Dual role of dissolved black carbon in sensitized ofloxacin photooxidation: Mechanism and influential factors.

Dissolved black carbon (DBC) is the more photoactive component of dissolved organic matter (DOM) pool, which plays a dual role in the photoconversion of aquatic contaminants, acting as both a photosensitizer and an inhibitor. However, little is known about the more systematic mechanism by which DBC exhibits a dual effect, which is closely related to the structure composition of DBC. In this study, the differences in characteristics of DBC obtained from 300 °C and 500 °C were compared via UV-vis absorption spectrum, Fluorescence excitation emission matrix spectra (3D-EEM), Fourier transform infrared (FT-IR), and X-ray photoelectron spectroscopy (XPS), and evaluated the promoting and inhibiting effects of DBC on ofloxacin (OFL) photodegradation. It was found that higher pyrolysis temperature reduced the UV absorbance, molecular weight, aromaticity, and phenolics of DBC while increasing the content of quinone/aromatic ketone and humic substances. Photochemical data showed that DBC*, O and ·OH were all participated in the DBC-mediated OFL photodegradation. Wherein, DBC (DBCT, where T = pyrolysis temperature) had strong light screening and dynamic quenching effect, but the formation ability of DBC*, O and ·OH was poor, which significantly retarded the photodegradation of OFL. While DBC exhibited a slight promotion effect due to its higher formation ability of reactive species and weak light screening effect. Moreover, DBC had higher steady-state concentration and (k) than DBC, which might be due to the higher contents of quinone/aromatic ketone and the lower contents of phenol in DBC, thus enhancing the reactivity of DBC* and OFL. Our research systematically revealed the trade-off mechanism of DBC on the photodegradation of fluoroquinolones, and provided an important theoretical guidance for the photodegradation of fluoroquinolones under the evolution of DBC composition.