Significant concerns have been raised over the presence of antibiotics including tetracyclines in aquatic environments. We herein studied single-walled carbon nanotubes (SWNT) and multi-walled carbon nanotubes (MWNT) as potential effective adsorbents for removal of tetracycline from aqueous solution. In comparison, a nonpolar adsorbate, naphthalene, and two other carbonaceous adsorbents, pulverized activated carbon (AC) and nonporous graphite, were used. The observed adsorbent-to-solution distribution coefficient (Kd, L/kg) of tetracycline was in the order of 10(4)-10(6) L/kg for SWNT, 10(3)-10(4) L/kg for MWNT, 10(3)-10(4) L/kg for AC, and 10(3)-10(5) L/kg for graphite. Upon normalization for adsorbent surface area, the adsorption affinity of tetracycline decreased in the order of graphite/ SWNT > MWNT >> AC. The weaker adsorption of tetracycline to AC indicates that for bulky adsorbates adsorption affinity is greatly affected by the accessibility of available adsorption sites. The remarkably strong adsorption of tetracycline to the carbon nanotubes and to graphite can be attributed to the strong adsorptive interactions (van der Waals forces, pi-pi electron-donor-acceptor interactions, cation-pi bonding) with the graphene surface. Complexation between tetracycline and model graphene compounds (naphthalene, phenanthrene, pyrene) in solution phase was verified by ring current-induced 1H NMR upfield chemical shifts of tetracycline moieties.

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The presence of sulfonamide antibiotics in aquatic environments has been recognized as an issue warranting consideration. In this study, we evaluated multiwalled carbon nanotubes (MWNT) as a potential effective adsorbent for removal of two sulfonamide antibiotics, sulfapyridine and sulfamethoxazole, from aqueous solutions. Nonporous, functionality-free graphite was included as a comparative adsorbent. Despite the very low hydrophobicity, the two sulfonamides adsorbed strongly to MWNT and graphite, a fact attributed to pi-pi electron coupling with the graphene surface of the adsorbent. For both sulfonamide antibiotics, similar patterns of pH-dependent adsorption were observed between MWNT and graphite, implying the predominance of graphene structures for the adsorption to MWNT. Moreover, the observed pH effects on adsorption indicate that the protonated neutral form of sulfonamide adsorbs much more strongly than the deprotonated anionic counterpart does. The effects of ionic strength (NaCl and CaCl2) and the presence of a dissolved soil humic acid on adsorption of the two antibiotics to MWNT and graphite were also assessed. Ring current-induced 1H NMR upfield chemical shifts further verified face-to-face complex formation between neutral sulfamethoxazole and model pi-electron donor compounds (naphthalene, phenanthrene, and pyrene) in solution.