Conversion of Malaysian low-rank coal to mesoporous activated carbon: Structure characterization and adsorption properties

Abstract

Malaysian Selantik low-rank coal (SC) was used as a precursor to prepare a form of mesoporous activated carbon (SC-AC) with greater surface area (SA) via a microwave induced KOH-activation method. The characteristics of the SC and SC-AC were evaluated by the iodine number, ash content, bulk density, and moisture content. The structure and surface characterization was carried out using pore structure analysis (BET), scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM–EDX), X-ray diffraction (XRD), Fourier Transform Infrared (FTIR), elemental analysis (CHNS), thermogravimetric analysis (TGA), and determination of the point of zero charge (pH PZC ). These results signify a mesoporous structure of SC-AC with an increase of ca. 1160 times (BET SA = 1094.3 m 2 ·g −1 ) as compared with raw SC without activation (BET SA = 1.23 m 2 ·g −1 ). The adsorptive properties of the SC-AC with methylene blue (MB) was carried out at variable adsorbent dose (0.2–1.6 g·L −1 ), solution pH (2–12), initial MB concentrations (25–400 mg·L −1 ), and contact time (0–290 min) using batch mode operation. The kinetic profiles follow pseudo-second order kinetics and the equilibrium uptake of MB conforms to the Langmuir model with a maximum monolayer adsorption capacity of 491.7 mg·g −1 at 303 K. Thermodynamic functions revealed a spontaneous endothermic adsorption process. The mechanism of adsorption included mainly electrostatic attractions, hydrogen bonding interaction, and π–π stacking interaction. This work shows that Malaysian Selantik low-rank coal is a promising precursor for the production of low-cost and efficient mesoporous activated carbon with substantive surface area.