Elucidating molecular characteristics of organic compounds during ozone micro-bubbles treatment based on GC ×...

The ozone micro-bubbles (OCBs) technology is increasingly gaining traction as a promising alternative method for organic compounds removal in wastewater. Nevertheless, there is a scarcity of literature addressing the molecular-level transformation of organic compounds during OCBs treatment. In this work, the secondary effluent from a wastewater treatment plant was treated with ozone milli-bubbles (OLBs) and OCBs, and the fate of organic compounds at the molecular level was investigated using comprehensive two-dimensional gas chromatography quadrupole time-of-flight mass spectrometry (GC × GC-QTOF-MS). The findings revealed that, compared to OLBs, OCBs increased the total mass transfer coefficient by 1.46 times and the half-life of ozone by 4 times. Consequently, OCBs enhanced the removal rates of COD_cr, NH₄⁺-N, UV₂₅₄, and TOC at the 30-min mark by 8.91%, 8.65%, 10.11%, and 2.15%, respectively. In the raw water, 710 organic compounds were detected, decreasing to 668 and 478 after treatment with OLBs and OCBs, respectively. Furthermore, the organic compounds with higher molecular weight and unsaturation degree were more prone to mineralization in the OCBs process. It was also identified that OCBs exhibited nearly 100% removal of amines, unsaturated hydrocarbons, aldehydes, phenols, and aromatic amides. It is noteworthy that, among the 15 identified emerging contaminants (ECs), the removal efficiency of OCBs (53.3%) was higher than that of OLBs (33.3%), with fewer by-products. More deeply, based on 30 common reactions, the primary reactions occurring in OLBs treatment were dealkylations, whereas the abundant hydroxyl radicals in OCBs treatment facilitated the oxidation reaction (+O). This study contributes to the exploration of the potential of OCBs technology in treating secondary effluent, providing invaluable insights for its rational application in practical scenarios.