Insight into Enhanced Reactive Intermediates Photogeneration Mechanism of Dissolved Black Carbon Derived from Co-Pyrolysis of Plastics and Biomass

The growing usage of plastic products during the COVID-19 period led to a major worsening of plastic pollution on land. In addition to being utilized as a modifier of the biochar as the soil conditioner, the plastic in the soil will unavoidably take part in the combustion of agricultural biomass. However, current studies are mainly focused on biochar materials, and the impact on the photochemical/photophysical characteristics for its dissolved black carbon (DBC) are still unclear. Here, polystyrene (PS), polylactic acid (PLA), and plastic mulch film (PMF) were selected as the plastic models. The photogenerated reactive intermediates (RIs) of DBC from the co-pyrolysis of plastic and pine needle were studied and compared with the original DBC. The results revealed that compared with the original DBC, the DBC from the modified biochar had a smaller molecular weight and more oxygen-containing groups. The yields of singlet oxygen (1O2) and DBC triplet states (3DBC*) from the modified DBC were much higher, especially for PMF-DBC. The 3DBC* formation rate for altered DBC was significantly lower than the original DBC. The secondary reaction rate constant of 3DBC* and TMP for the modified DBC were substantially higher, indicating the significant increment of the oxidation and reduction capacity for the modified DBC. The phototransformation experiments demonstrated that PMF-DBC was more photosensitive as compared to PS-DBC and PLA-DBC. The experiments about energy transfer and electron transfer of the high-/low-energy 3DBC* for 20 modified/unmodified biochar revealed that the impact of biochar modification to the photochemistry of the low-energy 3DBC* was more obvious in the electron transfer pathway than that in the energy transfer pathway. Our findings indicated that the co-pyrolysis of plastics and biomass produced DBC that had more excellent RS photogeneration and PMF that was more sensitive to alterations in the physicochemical characteristics of DBC than PS and PLA.