Microwave-enhanced catalytic degradation of p-nitrophenol in soil using MgFe2O4

• MgFe2O4 induced with MW showed a good removal for PNP in soil.
• Both OH and O2− were participate in the catalytic process.
• Water, oxygen, and the surface hydroxyl groups serve as reactive species donors.
• Thermal desorption, catalytic oxidation, and thermal stabilization were occurred.

The application of MgFe2O4 as microwave (MW) absorber and catalyst to removal p-nitrophenol (PNP) in soil has been investigated, including the catalytic performance, removal route, and reaction mechanisms. Five types of MgFe2O4 nanoparticles were synthesized with chemical co-precipitation (M1, M1–1, and M1–2), gel combustion (M2), and reverse micelle (M3) methods, respectively, and characterized by FTIR, XRD, and SEM technologies. The results showed that the chemical co-precipitation of MgFe2O4 (M1, calcined at 600 °C for 2 h) with higher O–H and H–O–H contents and higher Mg2+/Fe3+ occupying tetrahedral sites had the highest catalytic performance for PNP degradation. The enhancement of MW catalytic performance was attributed to the significant increased and hypodispersion of “hot spots” on MgFe2O4 surface (>1000 °C) and the generation of various active species (OH and O2−). Water and oxygen molecular as active species donors played an important role in the catalytic process. The main degradation products were phenol, hydroquinone, 1,2,4-trihydroxybenzene, and dihydroxynitrobenzene. Three stages were occurred in the MW catalytic process. A part of PNP was firstly thermal desorbed from soil with soil temperature increasing, and PNP was further catalytically oxidized by OH and O2− species from the disassociation of water and oxygen. Parts of PNP and its decomposed products were directly encapsulated in soil by thermal stabilization.