TiO₂-based (Fe₃O₄, SiO₂, reduced graphene oxide) magnetically recoverable photocatalysts for imazalil degradation in a synthetic wastewater

Magnetite (Fe₃O₄), a core-shell material (SiO₂@Fe₃O₄), and reduced graphene oxide-Fe₃O₄ (referred as rGO-MN) were used as supports of a specific highly active TiO₂ photocatalyst. Thermal treatments at 200 or 450 °C, different atmospheres (air or N₂), and TiO₂:support weight ratios (1.0, 1.5, or 2.0) were investigated. X-ray diffractograms revealed that magnetite is not oxidized to hematite when the core-shell SiO₂@Fe₃O₄ material-or a N₂ atmosphere (instead of air) in the thermal treatment-was employed to prepare the TiO₂-based catalysts (the magnetic properties being preserved). The materials treated with N₂ were first tested for degradation of imazalil (a well-known fungicide) in deionized water. The best compromise between the photocatalytic activity, magnetic separation, and Fe leached (1.61 mg L^-1, i.e., below the threshold for water reuse in irrigation) was found for the magnetic catalyst prepared with SiO₂@Fe₃O₄, an intermediate TiO₂:support ratio (1.5), and treated at 200 °C under N₂ atmosphere (i.e., SiO₂@Fe₃O₄-EST-1.5-200-N₂). This material was then tested for the treatment of imazalil in a synthetic wastewater, SW (with a chemical composition simulating an effluent resulting from fruit postharvest activity). This SW has a pH of 4.2 and the experiments were carried out at this natural pH₀ and at neutral conditions (keeping pH at 7 along the reaction). The magnetic catalyst was more active than bare TiO₂ for the treatment of imazalil in SW at natural pH. Since Fe leaching was observed (3.53 mg L^-1), added H₂O₂ enhanced both imazalil degradation and mineralization. Conveniently, these catalysts can be readily recovered by using a conventional magnetic field, as demonstrated over three consecutive recycling runs. Graphical abstract % Imazalil conversion using different magnetic catalysts and comparison with bare TiO₂.