Soil organic matter chemistry II. Effects of soil fulvic acids on the environmental photodecomposition of bromoxynil (3,5-dibromo-4-hydroxybenzonitrile) herbicide in water

Abstract Photochemistry of the herbicide 3,5-dibromo-4-hydroxybenzonitrile ( 1 ) (bromoxynil) was extensively investigated using ultraviolet radiation of wavelengths (λ) around 313 nm. Aqueous-phase photoreactions of 7.8 μM of 1 in water in the absence (pH 6.0–7.0 and presence of 5, 10, 15, 20, 40, 45, 60 and 100 mg 1 −1 fulvic acids (FAs) (pH 6.5–5.5) were carried out at 313 nm. For instance, for 15 min photolysis time, as the concentration of FAs increased, the rate of the photodestruction of 1 decreased. The first-order photolytic transformation rate constants ( k p, λ ) for 1 in the presence of 5–100mg 1 −1 FAs were in the range of (1.052 ± 0.011) × 10 −3 to (0.083 ± 0.006) × 10 −3 s −1 ; the intensity of light ( I λ ) at 313nm ranged from 3.70 to 3.80 μEl −1 s −1 . Light screening factor ( S λ ) at λ = 313 nm along with I λ and k p, λ data were used to determine the spectral response function ( X s, λ ) for the herbicide 1 in water containing various amounts of soil FAs using mathematical relationships described in the literature. These X s, λ parameters showed negative correlations with FAs concentrations. The experimentally determined X s, λ values at λ = 313nm, together with the solar irradiance ( Z λ ) data available in tabulated form in the literature, assisted the calculation of the sunlight indirect photodegradation half-lives [ (t 1 2 ) sp Ind. ] of bromoxynil in aquatic systems containing soil FAs. For example, [ (t 1 2 ) sp Ind. ] of the 7.8 μM herbicide 1 in aquatic environments containing 40 mg l −1 would be 3.481 ± 0.073 min. The quantum yields of photodegradation of 1 in H 2 O in the presence of soil FAs obeyed Stern-Volmer relationships. Though soil fulvic acids exerted diminishing effects on the rates of photodegradation of the herbicide 1 and on its quantum yield in water, the analysis of the latter data provided evidence that fulvic acids acted as photosensitizers during these reactions. Finally, the photoproducts of 1 both in the presence and absence of FAs were 3-bromo-4-hydroxybenzonitrile and 4-hydroxybenzonitrile, which were identified with the aid of GC-MS instrumentation.