Photocatalytic Degradation of p-Halophenols in TiO₂ Aqueous Suspensions: Halogen Effect on Removal Rate, Aromatic Intermediates and Toxicity Variations

The influence of the halogen upon the TiO₂ photocatalytic degradation of p-halophenols in water has been investigated. Phenol was used as the reference compound. Compared with its value for phenol, the apparent first-order rate constant of removal, k, was slightly but significantly higher for p-fluorophenol and p-chlorophenol, and slightly but significantly lower for p-bromophenol. For p-iodophenol, k was about half that of phenol. The relative values confirm that k is roughly correlated to the Hammett constant; this constant reflects the electron density on the aromatic ring and, accordingly, the reactivity towards electrophilic species generated by UV-irradiation of TiO₂. All compounds were found to be poorly adsorbed on TiO₂. Accordingly, k was not related to the differences observed in the very low adsorbed amounts. The detected aromatic intermediate products included hydroquinone (HQ), benzoquinone (BQ) and various halodihydroxybenzenes. HQ, BQ, 4-chloro-1,2 (and 1,3)-dihydroxybenzenes and 4-bromo-1,3-dihydroxybenzene were quantified. Mechanisms are tentatively suggested to interpret the differences in the degradation pathways of the p-halophenols. The organic intermediate products accounted for only a few percents of the total carbon during the degradation. The toxicity (1/EC₅₀) measured by the Microtox test almost did not vary in the course of the degradation of phenol, p-chlorophenol and p-bromophenol until complete removal of these compounds. By contrast, the value of 1/EC₅₀ was multiplied by ca. 2.5 when ca. 45% of p-iodophenol had been removed; concentrations of BQ higher than with the other p-halophenols are tentatively suggested to be at the origin of this increase. Interpretation of a surprising substantial increase in the 1/EC₅₀ value when the removal of p-fluorophenol increased from 80 to 95% requires further investigation.

Key Words:

• Photocatalytic degradation
• Halophenols
• Titanium dioxide
• Toxicity
• Photocatalytic water treatment.

ACKNOWLEDGMENTS

The authors wish to thank the European Commission for financial support under the CADOX Project (contract no. EVK1-CT-2002-00122). They also wish to express their gratitude to the Cooperation Program Environment EPFL-Colombia and V.P.F. Cooperation for the doctoral grant to S. Parra. Warm thanks are due to Mrs. M.N. Mozzanegga for her help throughout the work.