Photochemistry of halogenated benzene derivatives. X : effects of sodium chloride on the aquatic photodegradation of bromoxynil (3,5-dibromo-4-hydroxybenzonitrile) herbicide

Bromoxynil formed readily in natural waters from its potassium salt or esters, which were active ingredients in several herbicides used for weed control in Canadian prairie agriculture (1988). UV light absorption spectra were recorded for solutions of bromoxynil in water (7.8 uM) and in aqueous sodium chloride (0.5-25 mM), after 0-60 minutes irradiation at 313 nm. For photolysis of bromoxynil, quantum yields decreased from 0.052 at 0 mM to 0.017 at 25 mM sodium chloride. Four photoproducts formed in the presence of sodium chloride were identified by gas chromatography-mass spectrometry: 4-hydroxybenzonitrile (4-HB) and 3-bromo-4-HB, both of which could form without sodium chloride; also 3-bromo-5-chloro-4-HB and 3-chloro-4-HB, formed by photoincorporation of chloride ions into bromoxynil, a mechanism for which is suggested. Formation of 4-HB decreased with increasing sodium chloride concentration, and this secondary photoproduct was not detected in a bromoxynil (200 uM) and sodium chloride (0.5 M) mixture UV-irradiated for up to 3 h.