USING C-13-NMR SPECTROSCOPY TO EVALUATE THE BINDING MECHANISM OF BOUND PESTICIDE-RESIDUES IN SOILS .2. INVESTIGATIONS OF MODEL HUMIC ACIDS

Humic acids (HA) derived from humified, C-13-depleted maize were used for C-13-NMR spectroscopic investigations of bound residues of the fun gicide active ingredient anilazine (part 1 of this series, WAIS ET AL. , 1995). Due to the lowered C-13-NMR background signals of these HA a comparision to the C-13-NMR spectra of native HA was not possible. Fur ther investigations of HA derived from humified maize with natural car bon abundance became necessary. It was demonstrated that apart from di fferent humification procedures, after a period of 26 weeks the C-13-N MR spectra of the artificially prepared HA were almost identical to na tive HA samples. A time resolution of the humification indicates that the chemical structure of the model HA more closely resembles the non- humified material or the alkaline extracts of the straw itself during the first 8 weeks of the humification procedure. The GPC chromatograms of the HA resulting from these treatments underline this fact, becaus e of their decrease in molecular weight. The microbial activity detect ed by DMSO reduction was found to be nearly the same for all humificat ion procedures at the end of the experiment. Native HA of different or igin do not show significant differences in their C-13-NMR spectra. A comparision, however, of the model HA or the native HA to commercial H A or to the alkaline extract of the maize straw itself indicates great differences in chemical structure. This indicates that commercial HA do not fulfil the requirements for utilization as model substances for binding or adsorption studies with respect to environmental considera tions.