Sustainable synthesis and characterization of high-surface-area activated carbons from walnut and pistachio shell wastes via chemical activation

Valorization of agricultural residues into high-performance porous carbons is an effective route to sustainable adsorbents. Here, walnut green outer shell and pistachio pink outer shell were converted into activated carbons via chemical activation using KOH (1:1, 1:2, 1:3) and ZnCl₂ (1:1). Without any pretreatment, precursors were carbonized at 500 °C (1 h, nitrogen atmosphere), followed by activation at 800 °C (KOH, 1 h) or 500 °C (ZnCl₂, 1 h). Textural analyses revealed a strong dependence on both precursor type and activating agent. Walnut-shell carbons reached exceptionally high surface areas of 1028–2347 m² g⁻¹, with the maximum obtained for KOH (1:3), whereas pistachio-shell carbons achieved 788–1324 m² g⁻¹ under the same KOH series. In contrast, ZnCl₂ activation produced markedly lower areas (445–750 m² g⁻¹) and grinding caused only minor changes. FTIR/XRD/SEM/Elemental analyses collectively supported the formation of defect-rich, turbostratic carbon frameworks with well-developed porous morphologies, highlighting walnut shell as a particularly promising precursor for sustainable, high-surface-area activated carbons suitable for adsorption-driven environmental applications.