Textural and structural modifications of saponite from Cerro del Aguila by acid treatment

Abstract

The physicochemical properties of clays can be modified by acid treatment with
inorganic acids. This treatment is usually referred to as ‘acid activation’ , because it increases the
specific surface area and the number of active sites of the solids. In the present study, the acid
activation of saponite from Cerro del Aguila (Madrid, Spain) with HCl solutions was measured.
Illite, quartz and small amounts of feldspar were found as impurities in the raw saponite.
Acid treatments were carried out with different concentrations of HCl solutions. The samples
obtained were characterized by mineralogical and chemical analyses, XRD, FT-IR spectroscopy, N2
adsorption-desorption isotherms and TEM. The acid attack, under the conditions employed, produced
a progressive destruction of the structure of saponite by partial dissolution of the octahedral Mg(II)
cations. Amorphous silica coming from the tetrahedral sheet of saponite was generated. The specific
surface area of the most intensely treated sample (2.5% for 24 h) was doubled with respect to that of
natural saponite. This increase in the surface area is due to the increase in both the external and
internal surface areas.