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 different 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. Photodegradation experiments were performed in May through July 1998 at low concentrations in water samples (2-10 mg/L) and soil samples (5-20 mg/kg), which are close to usual field dosage. The photodegradation rates of all studied herbicides in different natural waters followed a pseudo-first order kinetics. The half-lives of the selected herbicides varied from 26 to 73 calendar days in waters and from 12 to 40 d in soil surfaces, showing that the degradation process depends on the constitution of the irradiated media. The presence of humic substances in the lake, river, and marine water samples reduces degradation rates in comparison with the distilled and ground water. On the contrary, the degradation in soil is accelerated as the percentage of organic matter increases. Generally, the photodegradation process in soil is faster than in water. The major photodegradation products identified by using 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.

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CPh 3 ), upon photolysis with 248-and 308-nm laser light, as well as by lamp irradiation with 254-nm light in acetonitrile and hexane solutions, undergo exclusively C-N bond homolysis to give the triphenylmethyl radical in a monophotonic process. Product analysis gives as main products Ph 3 CH, 9-Ph-fluorene, and PhNH 2 . The quantum yields for the formation of Ph 3 C • are high (0.6-0.8, 248-nm excitation) and independent of the solvent. This effective homolytic dissociation results from the low electronegativity difference between the carbon and nitrogen atoms constituting the bond to be broken, the low bond dissociation enthalpy of the C-N bond, the high excitation energy of the local chromophore (aniline), and probably from a favorable alignment of the C-N bond in a plane perpendicular to the anilino chromophore (due to the large steric requirements of the trityl group), thus enabling an effective hyperconjugative interaction with it. The above dissociation competes effectively with heterolytic cleavage, which is the pathway dominating, e.g., in the photolysis of Ph 3 C-Cl in MeCN under the same conditions. At high pulse intensities the trityl radicals formed above are excited by a second photon leading to either electrocyclization to 4a,4b-dihydro-9-phenylfluorenyl radical (DHPF • ), or photoionization to Ph 3 C + , the latter only in MeCN and only on 248-nm photolysis. A new intermediate (9-Ph-4aH-fluorene) on the way to the final product 9-Ph-fluorene is identified resulting from electrocyclization, supporting a mechanism proposed earlier. The optical measurements are supported by ESR studies (irradiation with 254-nm light).

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