Effect of energetic materials wettability on their outdoor effective elution rate
Introduction
Energetic materials (EM) are used in military ordnances and may present a threat for soil and water contamination in military training ranges and in war zones. EM residues are dispersed in the environment mainly by low order detonations [1], unexploded ordnance that are cracked open [2], [3], and from propellant residues deposited at the firing positions from incomplete combustion [4]. When EM particles are deposited on the soil surface they are exposed to weather conditions and are leached into the soil by water from rain and snowmelt. To some extent, they go through photolysis by sun ray and other degradation processes such as hydrolysis, or biodegradation before and during their infiltration into the vadose zone [4], [5], [6]. The dissolution rate of EM is the main process of mass reduction of EM particles at the soil surface and, it has been studied via different experimental set-ups [5], [6], [7], [8], [9]. However, no study has been published to see if a relationship can be made between wettability of EM and their environmental release (effective elution rate; EER) as observed in outdoor exposure experiments, and if so, what can be proposed to lower the environmental impact of EM on water quality. Furthermore, very few data on contact angle (θ) of EM are available in the literature helping to identify which chemical component(s) of a formulation may control its wettability [10].
The association between dissolution rate and wettability is widely known in pharmaceutical chemistry [11], [12], enhanced oil recovery [13] and environmental studies such as non aqueous phase liquid behavior [14], [15]. Interfacial tensions and wettability have been studied for the geological storage of CO2 [16], [17]. In this paper, the focus is on the wettability of the water phase on solid EM surfaces that can be dissolved by water in the presence of air.
θ measurements allow the evaluation of the water attraction to a solid surface. It is a basic physical parameter, easy to measure in the laboratory, used to specify wettability. It characterizes the tendency of a liquid (water) to spread on or to adhere to a solid surface (EM) in the presence of another immiscible fluid (air). θ, expressed in degrees, is the angle between the tangent of the liquid-air surface drawn on a static drop’s surface and the tangent of the solid-liquid surface. For a solid-liquid-air system (Fig. 1), θ can be determined following Young’s equation [18]:where σs-a is the solid-air surface tension; σs-l is the solid-liquid surface tension; and σl-a is the liquid-air surface tension. θ can be directly quantified via a video system.
θ is affected by the chemical composition, roughness, geometry and surface charge of the solid, by the liquid properties such as purity and polarity [19], [20] and to some extent by temperature and pressure. The relationship between the solid and the liquid may be repulsive which means the solid is considered as having a hydrophobic surface, or attractive where the solid is considered as having a hydrophilic surface. In the case of a hydrophilic surface, water adheres to the solid surface and occupies the smallest pores. Inner grain surfaces of the porous media can therefore be reached by water resulting in greater effective surface area coverage. In contrast, on a hydrophobic surface, water drops form spheres and are unable to enter into small pore spaces. Therefore, water is not able to adhere to the solid surface and occupies only the largest pores or their centers leading to a smaller surface area coverage. The contact time between water and a solid can be affected by the contact angle: Water tends to adhere more readily to the surface of a hydrophilic solid, while tending to be more rapidly released from the surface of a hydrophobic solid.
Legitimately, water able to wet hydrophilic solids may create cryo-fracturation due to its volume variation during its freezing-thawing cycles in small pore spaces. Consequently, the effective surface area of a hydrophilic surface can be raised affecting its dissolution rate which may not evolve at the same pace as for a hydrophobic surface.
EER of conventional energetic compounds (TNT and RDX crystals), conventional energetic formulations (Composition B, Octol, and C4), new energetic formulations (XRT, GIM, CX-85), and new propellants (HELOVA) were measured via long term outdoor exposure tests to weather conditions in Quebec between September 2007 and June 2011. EER is defined as the mass loss of EM samples during their outdoor exposure divided by the amount of water that had been directly in contact with those EM samples in relation to their specific surface area. Hence, it is possible to ascertain if new EM formulations, developed to improve ammunitions safety [21], [22], [23] also minimizes their environmental impact on water quality by leaching less EM compounds.
The objective of this paper is to determine: (1) Which chemical component of an EM formulation controls its wettability; (2) if a link can be made between wettability of EM and their environmental release (EER) observed in outdoor exposure experiments and; (3) what can be proposed to change wettability of EM to lower their environmental impact on water quality.
Section snippets
EM formulations preparation
Conventional energetic compounds (TNT and RDX crystals), conventional energetic formulations (Composition B, Octol and C4), as well as new EM formulations (XRT, GIM, CX-85 and HELOVA) were prepared and provided by Defence Research and Development Canada (DRDC-Valcartier). The whole content of these new formulations cannot be divulgated since they are Controlled Goods. However, a list of the main components is presented in Table 1. Composition B and Octol are obtained by dispersing RDX or
Results and interpretations
Table 3 and Fig. 2 show θ average values measured for each EM formulation with the coated glass slide, the plate and the direct methods.
Discussion
θ values of water droplets on conventional and new formulations of EM used in Canadian Forces ammunitions ranged from 78° to 101°. They can be classified in two distinct groups: (1) TNT, RDX, Octol, ETPE 1000 and HTPB-TDI are hydrophilic; and (2) Composition B, XRT, GIM, ETPE 2000, CX-85, C4 are hydrophobic. HELOVA grains were intermediate. The variability of each EM θ value was less than 9% and none of these variations affected the classification into hydrophilic or hydrophobic group (Fig. 2).
Conclusion
Static contact angle measurements of water droplets on energetic formulation surfaces can help predict the elution rate of energetic materials in the environment. It was demonstrated that a relationship exists between the contact angle of water droplets and the effective elution rate of energetic formulations. The addition of hydrophobic components to the matrix of explosives or propulsive formulations shields the formulation and decreases their environmental impact as they tend to release less
Acknowledgements
Authors wish to thank Director General Environment and </GS2>Director Land Environment of DND Canada</GS2> for their financial support, and Strategic Environmental Research and Development Program through research project ER-1481.
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