The effect of sulphuric acid activation on the crystallinity, surface area, porosity, surface acidity, and bleaching power of a bentonite

The Hançılı (Keskin, Ankara, Turkey) bentonite was activated with H2SO4 by dry method at 97 °C for 6 h to obtain optimum parameters for imparting a maximum bleaching power towards soybean oil. The H2SO4 content in dry bentonite-acid mixture was changed between 0% and 70%. The natural and activated samples were examined by X-ray diffraction (XRD), N2 adsorption–desorption, and n-butylamine adsorption (from the solution in cyclohexane). The specific surface area (S), specific micro–mesopore volume (V), mesopore size distribution (PSD), and surface acidity (nm) of the samples were determined. The bleaching power (BP) of each sample for alkali-refined soybean oil was determined. The S, V, nm, and BP increase after activation at various acid contents up to 40% H2SO4 without any considerable change in crystal structure of the smectite. The BP is controlled more by the PSD rather than other adsorptive properties of the bleaching earth. The optimum parameters for activation to obtain maximum bleaching power, are H2SO4% = 50–60, S = 250–230 m2 g−1, V = 0.46–0.47 cm3 g−1, nm = 9.0 × 10−4–8.4 × 10−4 mol g−1 and PSD mainly distributed between 1.4 and 6.0 nm.