Abstract: 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 ⋅O2- 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.

Key words: Dissolved black carbon (DBC); N-doping; Organic pollutants; Band gap; Photodegradation

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