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Long-Term RDX Sorption and Fate in Soil

School of Natural Resource Sciences, Univ. of Nebraska, Lincoln, NE 68583-0915.

^* Corresponding author (agro130{at}unlvm.unl.edu).

ABSTRACT

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. Sorption 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 Argiudoll) surface soil by equilibrating the soil with 32 mg RDX L^–1 (spiked with ¹⁴C-labeled RDX) for 168 d; similar experiments were performed with contaminated and uncontaminated subsurface soils. Surface soils exhibited rapid RDX sorption with 34% of the ¹⁴C sorbed within 30 min. 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 ¹⁴C was removed after eight extractions with 10% KOH in ethanol. Eight percent of the ¹⁴C-label was unextractable (bound) residue at 168 d; no ¹⁴C-bound residue formed in surface soil when solid-phase RDX was present in the equilibrating solution. Our experiments indicated limited RDX sorption and transformation in the Sharpsburg surface and subsurface soils. Most of the sorbed ¹⁴C was potentially available for transport, indicating the importance of remediating RDX-contaminated soil to protect groundwater quality.

no. 11906, Agric. Res. Div., Univ. of Nebraska-Lincoln.

Received for publication June 2, 1997.

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