Photodegradation of trifluralin in natural waters and soils: Degradation kinetics and influence of organic matter

The photochemical degradation of the herbicide trifluralin, a dinitroaniline, has been investigated in different types of natural waters (ground, marine, river, lake) and in distilled water as well as in soils characterized by different textures and compositions. Degradation kinetics was monitored under natural sunlight while the effect of humic acids (HA) on the degradation rate in natural waters was studied under laboratory conditions (Suntest apparatus). The photodegradation rate of the studied herbicide in different natural waters follows first-order kinetics with half-lives ranging from 12 to 29 h. The presence of dissolved organic matter (DOM) in the selected natural waters retarded the photoprocess relative to distilled water. Laboratory experiment has shown that the presence of HA inhibits the photolytic degradation of the herbicide. On the contrary, it was found that the degradation kinetics of trifluralin in soil is accelerated as the percentage content of organic matter increases. Photodegradation of trifluralin on the adsorbed phase (soils) gave depletion curves that can be well described by a first-order equation with half-lives varying from 15 to 38 h depending on the composition of the adsorbing media.