Photocatalytic degradation of pesticide pirimiphos-methyl -: Determination of the reaction pathway and identification of intermediate products by various analytical methods

The solar photocatalytic degradation of an organophosphorous pesticide (pirimiphos-methyl, PMM) has been mimicked in a microphotoreactor operating with an artificial light flux, which could be attenuated to values close to that of sun in Almeria. The catalyst was titania Degussa P-25 (50 m(2) g(-1)). The aim of this article was the identification of the maximum of possible intermediate products using a large set of powerful analytical techniques, such as GC, HPLC, GC-MS, TOC analysis, and, especially, LC-MS provided with the new atmospheric pressure ionization (API) interfaces (APCI and ES). In our conditions, practically total disappearance of PMM was achieved in 40 min, whereas total organic carbon (TOC) disappearance required 6 h. In parallel, the commercially formulated PMM was also degraded, but required a longer time (1 h) for total disappearance, because of either a stabilization effect due to the formulating agents and/or of a competition of these organic agents for degradation. Heteroatoms (P, S, N) were mineralized into phosphate, sulfate and nitrate anions, respectively. Interestingly microtox test was done during the photodegradation, indicating that the first intermediates formed during the first 20 min were more toxic than initial PMM. Toxicity tended, then, to zero, in parallel to TOC disappearance. A thorough analysis of the titania suspension performed with the analytic methods mentioned above, enabled one to identify 27 intermediate metabolites, which were into two tentative degradation routes. One was based on the initial photoassisted hydrolysis of the amino-aromatic N-C bond and the other one on the transient preservation of the thiophosphoric moiety. This work constitutes an example of a thorough chemical analysis study necessary for an extended knowledge of the successive steps in a solar-assisted water detoxification process. (C) 1999 Elsevier Science B.V. All rights reserved.