Résumé:
Chlorothalonil (CT ; 1,3-dicyano-2,4,5,6-tetrachlorobenzene), a broad-use fungicide, is likely to reach water supplies and contaminate them. Its phototransformation in water, assisted by acetonitrile (ACN) as a co-solvent, was studied under concentration and irradiation conditions close to those prevailing in the environment. It appears that direct photolysis is slow, but accelerated by photoproducts of CT or acetonitrile. The dissolved molecular oxygen acts strongly on the reaction rate, its suppression accelerating the reaction by a factor of 110. Four major photoproducts were identified. Their distribution varies considerably with the concentration of oxygen. In the water-rich medium, the major product is trichloro-1,3- dicyanobenzene (HCT, obtained by substitution of Cl by H). The formation of 4-OHCT (substitution of Cl at position 4 by OH) and CTOOH (removal of Cl, gain of OOH) is substantially favoured by enrichment of the medium with ACN and, in parallel, with O2. Conversely, the deoxygenation of the medium completely cancels the formation of these two products, in favour of CTH and a positional isomer of 4-OHCT, which quantum calculations assign to 2-OHCT. Whatever the conditions prevailing in the reaction medium, dechlorination, with the formation of HCl, is the privileged reaction pathway for degradation of CT and its photoproducts. Transient spectroscopy demonstrated the formation and participation of the triplet excited state of CT (3CT*) and the CTH• radical, derived from the triplet by addition of H, in the CT transformation. The kinetic and spectral characteristics of the triplet were evaluated, as well as the quantum efficiency of the intersystem crossing (ISC). The formation of singlet oxygen (1O2) was established by phosphorescence measurements. The quantum yield of 1O2 production varies from 0.85 (in 100% ACN) to 0.69 (in 30% ACN). In air saturated solutions, the triplet fraction that is trapped by oxygen ranged from 0.88 in pure ACN
to 0.48 in water-acetonitrile (95:5, % v/v). In agreement with the various results, a phototransformation mechanism is proposed. In surface water, one may expect that the disappearance of chlorothalonil will be promoted mainly by the organic compounds that this water contains and that are able of reducing the triplet. Chlorothalonil, in turn, generates high amounts of singlet oxygen, a species that is able of efficiently oxidizing a large number of pollutants in surface water.
