Aqueous-phase adsorption of aromatic compounds on water-soluble nanographite

Discotic polyaromatic macromolecules called water-soluble nanographites (WSNGs) have been prepared by the oxidative degradation of carbon black particles. The WSNGs consist of 12-19 fused benzene rings and hydrophilic functional groups bonding to the edges of the polyaromatic rings. The adsorptive characteristics of a water-soluble nanographite, WSNG3, were examined for nine aromatic compounds in aqueous solutions and were compared with those for a styrene-divinylbenzene porous polymer (XAD2), a carbon black heated at 2873 K (GCB), and a molecular sieving carbon (MSC5A). The intensity of interaction of WSNG3 with the adsorbate molecules was between those for XAD2 (low affinity) and GCB (high affinity). The interaction increases with the number of fused benzene rings of the adsorbates, and an electron-donating functional group of the adsorbate enhances the affinity, while electron-attracting groups suppress it. The maximum uptakes of aromatic acids and phenols on WSNG3 were independent of the size of the adsorbate molecules, which is quite different from the trend for the conventional hydrophobic adsorbents, XAD2 and GCB. This unique adsorptive characteristic is speculated to be due to the presence of molecular-size hydrophobic adsorption sites of ca. 1.5 nm on each WSNG3 molecule, which was called the “adsorption seat size effect”.