Adsorption of methylene blue on cashew nut shell based carbons activated with zinc chloride: The role of surface and structural parameters

Activated carbons were prepared from cashew nut shells by chemical activation with zinc chloride. Cashew nut oil was extracted from the shells, followed by various ratios of impregnation at 400 °C. The structural features of the carbons were characterized by nitrogen adsorption and scanning electron microscopy. The surface chemistry was studied by Fourier transform infrared spectroscopy and potentiometric titration. The sample activated at the ratio of 1.5:1 (zinc chloride/shells) showed the highest surface area, pore volume, and the amount of basic surface groups. Equilibrium adsorption data had the best fit to Langmuir equation for all the activated carbons. The highest maximum adsorption capacity was found for the sample with 1.5:1 ratio. The linear correlation was found between the adsorption capacity and the surface area, the adsorption capacity and the volume of pores with diameter of 1.6–2 nm, and the adsorption capacity and the amount of basic groups. Methylene blue molecules are likely attracted to the carbon surface by dipole-dipole interactions between nitrogen in methylene blue and phenolic groups of carbon. Finally, the effect of carbonization temperature on adsorption capacity was studied, and the carbonization temperature of 500 °C resulted in the best adsorption capacity value for methylene blue, 476 mg/g.