Hydroxylamine enhanced Fe(II)-activated peracetic acid process for diclofenac degradation: Efficiency, mechanism and effects of various parameters

•DCF removal was remarkably enhanced by adding hydroxylamine in Fe(II)/PAA process.•The addition of hydroxylamine promoted the redox cycle of Fe(III)/Fe(II).•FeIVO2+ and R-O• were primary reactive species responsible for DCF degradation.•Hydroxylamine/Fe(II)/PAA process possessed high selectivity towards micropollutants.•The eco-friendly gas of N2 was the major decomposition product of hydroxylamine. In this study, a commonly used reducing agent, hydroxylamine (HA), was introduced into Fe(II)/PAA process to improve its oxidation capacity. The HA/Fe(II)/PAA process possessed high oxidation performance for diclofenac degradation even with trace Fe(II) dosage (i.e., 1 μM) at pH of 3.0 to 6.0. Based on electron paramagnetic resonance technology, methyl phenyl sulfoxide (PMSO)-based probe experiments and alcohol quenching experiments, FeIVO2+ and carbon-centered radicals (R-O•) were considered as the primary reactive species responsible for diclofenac elimination. HA accelerated the redox cycle of Fe(III)/Fe(II) and itself was gradually decomposed to N2, N2O, NO2− and NO3−, and the environmentally friendly gas of N2 was considered as the major decomposition product of HA. Four possible degradation pathways of diclofenac were proposed based on seven detected intermediate products. Both elevated dosages of Fe(II) and PAA promoted diclofenac removal. Cl−, HCO3− and SO42− had negligible impacts on diclofenac degradation, while humic acid exhibited an inhibitory effect. The oxidation capacity of HA/Fe(II)/PAA process in natural water matrices and its application to degrade various micropollutants were also investigated. This study proposed a promising strategy for improving the Fe(II)/PAA process and highlighted its potential application in water treatment. [Display omitted]