PHYTOREMEDIATION OF MUNITIONS (RDX, TNT) WASTE BY A HYBRID POPLAR
|P.L. Thompson and J.L. Schnoor, University of Iowa, Department of Civil and Environmental Engineering, 116 Engineering Research Facility, 330 South Madison Street, Iowa City, IA 52242||
The production of munitions containing the explosives RDX (hexahydro-1,3,5-trinitro- 1,3,5triazine) and TNT (2,4,6-trinitro-toluene) has led to the contamination of numerous areas in the United States and worldwide. The Iowa Army Ammunition Plant (IAAP) in Middletown, Iowa is a Superfund site that is required to remediate tens of thousands of cubic yards of soil. Treatment technologies such as incineration can be costly, therefore alternatives such as phytoremediation, which has been proven to be an effective means of treating organic pollutants, are being considered.
The objectives of this research were to 1) determine the transport and fate of RDX and TNT within a hybrid poplar tree Populus deltoides X nigra DN34, and 2) assess the toxicity of these explosives to the plant. These studies evaluated 14C-RDX and 14C-TNT uptake and translocation throughout the plant by using bio-oxidizer and mass balance analyses. The fate of the explosives were determined through tissue extraction with acetone and subsequent analysis with HPLC and radiochromatography. Toxicity to poplar was evaluated by quantifying transpiration and plant growth.
Hydroponic bench-scale experiments were conducted in temperature controlled environments with artificial lighting (300 (moles/m2-s). Pre-rooted poplar cuttings were grown in 250 ml reactors and exposed to various concentrations of TNT or RDX. Daily transpiration and organic chemical uptake were monitored for periods ranging from two weeks to one month. Bench-scale experiments were also done with clean soil obtained from the IAAP. These soils were spiked with either RDX or TNT and plant uptake was assessed. Sorption isotherms were also performed in order to characterize the interaction of RDX and TNT with soil and roots.
The results of this research indicate that RDX and TNT behave quite differently in terms of rate of uptake, translocation, and chemical fate. The concentration of RDX in hydroponic solutions increases whereas TNT concentrations decrease quickly. Once the explosives are in the plant, over 60% of the RDX-related radioactivity is translocated to the leaves in as little as 48 hours while the majority of the TNT-related label is immobilized in the roots during this same period. These differences in translocation between RDX and TNT may be related to the chemical fate of the parent compounds within the plant.
Radio-chromatography analysis of plant extracts revealed that RDX was not significantly transformed whereas TNT transformation was evidenced by the presence of known metabolites such as 4-amino-2,6-dinitrotoluene, 2-amino-4,6-dinitrotoluene and 2,4-diamino-6nitrotoluene in the roots and stem material. A highly polar unknown compound comprised the only observable 14C-compound from TNT in the leaves. Mineralization experiments showed no significant production of 14C02 from either RDX or TNT. Experiments with the highly organic soil from the IAAP verified that the explosives were less bioavailable due to sorption competition.
Transpiration was shown to be a more sensitive indicator of toxicity to the trees than overall growth. Although acute toxicity assays (<14 days) showed the poplar had a significant tolerance to explosives concentrations of 5 mg/L, extended assays indicate that prolonged exposure of such doses adversely affects the survival of the tree.
Key words: phytoremediation, RDX, TNT, transformation
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