Photocatalytic degradability of substituted phenols over UV irradiated TiO2

Synthetic wastewaters consist of substituted phenols: two hydroxyphenol (hydroquinone (hydro) and resorcinol), 4-nitrophenol (4-NP), 2,4-dinitrophenol (2,4-DNP) and 2,4,6-trinitrophenol (2,4,6-TNP) were photocatalytically degraded in aqueous TiO2 suspension. The influence of the nature of the substituent on the photodegradation has been investigated by comparing the initial degradation rates (v0 relative to phenol) to the different substituted phenols. This reactivity follows Hammett law with regard to the effect of nature group on phenolic fonctionality. The plot of v0=f(σ) shows a poor correlation when considering all the substituted phenols, when the nitrophenols (NO2 substituents) are evaluated separately and the hydroxyphenol (OH substituent) is not taken into account, good correlations are observed in both groups of data. A considerably influence of pH upon the kinetics of chemical oxygen demand (COD) disappearance was observed. In general, acidic pH is preferred to COD removal for all phenolic compounds. Photocatalysis transforms the nitro group in nitrophenols into NO3− and NH4+ through the formation of NO2− intermediate. The amount of nitrate depends on the number of nitro group in nitrophenol. The selectivity in nitrate ions is equal to 80, 56 and 66%, respectively, for 4-NP, 2,4-DNP and 2,4,6-TNP. It was also found that all the photo-treated solutions which we evaluated the ratios 5-day biological oxygen demand (BOD5)/COD show values higher than those recorded in initial solutions which is an indication of the positive effect of the applied photo-treatment.