PHOTODEGRADATION OF BENZAMIDE IN TIO2 AQUEOUS SUSPENSIONS

Benzamide (BZA) was chosen as a model compound representative of water micropollutants with an amide functionality, and its degradation was studied in aerated TiO2 aqueous suspensions illuminated at lambda > 340 nm. The favorable photocatalytic effect of TiO2 was demonstrated by the much higher rate of disappearance of BZA in the presence of this photocatalyst than in its absence, even at lambda > 290 nm, despite competitive absorptivity of BZA in this latter spectral region. Among the aromatics formed in the photocatalytic degradation of BZA, the three hydroxy-benzamide (HBZA) isomers were identified as the main intermediates. However, their maximum concentration corresponded to only a very small percentage of the initial concentration of BZA and control experiments on the degradation of 2-HBZA or 4-HBZA under the same conditions showed that these compounds disappeared about as efficiently as BZA. Para-benzoquinone and hydroquinone were also detected as degradation products of BZA (traces) or 4-HBZA. Other intermediates were not identified but all of them disappear within about the same time as BZA: i.e., the complete elimination of aromatics was rapid. Although ca. 70% of the total organic carbon was degraded to CO2 when the dearomatization was achieved, complete mineralization needed a much longer illumination period. The slowness of the last mineralization stages is associated with the difficulty of converting the nitrogen atom of BZA into NO3-, as was confirmed by a study of the formation of nitrate ions from formamide.