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.

Keywords: (1)O(2) generation; CFD simulation; Cr(V) intermediate; EPR-spin trapping analysis; Simultaneous redox transformation; Ultrasonic cavitation.

Graphical abstract

Fig. 1

(a)Schematic diagram of the ultrasonic cavitation system. (b)Computational domain. (c) Tip displacement velocities of the ultrasonic probe at different vibration amplitudes. (d) Ultrasonic probes with varying tip diameters.

Fig. 2

EPR identification and quantitative analysis of reactive radicals during ultrasonic cavitation and their effects on Cr(VI) reduction and TC degradation. (a) DMPO-·OH/·H adducts, (b) DMPO-·O₂^– adducts, and (c) TEMPO (generated from TEMP/¹O₂ reaction). (d) Quantitative yields of ·OH, ·O₂^–, and ¹O₂. (e) Effects of radical quenchers and TC on Cr(VI) reduction. (f) Effects of radical quenchers and Cr(VI) on TC degradation. Experimental conditions: probe: Φ12, amplitude = 107 μm, T = 308.15 K (except for a-c: T = 288.15 K), pH = 7; [Cr(VI)]₀ = 0.3 mg/L, [TC]₀ = 3 mg/L, [DMPO]₀ = 100 mM, [TEMP]₀ = 50 mM, [CHCl₃]₀ = 10 mM, [CCl₄]₀ = 10 mM, [NaN₃]₀ = 10 mM, [TBA]₀ = 10 mM.

Fig. 3

Influence of probe tip diameter on pollutant removal, radical generation, and cavitation mechanics. (a) Cr(VI) reduction and (b) TC degradation. (c) Yields of ·OH, ·O₂^–, and ¹O₂. (d-f) Cavitation-induced mechanical energy fluctuations. Experimental conditions: amplitude = 92 μm, T = 308.15 K, pH = 7; [Cr(VI)]₀ = 0.3 mg/L, [TC]₀ = 3 mg/L.

Fig. 4

Influence of probe amplitude on pollutant removal, radical generation, and cavitation thermodynamics. (a) Cr(VI) reduction and (b) TC degradation. (c) Yields of ·OH, ·O₂^–, and ¹O₂. (d-f) Cavitation-induced internal energy dynamics. Experimental conditions: probe: Φ12, T = 308.15 K, pH = 7; [Cr(VI)]₀ = 0.3 mg/L, [TC]₀ = 3 mg/L.

Fig. 5

Influence of liquid temperature on pollutant removal, radical generation, and bubble dynamics. (a) Cr(VI) reduction and (b) TC degradation. (c) Yields of ·OH, ·O₂^–, and ¹O₂. (d) Initial bubble growth rate (dV/dt) during early expansion. Experimental conditions: probe: Φ12, amplitude = 107 μm, pH = 7; [Cr(VI)]₀ = 0.3 mg/L, [TC]₀ = 3 mg/L.

Fig. 6

Cr(VI)-mediated ·O₂^– to ¹O₂ conversion and its impact on pollutant removal. (a) Effect of Cr(VI) concentration on Cr(VI) reduction and (b) TC degradation efficiency. (c) Effect of pH on Cr(VI) reduction and (d) TC degradation efficiency. (e) EPR spectra of ¹O₂ under different conditions. (f) Proposed mechanistic pathway for ·O₂^– to ¹O₂ conversion and the coupled removal of Cr(VI) and TC. Experimental conditions: probe: Φ12, amplitude = 107 μm, T = 308.15 K (except for e, T = 288.15 K), pH = 7 unless otherwise specified; [Cr(VI)]₀ = 1.0 mg/L unless otherwise specified, [TC]₀ = 3 mg/L, [p-BQ]₀ = 50 μM, [TEMP]₀ = 50 mM.

Fig. 7

Evidence for Cr(V) formation and its dual role in TC degradation and Cr(VI) reduction. (a) EPR spectra of Cr(V) intermediates under different conditions. (b) Effect of Cr(VI) addition on TC degradation with ¹O₂ quenched. (c) Schematic illustration of the titration procedure. (d) TC degradation curves in TC and TC/Cr(VI) systems with Na₂SO₃ addition. (e) Cr(VI) reduction curves in Cr(VI) and TC/Cr(VI) systems with Na₂SO₃ addition. Experimental conditions: probe: Φ12; amplitude = 107 μm; T = 308.15 K (except for a, T = 288.15 K); pH = 4; [TC]₀ = 3 mg/L; [Cr(VI)]₀ = 1 mg/L; [EHBA]₀ = 100 mg/L; [NaN₃]₀ = 10 mM; [Na₂SO₃]₀ = 100 mg/L.

Fig. 8

Free-energy profiles of stepwise Cr(VI) reduction under different reaction scenarios. (a) TC/Cr(Ⅵ), (b) ·H/Cr(Ⅵ) and (c) TC/·H/Cr(Ⅵ).

Fig. 9

Toxicity evaluation and degradation pathways during ultrasonic cavitation-mediated co-removal of TC and Cr(VI). (a) Proposed transformation pathways of TC under cavitation. (b) Predicted acute toxicity (LC₅₀) toward Daphnia magna. (c) Predicted mutagenicity of TC and its major degradation intermediates. (d) Germination of wheat seeds exposed to ultrapure water. (e) Germination of wheat seeds exposed to the untreated TC-Cr(VI) mixed solution. (f) Germination of wheat seeds exposed to the cavitation-treated TC-Cr(VI) mixed solution. Ultrasonic cavitation conditions: probe: Φ12; amplitude = 107 μm; T = 308.15 K; pH = 7; [TC]₀ = 3 mg/L; [Cr(VI)]₀ = 1 mg/L.

Fig. 10

Proposed overall mechanistic pathway for synergistic removal of TC and Cr(VI) under ultrasonic cavitation.