Beyond top-hit nontarget screening: Diagnostic fragment analysis reveals nitrogen-containing heterocycles in iron and steel industry wastewater.

Nitrogen-containing heterocyclic compounds (NHCs), widely present in industrial wastewater, pose significant environmental and health risks, yet their identification and characterization remain poorly understood. Herein, we developed a diagnostic fragment list comprising 20 nitrogen-containing fragments for NHCs, by integrating chemical information from Pubchem with the NIST mass spectral library. Leveraging this list, we employed a diagnostic fragment-assisted nontarget screening approach and identified 151 NHCs in iron and steel industry wastewater. Notably, 57 of these NHCs were unidentified by conventional top-hit screening methods in gas chromatography high-resolution mass spectrometry (GC-HRMS) analysis. Furthermore, 121 of the identified NHCs represent the first reported occurrence in industrial wastewater. Semi-quantitative analysis revealed significant reductions in NHC concentrations during treatment: from 2.54 × 10 μg/L to 26.2 μg/L in coking wastewater (99.9 % removal rate) and from 32.4 μg/L to 1.17 μg/L in mixed wastewater (96.4 % removal rate). However, the effluent of mixed wastewater contained 18 NHCs classified as persistent, mobile, and toxic (PMT) or very persistent and very mobile (vPvM), underscoring their potential long-term environmental risks. This study significantly advances the identification of NHCs in industrial wastewater and provides a critical foundation for developing targeted strategies to mitigate the environmental and health risks posed by NHCs.