Hydrochloric versus sulfuric acid activation of krantau bentonite: From structural transformation to oil bleaching efficiency

Acid activation is an effective route to enhance the surface and sorption properties of natural clays. In this study, Ca-type bentonite from the Krantau deposit (Karakalpakstan, Uzbekistan) was beneficiated by elutriation and Na۲CO۳ treatment and then activated with HCl or H۲SO۴ (۵–۱۵ wt.%) at ۹۵–۱۰۰ °C for ۲–۶ h (solid/liquid = ۱:۶). FTIR and XRD indicated that calcite dissolution and cation exchange occur mainly within ۲ h; with H۲SO۴, residual CaSO۴ (gypsum) reflections appear. Chemical analysis confirmed a stronger decalcification by HCl (CaO decreased from ۲۱.۲۵ to ۱.۷۵ wt.%) than by H۲SO۴ (to ۷.۳۶ wt.%). Nitrogen adsorption at ۷۷ K gave type IV isotherms, consistent with predominantly mesoporous structures. The specific surface area increased from ۴۰.۲۷ m۲/g for the initial bentonite to ۶۴.۲۲ m۲/g after ۱۵% H۲SO۴ and to ۱۰۱.۶ m۲/g after ۱۵% HCl; micropore volume rose from ۰.۰۰۹۹ to ۰.۰۴۳ cm۳/g, and the mean pore radius shifted from ۵۷.۵ to ۵۴.۶ Å. SEM/EDS confirmed structural rearrangement and a strong drop in Ca (from ۱۲.۴ to ۰.۷۶ mass%). Performance was evaluated in sunflower-oil bleaching (۰.۵–۱.۰ wt.% adsorbent): the HCl-activated sample significantly lowered oil color and acid value (to ۰.۴ mg KOH/g) and reached ۹۵% β-carotene removal within ۴۰ min at ۷۰ °C. Overall, HCl activation under these conditions yields the most effective bentonite adsorbent for edible-oil refining.

کلیدواژه ها:

Bentonite ، Microporosity ، acid activation ، Cation exchange ، porosity ، Clay ، oil bleaching