Nitrate Photolysis Induces the Nitration of Anthracene in Surface Soil

Coexistence of nitrate (NO 3 – ) 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 O 2 •– and NO 2 from NO 3 – 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 (NO 2 + ) formation, enabling electrophilic nitration of ANT. Relative humidity (RH) and coexisting CO 3 2– /HCO 3 – 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 SiO 2, goethite, or magnetite, whereas hematite promoted 9-nitro-ANT formation, possibly due to electron transfer from ANT to Fe(III) to form Fe 2+, which further reduced NO 3 – to NO 2 . 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.