Highly efficient and selective Maerua subcordata tuber-derived activated carbons for enhanced removal of methylene blue from wastewater samples

Abstract

The discharge of methylene blue dye from textile and pharmaceutical industries increasingly threatens environmental safety and human health by introducing toxic contaminants. Because of the high cost of commercial activated carbon and the limited efficiency of many alternative adsorbents, the development of efficient and cost-effective materials remains highly desirable. In this study, the AC derived from Maerua subcordata tuber (MS-AC) was prepared using a simple and scalable method involving Na₂CO₃ activation followed by thermal treatment. The elemental composition, structural features, and morphology of the prepared MS-AC were examined using XPS, FTIR spectroscopy, XRD, SEM, and SEM/EDX elemental mapping. XRD, SEM, and FTIR spectroscopic analyses confirmed the enhanced amorphous structure, high porosity, and enriched surface functional groups after Na₂CO₃ activation. Batch adsorption tests were performed to investigate the ideal conditions for MB adsorption on MS-AC. The adsorption behavior followed the Langmuir isotherm model, with a maximum adsorption capacity of 158.7 mg g⁻¹ at 25 °C, which is superior to various materials reported earlier. The kinetic studies were best explained by the PSO model, while the Boyd model indicated film diffusion as the dominant rate-controlling step. Thermodynamic study disclosed negative ΔG° values, approving spontaneous adsorption, while a positive ΔH° value of 106.4 kJ mol⁻¹ indicated an endothermic chemisorption process. MS-AC exhibited excellent selectivity toward MB in mixed-dye systems and remarkable stability, retaining over 90% removal efficiency after four adsorption–desorption cycles using ethanol. Furthermore, MS-AC achieved 96.7% MB removal from real wastewater samples. The adsorption mechanism is mainly ascribed to hydrogen bonding, π–π stacking interactions, and electrostatic interactions between the methylene blue molecules and the MS-AC surface.