| When xenobiotic chemicals come into contact with soil they can be immobilized by three mechanisms: sorption, sequestration and covalent binding. In addition, microbial activity has the potential to alter a xenobiotic compound to a more reactive species, or transform the xenobiotic after immobilization.; A soil without a history of exposure to explosives was incubated with 15N-labeled 2,4,6-trinitrotoluene (TNT) and 14C-TNT and then amended with compost. The compost was inoculated, amended with glucose and starch, and incubated for 37 days under anaerobic growth conditions followed by 26 days of forced aerobic incubation. At the end of the aerobic phase, most of the radioactivity was associated with organic matter; only 8.7% could be extracted with water and methanol, but no TNT was present in the extracts as determined by high-performance liquid chromatography. The unextractable radioactivity was associated with humic acid (40.0 ± 1.0%), fulvic acid (14.3 ± 1.4%), and humin (28.2 ± 0.5%).; Binding of xenobiotics to soil organic matter is one of the ways of reducing bioavailability in soil. However, the stability of linkages between xenobiotics and soil organic matter is cause for concern because bound chemicals may eventually be released and result in delayed pollution problems. To address these uncertainties, a study was undertaken to assess the release of soil-bound xenobiotics (2,4-dichlorophenol, 2,4,6-trinitrotoluene and cyprodinil) during a 141-day exposure to microbial activity in fresh soil.; Trifluralin is a widely used herbicide for the control of broad leaf weeds in cotton, alfalfa and soybeans. Previous research indicated that in wet soils of high organic matter contents, trifluralin is likely to undergo binding and thus sustain significant losses in herbicidal activity. Bound residue formation may, at the same time, result in a delayed pollution problem. To evaluate trifluralin binding, experiments were undertaken, in which the 14C-labeled herbicide was incubated for 7 weeks with four soils of different organic matter contents under anaerobic growth conditions. The presence of fluorine (19F) in the trifluralin molecule was used to obtain structural information by 19F NMR spectroscopy. As determined by radiocounting, trifluralin binding ranged between 10 and 53% of the initial 14C depending on soil tested. The analyses of the methanol-extractable and unextractable 19F indicated that soil binding largely involved reduced metabolites of the herbicide. The 19F NMR spectra of the methanol-extractable transformation products did not match any of the spectra for the available trifluralin metabolite standards. Reacting trifluralin with zero valent iron generated a product that was identified as 2,6-diamino trifluralin (Fe-TR) by mass spectrometry. This product and one of the standard metabolites (1,2-diamine trifluralin or TR6) spontaneously formed associations with fulvic acid (FA), as indicated by the appearance of new resonances in the 19F NMR spectra taken periodically over a 3-week contact time. No unaltered Fe-TR or TR6 could be recovered by chloroform extraction of these complexes. At short contact times, TR6 and Fe-TR formed weak physical bonds with FA as the respective spin-spin relaxation times (T1) decreased from 1300–1831 msec for TR6 or Fe-TR analyzed in the absence of FA to 150–410 msec for TR6/FA or Fe-TR/FA mixtures. In general, the results indicated that trifluralin immobilization involved a variety of mechanisms that with time become increasingly stable. (Abstract shortened by UMI.)... |
