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 (pir imiphos-methyl, PMM) has been mimicked in a microphotoreactor operating wit h an artificial light flux, which could be attenuated to values close to th at 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 wit h the new atmospheric pressure ionization (API) interfaces (APCI and ES). I n our conditions, practically total disappearance of PMM was achieved in 40 min, whereas total organic carbon (TOC) disappearance required 6 h. In par allel, the commercially formulated PMM was also degraded, but required a lo nger time (1 h) for total disappearance, because of either a stabilization effect due to the formulating agents and/or of a competition of these organ ic agents for degradation. Heteroatoms (P, S, N) were mineralized into phos phate, sulfate and nitrate anions, respectively. Interestingly microtox tes t was done during the photodegradation, indicating that the first intermedi ates formed during the first 20 min were more toxic than initial PMM. Toxic ity tended, then, to zero, in parallel to TOC disappearance. A thorough ana lysis of the titania suspension performed with the analytic methods mention ed above, enabled one to identify 27 intermediate metabolites, which were i nto two tentative degradation routes. One was based on the initial photoass isted hydrolysis of the amino-aromatic N-C bond and the other one on the tr ansient preservation of the thiophosphoric moiety. This work constitutes an example of a thorough chemical analysis study necessary for an extended kn owledge of the successive steps in a solar-assisted water detoxification pr ocess. (C) 1999 Elsevier Science B.V. All rights reserved.