Pyrochar-derived DOM outshines hydrochar-derived DOM in photoreactivity: Insights from reactive species and organic...

The type of biochar-derived dissolved organic matter (DOM) determines photochemical pathways and differentially controls pollutant photolysis. However, comparative data on the photo-reactivity of pyrochar-derived DOM (DBC) vs. hydrochar-derived DOM (DHC) are lacking. Closing this gap is essential for predicting the photochemical impacts of contrasting biochars in field applications. In this study, we employed complementary techniques to investigate the photophysical properties of DBC and DHC, their roles in the photoproduction of reactive species, and their impact on tetracycline (TC) photolysis. DBC from pyrolysis exhibited a lower E₂/E₃ ratio and higher SUVA₂₅₄ and SUVA₂₈₀ values, indicating more aromatic structures with larger clusters. This was supported by molecular evidence of abundant condensed aromatic compounds with narrow molecular weight ranges, which boosted the production of ³DOM* and ¹O₂ in DBC. In contrast, DHC from hydrothermal carbonization contained more lignin-like compounds, such as phenolics, which partially suppressed reactive species generation. DBC significantly accelerated TC photolysis by producing high-energy ³DOM* and ¹O₂ that drive reductive demethylation, deamination, and ring-cleavage of TC. Conversely, the phenolic moieties in DHC quenched excited-state TC and reactive species. DBC exhibits enhanced photoreactivity in promoting TC photolysis, with its photodegradation rate influenced by water quality, yet the extent of photolysis remains unaffected. Our findings clarify the molecular-level photochemical differences between DBC and DHC and their relevance to aquatic contaminant photolysis.