Nitrate Photolysis Induces the Nitration of Anthracene in Surface Soil

Coexistence of nitrate (NO₃^-) and polycyclic aromatic hydrocarbons (PAHs) in overfertilized acidic soils may contribute to the formation of nitrated PAHs with enhanced toxicity. This study reveals that the Brønsted-acidity of montmorillonite (Mnt) plays a key role in generating O₂^•- and NO₂ from NO₃^- under irradiation, which can subsequently react with anthracene (ANT) to form 9-nitro-ANT (9-NA). In the dark, strong Brønsted acidity further promoted nitronium ion (NO₂⁺) formation, enabling electrophilic nitration of ANT. Relative humidity (RH) and coexisting CO₃^2-/HCO₃^- modulated Mnt surface acidity, thus regulating the nitration pathway. High RH inhibited nitration but facilitated hydroxylated ANT derivatives (e.g., hydroxyanthraquinone, 9-anthrol, anthradiol) via ^•OH attack. Nitration did not occur on SiO₂, goethite, or magnetite, whereas hematite promoted 9-nitro-ANT formation, possibly due to electron transfer from ANT to Fe(III) to form Fe²⁺, which further reduced NO₃^- to NO₂. In six representative agricultural soils, nitrated PAHs formation was consistently observed in Fe-rich Ultisol, especially under low pH and high irradiation intensity (110 mw·cm^-2). The nitration process was also observed for pyrene, but not naphthalene. This work provides direct evidence of natural PAH nitration in real soils under irradiation, offering new insights into nitrate-driven PAH transformations and soil environmental risk assessment.