Variation in photoactivity of dissolved black carbon during the fractionation process and the role in the photodegradation of various antibiotics.

The composition of dissolved black carbon (DBC) could be influenced by adsorption on minerals, subsequently affecting DBC's photoactivity and the photoconversion of contaminants. This study investigated the changes in photoactivity of DBC after absorption on ferrihydrite at Fe/C ratios of 0, 1.75, 7.50, and 11.25, compared the influences of DBC₀ and DBC_7.50 on the photodegradation of four typical antibiotics (AB) including sulfadiazine, tetracycline, ofloxacin, and chloramphenicol. The selective adsorption led to the compounds with high aromaticity, high oxidation states, and more oxygen-containing functional groups being more favorably adsorbed on ferrihydrite, further causing the steady-state concentrations of ³DBC*, ¹O₂, and •OH respectively to drop from 1.83 × 10^-13 M, 7.45 × 10^-13 M, and 3.32 × 10^-16 M in DBC₀ to 1.22 × 10^-13 M, 0.93 × 10^-13 M and 2.30 × 10^-16 M in DBC_11.25, while the light screening effect factor increased from 0.740-0.921 in DBC₀ with above four antibiotics to 0.775-0.970 for that of DBC_11.25. Unexpectedly, DBC after adsorption played a dual role in the photodegradation of various antibiotics. This difference might be caused by antibiotics' chemical composition, functional groups interacting with reactive intermediates, and the overlap in UV-vis spectra between antibiotics and DBC. Our data are valuable for understanding the dynamic roles of DBC in the photodegradation of antibiotics.