Study of ground and unground leached vermiculite Celia Maqueda a, ⁎, José Luís Perez-Rodriguez b , Jan Šubrt c , Natalia Murafa c a Instituto de Recursos Naturales y Agrobiología (CSIC) Apdo 1052, 41080-Sevilla, Spain b Instituto de Ciencia de Materiales de Sevilla (UNSE-CSIC) Americo Vespucio s/n, 41098 Sevilla, Spain c Institute of Inorganic Chemistry AS CR, 250 68 Řež, Czech Republic abstract article info Article history: Received 16 October 2008 Received in revised form 28 January 2009 Accepted 30 January 2009 Available online 14 February 2009 Keywords: Vermiculite Amorphous silica Acid leaching Porosity Electron microscopy Grinding Akaganeite Grinding of clays modifies their surfaces and can significantly affect their leaching behaviour. The acid reaction of vermiculite from Santa Olalla (Huelva, Spain) with HCl at various concentrations was affected by grinding and acid concentration. The acid leaching of ground vermiculite for 3 min with 1 M HCl solution at 80 °C for 24 h removed MgO and Al 2 O 3 almost completely, leaving a residue containing SiO 2 and Fe 2 O 3 . X-ray diffraction analysis showed the presence of akaganeite (β-FeOOH) and an amorphous phase (silica). Porosity studies showed a very high specific surface area for ground samples compared with unground vermiculite samples, attributed to the presence of iron in the residue coming from structural iron. High resolution transmission electron microscopy (HRTEM) confirmed the presence of iron oxyhydroxides embedded in the silica material. The particle morphology of the iron oxides corresponded well to akaganeite microcrystals precipitated from solution. The leached vermiculite residue also contained Cl - and a small amount of Ti 4+ , which were accumulated into the akaganeite microcrystals. © 2009 Elsevier B.V. All rights reserved. 1. Introduction Selective leaching with acid has been used to prepare porous silica from various clay minerals, including vermiculite (Suquet et al., 1991, 1994; Temuujin et al., 2003; Okada et al., 2006; Maqueda et al., 2007). Vermiculite clay is characterized by substitution in the tetrahedral sheet (Si 4+ by Al 3+ and Fe 3+ ) and in the octahedral sheet (Mg 2+ by Al 3+ , Fe 3+ and Fe 2+ ). The presence of iron in this mineral plays an important role in the specific surface area obtained after acid treatment, 672 m 2 g -1 and 720 m 2 g -1 by Temuujin et al. (2003) and Maqueda et al. (2007), respectively, which are higher than the values obtained using other clay minerals, including vermiculites, without iron in their structure. Temuujin et al. (2003) suggest that in vermiculites containing Si 4+ , Al 3+ and Fe 3+ in the tetrahedral sheet, the Fe 3+ leaches more slowly than Al 3+ . This may stabilize the tetrahedral sheet against framework structure formation, retaining the micropores and their high specific surface area. However, Maqueda et al. (2008) obtained similar specific surface areas using vermiculite without Fe 3+ in the tetrahedral sheet (from Santa Olalla, Huelva, Spain) suggesting that acid leaching is responsible for stabilising the high surface area formations. Grinding of clay minerals significantly influences their leaching behaviour and the formation of new phases upon heating (Sanchez- Soto and Perez-Rodriguez, 1989; Sanchez-Soto et al., 1993; Temuujin et al., 2002). Ground vermiculite shows a considerable particle size reduction with increasing grinding times, which induces structure degradation. The increased specific surface area, increased porosity and structural defects produced by grinding vermiculite all influence its leaching behaviour (Perez-Maqueda et al., 2004; Maqueda et al., 2007). The residues obtained by acid leaching of vermiculites reported by Temuujin et al. (2003) and Maqueda et al. (2007) show higher surface areas than other clay minerals including vermiculite. However, there are important differences in the experimental methods and the chemical composition of the residues obtained in each work. The residues that Maqueda et al. (2007) produced by leaching ground vermiculite showed a high percentage of Fe 2 O 3, this oxide was not significantly present in the residues obtained by Temuujin et al. (2003). Maqueda et al. (2008) also showed that the surface areas of unground samples were smaller than ground sample and no iron phase was found in the residues. Thus, result suggests that the iron plays an important role in the preparation of amorphous silica with very high surface area. Therefore it is important to further study the iron present in the residues after acid leaching. In this work iron oxide inclusion in amorphous silica formed during acid leaching of Santa Olalla vermiculite was studied by X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM). Applied Clay Science 44 (2009) 178–184 ⁎ Corresponding author. Tel.: +34 954624711; fax: +34 954624002. E-mail address: celia@irnase.csic.es (C. Maqueda). 0169-1317/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.clay.2009.01.019 Contents lists available at ScienceDirect Applied Clay Science journal homepage: www.elsevier.com/locate/clay