Study On The Photochemical Activity And Mechanisms Of Dissolved Black Carbon Under Different Environmental Conditions

Dissolved black carbon（DBC）is a water-soluble component of black carbon,which accounts for 4%～20%of the dissolved organic matter（DOM）pool in global waters.DBC has a highly conjugated aromatic hydrocarbon structure different from DOM,which can efficiently photogenerate reactive oxygen species（ROS）and photodegrade organic micropollutants.Currently,many studies have focused on the factors and mechanisms that affect DOM photochemistry,but the effects of different sources and water-soluble components on DBC photochemical behavior are unclear.In addition,plastic pollution is becoming increasingly serious,making plastic pollution in soil 4 to 23 times higher than in the ocean.Given that biochar in agricultural was mostly obtained through the in-situ combustion of straw,waste plastics in the soil would inevitably participate in the pyrolysis,which may affect the structure of biochar and further affect the photoactivity of DBC.However,the effect of co-pyrolysis of plastics and biomass on the photochemical activity of DBC is unkown.In our study,the research object was DBC derived from self-made biochar.From the perspective of photophysical properties and photochemical behavior,based on research methods,the effects of molecular structures of DBC from different sources on the photochemical behavior of DBC was investigated,and the impacts of common water-soluble components on DBC photogenerate ROS and DBC photodegrade organic micropollutants were explored.The main content and results were as follows:（1）The relationship between the ability of DBC to photoproduce ROS and spectral parameters(E₂/E₃ and S_275-295)at different pyrolysis temperatures and biomass sources was studied,and a prediction model for the ability of DBC to photodegrade sulfadiazine as a function of DBC spectral parameters was established.The results showed that the aromaticity and molecular weight of DBC molecules decreased with the increase of pyrolysis temperature,which may affect the ability of DBC to photoproduce ROS via impacting the content of electron acceptor groups（such as aromatic ketones and quinones）and electron donor groups（such as phenols）in DBC.It was also found that there was a good linear relationship between the efficiency of DBC photoproduce ROS and DBC spectral parameters(E₂/E₃ and S_275-295).The aromaticity,molecular weight,and donor/acceptor group content of DBC molecules affected the ability of DBC photoproduce ROS.The excited triplet state DBC（³DBC^*）was the main ROS in the indirect photolysis of sulfadiazine.The indirect photolysis of sulfadiazine photomediated by ³DBC^*has a linear relationship with the ³DBC^*quantum yield coefficient(f _TMP)and E₂/E₃.A simple f _TMP model with DBC spectral parameters as a function can be used to evaluate the ability of DBC to degrade pollutants with known spectral parameters.（2）Polystyrene,polylactic acid,and plastic mulch films were used as waste plastic models and co-pyrolysis with biomass to explore the effects of co-pyrolysis of different plastics and biomass on the photochemical activity of DBC.The results indicated that the DBC derived from the biochar that co-pyrolysis of plastics and biomass had a smaller molecular weight and the degree of condensation of conjugated aromatic hydrocarbons,which could more effectively photoproduce the singlet oxygen（¹O₂）and ³DBC^*.Average second-order reaction rate constants for ³DBC^*and TMP derived from the plastic and biomass co-pyrolysis biochar(k_p=7.42×10⁸M^-1·s^-1)was higher as compared to the original DBC(k_p=2.11×10⁸ M^-1·s^-1),suggesting an increase in the oxidation and reduction ability of DBC derived from the plastic and biomass co-pyrolysis biochar.The UV₂₅₄ variation of different DBC under simulated irradiation further demonstrated that the co-pyrolysis of plastics and biomass led to an increase in the DBC phototransformation.Compared to energy transfer,co-pyrolysis of plastics and biomass was more likely to affect the electron transfer of DBC.Moreover,as compared to polystyrene and polylactic acid,the co-pyrolysis of the same amount for plastic mulch films and biomass may significantly increase the content of DBC light-absorbing components and small molecular structures,and promote DBC phototransformation.（3）Using carbamazepine and 17β-estradiol as target pollutants,the impacts of halogen ions on the DBC photogenerate ROS and DBC photodegrade pharmaceutically active compounds were investigated.The results showed that halogen ions did not affect the UV visible absorption of DBC,but inhibited the electron transfer of high/low energy ³DBC^*through ionic strength effects.It was found that halogen ions had no significant effect on the formation rate of ³DBC^*,but can inhibit the quenching rate constant of ³DBC^*via the ionic strength effect,which led to the increase of the ³DBC^*steady-state concentration.The high-energy ³DBC^*was the main ROS for the indirect photolysis of carbamazepine,and the slowdown in carbamazepine photodegradation was mainly attributed to the specific effect of halogen ions on ³DBC^*;whereas for 17β-estradiol photodegradation,³DBC^*and ¹O₂were the primary ROS,the decrease in degradation was mainly attributed to the inhibition of electron transfer via the ionic strength effect of halogen ions,and the specific effect of halogen ions on ³DBC^*,further reducing ¹O₂.The second-order reaction rate constants for 17β-estradiol and different triplets were calculated,and it was observed that the average second-order reaction rate constant for ³DBC^*(k_3DBC*,E2=3.89×10¹⁰ M^-1·s^-1)was significantly higher than ³NOM^*(k_3DOM*,E2=2.48×10¹⁰M^-1·s^-1),indicating that ³DBC^*had a stronger ability to oxidize 17β-estradiol than³NOM^*.（4）Common metal ions(Mn²⁺,Cr³⁺,Cu²⁺,Fe³⁺,Zn²⁺,Al³⁺,Ca²⁺,and Mg²⁺)were selected as metal ion models to study the impacts of metal ions on the photochemical activity of DBC.The results suggested that paramagnetic metal ions(Mn²⁺,Cr³⁺,Cu²⁺,and Fe³⁺)had a stronger ability to complex DBC than non-paramagnetic metal ions(Zn²⁺,Al³⁺,Ca²⁺,and Mg²⁺),with the complexation constant order of Mn²⁺>Cr³⁺>Cu²⁺>Fe³⁺>Zn²⁺>Al³⁺.The complex system of metal ions and DBC mainly involved two types of complexes.The complexation of metal ions with DBC（static quenching）led to the formation of ground state complexes M-DBC,resulting in the fluorescent components quenching in DBC.The complexation of metal ions with ³DBC^*（dynamic quenching）formed a complex of[³DBC^*...M],inhibited the electron transfer and energy transfer of ³DBC^*,and decreased the formation of ¹O₂ and superoxide radicals（O₂·^-）.The good linear relationship between log K_M and the quenching rate constant further clarified the quenching mechanism of metal ions on ³DBC^*,that is,the M-DBC complex caused by static quenching promoted the quenching of ³DBC^*.In summary,this study found that the photochemical activity of DBC was highly dependent on different biochar sources and water-soluble components.In addition,our research also revealed the mechanism and regularity of the influence of DBC molecular structure on DBC photochemical activity,clarified the effect of plastic and biomass co-pyrolysis on the DBC photochemical structure and activity,explored the impacts of halogen ions on the DBC photodegrade pharmaceutically active compounds,and investigated the quenching mechanism of metal ions on DBC and ROS.The results from our study were helpful to provide a scientific understanding of the photochemical process of DBC in the water environment and provide a theoretical basis for evaluating the environmental fate of DBC and organic micropollutants in the aquatic environment.