Competitive adsorption of quinary heavy metal ions onto chestnut shell activated carbon

Abstract

For several decades, heavy metals have been a source of environmental pollution and the scientific community is continually asked for facing this threat. This study investigated the performance of an activated carbon made from chestnut shell (CNS-AC) in adsorbing Cd(II), Cr(VI), Ni(II), Cu(II), and Zn(II) in single and quinary-metal systems. The CNS-AC was characterized using FT-IR, TGA/DTG, FEG-SEM, EDX, and BET. Experimental results revealed that the adsorption follows the pseudo-second order kinetics, both in single and quinary systems. The equilibrium results reflect the Langmuir equilibrium model, with maximum adsorption capacities (mg.g−1) varying from 21.34 for Cd(II) to 73.16 for Zn(II); in single-metal systems, and from 9.80 for Cd(II) to 33.33 for Cu(II); in the quinary-metal systems. The overall maximum adsorption capacity in quinary-metal solution was of 113.22 mg g−1, higher than qm for each single-metal system. Furthermore, it was found that Cd(II) and Zn(II) pollution abatement to the standard limits was achieved in only one stage while two stages were enough to achieve the related pollution abatement for the other metals. A computational investigation was conducted using the Gaussian 09 and Density Functional Theory (DFT) methods, assuming the generation of organometallic complexes between the functionalized activated carbon and the metal ions. CNS-AC's carboxyl and amine functional groups seemed to be involved in the metal ions adsorption. The calculated bonding energies between the adsorbent and metal ions decreased in the following order Cu(II) < Cr(VI) < Ni(II) < Zn(II) < Cd(II). Correspondingly, experimental results revealed an actual competition between heavy metals for adsorption sites where Zn(II), Ni(II), and Cd(II) removal was hindered by the other metal ions occurrence while Cr(VI) and Cu(II) removal rates have remained broadly unchanged. The new insights brought by the theoretical study combined to the experimental investigation highlights the CNS-AC efficacy as adsorbent for Cd(II), Cr(VI), Ni(II), Cu(II), and Zn(II) removal.