High Performance Batch Adsorption of Methylene Blue Using Desert Date Seed Shell Activated Carbon: Characterization and Response Surface Methodology Optimization

This study explores the development of activated carbon from Desert Date Seed Shells (DDSSAC) for methylene blue removal. The activated carbon was synthesized using an innovative preparation method involving chemical activation with potassium hydroxide (KOH), carbonization at an unprecedented high temperature of 850 °C, and a controlled argon atmosphere. This method resulted in a remarkable BET surface area of 2768 m²/g, the highest reported for biomass-derived activated carbon. The TGA, XRD, SEM, and FTIR analyses demonstrated that the activated carbon exhibits high thermal stability, an amorphous carbon structure, a complex porous texture, and the presence of oxygenated functional groups and aromatic structures, respectively. The adsorption process was optimized using response surface methodology coupled with central composite design by studying the effect of adsorbent weight (3, 5, 7) mg, pH of the solution (4, 7, 10), and initial methylene blue concentration (100, 200, 300) mg/L. The fitted model, an inverse square root transformed reduced quadratic model, exhibited excellent predictive accuracy, as indicated by a high R² value of 0.9932. The RSM study showed that the adsorption capacity increased with the decrease in adsorbent weight and the increase in the pH of the solution and methylene blue (MB) concentration. The maximum adsorption capacity was 1684 mg/g, which was achieved at an adsorbent weight of 3 mg, pH of 10, and MB concentration of 300 mg/L due to enhanced electrostatic attraction at high pH, efficient utilization of active sites with low adsorbent weight, and a strong mass transfer driving force at higher dye concentration. The kinetic analysis showed that the Elovich model (R² = 0.9946) best described the adsorption process, indicating a rapid initial adsorption phase followed by gradual site saturation. The isotherm study revealed that the Temkin model provided the best fit (R² = 0.8994), highlighting uniform heat distribution and consistent interactions between the adsorbent and methylene blue molecules. The maximum Langmuir adsorption capacity of DDSSAC was 716 mg/g, outperforming other activated carbons derived from agricultural waste referring to hydrogen bonding, π–π stacking, hydrophobic interactions, and electrostatic attraction between MB and the carbon surface. These findings establish desert date seed shells activated carbon as a highly effective and eco-friendly adsorbent for methylene blue removal.