Superoxide radical driving the activation of persulfate by magnetite nanoparticles: Implications for the degradation of PCBs

Magnetite nanoparticles (MNPs) are ubiquitous components of the subsurface environment, and increasing attention has been paid to MNPs due to their highly reductive and heterogeneous catalysis reactivity for the degradation of organic contaminants. However, most previous research studies neglected the generation of reactive oxygen species (ROS) by MNPs, which plays an important role in the transformation of contaminants. In this paper, we investigated the activation of persulfate (PS) by MNPs for the degradation of 2,4,4′-CB (PCB28), a selected model compound, and the underlying mechanism was elucidated. The results indicated that the PS can be activated by MNPs efficiently for the degradation of PCB28 at neutral pH. Electron paramagnetic resonance (EPR) technique was used to detect and identify the radical species in these processes. The mechanism of the activation of PS by MNPs was that superoxide radical anion (O2−) generated by MNPs could activate the PS to produce more sulfate radicals (SO4−), which favored the degradation of PCB28. The conclusion was further confirmed by quenching studies with the addition of superoxide dismutase (SOD). The effects of Fe(II) and pH on the degradation of PCB28 by PS/MNPs as well as the generation of ROS by MNPs were also studied. Both sorbed Fe(II) on MNPs surface and increased pH led to production of more O2−, which activated the PS to give more SO4− to degrade PCB28. In addition, increasing the oxygen concentration in the reaction solution favored the generation of O2− as well as the degradation of PCB28. The findings of this study provide new insights into the mechanism of heterogeneous catalysis based on MNPs and the reactivity of MNPs toward environmental contaminants.