Sorption of the Chemical Warfare Agent VX to Clay Minerals and Soils.

Accurately predicting the fate and transport of the persistent chemical warfare agent O-ethyl-S-[2-(diisopropylamino)ethyl] methylphosphonothioate (VX) in environmental scenarios requires a better mechanistic understanding of the interaction of VX with environmental surfaces.  While sorption to soil components, such as clays and organics, is a major factor influencing fate and transport of commercial pesticides in soil environments, little data exists regarding the sorption of VX in these environments.  An enhanced mechanistic understanding of how VX interacts with environmental surfaces is critical to the development of effective countermeasures, predictive modeling capabilities, and enhanced detection capability for illicit use scenarios. 

This poster will present data on the sorption of VX with soil related sample matrices.  The sample matrices examined during this study included a montmorillonite clay, a kaolinite clay, and two natural soils.  Isothermal titration calorimetery was used to provide a direct measurement of heat flow during each experiment.  This heat flow data was then modeled to provide thermodynamic characterization of the interactions.  Complementary sorption/desorption profiles and batch equilibrium sorption isotherms were also generated during the study.  Kinetic time point samples were analyzed by a variety of analytical techniques to provide insights into the sorption pathway(s), and to distinguish sorptive from degradative processes occurring in the bulk solution.

No VX sorption was observed with kaolinite, but VX sorption by montmorillonite was rapid.  The predominant mechanism above the lower critical temperature (LCT) of VX was an exothermic physisorption process, consistent with an ion exchange mechanism.  Below the LCT, bimodal behavior was observed, with a second type of endothermic interaction occurring after the initial ion exchange.  The VX also interacts with both soils, but there are both exothermic and endothermic components to the interaction, suggesting multiple reaction sites.  Thermodynamic and kinetic characteristics of the interactions will be presented.  Data gaps and implications for agent fate assessments will also be discussed in the poster.