Environ. Sci. Technol. 2009, 43, 2322–2327
Mechanisms for strong adsorption of tetracycline to carbon nanotubes: A comparative study using activated carbon and graphite as adsorbents LIANGLIANG JI,† WEI CHEN,‡ L I N D U A N , ‡ A N D D O N G Q I A N G Z H U * ,† State Key Laboratory of Pollution Control and Resource Reuse/School of the Environment, Nanjing University, Jiangsu 210093, China, and Tianjin Key Laboratory of Environmental Remediation and Pollution Control/College of Environmental Science and Engineering, Nankai University, Tianjin 300071, China
Received November 18, 2008. Revised manuscript received January 16, 2009. Accepted January 29, 2009.
Significant concerns have been raised over the presence of antibiotics including tetracyclines in aquatic environments. We hereinstudiedsingle-walledcarbonnanotubes(SWNT)andmultiwalled 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 adsorbentto-solution distribution coefficient (Kd, L/kg) of tetracycline was in the order of 104-106 L/kg for SWNT, 103-104 L/kg for MWNT, 103-104 L/kg for AC, and 103-105 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, π-π electron-donor-acceptor interactions, cation-π bonding) with the graphene surface. Complexation between tetracycline and model graphene compounds (naphthalene, phenanthrene, pyrene) in solution phase was verified by ring currentinduced 1H NMR upfield chemical shifts of tetracycline moieties.
Introduction Tetracycline antibiotics are heavily used as veterinary therapeutics and growth promoters for animals. In the U.S., the annual consumption of tetracyclines in swine and poultry husbandry in the late 1990s reached 2.3 and 0.63 million kilograms, respectively (1). Most of the tetracyclines used in the farming industry are excreted via feces and urine as unmodified parent compounds, with only small fractions being metabolized (2, 3). Residues of veterinary pharmaceuticals, including tetracyclines discharged from municipal wastewater treatment plants and agricultural runoff, are * Corresponding author phone: +86 025-8359-6496; email: zhud@ nju.edu.cn. † Nanjing University. ‡ Nankai University. 2322
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frequently detected in surface water, groundwater, and even drinking water (4-7). Exposures to low-level antibiotics in the environment have raised significant concerns of the toxic effect, as well as the transfer and spread of antibiotic resistant genes among microorganisms (8, 9). It is thus of great importance to develop efficient and cost-effective treatment technologies for removal of such compounds. In the water treatment practices, sorbents of high binding affinity and capacity (e.g., activated carbon) are commonly employed to remove residues of undesirable organic chemicals from the aqueous phase. However, so far only a few studies have been conducted to investigate the sorption properties of tetracyclines, using activated sludge or activated carbons as the sorbent (10, 11). A larger effort has been made to understand the sorption mechanisms of tetracyclines to natural geosorbents, including soils, iron/aluminum hydroxides, clay minerals, humic substances, and clay-humic complexes (5, 12-19). An important findings from these studies was that the sorption is mainly controlled by cation/ anion exchange, cation bridging, and surface complexation (e.g., H-bonding) between the functional groups of tetracyclines and the respective charged/polar sites of sorbents, while hydrophobic effect is only a minor factor contributing to the overall sorption. Engineered carbon nanotubes have been demonstrated to be a very effective adsorbent for many aromatic compounds due to the large surface area and the capability of π-π electron coupling with target compounds (20-27). In a recent study (25), we proposed a mechanism of π-π electron-donor-acceptor (EDA) interaction between nitroaromatic compounds (π-electron-acceptors) and the π electron-rich regions on the graphene surface of carbon nanotubes and graphite. The strong electron-withdrawing ability of the nitro group causes the substituted aromatic rings to be electron-depleted and hence function as effective π-electron-acceptors. In addition to the mechanism of π-π EDA interaction, Lewis acid-base interaction with the surface O-functionalities has also been advanced as an extra important driving force for the strong adsorption of 1-naphthylamine to carbon nanotubes (26). Tetracyclines are amphoteric molecules and have multiple groups/moieties (phenol, amino, alcohol, enone) that are charged and/or capable of electronic coupling. Accordingly, various specific (e.g., π-π EDA, Lewis acid-base, electrostatic) interactions might exist between the multiple functionalities of tetracyclines and the corresponding structures on the surface of carbon nanotubes. To date the adsorption properties of tetracyclines to carbonaceous materials, including activated carbons and black carbons (char, charcoal, soot) that are well-studied for the adsorption of many organics, have not been adequately estimated (11). The overall objective of the present work is to investigate the adsorption mechanisms of tetracycline to carbon nanotubes. A pulverized activated carbon and nonporous, functionality-free graphite were included as additional adsorbents for assessing the roles of pore size distribution and surface chemistry in adsorption. Furthermore, naphthalene, a nonpolar and more hydrophobic, but less bulky compound, was included as a comparison to further understand the adsorption mechanisms. Solutionphase 1H nuclear magnetic resonance (NMR) studies were also conducted to probe possible complexation between tetracycline and selected polycyclic aromatic hydrocarbons (PAHs) as model compounds to represent the graphene structure of carbonaceous adsorbents. 10.1021/es803268b CCC: $40.75
2009 American Chemical Society
Published on Web 02/27/2009
TABLE 1. Elemental Compositions (Dry Weight-Based), Surface Area, and Average Pore Width for Single-Walled Carbon Nanotubes (SWNT), Multi-Walled Carbon Nanotubes (MWNT), Activated carbon (AC), and Graphite
adsorbent
C%
H%
N%
surface area (m2/g)a
SWNT MWNT AC graphite
81.86 94.01 85.86 99.99
0.35 0.15 1.32 BDLb
1.02 0.82 0.95 BDLb
370 148 624 4.5c
average pore width (Å)a 37.3 100.8 21.0
a Determined by N2 adsorption using the BrunauerEmmett-Teller (BET) method. b Below detectable level. c Adopted from Zhu et al. (34).
Experimental Section Materials. The two adsorbate compounds tested were naphthalene (99%, Aldrich) and tetracycline (hydrate, 99%, International Laboratory). Their selected physicochemical properties are listed in Table S1 of the Supporting Information (SI). The chemical structure of tetracycline is presented in Figure S1of the SI. In the solution-phase 1H NMR experiments, naphthalene, phenanthrene (98%, Fluka), and pyrene (98%, Aldrich), were used as π-electron-donors to model the graphene surface of adsorbents, and 1,4-dichlorobenzene (>99.0%, Fluka) was used as a control for non-π-donor. The four carbonaceous adsorbents tested were singlewalled carbon nanotubes (SWNT) (Nanotech Port, China), multi-walled carbon nanotubes (MWNT) (Nanotech Port), a pulverized activated carbon (AC) (Huajing Co., China), and nonporous graphite (Aldrich). Based on the information provided by the manufacturer, the SWNT contained >90% (by volume) of carbon nanotubes, and the content of SWNT with outer diameter less than 2 nm was >50%. The sizes of the outer diameter for the MWNT ranged from 10 to 30 nm. The samples of AC and graphite were used as received. The samples of carbon nanotubes were treated to remove amorphous carbon and trace metals using a previously developed method (28). Elemental composition, surface area, and average pore width of the adsorbents are summarized in Table 1. Batch Adsorption. Adsorption experiments were conducted using 40-mL glass vials equipped with polytetrafluoroethylene-lined screw caps. The aqueous solution of 0.02 M NaCl containing tetracycline at the desired concentration was preadjusted for pH with NaOH and HCl and purged by N2 to remove dissolved oxygen to prevent possible oxygenmediated degradation of tetracycline (29). The initial concentrations of tetracycline ranged from 0.0032 mmol/L to 0.19 mmol/L. Vials received a weighed amount of adsorbent (10-15 mg of carbon nanotubes, 10-20 mg of AC, and 30 mg of graphite) and a full volume of tetracycline aqueous solution or 0.02 M NaCl aqueous solution followed by naphthalene in a methanol carrier. If methanol was used, its volume ratio to solution was kept below 0.1% to minimize cosolvent effects. The samples were covered with aluminum foil from light (avoid possible photodegradation of tetracycline) and were mixed end-over-end at room temperature for 3 days. Afterward, the samples were left undisturbed for more than 12 h to allow complete settling of the adsorbent. Concentration of solute in an aliquot was analyzed by highperformance liquid chromatography (HPLC) with a UV detector using a 4.6 × 150 mm SB-C18 column (Agilent). Isocratic elution was performed under the following conditions: 0.01 M oxalic acid-acetonitrile-methanol (80:16:4, v:v: v)withawavelengthof360nmfortetracycline;methanol-water (75:25, v:v) with a wavelength of 254 nm for naphthalene. To take account for solute loss from processes other than
adsorbent sorption (i.e., sorption to septum and glass wall), calibration curves were built separately from controls receiving the same treatment and conditions (pH, temperature, etc.) as the adsorption samples but without adsorbent. Calibration curves included at least seven concentration levels over the test concentration range. Based on the obtained calibration curves, the adsorbed mass of solute was calculated by subtracting mass in aqueous solution from mass added. The equilibrium pH of samples, as measured at the end of adsorption, was 5.7 for SWNT, 6.0 for MWNT, 5.8 for AC, and 5.8 for graphite. A separate set of experiments were conducted to test the pH effect on single-point adsorption of tetracycline to graphite and to SWNT in 0.02 M NaCl over a pH range of 3.4-11.0 (pH adjusted with HCl and NaOH). Triplicate samples were prepared for the pH experiments, and duplicate samples were prepared for all other adsorption experiments. It should be pointed out that no peaks were detected in the HPLC spectra for potential degraded/transformed products of tetracycline. Solution-Phase 1H NMR. 1H NMR spectra of tetracycline (used as the received hydrate) in mixtures with model π-donor compounds (naphthalene, phenanthrene, and pyrene) and 1,4-dichlorobenzene as a non-π-donor control in chloroform-d (99.8% deuterium) were collected at room temperature using a Bruker-DRX 500 MHz spectrometer (Germany). To further test the deprotonation state of tetracycline on the complexation with PAH, two different tetracyclines were prepared by freeze-drying the saturated aqueous solution receiving no pH adjustment (pH 5.5) and pH adjustment with NaOH (pH 9.0), respectively. 1H NMR spectra were acquired for the obtained tetracyclines in mixtures with phenanthrene in cosolvents of chloroform-d and methanold4 (99.8% deuterium) (60:40, v:v). Methanol-d4 was used to overcome the low solubility of the alkaline tetracycline in pure chloroform-d.
Results and Discussion Adsorption Isotherms. Adsorption isotherms with the four carbonaceous adsorbents are presented in Figure 1 for tetracycline and Figure 2 for naphthalene. The data are fitted with the Freundlich sorption model: q ) KFCWn (weighed on 1/q), where q (mmol/kg) and CW (mmol/L) are the solidphase and aqueous-phase concentrations, respectively, at adsorption equilibrium, KF (mmol1-n Ln/kg) is the Freundlich affinity coefficient, and n (unitless) is the Freundlich linearity index. The model fitting parameters are summarized in Table S2 of the SI. The Freundlich model fits the adsorption data reasonably, and for all adsorbate/adsorbent combinations except naphthalene/AC, adsorption is highly nonlinear (departures of n from 1 imply nonlinear adsorption) within the tested concentration ranges. For all adsorbents, adsorption of tetracycline is much more nonlinear than adsorption of naphthalene, suggesting a more heterogeneous distribution of the adsorption (interaction) sites for tetracycline. In the present study, the obtained adsorption isotherms are used mainly for comparing the adsorption affinity between different adsorbate/adsorbent combinations. Figure 1a compares the adsorption of tetracycline between different adsorbents on unit mass basis. The adsorption affinity follows an order of SWNT > MWNT > AC > graphite. Within the tested concentration ranges, the adsorbent-tosolution distribution coefficient (Kd) is in the order of 104-106 L/kg for SWNT, 103-104 L/kg for MWNT, 103-104 L/kg for AC, and 103-105 L/kg for graphite. The observed Kd values of tetracycline with the two carbon nanotubes are greatly larger than the literature values reported on natural geosorbents, including soils (Kd ) 102-103 L/kg), humic substances (Kd ) 102-103 L/kg), and clay minerals (Kd ) 102-103 L/kg) (5, 12, 13, 18). The rapid growth in production and industrial applications of carbon nanomaterials has raised significant VOL. 43, NO. 7, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Adsorption isotherms of tetracycline plotted as solid-phase concentration (q) vs aqueous-phase concentration (CW) at adsorption equilibrium with different adsorbents: singlewalled carbon nanotubes (SWNT), multi-walled carbon nanotubes (MWNT), activated carbon (AC), and graphite. (a) q on unit mass basis. (b) q on unit surface area basis.
FIGURE 2. Adsorption isotherms of naphthalene plotted as solid-phase concentration (q) vs aqueous-phase concentration (CW) at adsorption equilibrium with different adsorbents: single-walled carbon nanotubes (SWNT), multi-walled carbon nanotubes (MWNT), activated carbon (AC), and graphite. (a) q on unit mass basis. (b) q on unit surface area basis. Data for SWNT and graphite are adopted from our previous study (27). concerns on the potential environmental impact of these materials (30-32). One of the concerns is that the strong adsorption affinity to carbon nanomaterials, as observed in the present work, could largely affect the environmental fate 2324
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and exposure of organic contaminants, with carbon nanotubes released into the aquatic or soil environments. Besides the environmental significance mentioned above, the extraordinarily strong adsorption affinity and capacity of tetracycline by carbon nanotubes make them a promising agent in water treatment. The point is supported by the fact that the two carbon nanotubes showed much higher (by 1-2 orders of magnitude) adsorption, especially at low solute concentrations, than AC, which is probably the most commonly used adsorbent for organic contaminants in water treatment. The large difference in adsorption affinity observed between SWNT and MWNT/AC was likely caused by molecular sieving effect. Tetracycline is bulky organic molecule (see critical volumes of compounds in Table S1 of the SI) and thus cannot access some of the innermost surfaces of MWNT. Moreover, the tested AC is highly microporous with the lowest average pore width (see Table 1) and has rigid pore structures, whereas in aqueous solution, carbon nanotubes form flexible microporous interstices (33) with averagely larger pore sizes (see average pore width values in Table 1). Therefore, AC is expected to show a more prominent molecular sieving effect than the carbon nonotubes, particularly when the adsorbate molecules are bulky. The molecular sieving effect of tetracycline can be better illustrated by comparing the surface area-normalized adsorption data among different adsorbents (results presented in Figure 1b). The normalized adsorption of tetracycline follows an order of graphite/SWNT > MWNT . AC, which correlates well with surface area accessibility of the adsorbents. The entire surface area of graphite and most surface area of SWNT are available for adsorption; however, AC has the highest microporosity and hence the lowest surface area accessibility to bulky tetracycline molecule due to the molecular sieving effect. Adsorbate size-dependent molecular sieving effect has been reported in previous studies in adsorption of organic compounds to multi-walled carbon nanotubes and a wood-derived charcoal (25, 34, 35). Figure 2a presents the adsorption data of naphthalene to the four adsorbents on unit mass basis. Clearly, naphthalene exhibits very different adsorption patterns from tetracycline (see Figure 1a). The adsorption affinity follows an order of AC > SWNT > MWNT > graphite. Upon surface area normalization of adsorbent, the order becomes AC > SWNT/ graphite > MWNT. The largest adsorption affinity of naphthalene to AC is likely caused by the pore-filling mechanism due to the closeness of the molecular size of naphthalene and the pore sizes of AC. In a previous study (34), the porefilling mechanism has also been proposed to account for the larger adsorption affinity of low molecular-sized compounds (benzene and toluene) to a highly microporous charcoal compared to graphite upon surface area normalization of adsorbent. To gain a more direct comparison of adsorption affinity between tetracycline and naphthalene, the adsorption data of these two compounds are plotted together for each individual adsorbent (SI Figure S2). It is obvious that tetracycline is adsorbed more strongly than naphthalene by SWNT, MWNT, and graphite, but an opposite trend is observed for AC. Adsorption Mechanisms. An important finding from the adsorption isotherms is that tetracycline can be adsorbed very strongly on the surface of carbonaceous materials if the molecular sieving effect is not in operation. Furthermore, compared with naphthalene, tetracycline exhibits pronouncedly stronger adsorption affinity to the two carbon nanotubes and graphite (see SI Figure S2) despite its much lower hydrophobicity, as measured by SW and KOW (values shown in Table S1 of the SI). Tetracycline has multiple polar/ ionizable functional groups, including phenol, alcohol, ketone, and amino, which makes the chemical relatively water-soluble but not very partitionable in a nonpolar hydrophobic phase (alkane-like structures). Previous studies
have invoked mechanisms of cation/anion exchange, cation bridging, and/or surface complexation (e.g., H-bonding) to explain the relatively strong sorption of tetracyclines by soils, iron/aluminum hydroxides, clay minerals, and humic substances (12-19). Within the tested pH of 5.7-6.0 in the present study, the zwitterion of tetracycline is predominated, and the surface of carbon nanotubes is also negatively charged mainly due to the deprotonated carboxyl groups (27). Therefore, cation exchange reactions, as well as surface complexation are expected to occur between the zwitterionic tetracycline molecules and the respective ionic/polar sites on the adsorbent surface. However, these mechanisms cannot be the major factors responsible for the strong adsorption of tetracycline on the carbon nanotubes for two main reasons. First, the adsorption affinity, measured by Kd, with SWNT and MWNT is greatly larger than that with sorbents having higher contents of ionic/polar sites (e.g., clay minerals and humic substances). Second, the strongest surface areanormalized adsorption is observed for graphite (Figure 1b), which contains pure carbon only (99.999% graphitized C, as provided by the manufacturer, and also verified in a separate elemental analysis) and is free of functionalities, and hence all the mechanisms mentioned above are not applicable for the adsorption to graphite. Therefore, the very strong adsorption of tetracycline to the carbonaceous adsorbents must have been caused primarily by strong interactions directly with the graphene surface. Based on our previous work and the literature, we propose the following mechanisms for the strong adsorptive interactions of tetracycline with the graphene surface: (1) van der Waals forces (permanent dipole-induced dipole forces and London dispersion forces); (2) π-π EDA interaction between the conjugated π-electron moieties and the graphene π-electrons; (3) cation-π bonding between the protonated amino group and the graphene π-electrons. The intensities of van der Waals forces of an adsorbed molecule are proportional to its contact surface area with the adsorbent and also the van der Waals index specific to the adsorbent surface (36). The graphene surface of carbonaceous adsorbents has a very high van der Waals index (graphite has a higher van der Waals index than paraffin or Teflon does) (36), and the tetracycline molecule has a large planar ring structure; therefore, strong van der Waals forces are likely to occur between the tetracycline molecule and the graphene surface of adsorbents. The π-π EDA interaction might be one of the most important nonhydrophobic adsorption driving forces for tetracycline. The conjugated enone structures of tetracycline molecule (see Figure S1 of the SI) can function as π-electronacceptors due to the strong electron-withdrawing ability of the ketone group, and hence interact strongly with the graphene surface (π-electron-donor) of carbonaceous adsorbents via π-π EDA interactions. Previous studies have suggested the π-π EDA interaction between quinonecontaining structures (π-electron-acceptors) in soil organic matter and PAHs (π-electron-donors) (37). More recently, the π-π EDA interaction has also been proposed for nitroaromatic compounds (π-electron-acceptors) with the graphene structures (π-electron-donors) of graphite, charcoal, and carbon nanotubes (25, 27, 34). Such specific interactions result in remarkably stronger adsorption of the nitroaromatic compounds relative to those non-π-acceptors (e.g., chlorinated benzenes) upon normalization of solute hydrophobicity. Another possible specific adsorption mechanism is the cation-π bonding between the protonated amino group on the ring C(4) (See SI Figure S1) and the graphene π-electrons. The amino group has a large pKa of 9.69 (SI Table S1) and is easily protonated under favorable environmental conditions. The cation-π bonding is dominated by the electrostatic
FIGURE 3. Effect of pH on distribution coefficient (Kd) for single-point adsorption of tetracycline to graphite (spiked at 4.8 × 10-3 mmol/L) and single-walled carbon nanotubes (SWNT) (spiked at 0.10 mmol/L) in 0.02 M NaCl. Averages of triplicates are shown with bidirectional error bars to represent standard deviations. force between the cation and the permanent quadrupole of the π-electron-rich aromatic structure and cation-induced polarization (38). Such bonding interaction between metal ions/protonated amino groups and π electron-rich structures has been found to be important for both chemical and biological systems (38-41). For example, the mechanism of cation-π interaction has been explored to explain the much higher selectivity for K+ over Na+ by the potassium channel in cells (38, 41). In more environmentally relevant studies (42-44), we proposed that the cation-π interactions with complexed transition metal ions and/or charged ammoniums are likely the reason for the enhanced uptake of PAHs on the surfaces of bacteria, biopolymer (phospholipids), and clay, respectively. Notably, the adsorbed tetracycline molecules should be oriented parallel to the graphene surface and form face-to-face complexes to maximize both van der Waals forces and π-π EDA interactions; however, because the ring C(4) is sp3-hybrided, the attached amino group can still effectively point to and interact with the graphene surface via cation-π bonding without interfering much with the face-to-face geometry of the complex. In contrast, the protons of the phenol, enol, and amide groups are coplanar with the tetracycline rings, and are thus prohibited from forming π-H bonds with the graphene surface (The proton needs to point to the plane of the aromatic ring perpendicularly in order to form a π-H complex, ref 45). The proposed mechanisms of π-π EDA interaction and cation-π bonding are supported by the observed pH effect on adsorption of tetracycline to graphite and to SWNT (presented in Figure 3). Given the experimental conditions, for graphite the Kd decreases from 6680 ( 80 L/kg (standard deviation calculated from triplicate samples) to 1600 ( 100 L/kg, by more than 4 times, over the tested pH range of 3.4-11.0. For SWNT, a similar trend of pH effect is observed, but with much stronger significance. The Kd decreases from 310 000 ( 10 000 L/kg to 18420 ( 90 L/kg, by approximately 17 times over a similar pH range. Because graphite is free of polar functionalities, the response of adsorption affinity to the change of pH must have been caused by pH-dependent VOL. 43, NO. 7, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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speciation of tetracycline. As a comparison, we have shown previously that changing the pH from 2 to 11 does not affect the adsorption of naphthalene, a nonpolar solute, to graphite (27). Increasing pH facilitates deprotonation of the charged amino group and the protonated enol groups (see structures presented in SI Figure S1) and hence weakens the electronacceptor ability of these moieties, therefore, suppressing the cation-π bonding and π-π EDA interaction with the graphene surface. Notably, the pH-mediated hydrophobicity change of tetracycline is very small and can be ruled out as the primary cause for the observed pH effect on adsorption to graphite. This is evidenced by the slight pH effect on tetracycline sorption to polyethylene beads (Aldrich) (Kd is lowered by only 12.8% over the pH range of 2.2-7.0, details not shown). Polyethylene is composed of the inert methylene structure, and therefore can only invoke hydrophobic effect in sorption. The similar pH dependency of SWNT to that of graphite indicates that the specific adsorption mechanisms (cation-π bonding, π-π EDA interaction) proposed for graphite might also govern the adsorption of tetracycline to SWNT. The larger pH effect observed on SWNT than on graphite is likely related to the difference in property and abundance of π-electron-rich sites on SWNT and on graphite. Solution-Phase Spectroscopic Studies. Placing a nucleus above or below an aromatic structure causes electronic shielding of the nucleus due to the “ring current” effect. Thus, the ring current-induced upfield chemical shifts of 1H/13C NMR can serve as a strong evidence for oriented complexation of the probed molecules with aromatic structures. In previous studies, the observed 1H/13C NMR upfield chemical shifts have been used to support the π-π EDA complexation between the π-electron-acceptor compounds and PAHs (πelectron-donors) (27, 34, 37), as well as the cation-π bonding between charged organic ammoniums and PAHs (43, 44). Prominent 1H NMR upfield shifts are observed for tetracycline in mixtures with π-electron-donor compounds in chloroformsup to 0.24 ppm for sCH3 of the amino group on C(4) with pyrene, and up to 0.36 ppm for sCH3 on C(6) with pyrene, wherein the shift magnitudes increase with π-donor strength of PAH (pyrene > phenanthrene > naphthalene) (Figure 4). The observed trends clearly demonstrate that tetracycline and PAH molecules form face-to-face complexes in chloroform, likely due to a combined effect of π-π EDA interaction and cation-π bonding. Thus, one can imagine that the adsorbed tetracycline molecules interact with the graphene surface of carbonaceous adsorbents in a similar fashion. It is worth noting that without the aid of directed forces (π-π EDA interaction, cation-π bonding) van der Waals forces alone are too weak to overcome the solvent effect for maintaining such a face-to-face geometry of tetracycline-PAH complexes. This is supported by the observation that 1,4dichlorobenzene, a non-π-donor control, induces only negligible 1H NMR upfield chemical shifts (
