Poly(acrylic acid) grafted multiwall carbon nanotubes (MWCNT-g-PAA) was synthesized by using plasma techniques, and was applied as a adsorbent to remove Co(II) from aqueous solution. Fourier transferred infrared spectroscopy (FT-IR) and thermo gravimetric analysis (TGA) demonstrate that poly(acrylic acid) was successfully grafted on the surface of MWCNTs. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) show that the bulk properties of MWCNTs were not changed after plasma treatment. The sorption capacity of Co(II) is improved conspicuously after plasma treatment due to the oxygen-containing functional groups. The removal of Co(II) by MWCNT-g-PAA occurs rather quickly, which can be well described by the pseudo-second-order model. The sorption of Co(II) is strongly dependent upon the pH values. The sorption isotherms fit the Langmuir sorption model well. The thermodynamic data (Delta H degrees, Delta S degrees and Delta G degrees) are calculated from the temperature-dependent sorption isotherms, and the results suggest that the sorption process of Co(II) on MWCNT-g-PAA is spontaneous and endothermic. Crown Copyright (C) 2012 Published by Elsevier B.V. All rights reserved.