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 soil-bound trifluralin residues, 14C-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 14C depending on the soil tested. 19F 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 19F 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 (T1) decreased from the range 1300-1831 ms for TR6 or Fe-TR analyzed in the absence of FA to the range 150-410 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.