Molecular Drivers of Electron-Donating Capacity in Dissolved Black Carbon from Nitrogen-Rich Pyrogenic Carbon

High Resolution Image Download MS PowerPoint Slide Dissolved black carbon (DBC) from nitrogen-rich feedstock-derived pyrogenic carbon may influence aquatic photochemistry and byproduct formation due to its electron-donating capacity (EDC). Yet, the molecular drivers of EDC remain unclear. Here, we developed an integrated analytical framework to characterize DBC leached from nitrogen-rich biochar pyrolyzed at 350, 450, and 550 °C (DBC350, DBC450, and DBC550) under simulated intermittent rainfall over 30 days. Through two-dimensional correlation spectroscopy (2D-COS) and Fourier transform ion cyclotron resonance mass spectrometry, we analyzed sequential responses and synergistic relationships of thousands of individual DBC molecules with various functional groups. The EDC increased with leaching time, particularly in DBC350, coinciding with a shift toward lower m / z, more unsaturated, and aromatic compounds. Spearman’s analysis showed that EDC-related molecules were predominantly nitrogen-bearing (61–76%), highly unsaturated, and low-oxygen. Our 2D-COS analysis on EDC-related molecules and functional groups identified (hetero)aromatic structures as key EDC contributors. Tandem mass spectrometry and X-ray photoelectron spectroscopy further confirmed the prevalence of carboxylic, pyrrolic, and/or amide groups. Extended (hetero)aromatic structures contributed to the higher EDC in DBC350 than in DBC450 and DBC550. Our study offers the first molecular and functional group-level insight into EDC-related DBC compositions, with implications for biochar-related and postwildfire water quality management.