Synthesis of a high characteristics activated carbon from walnut shell for the removal of Cr (VI) and Fe (II) from aqueous solution: single and binary solutes adsorption

A high performance activated carbon was synthesized using walnut shell as a solid waste through a two-step zinc chloride chemical activation-thermal pyrolysis process. Characterization results demonstrated its porous structure with very good textural properties such as high BET surface area (1223 m2/g) and high total pore volume (0.85 cm3/g). The final adsorbent was used for adsorption of Fe (II) and Cr (VI) from aqueous solution. Effect of pH, initial concentration of metal ions, temperature, and contact time on adsorption capacity of the adsorbent was investigated. Adsorption results revealed that the maximum removal of Fe (II) and Cr (VI) ions, occurred at pH 4.5 and 2 respectively, were 96.2% and 99% at 313K. The equilibrium and kinetics data for adsorption of single-component ions were well described by the Sips isotherm and the pseudo-nth-order models, respectively. The impact of competing ions was studied by adsorption of a binary solution of Fe (II) and Cr (VI) ions. The binary adsorption isotherm was described by the modified Langmuir model and model parameters were found following an optimization procedure by genetic algorithm. Finally, the developed walnut-shell based activated carbon showed higher adsorption efficiency compared to other activated carbons at similar conditions.