A Predictive Model For Photochemical Activity Of Dissolved Black Carbon

Dissolved black carbon(DBC)is the water-soluble fraction of black carbon.Being easily transported and transformed,DBC is the vital chemical speciation in the global cycle of black carbon.Photodegradation is the major pathway in its degradation processes.However,the optical properties and photoactivity of DBC are still largely unknown.As an important subgroup of the dissolved organic matter(DOM)pool,understanding the structure and photochemical properties of DBC,while compared to other important fractions of DOM,is of great significance to comprehending its role in the photochemistry of natural waters.In this study,dissolved black carbon were prepared using water extraction of bulk BC derived from both C3 and C4 plants.The elemental composition and structural properties of DBC were examined.A range of optical properties of DBC,as well as humic substances(HS)from the International Humic Substance Society(IHSS),were characterized by ultraviolet-visible spectroscopy;fluorescence spectroscopy and proton nuclear magnetic resonance to form a characterization library.Also,the apparent singlet oxygen quantum yields(Φsinglet oxygen)under simulated sunlight were determined.The correlations between the Φsinglet oxygen and indices in the characterization library were then investigated in order to find the quantitative structure-activity relationship between structure and photochemical activities of organic matter.The major findings are as follows:(1)DBC can be readily released from bulk materials.Compared with bulk BC,DBC has more oxygen-containing functional groups and lower aromaticity.Also,the crystallite size of DBC is smaller,indicating lower degree of graphitization.The structural signature of DBC is the small aromatic clusters heavily substituted by carboxyl groups.(2)The absorption spectra of DBC have similar shape as that of HS.However,DBC has significantly higher E2/E3,S275-295 and SR than HS,more dissolved lignin content and protein-like components as well as higher aromatic protons,suggesting its lower average molecular weight but higher aromaticity.DBC can be readily differentiated from HS using spectroscopic indices.(3)The Φsinglet oxygen of DBC under simulated sunlight varies from 3.46%to 6.13%,significantly higher than that of HS,1.26%to 3.57%,suggesting that DBC is the more photoactive component in the DOM pool.(4)The spectral slope coefficient S275-295 and absorbance ratio E2/E3 are good predictors of the Φsinglet oxygen,indicating the similar photophysical basis for the photoactivity of DBC and HS.The charge transfer model can well explain the relationship between optical and photochemical properties of organic matter.The average molecular weight and aromatic ketones/quinones content are key factors that influence the generation of singlet oxygen.The higher aromaticity but lower average molecular weight of DBC consequently contributes to its higher photoactivity.