Using 19F NMR spectroscopy to determine trifluralin binding to soil

Trifluralin is a widely used herbicide for the control of broad leaf weeds in a variety of crops. Its binding to soil may result in significant losses in herbicidal activity and a delayed pollution problem. To investigate the nature of soilbound trifluralin residues, C-14-labeled herbicide was incubated for 7 weeks with four soils under anoxic conditions. As determined by radiocounting, trifluralin binding ranged between 10 and 53% of the initial C-14 depending on the soil tested. F-19 NMR analyses of the methanol extracts and different fractions of the extracted soil suggested that bound residue formation largely involved reduced metabolites of the herbicide. A 2,6-diamino product of trifluralin reduction with zero-valent iron (Fe-TR), and the standard of a 1,2-diaminotrifluralin derivative (TR6) formed covalent bonds with fulvic acid (FA), as indicated by the F-19 NMR spectra taken periodically over a 3-week contact time. At short contact times, TR6 and Fe-TR formed weak physical bonds with FA, as the respective spin-lattice relaxation times (T-l) decreased from the range 1300-1831 ms for TR6 or Fe-TR analyzed in the absence of FA to the range 150410 ms for TR6/FA or Fe-TR/FA mixtures. In general, the results indicated that trifluralin immobilization involved a variety of mechanisms (covalent binding, adsorption, sequestration), and with time it became increasingly stable.