) to explore other early intermediates in more detail. We observed a transient LC-MS peak with a [M−H] at 192 Da that was tentatively identified as 4,6-dinitro-2,4,6-triaza-hexanal (O2NNHCH2NNO2CH2NHCHO, III) considered as the hydrolyzed product of II. In addition, we detected another novel intermediate with a [M−H] at 148 Da that was tentatively identified as a hydrolyzed product of III, namely, 5-hydroxy-4-nitro-2,4-diaza-pentanal (HOCH2NNO2CH2NHCHO, IV). Both III and IV can act as precursors to 4-NDAB. In the case of the polycyclic nitramine 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (CL-20), denitration (two NO2 -) also led to the formation of HCOOH, NH3, and N2O, but neither HCHO nor 4-NDAB were detected. The results provide strong evidence that initial denitration of cyclic nitramines in water is sufficient to cause ring cleavage followed by spontaneous decomposition to form the final products.