Occurrence, molecular transformation and prioritizing environmental risks of organic nitrogen in aqueous phase from...

Hydrothermal carbonization (HTC) is an energy-efficient and environmentally friendly method for treating waste activated sludge (WAS). However, the aqueous phase by-product (carbonization liquid, CL) presents a significant challenge because of its high nitrogen content and complex organic composition, which remains inadequately explored. This study uses non-targeted screening and mass-difference analysis to research organic nitrogen compounds (Org-Ns) in CL. Results indicated that a total of 1533 features were identified in WAS and five CL samples through non - targeted screening. Aliphatic compounds were dominant in both quantity and content, accounting for 41.7 % and 69.9 % on average, respectively. At 260 °C, heterocyclic compounds reached their highest formation rate (92.2 %). The Org-Ns were mainly CHON-type molecules (69.3 %), containing 1 or 2 N atoms, and were predominantly nitrogen-heterocyclic compounds (NHCs). Non-targeted mass-difference analysis indicated that dealkylation and deoxygenation were the main reactions contributed to the formation of aliphatic compounds and Maillard reaction precursors. NHCs were mainly derived from the Maillard reaction. Based on occurrence, persistence, bioaccumulation, and toxicity, 8 high-risk NHCs with ToxPi scores above 0.6 were identified as major concerns. These were mainly pyrroles, imidazoles, and quinolines, biotoxicity to aquatic organisms. This study offers theoretical support and methodological guidance for optimizing the HTC process and assessing the risk of Org-Ns. It provides a reference for the targeted degradation of high-risk Org-Ns in CL.