Bentonite, biotite, illite, kaolin, vermiculite and zeolite were acidified or alkalized with hydrochloric acid or sodium hydroxide at concentrations of 0.1, 1.0 and 5.0 mole dm−3 at room temperature for two weeks. In acid treatments, dissolution of Al prevailed over Si and the opposite was observed in alkali treatments. The XRD patterns showed severe alteration of the crystal structure after acid treatments, whereas sharpening of the XRD peaks after alkali treatments was observed. Illite and kaolin were most resistant to acid attack. with a few exceptions, the surface areas of the minerals computed from both water and nitrogen adsorption isotherms increased with acid and alkali treatments. with increasing reagent concentration, the nitrogen surface area increased faster than the water surface area. well-defined trends were not noted in either changes of average water or nitrogen adsorption energies or in relative amounts of adsorption sites, indicating that the effects of acid and alkali attack are controlled by the individual character of the minerals.

A b s t r a c t. Adsorption of nitrogen on the rapeseed oil cake and rapeseed oil cake with wheat meal extrudates was investigated. The results are presented as adsorption-desorption isotherms. The Brunauer-Emmet and Teller equation was used to analyse the experimental sorption data. To obtain estimates of the surface area and surface fractal dimension, the sorption isotherms were analyzed using the Brunauer-Emmet and Teller and Frenkel-Halsey-Hill equations. Mesopore analysis was carried out using the Dollimore and Heal method. The properties and surface characteristic of rapeseed oil cake extrudates are related to different basic properties of particular samples and duration of the extrusion process. Extrusion conditions lead to essential differences in particular products. For all kinds of rapeseed oil cakes the amount of adsorbed nitrogen was different, but for the rapeseed oil cake extrudates a large amount of adsorbed nitrogen was observed. The average surface area of the rapeseed oil cake extrudates was about 6.5-7.0 m 2 g -1 , whereas it was equal to about 4.0-6.0 m 2 g -1 for rapeseed oil cake with the wheat meal extrudates. In the case of non-extruded rapeseed oil cake and wheat meal, the dominant group included ca. 2 and 5 nm pores. The values of surface fractal dimension suggested that the surface of the extrudates was more homogenous than that of the raw material. Duration of the extrusion process to 80 s resulted in a decrease in the specific surface area, surface fractal dimension, and porosity of the extrudates.K e y w o r d s: rapeseed oil cake extrudates, nitrogen sorption, specific surface area, fractal dimension, porosity INTRODUCTION Physical gas adsorption is often used as a method to study the surface and pore characteristics of solid materials. The technique accurately determines the amount of a gas retained by a solid body, which is a direct measure for the surface and porous properties. The method is relatively fast and relatively easy in operating the equipment. The isotherm obtained from experiments can provide information about the surface area, surface energetic and geometric heterogeneity, pore volume and pore size distribution (PSD). Nitrogen adsorption at 77 K is a standard method for investigation of porous solids and can be used for routine quality control, as well as for investigation of new materials (Rouquerol et al., 1994;Sing, 2001).The theory of fractals is becoming more and more widely used for describing the structure of disordered media and the processes occurring in them (Senesi and Wilkinson, 2008). Many methods have been developed to obtain surface fractal dimensions (D s ) of adsorbents on the basis of different experiments. Among those, the methods based on analysis of adsorption isotherms play an important role, since they require only one complete adsorption isotherm for a given adsorbent to calculate the surface fractal dimension (Terzyk et al., 2003). A simple method for calculation of the fractal dimension from a single adsorption isotherm was developed by J...