Julio G. Briano

Transport of explosives I: TNT in soil and its equilibrium vapor

Landmine detection is an important task for military operations and for humanitarian demining. Conventional methods for landmine detection involve measurements of physical properties. Several of these methods fail on the detection of modern mines with plastic enclosures. Methods based on the detection signature explosives chemicals such as TNT and DNT are specific to landmines and explosive devices. However, such methods involve the measurements of the vapor trace, which can be deceiving of the actual mine location because of the complex transport phenomena that occur in the soil neighboring the buried landmine. We report on the results of the study of the explosives subject to similar environmental conditions as the actual mines. Soil samples containing TNT were used to study the effects of aging, temperature and moisture under controlled conditions. The soil used in the investigation was Ottawa sand. A JEOL GCMate II gas chromatograph ñ mass spectrometer coupled to a Tunable Electron Energy Monochromator (TEEM-GC/MS) was used to develop the method of analysis of explosives under enhanced detection conditions. Simultaneously, a GC with micro cell 63Ni, Electron Capture Detector (μECD) was used for analysis of TNT in sand. Both techniques were coupled with Solid-Phase Micro Extraction (SPME) methodology to collect TNT doped sand samples. The experiments were done in both, headspace and immersion modes of SPME for sampling of explosives. In the headspace experiments it was possible to detect appreciable TNT vapors as early as 1 hour after of preparing the samples, even at room temperature (20 °C). In the immersion experiments, I-SPME technique allowed for the detection of concentrations as low as 0.010 mg of explosive per kilogram of soil.

Modeling of TNT transport from landmines: numerical approach.

The fate and transport of TNT has been studied as part of a research program to develop chemical sensors for detecting landmines. The spatial and temporal concentration profiles of TNT and its degradation products depend primarily on the mobility of the water phase. The fate and transport of TNT released from a mine determine the location of the maximum concentration of chemicals on the surface. Chemical detection on the other hand may also provide such spatial location, but the actual position of the landmine is not necessary under the surface of that point. Although this hypothesis seems logical, it has never been proven. This paper presents numerical simulations in 3D (using LaGriT and FEHM codes developed at Los Alamos National Laboratories) of the fate and transport of TNT released from a landmine under changing environmental conditions (such as rainfall and solar radiation). To assess the numerical techniques, we compare numerical simulations to experimental data previously obtained.

Effect of environmental parameters on the chemical signature of TNT in soil

As part of a large research program aiming to the development of chemical sensor for detecting landmines, we have studied the fate and transport of TNT subject to different ambient parameters. The space and temporal concentration profiles of TNT, and its degradation compounds have been measured using soil tanks. The following ambient parameters were controlled to emulate environmental factors: water content, temperature, relative humidity, and UV-VIS radiation. A series of soil tanks were kept under controlled conditions for longer than a year and sampled periodically at the surface. After several months, all tanks were sampled vertically and disposed of. Chromatography (GC-&mgr;ECD) with direct injection was used for the analysis of the samples. Of particular interest is the presence of several degradation compounds, as time evolves, responding to the ambient parameters imposed. The vertical concentration profiles of the several chemicals found, gives an interesting view of the degradation process as well as of the transport mechanisms. The results agreed with our computer simulations, and are used to validate previous numerical analyses.

Detection of chemical signatures from TNT buried in sand at various ambient conditions: phase II

New analytical methods have been developed and existing methods have been improved for the detection of explosives and their degradation products by increasing their sensitivity and selectivity. Some of the analytical methods available for detection of explosives and degradation products are gas chromatography, mass spectrometry, high performance liquid chromatography, and gas chromatography with mass spectrometry. This work presents the design and development of the experiments for the detection of the spectroscopic signature of TNT buried in sand and its degradation products. These experiments are conducted using a series of soil tanks with controlled environmental conditions such as: temperature, soil moisture content, relative humidity and radiation (UV and VIS). Gas chromatography and solid-liquid extraction with acetonitrile were used for the analysis of explosives. Sampling of tanks was performed in three points on the surface. The results show that TNT and 2,4-DNT are the main explosives that reach the surface of tanks. Temperature and water content play a most important role in the degradation and diffusion of TNT. Finally, the tanks were disassembled and sampling in deep with the objective to obtain a concentration profile. The results demonstrated that the highest concentration was located at 5 cm from surface.

Spectroscopic signatures of PETN: Part II. Detection in clay

Infrared Spectroscopy is a well established tool for standoff detection of chemical agents in military applications. Vibrational IR spectroscopic analysis can also be used in Chemical Point Detection mode and to the arena of explosives identification and detection when energetic compounds are in contact with soil. PETN is an important nitroaliphatic explosive for military applications. Due to its intrinsic explosive power, it can be used in laminar form or mixed with RDX to manufacture Semtex plastic explosive and in the fabrication of Improvised Explosive Devices (IEDs). This investigation focused on the study of spectroscopic signatures of PETN in contact with soil. For this study, clay was mixed in different proportions with PETN. Detection of the vibrational signatures of PETN constitutes the central part of the investigation. The mixtures were submitted to the effect of water, acid and alkaline solutions, heat and deep UV light (234 nm) in order to establish the effect on these environmental parameters on the vibrational signatures of the explosive in the mixtures. The results reveal that the characteristic bands of PETN are highly persisted, degraded only by extreme conditions of UV radiation and exposure to high temperature for prolonged time. These results could be used in the development of sensitive sensors for detection of landmines, and improvised explosives devices (IDEs).

Density-functional-theory calculations of TNT and its interaction with siloxane sites of clay minerals

2,4,6-trinitrotoluene (TNT) is the most used explosive as main charge in landmines. There have been found contamination of soil and groundwater with munitions residues of TNT due to buried landmines. We are investigating the molecular structure, vibration behavior and the binding energy of TNT with the siloxane surface site of clay minerals in order to determine the spectroscopic signature of TNT in soil. Two different molecular symmetry structures were found with density functional theory (DFT) B3LYP method with 6-31G, 6-31G*, 6-311G, 6-311G*, and 6-311+G** basis sets from the Gaussian 98 systems of programs. Different deformations of the phenyl ring and distortions of the nitro and methyl groups with the ring were observed. In both structures, C1 and Cs, the nitro groups in positions 2 and 6 are out of plane with the phenyl ring due to steric interaction with the methyl group while the nitro group in position 4 is planar to the phenyl ring. The difference between the two structures is the internal rotation of the methyl group and 2, 6-nitro groups. Comparison of the calculated energies of the two structures in several basis sets reveals that the lowest-energy geometry for the TNT structure corresponds to Cs symmetry with B3LYP/6-311+G**. FTIR spectra of TNT are presented and assigned assisted by B3LYP/6-311+G** result. The lowest-energy molecular structure of TNT was interacted with the basal siloxane surface of clay minerals to determine the binding energy (Eb) between them. The binding energy was obtained by optimizing the vertical distance, the rotational and inclination angles between TNT and siloxane surface using the B3LYP hybrid functional with different basis sets.

Spectroscopic signatures of PETN in contact with sand particles

The detection of explosive materials is not only important as an issue in landmines but also for global security reasons, unexploded ordnance, and Improvised Explosive Devices detection. In such areas, explosives detection has played a central role in ensuring the safety of the lives of citizens in many countries. Raman Spectroscopy is a well established tool for vibrational spectroscopic analysis and can be applied to the field of explosives identification and detection. The analysis of PETN is important because it is used in laminar form or mixed with RDX to manufacture Semtex plastic explosive and in the fabrication of Improvised Explosive Devices (IEDs). Our investigation is focused on the study of spectroscopic signatures of PETN in contact with soil. Ottawa sand mixed in different proportions with PETN together with the study of the influence of pH, temperature, humidity, and UV light on the vibrational signatures of the mixtures constitute the core of the investigation. The results reveal that the characteristic bands of PETN are not significantly shifted but rather appear constant with respect of the ubiquitous band of sand (~463 cm-1). These results will make possible the development of highly sensitive sensors for detection of explosives materials and IDEs.

Numerical simulation of the chemical-signature-compounds transport from a mine field

The transport of the chemical signature compounds from buried landmines in a three-dimensional minefield array has been numerically modeled using the finite-volume technique. Compounds such as trinitrotoluene and dinitrotoluene are semi-volatile and somewhat soluble in water; furthermore, they can strongly adsorb to the soil and undergo chemical and biological degradation. Consequently, the spatial and temporal distributions of such chemicals depend on the mobility of the water and gaseous phases, their molecular and mechanical diffusion, adsorption characteristics, soil water content and compaction, and environmental factors. Surface concentrations decrease, when precipitation occurs due to advective flux around the object. Deformation in the concentrations contours after rainfall is observed in the inclined surface case and it is attributed to both: the advective flux, and to the water flux at the surface caused by the inclination. The LaGrit code developed at Los Alamos National Laboratory (LANL) was used to generate the 3D grid array and to place several landmines at different underground positions. The simulations were performed by using the Finite-Element Heat and Mass-transfer code also developed originally at LANL.

Theoretical studies of the molecular structures of dinitrotoluenes and their interactions with siloxane site surface of clays

Among the many different signature compounds emitted from a landmine in the vapor phase, 2,4-dinitrotoluene (2,4-DNT) is the most common nitroaromatic compound in terms of detecting buried landmines, although it is a byproduct in the synthesis of TNT. 2,4-DNT is used as an ingredient in mining explosives and also prevalent on the soil surface but is somewhat seasonally dependent. The B3LYP hybrid functional was used to obtain the lowest-energy structure of both 2,4 and 2,6-DNT. Increasing basis sets from the 3-21G up to the 6-31++G (d, p) are used to predict structural parameters, vibrational frequencies, IR intensities and Raman activities for the explosives molecules. The calculated energies show that the 2,4-dinitrotoluene isomer is more stable than 2,6-dinitrotoluene isomer due to the lesser levels of steric effects between the nitro groups and the methyl group. The optimized structures were interacted with the siloxane site of clay minerals, using the density functional level of theory and the basis sets used to optimize the geometry of the DNT molecules. The binding energy (Eb) between the optimized molecules and the basal siloxane site surface of clay minerals was calculated at distances in a range between 2.5 to 8.5 Å.

3D Numerical simulation of the transport of chemical-signature-compounds from buried landmines

The transport of the chemical signature compounds from buried landmines in a three-dimensional (3D) array has been numerically modeled using the finite-volume technique. Compounds such as trinitrotoluene, dinitrotoluene, and their degradation products, are semi volatile and somewhat soluble in water. Furthermore, they can strongly adsorb to the soil and undergo chemical and biological degradation. Consequently, the spatial and temporal concentration distributions of such chemicals depend on the mobility of the water and gaseous phases, their molecular and mechanical diffusion, adsorption characteristics, soil water content, compaction, and environmental factors. A 3D framework is required since two-dimensional (2D) symmetry may easily fade due to terrain topography: non-flat surfaces, soil heterogeneity, or underground fractures. The spatial and temporal distribution of the chemical-signature-compounds, in an inclined grid has been obtained. The fact that the chemicals may migrate horizontally, giving higher surface concentrations at positions not directly on top of the objects, emphasizes the need for understanding the transport mechanism when a chemical detector is used. Deformation in the concentration contours after rainfall is observed in the inclined surface and is attributed to both: the advective flux, and to the water flux at the surface caused by the slope. The analysis of the displacements in the position of the maximum concentrations at the surface, respect to the actual location of the mine, in an inclined system, is presented.