Adsorption of hexavalent chromium from aqueous solution by activated carbon prepared from longan seed: Kinetics, equilibrium and thermodynamics

Activated carbon was prepared from longan seed by chemical activation with sodium hydroxide (NaOH). The longan seed activated carbon (LSAC) was characterized for surface area and pore volume and used as an adsorbent for Cr(VI) removal from aqueous solutions in batch adsorption experiments. The effect of shaking speed, pH, LSAC dosage and temperature were studied. The results show that LSAC has a high surface area (1511.8 m2/g) and porous structure, which is favorable for the adsorption. The effect of external diffusion is eliminated at 170 rpm, the best pH value is 3.0 and the removal percentage of Cr(VI) increases with the increase of the LSAC dosage. The adsorption process can be well described by the pseudo second-order kinetic model and the activation energy is 84.4 kJ/mol. The adsorption isotherms data of Cr(VI) on LSAC obtained at 298.2, 308.2, 313.2 and 318.2 K were analyzed by Langmuir and Freundlich isotherm models and results show that it is better represented by the Langmuir isotherm model. The thermodynamics parameters, ΔH = 2.73 kJ/mol, ΔS = 28.14 J/mol K and ΔG < 0, show that the adsorption of Cr(VI) on LSAC is spontaneous and endothermic. The results show that LSAC is an efficient adsorbent for the removal of Cr(VI) from aqueous solutions.

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