Characterization of Aromatic Compound Sorptive Interactions with Black Carbon (Charcoal) Assisted by Graphite as a Model

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 pi-pi electron donor-acceptor (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 graphite-water 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). Charcoal-graphite isotherms calculated from charcoal-water and graphite-water 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 pi-pi EDA interactions of the acceptors-and possibly donors, although the calibration set may underestimate the hydrophobic effect for fused ring systems-with 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 0 functionality on the edges of graphene sheets.