The controllable generation of ·OH in NH₂OH-enhanced Fe(III)/H₂O₂ system: the overlooked role of N-containing species

Hydroxylamine (NH₂OH) has significant effects on accelerating Fenton and Fenton-like systems, while little attention was paid to the contribution of N-containing species to the generation of ROS, resulting in the limited utilization of enhancement capacity of NH₂OH. Herein, the generation of ·OH was initially evaluated in Fe(III)/H₂O₂ system under different NH₂OH dosing modes. With the same total dosages of NH₂OH or Fe(III), continuous dosing of NH₂OH always achieved a uniform production of ·OH with the accumulated concentration 6 %-61 % higher than that in the single-dosing system, which was different from the typical two-stage ·OH generation profile. The reaction stoichiometry between Fe(III) and NH₂OH was calculated to be around 1–1.3, while the value of NH₂OH utilization efficiency for ·OH production (R_OH-HAm) kept around 2, indicating the existence of ignored ·OH generation pathway through the direct participation of NH₂OH. The combination of transformation products analysis, ESR spectra and kinetic modeling demonstrated that NH₂O· generated from the reaction between Fe(III) and NH₂OH could activate H₂O₂ to generate ·OH, which contributed >25 % of the accumulated ·OH in the continuous-dosing system. On the contrary, single-dosing of NH₂OH facilitated the dimerization of NH₂O·, decreasing its contribution to ·OH production and attenuating the enhancement capacity of NH₂OH in the end. By selecting the dosing rate within a proper range related to the total dosages of NH₂OH and Fe(III) (i.e. dosing rate ≤ 0.13·[NH₂OH]_tot·k_{Fe(III), NH2OH}·[Fe(III)]₀), the rate-limiting step for ·OH production would switch from the reduction of Fe(III) to the dosing of NH₂OH, which established the dynamic equilibrium of both Fe(III) and NH₂OH, thereby achieving the controllable generation of ·OH regulated by NH₂OH dosing rate. This controllable ·OH generation system was successfully used in predicting the exact demand of NH₂OH at desired treatment efficiency and evaluating the ·OH scavenging capacity of real water, which was conductive to the experimental design and parameter selection for the future studies of reductant-enhanced Fenton-like systems.