Full insights into the roles of pH on hydroxylation of aromatic acids/bases and toxicity evaluation.

Abstract Advanced oxidation processes (AOPs) based on hydroxyl radicals (•OH) are the most important technologies for the removal of bio-recalcitrant organic pollutants in industrial wastewater. The pH is one of the crucial environmental parameters that affect the removal efficiency of pollutants in AOPs. In this work, the mechanistic and kinetic insights into the roles of pH on the hydroxylation of five aromatic acids and bases in UV/H2O2 process have been investigated using theoretical calculation methods. Results show that the reactivity of •OH towards the twelve ionic/neutral species is positively correlated with electron-donating effect of substituents, which contributes to the positively pH-dependent reactivity of aromatic acids and bases towards •OH. The hydroxylation apparent rate constants (kapp, M−1 s−1) (at 298 K) increase as the pH values increase from about 1 to 10, but they decrease as the pH values increase from about 10 to 14. However, the best pH values for degradation are not around 10 because the [•OH] decreases continuously with the increasing pH values from 3 to 9.5. Combining the factors of kapp and [•OH], the best degradation pH values are around 5.5∼7.5 for p-hydroxybenzoic acid, p-aminophenol, aniline and benzoic acid, 3.0∼7.5 for phenol and 5.5∼7.5 for mixed pollutants of these aromatic acids/bases in UV/H2O2 process. Moreover, a significant number of hydroxylation by-products are more toxic or harmful to aquatic organisms and rat (oral) than their parental pollutants. Altogether, this work provides comprehensive understanding of the roles of pH on •OH-initiated degradation behavior of aromatic acids and bases.