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Walnut shell-based activated carbon with excellent copper (II) adsorption and lower chromium (VI) removal prepared by acid–base modification

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In this work, acid and acid–base modified carbons were developed from walnut shell, and batch experiments were carried out to study their adsorption behaviors toward Cu(II) and Cr(VI). Boehm titration data, elemental analysis, and FTIR studies suggested that the surface chemistry of the activated carbons was significantly changed by both acid and acid–base modification. N2/77 K adsorption isotherm methods revealed a significant decrease in SBET with acid treatment, a dramatic decrease was also observed in SBET in acid–base modified carbon. Batch equilibrium adsorption tests including the effect of pH of the solution, metal initial concentration, and temperature were conducted to evaluate the solids adsorption performance. The process kinetics was evaluated by pseudo-first-order and pseudo-second-order models, and the kinetics data agreed well with pseudo-second-order model. The applicability of the Langmuir, Freundlich, and D–R adsorption isotherms has been tested for the interpretation of equilibrium adsorption data. The results revealed that the acid–base carbon exhibited excellent copper (II) adsorption performance with a maximum adsorption capacity of 204.08 mg/g. The mean free energy value evaluated from D–R model indicated the adsorption of Cu(II) onto acid–base carbon involved chemical ion-exchange process. Thermodynamic studies illustrated adsorption of Cu(II) onto ACHN was endothermic and spontaneous. According to the overall results, the solution pH and the surface chemistry of the carbons were found to play a critic role in the uptake of copper ions from aqueous solutions rather than the carbon texture characteristics, while the adsorption of Cr(VI) closely related to the solution pH and its texture properties. © 2012 American Institute of Chemical Engineers Environ Prog, 32: 688–696, 2013