Scavenger-Probed Mechanisms in the Ultrasound/Chlorine Sono-Hybrid Advanced Oxidation Process

Sonochlorination (US/chlorine) is an emerging sonohybrid advanced oxidation process whose performance reportedly surpasses that of its individual components. However, the underlying oxidant budget is still being debated. We mapped the mechanism by systematically probing the US/chlorine system with selective scavengers (ascorbic acid, nitrobenzene, tert-butanol, 2-propanol, and phenol), competing anions (nitrite), and natural organic matter (humic acid). The kinetic hierarchy US/chlorine > US > chlorine remained consistent across all conditions, though its magnitude depended heavily on the matrix composition. Efficient •OH traps, such as alcohols and nitrobenzene, only partially suppressed the US/chlorine system. However, they greatly slowed sonolysis. This reveals a substantial non-•OH channel in the hybrid process. Ascorbic acid eliminated synergy by stoichiometrically removing free chlorine. Phenol quenched HOCl and chlorine-centered radicals. Nitrite imposed a dual penalty by scavenging •OH and consuming HOCl via the nitryl chloride (ClNO2) pathway. Humic acid acted as a three-way sink for •OH, HOCl, and chlorine radicals. These patterns suggest that reactivity is co-controlled by Cl•, Cl2•−, and ClO•. The results obtained are mechanistically consistent with cavitation-assisted activation of HOCl/OCl− at pH 5–6, where HOCl concentration is maximal. This yields a mixed oxidant suite in which Cl2•− is the dominant bulk oxidant, Cl• provides fast interfacial initiation, and ClO• offers selective support.