Uncovering Nitro Compounds in Water: Photolysis-Based Analytical Methods and Insights into Their Formation during Ozonation

Nitro compounds, including aliphatic, aromatic, and halogenated structures, are potentially toxic oxidation products formed during water treatment, yet remain poorly characterized beyond a few known species. This study presents two novel analytical methods for nitro compound quantification without the need for compound-specific standards: a batch UV₂₅₄ photolysis coupled with colorimetry for total nitro, and liquid chromatography with postcolumn photolysis (LC-PCUV) for individual compounds. Both methods rely on photolytic nitrite formation as a proxy for nitro compounds, yielding average molar nitrite yields of 68 ± 17% for aliphatic compounds and 11 ± 11% for aromatic compounds. The methods demonstrated high sensitivity in complex matrices, achieving detection limits down to a few nM nitro-equivalent when combined with solid phase extraction. A proof-of-concept ozonation experiment with model amines and amino acids confirmed the applicability of the methods, providing new insights into γ-aminobutyric acid ozonation. Notably, it is reported here for the first time that over 20% of amine-containing moieties in extracted natural organic matter can be converted to nitro during ozonation. Significant nitro concentrations (140-180 nM) were also detected in wastewater effluents and found to further increase upon ozonation. Together, these methods offer valuable tools to investigate the fate of reactive nitrogen moieties and nitro formation, including (halo)nitroalkanes during oxidative water treatment.