The mechanism of atrazine degradation in the aqueous phase was investigated under sonolysis at 20 kHz, ozonation, photolysis at 254 nm and photocatalysis on TiO₂ under irradiation in the 315 - 400 nm wavelength range, employed either separately or in combination (1). Aim of the work was to compare the efficiency of such different advanced oxidation techniques in inducing not only atrazine degradation in water, but also its overall detoxification. Particular attention was focused on the influence of the irradiation wavelength in photoinduced water treatments and on the existence of possible synergies in the combined use of the investigated oxidation techniques, especially for achieving water detoxification. Information on the mechanism and efficiency of atrazine degradation under the different experimental conditions was obtained from the concentration profiles of the main degradation intermediates.

Ozonation and photocatalysis, both proceeding mainly through ^•OH radical attack, induced atrazine de-alkylation, followed by slower de-chlorination, while direct photolysis at 254 nm produced very efficient de-chlorination, through the homolytic cleavage of the C-Cl bond occurring from the electronically excited state of atrazine. Simultaneous sonolysis had beneficial effects on ozonation and photocatalysis, especially by increasing the rate of ^•OH radical initiated de-alkylation, and no effect on the unimolecular photolytic de-chlorination of atrazine. Thus, synergistic effects induced by sonolysis simultaneous to photocatalysis and ozonation, which have been observed in the degradation of different pollutants (2-4), in the case of atrazine degradation can mainly be attributed to the ultrasound-induced enhancement of mass transport phenomena and possibly to an increase of photocatalyst dispersion. An effectively higher concentration of reactive radical species produced by cavitation effects should be excluded by the fact that under sono-photocatalysis and sono-ozonation the same set of intermediate species was produced as under photocatalysis and ozonation, with ultrasound apparently inducing an increase of the rate constants of all degradation steps.

Complete atrazine mineralization, as monitored by TOC analysis, did not proceed beyond ca. 65%, corresponding to the removal of 5 out of the 8 carbon atoms originally present in the molecule. This confirms the well recognized stability of the s-triazine ring toward conventional oxidation (5), atrazine oxidative degradation proceeding only up to cyanuric acid formation. However, atrazine degradation and overall detoxification, as related to the disappearance of chlorinated by-products, proceeded at the highest rate when photolysis at 254 nm was combined with ozonation.

1. Bianchi, C.L., et al. (2006). Appl. Catal. B: Environ. in press.

2. Ragaini, V., et al. (2001). Ultrason. Sonochem. 8: 251-258.

3. Selli, E. (2002). Phys. Chem. Chem. Phys. 4: 6123-6128.

4. Selli, E. and Bertelli, M. (2004). Appl. Catal. B: Environ. 52: 205-212.

5. Pelizzetti, E., et al. (1990). Environ. Sci. Technol. 24: 1559-1565.

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