Transport and immobilization of 2, 4, 6 15N-trinitrotoluene in soil microcosms subjected to long term incubation under aerobic conditions

15N-labeling and solid-state 13C and 15N nuclear magnetic resonance (NMR) spectroscopy was applied to study the immobilization of 2,4,6 trinitrotoluene (TNT) into soil organic matter (SOM). Uncontaminated soil from the Ap horizon of a Luvisol was mixed with 15N-TNT (enrichment: 99 atm%) and laid over an unspiked layer of the same material. The latter covered soil from the Bt horizon. The microcosms were aerobically incubated under laboratory conditions for up to 11 months. After 1 week, within the total microcosm approximately 90% of the added 15N (15Nadd) were recovered, mostly in the top layer (87%). After 11 months, this amount decreased to 71%, indicating losses due to denitration or transamination. Within two months, half of 15Nadd had been immobilized in the residues not extractable with organic solvents and water. The amount of the sequestered 15Nadd remained fairly constant until the end of the experiment pointing towards a high stability of TNT-SOM associates. Solid-state 15N NMR revealed their formation by covalent binding, most tentatively as amides. Complete reduction of TNT to triaminotoluene (TAT) was not prerequisite. The most pronounced downwards movement of 15N-TNT occurred during the first two months. The major part of it, however, experienced quick immobilization, leaving approximately 10% of 15Nadd recovered in the leachate at the end of the experiment. Calculations indicated contributions of inorganic 15Nadd. Approximately 25% of its organic 15Nadd originated from condensed N, suggesting that in soils the transport of partly reduced TNT is in close association with the organic matter of the soil solution to which they are covalently bound.

Peer reviewed