Comparative formation of nitrogenous by-products in hydroxylamine-enhanced Fe(II)/PDS and Fe(II)/HO systems during contaminant degradation.

This study systematically investigated the degradation of phenolic (phenol) and aromatic carboxylic (benzoic acid (BA)) compounds and formation of nitrogenous by-products in Fe(II)-activated peroxide systems with hydroxylamine (HA). Significant differences were observed between the peroxydisulfate (PDS)-based and hydrogen peroxide (HO)-based systems. The Fe(II)/PDS/HA system produced substantial yields of nitrosated and nitrated by-products, reaching 10-35 % for phenol and 2-17 % for BA. In contrast, the Fe(II)/HO/HA system showed minimal nitrosated and nitrated by-product formation, remaining below 1 %. Reactive species characterization identified Fe(IV), sulfate radical (SO), hydroxyl radical (•OH) and various reactive nitrogen species (RNS) including nitric oxide radical (•NO), nitrogen dioxide radical (•NO), and peroxynitrous acid (ONOOH) in the PDS system, while only •OH and •NO were detected in the HO system. The divergent nitrogenous by-product formation originated from distinct reaction mechanisms. In the PDS system, SO oxidized substrates to phenoxyl radicals that rapidly combined with RNS. In contrast, •OH addition in the HO system predominantly yielded hydroxylated intermediates with low reactivity toward RNS. Operational parameters including Fe(II) concentration, HA dosage, and pH significantly influenced both contaminant degradation and nitrogenous by-product formation. Notably, Cl promoted the formation of nitro(so) by-products in both PDS and HO systems through formation of nitrosyl and nitryl chlorides. This work presents the first direct evidence of HA-derived nitrogen incorporation into organic by-products via RNS-mediated transformation pathways in Fenton and Fenton-like systems. These findings highlight critical environmental implications for the application of HA-enhanced advanced oxidation processes (AOPs) in water and wastewater treatment, particularly concerning the potential formation of toxic nitrogenous transformation products.