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+).
Atomistic molecular dynamics (MD) simulations are performed in order to derive thermodynamic properties important to understand the extraction of gadolinium (Gd) and uranium dioxide (UO) with dibenzo crown ether (DBCE) in nitrobenzene (NB) and octanol (OCT) solvents. The effect of polystyrene graft length, on DBCE, on the binding behavior of Gd and UO is investigated for the first time. Our simulation results demonstrate that the binding of Gd and UO onto the oxygens of crown ethers is favorable for polystyrene grafted crown ether in the organic solvents OCT and NB. The metal ion binding free energy (ΔG) in different solvent environments is calculated using the thermodynamic integration (TI) method. ΔG becomes more favorable in both solvents, NB and OCT, with an increase in the polystyrene monomer length. The metal ion transferability from an aqueous phase to an organic phase is estimated by calculating transfer free-energy calculations (ΔG). ΔG is significantly favorable for both Gd and UO for the transfer from the aqueous phase to the organic phase (i.e., NB and OCT) via ion-complexation to DBCE with an increase in polystyrene length. The partition coefficient (log P) values for Gd and UO show a 5-fold increase in separation capacity with polystyrene grafted DBCE. We corroborate the observed behavior by further analyzing the structural and dynamical properties of the ions in different phases.
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