Феригідрит: лабораторний синтез, структура та фазові перетворення

Abstract

Феригідрит належить до метастабільних залізокисневих фаз, розповсюджених у природних системах. Він є перспективним матеріалом для практичного застосування в техніці, біології та медицині. До теперішнього часу залишаються дискусійними питання щодо його загальної формули, структуральної та поверхневої моделей, координації та валентності заліза, кількості модифікацій та ін. У роботі зроблено спробу проаналізувати літературні першоджерела, присвячені вивченню структури та фазових перетворень феригідриту.

Ферригидрит относится к метастабильным железокислородным фазам, распространенным в природных системах. Он служит перспективным материалом для практического применения в технике, биологии и медицине. До настоящего времени остаются дискуссионными вопросы о его общей формуле, структуральной и поверхностной моделях, координации и валентности железа, количестве существующих модификаций и др. В работе предпринята попытка проанализировать литературные источники, посвященные исследованию структуры и фазовых превращений ферригидрита.

The search of new materials with nanosized particles leads to the interest in nature disperse minerals particularly iron oxides and hydroxides and their synthesized analogs. Nowadays such structures are widely used for technical and medico-biological applications. One of the iron-oxygen structures which has great prospects for practical aims is ferrihydrite. It was found by academic F.V. Chukrov and was recognized as a mineral by the International Mineralogical Association in 1975. But the information about ferrihydrite is too contradictory, because this mineral belongs to metastable phases and is a precursor for the formation of goethite and hematite. Now there is no common opinion about its general formula, structural mo del (mono or polyphase), iron coordination (octahedral or both octahedral and tetrahedral positions) in the lattice and in the presence of ferric or ferrous iron forms in lattice, number of modifications (only two- and six-lines or their intermediate forms), as well as properties and mechanisms of phase transformation. The purpose of the present work is the review of the literature sources devoted to investigations of ferrihydrite. The synthesis of ferrihydrite in laboratory conditions has been carried out by different methods, i. e.: hydrolysis and precipitation of ferric salts, phase transformation Fe(II)—Fe(III) LDH, sol-gel syn thesis, formation of the particles in organic-inorganic polymer matrixes, and pyrolysis of sprays. Depending on the way of synthesis the ferrihydrite particles have different size and crystallinity. In the first part of the review we showed the structural peculiarity of two- and six-line ferrihydrite. The development of the classical structural models by K.M. Towe and W.F. Bradley; J.D. Russell; R.A. Eggleton and R.W. Fitzpatrick; V.A. Drits et al. A. Manceau and modern positions about this question were monitored. For the structural studying of ferrihydrite X-ray diffraction, M ssbauer spectroscopy, IR-FT, EXAFS, XANS, XANES, NEXAFS, TEM and SEM are used as the main methods. Ferrihydrite can transform not only into the phases of structural α-row but into iron oxides and hydroxides of other structural modifications. The chemical composition and pH values of dispersion medium, temperature of carrying out the process and red-ox condition belong to the most important parameters which influence the mechanisms of the phase tran sfor mation. The examples of the phase transformation of 2-line ferrihydrite to 6-line ferrihydrite, their transfor mation into goe thite, hematite, lepidocrocite and mag netite are shown in numerous literature sources. This opens the way for direct synthesis of pure iron-oxygen nano- and microsized structures from ferrihydrite precur sors. In the second part of the review the sorption and magnetic properties of ferrihydrite, its interaction with species of environment, the specific character of biogenic and corrosion ferrihydrite as well as usage of the ferrihydrite particles for medicobiological applications will be presented.