(86c) SORPTION of Explosive Chemicals to Leaves and Leaf Litters of Tropical Plants FROM Hawaii

Abstract: Several areas in Hawaii contain unexploded ordnance (UXO) as a consequence of military training during the Second World War. This study was conducted to investigate the sorption of UXO chemicals to leaves and to examine their transport in the litter of different plants in Hawaii. The explosives studied, were selected based on their physico-chemical properties, frequency of occurrence in the environmental, and the environmental fate. Batch sorption experiments were conducted with 2,4,6-trinitrotoluene (TNT), 2,4-dinitrotoluene (2,4 DNT), hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX), and 1,3,5,7-tetranitro-1,3,5,7-tetrazocane (HMX)using dried leaves of Casuarina, Araucaria heterophylla, and Eucalyptus and using litter, top soil, leaves and branches, of Casuarina, Araucaria and grass. Saturated column transport experiments were conducted with TNT, 2-4-DNT, RDX, HMX and bromide in the litters of Casuarina, Araucaria and grass. For all the UXO compounds, breakthrough curves were observed later than that for the conservative tracer indicating adsorption to the solid phase. Introduction and Background: Unexploded ordnance (UXO) are military munitions that have been prepared for action, fired, dropped or buried and remain undetonated, posing a hazard to operations, personnel, or material [1]. In the US, the Army alone has estimated that over 1.2 million tons of soil has been contaminated with explosives, and the impact of explosives contamination in other countries is of similar magnitude [2]. The primary release mechanism includes corrosion, leaking, or rupture of the containment system. Once the UXO containment system has been breached, dissolution of the explosive occurs. In the case of rupture and resulting leakage, the explosives may exist as a free product in the soil due to spillage or may be partially contained within the delivery system. After dissolution of the explosives has begun, fate and transport processes, such as advection, dispersion, adsorption-desorption, diffusion, biotic transformation, oxidation-reduction, covalent binding, polymerization, photolysis, infiltration and plant-root uptake, interact with the dissolved contaminant [1]. TNT and DNTs are classified as nitroaromatic explosives having ring structures, whereas RDX and HMX are nitramine explosives possessing N-nitro groups [3]. TNT and DNTs have higher octanol ? water partition coefficients (Kow) values than RDX, suggesting that they are strongly bound to soil organic matter, whereas RDX is mobile due to poor sorption [4]. The environmental fate of these compounds is different. TNT tends to accumulate in plant roots (more then 75%), only a small portion, less than 25% is translocated into the aerial parts of plants. More than 80% of RDX taken up is translocated to aerial tissues. Several areas of Hawaii have significant UXO contamination as a consequence of military training. There is a continuing need for the military to be trained at live-fire ranges, and these military training lands are often located in tropical islands, for example, in Hawaii. The presence of unexploded ordnance and the release of chemical compounds are the main causes of environmental concern. At firing ranges, leaves and litter are particularly susceptible to contamination with energetic compounds such as nitroaromatic and nitroamine explosives. Contaminants present on the surface of leaves and on plant litter can be easily transported by natural means, for example birds, and animals and enter the food chain, soil, and water. Approach / Experimental: Chemical analysis of TNT, RDX, HMX and 2,4-DNT were performed using high performance liquid chromatography (HPLC) according to EPA Method 8330. The mobile phase was 43:57 methanol:water (vol:vol). Bromide breakthrough curves were measured by a Dionex DX-120 ion chromatography (IC) system. Kinetic experiments were conducted to determine the length of time necessary for the UXOs in solution to obtain sorptive equilibrium with samples of leaves or litters. The experiments were performed in batch reactors that were 40 mL glass vials stoppered with Teflon caps. Experiments were performed in triplicate for each kind of leaf and litters tested. For Casuarina leaves, 0.25 g of leaf was combined with a liquid phase of 20 mL of deionized water (1 mM CaCl2) containing the UXO stock solution (concentration of 1 mg/L). After preparation, the reactors were placed on a rotating shaker at 10 rpm in room temperature. The 40 mL glass vials were withdrawn and tested at intervals of: 1, 2, 4, 8, 16, 24 and 48 h. Before analysis, each vial was centrifuged for 30 min at 5000 rpm (G force is 560 units) in order to separate the solid and liquid fractions for testing. Sorption and desorption isotherms for each explosive compound were determined for Casuarina, Araucaria and grass. The experiments were performed as described for the kinetic studies, except that different concentrations (1 mg/L, 0.75 mg/L, 0.5 mg/L, 0.25 mg/L and 0.1 mg/L) of UXOs solution were tested and the aqueous concentration of each was measured after the equilibrium time (16 hr) determined from the kinetic study. For the all the litters, 2 g of litter were mixed with 20 mL of deionized water (0.1 mM CaCl2). The same contact time of 16 hours was used for the three litters. Linear distribution coefficients (Kd) for the sorption of explosive compounds to the uncontaminated leaves and litters were determined for triplicate samples. Three flux-controlled flow-through columns were used in this study. They consisted of 4.70 cm internal diameter by 7.60 cm height stainless steel columns (Alltech HPLC Columns) and caps on the top and the bottom. An aqueous tracer (bromide) and UXO solution was introduced into the columns via a valve in the top. A Series I HPLC Digital Pump Lab Alliance (State College, PA), with Flow Accuracy 3% and Flow Precision 0.5 % RSD, was used to deliver the solution containing UXO on top of each column. Outflow samples were collected continuously into 20-mL vials using an automatic fraction collector. The fraction collector, model Retriever II from Teledyne Isco (Lincoln, NE), and the bottles containing background solution (1 mM CaCl2) and the contaminant solution were placed in a cooler. Low temperature was maintained to slow the degradation of chemicals in the influent and the effluent. Results and Discussion: For sorption of all the UXOs compounds to Casuarina equilibrium was achieved within 12 hours; however, in order to be certain that sorptive equilibrium was achieved during the experiments a contact time of 16 hours was used in all sorption tests performed. The Kd values for all the UXOs compounds were less than 0.5 mL/g for Casuarina, indicating that there was little interaction between the explosives and the leaves. A Kd of 3.2 mL/g was found for Eucalyptus for TNT, which could be partly due to degradation of TNT. No significant value of the linear adsorption coefficient was found in Araucaria. The Kd for all the litters was higher than 1 mL/g for HMX, RDX and 2-4DNT. The Kd values Grass and Araucaria heterophylla were similar, while Casuarina showed higher Kd value. RDX, HMX and 2,4-DNT had the same linear trend for all the litters. TNT degraded in all the litters. Breakthrough of the conservative tracer (bromide) occurred before one pore volume, indicating the absence of preferential pathways for flow. No air entrainment within the column was observed. For RDX and HMX breakthrough curves were observed later than for the conservative tracer indicating adsorption to the solid phase. No breakthrough curves where observed for TNT and DNT. One of the possible reasons is that the flow was stopped early. Summary and Conclusions: The study shows differing sorption behavior of the UXO compounds in litter and in leaves. Adsorption is stronger to litters than leaves. The Kd for HMX, RDX and 2,4-DNT follows the order: Grass > Araucaria heterophylla > Casuarina. The breaktrough curves of the three litters showed similar behavior. Acknowledgements: We acknowledge help of Mr. Joseph Lichwa of the Water Resources Research Center, University of Hawaii for helping in the analysis of the UXO chemicals. References: [[1] Conceptual Model and Process Descriptor Formulations for Fate and Transport of UXO, US Army Corps of Engineers, Technical Report IRRP-99-1 February 1999. [2] Lewis, T.A.; Newcombe, D.A.; Crawford, R.L. Bioremediation of soils contaminated with explosives. J. Environ. Mgmt. 2004, 70 (4), 291?307. [3] Hannink NK et al. (2002) Phytoremediation of explosives. Critical Reviews in Plant Sciences 21 511?538. [4] Jong Moon Yoon, David J. Oliver and Jacqueline V. Shanks Iowa State University Ames, IA