Effect of acid activation on the sorption properties of bentonite clay
DOI:
https://doi.org/10.17308/sorpchrom.2026.26/14178Keywords:
bentonite, montmorillonite, acid activation, dry activation process, slurred activation, β-carotene adsorp-tion, cation exchange capacityAbstract
Acid activation of bentonite clay in paste form consists of heat treatment of a paste-like mixture of clay with acid. This method reduces acid consumption for clay activation compared to the conventional wet activation process. The clay was mixed with acid and granulated using a household meat grinder. The granules were kept at t=200-350°C in a muffle furnace and washed with water to remove excess acid. For the activation products, the adsorption value of β-carotene in n-hexane and the cation exchange capacity (CEC) based on the adsorption of the Cu-triethylenetetramine complex were determined spectrophotometrically. The dependence of the adsorption value on the amount of sulfuric acid used for activation exhibits an extremum corresponding to 30 g of acid (ρ=1.83 g/cm³) per 100 g of dry clay. With increasing acid concentration, the cation exchange capacity of the activation products decreases from 57.9 mg-eq/100g for natural clay to 17.1 mg-eq/100g for clay activated with 50 g of acid per 100 g of dry clay. The dissolution rate of montmorillonite in sulfuric acid is limited by the diffusion of dissolved octahedral and tetrahedral cations from the unreacted surface. The dissolution of montmorillonite is accompanied by the formation of new pores and the enlargement of existing ones. During pore enlargement, local accumulation of octahedral cations occurs near the dissolving surface, and the reaction rate slows down. Therefore, the dependence of the adsorption value on treatment duration exhibits a plateau caused by the cessation of pore growth on the external surface of the crystals. In the activation temperature range of 200-350°C, the adsorption value of β-carotene in n-hexane reaches 86.1-96.5 mg/g. The dissolution of heterovalently substituted cations of the octahedral sheet of montmorillonite causes a decrease in cation exchange capacity. With an increase in activation temperature from 250 to 350°C, a consistent decrease in cation exchange capacity occurs at equal activation durations. Unlike β-carotene adsorption, cation exchange capacity does not depend on the average pore size of the adsorbent but is determined by the magnitude of the negative surface charge of the clay particle layers. Diffusion of reaction products does not limit the formation of new pores; therefore, pore formation occurs throughout the entire activation duration, and cation exchange capacity continuously decreases. To intensify the diffusion of dissolved cations from the external surface of montmorillonite crystals, distilled water was added to the clay–acid mixture in an amount of 10 g per 100 g of dry clay. An increase in the volume of free liquid contributed to an increase in the adsorption value of β-carotene by 9.2%.
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