Unraveling the Radical Chemistry of Nitrophenols and Biomass-Burning Brown Carbon in Waters

More frequent global wildfire events intensify the exposure risks of nitrophenols through brown carbon (BrC) emissions. Radical reactions dominate the fates of nitrophenols in surface, municipal, and atmospheric waters, yet the involved reaction kinetics and mechanisms remain poorly understood. In this work, by combining transient and steady-state multispectral analyses, the second-order rate constants (k) for nitrophenols and biomass-burning BrC reacting with HO^• as well as common secondary radicals (e.g., Cl₂^•-, CO₃^•-, and NO₃^•) were determined (k ≈ 10⁵-10¹⁰ M^-1 s^-1 at pH 2.0-10.0 for both protonated and deprotonated nitrophenols), most of which are reported for the first time. We also experimentally demonstrated that the reactions of nitrophenols and BrC with various radicals could produce a substantial amount of HONO/NO₂^- (a key precursor of tropospheric HO^•), with molar yields reaching ∼50%. Mechanistic investigations revealed that the phenoxy radical (PhO^•), generated from either single electron transfer or addition-elimination reactions during the radical reactions of nitrophenols, acted as a universal intermediate for the denitration. Hence, the new kinetic and mechanistic insights suggest that the radical-induced denitration process of BrC and nitrophenols in acidic waters may serve as a previously overlooked HONO source, but such denitration does not guarantee detoxification due to the simultaneous generation of high-risk biphenols. This work highlights the critical role of secondary radicals in nitrophenols and BrC transformation across different waters, shedding light on several unforeseen chemical impacts driven by intensified global wildfires.