Unexpected activity of magnetically separable Fenton catalyst in clay slurries

Maghemite/silica nanocomposite microspheres (γ-Fe₂O₃/SiO₂ MS) have been tested as magnetically separable heterogeneous Fenton catalyst in beidellite clay slurries in the presence of methylorange (MO) as a model pollutant. Due to its strong magnetic susceptibility, the catalyst was easily recovered from the clay slurries by a selective magnetic sorting, even in highly concentrated slurries. The Fenton oxidation of MO was strongly enhanced in the clay slurries in comparison to the reaction in aqueous phase, highlighting a synergistic catalytic effect. The influence of different experimental parameters such as the presence of specific Reactive Oxygen Species (ROS) scavengers, pH, H₂O₂, catalyst and clay concentrations, on the catalytic activity was also evaluated. Contrary to what was observed in aqueous phase, generation of hydroxyl radicals via the Haber-Weiss mechanism was not the unique pathway that caused MO degradation in the beidellite slurries. We also observed that the synergistic effect is more marked at pH near neutrality and low amounts of H₂O₂ and clay. Furthermore, the unchanged activity of the catalyst during five successive cycles together with the low level of iron leaching, makes it suitable candidate for an application as a magnetic heterogeneous Fenton catalyst in wastewater treatment, especially for surface waters or sludges naturally containing fine clay particles.