Environmental Photochemistry of the Herbicide Bromoxynil in Aqueous Solution Containing Soil Fulvic Acids

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Abstract
Photochemistry of the herbicide 3,5-dibromo-4-hydroxybenzonitrile (1) (bromoxynil) was investigated using a narrow band of ultraviolet radiation at 313 nm. UV absorption spectrometry of 7.8 μM 1 in water without and with soil fulvic acids (FAs) (50.0 mg L−1) showed that FAs exerted several fold additive effects on the spectrum of 1 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 L−1 FAs (pH 5.5–6.5) were carried out at 313 nm. For instance, for 15 min photolysis time, as the amount 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 5-100 mg L−1 were in the range of (1.052 ± 0.011) × 10−3 to (0.083±0.006) × 10−3 sec−1 The intensity of light (Iλ) at 313 nm ranged from 3.70 to 3.80 μEL−1 sec−1. The light screening factor (S λ) at λ = 313 nm along with Iλ 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. The experimentally determined Xs.λ values at λ = 313 nm together with thesolar irradiance (Z λ) data available in tabulated form in the literature assisted the calculation of the sunlight indirect photodegradation half-lives [t 1/2)Ind. sp] of bromoxynil (1) in aquatic systems containing soil FAs. For example, in summer, the [(t 1/2)Ind. sp] of 7.8 μM 1 in aquatic environments containing 5, 10, 15, 20, 40, 45, 60 and 100 mgL−1 would be 1.99±0.02, 2.02±0.02, 2.17±0.02, 2.76±0.04, 3.48±0.07, 4.06±0.06, 5.03±0.09 and 17.60±1.33 min, respectively. The photoproducts of 1 were identified as 3-bromo-4-hydroxybenzonitrile and 4-hydroxybenzonitrile by GC-MS.