Fate of Plant Tissue Associated RDX in Surface Soil

Dr. Charles Reynolds | U.S. Army ERDC/CRREL

ER-1412

Objective

RDX, an explosive used in a variety of munitions and currently found in soils at training ranges, is a contaminant of concern for the Department of Defense (DoD). Low-order and incomplete detonation has resulted in the surface distribution of RDX that is both heterogeneous and widely dispersed. RDX leaching from contaminated surface soils on ranges has been implicated in groundwater contamination. Due to the need for continued training and the hazard of unknown unexploded ordnance on ranges, few remediation strategies are considered safe and applicable to remediate RDX contamination. Two low-cost approaches, natural attenuation and phytoremediation, are increasingly viewed as acceptable treatment technologies; however, a critical concern is the permanence of the remediation. Dissolved RDX has been shown to undergo a series of reductive transformations under sufficiently reduced conditions in soil, yet surface soils are generally well aerated and become wet only ephemerally. As such, surface soils on ranges can serve as an RDX source from which surface and subsurface waters become contaminated. A potential treatment technology ties together phytoremediation and photodegradation in the plant tissue and humification of plant tissue-associated RDX residues that return to the soil following plant senescence. 

The overall objective of this project was to improve the understanding of RDX transformation in plant tissues and the subsequent cycling of tissue-associated RDX and RDX daughter products among soil mineral and humic fractions following plant senescence. The hypothesis was that environmental risks from RDX at military training ranges can be reduced, and possibly eliminated, through a series of coupled processes involving plant uptake, plant enzyme mediated transformation, photodegradation in the plant, and finally humification of plant tissue-associated RDX conjugates into soil organic matter after plant senescence and leaf drop. Although the effect of each individual process may be small, the combined effects of the processes taken as a system for sustainability may have a significant impact on RDX residues on surface soils. If so, they may lead to feasible range sustainability management practices.

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Technical Approach

For uptake and with-leaf fate of RDX, plants within the family Labiatea (genera Mentha and Coleus) were investigated to determine their relative sensitivities to RDX and to determine the uptake rates for RDX for the selected plants. Plants in the family Labiatea have been shown to have high enzyme activities associated with their pigmentation.

The phytophotolysis of 2,4,6-trinitrotoluene (TNT) also was examined using simulated plant cell conditions as the degradation of TNT pollution has application to RDX. The study was made up of two different experimental approaches, including multifactor experiments in aqueous solution and single factor experiments using iceberg lettuce extract.

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Results

Plant tissue with higher chlorophyll content was found to contain higher concentrations of RDX, while the presence of anthocyanin appeared to have no impact. Of the four varieties of mint tested, chocolate mint, a variety of spearmint [Mentha spicata], had significantly lower RDX concentrations in its leaf tissues. Further research is needed to determine what processes are responsible for the reduced RDX content.

Ascorbate, pH, and glutathione (GSH) were found to be statistically significant factors in the photodegradation of TNT, a process applicable to RDX. Ascorbate and pH increased the rate of TNT degradation, whereas GSH inhibited it. Photo-induced degradation of TNT occurs at approximately the same rate in extract-based solution. The results indicate that ascorbate and pH increase the rate of photolysis of TNT, whereas GSH decreases it.

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Benefits

In sufficiently reduced systems, RDX has been shown to attenuate, but the specific reactions and characterization of the residues that are produced have not been completely determined. Recent studies have demonstrated that both bacteria and fungi can also mineralize RDX, but, again, the pathways and intermediates formed are poorly understood. Because precedence has been established for RDX transformation, and explosives have been shown to bind covalently to soil humic fractions or organic material in compost, a humification approach may have significant utility in treating surface soils on impact and training ranges.

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Points of Contact

Principal Investigator

Dr. Charles Reynolds

U.S. Army ERDC/CRREL

Phone: 603-646-4394

Program Manager

Environmental Restoration

SERDP and ESTCP

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