57Fe-Mössbauer study and methylene blue decomposing effect of nanoparticle mixtures composed of metallic iron and maghemite

A relationship between local structure and methylene blue degradation ability of nanoparticle (NP) mixtures composed of metallic iron (Fe0) and maghemite (γ-Fe2O3) was studied by means of 57Fe-Mössbauer spectroscopy, X-ray diffraction (XRD), transmission electron microscopy (TEM), thermogravimetry-differential thermal analysis (TG-DTA), ultraviolet-visible light absorption spectroscopy (UV-Vis) and electrospray ionization mass spectroscopy (ESI-MS). Organic compounds decomposing behaviour of NPs was evaluated by 10-day degradation test with methylene blue (MB) aqueous solution. (Fe0 + γ-Fe2O3) NPs with a molar ratio of 1: 3 showed a remarkable decrease in the MB concentration from 20.0 to 0.85 μM with the largest first-order rate constant (kMB) of 2.84 day−1. The ESI-MS spectrum of MB aqueous solution after 1-day reaction with the (Fe0 + γ-Fe2O3) NPs (1:3) showed several peaks at the m/z values of 109, 129 and 165 etc. indicating that MB was decomposed by the NP mixture. Mössbauer spectrum of the NP mixture measured after 1-day degradation test, composed of two relaxation spectra (sextets): one with isomer shift (δ) of 0.35±0.01 mm s−1 and internal magnetic field (Hint) of 36.3±0.1 T, which was ascribed to tetrahedral Fe3+, and the other with δ of 0.65±0.01 mm s−1 and Hint of 33.1±0.1 T due to octahedral Fe2.5+. The precipitation of Fe3O4 NPs was also confirmed by the Mössbauer spectrum of the same sample measured at 78 K. Isothermal annealing of the Fe3O4 NPs at 250 and 450 °C proved an NP formation of γ-Fe2O3, suggesting that the (Fe0 + γ-Fe2O3) NPs could be applied as the environmental purifying material.