Low-molecular-weight dissolved organic nitrogen transformation behavior in concentrated leachate by O and OH: Partition, molecular insight, and potential precursor-product relationship.

Low-molecular-weight dissolved organic nitrogen (LMW-DON) is an emerging issue in concentrated leachate (CL). Ozonation is crucial to remove LMW-DON, but selectivity mechanisms of different reactive oxygen species were unknown. Here, reactions of O and OH with LMW-DON at different dosages were determined from composition, unsaturation/redox potential, and precursor-product relationship. The molecular weight of LMW-DON in CL presented a normal distribution and 76.5% was below 450 Da. LMW-DON with 400-1000 Da increased to 55.6%-66.7% and O/C increased by over 40.0% due to electrophilic substitution of O. LMW-DON with <400 Da and 550-1000 Da were preferentially degraded by OH at the low and high O dosage, respectively. O preferred to remove lipid-like (69.1%), protein-like (58.2%), and amino sugars-like (72.8%) LMW-DON, whereas OH preferred to the refractory LMW-DON, such as carbohydrates-like (71.1%), lignin-like (49.6%), and tannins-like (72.5%) LMW-DON. Forty-three transformation reactions were quantified using mass difference analysis, and O preferred to oxygen addition (e.g., +2O) and conversed amino to nitro groups, and saturated LMW-DON increased via unsaturated bonds rupture. OH attacked the carbon groups (e.g., -CH) and nitrogen groups (e.g., -NH+O, -NO+H). These findings provide molecular evidence for the selectivity of oxidants with LMW-DON and improve the ozonation application in wastewater treatment.