Formation of nitro(so) by-products of concern during the treatment of phenolic compounds by the hydroxylamine-enhanced Fe(II)/peroxymonosulfate process

We report the formation of p-nitrosophenol (p-NSP) and nitrophenols (NPs), including 2-nitrophenol (2-NP) and 4-nitrophenol (4-NP), during the treatment of phenolic compounds using the Fe(II)/PMS/HA system, which has largely been neglected so far. Various reactive nitrogen species (RNS; e.g., HNO, ·NO, ·NO₂, N₂O₃, N₂O₄) were generated during the reaction of HA with Fe(III) and PMS, which was confirmed by electron paramagnetic resonance (EPR) analysis and using specific quenching agents. Both SO₄˙⁻ and RNS participated in the formation of undesirable nitro(so) products. Taking phenol as an example, our results demonstrate that p-NSP is generated through either the direct attack of N₂O₃ on phenol or coupling of ·NO with the phenoxy radical (PhO·) formed from the reaction of phenol with SO₄˙⁻ and ·NO, whereas, NPs are generated by the fast coupling of ·NO₂ with PhO·. The sum of the yields of p-NSP and NPs was quantified to be 20–40% at various Fe(II) (5–100 μM) and HA doses (10–600 μM) under acidic conditions (pH = 3.0–6.0). This is the first report on the formation of toxic nitro(so) products during the treatment of phenolic compounds using the Fe(II)/PMS/HA system, which should be taken into consideration before the potential application of this technology in water treatment.