Upgrading low rank coal into mesoporous activated carbon via microwave process for methylene blue dye adsorption: Box Behnken Design and mechanism study

Abstract

This research aims to upgrade Malaysian low rank coal namely Merit Kapit (MK) into mesoporous activated carbon (MKAC) via fast and convenient microwave-assisted H3PO4 activation. The best activation conditions for MKAC were observed at 1:2 (MK to H3PO4) impregnation ratio, radiation power 600 W, and radiation time 20 min. These optimum activation conditions are responsible for producing a mesoporous structure of MKAC with an increase of ca. 848.7 folds (specific surface area (BET) of MKAC = 848.7 m2/g) as compared with natural MK without thermal modification (specific surface area (BET) of MKAC = 1.00 m2/g). The adsorptive performance of MKAC towards the removal of methylene blue dye (MB) was statistically optimized by using Box Behnken Design (BBD). The BBD results indicate the best removal (86.6 %) of 50 mg/L MB can be achieved at the following optimum adsorption conditions; MKAC dose = 0.06 g, solution pH = 7, and process time = 50 min. The adsorption process of MB by MKAC was well described by Freundlich isotherm model and second-order kinetic, with a maximum adsorption capacity of 186.4 mg/g at 25 °C ± 2. Thus, several types of interactions such as electrostatic interaction, H-bonding, and pi-pi interaction are responsible for capturing MB onto MKAC surface.