Temperature-dependent molecular evolution of dissolved black carbon and its interaction mechanism with dissimilatory iron reduction.

Pyrolysis temperature regulates the molecular composition of dissolved black carbon (DBC), which influences its electron transfer capacity and stability. However, the impact of these temperature-induced changes on DBC's ability to facilitate microbial dissimilatory iron reduction (DIR), as well as its molecular transformations during the process, remains poorly understood. In this study, DBC samples derived from biochars pyrolyzed at 300, 450, and 600 °C (DBC300, DBC450, and DBC600) were examined for their effectiveness in mediating the microbial reduction of ferrihydrite (Fh). The results showed that DBC increased Fh reduction by 5.3 to 6.4 times. DBC's effect increased with pyrolysis temperatures, with DBC600 showing the strongest impact, even at a low concentration of 2 mgC L-1. DBC promoted Fh reduction mainly through electron shuttle mediated by semiquinone functional groups. This promotion correlated positively with the electron acceptance capacity (EAC) of the DBCs, and their highly condensed aromatic cores served as the key structural basis for enhancing DIR. FT-ICR MS analysis revealed that molecular characteristics governed by pyrolysis temperature strongly influence DBC's role in DIR. Compared with DBC300, DBC600 with high EAC and abundant CAS accelerated reduction dynamics by acting as a stable and efficient electron shuttle. The supernatant organic molecules exhibited higher Gibbs free energy and lower enthalpy after DIR, suggesting a transition to more thermodynamically stable forms. These findings provide mechanistic insight into how biochar-derived DBC may influence iron transformation under anoxic conditions.