Mössbauer study of metallic iron and iron oxide nanoparticles having environmental purifying ability

Abstract

A relationship between local structure and methylene blue (MB) decomposing ability of nanoparticles (NPs) of metallic iron (Fe0) and maghemite (γ‐Fe2O3) was investigated by 57Fe Mössbauer spectroscopy, X-ray diffractometry and UV-visible light absorption spectroscopy. γ‐Fe2O3 NPs were successfully prepared by mixing (NH4)2Fe(SO4)2⋅6H2O (Mohr's salt) and (NH4)3Fe(C2O4)3⋅3H2O aqueous solution at 30 °C for 1 h, while those of Fe0 were obtained by the reduction of Mohr's salt with NaBH4⁠. From the Scherrer's equation, the smallest crystallite sizes of γ‐Fe2O3 NPs and Fe0 NPs were determined to be 9.7 and 1.5 nm, respectively. 57Fe Mössbauer spectrum of γ‐Fe2O3 NPs consists of a relaxed sextet with isomer shift (δ) of 0.33±0.01 mm s−1, internal magnetic field (Hint) of 25.8±0.5 T, and linewidth (Γ) of 0.62±0.04 mm s−1. 57Fe Mössbauer spectrum of Fe0 NP is mainly composed of a sextet having δ, Δ, and Hint of 0.00±0.01 mm s−1 0.45±0.01 mm s−1, and 22.8±0.1 T, respectively. A bleaching test of the mixture of Fe0 and γ‐Fe2O3 NPs (3:7 ratio, 100 mg) in MB aqueous solution (20 mL) for 6 h showed a remarkable decrease of MB concentration with the first-order rate constant (kMB) of 6.7 × 10−1 h−1. This value is larger than that obtained for the bleaching test using bulk Fe0+γ‐Fe2O3 (3:7) mixture (kMB = 6.5×10−3h−1)⁠. These results prove that MB decomposing ability is enhanced by the NPs mixture of Fe0 and γ‐Fe2O3⁠.