Photocatalytic degradation of methyl parathion: Reaction pathways and intermediate reaction products

The photocatalytic degradation of O,O-dimethyl O-p-nitrophenyl phosphorothioate (common name methyl parathion) in aqueous TiO2 suspensions is reported in this study. The aim of this article is to identify the intermediate organic reaction products using a number of analytical techniques, such as UV–vis, HPLC, GC–MS and 1H NMR. Total organic carbon (TOC) and the concentration of inorganic ions in the reaction mixture, as a function of reaction time, are also measured to monitor the mineralization of organic chemical species. Under both, acidic and basic conditions, the primary product is methyl paraoxon, which eventually is degraded to 4-nitrophenol. This aromatic compound is mainly transformed into hydroquinone, which is later oxidized to 1,2,4-benzenetriol. GC–MS studies show that this aromatic triol is preferentially converted via an oxidative pathway into 5-hydroxymethyl-5H-furfuran-2-one. Under basic conditions, methyl parathion is converted via a reaction pathway involving methyl paraoxon, 4-nitrophenol, hydroquinone and aliphatic acids with the resulting species being refractory to be further oxidized and mineralized. On the other hand, under controlled acidic conditions, methyl parathion and various intermediates quickly degrade in a rather fast oxidation process.