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

Advanced oxidation processes (AOPs) based on hydroxyl radicals (center dot 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 center dot 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 center dot OH. The hydroxylation apparent rate constants (k(app), 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 [center dot OH] decreases continuously with the increasing pH values from 3 to 9.5. Combining the factors of k(app) and [center dot OH], the best degradation pH values are around 5.5 similar to 7.5 for p-hydroxybenzoic acid, p-aminophenol, aniline and benzoic acid, 3.0 similar to 7.5 for phenol and 5.5 similar to 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 center dot OH-initiated degradation behavior of aromatic acids and bases. (c) 2020 Elsevier Ltd. All rights reserved.

Automated summary

This study investigated the mechanistic and kinetic insights into the roles of pH on the hydroxylation of five aromatic acids and bases in UV/H2O2 process. The reactivity of center dot OH towards the species is positively correlated with electron-donating effect of substituents, contributing to the positively pH-dependent reactivity of aromatic acids and bases. The optimal degradation pH values are around 5.5 to 7.5.