Adsorption of hexahydro-1,3,5-trinitro-1,3,5-triazina (RDX) on soil and clay mineral surfaces

Hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) is an energetic compound that is commonly used as a military explosive. Studies indicate that nitroamines compounds, particularly those exhibiting several NO2 groups or other electron-withdrawing substituents, may adsorb strongly and reversibly from aqueous solutions to natural clay minerals. To determine the fate and transport mechanisms of explosives contained on buried landmines it is essential to understand the adsorption process on soil and clay minerals. In this research, the adsorption behavior of RDX was evaluated in soil samples and clay fractions from the Ap, and A horizons of the Jobos Series at Isabela, Puerto Rico and the University of Puerto Rico Mayaguez (UPRM campus). The clay fraction was separated from the other soil components by centrifugation. We analyzed the mass of solute sorbed per unit mass of soil at equilibrium (mug/g) and the aqueous equilibrium phase solute concentration (L/kg) using high-performance liquid chromatography (HPLC). Adsorption coefficients (Kd) for the RDX--soil and RDX-clay interaction were determined. The adsorption process for RDX-soil was described by the Freundlich model. The higher adsorption coefficient was observed in the UPRM soil (0.99 L/Kg). The Freundlich algorithm also described the adsorption process for RDX-clay interaction. The relative adsorption capacity of the clays for RDX was higher in the A horizon (4.42 L/Kg). These results suggest that adsorption by soil organic matter predominates over adsorption on clay minerals when significant soil organic matter content is present. It was also found that properties like cation exchange capacity, surface area, type of exchangeable cations and clay minerals present in the clay fractions are important factors in the adsorption of RDX on clay and soils. The experimental adsorption enthalpy (DeltaHads = -18.46 KJ/mol) found for the RDX-soil interaction, suggests that these interactions are of the Van der Waals type.