Long-term TNT sorption and bound residue formation in soil

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 solid-phase 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 C-14-TNT with the soil revealed a gradual increase in amount sorbed and formation of unextractable (bound) C-14 residues. The presence of solid-phase TNT did not initially affect the amount of C-14 sorbed during a 168-d equilibration. After 168 d, 93% of the added C-14 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 C-14 in soil organic matter. More C-14 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 C-14 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.