Comparative degradation kinetics of alachlor in water by photocatalysis with FeCl3, TiO2 and photolysis, studied by solid-phase disk extraction followed by gas chromatographic techniques

A comparative degradation study of alachlor spiked at 30-60 mu g/l in water using photocatalysis with FeCl3, TiO2 and photolysis using a xenon are 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 FeCl3 or TiO2 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.