Environmental remediation by an integrated microwave/UV-illumination technique: IV. Non-thermal effects in the microwave-assisted degradation of 2,4-dichlorophenoxyacetic acid in UV-irradiated TiO2/H2O dispersions

After establishing proof of concept in the first three articles of this series, in which we used the rhodamine-B dye to test the methodology, we herein report a study on the photocatalyzed degradation of 2,4-dichlorophenoxyacetic acid (2,4-D) in aqueous TiO2 dispersions that were subjected to simultaneous irradiation by UV light and microwave (MW) radiation. The characteristics of the process were monitored for the effect of microwave radiation by examining the fate of each substituent group, viz. the carboxylic acid group, the chlorine substituents and the benzene ring of the 2,4-D structure. The degradation dynamics of the microwave-assisted process were examined by decrease of UV absorption, loss of total organic carbon (TOC), formation of Cl− ions (dechlorination), and identification of intermediates by electrospray mass spectral techniques. The thermal effect of the microwave radiation to the system was considered by comparing results from MW-generated heat versus externally applied heat. The effect of dissolved oxygen or absence of oxygen on the degradative process was also investigated. The greater efficiency of the MW-assisted process is ascribed to a non-thermal effect of microwave radiation on the break-up of the aromatic ring of 2,4-D (oxidation) but evidently not on the dechlorination process (reduction) for which MW radiation seems to have a negligible influence, if any. Factors involved in this non-thermal component and a mechanism are proposed for the initial stages of the degradation.