Comparative degradation kinetics of alachlor in water by photocatalysis with FeCl 3, TiO 2 and photolysis, studied by solid-phase disk extraction followed by gas chromatographic techniques

Abstract

A comparative degradation study of alachlor spiked at 30–60 μg/1 in water using photocatalysis with FeCl 3, TiO 2 and photolysis using a xenon arc photoreactor was performed. After irradiation the water samples were preconcentrated using solid-phase disk extraction with C 18 and analyzed by gas chromatography-electron capture and mass spectrometric detection (GC-ECD and GC-MS, respectively). Three photoproducts were unequivocally identified corresponding to 2-hydroxy-2,6′-diethyl-N-methylacetanilide, 8-ethyl-1-methoxymethyl-4-methyl-2-oxo-1,2,3,4-tetraquionoline and hydroxyalachlor. Two photoproducts were identified by GC-MS although their chemical structure is unknown and corresponded to two compounds with base peaks at m/ z values of 146 and 218 and molecular masses of 221 and 233, respectively. Although alachlor showed high stability in water when exposed to natural sunlight irradiation for a period of more than 120 h, by the use of photocatalysis with of FeCl 3 or TiO 2 the degradation was very fast with half lives varying from 10–17 min. The degradation kinetics followed a first order reaction and the relative derivation, for n = 4, varied from 2–16 or 27–33% under photocatalysis or photolysis.