Iron oxide nanoparticles as heterogeneous electro-Fenton catalysts for the removal of AR18 azo dye

Heterogeneous electro-Fenton mineralization of Acid Red 18 (AR18) in aqueous solution was studied with magnetite Fe₃O₄ (MNPs) and haematite Fe₂O₃ (HNPs) nanoparticles as catalysts. High mineralization yields of AR18 were obtained with magnetite, 81% TOC removal after 180 min of electrolysis in 40 mg L^-1 Fe₃O₄, pH 3.0, at 50 mA of current intensity and in 50 mM Na₂SO₄. In order to explain the obtained mineralization yield achieved with MNPs, the quantification of hydrogen peroxide (H₂O₂), hydroxyl radical (•OH) and iron leaching were performed at 50 and 100 mA. From the high iron concentration found in the bulk solution and the slight impact of the catalyst mass concentration on TOC removal, the formation of hydroxyl radicals occurs mainly through homogeneous process. In the presence of hydroxyl radical scavenger, degradation remained total after 15 min showing the involvement of a direct electrochemical oxidation of the dye at the electrode surface. The hydroxyl radical oxidation is responsible for at least 50% of mineralization.