Comparison between activated carbon and multiwall carbon nanotubes in the removal of cadmium(II) and chromium(VI) from water solutions

This paper evaluates the aqueous cadmium(II) and chromium(VI) adsorption efficiency on multiwall carbon nanotubes and activated carbon (AC). The effects of pH, initial metal ion concentrations, and adsorbent dosage on the adsorption of heavy metal by carbon adsorbents were also evaluated. In addition, this study used the Langmuir and Freundlich isotherms to describe behavior of equilibrium adsorption. It has been shown that the carbon nanotubes (CNTs) sampled exhibited a pH of point of zero charge (pH_pzc) of around 6.1 while the AC sample had a pH_pzc of about 6.8. Maximum adsorption efficiencies were found at pH 3 for both CNTs and AC. Also, carbon nanotubes have higher adsorption efficiency towards both studied metal ions in comparison with ACs. The capacity of CNTs to adsorb Cd(II) and Cr(VI) was 4.5 and 3.1 mg/g, respectively. Additionally, the capacity of AC to adsorb Cd(II) and Cr(VI) was 2.8 and 2.4 mg/g, respectively. The adsorption percentage of metal ions onto both carbon adsorbents increased as adsorbents dosage and initial metal ion concentrations increased. The equilibrium adsorption of the studied metal ions is best fitted using the Freundlich isotherm.