Pyrolysis-Tuned Dual-Function Biochar-Derived Dissolved Organic Matter Drives Redox Coupling for Deep PFAS Defluorination in UV/Sulfite Systems.

The growing use of biochar is increasing the release of biochar-derived dissolved organic matter (BDOM) in surface waters. Concurrently, remediation of persistent per- and polyfluoroalkyl substances (PFAS) remains challenging. Although natural DOM can both photosensitize and scavenge reactive species, how BDOM modulates PFAS transformation in waters is unclear. Here, we show that low-level BDOM (1 mgC L-1) unexpectedly promotes PFAS defluorination in the UV/sulfite (UV/S) system by simultaneously boosting hydrated electron (eaq-) production and generating oxidizing radicals via triplet states (3BDOM*). The hydrophilic, moderately condensed aromatic, fulvic/humic-like fraction of BDOM is most effective across all tested feedstocks and pyrolysis temperatures. BDOM derived at 500 °C raises eaq- yield 2.6-fold over UV/S alone while co-producing •OH. Crucially, BDOM appears to reroute the eaq--mediated decarboxylation-hydroxylation-elimination-hydrolysis pathway: instead of water addition, •OH may directly attack perfluorinated radical, making key steps more thermodynamically favorable and accelerating C-F cleavage. Leveraging naturally derived BDOM in waters, this eaq-/•OH-mediated reduction-oxidation coupling may reduce reliance on externally added chemicals while achieving complete degradation of perfluorocarboxylic acids and GenX within 1 h and ∼80% defluorination within 6 h. The findings establish BDOM as an intrinsic photosensitizer and electron ejector that reshapes radical chemistry and PFAS reaction pathways, supporting more efficient and sustainable UV/S-based remediation.