Photocatalytic degradations of alachlor in TiO{sub}2 suspensions with and without the use of hydrogen peroxide were studied using two different monochromatic UV irradiations (300 and 350 nm). Direct photolysis of alachlor was a rather slow process, but the addition of TiO{sub}2 enhanced the reaction rates by 12 and 26 times using 300 and 350 nm UV irradiation, respectively. The results showed that a low H{sub}2O{sub}2 dosage in photocatalysis using 300 nm UV would enhance the rates by 3.3 times, but an overdose of H{sub}2O{sub}2 will retard the rate due to the hydroxyl radicals are consumed. However, this process is impracticable at 350 nm due to the absorption characteristic of H{sub}2O{sub}2. A neutral initial pH level was found to favor the H{sub}2O{sub}2 assisted photocatalysis at 300 nm UV illumination. Eleven major intermediates were identified by liquid chromatography-electrospray ionizationmass spectrometry (LC-ESI-MS) and MS/MS. The major degradation mechanisms of H{sub}2O{sub}2-assisted alachlor photocatalysis include dechlorination, dealkylation, hydroxylation, cyclization, scission of C-O bond, and N-dealkylation. Bell-shaped evolution profiles of different intermediates were observed. Degradation pathways were proposed accordingly to illustrate series of degradation steps. The TOC analysis revealed the different stages of the reaction.
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