SOIL ORGANIC-MATTER CHEMISTRY .2. EFFECTS OF SOIL FULVIC-ACIDS ON THE ENVIRONMENTAL PHOTODECOMPOSITION OF BROMOXYNIL (3,5-DIBROMO-4-HYDROXYBENZONITRILE) HERBICIDE IN WATER

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 (kp,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 (Iλ) at 313nm ranged from 3.70 to 3.80 μEl-1 s-1. Light screening factor (Sλ) at λ = 313 nm along with Iλ and kp,λ data were used to determine the spectral response function (Xs,λ) for the herbicide 1 in water containing various amounts of soil FAs using mathematical relationships described in the literature. These Xs,λ parameters showed negative correlations with FAs concentrations. The experimentally determined Xs,λ values at λ = 313nm, together with the solar irradiance (Zλ) data available in tabulated form in the literature, assisted the calculation of the sunlight indirect photodegradation half-lives [(t 1 2)spInd.] of bromoxynil in aquatic systems containing soil FAs. For example, [(t 1 2)spInd.] 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 H2O 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. © 1990.