Heavy Metals Removal from Aqueous Solutions by Multiwall Carbon Nanotubes: Effect of MWCNTs Dispersion

Abstract

Carbon nanotubes (CNTs) are one of the most studied nanoparticles due to their physical, chemical and electronic properties. However, strong Van der Waals bonds, which promote CNTs aggregation are usually present, affecting their unique properties. Avoiding CNTs aggregation is one of the main difficulties when using these nanoparticles. Regarding the adsorption capacity of CNTs, the tendency of CNTs to aggregate decreases the surface area available to retain contaminants. One way to overcome this issue is by changing the surface energy of CNTs through chemical (covalent and noncovalent methods) or mechanical stabilization, but there is not yet a unique solution to solve this problem. In this work, a chemical noncovalent method (addition of surfactants) combined with mechanical energy (ultrasounds) was applied for CNTs stabilization, and the influence in heavy metal ions removal, Pb (II), Cu (II), Ni (II) and Zn (II), an area of high environmental relevance, was evaluated. It was proved that high amounts of metals could be removed from water during the first eighteen hours. Competitive adsorption between heavy metals, during adsorption tests with the simultaneous presence of all ions, was also studied and it was possible to prove that the electronegativity and atomic radius of cations influence their removal. Pb (II) and Cu (II) were the metals removed in higher percentages, and Ni (II) and Zn (II) were the metals less removed during competitive adsorption. Finally, the results obtained show that MWCNTs, if adequately dispersed, present a good solution for the treatment of water contaminated with highly toxic heavy metals, even when using very low concentrations of Multiwall Carbon Nanotubes (MWCNTs).