Decomposition mechanism of methylene blue caused by metallic iron-maghemite mixture

Abstract

Decomposition mechanism of methylene blue (MB) caused by a mixture of metallic iron-maghemite (Fe⁰-γFe₂O₃) was investigated by means of ⁵⁷Fe-Mössbauer spectroscopy, X-ray diffractometry (XRD), Ultraviolet-Visible Absorption Spectroscopy (UV-vis) and electrospray-ionization mass spectroscopy (ESI-MS). Ten day-leaching test of 10 μmol L^− 1 MB aqueous solution and Fe⁰-γFe₂O₃ mixture (mass ratio 3:7) showed a decrease in the concentration from 10.5 to 4.45 μmol L^− 1 with first-order rate constant (k) of $1.5_{7} \times 10^{-1}$ day^− 1. An ESI-MS study of Fe⁰-γFe₂O₃ mixture (3:7) after the leaching test revealed new peaks at m/z of 100, 110 and 137 due to fragmentation of MB, in addition to those observed at m/z of 284, 270 and 256 which were ascribed to MB, Azure B and Azure A, respectively. ⁵⁷Fe-Mössbauer spectra of Fe⁰-γFe₂O₃ mixture (3:7) resulted in a decrease in absorption area (A) for Fe⁰ (δ = 0.00_±0.01 mm s^− 1, H _int = 33.0_±0.1 T) from 37.5 to 21.3, 9.7, 7.9, 7.0 and 4.5_±0.5 %, together with an increase in A from 0.0 to 5.0, 13.8, 17.2, 21.0 and 22.4_±0.5 % for octahedral (O _h) iron (Fe^II + Fe^III) of Fe₃O₄. XRD study of these samples indicated that the peak intensity at 2Θ of 44.7° being ascribed to Fe⁰ was decreased, while that of 35.6° due to Fe₃O₄ was almost constant; relative peak ratio of Fe$^{0}/\mathrm{Fe}_{3}$O₄ was decreased from 26.3 to 2.76 after the leaching, indicating that Fe⁰ in the Fe⁰-γFe₂O₃ mixture was oxidized to Fe₃O₄. These experimental results suggest that Fe⁰-γFe₂O₃ mixture could be utilized for the cleaning or decomposition of toxic organic compounds like trichloroethylene.