A comparative study of Merit Kapit activated coal prepared via microwave and tube furnace pyrolysis processes for Methyl Violet 2B dye removal

This study: a comparative study of Merit Kapit activated coal prepared via microwave and tube furnace pyrolysis processes for methyl violet 2B dye (MV dye) removal explores the conversion of Malaysian low-rank coal, known as Merit kapit coal (MKC), into high-surface-area mesoporous activated carbon adsorbents via ZnCl₂-assisted activation using microwave and tube furnace pyrolysis methods. MV dye is selected for its applications in textiles, cosmetics, and the paper industry because of its vibrant color properties. However, its presence in wastewater poses significant health risks to aquatic and human lives because it is non-biodegradable and contaminates the water bodies. Merit Karpit coal is selected as a precursor for activated carbon due to its low cost and its ability to aid in converting waste into valuable materials, as it is regarded as waste. The activated carbon produced from microwave pyrolysis is designated as MK-ACMP, while that produced from the tube furnace pyrolysis is referred to as MK-ACTP. The adsorption capabilities of MK-ACMP and MK-ACTP were evaluated for removing MV dye, a model cationic pollutant. Microwave-assisted activation produced mesoporous MK-ACMP with a surface area of 798.18 m²/g and an average pore diameter of 3.67 nm, whereas tube furnace pyrolysis yielded MK-ACTP with a higher surface area of 1229.1 m²/g and a pore diameter of 2.9 nm. Brunauer Emmett-Teller (BET), x-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and the determination of the pH at the point of zero charge (pHpzc) analyses were performed to evaluate the physicochemical properties. A Box-Behnken design (BBD) was used to optimize adsorption parameters, including adsorbent dosage (0.02–0.1 g), solution pH (4–10), and contact time (5–25 min) for MK-ACMP and adsorbent dosage (0.02–0.1 g), solution pH (4–10), and contact time (2–12min) for MKACTP. Adsorption experiments demonstrated that both MK-ACMP and MK-ACTP had significant adsorption capacities, with maximum values (qmax) of 134.1 mg/g and 238.6 mg/g, respectively. The equilibrium adsorption of MK-ACMP followed the Freundlich isotherm, while MK-ACTP adhered to the Langmuir model. Both adsorbents exhibited pseudo-second-order kinetic behaviour, indicating chemisorption. Mechanistic analysis revealed that MV dye adsorption was facilitated by electrostatic attraction, π–π stacking, pore diffusion, and hydrogen bonding. This study identifies the feasibility of converting low-grade Malaysian coal into high-performance adsorbents for effective dye removal, offering a sustainable solution for wastewater treatment.