New insight into the activation mechanism of hydrogen peroxide by greigite (Fe3S4) for benzene removal: The combined action of dissolved and surface bounded ferrous iron

Iron sulfides have attracted growing concern in heterogeneous Fenton reaction. However, the structure of iron sulfides is different from that of iron oxides and how the structures affect the activation property of hydrogen peroxide (H2O2) remains unclear. This study investigated benzene removal through the activation of H2O2 by the synthesized magnetite (Fe3O4) and greigite (Fe3S4). The structures of Fe3O4 and Fe3S4 were characterized by XRD and EPR, the electron transfer properties of Fe3O4 and Fe3S4 were analyzed by electrochemical workstation, XPS and DFT. It is revealed that the effective benzene removal rate of 88.86% in the Fe3S4/H2O2 was achieved, which compared to 15.58% obtainable from the Fe3O4/H2O2, with the apparent rate constant in the Fe3S4/H2O2 being approximately 65 times over that in the Fe3O4/H2O2. The better H2O2 activation by Fe3S4 was attributed to the significant roles of S (-II) and S vacancies in regulating the dissolution of ferrous iron ions, thus generating abundant free •OH radical. In addition, surface bounded ferrous iron of Fe3S4 could transfer more electrons to H2O2 and O2 to generate more surface bounded •OH and •O2−. This study revealed the combined action of dissolved and surface bounded ferrous iron of greigite on H2O2 activation, and provides an efficient heterogeneous H2O2 activator for the remediation of organic contaminants in groundwater.