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The Behavior of Fe in Ground and Acid-Treated Vermiculite from Santa Olalla, Spain

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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 (SBET) 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 80ºC over a 24 h period was decomposed to X-ray amorphous silica with a very large specific surface area (SBET = 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.
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Keywords: ACID LEACHING; GRINDING; MÖSSBAUER SPECTROSCOPY; SURFACE PROPERTIES; VERMICULITE; X-RAY AMORPHOUS SILICA

Document Type: Research Article

Publication date: June 1, 2008

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  • The JOURNAL publishes articles of interest to the international community of clay scientists, including but not limited to areas in mineralogy, crystallography, geology, geochemistry, sedimentology, soil science, agronomy, physical chemistry, colloid chemistry, ceramics, petroleum engineering, foundry engineering, and soil mechanics. Clays and Clay Minerals exists to disseminate to its worldwide readership the most recent developments in all of these aspects of clay materials. Manuscripts are welcome from all countries.

    Clays and Clay Minerals is the official publication of The Clay Minerals Society.

    The Editor-in-Chief is Professor Joseph W. Stucki [email protected]

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