Green iron nanoparticles supported on amino-functionalized silica for removal of the dye methyl orange

This work describes the synthesis of green iron nanoparticles (FeNPs) supported on amino-functionalized silica, carried out in presence of the extract of the plants Yerba Mate (Ilex paraguariensis) or black tea (Camellia sinensis), or in presence of the biodiesel co-product glycerol, as green reducing and stabilizing agents. The materials were characterized by AAS, SEM, TEM, FTIR, XRD, ASAP, Mossbauer spectroscopy and thermogravimetric analysis, which revealed the formation of amorphous FeNPs of iron oxyhydroxides, such as akaganeite and ferrihydrite. FeNPs in the order of 5 to 20 nm were obtained from tea. Besides, the plant extracts were able to reduce the Fe3+ precursor ion to Fe2+ species bound to the plant polyphenols. The FeNPs were applied in the removal of the azo dye methyl orange by adsorption and heterogeneous Fenton degradation, at different reaction conditions, such as catalyst dosage, pH, hydrogen peroxide concentration, and temperature. All iron compounds prepared removed the dye completely after three hours of reaction, and could be successfully recycled. In particular the FeNPs prepared with glycerol were active up to the fifth cycle in the Fenton degradation tests, due to the lower iron and organic compounds leaching. Kinetics showed that the adsorption is a key step in the dye removal, and that the adsorption sites are composed by the FeNPs bound to polyphenols/glycerol. The superior activity of the supported FeNPs in relation to the free analogues showed the benefits of the functionalization of the silica by the amino groups to stabilize the green iron nanoparticles.