Comparative study of Ni(ii) adsorption by pristine and oxidized multi-walled N-doped carbon nanotubes

摘要

The principles and mechanisms of adsorption of Ni( II ) ions by well characterized pristine and oxidized N-doped multi-walled carbon nanotubes (N-CNTs) are described and discussed. The samples were synthesized by CCVD method using n -butylamine as the carbon source and Ni(NO _(3) ) _(2) + MgO as the catalyst and purified by treatment with HCl. The surface functionalization was performed using oxidation with a mixture of concentrated H _(2) SO _(4) and HNO _(3) . The morphology, nature and charge of surface groups were characterized by HRTEM, XPS, FTIR and micro-electrophoresis methods. It has been shown that: adsorption of Ni( II ) reaches an equilibrium value within 20–30 min; the degree of extraction of nickel ions from the solution increases with its dilution; adsorption of Ni( II ) results in an insufficient decrease in the suspension pH for pristine N-CNTs (0.5–0.6 pH unit) and considerable lowering of the pH for the oxidized sample (up to 2.5 pH unit); the adsorption isotherms are described by the Langmuir equation; the plateau amounts of adsorption (35–40 mg g ~(?1) ) are almost the same for both as-prepared and oxidized samples; at pH 8 and higher a sharp increase in adsorption is observed which is caused by nickel hydroxide precipitation. The spectroscopic, adsorption, electrophoretic and pH measurement data testify that below pH 8 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 the nanotube lattice and vacant d-orbital of the adsorbing Ni( II ) ions. For the oxidized N-CNTs ion-exchange processes with a release of H ~(+) play a decisive role.