Toxicity and color formation during ozonation of mono-substituted aromatic compounds

This research investigates the aqueous toxicity occurring during color formation due to ozonation in a semi-batch reactor. The tested mono-substituted aromatic compounds were aniline, phenol (-NH2' -OH, electron-donating group), nitrobenzene and benzoic acid (-NO2' -COOH, electron-withdrawing group). The results revealed that the aqueous toxicity of the Microtox assay from oxidized samples during the early stage of ozonation increased when the color formation was observed, except in the case of nitrobenzene under high pH conditions. Toxicity typically reached a peak at or near the moment of maximum color intensity. Ozonation of these aromatic compounds showed a large increase in toxicity, and the 50% effective concentration (EC50) expressed as chemical oxygen demand (COD) (mg 1(-1)) decreased to 1.4 (aniline), 1.2 (benzoic acid), 0.8 (phenol) and 0.6 (nitrobenzene) orders-of-magnitude. Moreover, the aromatic compounds with an electron-donating group promoted color formation more than did the electron-withdrawing group substituted compounds.