Influence of pH and irradiation wavelength on the photochemical degradation of sulfonylureas

The photochemical behavior of two sulfonylurea herbicides have been investigated in solution at different irradiation wavelength ranges and pH. Spectrophotometric methods have permitted to determine the pKa values in the ground state and in the first singlet excited state. Kinetics and quantum efficiency measurements have shown that photodegradation was faster in acidic medium but more efficient in basic medium, when the sulfonylureas were in their ionic form. The identification of the photoproducts and their evolution during irradiations have indicated that three competitive pathways occurred: carbon–sulfur (C–S) cleavage, nitrogen–sulfur (N–S) cleavage, and photohydrolysis of the sulfonylurea bridge. Their preponderance depended on the irradiation wavelengths.