Comparative study of Pb(II) sorption on XC-72 carbon and multi-walled carbon nanotubes from aqueous solutions

Sorption properties of XC-72 carbon and multi-walled carbon nanotubes (MWCNTs) for Pb(II) removal from aqueous solution as a function of pH, ionic strength, solid content and initial Pb(II) concentration were investigated to evaluate their availability performance in wastewater treatment. The results indicate that Pb(II) sorption on XC-72 carbon and on MWCNTs are dependent on pH. The maximum sorption capacities of Pb(II) calculated by the Langmuir model are ∼125.0 mg/g for XC-72 carbon and ∼17.5 mg/g for MWCNTs at pH = 6.0. The results reveal that XC-72 carbon possesses better performance of Pb(II) sorption than MWCNTs in Pb(II) removal from large volumes of aqueous solutions. The sorption isotherms of Pb(II) on XC-72 carbon and MWCNTs are simulated by constant capacitance model with the aid of FITEQL 3.1 code. At low pH, the major species of Pb(II) sorbed on XC-72 carbon is XOHPb2+, and the low Pb(II) sorption can be partly attributed to the competition between H+ and Pb2+ ions on the surface sites. At pH 5.9–7.0, the removal of Pb(II) is accomplished by simultaneous sorption of Pb2+ and Pb(OH)+, and the species of Pb(II) sorbed on XC-72 carbon are XOHPb2+ and XOPbOH. At pH > 7.0, the removal of Pb(II) is accomplished by simultaneous precipitation of Pb(OH)20 and the sorption of Pb(OH)+. As for MWCNTs, the species of sorbed Pb(II) are XOHPb2+ at pH < 5.7, XOHPb2+ and XOPbOH+ at pH 5.7–8.5, and XOPbOH at pH > 8.5.