The kinetics and mechanisms of removing atrazine by catalytic oxidation processes with or without the UV irradiation

Abstract

The degradation of herbicide, atrazine (ATZ), 2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine, by catalytic oxidation process (dark-COP) and UV-assisted catalytic oxidation process (UVCOP) was investigated in terms of Fe(II) and H2O2 concentrations (COP doses) and light intensities in a batch reactor. In dark-COP, around 66% to 97% of the ATZ was observed to be removed at the end of the experiment. In UVCOP, an exceptionally high reaction rate due to the "compound effect" was observed in the processes compared with the simple combination of the two individual reaction rates of direct photolysis (sole-UV) and dark-COP. This suggested that the UVCOP greatly improved the removal of ATZ. Such effect was further investigated through the examination of the kinetic data of all of the processes (dark-COP, sole-UV, and UVCOP), and quantified in terms of the UV intensities and dosages of the reagent. In addition, an enhancement index was introduced. It was useful for verifying the effectiveness of the compound effect and predicting the performance of the UVCOP. The transformation mechanisms of ATZ degradation in all the mentioned processes were also examined and compared. Depending on the selection of the processes, one to fourteen ATZ derivatives were detected by liquid chromatography electrospray tandem mass spectrometry (LC/ESI-MS/MS). It was found that alkylic-oxidation, dealkylation, and dechlorination-hydroxylation were the leading pathways in the dark-COP and the UVCOP. In addition, according to a toxicity index that introduced in this study, the UVCOP was suggested to be a more environmental-friendly process than the dark-COP. Furthermore, the photolytic destruction of ATZ following a surfactant-aided soil-washing process was investigated. Surfactant was found to be a good solving agent to extract ATZ without causing any retardation effect on the photolysis process. However, the co-extracted humic acid in the solution was found to be capable of improving the photolysis of ATZ at low humic concentration, while quenching the photodegradation of ATZ at high humic concentration. Accordingly, the quantum yields of the system with respect to the proposed reaction mechanisms of the associated excited states of ATZ were investigated and modeled. The relative kinetic rates of the dominant reaction mechanisms were therefore compared.