Accelerated Transformation of Nitrite to Nitro-organic Compounds in Frozen Water in the Presence of Natural Organic Matter

Nitrite (NO 2 – ) is a key intermediate in nitrogen cycling, yet its abiotic fate in frozen environments remains insufficiently understood, particularly its conversion to organic nitrogen. Here, we demonstrate that freezing dramatically accelerates NO 2 – oxidation and promotes its incorporation into natural organic matter (NOM), forming hazardous nitroaromatic compounds. At −20 ° C and pH 4.0, nitrite removal exceeded 85% within 24 h, compared to negligible loss in liquid water. Confocal Raman and fluorescence imaging revealed strong enrichment of NO 2 – and NOM within quasi-liquid grain boundaries, accompanied by a localized pH drop from 4.0 to ∼1.2. These microenvironments favor the formation of reactive nitrogen species (HNO 2 and NO + ), which nitrate electro-rich moieties in NOM and initiate oxidative coupling reactions. Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) identified a marked increase in nitrogen-containing formulas (from 1290 to 1440) and higher average molecular weight ( M n ) of NOM (from 369.191 to 382.594 Da). Phenol served as a model substrate, confirming nitration and oligomerization pathways. These findings demonstrate that freeze-concentration effect induced acidification and nitrite speciation change act as significant drivers for the molecular-level transformation of NOM in ice, providing new insights into nitrogen geochemical cycling, nitroaromatic pollutants formation, and ecological risk in cryosphere and seasonally frozen waters under climate change.