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Abstract:

Molecular interactions controlling the sorption of pollutants to environmental black carbons (soot, charcoal) are not well-resolved. Sorption of a series of aromatic compounds was studied to wood charcoal and nonporous graphite powder as a model adsorbent. Issues of concern were the possible involvement of pipi electron donoracceptor (EDA) interactions of electron-poor and electron-rich solutes with the graphene (polycyclic aromatic) surface and size exclusion effects. Sorption of pi-acceptors, benzonitrile (BNTL), 4-nitrotoluene (MNT), 2,4-dinitrotoluene (DNT), and 2,4,6-trinitrotoluene (TNT), and to a lesser extent pi-donor solutes, naphthalene (NAPH) and phenanthrene (PHEN), was greater than predicted by hydrophobic driving forces in accord with their acceptor or donor strength. Hydrophobic effects were estimated using a concentration-dependent free energy relationship between adsorption and partitioning into an inert solvent (n-hexadecane or benzene) for a non-donor/non-acceptor calibration set (benzene and chlorinated and methylated benzenes). Molecular complexation between acceptors and model graphene donors, NAPH, PHEN, and pyrene (PYR), in chloroform and benzene was tracked by ring-current induced upfield shifts in the 1H NMR spectrum and by charge-transfer bands in the UV/visible spectrum. The EDA component of graphitewater adsorption for the acceptors correlated with the NMR-determined complexation constant with the model donors in chloroform, which, in turn, correlated with pi-acceptor strength (TNT > DNT > MNT > BNTL) and pi-donor strength (PYR > PHEN > NAPH). Charcoalgraphite isotherms calculated from charcoalwater and graphitewater isotherms indicated molecular sieving effects on charcoal for tetrasubstituted benzenes (tetramethylbenzenes and TNT) and some trisubstituted benzenes (1,3,5-trichlorobenzene, possibly DNT). When steric effects are taken into account, the order in adsorption among acceptors was qualitatively similar for graphite and charcoal. The results suggest pipi EDA interactions of the acceptorsand possibly donors, although the calibration set may underestimate the hydrophobic effect for fused ring systemswith both graphite and charcoal surfaces. For graphite, it is postulated that pi-acceptors interact with electron-rich regions of the basal plane near edges and defects and that pi-donors interact with electron-depleted regions further away. A similar mechanism may operate on the charcoal but would be modified by the (mostly) electron-withdrawing effects of O functionality on the edges of graphene sheets.

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