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THE BEHAVIOR OF Fe IN GROUND AND ACID-TREATED VERMICULITE FROM SANTA OLALLA, SPAIN

¹ Instituto de Recursos Naturales y Agrobiología (CSIC), Apdo 1052, 41080 Sevilla, Spain
² Instituto de Ciencia de Materiales de Sevilla (UNSE-CSIC), Americo Vespucio 49, 41092 Sevilla, Spain
³ Walther-Meissner-Institut, Bayerische Akademie der Wissenschaften, Walter-Meissner-Strasse 8, Garching D-85748, Germany
⁴ Physik-Department EI5, Technische Universität München, Garching D-85748, Germany

^* E-mail address of corresponding author: celia{at}irnase.csic.es

The preparation of porous materials from clay minerals by selective leaching is of interest because it yields residues with large specific surface areas that can be used as adsorbents of contaminants or as catalysts. Grinding produces surface modifications and therefore may significantly influence the leaching behavior. The aim of this paper is to study the effect of grinding and leaching on the structure of the vermiculite from Santa Olalla, Spain, using ⁵⁷Fe Mössbauer spectroscopy, X-ray diffraction, infrared spectroscopy, and specific surface area (S_BET) measurements. The study shows that grinding destroys the long range order of the vermiculite, but leaves the local structure in the environment of the Fe atoms intact, at least up to a grinding time of 10 min. The Mössbauer study shows that there is no Fe³⁺ in the tetrahedral sheets and that grinding does not lead to a significant oxidation of the structural Fe. Vermiculite ground for 4 min and leached with 1 M HCl solution at 80°C over a 24 h period was decomposed to X-ray amorphous silica with a very large specific surface area (S_BET = 720 m²g^–1) and with total pore volume of 0.586 cm³ g^–1, whereas an unground sample leached with the same acid concentration yielded a specific surface area of only 504 m² g^–1. Most of the Mg²⁺ and Al³⁺ are removed from the ground sample after leaching with 1 M HCl, while large percentages of Fe₂O₃ remain with the X-ray amorphous silica. In unground vermiculite leached with 1 M HCl, a considerable amount of vermiculite remains in the residue. A sample ground for 4 min and treated with 0.25 M HCl also shows the typical vermiculite Mössbauer spectrum with an Fe²⁺/Fe³⁺ ratio similar to that of the unground vermiculite. The samples ground for 2 or 4 min and treated with 1 M HCl solution have an orange color and, according to the Mössbauer spectra, only Fe³⁺ remains. Mössbauer spectra of these samples taken at 4.2 K reveal the presence of akaganéite.

Key Words: Acid Leaching • Grinding • Mössbauer Spectroscopy • Surface Properties • Vermiculite • X-ray Amorphous Silica

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