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

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(NO₃)₂ + MgO as the catalyst and purified by treatment with HCl. The surface functionalization was performed using oxidation with a mixture of concentrated H₂SO₄ and HNO₃. 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.