Free radical switching behavior between initial neutral and buffer neutral condition during Fenton-like degradation over pyrite/hematite mineral.

Heterogeneous Fenton-like reactions have broadened the pH adaptation window of traditional homogeneous Fenton during water purification. However, the sharp decrease in their activity under macro-neutral conditions is still a large challenge. More importantly, although it has been realized that the pH value always changes during the heterogeneous Fenton-like process, there are still a few research focuses on the degradation mechanisms in different pH systems, especially the difference between initial neutral and the buffered neutral system. This study reports a pyrite/hematite (FeS2/Fe2O3) nanocomposite mineral capable of degrading organic pollutants under different pH system. Interestingly, the free radical switching from hydroxyl radical dominated in initial neutral system into mainly superoxide radical in buffer neutral condition. Moreover, there is a relatively fast electron transfer between the two types of iron ore through formation of Fe-S bond. The spontaneous formation of sulfur vacancies during the degradation process also promotes the Fe3+/Fe2+ cycle. In addition, the photoactivated Fe3+/Fe2+ valence cycle restores the degradation rate of the deactivated catalyst in the dark reaction to 80 %, breaking through the bottleneck of "iron ion precipitation inactivation" in the traditional Fenton system. This research provides a new strategy for constructing composite nano-iron ore and eliminates the pH limitation for Fenton-like wastewater control technologies.