PHOTOSENSITIZED TRANSFORMATION OF THE HERBICIDE 4-CHLORO-2-METHYLPHENOXY ACETIC-ACID (MCPA) IN WATER

The photochemical transformation of the chlorophenoxy herbicide MCPA w as investigated under a variety of reaction conditions using distilled water, Thames river water and solutions of naturally occurring photos ensitizers. 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 in significant, 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 sunl ight (late September to early October, 51 degrees N), MCPA was transfo rmed in river water with a pseudo first-order k(exp) of 0.009 h(-1) an d an experimental t(1/2) of 7 days. (However, as experiments were cond ucted in test-tubes, this experimental t(1/2) needs to be multiplied b y 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 U V in the presence of nitrate, nitrite and humates tested singly and in combination in distilled water. Thus,MCPA is photo-oxidized in the pr esence of hydroxyl radicals, which are generated by nitrate and nitrit e following absorption of UV light and also by photo-oxidants generate d by humates. In tests where combinations of sensitizers were varied o ver 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 resul ts indicate that although hydroxyl radicals produced by nitrate and ni trite appear to enhance rates of photo-oxidation, their relative impor tance is significantly diminished in mixtures of sensitizers due to sc avenging of hydroxyl radicals by humates. Thus, photo-oxidants generat ed by humates appear to be of greater environmental significance than hydroxyl radicals in the phototransformation of MCPA. (C) 1998 Elsevie r Science Ltd. All rights reserved.