Phytophotolysis of Hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) in Leaves of Reed Canary Grass

Abstract

Hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) was degraded in reed canary grass leaves exposed to simulated sunlight to primary products nitrous oxide and 4-nitro-2,4-diazabutanal. This is the first time that 4-nitro-2,4-diazabutanal, a potentially toxic degradate, has been measured in plant tissues following phytotransformation of RDX. These compounds, along with nitrite and formaldehyde, were also detected in aqueous RDX systems exposed to the same simulated sunlight. Results showed that the initial products of RDX photodegradation in translucent plant tissues were similar to products formed from aqueous photolysis of RDX. Combustion analysis of leaves following 14C-RDX uptake and subsequent light exposure revealed the presence of tissue-bound material that could not be extracted with acetonitrile. No detectable formaldehyde was emitted from the leaves. The detection of similar RDX degradation products in both aqueous and plant-based systems suggests that RDX may be initially transformed by similar mechanisms in both systems. Direct photolysis of RDX via ultraviolet irradiation passing into the leaves is hypothesized to be responsible for the observed transformations. In addition, membrane-bound "trap chlorophyll" in the chloroplasts may shuttle electrons to RDX as an indirect photolysis transformation mechanism. Results from this study indicate that reed canary grass facilitates photochemical degradation of RDX, and this mechanism should be considered along with more established phytoremediation processes when assessing the fate of contaminants in plant tissues. Plant-mediated phototransformation of xenobiotic compounds is a process that may be termed "phytophotolysis".

Metabolism of the Aliphatic Nitramine 4-Nitro-2,4-Diazabutanal by Methylobacterium sp. Strain JS178

“…The results reveal the potential for the sequential participation of rhodococci and methylobacteria to effect the complete degradation of RDX. Methylobacterium species might play a major role in phytoremediation of nitramine compounds, especially in view of the recent report that NDAB formed in leaves of hydroponic cultures of reed canary grass contaminated with RDX and exposed to simulated sunlight (15). The role of methylobacteria as endophytes would be particularly relevant.…”

Phytoremediation of Toxic Explosives

“…Based on studies published to date, a working hypothesis for how plants deal with organic chemical contaminants such as RDX and TNT is based on three phases (Harvey et al 1990;Sandermann 1992;Coleman et al 1997;Best et al 1999Best et al , 2005Best et al , 2006Bhadra et al 1999aBhadra et al , b, 2001Larson et al 1999;Hannink et al 2002;Ekman et al 2003Ekman et al , 2005Just and Schnoor 2004;Van et al 2004;Van Dillewijn et al 2008;Rylott and Bruce 2009): phase I (transformation or activation) -a transformation phase of metabolism of the chemical, phase II (conjugation) -conjugation of the chemical contaminant to endogenous hydrophilic molecules to facilitate compartmentalization of the contaminant, and phase III (compartmentation) -movement of the contaminants and breakdown products into vacuoles to reduce their toxicity.…”

Identification of Metabolic Routes and Catabolic Enzymes Involved in Phytoremediation of the Nitro- Substituted Explosives TNT, RDX, and HMX

“…Identified transformation products include hexahydro-1-nitroso-3,5-dinitro-1,3,5-triazine (MNX), 4-nitro-2,4-diazabutanal, formaldehyde, methanol, nitrous oxide, and nitrite Hawari, 2000;Just and Schnoor, 2004;. Unidentified high molecular weight and polar metabolites have also been reported (Best et al, 1997;Larson et al, 1999;Thompson et al, 1999;Just and Schnoor, 2004;. Experiments with [U-…”

“…Active uptake of RDX by both terrestrial and wetland plants is well documented (Harvey et al, 1991;Best et al, 1997;Best et al, 1999;Larson et al, 1999;Thompson, Ramer, and Schnoor, 1999;Bhadra et al, 2001;Price et al, 2002;Just and Schnoor, 2004). Following uptake, RDX is translocated preferentially to the leaves where it undergoes a limited transformation (Harvey et al, 1991;Best et al, 1997;Just and Schnoor, 2004;.…”

“…Following uptake, RDX is translocated preferentially to the leaves where it undergoes a limited transformation (Harvey et al, 1991;Best et al, 1997;Just and Schnoor, 2004;. Identified transformation products include hexahydro-1-nitroso-3,5-dinitro-1,3,5-triazine (MNX), 4-nitro-2,4-diazabutanal, formaldehyde, methanol, nitrous oxide, and nitrite Hawari, 2000;Just and Schnoor, 2004;. Unidentified high molecular weight and polar metabolites have also been reported (Best et al, 1997;Larson et al, 1999;Thompson et al, 1999;Just and Schnoor, 2004;.…”