We investigated the influence mechanism of N-doping for dissolved black carbon (DBC) photodegradation of organic pollutants. The degradation performance of N-doped dissolved black carbon (NDBC) for tetracycline (TC) (71%) is better than that for methylene blue (MB) (28%) under irradiation. These levels are both better than DBC degradation performances for TC (68%) and MB (18%) under irradiation. Reactive species quenching experiments suggest that h+ and \(\cdot{\rm{O}}_2^ - \) are the main reactive species for NDBC photodegraded TC, while ·OH and h+ are the main reactive species for NDBC photodegraded MB. ·OH is not observed during DBC photodegradation of MB. This is likely because N-doping increases valence-band (VB) energy from 1.55 eV in DBC to 2.04 eV in NDBC; the latter is strong enough to oxidize water to form ·OH. Additionally, N-doping increases the DBC band gap of 2.29 to 2.62 eV in NDBC, resulting in a higher separation efficiency of photo-generated electrons-holes in NDBC than in DBC. All these factors give NDBC stronger photodegradation performance for TC and MB than DBC. High-performance liquid chromatography-mass spectrometry (HPLC-MS) characterization and toxicity evaluation with the quantitative structure-activity relationship (QSAR) method suggest that TC photodegradation intermediates produced by NDBC have less aromatic structure and are less toxic than those produced by DBC. We adopted a theoretical approach to clarify the relationship between the surface groups of NDBC and the photoactive species produced. Our results add to the understanding of the photochemical behavior of NDBC.
