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

Abstract

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.