Dendrimer-modified carbon nanotubes for the removal and recovery of heavy metal ions from water

Effective removal of trace heavy metal ions from aqueous bodies is a pressing problem and requires significant improvement in the area of absorbent material in terms of removal efficiency and sustainability. We propose an efficient strategy to enhance the adsorption efficiency of carbon nanotubes (CNTs) by growing dendrimers on their surface. First, CNTs were pre-functionalized with maleic acid (MA) via Diels-Alder reaction in presence of a deep eutectic solvent under ultrasonication. Subsequently, dendrimers of varying length were grown by the repeated reaction of ethylene diamine and MA. Raman spectroscopy was specifically used to confirm the Diels-Alder reaction on the surface of CNTs, and other characterization techniques (SEM, EDX, XRD, TGA, and FTIR) were applied to confirm the successive growth of the dendrimers. Highly dendrimerized CNTs were found to be more effective in removing heavy metal ions (Pb²⁺ and Cd²⁺) from aqueous solutions with enhanced recyclability than less dendrimerized CNTs. Kinetic studies have revealed that the adsorption process followed a pseudo-second order kinetic model, and the rate-limiting step was mainly chemisorption. This study has not only excluded the involvement of harmful chemicals to pre-functionalize the CNTs with high loading but also provided an effective way to enhance the adsorption of heavy metal ions.