Removal of Cationic Dye by Adsorption onto Activated Carbon: Optimization by Response Surface Methodology

Population growth is negatively impacting water quality, despite strict wastewater treatment policies, and the problem remains a serious concern. Therefore, preserving the environment for aquatic and human life is imperative. To this end, the physicochemical properties of Activated Carbon (AC) were evaluated using various analytical techniques, including pHpzc, BET method, FTIR spectroscopy, and Scanning Electron Microscope (SEM-EDX analysis). A Box-Behnken experimental design was used to optimize three key parameters namely the initial BF concentration (C_o: 100-200 mg/L), pH (6-10), and the AC dose (0.1-0.5 g) for BF removal. The experimental data were discussed by Analysis of Variance (ANOVA), subjected to a second-order polynomial equation using multiple regression analysis. The optimal conditions achieved by exploiting the 3D contours and surfaces are: C_o: 178.074 mg/L, pH 6.78 and AC dose: 0.3837 g at temperature: 25 °C and different isotherms were used to fit the experimental data. The results indicate that the Langmuir model offers the best correlation, with maximum adsorption capacities of 34.482, 66.667, and 93.467 mg/g at 20, 30 and 40 °C, respectively. Adsorption follows a pseudo-second-order kinetic model with a correlation coefficient (R²) of 0.999. The effect of temperature on the adsorption isotherms allowed for the evaluation of the thermodynamic functions. The free energy ΔG° (-11.455 → -6.415 kJ/mol) and a positive enthalpy ΔH° (67.421 kJ/mol) confirm the spontaneous and endothermic nature of BF adsorption on AC. The positive entropy ΔS° (252 J/mol·K) suggests the increased randomness at the Solid/Solution interface during the BF adsorption, which can be explained by different interactions, including π-π stacking, electrostatic forces, pore filling and hydrogen bonding. The AC adsorbent on fresh water, where the germination rate was found to be similar to that of freshwater (approximately 80%), strongly supports the relatively less toxic nature of the treated dye solution.