Walnut-shell-derived porous carbon for efficient room-temperature adsorption of organic dyes

Sustainable porous carbons derived from agricultural waste represent promising materials for wastewater remediation. In this study, activated carbons were synthesized from walnut shells via a single-step carbonization-chemical activation process using KOH and K₂CO₃ at 700 and 800 °C. The optimal material, WS-K₂CO₃-800 °C, exhibited a high specific surface area of 1786.64 m² g^-1 and a hierarchical pore structure, as confirmed by N₂ adsorption-desorption analysis. Its adsorption performance was evaluated for methylene blue (MB), methyl orange (MO), and Congo red (CR) at room temperature. Rapid adsorption was observed, achieving near-complete dye removal within 30 min at initial concentrations of 20 ppm for MB and MO, and 10 ppm for CR. Kinetic modelling showed excellent agreement with the pseudo-second-order model, with correlation coefficients (R²) of 0.998, 0.996, and 0.995 for MB, MO, and CR, respectively, indicating chemisorption-dominated uptake. The calculated equilibrium adsorption capacities reached 185.8 mg g^-1 for MB, 133.7 mg g^-1 for MO, and 88.9 mg g^-1 for CR. Isotherm analyses indicated predominantly heterogeneous and multilayer adsorption behaviour, with the Freundlich and Temkin models providing the best fits. The superior performance of WS-K₂CO₃-800 °C is attributed to its high surface area, hierarchical porosity, and surface functionality, highlighting walnut-shell-derived activated carbon as a low-cost, renewable, and effective adsorbent for dye-contaminated wastewater.