Batch removal of Pb (II) and Cd (II) from aqueous solutions using multi-wall carbon nanotubes

Water resource pollution with heavy metals is one of the most environmental problems at the global level. Removal of Pb (II) and Cd (II) ions from contaminated water is essential due to their high toxicity and tendency to accumulate in living organisms. Today, nanomaterials such as multi-wall carbon nanotubes (MWCNTs) have wide environmental applications due to their high surface area, small size, and chemical stability. In this context, the performance of MWCNTs was evaluated by removing Pb (II) and Cd (II) ions in a batch mode experiment. To study the adsorption process, the influence of different adsorption parameters, such as contact time, temperature, initial concentration of pollutant, the dosage of adsorbent, and pH values of the solutions were investigated. The experimental data were fitted to the pseudo-first-order and pseudo-second-order to analyze the kinetics adsorption studies of metal ions. Also, to determine the isotherm studies of metal ions, the Freundlich, Langmuir, and Langmuir–Freundlich models were used. The MWCNTs were characterized by several techniques including BET, FT-IR spectroscopy, scanning electron microscopy, and zeta potential. The results showed that the maximum metal adsorption occurred at pH, 6-8, contact time of 5 minutes, and adsorbent dosage of 0.05 gram. The pseudo-second-order type model satisfactorily (R2= 1) described the experimental metal data. Also, comparing the adsorption isotherm equations showed that the Freundlich-Langmuir model [(R2=0.984, SEE=4.42) for Pb (II) ions and (R2=0.969, SEE=5.27) for Cd (II)] was more proper in describing adsorption process. The maximum adsorption capacities of Pb (II) and Cd (II) ions were obtained at 137.86 and 110.82 mg g-1, respectively. The results obtained from the FTIR technique confirmed that the O–H groups on the MWCNTs surface have an essential role in metals adsorption.