ENVIRONMENTAL PHOTOCHEMISTRY OF THE HERBICIDE BROMOXYNIL IN AQUEOUS-SOLUTION CONTAINING SOIL FULVIC-ACIDS

Photochemistry of the herbicide 3,5-dibromo-4-hydroxybenzonitrile (I) (bromoxynil) was investigated using a narrow band of ultraviolet radiation at 313 nm. UV absorption spectrometry of 7.8 1 in water without and with soil fulvic acids (FAs) (50.0mgL−1) showed that FAs exerted several fold additive effects on the spectrum of 1. Aqueous phase photoreactions of 7.8μM of 1 in water in the absence (pH 6.0-7.0) and presence of 5, 10, 15, 20, 40, 45, 60 and l00 mgL−1FAs (pH5.5-6.5) were carried out at 313nm. For instance, for 15 min photolysis time, as the amount of FAs increased, the rate of the photodestruction of I decreased. The first-order photolytic transformation rate constants (kp λ)for 1 in the presence of 5-100mgL−lwere in the range of (1.052±0.011) x 10'3to (0.083±0.006) x 10−3 sec”1, the intensity of light (/A) at 313 nm ranged from 3.70 to 3.80 μEL −1 sec− s. The light screening factor (SA) at λ= 313nm along with/λand kp A data were used to determine the spectral response function (a'sλ) for the herbicide 1 in water containing various amounts of soil FAs. The experimentally determined Xs λvalues at λ= 313nm together with the solar irradiance (ZA) data available in tabulated form in the literature assisted the calculation of the sunlight indirect photodegradation half-lives [t1/2)sp ind.] of bromoxynil (1) in aquatic systems containing soil FAs. For example, in summer, the [(t1/2)sp ind.]of 7.8μM 1 in aquatic environments containing 5, 10, 15, 20, 40, 45, 60 and l00 mgL−1would 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. © 1990, Taylor & Francis Group, LLC. All rights reserved.