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The adsorption behaviors of heavy metal ions Cd(2+), Cu(2+), Pb(2+), and Hg(2+), in aqueous media using functionalized single-walled carbon nanotube (SWCNT) with functional groups -COO(-), -OH, and -CONH2 are studied using molecular dynamics (MD) simulations. The results show that adsorption capacity is improved significantly using surface modification of SWCNT with carboxyl, hydroxyl, and amide functional groups. In addition, the adsorption capacity is found to increase with increasing metal-ion concentration. It is observed that the CNT-COO(-) surface effectively adsorbs over 150-230% more metal ions than the bare CNT surface. On the contrary, -OH and -CONH2 are relatively weak functional groups where excess metal-ion adsorption compared to the bare CNT is in the range 10-47%. The structural properties, self-diffusion coefficients, and adsorption isotherms of the metal ions are computed and analyzed in detail. Moreover, the potential of mean force (PMF) is computed to understand the free energy of metal ions, in the presence of functional groups, which is remarkly higher in absolute terms, leading to significant affinity for adsorption compared to the case for the bare CNT. In general, the following order of adsorption of the metal ions on functionalized CNT is observed: Pb(2+) > Cu(2+) > Cd(2+) > Hg(2+).

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