Water treatment by TiO2 photocatalysis and/or ultrasound: degradations of phenyltrifluoromethylketone, a trifluoroacetic-acid-forming pollutant, and octan-1-ol, a very hydrophobic pollutant

Abstract

TiO2 photocatalysis and ultrasound are advanced oxidation processes for water treatment. Our study aimed at showing, via the purposely chosen title compounds, that these techniques can be complementary. For C6H5COCF3 (PTMK), the photocatalytic removal rate was higher than the ultrasonic (515 kHz) removal rate in the presence of TiO2 in the dark, whereas it was the opposite for octan-1-ol under the conditions employed. Simultaneous UV and ultrasound irradiation of the TiO2 suspension led, for PTMK, to a removal rate about equal to the sum of the removal rates observed for separate irradiations, and decreased by a factor of approximately 20% for octan-1-ol as if the photocatalytic degradation was suppressed by the dominant distribution of octan-1-ol to the cavitation bubbles. This distribution was substantiated by the large detrimental effect of octan-1-ol on the PTMK ultrasonic removal rate. The concurrent use of both techniques allowed a faster removal of both pollutants in binary mixtures. The amount of CF3COOH from PTMK was approximately eight times lower in sonicated, than in UV-irradiated, TiO2 suspensions. Several intermediate products showed the occurrence of chemical transformations occurring in and/or on the cavitation bubbles. COD decline and CO2 formation were initially higher for ultrasonic than for photocatalytic treatment. However, complete mineralization (except for CF3COOH) was achieved more rapidly by photocatalysis and even more rapidly by simultaneous use of both techniques.