Microwave-enhanced catalytic degradation of organic compounds with silica-coated iron oxide nanocrystals via fenton-like reaction pathway

Abstract

Microwave (MW)-enhanced catalytic oxidation processes are emerging and effective techniques for the degradation of organic compounds in water and wastewater treatment processes. In this study, through applied MW irradiation, monodisperse, superparamagnetic iron oxide nanocrystals (IONCs) with thin, amorphous silica coatings are demonstrated to rapidly catalyze the degradation of organic compounds in water through a thermally enhanced, Fenton−type process. For this, we precisely synthesize amorphous silica-coated various metal oxide (single domain) nanocrystals, and then evaluate the degradation of methyl orange (MO) and benzoic acid (BA), chosen as model organic molecules. We examine (and optimize) the effects of core (nanocrystal) composition, size, and concentration, along with solution pH and hydrogen peroxide (H₂O₂) concentration. Further, we describe the catalytic degradation of BA with IONCs under MW irradiation through radical scavenger controls and electron paramagnetic resonance (EPR) analysis, which support the proposed reaction mechanism. For materials evaluated, the amorphous silica coating not only prevents the loss of nanocrystal integrity but also provides a reactive, yet stable, interface between nanocrystals and bulk solutions, where the degradation of organic compounds can occur. Synthesized IONCs show high performance, which is repeatable for over five cycles without any deterioration of the nanocrystals core or metal leaching. Taken together, this research highlights the potential of enhanced MW-enhanced oxidation processes appropriately coated (i.e., designed) MW absorbers (here as superparamagnetic IONCs) for advanced water treatment.