Fate of Nerve Agent Tabun in Concrete and Soil: Evaporation and Decontamination

Abstract

This study reports on evaporation and decontamination of the nerve agent tabun from common civil surfaces, concrete and soil. Evaporation of various agent drop sizes was measured using a laboratory-sized wind tunnel and thermal desorber in combination with gas chromatography. Samples were periodically analyzed to determine the half-life of tabun at various temperatures, with and without a decontamination process. Results showed that in concrete, a drop of the agent was rapidly absorbed and spread into the matrix, while evaporating. In soil, a drop of the agent remained on the surface without spreading, while slowly evaporating. Not only does tabun remain longer in soil than in concrete but also the time required for evaporation was prolonged for larger drops. On the other hand, drops of the agent absorbed into the matrices remained for times after decontamination, following a pseudo first-order rate of degradation and decontamination. Estimated half-lives of tabun were 2 days in concrete and 5 days in soil at 25°C, which were shortened to 1.1 days in concrete and 1.8 days in soil at 50°C. The half-lives were significantly shortened after decontamination, to 4.6, 1.2, and 0.7 min in concrete and 5.9, 2.5, and 1.0 min in soil at 25°C, 35°C, and 50°C, respectively.

Introduction

The Chemical Weapons Convention (CWC) banned the development, production, and use of the chemical warfare agents (CWAs) in 1997. However, the agreement was not ratified by all countries, and stockpiles of CWAs still exist in several countries, thereby continuing to pose a threat to the military and to civilians (Jung and Lee, 2014). In particular, in big cities with high population and housing densities, a terroristic attack involving CWAs could yield a great number of casualties and vast economic loss.

CWAs can exist as solids, liquids, or gases. Most liquids and solids can be prepared in aerosol or smoke for rapid dispersion over a large area. After release of the agent, a suspension of smaller droplets will form a primary cloud, which under favorable atmospheric conditions may spread over large distances. Large droplets will tend to remain at or close to the point of release, contaminating the immediate surroundings. In case of surface contamination, the physical and chemical properties of the CWA will influence the contact time and the mechanism of spreading over and into the contaminated surface. A substantial amount of work has been published on the fate of CWAs on the natural and built environment, largely as a result of military research (Wagner et al., 2004; Tang et al., 2006; Brevett et al., 2009; Columbus et al., 2018; Jung and Choi, 2018). A large number of the reports available have focused on the fates of mustard agent (bis(2-chloroethyl)sulfide [HD]) or venomous agent X (VX, O-ethyl S-2-(N,N-diisopropylamino)ethyl methylphosphonothioate) in the natural outdoor environment, and their influences over material (Mizrahi and Columbus, 2005; Mizrahi et al., 2011; Columbus et al., 2012). Concurrently, considerably less research has been published in the open literature regarding the fate of tabun on common civil surfaces.

Tabun (O-ethyl-N,N-dimethylamidocyanophosphate, also known as GA in military terminology) is an extremely toxic chemical agent classified as a nerve agent, which fatally interferes with the normal functioning of the nerves (Vesela et al., 2006; Barba-Bon et al., 2015). Although it was originally developed as a pesticide in Germany in 1936, tabun was used in chemical warfare during the Iran-Iraq War in the 1980s (Greenfield et al., 2002). As a man-made CWA, it still has potential to be used as a chemical weapon by terrorists, especially in urban environments.

When a liquid droplet of tabun is deposited on a surface, it will rest on the surface in the form of splashes or droplets. The size (volume) and distribution of the droplet are largely dependent on the physicochemical properties of the agent itself and the material of the surface. The droplet may then be absorbed, evaporate, undergo a chemical change (a reaction with the surface or with the ambient atmosphere, e.g. hydrolysis), or all the above (Bartelt-Hunt et al., 2006). In comparison to other notorious nerve agents, tabun is more volatile than VX, but less volatile than sarin. Therefore, it can either remain on the exposed surfaces or evaporate back into the air for some period of time. Even though tabun in a liquid form evaporates, it will eventually sink to low-lying areas because its vapor is denser than air, thereby posing a greater exposure hazard in such areas.

In this study, we investigated various aspects of the physical and chemical interactions of tabun with surfaces, and their influence on persistence and fate of tabun. Evaporation was measured with a laboratory-sized wind tunnel, wherein samples were contained within a controlled environment and monitored by video analysis to document changes to the agent. The half-lives of tabun deposited on concrete and soil were examined at operationally relevant temperatures with and without the use of a decontamination process. The surfaces tested were concrete and soil, which were chosen because of their widespread occurrence in building foundations, sidewalks, and so on.

Experimental

Materials

Tabun, soman (O-pinacolyl-methylphosphonofluoridate, GD), sulfur mustard (HD), and VX were sourced from the Chemical Analysis Test and Research Lab of the Agency for Defense Development (ADD, South Korea) and had purities of ≥95% as determined by GC-MS (Agilent) analysis, ¹H and ¹³C Nuclear Magnetic Resonance (NMR) analyses (Caution: these chemical agents are highly toxic, and should be handled with extreme care only by trained personnel at an approved facility). Fresh standard concrete (42.6% cement and 49.2% coarse limestone aggregates) was obtained from the Advanced Construction Material Testing Center (South Korea) and kept at room temperature. Soil was taken from a mountainous region in Dong-Gu, Daejeon, South Korea. The soil was clay loam (sand 21%, silt 48%, and clay 31%), and was dried at 70°C for 1 day and sieved (∼2 mm) from twigs and stones before use.

Wind tunnel

A laboratory-sized wind tunnel was specifically designed to produce full-scale vertical velocity profiles on the basis of the defined wind-induced boundary layers, and to measure and monitor evaporation processes. A detailed description of the tunnel has been reported previously (Jung and Choi, 2018). In brief, drops of tabun (total 6 μL, neat) were deposited on the surface of glass (a 38-mm-diameter circle), soi1 (6 g in a Teflon on cup), and concrete (a 35 mm diameter and a 15 mm thickness). Samples of interest were then inserted through a Teflon piston into the test section of the tunnel.

A series of feedback circuits and sensors automatically controlled and recorded the wind velocity, relative humidity (RH), and pressure within the test section. Eight independent thermocouple and heater sets monitored and controlled portions of the wind tunnel to ensure the entire apparatus maintained the set temperature. A custom computer code-based program controlled the wind tunnel in terms of the environmental parameters and conditions inside the tunnel, and securely logged the data for analysis. Video equipment was incorporated into the tunnel for visually monitoring and measuring the evaporation process.

The tunnel was set to the appropriate parameter values and allowed to equilibrate before use. The samples were conditioned in the tunnel for several hours under the test conditions. The environmental conditions selected were representative of a temperate arid climate, with negligible RH, air temperatures ranging from 25°C to 35°C and wind speed of 1.75 m/s measured 2 m from the surface. Temperature-controlled air from a Miller-Nelson environmental control unit was passed over the sample, and the vapors were collected in thermal desorption (TD) tubes filled with Tenax^® at the vapor sampling inlet. The typical flow in the TD experiments was 100 mL/min.

Analysis of tabun vapors: TD and GC

Vapors adsorbed in the TD tubes were desorbed using a thermal desorber (Markes UNITY/ULTRA TD System; Markes International, United Kingdom) and analyzed using a GC system (Agilent 7890B). Under a flow of N₂ for 10 min at 250°C, the tabun vapor was transferred to the GC system with a split ratio of 20:1. The transfer line from the desorber to the GC was heated to 120°C. The vapor was analyzed by the GC system with an HP5 column (30 m long and 0.5 mm i.d.) and a flame photometric detector under a temperature program that began at 80°C and heated up to 230°C at a rate of 15°C/min. N₂ was used as the carrier gas at a flow rate of 1.2 mL/min and a constant pressure of 10.3 psi. The retention time (R_t) for tabun was 4.1 min. The amount of tabun vapor was quantified by integration of the peaks reported by the GC using a standard calibration curve. The vapor (%) was calculated on the basis of the known sample volume and air flow rate in the tunnel.

Half-lives

Tabun (3.0 mg, neat) in a liquid form was introduced to the surface of crushed concrete and soil (2.5 g per vial) placed in a series of screw-top vials in a sand bath with a controlled temperature (25°C, 35°C, and 50°C). After being spiked, the samples were sealed until extraction. Tabun remaining in the substrate was extracted with ethyl acetate and a 1:1 (v/v) mixture of ethyl acetate and methanol periodically to study the kinetics of degradation. The extract was filtered, placed into a GC vial, and analyzed by GC-MS. The GC oven temperature was programmed to heat from 80°C (held for 3 min) to 260°C at a rate of 15°C/min. A volume of 1 μL of the sample was injected into a split/splitless port at 250°C. All tabun deposition was carried out in a chemical hum hood.

Decontamination

The surface of the substrate specimens in the screw-top vials was contaminated with a total concentration of 5 g/m² of neat tabun and kept in the temperature-controlled sand bath (25°C, 35°C, and 50°C). Approximately 2.5 g of concrete and soil was used in each vial for this experiment. The contaminated substrate was later covered with a cap to ensure realistic contact time of tabun with the substrate materials. The decontamination solution was prepared by dissolving 2.5 g of dichloroisocyanuric acid sodium salt and 2.5 g of a 4:1 mixture of sodium carbonate and potassium hydrogen carbonate in Milli-Q water, resulting in a final volume of 50 mL. The decontamination solution (1.5 L/m²) was evenly dispersed over the contaminated surface, and the decontamination was left to proceed over a specific time at 25°C, 35°C, and 50°C to study the kinetics of degradation.

Results and Discussion

Evaporation of tabun deposited on concrete and soil

The evaporation of the agent tabun after being deposited on surface in liquid form has been the subject of both experimental and theoretical studies, and many aspects of the liquid–solid interactions can be modeled, analyzed, and described (Columbus et al., 2018). In general, when the drop collides with the surface of a rigid and impermeable solid, it may spread, or evaporate, depending on the kinetic energy of the drop upon impact and the surface tension of the liquid (Zhang and Basaran, 1997). In the case of a solid that can be penetrated by the liquid, the evaporation process is more complicated due to the permeation, absorption, and chemical reactions of the liquid with the surface. A drop of liquid spreads in the lateral direction and at the same time penetrates the porous medium. Pores are usually sufficiently small in diameter to be considered an array of capillary tubes, and the interactions between the liquid and the walls of the capillaries influence the persistence of the agent and its evaporation from the surface (Reis Jr et al., 2008). Nonetheless, evaporation is the dominant fate for CWAs.

The evaporation of chemical agents depends on numerous factors, including the chemical itself, temperature, air pressure, wind velocity, and the surface with which the agent is in contact (Jung and Choi, 2017). Compounds evaporate more readily at higher temperatures, in stronger winds, or in contact with nonporous surfaces such as glass in comparison to porous materials (Brevett et al., 2009). Evaporation of tabun drops of various sizes (total amount of tabun 6 μL) from concrete and soil is visually depicted in Fig. 1. Various drop sizes (1, 3, and 6 μL) were applied to the surface. Upon application of the drops to concrete, they were absorbed immediately into the matrix, while spreading and evaporating. When the drops were applied to the soil surface, they soaked immediately into the soil without spreading laterally. The time it took for the agent to visibly fade from the surface was longer on soil than on concrete and increased as the drop size was increased from 1 to 6 μL.

FIG. 1.

Photographs of neat tabun drops (total amount 6 μL) in various sizes (a) in concrete and (b) in soil at 25°C and wind speed of 1.7 m/s: 1 × 6 μL drop, 3 × 2 μL drops, and 6 × 1 μL drop. The photographs were taken by a monitor-type borescope fixed at the ceiling of wind tunnel from the top side of the surface.

Data on the evolution of the tabun droplets over time are presented in Fig. 2. The evaporation of the vapor was measured continually until no further decrease in the vapor concentration was detected, at which time, the experiment was terminated. Data for the evaporation of GD, HD, and VX from a glass surface are presented alongside the data for tabun for comparison, in Fig. 2a. Compared to other nerve agents such as GD and VX, tabun exhibited evaporation characteristic similar to that of HD. The loss of tabun applied to concrete and soil over time due to evaporation was linear for a considerable proportion of the total drop life, as seen in Fig. 2b. However, the evaporation of tabun was relatively slower from soil than from concrete, resulting in an extended and slow release of the agent. This may be attributed to the efficient sorption of tabun into the layer of the soil where it remained without spreading, as shown in Fig. 1b. The average percentages of the applied mass of tabun (one 6 μL drop) released as vapor amounted to ∼30% and ∼20% from concrete and soil, respectively, which are relatively low in comparison to that of ∼80% from glass. It thus seems that concrete and soil emit the similar amount of tabun vapors. We propose that the interaction between tabun and the surface may primarily affect the initial evaporation rate, and thus the total evaporation times. Indeed, the initial rate of evaporation of the agent (one 6 μL drop) was approximately three times faster from concrete than from soil, with rates of 11.6 μg/min in concrete and 4.3 μg/min in soil, respectively. It should also be noted that the amount of tabun released as vapor from both concrete and soil was significantly increased by decreasing the drop size to 1 μL, reaching as high as ∼80%.

FIG. 2.

Evaporation curves at different temperatures and drop sizes; (a) 6 μL drop of GD, HD, VX, and tabun evaporating from glass, (b) concrete, and (c) soil. Curves of (b) and (c) indicate tabun vapor (%) versus time, 1 × 6 μL drop, 3 × 2 μL drop, and 6 × 1 μL drop, at 25°C and wind speed of 1.7 m/s. GD, O-pinacolyl-methylphosphonofluoridate; HD, bis(2-chloroethyl)sulfide; VX, venomous agent X (O-ethyl S-2-(N,N-diisopropylamino)ethyl methylphosphonothioate).

During the first 200 min (one 6 μL drop of tabun on concrete) and 400 min (one 6 μL drop of tabun on soil), the vapors emanating from the substrates reached tabun concentrations that were close to the immediately dangerous to life/health limit (IDLH) of 1 × 10⁻¹ mg/m³, and consistently above the short-term exposure limit of 1 × 10⁻⁴ mg/m³ (Fatz, 2004; Brevett et al., 2009). After these initial periods, our data indicate that the surface no longer posed hazard in terms of airborne exposure limits.

Half-lives and decontamination of tabun on concrete and soil

The persistence of chemical agent, which describes the period of time over which the agent retains its toxicity in the air or on the ground, is an important characteristic since persistent agents are able to cause long-lasting contamination of terrain and materials, hampering their use (MacGregor et al., 2008). However, estimating the exact persistence is very complex, since it depends on many factors, such as the physical and chemical characteristics of the agent, environmental conditions including temperature, humidity, and terrain topography, and the state of the affected surfaces (Checkai et al., 2017). In particular, temperature may significantly affect the effectiveness of the agent. It is plausible that lower temperatures may significantly increase the persistence of an agent. For instance, tabun is known to remain in the air for 4 days in the winter, but only for 1 day in the summer (Gorzkowska-Sobas, 2013).

In this study, we attempt to better understand the persistence of tabun based on the half-life of the compound in scenarios with and without a decontamination process and at various temperatures. The evolution over time of tabun remaining in the matrices at different temperatures (25°C, 35°C, and 50°C) with and without the decontamination process is shown in Fig. 3. First, tabun disappeared from concrete and soil naturally in a matter of days, as seen in Fig. 3. Tabun disappeared from concrete at a faster rate than that observed in soil. The data points fit reasonably to a pseudo first-order kinetic process, leading to calculations of the half-life of 2 days in concrete and 5 days in soil at 25°C, which are much shorter than those at 35°C (1.5 days in concrete and 2.3 days in soil) and 50°C (1.1 days in concrete and 1.8 days in soil).

FIG. 3.

Pseudo first-order kinetic plots for tabun on concrete and on soil without decontamination at (a) 25°C, (b) 35°C, and (c) 50°C, and with decontamination at (d) 25°C, (e) 35°C, and (f) 50°C.

Concrete is a ceramic material with a high surface energy, owing to the presence of both covalent and ionic bonds, but it also exhibits a certain degree of porosity, allowing for penetration of the agent into its internal structure (Roy et al., 1993). Cement, which accounts for ∼10% to 15% of concrete, is a mixture of calcium hydroxide and oxides of silicon, aluminum, and iron. In the presence of moisture, calcium hydroxide dissociates into Ca²⁺ and OH⁻, which are responsible for the alkalinity of concrete (pH of 12.0), and likely participate in the degradation of tabun on the concrete surface.

Decontamination is a complex process and can be carried out in different ways from a methodological point of view, including mechanical, physical, and chemical decontamination. We applied chemical decontamination using a mixture of an oxidizing agent (dichloroisocyanuric acid) and hydrolytic agent (carbonates), as described in the Experimental. The advantage of chemical decontamination in the case of CWAs is the conversion of the toxic CWAs into innocuous products, which can safely be handled. The variables affecting decontamination are contamination time, temperature, contamination density, the decontamination medium, the nature of the agent and decontaminants, and interactions with the surface on which contamination is present (Beer Singh et al., 2010).

It was found that the half-life of tabun was shortened significantly after decontamination, to 4.6 (25°C), 1.2 (35°C), and 0.7 min (50°C) in concrete, and 5.9 (25°C), 2.5 (35°C), and 1.0 min (50°C) in soil. As summarized in Table 1, tabun was decontaminated more effectively in concrete than in soil. The half-life of tabun in concrete was approximately half of that in soil at an elevated temperature of 35°C. We attempted to analyze the spreading factor of the drop of tabun in concrete and in soil, which is the ratio between the diameter of the wetted area in surface and the diameter of the drop (Jung and Lee, 2015). The 6 μL drop of tabun was ∼2.25 mm in diameter, based on calculations. The wetted areas were measured at 15 mm in concrete and 5 mm in soil, as calculated by approximating the wetted area to a circle. The spreading factors were 6.6 in concrete and 2.2 in soil, indicating that the drop of tabun spread three times more in concrete than in soil. This suggests that the spreading drop of tabun in concrete may effectively interact with the decontaminants within a relatively short period of time.

Table 1.

Summary of Kinetic Data and Maximum Conversions of Tabun in Concrete and in Soil, With and Without Decontamination

Matrix		k, day⁻¹	t 1/2, day	Conv., %	Temp., °C
Concrete	Without decontamination	0.34	2.0	91.0	25
		0.46	1.5	96.0	35
		0.61	1.1	92.0	50
Soil		0.14	5.0	86.0	25
		0.30	2.3	88.0	35
		0.37	1.8	85.0	50

Matrix		k, min⁻¹	t 1/2, min	Conv, %	Temp., °C
Concrete	With decontamination	0.15	4.6	86.9	25
		0.57	1.2	90.8	35
		0.96	0.7	79.7	50
Soil		0.12	5.9	91.3	25
		0.28	25	79.8	35
		0.69	1.0	90.8	50

It should be noted that in reality, both concrete and soil are intricately porous and thus difficult to decontaminate. Both materials are full of microscopic pores, allowing the structures to absorb or adsorb chemicals and sometimes release them back out. This implies that even if the surface is cleaned, toxic chemicals, such as those from a chemical warfare event, could still be present below the surface. When the wind tunnel experiments were performed with the decontaminated substrates, it was found that trace residual amounts of the agent present in the substrate were released as vapor (Supplementary Fig. S1). However, the amount of vapor released was relatively low in comparison with the level of contamination of the contaminated surface, and therefore, the decontamination still decreased the hazard (data shown in Supplementary Data). Nonetheless, such materials could still pose a long-term threat to exposed individuals and the environment.

Conclusions

In summary, the nerve agent tabun was absorbed rapidly into the surface of concrete upon deposition, while simultaneously spreading. However, tabun remained in soil without visible spreading, resulting in the relatively longer time required for the evaporation of tabun from soil than from concrete. In terms of persistence, tabun was eliminated naturally in a matter of days following a pseudo first-order kinetic process. Upon decontamination, the rates of disappearance of tabun increased significantly, to within a few minutes. The half-life of tabun was shortened upon increasing the temperature from 25°C to 50°C. The importance of a proper understanding of the agent-surface interactions, as well as the nature of the interface between them, cannot be overlooked. Such understanding is crucial for the disposal of CWAs, and for the development of efficient methods of preventing the contamination or minimizing the consequences resulting from events involving CWAs.

Footnotes

Acknowledgments

The authors would like to thank the Chemical Analysis Test and Research Laboratory for providing chemical agents (tabun, GD, HD, and VX). They also thank the reviewers for their insightful comments on the article.

Author Disclosure Statement

No competing financial interests exist.

Supplementary Material

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Supplementary Material

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