Removing low levels of Cd(II) and Pb(II) by adsorption on two types of oxidized multiwalled carbon nanotubes

This work investigates the possibility of applying two types of oxidized multiwalled carbon nanotubes (oxMWCNTs) as effective adsorbents for removing low concentrations of Cd(II) and Pb(II) (<= 5 mg L-1) from aqueous media, as well as the influence of the functionalization process duration (3 h (oxMWCNT3h) or 6 h (oxMWCNT6h)) on the performance of the materials tested. Both oxMWCNTs were thoroughly characterized by SEM/EDS, TEM, FTIR, N-2 adsorption/desorption and pHpzc techniques. An extensive study of Cd(II) and Pb(II) adsorption and the mechanisms responsible was conducted by evaluating the effects of contact time, initial Cd(II) and Pb(II) concentration, pH and ionic strength. Adsorption kinetic data show that the two materials can be considered extremely effective, as less than 20 min was required to reach a state of equilibrium. Freundlich, Langmuir, Dubinin-Radushkevic and Temkin adsorption isotherm models fit the data well and suggest that the investigated adsorption processes are favorable, mainly electrostatically driven and exothermic. The maximum adsorption capacities obtained by the Langmuir model were 10.5 and 13.5 mg g(-1) for Cd(II) and 23.4 and 27.9 mg g(-1) for Pb(II) (thus: oxMWCNT6h > oxMWCNT3h and Pb(II) > Cd(II)). The adsorption of Cd(II) and Pb(II) is strongly and positively influenced by increasing the pH and negatively affected by increasing the ionic strength of the solutions. In general, the obtained results are very promising, with the application of oxMWCNTs proving effective for the purification of aqueous media contaminated with low concentrations of Cd(II) and Pb(II), regardless of the duration of the functionalization process.