Adsorption of nonionic aromatic compounds to single-walled carbon nanotubes: effects of aqueous solution chemistry

We systematically studied effects of pH, ionic strength, and presence of Cu2+ (50 mg/L) or a dissolved humic acid (HA, Fluka) (50 mg/L) on adsorption of three nonionic aromatic compounds, naphthalene, 1,3-dinitrobenzene, and 1,3,5-trinitrobenzene to single-walled carbon nanotubes. Presence of Cu2+ or variance in the ionic strength between 0.02 and 0.1 M (NaNO3) only slightly affected adsorption affinities. Presence of HA reduced adsorption of the three compounds by 29-57% for CNTs, as measured by change in distribution coefficient(Kd), and by 80-95% for graphite. In contrast to nonporous graphite, whose surface area was completely accessible in adsorption, CNTs formed aggregates with microporous interstices in aqueous solution, which blocked large HA molecules from competing with the surface area. Changing the pH from 2 to 11 did not affect adsorption of naphthalene, while it increased adsorption of 1,3-dinitrobenzene and 1,3,5-trinitrobenzene by 2-3 times. Increasing pH apparently facilitated deprotonation of the acidic functional groups (-COOH, -OH) of CNTs, which promoted the pi-electron-donor ability of the graphene surface, therefore enhancing pi-pi electron-donor-acceptor (EDA) interactions of the two nitroaromatics (pi-electron acceptors).