pubs.acs.org/Langmuir © 2009 American Chemical Society
Adsorption of Sulfonamide Antibiotics to Multiwalled Carbon Nanotubes Liangliang Ji,† Wei Chen,‡ Shourong Zheng,† Zhaoyi Xu,† and Dongqiang Zhu*,† †
State Key Laboratory of Pollution Control and Resource Reuse/School of the Environment, Nanjing University, Jiangsu 210093, China, and ‡College of Environmental Science and Engineering/Tianjin Key Laboratory of Environmental Remediation and Pollution Control/Ministry of Education Key Laboratory of Pollution Processes and Environmental Criteria, Nankai University, Tianjin 300071, China Received May 4, 2009. Revised Manuscript Received August 12, 2009
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 π-π 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 π-electron donor compounds (naphthalene, phenanthrene, and pyrene) in solution.
Introduction Sulfonamide antibiotics are produced in large quantities and heavily used in human therapy and livestock production. Residues of sulfonamide compounds and metabolites discharged from municipal wastewater treatment plants and agricultural runoff have a high potential to enter surface- and groundwater.1-6 Concerns arising from exposure to sulfonamide antibiotics in aquatic environments include acute and chronic toxic effects and microorganism antibiotic resistance.7-9 However, the removal of antibiotics by existing water treatment technologies is incomplete.10 Engineered carbon nanotubes have shown great potential for many nanotechnology applications, including effective adsorbents for removal of undesirable organic contaminants in water treatment.11,12 Because of the large surface area and high surface hydrophobicity, engineered carbon nanotubes tend to adsorb many hydrophobic organic compounds such as polycyclic
aromatic hydrocarbons (PAHs) and dioxins very strongly.12-20 In these studies, a mechanism of π-π electron coupling and stacking between the aromatic compound and the graphene surface of carbon nanotubes has also been proposed. Notably, studies on adsorption of emerging organic contaminants, including antibiotics and hormones, to carbon nanotubes are still very limited in the literature.21-24 The strong adsorption affinity of carbon nanotubes for sulfonamides has been explored in developing efficient solid-phase extraction methods for the chromatographic analysis of these compounds.21,22 However, the underlying mechanism(s) controlling adsorption of sulfonamides to carbon nanotubes is still largely unknown. In a recent study,24 we reported that carbon nanotubes and functionality-free, nonporous graphite exhibited extraordinarily strong adsorption for tetracycline antibiotics, due to the strong specific adsorptive interactions [π-π electron donor-acceptor (EDA) interactions and cation-π bonding] with the highly polarized graphene structure (π-electron donor) of the adsorbents. Adsorption of tetracycline decreases as the pH increases because the electron
*To whom correspondence should be addressed. Telephone and fax: +86 025-8359-6496. E-mail:
[email protected]. (1) Burkhardt, M.; Stamm, C.; Waul, C.; Singer, H.; M€uller, S. J. Environ. Qual. 2005, 34, 1363–1371. (2) G€obel, A.; Thomsen, A.; McArdell, C. S.; Joss, A.; Giger, W. Environ. Sci. Technol. 2005, 39, 3981–3989. (3) Burkhardt, M.; Stamm, C. J. Environ. Qual. 2007, 36, 588–596. (4) Kim, S. C.; Carlson, K. Environ. Sci. Technol. 2007, 41, 50–57. (5) Stoob, K.; Singer, H. P.; Mueller, S. R.; Schwarzenbach, R. P.; Stamm, C. H. Environ. Sci. Technol. 2007, 41, 7349–7355. (6) Xu, W. H.; Zhang, G.; Li, X. D.; Zou, S. C.; Li, P.; Hu, Z. H.; Li, J. Water Res. 2007, 41, 4526–4534. (7) Thiele-Bruhn, S.; Beck, I.-C. Chemosphere 2005, 59, 457–465. (8) Park, S.; Choi, K. Ecotoxicology 2008, 17, 526–538. (9) Hammesfahr, U.; Heuer, H.; Manzke, B.; Smalla, K.; Thiele-Bruhn, S. Soil Biol. Biochem. 2008, 40, 1583–1591. (10) Ternes, T. A.; Joss, A.; Siegrist, H. Environ. Sci. Technol. 2004, 38, 392A– 399A. (11) Masciangioli, T.; Zhang, W. X. Environ. Sci. Technol. 2003, 37, 102A–108A. (12) Pan, B.; Xing, B. Environ. Sci. Technol. 2008, 42, 9005–9013. (13) Long, R.; Yang, R. J. Am. Chem. Soc. 2001, 123, 2058–2059.
11608 DOI: 10.1021/la9015838
(14) Peng, X.; Li, Y.; Luan, Z.; Di, Z.; Wang, H.; Tian, B.; Jia, Z. Chem. Phys. Lett. 2003, 376, 154–158. (15) Fagan, S. B.; Filho, A. G. S.; Lima, J. O. G.; Filho, J. M.; Ferreira, O. P.; Mazali, I. O.; Alves, O. L.; Dresselhaus, M. S. Nano Lett. 2004, 4, 1285–1288. (16) Gotovac, S.; Hattori, Y.; Noguchi, D.; Miyamoto, J.; Kanamaru, M.; Utsumi, S.; Kanoh, H.; Kaneko, K. J. Phys. Chem. B 2006, 110, 16219–16224. (17) Zhao, J.; Lu, J. Appl. Phys. Lett. 2003, 82, 3746–3748. (18) Fagan, S. B.; Santos, E. J. G.; Filho, A. G. S.; Filho, J. M.; Fazzio, A. Chem. Phys. Lett. 2007, 437, 79–82. (19) Chen, W.; Duan, L.; Zhu, D. Environ. Sci. Technol. 2007, 41, 8295–8300. (20) Chen, W.; Duan, L.; Wang, L.; Zhu, D. Environ. Sci. Technol. 2008, 42, 6862–6868. (21) Fang, G.; He, J.; Wang, S. J. Chromatogr., A 2006, 1127, 12–17. (22) Niu, H.; Cai, Y.; Shi, Y.; Wei, F.; Liu, J.; Mou, S.; Jiang, G. Anal. Chim. Acta 2007, 594, 81–92. (23) Pan, B.; Lin, D.; Mashayekhi, H.; Xing, B. Environ. Sci. Technol. 2008, 42, 5480–5485. (24) Ji, L.; Chen, W.; Duan, L.; Zhu, D. Environ. Sci. Technol. 2009, 43, 2322– 2327.
Published on Web 09/03/2009
Langmuir 2009, 25(19), 11608–11613
Ji et al.
Article
acceptor abilities of the cationic amine group and the enol-enone groups are weakened by deprotonation of these moieties at high pH. Similar to tetracyclines, sulfonamides are amphoteric molecules and may also undergo pH-dependent speciation reactions. Therefore, pH is expected to be a key solution chemistry factor affecting adsorption of sulfonamides to carbon nanotubes. To date, sorption studies of sulfonamide antibiotics have mainly focused on natural soil, humic substance, and clay minerals.3,25-29 These studies imply that the sorption is dominated by cation exchange and complexation reactions such as H-bonding between the multiple groups and/or moieties of sulfonamides and the respective charged and/or polar sites of sorbents, while the hydrophobic effect plays only a minor role in sorption. For example, cation exchange appears to be the main factor contributing to sorption to expandable clay minerals.27,29 Alternatively, sorption to soil humic substance is mainly controlled by complexation with phenolic and carboxylic groups, N-heterocyclic structures, and lignin decomposition products.26,28 Thus, it seems reasonable to hypothesize that strong complexation reactions (electrostatic forces and H-bonding) might also exist between sulfonamide molecules and the surface functionalities of carbonaceous materials. However, the relative importance of the graphene structure and the surface functional groups in adsorption of sulfonamides to carbon nanotubes has not been evaluated. The main objective of this study is to investigate the mechanism(s) and predominant factors controlling the adsorption of sulfonamides to multiwalled carbon nanotubes. Two commonly used sulfonamide antibiotics, sulfapyridine and sulfamethoxazole, were examined as the adsorbates in batch adsorption experiments. The two selected sulfonamides have similar hydrophobicities but contain different substituted N-heterocyclic groups. Nonporous, functionality-free graphite was explored as a comparative adsorbent for evaluating the roles of surface functional groups of carbon nanotubes in adsorption. Impacts of solution chemistry conditions (pH, ionic strength, and dissolved soil humic acid) on adsorption were also evaluated. Furthermore, solution-phase 1H nuclear magnetic resonance (NMR) studies were performed to test the possible complexation between selected sulfonamide and PAHs as model compounds to mimic the graphene structure of carbonaceous adsorbents.
Experimental Section Materials. Two sulfonamide antibiotics, sulfapyridine (99%, Sigma) and sulfamethoxazole (99%, Sigma), were used as adsorbate compounds. The chemical structures and associated protonation/deprotonation speciation of the two compounds are presented in Figure S1 of the Supporting Information. The aqueous solubility (SW), n-octanol-water partition coefficients (KOW), and acidic dissociation constants of the two compounds are listed in Table 1. In the solution-phase 1H NMR experiments, naphthalene (>99%, Sigma), phenanthrene (98%, Fluka), and pyrene (98%, Aldrich) were used as π-electron donors to mimic the graphene surface of adsorbents, and 1,4-dichlorobenzene (>99.0%, Fluka) was used as a control for non-π-donors. Nonporous, pure graphite was purchased from Aldrich (99.999% graphitized C, as provided by the manufacturer, and also verified in a separate elemental analysis) and was used as (25) Boxall, A. B. A.; Blackwell, P.; Cavallo, R.; Kay, P.; Tolls, J. Toxicol. Lett. 2002, 131, 19–28. (26) Thiele-Bruhn, S.; Seibicke, T.; Schulten, H.-R.; Leinweber, P. J. Environ. Qual. 2004, 33, 1331–1342. (27) Gao, J. A.; Pedersen, J. A. Environ. Sci. Technol. 2005, 39, 9509–9516. (28) Kahle, M.; Stamm, C. Environ. Sci. Technol. 2007, 41, 132–138. (29) Kahle, M.; Stamm, C. Chemosphere 2007, 68, 1224–1231.
Langmuir 2009, 25(19), 11608–11613
Table 1. Adsorbate Water Solubility (SW), n-Octanol-Water Partition Coefficients (KOW), and Acid Dissociation Constants (pKa) compound
SW (mmol/L)
KOWa (L/L)
sulfamethoxazole 1.46b 7.76 2.24 sulfapyridine 1.08c a From ref 27. b From ref 41. c From ref 26.
pKa valuesa 1.8(1), 5.6(2) 2.3(1), 8.4(2)
received. Multiwalled carbon nanotubes (MWNT) were purchased from Nanotech Port Co. (Shenzhen, Guangdong Province, China). On the basis of the information provided by the manufacturer, the MWNT contained more than 95% carbon nanotubes and less than 5% impurities, mainly amorphous carbon (
