Sorption of nonionic organic compounds (NOCs) to carbonaceous geosorbents (CGs) is complex due to the interior porosity of CG, which is thought to consist mostly of micropores (<2 nm).  These pores sorb NOCs strongly, particularly small compounds, while the mesopores (2-50 nm) are thought to sorb larger three-ring molecules such as the dibenzo-p-dioxins (DD).  We postulate that such adsorption selectivity could be due to adsorbate size-exclusion effects and/or sterically hindered diffusion processes from mesopores to micropores. Our recent data show that smaller molecules like naphthalene are sorbed much more strongly by CG than larger molecules like DD and chlorinated DDs, despite the much larger aqueous solubility for naphthalene.  We hypothesize that micropores (<2 nm) are the most favorable adsorption sites in CGs, but simultaneously act as size-selective molecular sieves.  To compute sorption energies and develop structural hypotheses for both equilibrium and kinetic behavior, we employed classical molecular dynamics simulations.  Graphene sheets, C nanotubes, and hybrids between the two were used to model cylindrical pores and pore entrances of CGs.  Pore diameters range from 0.6 nm to 3 nm.  Force fields from the literature were applied, and MD employed the LAMMPS code in the NPT ensemble at 298 K and 1 atm.  Aqueous TCDD or PAH molecules began in bulk water and then moved toward the pores using steered molecular dynamics.  Trajectories were analyzed for the potential of mean force to estimate the free energy of sorption processes. Results were consistent with the hypotheses above: TCDD molecules tend to block, rather than enter, simulated 0.9-nm pores in CG while naphthalene is strongly sorbed in such pores.

Molecular sieving by micropores of carbonaceous geosorbents: Steered classical mechanics simulations

246th National Meeting (2013)

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