Simultaneous removal of methylene blue and Cd(II) by pecan shell-based activated carbon: adsorption kinetics, isotherms, and physicochemical characterization.

The coexistence of organic dyes and heavy metals in wastewater requires efficient and sustainable adsorbents capable of simultaneous contaminant removal. In this study, pecan shell-based activated carbons (PSC-0-PSC-3) were prepared by KOH activation using different impregnation ratios; their physicochemical properties and adsorption performance toward methylene blue (MB) and Cd(II) were systematically investigated under single- and binary-component systems. Among the prepared adsorbents, PSC-3 exhibited the highest BET specific surface area (2857 m2 g-1) and total pore volume (1.61 cm3 g-1). MB adsorption was best described by the Freundlich isotherm model (R2 = 0.998), indicating heterogeneous adsorption behavior. The Langmuir model estimated a maximum adsorption capacity of 529.5 mg g-1. Under binary-component conditions, PSC-3 achieved nearly complete MB removal (≈ 100%), whereas Cd(II) removal was strongly dependent on solution pH and adsorbent dosage. The adsorption of Cd(II) was primarily governed by electrostatic attraction and surface complexation with oxygen-containing functional groups. Adsorption-desorption experiments demonstrated that PSC-3 retained approximately 85% and 80% of its initial adsorption efficiency for MB and Cd(II), respectively, after five regeneration cycles, indicating good reusability. These findings demonstrate that PSC-3 is a low-cost, sustainable, and highly efficient adsorbent with strong potential for the simultaneous removal of organic dyes and heavy metal ions from mixed wastewater.