Investigations on iron leaching from oxides and its relevance for radical generation during Fenton-like catalysis

Crystalline hematite, goethite, and amorphous iron oxide supported on silica were used to investigate the leaching of iron during heterogeneous Fenton catalysis. The effects of relevant variables, comprising pH, radiation incidence, the presence of reductive species (hydroquinone), and hydrogen peroxide concentration, were evaluated using a first-order factorial design experiment. Results revealed a significant influence of pH, hydroquinone concentration, and radiation incidence for hematite and for the amorphous iron oxide, resulting in acidic and reductive leaching. As for goethite, radiation incidence and pH were the most significant variables. Higher concentrations of iron, which were measured during reactions conducted with iron oxide supported on silica, were attributed to its low crystalline structure. Electron paramagnetic resonance analyses proved the reductive environment provided by radiation and by the presence of reductive species is essential to promote the hydroxyl radical generation by the soluble iron species. The findings suggest that different iron oxides present different mechanisms for iron lixiviation and emphasize the importance of assessing the leaching of iron during Fenton-like reactions.