Characterization of the photodegradation products of metolachlor: structural elucidation, potential toxicity and persistence.

Aqueous solutions of metolachlor and metolachlor-d(6) were photolyzed with UV-visible radiations. The structures of 15 by-products of metolachlor were determined through gas chromatography-mass spectrometry analyses using electron and chemical ionization combined with multistage mass spectrometry. The photolysis by-products of metolachlor resulted mainly from dehalogenation and hydroxylation, in some cases accompanied by cyclization. In silico tests for toxicity prediction showed that the toxicity of some photolysis products is expected to be greater than that of metolachlor. Persistence studies showed that the by-product relative abundances vary in large amounts with the irradiation time. The post-photolysis evolution of the solution was also studied, in order to determine the persistence of the main by-products. It allowed to establish that most of the by-products can be found more than 12 h after the end of the photolysis, which is of a great concern as treated water is generally available for consumption only a few hours after treatment in most of industrial processes.