Mechanism of real-time capture of organics by in-situ-formed microbubble flocs to enhance organics removal in hybrid ozonation-coagulation process

The over-oxidation of organic pollutants by pre-ozonation leads to a decreased removal efficiency of coagulation. In this study, it was demonstrated that organics modified by ozonation can be captured in real time by microbubble flocs mediated by the interaction among ozone, Al species, and other organics using the hybrid ozonation-coagulation (HOC) process, which avoids further oxidation of the modified organics. Compared with the pre-ozonation-coagulation process, this real-time capture strategy during the HOC process can increase organics removal efficiency, and broaden the ozone dosing range. The synergistic effect of Al species and ozone enhanced hydroxyl radicals (·OH) formation, which increased the content of carboxyl and hydroxyl groups in the modified intermediates while retaining their original macro aromatic structure. That modified intermediates can be captured and complexed by Al species in the HOC process in real time was verified by differential absorbance spectroscopy (DAS). Al species are complexed with carboxyl and hydroxyl groups of organic compounds to form Al-organic complexes that have C–O–Al coordination bonds. These complex structures protect the modified organic compounds during immediate contact with and decomposition by ozone. The existence of ozone can increase the formation of poly Al species via hydroxyl bridges, thus causing the soluble small-scale metal complex to form insoluble flocs. The surface of flocs can be oxidized and modified, thus strengthening the surface coordination of residual organics in water and further improving their removal efficiency.