The photocatalytic degradation of alachlor and dicamba in TiO2 suspension

Abstract

Direct photolysis and photocatalytic degradations of two widely used herbicides: alachlor and dicamba were studied using three different monochromatic UV lamps (254 nm, 300 nm and 350 nm). The photocatalytic reactions were studied under various conditions including the examination of the effects of different TiO2 sources, TiO2 dosages, initial pH and proton sources. The influence of the addition of hydrogen peroxide in photocatalysis of alachlor and dicamba was also investigated using the UV at 300 and 350 nm. Degradation mechanisms of alachlor were studied through intermediates identification. Both the direct photolysis and photocatalytic degradations of alachlor and dicamba follow pseudo first-order decay kinetics. Direct photolysis was a rather slow process, but the rate constants for alachlor photocatalysis were increased by about 11 and 26 times at 300 and 350 nm respectively, whereas the rate constants for dicamba photocatalysis were increased by about 3 and 5 times. TiO2-P25 was found to be an effective photocatalyst compared to TiO2-BDH. The direct photolysis of alachlor and dicamba was dominant at 254 nm even if TiO2 was present in the solution. Among the three UV wavelengths used, the highest photocatalysis quantum yield was obtained at 300 nm. The photocatalytic degradation rate of alachlor and dicamba increased with the dosages of TiO2, but an overdose of TiO2 would retard the reaction due to light attenuation. Photocatalysis rate of alachlor were slightly enhanced in an alkaline medium, and the different proton sources causing various degrees of rate retardation were due to the presence of the corresponding counter anions. At low to medium pH ranges, photocatalysis rate of dicamba were increased with pH because of the increase of hydroxide ions, but the reaction was gradually retarded at alkaline medium when the effect of charges repulsion was dominant. Rate retardation caused by the presence of the corresponding counter anions was also observed. Photocatalytic degradations of alachlor and dicamba in TiO2 suspensions with and without the use of hydrogen peroxide were studied using two different monochromatic UV irradiations (300 and 350 nm). The results of H2O2 assisted photocatalysis experiments showed that a low H2O2 dosage in photocatalysis using UV at 300 nm would enhance the pseudo first order decay rates by 3.3 times for alachlor and 2.4 times for dicamba, but overdose of H2O2 will retard the rate because of the expenditure of hydroxyl radicals. However, this process was found impracticable at UV 350 nm due to the absorption characteristic of H2O2. In photocatalysis or H2O2-assisted photocatalysis of alachlor, a total of thirteen major intermediates were successfully identified using Liquid chromatographyelectrospray ionization-mass spectrometry (LC-ESI-MS) and MS/MS. The major degradation mechanisms of alachlor photocatalysis include dechlorination, dealkylation, hydroxylation, scission of C-O bond, cyclization and N-dealkylation. Bell-shaped evolution profiles of different intermediates were observed. Accordingly, the degradation pathways of alachlor photocatalysis were proposed in relation to the intermediates evolution profiles to successfully illustrate series of degradation steps.