Bael fruit shell–derived activated carbon adsorbent: effect of surface charge of activated carbon and type of pollutants for improved adsorption capacity

A recyclable activated carbon adsorbent with a highly negative surface charge was synthesized from bael fruit shell waste. The structural, morphological, and surface properties of the activated carbon adsorbent were evaluated using XRD, FE-SEM, FTIR, zeta potential, and BET surface area analysis. The results demonstrated that the surface area of the activated carbon adsorbent got significantly enhanced (944.09 m2/g) as compared to that of the carbonized material (2.43 m2/g) which was validated through FE-SEM analysis. The zeta potential value of activated carbon (− 16.2 mV) reveals that the net surface charge of the activated carbon is negative, which leads to 94.15% removal of methylene blue dye (cationic) as compared to 65.71% and 50.59% removal of rhodamine B (zwitterionic) and reactive red (anionic) dyes, respectively. This could be due to the electrostatic attraction between the cationic methylene blue dye and the negative surface of activated carbon. Furthermore, the adsorption data revealed that 300 mg of activated carbon, 50 ppm initial concentration, and pH of 6.56 are the optimum conditions for enhanced adsorption capacity of activated carbon towards methylene blue pollutant. The adsorption experimental data were best fitted into the Langmuir isotherm and second-order kinetic model (R2 = 0.99–1.00). In addition, the activated carbon was successfully regenerated and reused up to 5 cycles of adsorption experiments where the adsorption efficiency was retained. Therefore, the bael fruit shell waste–derived activated carbon adsorbent with a high negative surface charge could be a potential alternative to commercial counterparts in the selective removal of pollutants removed from the wastewater stream.