Valorization of Pumpkin Seed Shells into Activated Carbon for Phenol Adsorption: Characterization, Mechanisms, Isotherms, and Thermodynamics.

Abstract

The valorization of agricultural residues into functional adsorbents offers a sustainable approach for the removal of phenolic contaminants from water. In this study, activated carbon was prepared from pumpkin seed shells through carbonization and chemical activation and evaluated for phenol removal from aqueous solution. The physicochemical properties of the resulting carbon were investigated by scanning electron microscopy (SEM) and Brunauer–Emmett–Teller (BET) analysis, revealing a well-developed porous structure with a specific surface area of 629.84 m² g⁻¹, pore volume of 0.227 cm³ g⁻¹, and average pore diameter of 54.26 Å. Batch adsorption experiments were performed to assess the effects of adsorbent dosage, contact time, and solution pH. Under the investigated conditions, an optimum adsorbent dosage of 0.20 g and pH close to 7 afforded an experimental adsorption capacity of 31.75 mg g⁻¹, with equilibrium approached after approximately 150 min. Among the tested kinetic models, the pseudo-second-order model provided the best fit (R² = 0.998), with a calculated equilibrium capacity of  34.01 mg g⁻¹. Equilibrium data were better described by the Freundlich model (R² = 0.981) than by the Langmuir and Temkin models, indicating adsorption on a heterogeneous surface. Thermodynamic analysis yielded negative ΔG° values and a negative ΔH° (−3.50 kJ mol⁻¹), indicating a spontaneous and exothermic adsorption process. Overall, the results demonstrate the potential of pumpkin seed shell-derived activated carbon as a low-cost and sustainable adsorbent for phenol removal from aqueous media.