Photodegradation of selected herbicides in various natural waters and soils under environmental conditions

The photochemical degradation of herbicides belonging to different chemical groups has been investigated in different types of natural waters (ground, river, lake, marine) and distilled water as well as in soils with differen t texture and composition. Studied herbicides and chemical groups included atrazine, propazine, and prometryne (s-triazines); propachlor and propanil (acetanilides); and molinate (thiocarbamate), The degradation kinetics were monitored under natural conditions of sunlight and temperature. Photodegra dation experiments were performed in May through July 1998 at low concentra tions in water samples (2-10 mg/L) and soil samples (5-20 mg/kg), which are dose to usual field dosage, The photodegradation rates of all studied herb icides in different natural waters followed a pseudo-first order kinetics. The half-lives of the selected herbicides varied from 26 to 73 calendar day s in waters and from 12 to 40 d in soil surfaces, showing that the degradat ion process depends on the constitution of the irradiated media. The presen ce of humic substances in the lake, river, and marine water samples reduces degradation rates in comparison with the distilled and ground water. On th e contrary, the degradation in soil is accelerated as the percentage of org anic matter increases, Generally, the photodegradation process in soil is f aster than in water. The major photodegradation products identified by usin g gas chromatography-mass spectrometry (GC-MS) techniques were the hydroxy and dealkylated derivatives for s-triazines, the dechlorinated and hydroxy derivative for the anilides, and the keto-derivative for the thiocarbamate, indicating a similar mode of degradation for each chemical category.