The pH-dependent contributions of radical species during the removal of aromatic acids and bases in light/chlorine systems

The solar/chlorine and UV/chlorine systems as emerging advanced oxidation technologies (AOTs) are used for degrading trace organic contaminants (TrOCs) through generating various radical species. The pH is a vital parameter that significantly affects the degradation efficiency of contaminants. In this study, six aromatic acids and bases (AABs) are selected to investigate the pH-dependent degradation mechanisms and kinetics by two newly-discovered radicals (ClO• and BrO•). Among the 16 dissociation species, the structures with electron-rich rings possess stronger reactivities to ClO• and BrO• than those with electron-poor rings, which is similar to the result of HO•. However, ClO• and BrO• are considered to be more selective and pH-sensitive reacting with AABs than HO• based on the corresponding second-order rate constants (M−1 s−1). Compared with acidic pH, the basic pH could improve the degradation rate of most aromatics in both systems. As pH increases from 6 to 8, the contribution percentages of ClO• in terms of the removal of the aromatics (except for benzoic acid) in solar/chlorine rise more rapidly (from 0.33 ∼ 61.34% to 94.23 ∼ 99.09%) than those in UV/chlorine system (from 19.14 ∼ 99.60% to 96.75 ∼ 99.88%). The pH-dependent contributions of various radical species are attributed to structure-dependent reactivities of compounds and pH-dependent concentrations of radical species. As the dose of Br- increases from 0 to 10 μM, the contributions of BrO• to the removal of aromatics (except for benzoic acid) increase from 0% to 10.28 ∼ 19.32%, thanks to the increased [BrO•]ss. This work is necessary for enhancing the understanding of the pH-dependent contributions of individual species during the removal of dissociable aromatic contaminants in light/chlorine systems.