Hydroxylamine-Enhanced NiFe2O4/H2O2 Fenton-like System for Phenol Degradation at an Initial pH of 7: Performance and Mechanistic Insights

A major limitation hindering the practical application of heterogeneous Fenton-like systems is their inherently slow reaction kinetics, particularly near neutral pH. To address this issue, a hydroxylamine (HA)-enhanced NiFe2O4/H2O2 system was developed for phenol degradation. At an initial pH of 7, with 5 mmol/L HA and 10 mmol/L H2O2, the system achieved 97.8% phenol degradation within 60 min, compared with 17.4% in the HA-free system. XPS analysis showed that the Fe2+ proportion increased from 49.1% to 53.6% and the Ni2+ proportion increased from 52.1% to 63.4% after reaction. These changes are consistent with HA facilitating the formation or regeneration of reduced metal species, suggesting that HA may be related to accelerated Fe3+/Fe2+ or Ni3+/Ni2+ cycling. Radical scavenging experiments and electron paramagnetic resonance (EPR) results indicated that identified hydroxyl (•OH) and superoxide (O2•−) radicals were the predominant reactive species. Metal-leaching and catalyst-removal experiments indicated that both heterogeneous and homogeneous processes contributed to phenol degradation. The results provide mechanistic insights into HA-enhanced H2O2 activation while also highlighting the need to consider metal leaching, HA-derived nitrogen products, and catalyst reusability.