Abstract
This study systematically investigated the feasibility of the microbubble ozonation process to degrade the 17α-ethinylestradiol, ibuprofen, and atenolol through the comparison with the millibubble ozonation process for elucidating the degradation behavior and mechanisms during the microbubble ozonation processes. The proportions of small microbubbles (diameter 1–25 μm) were increased with increasing the cavity pump frequency (40 Hz: 51.4%; 50 Hz: 57.5%; 60 Hz: 59.9%). The increased concentrations of O3 and OH radicals due to the higher specific area of O3 microbubbles compared to O3 millibubbles could facilitate their mass transfer at the gas–water interface. Furthermore, the elevated reactivity of O3 by increasing the temperature might improve the degradation of the pharmaceutical compounds, which was more pronounced for the microbubble ozonated waters than the millibubble ozonated waters. Although the degradation efficiency of the pharmaceutical compounds during the microbubble ozonation processes was significantly influenced by the existence of humic acids compared to the millibubble ozonation process, the increased solubilization rate of O3 and OH radicals by collapsing O3 microbubbles enhanced the degradation of the pharmaceutical compounds. Overall, these results clearly showed that the microbubble ozonation process could be an alternative option to conventional ozonation processes for the abatement of the pharmaceutical compounds.
Highlights
In recent years, the occurrence and fate of trace organic compounds, including pharmaceutical compounds, personal care products, and endocrine-disrupting chemicals in surface water, wastewater, and groundwater have become an emerging concern over the world since a considerable amount of used and/or metabolized organic compounds are discharged from municipal wastewater treatment plants (WWTPs) to the receiving surface water bodies in highly urbanized areas [1,2,3]
In the tested microbubble ozonation process, the generation of microbubbles was not detected at the cavity pump frequency of 30 Hz, whereas microbubbles seemed to generated stably when the cavity pump frequency was higher than 40 Hz
The fractions of microbubbles whose diameters ranged from 1–25 μm gradually increased with increasing the cavity pump frequency (40 Hz: 51.4%; 50 Hz: 57.5%; 60 Hz: 59.9%)
Summary
The occurrence and fate of trace organic compounds, including pharmaceutical compounds, personal care products, and endocrine-disrupting chemicals in surface water (i.e., lake water and river water), wastewater, and groundwater have become an emerging concern over the world since a considerable amount of used and/or metabolized organic compounds are discharged from municipal wastewater treatment plants (WWTPs) to the receiving surface water bodies in highly urbanized areas [1,2,3]. O3 gases injected into the aqueous phase generally remains in the gaseous phase because of its low solubility in water (1.0 × 10−6 mol m−3 ·Pa–1.3 × 10−4 mol m−3 ·Pa), which may allow O3 gases existing in surface water and wastewater to pass through the reacting zone of the O3 reactors without the chemical reactions with trace organic compounds [13] Based on these reasons, it is required to develop a novel method for improving the solubilization rate of O3 gases injected in surface water and wastewater closely associated with the removal of pharmaceutical compounds [14]
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