Long-Term TNT Sorption and Bound Residue Formation in Soil

Abstract

Soils surrounding former munitions production facilities are highly contaminated with 2,4,6-trinitrotoluene (TNT). Long-term availability and fate of TNT and its transformation products must be understood to predict environmental impact and develop appropriate remediation strategies. Sorption and transport in surface soil containing solidphase TNT are particularly critical, since nonlinear sorption isotherms indicate greater TNT availability for transport at high concentrations. Our objectives were to determine long-term sorption and bound residue formation in surface and subsurface Sharpsburg soil (Typic Argiudoll). Prolonged equilibration of ¹⁴C-TNT with the soil revealed a gradual increase in amount sorbed and formation of unextractable (bound) ¹⁴C residues. The presence of solid-phase TNT did not initially affect the amount of ¹⁴C sorbed during a 168-d equilibration. After 168 d, 93% of the added ¹⁴C was sorbed by uncontaminated soil, while 79% was sorbed by soil containing solid-phase TNT. In the absence of solid phase, pools of readily available (extractable with 3 mM CaCl₂) and potentially available (CH₃CN-extractable) sorbed TNT decreased rapidly with time and coincided with increased ¹⁴C in soil organic matter. More ¹⁴C was found in fulvic acid than in the humic acid fraction when no solid-phase TNT was present. After sequential extractions, including strong alkali and acid, 32 to 40% of the sorbed ¹⁴C was irreversibly bound (unextractable) in Sharpsburg surface and subsurface soil. Results provide strong evidence for humification of TNT in soil. This process may represent a significant route for detoxification in the soil-water environment. Publication no. 11551, Agric. Res. Div., Univ. of Nebraska — Lincoln, Lincoln, NE 68583-0915.