CONSEQUENCES OF THE PARTITION BETWEEN WATER AND MOLECULAR AGGREGATES ON THE PHOTODEGRADATION PATTERN AND KINETICS OF A PESTICIDE OF THE COUMARIN FAMILY

In natural waters, hydrophobic pollutants are predominantly associated either with the ''dissolved'' organic matter or with biological struc tures. Under these conditions, the pattern of degradation, particularl y of photodegradation, and its kinetics may be drastically modified. T his fact results from the effect of the interface upon the reaction me chanism and also from the microcompartmentalization of reactants. Whil e the modifications induced by the presence of interfaces have been th e subject of much research, the consequences of microcompartmentalizat ion have, so far, been neglected. The objective of this paper is to st udy the effect of the distribution of the pollutant in microreactors a nd to demonstrate that this may result in marked differences in the na ture and relative amounts of the degradation products. A model system consisting of a pesticide, coumaphos, in SDS micelles is studied. It i s shown that there are no significant differences in the nature of the photodegradation products formed in air-equilibrated aqueous media an d in the presence of SDS. In both cases, two isomer dimers and a produ ct resulting from the opening of the coumarinic ring are obtained. How ever, their relative yields strongly depend on the relative concentrat ion of pesticide and surfactant. This result is quantitatively explain ed using a kinetic model for the analysis of photoreactions in microco mpartmentalized systems that is developed in the paper.