Adsorption mechanisms of thiazafluron in mineral soil clay components

The adsorption of the herbicide thiazafluron, 1,3-dimethyl-1-(5-trifluoromethyl-1,3,4-thiadiazol-2-yl)urea, by three smectites, illite, kaolinite, ferrihydrite and the clay fraction of an illitic soil (54.9% illite, 17.0% montmorillonite and 24.9% kaolinite) and a montmorillonitic soil (33.9% illite, 55.0% montmorillonite and 11.1% kaolinite) has been determined. Thiazafluron adsorbed on neither kaolinite nor iron oxide. The adsorption isotherms on smectites and illite conformed to the Freundlich equation. Values of K_f-obtained for smectites were larger than for the illite and increased as the layer charge of the smectite decreased. Desorption of thiazafluron on smectites was shown to be highly irreversible. Adsorption isotherms of thiazafluron on different homoionic montmorillonite samples suggest an important role of the exchangeable cations in the adsorption. Infrared spectra and X-ray diffraction analysis of the complexes of thiazafluron with homoionic montmorillonites indicated that thiazafluron adsorbs in the interlamellar space of the smectites, mainly by substitution of water molecules associated with the exchangeable cations through the carbonyl-amide group and formation of H-bonds or waterbridge between the NH group of the amide and the basal oxygens of the montmorillonite. The illitic soil clay adsorbed more of the herbicide than the montmorillonitic one did, suggesting that illite and montmorillonite may be present in soils in altered forms giving rise to different adsorption capacities from those of the pure minerals.