LONG-TERM RDX SORPTION AND FATE IN SOIL

Soils at former munitions production facilities are often contaminated with hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX). Contamination can be excessive and soils often contain precipitated or solid-phase RDX, resulting in soil solution concentrations at or near saturation. Sorp tion and long-term fate must be understood to predict RDX availability and develop remediation strategies. We characterized RDX sorption and availability in Sharpsburg (a fine, montmorillontic, mesic Typic Argi udoll) surface soil by equilibrating the sail with 32 mg RDX L-1 (spik ed with C-14-labeled RDX) for 168 d; similar experiments were performe d with contaminated and uncontaminated subsurface soils. Surface soils exhibited rapid RDX sorption with 34% of the C-14 sorbed within 30 mi n. This sorbed fraction increased to only 37% at 168 d. During the 168 -d equilibration, readily available RDX (sorbed RDX extractable with 3 mM CaCl2) decreased from 75 to 52%, while potentially available RDX ( acetonitrile-extractable) increased from 24 to 32%. Carbon-14 in the 0 .5 M NaOH-extractable organic matter fraction increased from 0.8 (T = 30 min) to 3.8% (T = 168 d). Little C-14 was removed after eight extra ctions with 10% KOH in ethanol. Eight percent of the C-14-label mas un extractable (bound) residue at 168 d; no C-14-bound residue formed in surface soil when solid-phase RDX mas present in the equilibrating sol ution. Our experiments indicated limited RDX sorption and transformati on in the Sharpsburg surface and subsurface soil. Most of the sorbed C -14 was potentially available for transport, indicating the importance of remediating RDX-contaminated soil to protect groundwater quality.