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THE EFFECT OF THERMAL TREATMENT ON SOME OF THE PHYSICOCHEMICAL PROPERTIES OF A BENTONITE

Yüksel Sarikaya, Müerref Önal, Bülent Baran and Tülay Alemdarolu

University of Ankara, Faculty of Science, Department of Chemistry, 06100 Beevler, Ankara, Turkey

E-mail of corresponding author: sakaya{at}science.ankara.edu.tr

A white calcium bentonite (CaB) from the Kütahya region, Turkey, contains 35 wt. % opal-CT and 65 wt. % Ca-rich montmorillonite (CaM). Samples were heated at various temperatures between 100–1300°C for 2 h. Thermal gravimetric (TG), derivative thermal gravimetric (DTG), and differential thermal analysis (DTA) curves were determined. Adsorption and desorption of N₂ at liquid N₂ temperature for each heat-treated sample was determined. X-ray diffraction (XRD) and cation-exchange capacity (CEC) data were obtained. The change in the d(001) value and the deformation of the crystal structure of CaM depend on temperature. Deformation is defined here as changes of the clay by dehydration, dehydroxylation, recrystallization, shrinkage, fracture, etc. The activation energies related to the dehydration and dehydroxylation of CaB calculated from the thermogravimetric data are 33 and 59 kJ mol^–1, respectively. The average deformation enthalpies, in the respective temperature intervals between 200–700°C and 700–900°C, were estimated to be 25 and 205 kJ mol^–1 using CEC data and an approach developed in this study. The specific surface area (S) and the specific micropore-mesopore volume (V) calculated from the adsorption and desorption data, respectively, show a "zig zag" variation with increasing temperature to 700°C, but decrease rapidly above this temperature. The S and V values were 43 m² g^–1 and 0.107 cm³ g^–1, respectively, for untreated bentonite. They reach a maximum at 500°C and are 89 m² g^–1 and 0.149 cm³ g^–1, respectively. The XRD data clearly show that, at 500°C, where the irreversible dehydration is completed without any change in the crystal structure, the porosity of CaM reaches its maximum.

Key Words: Adsorption • Bentonite • Cation-Exchange Capacity • Dehydration • Dehydroxylation • Montmorillonite • Pore Volume • Surface Area • Thermal Analysis

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