Inefficient utilization of hydroxyl radicals during contact between dissolved ozone and granular activated carbon in...

Ozone (O₃) treatment generates hydroxyl radicals (·OH) and enables the removal of micropollutants, such as odor-causing compounds; therefore, it is widely used in drinking water treatment. In most cases, granular activated carbon (GAC) treatment is implemented downstream of ozonation; consequently, water containing dissolved O₃ may come into contact with the GAC. In this study, the effectiveness of ·OH-mediated reactions in bulk water and on GAC was compared. Benzenesulfonic acid (BSA) was established as a novel ·OH probe that exhibits relatively low adsorption onto GAC and can be sensitively quantified using liquid chromatography with fluorescence detection (limit of quantification 1.3 nM). Across a range of temperatures, pH values, and O₃ doses, BSA removal attributable to ·OH reactions on GAC, normalized per unit O₃ consumed, was consistently less than one-eleventh of that in bulk water. Operating without residual O₃ upstream of the GAC typically maintained BSA removal efficiency while markedly reducing bromate formation (a major concern in ozonation). Thus, when ozonation is operated with the objective of maximizing ·OH-driven reactions, maintaining residual O₃ upstream of the GAC tank is considered an inefficient operational strategy. Because achieving zero residual O₃ at the GAC influent requires only adjustment of the applied O₃ dose and does not necessitate major plant retrofits, this study provides practical and actionable operational guidance for ozonation.