Direct acid activation of kaolinite and its effects on the adsorption of methylene blue

• Direct sulfuric acid treatment of raw kaolinite produces porous AAK.
• The acid-activated kaolinite possesses high surface area up to 257.8 m2 g− 1.
• The AAK shows a higher removal efficiency for methylene blue than raw kaolinite.
• High concentration of methylene blue can be rapidly removed by adsorption.

Coal-bearing kaolinite was directly treated with concentrated sulfuric acid to improve its surface properties and adsorption ability. Acid treatment was carried out at various temperatures (i.e., room temperature − 250 °C), by varying time of treatment from 0 to 120 min. The samples were characterized by X-ray diffraction analysis, elemental analyses, thermogravimetric analysis, N2 adsorption-desorption analysis, high-resolution transmission electron microscopy, and Fourier transformed infrared spectroscopy. The activation of kaolinite strongly depended on the acid treatment including treatment temperature and time. Acid treatment at room temperature did not cause significant alterations either in the chemical composition or in the structure of the kaolinite. On the other hand, treatment at increased temperature led to the removal of Al3 + ions and thus increased the porosity of the material. The surface area and the pore volume of original kaolinite could be greatly changed as a function of treatment temperature and time of treatment, and they increased from 13.6 to 257.8 m2 g− 1 and from 0.045 to 0.25 cm3 g− 1, respectively, when the kaolinite was treated at 200 °C for 30 min. The adsorption ability of acid-activated kaolinite (AAK) was investigated using methylene blue (MB) as a typical pollutant. For this, the effects of contact time, pH, initial MB concentration and temperature were studied in batch mode. Gibb's free energy (ΔG0), entropy (ΔS0) and enthalpy (ΔH0) changes for MB adsorption were calculated. Owing to its high surface area, the AAK showed higher removal efficiency for MB than for original kaolinite, with a maximum adsorption capacity of 101.5 mg g− 1.

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