Photocatalytic transformation of pesticides in aqueous titanium dioxide suspensions using artificial and solar light: intermediates and degradation pathways

Abstract

It has been shown that a wide range of organic compounds in aqueous solutions is photocatalytically oxidized to carbon dioxide in the presence of titanium dioxide with artificial and solar light. The photocatalytic mineralization of the most important classes of pesticides in water by illuminated TiO 2 has been reviewed. The study focuses on the determination of the nature of the principal organic intermediates and the evolution of the mineralization as well as on the degradation pathways followed during the process. The photocatalyzed degradation of pesticides does not occur instantaneously to form carbon dioxide, but through the formation of long-lived intermediate species. Their detection and identification are required in order to determine which kind of chemical structures are left at the end of the process. Various analytical techniques such as liquid–liquid extraction and solid phase extraction followed by gas chromatography-mass spectrometry (GC–MS), liquid chromatography–mass spectrometry (LC–MS) or 1 H -NMR were used for the determination of organic intermediates. Gas chromatography, ion chromatography, spectrophotometry, titration and other techniques were used for the monitoring of the so-called end-products such as CO 2, Cl −, SO 4 2−, NH 4 +, NO 3 −, etc. The main intermediates of pesticide degradation are of five types: (a) hydroxylated products and derivatives usually after dehalogenation of the parent pesticide, if halogen substituents are present (b) products of oxidation of the side chain, if present (c) ring opening products for aromatic pesticides (d) decarboxylation products and (e) isomerization and cyclization products. The identification of possible formation of highly toxic compounds even at low concentrations is essential for the assessment of treated water. Therefore, toxicity tests for the irradiated samples are also important to be conducted in combination with the identification of the intermediates.