Ultrasonic cavitation-induced radical processes for tetracycline degradation and Cr(VI) reduction: Highlighting the pivotal role of Cr(V) intermediates.

Abstract

Tetracycline (TC) and Cr(VI) often co-occur in wastewaters, where their synergistic toxicity and contrasting redox behaviors complicate remediation. In this study, ultrasonic cavitation was employed as a employed as a green and efficient process to simultaneously degrade TC and reduce Cr(VI). Under optimal conditions (20 kHz, 12 mm probe, 131 μm amplitude, 308 K), TC and Cr(VI) removal reached 62.9 % and 70.8 % within 2 h, respectively. The synergistic mechanism was elucidated through radical quenching, electron paramagnetic resonance (EPR) spectroscopy, and complementary computational fluid dynamics (CFD) simulations and density functional theory (DFT) calculations. Cavitation bubble collapse generated both oxidative and reductive radicals, enabling concurrent oxidation and reduction. Radical identification showed that ·O₂^- and ·H were the dominant reductants responsible for Cr(VI) reduction, whereas ¹O₂ and ·OH primarily controlled TC degradation. CFD simulations further demonstrated that mechanical energy, internal energy, and bubble growth rate during cavitation were positively correlated with radical generation. Importantly, experimental evidence suggested that Cr(VI) promoted the conversion of ·O₂^- into ¹O₂, establishing a coupled radical pathway that linked metal detoxification with antibiotic degradation. Furthermore, Cr(V) intermediates were detected as key transient species, exhibiting strong oxidative capacity toward TC and accelerating Cr(VI) reduction. Complementary DFT calculations confirmed that the coexistence of TC and ·H markedly lowered the energy barrier of Cr(VI) reduction. Overall, this work provides new mechanistic insight into radical transformation and metal-organic coupling under ultrasonic cavitation, and highlights its potential as a sustainable strategy for treating waters co-contaminated with metals and antibiotics.