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 kexp of 0.029 h−1 and a t1/2 of 23.8 h.
In contrast, transformation in distilled water was insignificant, with a first-order experimental rate constant kexp of 0.002 h−1 and an experimental half-life (t1/2) of 309 h. Under sunlight (late September to early October, 51°N), MCPA was transformed in river water with a pseudo first-order kexp of 0.009 h−1 and an experimental t1/2 of 7 days. (However, as experiments were conducted in test-tubes, this experimental t1/2 needs to be multiplied by a factor of 2 to provide an estimate of t1/2 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.
Stangroom, S.J., Macleod, C.L. and Lester, J.N., 1998. Photosensitized transformation of the herbicide 4-chloro-2-methylphenoxy acetic acid (MCPA) in water. Water Research, 32(3), pp.623-632.
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