The hydrogen peroxide-assisted photocatalytic degradation of alachlor in TiO₂ suspensions

Photocatalytic degradations of alachlor in TiO2 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 TiO2 enhanced the reaction rates by 12 and 26 times using 300 and 350 nm UV irradiation, respectively. The results showed that a low H2O2 dosage in photocatalysis using 300 nm UV would enhance the rates by 3.3 times, but an overdose of H2O2 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 H2O2. A neutral initial pH level was found to favor the H2O2 assisted photocatalysis at 300 nm UV illumination. Eleven major intermediates were identified by liquid chromatography-electrospray ionization-mass spectrometry (LC-ESI-MS) and MS/MS. The major degradation mechanisms of H2O2-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.