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

Decomposition mechanism of methylene blue (MB) caused by a mixture of metallic iron-maghemite (Fe0-γFe2O3) was investigated by means of 57Fe-Mossbauer 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-Mossbauer 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 (FeII + FeIII) 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.