Dissolved black carbon enhanced the aquatic photo-transformation of chlortetracycline via triplet excited-state species: The role of chemical composition

Dissolved black carbon (DBC), widely distributed in the aquatic environments, can accelerate sunlight-driven photo-transformation of micropollutants, however the photosensitization mechanisms are not clear. Herein, the DBC was extracted from bamboo biochar and fractionated by molecular weight (i.e. <10 k, <3 k, and <1 k Da). The effects of DBC on chlortetracycline (CTC) photolysis behaviors, and the role of chemical composition (i.e., molecular weight and chemical structure) in DBC-mediated photo-transformation were investigated. The results showed that DBC could accelerate CTC photodegradation significantly. At low DBC concentrations (<6.0 mg C/L), the photodegradation rate constant of CTC increased from 0.0299 to 0.0416 min-1 with the increasing DBC concentration. Via quenching experiment, the triplet excited-state of DBC was identified as the dominant reactive intermediate with >90% contribution to total CTC photodegradation. In addition, it was found that the photosensitive efficiency of DBC increased as the molecular weight decreased, and the stronger photosensitization ability exhibited in DBC with low-molecular weight was potentially attributed to its higher content of carbonyl compounds. The observed photosensitive efficiency of DBC sharply decreased after reduction by NaBH4, further confirming the key role of carbonyl compounds in the photosensitization process. Moreover, based on the result of photoproducts, the amidogen in CTC was verified to be susceptible to react with 3DBC*.