Environ. Sci. Technol. 2010, 44, 3806–3811
Contribution of Different Sulfamethoxazole Species to Their Overall Adsorption on Functionalized Carbon Nanotubes D I Z H A N G , † B O P A N , * ,† H U A N G Z H A N G , † PING NING,† AND BAOSHAN XING‡ Faculty of Environmental Science & Engineering, Kunming University of Science & Technology, Kunming, China, 650093, and Department of Plant, Soil and Insect Sciences, University of Massachusetts, Amherst, Massachusetts 01003
Received December 19, 2009. Revised manuscript received April 1, 2010. Accepted April 2, 2010.
Antibiotics pose environmental risks, but their adsorption mechanisms are still unclear. Identifying the contributions of different mechanisms is vital in predicting antibiotic environmental behavior and consequently understanding their environmental risks. This study used functionalized carbon nanotubes (CNTs), namely hydroxylized (MH), carboxylized (MC), and graphitized (MG) multiwalled CNTs, as adsorbents and sulfamethoxazole (SMX) as an adsorbate to study the adsorption mechanisms of ionizable organic contaminants on solid particles. At pH around 3.7, SMX always showed the highest adsorption on different CNTs and the adsorption followed the order of MH > MG > MC. Combining the results on SMX specie analysis, the pH-dependent adsorption is well-explained by hydrophobic and electron-donor-acceptor interactions. The adsorption of neutral SMX is always dominant by contributing generally over 80% to the overall adsorption. A significant contribution of cationic SMX at pH < 3.5 suggested significant contribution of hydrogen bonds to SMX adsorption. The strength of bisphenol A (BPA) inhibiting SMX adsorption was dependent on the concentration ratio of BPA to neutral SMX, instead of to the overall SMX concentration. This result emphasized the importance of identifying the dominant mechanisms or species for the adsorption of antibiotics.
Introduction Sulfamethoxazole (SMX) is a type of sulfonamide (SA) that is widely used in human and veterinary pharmaceuticals to prevent and/or treat disease (such as diminishing inflammation) and to promote livestock growth (1). These antibiotics are unavoidably discharged into the aquatic and soil environments mostly via domestic wastewater effluent, disposal of expired pharmaceuticals, and excretion as the original or metabolized forms (2-4). These compounds are potentially toxic to aquatic organisms and eventually to humans through the food chain and drinking water (5, 6). It is reported that SMX could cause acute toxicity as well as chronic toxic effects with low milligrams per liter level exposure (7). In addition, SMX is mutagenic (8). These toxic effects are enhanced when * Corresponding author phone/fax: 86-871-5170906; e-mail:
[email protected]. † Kunming University of Science & Technology. ‡ University of Massachusetts. 3806
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various antibiotics are presented simultaneously. For example, growth-inhibiting effects are intensified through simple addition of binary mixtures of sulfonamides or even synergistic effects of trimethoprim and sulfonamides (9). More importantly, antibiotic-resistance is promoted by prolonged exposure of bacteria to antibiotics in the environment, leading to the failure of antibiotics in clinical applications (3-5). Thus, the environmental behavior and risk assessment of antibiotics have attracted special research attention. Several studies have reported the occurrence of SMX in various countries (10, 11), but SMX environmental behavior is still unclear (12, 13). One of the most important processes that control the environmental behavior of SMX is its adsorption on solid particles. In comparison to traditional hydrophobic organic chemicals, SMX adsorption is relatively more complicated, because it may exist as three types of species at different pHs, namely SMX+, SMX0, and SMX-. The adsorption of SMX could be greatly affected by pH because the adsorption of different SMX species may be controlled by different mechanisms. In order to understand the environmental behavior of SMX, we need to systematically investigate the contribution of different species of SMX and different adsorption mechanisms to its overall adsorption. This line of study requires well-characterized model adsorbents so that other uncertain influences could be excluded. Carbon nanotubes (CNTs) seem a good model adsorbent because of their definite structures and uniform surfaces compared to activated carbon (14). In addition, the strong interactions between CNTs and organic chemicals have great potential in environmental analysis and water treatment (14). Therefore, many studies have been conducted along this line of research. It was reported that various mechanisms simultaneously play roles in organic chemical adsorption, such as hydrophobic, electrostatic, hydrogen bond, and π-π interactions (13, 14). But the relative contributions of individual mechanisms were seldom addressed, which is vital to understand and predict organic chemical adsorption based on CNT properties. For example, if hydrogen bonds are predominant, CNT oxidation may increase organic chemical adsorption (15, 16). However, if hydrophobic interaction is the overwhelming mechanism, CNT oxidation could decrease organic chemical adsorption (17-19). Thus, if the contributions of different mechanisms are unknown, an incorrect conclusion could be made in predicting the effects of CNT oxidation. Fortunately, there are different functionalized CNTs available on the market, such as hydroxylized, carboxylized, and graphitized CNTs. Investigating the adsorption mechanisms on different types of CNTs may provide important information on understanding CNT-organic chemical interactions. Therefore, the objectives of this work were (1) to determine the adsorption contribution of various SMX species to its total adsorption and (2) to examine the effect of CNT functional groups on SMX adsorption. Both goals will provide important data to understand SMX adsorption mechanisms on solid particles.
Experimental Section Materials. The CNTs used in the study were three multiwalled CNTs (purity >95%) which were hydroxylized (MH), carboxylized (MC), and graphitized (MG), respectively. They were purchased from Chengdu Organic Chemistry Co., Chinese Academy of Sciences. These CNTs were synthesized in the CH4/ H2 mixture at 700 °C (MG at 2800 °C) by the chemical vapor deposition method. The antibiotic, SMX, was obtained from Bio Basic Inc. All the other chemicals were higher than analytical grade. All the CNTs were characterized for their ζ-potential, 10.1021/es903851q
2010 American Chemical Society
Published on Web 04/15/2010
surface area, elemental composition, and surface functional groups (X-ray photoelectron spectroscopy). Batch Adsorption Experiments. Sulfamethoxazole (100 mg/L) was dissolved in background solution containing 0.01 M NaCl and 200 mg/L NaN3 (bioinhibitor) as a stock solution. Three types of adsorption experiments were conducted in this study: (a) adsorption isotherms of SMX, (b) sorption edge study of SMX on CNTs at different pHs, and (c) coadsorption of SMX and bisphenol A (BPA). Adsorption experiments (a) were conducted in 40 mL glass vials with Teflon-lined screw caps. According to preliminary studies, the aqueous:solid ratios were 4000:1, 8000:1 and 2000:1 (w/ w) at pH ) 1.0, 3.7, and 7.5, respectively. Although these pHs (1.0 and 3.7) are not of significantly environmental relevance, adsorption study at these pHs gives important insight on SMX species distribution and adsorption mechanisms. The stock solutions of SMX were diluted with the background solution to eight different concentrations (0-100 mg/L). Samples of SMX solutions without solid particles were kept in the same condition as other samples and are referred to as the initial concentration references. All of the vials were kept in the dark and were shook in an air-bath shaker at 25 °C for 7 d. This was sufficient to reach apparent equilibrium (20). During this time period, SMX was stable and no apparent degradation was observed (Figure S1, Supporting Information). After equilibrating for 7 d, all of the vials were centrifuged at 1000g for 10 min and the supernatants were subjected to quantification of the solute. Solution pH at equilibrium was measured. For the sorption edge study (b), the initial concentrations of SMX were fixed at 9.5 mg/L. The aqueous:solid ratio was 2000:1 (w/w). The pH of the suspension was adjusted in the range 1.5-12 using HCl or NaOH. The other experimental procedure was the same as experiment a. The SMX-adsorbed CNTs were subject to XPS measurement for their surface elemental compositions. For the coadsorption experiments (c), BPA was selected as the coadsorbate for SMX because of the unchanged BPA speciation in this pH range and the strong adsorption of BPA on CNTs as presented in our previous study (20). The stock solution of BPA (100 g/L, dissolved in methanol) was diluted by the background solution to eight different concentrations (0-32 mg/L). The initial concentrations of SMX were fixed at 50 mg/L. The pH of the suspension was fixed at 1, 3.7, and 7.5, respectively. The other conditions were the same as in experiment a. After centrifugation, the supernatants were subjected to SMX or/and BPA analysis. Detection of SMX and BPA. The concentrations of SMX and BPA in the supernatants were quantified by HPLC (Agilent Technologies 1200) equipped with a reversed-phase C8 column (5 µm, 4.6 × 150 mm) and an UV detector. Sulfamethoxazole was quantified at 265 nm while BPA at 280 nm. The mobile phase was 40:60 (v:v) acetonitrile:deionized water with 0.1% acetic acid and the flow rate was 1 mL/min. The detection limits were 0.05 mg/L for SMX and 0.1 mg/L for BPA. Because of the small amount of the injected samples (10 µL) compared with mobile phase in HPLC, UV-vis response of SMX did not change significantly with the change of sample pH. The spectra properties of SMX at different pHs were also investigated using a UV-vis spectrometer (Shimadzu UV-2450). Data Analysis. Adsorption isotherms were fitted using the Freundlich and Dubinin-Ashtakhov models (21) with SigmaPlot 10.0. Freundlich model (FM)
log Qe ) log KF + N log Ce
Dubinin-Ashtakhov model (DA) log Qe ) log Q0 - (ε/E)b where KF [(mg/kg)/(mg/L)N] is the Freundlich adsorption coefficient and N is the nonlinearity factor. Qe (mg/kg) and
TABLE 1. Characterization of Original CNTs and SMX-Adsorbed CNTs original CNTs MHb elemental analysis (atom based) (%)
C 94 N H O
XPS (atom based) (%)
97
0.2 1.5 pH ) 1.0 > pH ) 7.5 (Figure 1A). The same results were obtained for MC and MG at various pHs. This mainly was attributed to strong electrostatic repulsions between SMX (negative charge) and CNTs (negative charge) at pH ) 7.5 and will be discussed in the following sections. Possible Adsorption Mechanisms of SMX on CNTs. In the adsorption edge study, the experiments were conducted at a series of pHs, providing more detailed information to compare SMX species distribution and their adsorption coefficients. Kd of SMX on three different CNTs as a function of pH showed similar trends: in the range of pH < 3.5, Kd of 3808
Kd (L/kg) 2
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SMX increased with increasing pH; however, in the range of pH > 3.5, Kd declined greatly (Figure 2). Obviously, Kd of SMX reached a maximum value at around pH ) 3.5. This result is consistent with the adsorption isotherm study. Electrostatic interactions greatly control the adsorption of ionic compounds. For example, the synergistic adsorption of sulfate and Cu2+ on goethite was explained by the alteration of goethite surface properties after adsorbing Cu2+ and sulfate (24). Electrostatic interaction mechanisms could also explain our sorption data. The pHzpc values (zero point of charge) of MH and MC are around 4 (Figure S4, Supporting Information). Thus, MH and MC are positively charged at pH < 4 and negatively charged at pH > 4. In the range of pH > 3.5, anionic SMX species increased with increased pH and the electrostatic repulsion between SMX and MH or MC resulted in decreased Kds. At pH < 3.5, apparent Kds decreased greatly with decreased pH because of the electrostatic repulsion between cation SMX and positively charged MH or MC. Decreased adsorption of sulfonamides with increasing pH (pH > 3) was also observed on Na+-saturated montmorillonite (25) and multiwalled CNTs (13). Because those experiments were conducted at pH > 3, the decreased sorption at lower pH was not observed. It should be noted that pHzpc of MG could not be obtained as indicated by huge variation of zeta potential in the pH range of 3-8 (Figure S4, Supporting Information). This result is understandable because no functional groups were introduced in MG. The highest adsorption of SMX on MG in comparison to other CNTs at pH ) 7.5 (Table 2) may be resulted from lack of electrostatic repulsion between SMX and MG. The solubility of SMX was affected greatly by pH. As measured by Dahlan et al. (26), the minimum solubility was 281 mg/L at pH ) 3.22 (25 °C). However, at base or acid pH, SMX solubility could be increased to 17 900 mg/L (pH ) 7.5, calculated based in the provided equation) or 560 mg/L (pH ) 1.7). These results clearly indicate that the hydrophobicity of SMX changed greatly with pH. The apparent adsorption is negatively related with solubility, indicating hydrophobic interaction was one of the important adsorption mechanisms. The important contribution of hydrophobic interaction to
TABLE 3. Calculated Sorption Coefficients for Three SMX Species Kd- (104 L/kg) Kd0 (104 L/kg)
pH 3.5 MH MC 1.5-12 MG -
Kd+ (104 L/kg)
1.47 ( 0.15 0.097 ( 0.016 0.72 ( 0.05 0.091 ( 0.044 1.47 ( 0.15 31.9 ( 1.09 0.72 ( 0.05 3.78 ( 0.56 0.97 ( 0.03 -
radj2 0.990 0.993 0.997 0.996 0.963
This adsorption edge study was also conducted at 1 mg/L (Figure S5, Supporting Information) and the resulted pHdependent adsorption showed the same trend as presented in Figure 2. This result indicated that the discussion on possible mechanisms for SMX adsorption on CNTs is applicable at lower concentrations. However, it is reasonable to expect different dominant adsorption mechanisms at different concentrations because of the site-specific adsorption. Future study with more specific experimental design is needed. Contribution of SMX Species to the Overall Adsorption. To quantitatively identify the contribution of different SMX species to the overall adsorption (30), the following empirical models could be used to calculate sorption coefficients for individual SMX species using SPSS 17.0: FIGURE 2. pH-dependent SMX adsorption on CNTs. (A) Adsorption coefficient (Kd) of SMX on CNTs at different pHs. The regression curves denote the best fit of eqs 3-5 to the adsorption data. (B) Possible adsorption mechanisms of SMX on CNTs at different pHs. The experiments were carried out at SMX initial concentration of 9.5 mg/L. Kd decreased with increasing pH at pH > pHzpc and decreased with decreased pH at pH < pHzpc because of electrostatic repulsions. At pH around pHzpc, various adsorption mechanisms (such as hydrophobic interaction and π-π and hydrogen bonds) contribute to the overall SMX adsorption. organic chemical adsorption on CNTs is widely accepted in literature (27, 28). The differences between CNTs are their functional groups and surface area. The largest surface area (228 m2/g) and the highest extent of functionalization (4.28%) (Table 1) were observed for MH. It should be noted that at pH around 7.5, the highest adsorption was observed on MG, which has the lowest surface area, indicating surface area (measured by N2 adsorption) may not be the controlling factor to SMX adsorption on CNTs in this study. Significant adsorption differences between CNTs were observed at pH < 5.7 when neutral and positive SMX dominate. This result could be understood from the different functional groups of CNTs. The SMX molecule is a strong π-acceptor because of its amino functional groups and N-heteroaromatic rings. The adsorbent MC is also strong π-acceptor due to carboxyl functional groups on benzene rings (especially, protonated carboxyl groups at low pH) (29). But MH is a π-donor because of the hydroxyl groups on benzene rings (20). Interactions between a π-donor and π-acceptor are much stronger than that of donor-donor or acceptor-acceptor pairs (29). Thus π-π electron donor-acceptor interactions (EDA mechanism) could explain the different Kds of MH, MG, and MC at pH around 3.5. It is interesting to notice in Figure 2A that the variation of Kd did not show a symmetric pattern around pH ) 3.5. Relatively higher adsorption could be observed at low pHs. One of the possible reasons is the contribution of EDA interaction, as discussed earlier. Another possibility is that hydrogen bonds may play roles in the overall adsorption (14).
Kd ) Kd-δ- + Kd0δ0 + Kd+δ+
pH < 3.5
(1)
Kd ) Kd+δ+ + Kd0δ0 + Kd-δ-
pH > 3.5
(2)
where Kd (L/kg) is the overall adsorption coefficient and Kd-, Kd0, and Kd+ are sorption coefficients for anionic, neutral, and cationic SMX, respectively. δ-, δ0, and δ+ are the percentages of anionic, neutral, and cationic SMX at a certain pH, respectively. The fitted results showed negligible Kd- values [(0.02340.0315) × 104 L/kg] at pH > 3.5, which may have resulted from electrostatic repulsion between negatively charged SMX and MH or MC. However, at pH < 3.5, when both SMX and MH or MC were positively charged, Kd+ values were 1 order of magnitude higher than Kd- at pH > 3.5. As discussed earlier, hydrogen bond and EDA interaction may play roles at pH < 3.5. Therefore, the equations used to obtain the contribution of different SMX species were modified as eqs 3-5. For MH and MC: Kd ) Kd-δ- + Kd0δ0 + Kd+δ+ Kd ) Kd+δ+ + Kd0δ0
pH < 3.5
(3)
pH > 3.5
(4)
For MG: Kd ) Kd0δ0
pH ) 1.5-12
(5)
For MH and MC, the contributions of three SMX species were considered at pH < 3.5; however, the contribution of negatively charged SMX was neglected at pH > 3.5. Electrostatic interaction could be neglected for MG because of its lack of functional groups. Thus, only the neutral SMX was used in the equation fitting. The calculated results are listed in Table 3 and regression curves are shown in Figure 2A. The result showed that sorption coefficients of anionic SMX [(50.4-432) × 104 L/kg] (pH < 3.5) and cationic SMX [(3.78-31.9) × 104 L/kg] (pH > 3.5) were much higher than that of neutral SMX (0.721 × 104 to 1.47 × 104 L/kg). However, the contribution of neutral SMX to the overall adsorption is always higher than 50% and mostly higher than 80% (Figure S6, Supporting Information) indicating adsorption of neutral VOL. 44, NO. 10, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 3. Inhibition ratio of SMX adsorption on MH (A), MC (B), and MG (C) as a function of BPA aqueous concentrations. R was calculated according to eq 6. SMX was the main contribution to the overall adsorption. Although the percentage of anionic SMX ( MG > MC clearly provides evidence of EDA interactions of SMX adsorption. The pHdependent SMX adsorption could be well-explained by considering various functioning adsorption mechanisms. Interestingly, the inhibition strength of BPA on SMX adsorption varied with pH. This result further illustrated that the overlap of adsorption mechanisms changes with pH because of the change in the dominant species. Clearly, understanding the exact contribution of a certain adsorption mechanism is vital in predicting the environmental fate of SMX. However, current studies could not provide enough information for this quantitative goal. Improved experimental designs are demanded. With the widespread use of SAs and CNTs, they are/or will be unavoidably discharged into the environment, particularly in wastewater and solid waste from wastewater treatment plants. Their strong interactions would greatly affect the environmental behavior and fate of both SAs and CNTs. This study presented the results on SA-CNT interactions in a very simple system. For the ultimate goal of estimating environmental behavior and risks of SAs and CNTs, various environmental factors need to be considered, such as the presence of other chemicals (cations, anions, and neutral molecules), natural organic matter (may bind with SAs, adsorb on CNTs, and disperse CNT aggregates), and chemical reactions of CNTs in real environments.
Acknowledgments This research was supported by the National Scientific Foundation of China (40973081, 40803034) and Research Grant for Future Talents of Yunnan Province. The authors are grateful to Dr. Xikun Yang for his assistance with XPS measurements. The authors thank the anonymous reviewers for their valuable comments to improve our manuscript.
Supporting Information Available Figures S1-S9. This material is available free of charge via the Internet at http://pubs.acs.org.
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