Linking molecular signatures of dissolved black carbon to photochemical fate: FT-ICR-MS analysis and mechanistic effects on α-nitroso-β-naphthol degradation

Abstract

Dissolved black carbon (DBC) plays a critical but poorly understood role in the photochemical transformation of organic pollutants. Its structural heterogeneity, determined by biomass source and pyrolysis temperature, strongly influences its photosensitization efficiency. This study systematically investigated the molecular characteristics of DBC derived from rice, wheat and corn straw and examined their effects on the photodegradation kinetics and pathways of α-nitroso-β-naphthol (αNβN). Fourier transform ion cyclotron resonance mass spectrometry analysis revealed that DBC primarily comprises CHO compounds with lignin-derived and fused-ring aromatic structures. With increasing pyrolysis temperature, lignin transforms into highly condensed polycyclic aromatic frameworks characterized by enhanced unsaturation and aromaticity. This molecular transformation directly governs the photochemical activity of DBC, as evidenced by linear correlations between the molecular features of DBC (i.e., unsaturation and aromaticity) and its optical properties, steady-state concentrations of reactive intermediates (RIs) and αNβN photolysis rates. Mechanistic studies identified the triplet-excited state of DBC (3DBC*) as the dominant RI responsible for αNβN degradation via dehydrogenation, condensation and redox reactions. While most photoproducts exhibit lower ecotoxicity than αNβN, exceptions such as 2-nitro-1-naphthol and coumarin underscore source-dependent risk variations. These findings advance the molecular-level understanding of the environmental implications of DBC, emphasizing its dual function as both a photochemical promoter and a modulator of pollutant behavior.