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Fe³⁺ intercalation/deposition on vermiculites and interaction with pyrrole: a Mössbauer spectroscopic investigation

¹ Materials Science Institute of Seville, CSIC-US, Av. Américo Vespucio, 49, 41092 Sevilla, Spain, ² Walther Meissner Institut, Bayerische Akademie der Wissenschaften, D-85748 Garching, Germany, and ³ Physik Department, Technische Universität München, D-85748 Garching, Germany

^* E-mail: veronica{at}icmse.csic.es

(Received 22 June 2007; revised 24 March 2008)

The Fe³⁺ uptake from aqueous and methanol solutions of FeCl₃·6H₂O in two vermiculites from Santa Olalla and Ojén (Spain) and in a montmorillonite from Wyoming has been investigated by Mössbauer spectroscopy at room temperature and 4.2 K. The samples were further characterized by X-ray diffraction. The low temperature Mössbauer spectra reveal the deposition of akaganeite and perhaps some ferrihydrite on the external surface of the clay minerals. The room temperature spectra exhibit two Fe³⁺ quadrupole doublets in addition to the Fe²⁺ components of the structural iron in the sheet silicates. On treatment in 0.04 M Fe³⁺ solutions the Fe³⁺ quadrupole splittings of 0.90 mm/s and 0.55 mm/s can be assigned to the mineral akaganeite; at smaller Fe³⁺ concentrations one of these doublets has a quadrupole splitting similar to that of the Fe³⁺ in the original vermiculites, the other one a quadrupole splitting of 0.6–0.7 mm/s, which can be considered as a hint for an uptake of Fe dimers in the interlayer space irrespective of the solvent used for iron deposition. Preliminary results show that the adsorption of pyrrole in all of our Fe³⁺-containing samples, regardless of the preparation in aqueous or methanol solution, results in the formation of polypyrrole and a concomitant reduction of some of the Fe³⁺. An uptake of pyrrole into the interlayer space has been observed only in the Fe³⁺-containing Wyoming montmorillonite and in the Ojén vermiculite treated with the methanol solution of iron chloride.

KEYWORDS: vermiculite, montmorillonite, ferrihydrite, akaganeite, pyrrole, nano-composite, Mössbauer spectroscopy, XRD