Walnut Shell as a bio-activated carbon for elimination of malachite green from its aqueous solution: Adsorption isotherms, kinetics and thermodynamic studies

Large amounts of hazardous and synthetic dye are produced by the textile industry. Nevertheless, the literature still lacks an effective method for removing hazardous synthetic dye, specifically malachite green, from wastewater. To fill this gap, this study used a batch method to evaluate the adsorption of malachite green onto an adsorbent derived from walnut shells (ACWS). According to previous studies, zinc chloride is rarely used as a chemical agent for activating walnut shells. Moreover, very little carbon has been used to eliminate the dye. To identify the ideal conditions, this paper examined the effects of washing, carbonization temperature, contact duration, adsorbent dosage, and dye concentration on the adsorption capacity and removal percentage. The Freundlich and Langmuir models were used to analyze the experimental isotherm data. The equilibrium adsorption data fit the Langmuir equation more closely, as evidenced by a high correlation factor (R² = 0.9970). The data also showed an acceptable fit with the Freundlich model, with the “n” constant ranging from 2 to 10. The parameter values for the aforementioned models were as follows: Qm = 588.23 mg/g, KL = 0.0602 L/mg, KF = 201, and 1/n = 0.1838. The adsorption process of malachite green onto ACWS was further investigated using both first- and second-pseudo-order models. The kinetics of adsorption, with a determination coefficient (R²) of 0.9943, closely followed a pseudo-second-order kinetic model. The rate constants were found to be 0.0274 min⁻¹ for the pseudo-first-order and 0.0033 g/min·mg for the pseudo-second-order kinetics. The thermodynamic parameters were determined from the adsorption isotherms at various temperatures, yielding free energy ΔG° values ranging from −5.5241 to −9.27 kJ/mol, a positive enthalpy change ΔH° of 31.0538 kJ/mol, and an entropy change ΔS° of 124.84 J/mol·K. The positive ΔH° and negative ΔG° indicate that the overall adsorption of MG is endothermic and spontaneous.