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Mössbauer study of magnetite and maghemite nanoparticles prepared for water cleaning

Trichloroethylene (TCE) has the toxicity like carcinogenicity. Our previous studies revealed that industrially-produced Fe^0-γ-Fe2O3 mixture decreased concentration of TCE in aqueous solution from 10 to 0.5 ppm after 7-day leaching. It revealed that Fe^0-γ-Fe2O3 mixture (3:7) decomposed methylene blue (MB)with a first order rate constant of 1.6×10^-1 day^-1. These cleaning effects were observed in a leaching test with bulk powder. It is expected that organic compounds like TCE or dioxine could be decomposed more effectively by the Fe^0-γ-Fe2O3 mixture of smaller particle size. particle size. Generally, magnetite nanoparticles can be synthesized using different chemical methods, such as hydrothermal route, thermal decomposition, sol-gel, chemical precipitation, etc. In this study, nanoparticles (NPs) of Fe3O4 and γ-Fe2O3 were synthesized using a)hydrothermal synthesis from alcoholic solution of Fe(III)-Fe(II) salts with a base addition and b)oxidation-precipitation method from aqueous Fe(II) salt solution, a base and a mild oxidant (nitrate ions).(NH4)2Fe(SO4)2•6H2O (Mohr salt) was used as a Fe^2+ source. γ-Fe2O nanoparticles were prepared by heating magnetite nanoparticles at 250 ºC in air. The precipitates structural characterization was carried out by 57^FeMössbaur and Fourier transform infrared (FT-IR)spectroscopies, X-ray diffractometry (XRD) and scanning electron microscopy (FE-SEM). Mössbauer spectra recorded at 20 ºC of magnetite NPs prepared by hydrothermal procedure and oxidation-precipitation method showed spectral features characteristic for nonstoichiometric magnetite, whereas oxidized magnetite NPs showed the characteristic maghemite Mössbauer spectrum. The corresponding calculated 57^ Fe Mössbauer parameters were used for magnetite NPs stoichiometry determination. Magnetite NPs obtained by hydrothermal and oxidation-precipitation method showed (Fe^2+)/(Fe^3+) ratio of 0.234 and 0.214, giving magnetite chemical formula Fe3-δO4, δ = 0.154 or 0.167, respectively. Particles size of magnetite NPs were determined by FE-SEM inspection and were in the range ~20-30 nm or ~20-50 nm depending on the synthesis procedure. On the other hand the particle size of maghemite NPs which were prepared by oxidation of hydrothermally synthesized magnetite NPs did not change significantly compared to precursor particle size.

Magnetite; Maghemite; Nanoparticles; Synthesis; FE-SEM; FT-IR; XRD; Mossbauer spectroscopy

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