Formation of highly toxic p-benzoquinones byproducts during ozonation of cresols.

This study aims to identify the highly non-specific toxic by-products during ozonation of three cresols in wastewater. In ozonated effluents, biotoxicity increased along with increasing reaction time, followed by a gradual decrease. The peak biotoxicity for ozonated o-cresol (o-C), m-cresol (m-C), and p-cresol (p-C) was estimated to be 17.4, 14.8 and 5.5 times higher than that of untreated wastewater, respectively. A redox-directed approach with high-resolution mass spectrometry detection and toxicity prediction revealed that monomeric para-benzoquinones (p-BQs), hydroxylated p-BQs, and dimeric p-BQs in ozonated cresols were the primary contributors to the increased toxicity. Calculations based on density functional theory indicated formation pathways of p-BQs byproducts, e.g., the formation of 2-methyl-p-benzoquinone was likely induced by ozone molecules rather than hydroxyl radicals in ozonated o-C and m-C, and the formation of p-BQs during ozonation of p-C was attributed to the oxidation of methyl group to carboxyl group and subsequent decarboxylation initiated by hydroxyl radicals. Electron paramagnetic resonance and spin density calculation showed that the presence of carbon-centered cresoxyl radicals was responsible for dimeric p-BQs formation. Collectively, these results underscore significant contribution of non-halogenated p-BQs to non-specific toxicity increase in ozonated effluents.