Insight into enhanced photogeneration mechanism of reactive intermediates from dissolved black carbon by co-pyrolysis of plastics and biomass

The dissolved black carbon (DBC) derived from co-pyrolysis of plastic and biomass be released into aquatic environment. DBC was an important photoactive constituent, however, the photo-chemical activity of DBC from the co-pyrolysis plastic and biomass remains unclear. Herein three popular plastic wastes (polystyrene, PS; polylactic acid, PLA; plastic mulch film, PMF) and pine needle biomass prepared co-pyrolysis biochar, and effects of co-pyrolysis plastics on DBC's photogeneration ability of reactive intermediates, affecting pollutant photodegradation, were investigated. In comparison to individual DBC, the mixed DBC from co-pyrolysis biochar had a lower molecular weight and more oxygen-containing groups. Quantum yields of singlet oxygen (1O2) and triplet state DBC (3DBC*) from mixed DBC were higher, especially PMF-DBC since its lower aromaticity and molecular weight connected with its higher E2/E3 and lower SUVA254 values. Steady-state photochemical experiment demonstrated 3DBC* formation rates for mixed DBC were lower than individual DBC, while the second-order reaction rate constants of 3DBC* with 2,4,6-trimethylphenol for mixed DBC were higher than that of individual DBC. Quenching experiment by sorbic acid to distinguish high/low-energy triplets revealed that contributions of low-energy triplets to 3DBC* and 1O2 generation decreased relative to individual DBC, indicating the transformation of reductive groups to oxidative groups, which was supported by the photo-transformation kinetics. Quantum yields of 3DBC* and 1O2 were positively linear correlations with E2/E3 using twenty DBC samples. These findings are helpful in understanding DBC's photochemical activity from co-pyrolysis of plastics and biomass, especially in aquatic environ-ments adjacent to soil amended by biochar.