Environ. Sci. Technol. 2009, 43, 3400–3401
Response to Comment on “Adsorption of Hydroxyl- and Amino-Substituted Aromatics to Carbon Nanotubes” We appreciate the comments by Jiang et al. on our paper (1) regarding the adsorption mechanisms of hydroxylsubstituted aromatics to carbon nanotubes. We offer the following responses to the comments. Oxidative coupling with molecular oxygen has been proposed to explain the apparent adsorption enhancement of phenolic compounds to granular activated carbons, activated carbon fibers, and graphite (2-7). Primary evidence included lower recovery of adsorbate by solvent extraction from the adsorbent when molecular oxygen was present, simultaneous consumption of molecular oxygen during adsorption, and identification of reaction products by mass spectroscopy. Thus, Jiang et al. argued that the strong adsorption of 2,4-dichlorophenol and 2-naphthol to graphite and carbon nanotubes and the associated pH effect observed in our study were due to oxidative coupling rather than π-π electron-donor-acceptor (EDA) interaction. However, after carefully examining the data in the literature and our previous study and conducting additional experiments, we have concluded that oxidative coupling could not be a major mechanism for the observations in our paper. In our previous study when normalized for hydrophobic effect, 2-naphthol exhibited much greater adsorption affinity (more than 1 order of magnitude) to carbon nanotubes and graphite than structurally similar naphthalene. However, the literature reported adsorption enhancement of phenolic compounds (phenol, methylated and chlorinated phenols) to activated carbons due to oxidative coupling was much weakersthe largest difference was only several times between oxic and anoxic conditions (3, 5). Thus, it seems that oxidative coupling alone could not fully account for the strong adsorption of 2-naphthol observed in our previous study. It has been reported that the surface functional groups and pore size distribution of carbonaceous adsorbents are important factors controlling the degree of oxidative coupling of phenolic compounds. For example, no significant effect of molecular oxygen on the adsorption of phenolic compounds was observed for an activated carbon fiber with an average pore size
