Photosensitized transformation of the herbicide 4-chloro-2-methylphenoxy acetic acid (MCPA) in water

The photochemical transformation of the chlorophenoxy herbicide MCPA was investigated under a variety of reaction conditions using distilled water, Thames river water and solutions of naturally occurring photosensitizers. Under low intensity UV (300–450 nm), MCPA was transformed in river water with a pseudo first-order k exp of 0.029 h −1 and a t 1/2 of 23.8 h. In contrast, transformation in distilled water was insignificant, with a first-order experimental rate constant k exp of 0.002 h −1 and an experimental half-life ( t 1/2) of 309 h. Under sunlight (late September to early October, 51°N), MCPA was transformed in river water with a pseudo first-order k exp of 0.009 h −1 and an experimental t 1/2 of 7 days. (However, as experiments were conducted in test-tubes, this experimental t 1/2 needs to be multiplied by a factor of 2 to provide an estimate of t 1/2 for the flat surface of a natural water body.) MCPA was also degraded under low intensity UV in the presence of nitrate, nitrite and humates tested singly and in combination in distilled water. Thus, MCPA is photo-oxidized in the presence of hydroxyl radicals, which are generated by nitrate and nitrite following absorption of UV light and also by photo-oxidants generated by humates. In tests where combinations of sensitizers were varied over environmental concentrations, the pseudo first-order k exp ranged from 0.03 h −1 (s.e. 0.0008) to 0.048 h −1 (s.e. 0.0022). The results indicate that although hydroxyl radicals produced by nitrate and nitrite appear to enhance rates of photo-oxidation, their relative importance is significantly diminished in mixtures of sensitizers due to scavenging of hydroxyl radicals by humates. Thus, photo-oxidants generated by humates appear to be of greater environmental significance than hydroxyl radicals in the phototransformation of MCPA.