Preparation and characterization of nanoporous powders from bentonite by hydrochloric acid leaching and using as bleaching earth

Abstract

A calcium bentonite (CaB) was leached by hydrochloric acid (HCl) solutions to improve the adsorptive properties such as nanoporosity, acidity, and decoloration power. The mass ratio of HCl/CaB was changed between zero and 1.00 according the anhydrous CaB + HCl mixture. Different powders were obtained from the heating of the aqueous mixtures at 90 °C for 16 h under a reflux condenser. The raw CaB and obtained powders were investigated by different methods such as X-ray diffraction, chemical analyses, adsorption, pH-metry, and tintometry techniques. A calcium rich-smectite (CaS) are found in the CaB as major clay mineral, whereas illite (I) and metahalloysite (MH) as minor ones. The impurities are opal-A (OA), opal-CT (OCT), quartz (Q), calcite (C), orthoclase (O), and clinoptilolite (CLN). The fraction of the dissolved cations from the C, CaS, I, and CLN increases with the increasing of HCl concentration. The remaining K+ and Al3+ cations are located in the undissolved orthoclase. An amorphous silica OA (SiO2·nH2O) remained at the end of completed leaching. The gradual increase in the adsorptive properties, acidity, and bleaching power is due to the formation of new nanopores. Gradual decrease after their maxima is caused from the degradation of the layered structure for clay minerals. The maximum bleaching power tentatively intensifies with the increasing of the used powder mass.