Activated carbon from chili straw: K2CO3 activation mechanism, adsorption of dyes, and thermal regeneration

Activated carbon (AC(cs)) from chili straw was prepared by sequential carbonization and K2CO3 activation and applied for adsorption of cationic (methylene blue (MB)) and anionic (methyl orange (MO)) dyes with similar molecular sizes from aqueous solution. The physicochemical properties of chili straw char (Char(cs)) and AC(cs) were compared based on N-2-adsorption/desorption, SEM, XRD, Raman spectroscopy, and FTIR. Activation mechanism was revealed with the aid of TG-FTIR. Batch experiments were performed to determine the isotherm models for adsorption of dyes. Results showed that AC(cs) possessed a large specific surface area of 1868 m(2) g(-1). The formation of pores was attributed to the reactions between oxygen-containing functional groups in Char(cs) and K2CO3 at temperatures of 670 to 850 degrees C. Sips model was applicable for the adsorption isotherm data for both MO and MB with outstanding maximum adsorption capacities of 1542 and 989 mg g(-1), respectively. Thermodynamic analysis revealed that the adsorption processes of MO and MB were endothermic and spontaneous, and the adsorption was more spontaneous in the case of MO. Zeta potential analysis revealed that electrostatic interactions were responsible for the significant difference between the adsorption capacities for MO and MB. Thermal desorption MO from AC(cs) was investigated by TG-FTIR, and thermal regeneration of MO-saturated AC(cs) was carried out at 650 degrees C for 30 min, and the regeneration efficiency remained higher than 60% after five cycles.