Mechanisms of Enhancing Tetracycline Oxidation in Wastewater by Ozone Micro-Nano Bubbles

To address the low efficiency of tetracycline (TC) ozonation caused by low ozone solubility, short aqueous half-life, and mass-transfer limitations, an ozone micro-nano bubble (O3-MNBs) oxidation system was designed and systematically compared with conventional ozone sparging (Conv-O3). Thus, this study assessed the bubble size distribution, zeta potential, ozone dissolution and decay behaviors in water, ·OH concentration, and TC oxidation products, elucidating the degradation pathways and underlying mechanisms enabled by O3-MNBs. Relative to Conv-O3, O3-MNBs increased the steady-state dissolved ozone concentration by 2.57–4.33 times, reduced the ozone decay rate constant by 41.3%, and enhanced ·OH generation by 2.3 times. TC degradation in the O3-MNB system exhibited a distinct two-stage kinetic behavior, following second-order kinetics in the initial period (0–30 s) and first-order kinetics thereafter (30–120 s). Accordingly, the TC removal efficiency of O3-MNBs reached 96.25% within 120 s, which was 81.25% higher than that of Conv-O3. Notably, TC removal under Conv-O3 obeyed first-order kinetics throughout, with an apparent rate constant only 7.14% of that obtained with O3-MNBs. These improvements were attributed to the sustained and efficient supply of oxidants, high dissolved ozone and ·OH radicals, promoting the conversion of TC intermediates toward low m/z small-molecule end products, with greater ring opening and skeletal fragmentation. Our findings suggest that the enhanced biodegradability results in a markedly reduced burden and environmental risk for subsequent biological or advanced treatment processes. Therefore, this study highlights the potential of O3-MNBs to enhance ozone utilization and oxidation intensity, providing mechanistic insights and technical support for rapid pretreatment of antibiotic-containing wastewater.