engleski

From ferrihydrite to magnetite and vice versa

The syntheses of nanosize magnetite particles by wet chemical oxidation of Fe2+ have been extensively investigated. In the present investigation the nanosize magnetite particles were synthesised without using the Fe(II) precursor. This was achieved by gamma-irradiation of water-in-oil microemulsion containing only the Fe(III) precursor. The corresponding phase transformations were monitored. Microemulsions (pH~12.5) were gamma-irradiated at a relatively high dose rate of ~22 kGy/h. Upon 1 h of gamma-irradiation the XRD pattern of the precipitate showed goethite and unidentified low-intensity peaks. Upon 6 h of gamma-irradiation, reductive conditions were achieved and substoichiometric magnetite (~ Fe2.71O4) particles with insignificant amount of goethite particles found in the precipitate. Hydrated electrons (e-aq), organic radicals and hydrogen gas as radiolytic products were responsible for the reductive dissolution of iron oxide in the microemulsion and the reduction Fe3+ - Fe2+. Upon 18 h of gamma-irradiation the precipitate exhibited dual behaviour, it was a more oxidised product than the precipitate obtained after 6 h of gamma-irradiation, but it contained magnetite particles in a more reduced form (~ Fe2.93O4). It was presumed that the reduction and oxidation processes existed as concurrent competitive processes in the microemulsion. After 18 h of gamma-irradiation the pH of the medium shifted from the alkaline to the acidic range. The high gamma-dose rate of ~22 kGy/h was directly responsible for this shift to the acidic range. At a slightly acidic pH a further reduction of Fe3+ - Fe2+ resulted in the formation of more stoichiometric magnetite particles, whereas the oxidation conditions in the acidic medium permitted the oxidation Fe2+ - Fe3+. The Fe3+ was much less soluble in the acidic medium and it hydrolysed and recrystallised as goethite. The gamma-irradiation of the microemulsion for 25 h at a lower dose rate of 16 kGy/h produced pure substoichiometric nanosize magnetite particles of about 25 nm and with the stoichiometry of Fe2.83O4. The phase composition of precipitates obtained by gamma-irradiation of ferrihydrite precipitate dispersed in aqueous or organic medium was also investigated. The present investigation has demonstrated the possibility of applying gamma-irradiation in the synthesis of nanosize magnetite particles starting only from Fe(III) precursor. By controlling the gamma-dose and gamma-dose rate one can control the phase composition, stoichiometry and size of magnetite particles

ferrihydrite; magnetite; gamma-irradiation; microemulsion; Mossbauer

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