Oxidation on zerovalent iron promoted by polyoxometalate as an electron shuttle

Most studies on zerovalent iron (ZVI) were mainly focused on the reductive transformation of halo- or nitro-compounds. Oxidation reactions occurring on ZVI have been recently recognized. In this study, we demonstrate that the oxidation pathways on ZVI can be accelerated by the presence of polyoxometalate (POM: nanosized metal-oxygen cluster anion) serving as an electron shuttle. The ions, SiW12O404- and PW12O403-, can mediate the electron transfer from the Fe-0 surface to O-2 while enhancing the production of H2O2, which subsequently initiates the OH radical-mediated oxidation through a Fenton-type reaction. The oxidation reaction was completely quenched by adding methanol as an OH radical-scavenger. On the other hand, PMo12O403- completely inhibited the oxidative degradation by irreversibly scavenging an electron and holding it. We systematically investigated the effects of iron loading, the concentration of POM, and pH on the oxidative degradation kinetics of 4-chlorophenol in the POM-mediated ZVI system. The POM-mediated oxidations on ZVI were additionally tested for 12 organic contaminants and the rates were compared. Their oxidative degradation on ZVI was mostly enhanced in the presence of POM (SiW12O404-). The present study provides a good model system upon which the ZVI-based oxidation technologies can be successfully enhanced and modified for further developments.