Kinetics of photodegradation of the fungicide fenarimol in natural waters and in various salt solutions: Salinity effects and mechanistic considerations

The rate of photodegradation of the Fungicide fenarimol (alpha-(2-chlorophe nyl)-alpha-(4-chlorophenyl)-5-pyrimidine-methanol) by solar radiation has b een studied in natural waters with different salinities. Photodegradation r ates and quantum yields are found to decrease with increasing salinity. To explain this behaviour, experiments were pei formed with different salt solutions (NaCl, NaBr, BaCl2 and ZnCl2) using both solar irradiation and a photoreactor (lambda(excit) 313 nm). As with natural waters, marked effects of added salt were observed on the photolysis rater which decreased signif icantly from 0.67 h(-1) in the absence of salt to 0.11 h(-1) in the presenc e of BaCl2. In photoreactor studies, the observed order of quenching was Ba Cl2 > ZnCl2 and NaBr > NaCl. In contrast, neither the triplet state quenche r sorbic acid nor oxygen had any effect on the photolysis, supporting the i dea that photodegradation goes by the lowest excited singlet state of fenar imol. To understand the origin of the effect of added salts, fluorescence quenchi ng studies have been performed with various halide and nonhalide salts and the respective quenching constants determined. With NaCl and NaBr, the rati o of fluorescence quenching constants is identical to the ratio of reciproc al quantum yields for photodegradation of fenarimol, indicating a common me chanism. The correlation of fluorescence quenching rates with halide ion ox idation potentials strongly suggests that this involves an electron transfe r mechanism. It is suggested that the effect may be used to stabilise the f ungicide towards photodegradation. (C) 2000 Elsevier Science Ltd. All right s reserved.