Comparative Formation of Nitrogenous By-products in Hydroxylamine-Enhanced Fe(II)/PDS and Fe(II)/H2O2 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 (H₂O₂)-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)/H₂O₂/HA system showed minimal nitrosated and nitrated by-product formation, remaining below 1%. Reactive species characterization identified Fe(IV), sulfate radical (${\text{SO}}_4^{•-}$${\text{SO}}_4^{•-}$), hydroxyl radical ($•\text{OH}$$•\text{OH}$) and various reactive nitrogen species (RNS) including nitric oxide radical ($•\text{NO}$$•\text{NO}$), nitrogen dioxide radical ($•\mathrm{N}{\mathrm{O}}_2$$•\mathrm{N}{\mathrm{O}}_2$), and peroxynitrous acid (ONOOH) in the PDS system, while only $•\text{OH}$$•\text{OH}$ and $•\text{NO}$$•\text{NO}$ were detected in the H₂O₂ system. The divergent nitrogenous by-product formation originated from distinct reaction mechanisms. In the PDS system, ${\text{SO}}_4^{•-}$${\text{SO}}_4^{•-}$ oxidized substrates to phenoxyl radicals that rapidly combined with RNS. In contrast, $•\text{OH}$$•\text{OH}$ addition in the H₂O₂ 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 H₂O₂ 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.