Activated Carbon from Tropical Almond Shell ( Terminalia catappa L. ): Synthesis, Characterization, Application, and Regeneration for Chromium (VI) Adsorption

Abstract This study reports the synthesis of activated carbon (AC–H3PO4) from tropical almond shell (Terminalia catappa L.), an agro-waste material, using phosphoric acid and evaluates its effectiveness in removing hexavalent chromium (Cr(VI)) from aqueous media, emphasizing regeneration and reuse as strategies aligned with circular economy principles. The adsorbent was characterized in terms of its textural properties and chemical composition using Brunauer–Emmett–Teller (BET) surface area analysis and scanning electron microscopy, zero charge point (PCZ), EDX, and elemental analysis (CHN). Batch adsorption studies were performed to evaluate the removal of Cr(VI) from aqueous solutions. Textural analysis showed that the material CA-H3PO4 has a high mesoporous area (54%), with surface area BET of 1489.72 m2 g–1. The pH at which charges were neutralized between the adsorbent surface and the solution was equivalent to 2. At this pH, Cr(VI) removal exceeded 80.0% under the experimental conditions of 100 mL of solution containing 50.0 mg L–1 Cr(VI) and 0.05 g of the adsorbent. The adsorption kinetics were evaluated using the pseudo-first-order, pseudo-second-order, and Weber–Morris intraparticle diffusion models, with the pseudo-second-order model providing the best fit. Likewise, the equilibrium data were analyzed using the Langmuir, Freundlich, and Temkin isotherm models, with the Freundlich model showing the best agreement with the experimental data. The adsorption process was rapid, exhibiting a rate constant of 0.002 g mg–1 min–1, and was found to be spontaneous and endothermic, as indicated by the values of ΔG (−0.2 to −0.5 kJ mol–1) and ΔH (13.3 kJ mol–1). Desorption experiments demonstrated that the adsorbent could be regenerated using 0.5 mol L–1 NaOH, retaining a Cr(VI) adsorption capacity of 83.6 mg g–1 after the fourth regeneration cycle, corresponding to 55.0% of its original adsorption capacity. These findings suggest that CA-H3PO4 has considerable potential as an adsorbent for the remediation of waters contaminated with hexavalent chromium.