Application of ZnCl2-modified Biowaste to the removal of highly polluted dye: A case study of investigating the kinetics and adsorption isotherms

The purpose of this study is to generate ZnCl₂-modified walnut shell activated carbon (ZnCl₂-WSAC) for the removal of methylene blue (MB) from an aqueous solution by evaluating the adsorption capabilities of raw walnut shell powder following treatment with zinc chloride. To do this, the ZnCl₂-WSAC’s mass (0.04–0.12 g), the starting MB concentration (50, 80, and 100 mg/L), pH solution (2–10), the contact period (0–180 min), and the temperature (20–50 °C) were selected as the effective parameters. The ZnCl₂-WSAC was characterized using various analytical techniques, including identification based on its diffraction pattern, examination of morphological features and surface characteristics before and after methylene blue treatment, and assessment of malachite green adsorption on the fig leaf carbon. The results demonstrated high removal percentages and excellent adsorption efficiency In the present study, two common models, Langmuir and the Freundlich models, were used to examine the experimental isotherm data after the adsorption of MB dye. The highest adsorption capacity was 35.4 mg/g. After analyzing the experimental kinetic data, it was determined that the pseudo-second-order model, which had an excellent determination coefficient (R² = 0.9989), was better suited to describe the adsorption process. By using the Van't Hoff equation to compute the exchanged standard enthalpy (∆H° = 26.3279 KJ/mol), the adsorption process was exothermic. The adsorption process of the MB dye on ZnCl₂-WSAC exhibited spontaneous behavior at different temperatures, in which standard Gibbs free energy values (∆G°) ranged from -2.348 to -5.284 KJ/mol. It was determined that ZnCl₂-WSAC might be used as a new, efficient, and reasonably priced adsorbent to remove the dye from solution.