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-chlorophenyl)-alpha-(4-chlorophenyl)-5-pyrimidine-methanol) by solar radiation has been studied in natural waters with different salinities. Photodegradation rates 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 significantly from 0.67 h(-1) in the absence of salt to 0.11 h(-1) in the presence of BaCl2. In photoreactor studies, the observed order of quenching was BaCl2 > ZnCl2 and NaBr > NaCl. In contrast, neither the triplet state quencher sorbic acid nor oxygen had any effect on the photolysis, supporting the idea that photodegradation goes by the lowest excited singlet state of fenarimol. To understand the origin of the effect of added salts, fluorescence quenching studies have been performed with various halide and nonhalide salts and the respective quenching constants determined. With NaCl and NaBr, the ratio of fluorescence quenching constants is identical to the ratio of reciprocal quantum yields for photodegradation of fenarimol, indicating a common mechanism. The correlation of fluorescence quenching rates with halide ion oxidation potentials strongly suggests that this involves an electron transfer mechanism. It is suggested that the effect may be used to stabilise the fungicide towards photodegradation. (C) 2000 Elsevier Science Ltd. All rights reserved.