Comparative Study of Cu(II) Adsorption by As-prepared and Oxidized Multi-walled N-Doped Carbon Nanotubes

Abstract

The laws and mechanisms of adsorption of Cu(II) ions by well characterized and oxidized N‑doped multi-walled carbon nanotubes (N-CNTs) are discussed. The samples were synthesized by catalytic chemical vapour deposition method using n-butylamine as a carbon source and their surface was functionalized by oxidation with mixture of concentrated H2SO4 and HNO3. The morphology, chemistry and charge of surface groups were characterized by transmission electron microscopy, X-ray photoelectron spectroscopy, Fourier-transform infrared spectroscopy and micro-electrophoresis methods. It has been shown that: adsorption of Cu(II) reached equilibrium value within 60 min; the degree of extraction of these ions from the solution increased with its dilution; adsorption resulted in a moderate decrease in the suspension pH for as-prepared N-CNTs (1.5 pH unit) and its considerable lowering for the oxidized sample (up to 2.5 pH unit); the adsorption isotherms can be described by the Langmuir model and the plateau amounts of adsorption (28–35 mg/g) were almost the same for both as-prepared and oxidized samples; at pH 6.5 and higher a sharp increase in adsorption was observed which was attributed to hydroxides precipitation. The spectroscopic, adsorption, electrophoretic and pH measurements data showed that below pH of hydroxide precipitation, the major mechanism of adsorption by as-prepared N-CNTs is the donor-acceptor interaction between the free electron pair of N atoms incorporated into nanotubes lattice and vacant d orbital of the adsorbing Cu(II) ions. For the oxidized N-CNTs ion-exchange processes with a release of H+ ions play also a role.