Synthesis and characterization of hybrid materials based on kaolinitic clay for the adsorption of azo dyes

P. Ngue Song1, J. G. Mahy1, 2, C. Calberg1, N. Fagel3, S. D. Lambert1

1Department of Chemical Engineering – Nanomaterials, Catalysis, Electrochemistry, University of Liège, Belgium
2Institut National de la Recherche Scientifique (INRS), Centre-Eau Terre Environnement, University of Quebec, Canada
3Department of geology – Clays, Geochemistry and Environment, University of Liège, Belgium  


Clays are natural nanomaterials with various microstructures, such as nanoplatelets and nanofibres. They present many advantages as they are abundant in nature, inexpensive and environmentally friendly. Kaolinite is a naturally abundant clay mineral. It is an aluminosilicate of type 1:1 ; that is, tetrahedral sheets of silicate units (SiO4) are covalently bonded to octahedral sheets of hydroxylated aluminates (AlO6-x(OH)x), with the general composition Al2Si2O5(OH)4. The presence of hydrogen bonds between neighbouring or adjacent kaolinite layers, increase their stability, and makes it difficult to modify this mineral in terms of exfoliation or delamination. In addition, this property prevents the free expansion of the interfoliar space as in the 2:1 swelling clay minerals. 


In this work, three metakaolinite samples were prepared by calcining a natural Cameroonian kaolinite (Kribi gisement) (specific surface area, SBET, equal of 20 m2/g) at 600, 700 and 800 °C. Indeed, during this thermal treatment, the kaolinite is dehydroxylated and the strong hydrogen bonds between the adjacent layers were broken. Furthermore, these three samples, in addition to the parent kaolinite, were subjected to an acid treatment (HCl 6M) at high temperature (110 °C) under reflux conditions for 5 h. 


It was observed that the kaolinite was resistant to acid leaching, while the more reactive metakaolinite samples favoured the extraction of Al3+ cations from most octahedral sites accompanied by the formation of an amorphous silica phase. So the three metakaolinite samples presented SBET values around 315 m2/g with the presence of micropores and mesopores. We determined the microporous specific surface area, Smicro, from DubininRadusckevitch theory, and the values of Smicro are equal to 125 m2/g for MK-600°C-HCl, 130 m2/g for MK-700°C-HCl and 140 m2/g for MK-800°C-HCl respectively. 


All these samples are used for the adsorption of a model dye, like Congo Red. So it is observed that the percentage of adsorbed Congo Red increases with the specific surface area of the samples. Furthermore, the isoelectric point of all samples has to be determined also to check the eventual interactions between the surface of modified clays and the dye.