Catalytic activity of different iron oxides: Insight from pollutant degradation and hydroxyl radical formation in heterogeneous Fenton-like systems

The catalytic performance of iron oxides in 2,4,4'-trichlobiphenyl (PCB28) degradation following an order of goethite > magnetite > hydrated iron(III) oxide > hematite was observed in four iron oxides catalyzed Fenton-like reactions, which was not consistent with the stoichiometric efficiency of center dot OH generation. This result indicates that center dot OH was not the sole active species responsible for PCB28 degradation in four iron oxides catalyzed Fenton-like systems. The reductive degradation of hexachloroethane (HCE) was 76.8%, 58.7%, 46.1% and 37.6% for magnetite, goethite, hematite and hydrated iron(III) oxide, respectively, further suggesting that both oxidative species (center dot OH) and reductive species (HO2 center dot) simultaneously contributed on PCB28 degradation. The addition of tert-butyl alcohol (center dot OH scavenger) decreased the amount of 7-hydroxycoumarin (7-HC, center dot OH indicator) by 42%, 77%, 97%, and 97% for magnetite, goethite, hematite, and hydrated iron(III) oxide, respectively. Meanwhile, the addition of p-benzoquinone (HO2 center dot scavenger) only reduced the amount of 7-HC by 77%, 27%, and 33.5% for magnetite, hematite, and hydrated iron(III) oxide, respectively; but increased the amount of 7-HC by 126.7% in the goethite/H2O2 system. This result indicates that the mechanism of center dot OH generation from H2O2 catalyzed by four iron oxides was different. For goethite, the surface lattice iron was primarily responsible for activating H2O2 to form reactive species (center dot OH and HO2 center dot) and thereby degrade pollutants on the oxide surface. For magnetite, both the surface lattice iron and the dissolved Fe in solution played an equally important role in catalyzing H2O2 decomposition to form and propagate reactive species for pollutant degradation. For hematite and hydrated iron(III) oxide, the solution phase chain reaction effectively propagated by dissolved Fe should be the primary catalytic mechanism although the chain reaction was initiated by the surface processes.