Characterization of 15N-TNT residues after an anaerobic/aerobic treatment of soil/molasses mixtures by solid state 15N NMR spectroscopy.: 1.: Determination and optimization of relevant NMR spectroscopic parameters

Solid-state (15)N NMR was applied to a humic acid, extracted from (15)N-2,4,6-trinitrotoluene (TNT) enriched soil treated in an anaerobic/aerobic composting system to characterize the nitrogen functionality of the transformation products bound to the soil organic material. Signals assignable to aniline derivatives and condensation products were identified, indicating that the anaerobic/aerobic treatment caused a reduction of nitro groups followed by condensation reactions with the soil organic material. Relevant parameters for routine application of the cross polarization magic angle spinning technique were determined and optimized. The proton spin-lattice relaxation times of all peaks in the (15)N NMR spectrum of the humic acid did not exceed 30 ms. Due to the fast relaxation, the application of (15)N NMR spectroscopy to soils with lower enrichment of (15)N-TNT is feasible. The influence of spinning sidebands on the intensity distribution was shown to be minimal at spinning speeds between 5.5 and 6.5 kHz. Contact times between 0.7 and 1 ms resulted in spectra with representative intensity distribution of all visible (15)N-TNT transformation products. However an underestimation of unreacted TNT must be considered. The results imply that CPMAS (15)N NMR is a valuable tool for the examination of bound residues of TNT in soils. (Figure 1).