Effective removal of dyes from aqueous systems by waste-derived carbon adsorbent: physicochemical characterization and adsorption studies

Due to their cost-effectiveness and high surface area, activated carbons are commonly used for the adsorption of dyes from aqueous solutions. In this study, activated carbon was synthesized from walnut shell waste via KOH activation (1:3 ratio), yielding a surface area of 2347.4 m²/g. Reactive Blue 19 and Reactive Red 195 adsorption behavior were studied under varying experimental conditions. These included natural pH values (6.8-7.2), dye concentrations between 50 and 1250 mg L⁻¹, and adsorbent dosages ranging from 0.1 to 1.0 g. Adsorption equilibrium was achieved within 150 min. The maximum adsorption capacities were found to be 1227.17 mg g⁻¹ for RB 19 and 235.74 mg g⁻¹ for RR 195. Isotherm modeling was conducted using Langmuir, Freundlich, Temkin, and Dubinin-Radushkevich models, with Freundlich providing the best fit for both dyes, indicating multilayer adsorption on heterogeneous surfaces. Thermodynamic analysis revealed that the adsorption processes were spontaneous and endothermic, with negative Gibbs free energy (ΔG°), positive enthalpy (ΔH°), and positive entropy (ΔS°) values. These results highlight the high adsorption performance and practical potential of walnut shell-derived activated carbon for dye removal from wastewater.