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

Abstract

Nitrite (NO2–) 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 NO2– 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 NO2– 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 (HNO2 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 (Mn) 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.