Adsorption of tetracycline on singlewalled and multiwalled carbon nanotubes as affected by aqueous solution chemistry

Environmental Toxicology and Chemistry, Vol. 29, No. 12, pp. 2713–2719, 2010 ADSORPTION OF TETRACYCLINE ON SINGLE-WALLED AND MULTI-WALLED CARBON NANOTUBES AS AFFECTED BY AQUEOUS SOLUTION CHEMISTRY LIANGLIANG JI,y WEI CHEN,z JUN BI,y SHOURONG ZHENG,y ZHAOYI XU,y DONGQIANG ZHU,*y and PEDRO J. ALVAREZ§ yState Key Laboratory of Pollution Control and Resource Reuse, Nanjing University, Jiangsu 210093, China zTianjin Key Laboratory of Environmental Remediation and Pollution Control, Nankai University, Tianjin 300071, China §Department of Civil and Environmental Engineering, Rice University, Houston, Texas, USA (Submitted 12 May 2010; Returned for Revision 15 July 2010; Accepted 2 August 2010) Abstract—Carbon nanotubes have shown great potential as effective adsorbents for hydrophobic organic contaminants in watertreatment. The present study investigated the influence of aqueous solution chemistry on the adsorption of tetracycline to carbonnanotubes. Specifically, the effects of ionic strength (NaCl and CaCl2) and presence of Cu2þ ion (7.5 mg/L) or dissolved soil or coalhumic acids (50 mg/L) on adsorption of tetracycline to single-walled carbon nanotubes (SWNT), multi-walled carbon nanotubes(MWNT), and nonporous pure graphite as a model of the graphite surface were systematically estimated. The presence of humic acidssuppressed tetracycline adsorption on graphite and MWNT prominently, with stronger effects observed on graphite, but only slightlyaffected tetracycline adsorption on SWNT. The relatively large humic acid components could not readily access the small interstitialspaces of SWNT and thus were less competitive with tetracycline adsorption. The presence of Cu2þ ion increased tetracycline adsorptionto both SWNT and MWNT through the mechanism of cation bridging, with much larger effects observed on MWNT. This was probablybecause when compared with the Cu2þ ions complexed on the surface of SWNT, those on the surface of MWNT having largermesoporous interstices were more accessible to the relatively bulky tetracycline molecule. Increasing the ionic strength from 10 mM to100 mM decreased tetracycline adsorption on both SWNT and MWNT, which was attributed to electronic shielding of the negativelycharged surface sites. These results show that aqueous solution chemistry is important to tetracycline adsorption on carbon nanotubes.
Environ. Toxicol. Chem. 2010;29:2713–2719. # 2010 SETAC crucial issue for both their environmental applications and Tetracycline is one of the most commonly used antibiotics in livestock production for disease treatment and growth promo- Previous studies on sorption of antibiotics, such as tetracy- tion. Because most antibiotics are poorly metabolized and clines and sulfonamides, have mainly involved natural geo- absorbed by the treated animal body, large fractions are sorbents, including soils, humic substances, and clay minerals excreted through urine and feces as unmodified parent com- [12–16]. Sorption is primarily driven by the specific mecha- pound and consequently reach aquatic and soil environments nisms of cation exchange/bridging and surface complexation [1–4]. One of the concerns raised by environmental antibiotic reactions (H-bonding and other polar interactions) between the residues is the antibiotic resistance propagation in microorgan- multi-functionalities (amino, carboxyl, and phenol) of the anti- isms [5,6]. The removal of pharmaceutical antibiotics by con- biotic molecules and the corresponding interactive sites of the ventional water and wastewater treatment technologies is sorbents, while hydrophobic effect is only a minor driving force generally incomplete [7]. Thus, there is an increasing demand for sorption. A number of studies have also been performed to for the development of more effective technologies to treat such characterize adsorption of organic compounds on carbon nano- tubes [17–20]. These studies propose a common adsorption Carbon nanotubes have shown great promise for many mechanism of p-p electron coupling/stacking between the nanotechnology applications, including effective adsorbents graphite surface of carbon nanotubes and the p-electrons of for removal of undesirable organic chemicals in water treatment aromatic compounds such as polycyclic aromatic hydrocarbons [8–10]. Moreover, the rapid growth in industrial production and (PAHs) and chlorinated benzenes. However, reference papers use of carbon nanotubes has raised serious concerns over the on adsorption of emerging organic contaminants (antibiotics potential environmental impact of these materials [9,11]. For and hormones) to carbon nanotubes are still very limited [21– example, carbon nanotubes released to the aquatic environment 24]. It was recently reported that the surface of carbon nano- might play an important role in the fate, bioavailability, expo- tubes can strongly retain tetracycline because the enone struc- sure, and reactivity of organic contaminants due to the very tures and the protonated amino group of tetracycline strongly strong adsorption affinity and capacity of carbon nanotubes.
interact with the polarized electron-rich graphene structures of Thus, understanding the mechanisms and factors controlling the carbon nanotubes through p-p electron-donor-acceptor (EDA) adsorption of organic contaminants to carbon nanotubes is a interaction and cation-p bonding, respectively [21].
Aqueous solution chemistry (pH, ionic strength, and pres- ence of heavy metal ion or dissolved humic substances) is * To whom correspondence may be addressed expected to play an important role in adsorption of antibiotics to carbon nanotubes. Because antibiotics often have multifunc- Published online 9 September 2010 in Wiley Online Library tionalities susceptible to pH-mediated speciation reactions, changing pH can easily affect the adsorbate physiochemical two humic acids have been quantified in detail elsewhere by properties (charge and hydrophobicity) and, hence, the adsorp- elemental analysis, solid-state 13C nuclear magnetic resonance tive interactions on the carbon nanotube surface. It has been (NMR), acid-base titration, and Zeta potential measurement reported previously that increasing pH suppresses tetracycline [27]. In summary, the soil humic acids are mainly composed of adsorption on single-walled carbon nanotubes and graphite young materials of lignin, carbohydrates and peptides, and because the specific p-p EDA interaction and cation-p bonding oxidized charcoal, while the Fluka coal humic acids primarily are both impeded by dissociation of the protonated groups of consist of poly(methylene)-rich aliphatics with more aromatic tetracycline [21]. Likewise, coexisting heavy metal ions are C-O and higher negative surface charge.
able to complex with antibiotic functionalities, as well as the Nonporous, pure graphite (Aldrich) containing 99.999% surface functional groups of carbon nanotubes, and thus impact graphitized C (as provided by the manufacturer) was used as antibiotic adsorption on carbon nanotubes. Moreover, adsorp- received. Single-walled carbon nanotubes (SWNT) and multi- tion of antibiotics on carbon nanotubes can be prominently walled carbon nanotubes (MWNT) were purchased from the influenced by the presence of dissolved humic substances that Nanotech Port Company. Based on the information provided by the manufacturer, SWNT contained >90% (by volume) of It is quite common for organic contaminants to be present carbon nanotubes, and the content of SWNT with outer diam- together with a complex suit of dissolved humic substances, eter <2 nm was >50%; MWNT contained >95% of carbon heavy metals, and many other ionic species in contaminated nanotubes, and the sizes of the outer diameter of MWNT ranged water. For example, the concentration of dissolved humic from 10 to 30 nm; the length of both carbon nanotubes was substances in soil pore water can be as high as 100 mg/L between 5 and15 mm. The samples of SWNT and MWNT were [25]. Thus, to better understand the effect of aqueous solution treated to remove amorphous carbon by heating, and trace chemistry on adsorption is imperative for exploring carbon metals by sodium hypochlorite under sonication as described nanomaterials as effective adsorbents for the removal of anti- biotics in water treatment. However, to our knowledge, fewrelevant studies have been conducted thus far. In the present study, the batch technique was performed to systematically Surface elemental compositions of carbon nanotubes were evaluate the influences of ionic strength (NaCl and CaCl2) and determined using an X-ray photoelectron spectrometer (Perkin presence of cosolute of heavy metal ion (Cu2þ) or dissolved Elmer PHI 550 ESCA/SAM). Zeta potential (z) of carbon humic acids on tetracycline adsorption to both single-walled nanotubes suspended in 1 mM NaCl solution was measured and multi-walled carbon nanotubes. Sodium and calcium are at different pH (equilibrated for 2 d) using a Zeta potential predominated cations in soil systems. Copper was selected as a analyzer (Zeta PALS, Brookhaven Instruments). Surface areas representative heavy metal that is commonly present in aquatic and pore size distributions were characterized by N2 adsorption/ desorption on a Micrometrics ASAP 2020 (MicromeriticsInstruments) apparatus at À1968C (77 K). The surface area was determined by the Brunauer–Emmett–Teller (BET)method, and the pore size distribution profile was calculated by the slit Density Function Theory (DFT).
Tetracycline (99%, hydrate) was purchased from Interna- tional Laboratory and was used as received. Chemical structure and the three acidic dissociation constants (pKas) are given in The experiments were conducted using a batch approach Figure 1. Soil humic acids were extracted from a soil collected developed in our previous studies [21,22]. Duplicate samples from Shenyang, Liaoning Province in northeast China using were performed for the isotherm experiments, and triplicate standard methods [26]. Coal humic acids were purchased from samples were performed for the ionic strength experiments. To Fluka with a further deashing treatment by HCl/HF using the prepare bulk solution of dissolved soil humic acids (DSHA) or same literature method [26]. Structural characteristics of the dissolved coal humic acids (DCHA), 50 mg of humic acids weredissolved in 5 ml of 0.1 M NaOH, and then mixed with distilledwater to reach an apparent concentration of 50 mg/L, corre-sponding to 25 mg-C/L for DSHA and 22 mg-C/L for DCHA, asmeasured by a total organic carbon (TOC) analyzer (TOC5000A). The humic acid solution was adjusted to pH 6.0 with0.1 M HCl, followed by filtration through a 0.45 mm membrane.
The obtained humic acid solution was then used to preparean operational background solution containing 0.02 M NaCl.
Single-point adsorption of the humic acids on SWNT, MWNT,and graphite was measured separately on the basis of TOC.
A weighed amount of CuCl2 was added to 0.02 M NaCl solutionto prepare a background solution containing 7.5 mg/L Cu2þ.
Single-point adsorption of Cu2þ on SWNT and MWNT wasmeasured separately using an atomic absorption spectrometer(Thermo Scientific Electro GF95Z). Aqueous background sol-utions of NaCl and CaCl2 at concentrations of 0.01 M, 0.02 M,0.05 M, and 0.1 M were also prepared for the ionic strengthexperiments.
Fig. 1. Chemical structure of tetracycline. The regions framed by dashed To initiate the adsorption experiments, a 40-ml amber lines represent the structural moieties associated with the three acidicdissociation constants pKa1, pKa2, and pKa3 adopted from Tolls [4].
vial with polytetrafluoroethylene-lined screw cap received a Tetracycline adsorption on carbon nanotubes weighed amount of adsorbent (30 mg of graphite and 10 mg of carbon nanotubes), followed by aqueous stock solution of tetracycline and a full volume of background solution. The pH of background solution was preadjusted by considering the acid/base-buffering ability of the adsorbent. The samples werecovered with aluminum foil to avoid possible photodegradation of tetracycline [29] and mixed end-over-end at room temper-ature for 3 d. The time was sufficient to reach apparent adsorption equilibrium (no further uptake) based on the adsorp-tion kinetics determined in our previous study [30]. Two replicates were used to characterize adsorption isotherms, and three replicates were used in the ionic strength experiments.
After centrifugation at 2,000 rpm for 10 min, tetracycline in the aqueous phase was analyzed directly by high-performance Fig. 2. Pore size distributions of single-walled carbon nanotubes (SWNT) liquid chromatography (HPLC) with an ultraviolet detector and multi-walled carbon nanotubes (MWNT) with and without the presence using a 4.6 Â 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) witha wavelength of 360 nm. To account for possible solute loss from processes other than adsorbent sorption (sorption to glass-ware and septum), calibration curves were obtained separately Adsorption isotherms. Adsorption isotherms of tetracycline from controls receiving the same treatment as the adsorption on SWNT, MWNT, and graphite are presented in Figure 4. The samples but no adsorbent. Calibration curves included at least adsorption data are fitted to the Freundlich model, q ¼ KFCn, by 14 standards over the tested concentration ranges. Based on the nonlinear regression (weighed on 1/q), where q (mmol/kg) and obtained calibration curves, the adsorbed mass of tetracycline Ce (mmol/L) are the adsorbed concentration and aqueous con- was calculated by subtracting mass in the aqueous phase from centration, respectively, at adsorption equilibrium; KF (mmol1À mass added. It should be pointed out that no peaks were detected Ln/kg) is the Freundlich affinity coefficient; n (unitless) is the in the HPLC spectra for possible degraded or transformed Freundlich linearity index. The fitting parameters are summar- products of tetracycline. The sample pH was measured at the ized in Table 2, along with the upper and lower boundary values end of batch experiments and was 5.0 Æ 0.2.
of the adsorption distribution coefficient (Kd) measured withinthe examined concentration ranges. The Freundlich model fitsthe adsorption data reasonably except for the graphite-only condition. For all adsorption data, the linearity index (n) is much smaller than 1, reflecting the high adsorption nonlinearity.
The information of surface elemental compositions and BET Adsorption of tetracycline on the three carbonaceous adsorbents surface areas of the adsorbents is summarized in Table 1. The is very strong. Within the examined concentration ranges, the adsorbents are predominantly graphitized C (>91%, dry weight Kd is in the order of 104 to 106 L/kg for SWNT, 103 to 105 L/kg based) on the surfaces; however, a relatively high content of O- for MWNT, and 103 to 105 L/kg for graphite. Mechanisms of containing groups still exist on the surfaces of carbon nanotubes strong interactions (van der Waals forces, p-p EDA interac- (7.25% for SWNT and 8.34% for MWNT). The pore size tions and cation-p bonding) with the graphite surface have been distribution profiles (Fig. 2) demonstrate that MWNT contains proposed to account for the high adsorption affinity of tetracy- larger portions of mesopore volumes than SWNT. The Zeta cline on carbon nanotubes and graphite [21]. The graphite potential (z) of SWNT and MWNT as a function of pH ispresented in Figure 3. The two carbon nanotubes show similarz-pH relationships and are both negatively charged under the tested pH conditions, resulting from dissociation of the acidic surface functional groups. Graphite is expected to have no net surface charge due to the absence of dissociable functionalities.
Table 1. Surface elemental compositions (dry-wt based) and surface areasof single-walled carbon nanotubes (SWNT), multi-walled carbon nanotubes a BDL ¼ below detectable level; ND ¼ not determined.
Fig. 3. Zeta potential (z) of single-walled carbon nanotubes (SWNT) and b Determined by X-ray photoelectron spectroscopy (XPS).
multi-walled carbon nanotubes (MWNT) as a function of pH. Each data c Determined by N2 adsorption using the Brunauer–Emmett–Teller (BET) point was based on three (MWNT) or four (SWNT) replicate samples.
Bidirectional error bars represent standard deviations.
c Gra phit
Fig. 4. Adsorption isotherms plotted as adsorbed concentration (q) versus aqueous-phase concentration (Ce) at equilibrium pH of approximately 5.0 underdifferent aqueous solution chemistry conditions: in 0.02 M NaCl only, in the presence of dissolved soil humic acids (DSHA) (initially at 50 mg/L) in 0.02 M NaCl,in the presence of dissolved coal humic acids (DCHA) (initially at 50 mg/L) in 0.02 M NaCl, and in the presence of Cu2þ ion (initially at 7.5 mg/L) in 0.02 M NaCl.
(a) Single-walled carbon nanotubes (SWNT). (b) Multi-walled carbon nanotubes (MWNT). (c) Graphite.
surface has a very high van der Waals index, and the tetracy- 90 and 50%, respectively. Nonetheless, tetracycline adsorption cline molecule has a planar geometry, giving rise to strong van on SWNT is not much affected by the presence of dissolved der Waals forces in adsorption on carbonaceous adsorbents.
humic acids. Despite the structural differences, DSHA and Due to the strong electron-withdrawing ability of the ketone DCHA have similar effects on tetracycline adsorption. Under group, the enone structures of tetracycline (Fig. 1) are consid- the tested pH conditions (5.0 Æ 0.2), humic acid molecules are ered p-electron-acceptors and may interact strongly with negatively charged due to the dissociation of carboxyl groups, the polarized electron-rich regions (p-electron-donor) on the and thus may invoke repulsive electrostatic interactions with the graphite surface of carbonaceous adsorbents through p-p EDA same negatively charged carbon nanotube surface. However, interactions. Additionally, the protonated amino group of tet- the various structural components in humic acids are expected racycline may facilitate cation-p bonding with p-electrons on to interact strongly with the graphite surface through van der the graphite surface of carbon nanotubes and graphite.
Waals forces and other specific mechanisms such as p-p Effect of dissolved humic acids. It is evident from Figure 4 electron coupling and H-bonding. As a result, the two dissolved that the presence of DSHA or DCHA decreases tetracycline humic acids show high adsorption affinity to the carbonaceous adsorption on graphite and MWNT markedly, up to about adsorbents, consequently causing competitive effect on tetra- Table 2. Freundlich model parameters KF and n Æ standard deviation and adsorption distribution coefficient (Kd) for adsorption isotherms of tetracycline on single-walled carbon nanotubes (SWNT), multi-walled carbon nanotubes (MWNT), and graphite under different aqueous solution chemistry conditionsa a DSHA ¼ dissolved soil humic acids at an initial concentration of 50 mg/L; DCHA ¼ dissolved coal humic acids at an initial concentration of 50 mg/ L; Cu2þ ¼ Cu2þ at an initial concentration of 7.5 mg/L.
b In background solution of 0.02 M NaCl with presence of additional solute (if applied) as noted.
c Reported as upper and lower boundary values within the examined concentration ranges.
Tetracycline adsorption on carbon nanotubes cycline adsorption. The Kd on SWNT, MWNT, and graphite adsorbent pore size distribution by occupying the large pores.
measured from single-point adsorption is 1,500 Æ 200 L/kg As a result, the heterogeneity of adsorption sites is lessened and, (standard deviation based on four replicates), 1,100 Æ 200 L/ in turn, adsorption becomes more linear.
kg, and 120 Æ 20 L/kg for DSHA, and 2,000 Æ 100 L/kg, Effect of Cu2þ ion. The effect of Cu2þ ion on tetracycline 900 Æ 300 L/kg, and 64 Æ 5 L/kg for DCHA, respectively. Con- adsorption to the three adsorbents is also shown in Figure 4. In sistent results were reported in a previous study [31] that the presence of Cu2þ, adsorption to MWNT is doubled; how- coadsorption of humic acids suppresses adsorption of aromatic ever, adsorption increases only slightly on SWNT and keeps compounds on multi-walled carbon nanotubes, and the sup- nearly constant on graphite with the presence of Cu2þ ion.
pressive effects negatively correlate with adsorbent surface area Recently, it has also been reported that phenol adsorption on N- doped carbon nanotubes is facilitated by preadsorption of Cd2þ One may argue that the suppressed tetracycline adsorption ion, which is attributed to the alleviated repulsive interaction on MWNT and graphite is due to competitive complexation of between phenol and the adsorbent surface due to Cd2þ com- tetracycline with the free dissolved humic acids in aqueous plexation with the surface O-containing groups [32]. In contrast, solutions. This seems reasonable, considering that humic sub- heavy metal-induced suppressive effects have been shown for stances bind the tetracycline molecule very strongly through a adsorption of organic compounds on black carbon (wood-made variety of specific mechanisms, including ligand exchange and charcoal and crop residue-burning ash) [33,34]. For example, complexation reactions such as H-bonding [13,27]. However, the presence of Cu2þ ion at 50 mg/L decreases adsorption this hypothesis can be ruled out by comparing the sorption ratios of both polar (2,4-dichlorophenol) and nonpolar compounds of tetracycline on the carbonaceous adsorbents versus on the (1,2-dichlorobenzene and naphthalene) on highly microporous dissolved humic acids as single sorbents. Graphite exhibits the wood-made charcoals up to 30 to 60%, as measured by strongest humic acid-suppressed adsorption of tetracycline, and changes in Kd [33]. It is proposed that Cu2þcomplexation with thus is used as an example for the comparison. According to our the surface functional groups forms hydration shells of dense previous study [27], the Kd for tetracycline sorption to the solid- water to directly compete with the organics for adsorption state humic acids (surrogates for the dissolved form) is in the order of 103 L/kg, which is up to two orders of magnitude lower It is well recognized that coadsorption of multivalent metal than the Kd for tetracycline adsorption on graphite (Table 2).
ions (Cu2þ, Al3þ, and Fe3þ) can increase tetracycline sorption The difference in magnitudes would be even larger for the to humic substances and mineral surfaces considerably through carbon nanotubes. Moreover, the difference in adsorption ratio cation bridging between the metal ion and tetracycline and between the two sorbents would become even larger after taking sorbent ligand groups [35,36]. Under the tested pH conditions into account the sorbent amount applied (on a mass basis, 30 mg (5.0 Æ 0.2), tetracycline is predominated by the zwitterions of graphite and maximum 2 mg of dissolved humic acids which contain deprotonated hydroxyl group and amide group without abatement from adsorption to graphite). Therefore, to enable Cu2þ ion coordination; in parallel, the surface acidic compared with adsorbed humic acids, the competitive effect functional groups (carboxyl and hydroxyl) of carbon nanotubes caused by dissolved humic acids on tetracycline adsorption to can strongly bind Cu2þ ions through ligand-exchange reactions.
the carbonaceous adsorbents is considered negligible.
Hence, ternary complexes are expected to form between Cu2þ The discrepancies of suppressed tetracycline adsorption ions and tetracycline and carbon nanotube functional groups, between the three carbonaceous adsorbents can be well resulting in Cu2þ-enhanced tetracycline adsorption on the explained by the accessibility of adsorption sites for humic carbon nanotubes. In agreement with the cation bridging mech- acids regulated by adsorbent porosity. The entire surface area of anism, the presence of Cu2þ ion causes negligible effect of nonporous graphite should be available to all sized humic acid tetracycline adsorption on graphite because it is free of surface components. The available adsorption sites of carbon nanotube complexing functionality. Additionally, similar to the two bundles mainly include the external surface and the interstitial dissolved humic acids, the presence of Cu2þ ion also decreases and groove spaces between individual carbon nanotubes. How- the nonlinearity of tetracycline adsorption on the three adsorb- ever, a large portion of the surface area (especially that asso- ents, but in much less degrees (see the Freundlich n values in ciated with micropores) of carbon nanotubes is expectedly Table 2). This is probably because the distribution of adsorption inaccessible to the large-sized humic acid molecules due to sites for tetracycline becomes less heterogeneous when tetra- size exclusion. The pore size distribution information (Fig. 2) cycline and/or carbon nanotube functional groups coordinate indicates that MWNT is less microporous than SWNT. Hence, the percentage of surface area available for humic acid adsorp- However, the stronger Cu2þ-enhanced tetracycline adsorp- tion is higher on MWNT than on SWNT. A better illustration of tion observed on MWNT than on SWNT cannot be explained by the proposed mechanism can be obtained by comparing surface the cation bridging mechanism alone. Notably, the binding area-normalized adsorption of humic acids between the three affinity of Cu2þ to SWNT is higher than that to MWNT; adsorbents. Based on the single-point data, the normalized the measured Kd of Cu2þ is 3,900 Æ 300 L/kg (based on adsorption of the two humic acids is ordered as follows: graph- single-point adsorption data with five replicates) for SWNT ite >> MWNT > SWNT, which is in accordance with the and 1,570 Æ 80 L/kg for MWNT. Additionally, the differences observed suppressive effects on tetracycline adsorption. It is in surface area (up to 10%) and pore size distribution between also interesting to note that the presence of DSHA or DCHA the pristine carbon nanotubes and the Cu2þ-complexed decreases the nonlinearly of tetracycline adsorption on the three carbon nanotubes are too small to cause any noticeable effect adsorbents (reflected by the enhanced Freundlich n values, on tetracycline adsorption. It is proposed that the stronger Table 2) in the same order as the suppressive effects on degree of Cu2þ-enhanced adsorption on MWNT is due to the adsorption affinity. To take graphite as an example, the Freund- larger mesoporous interstices of MWNT (see pore size distri- lich n value is increased remarkably from 0.074 to 0.40 by bution data in Fig. 2); therefore, the Cu2þ ions complexed on the coadsorption of DSHA. Humic acid adsorption on the carbona- surface of MWNT are more accessible to the relatively bulky ceous adsorbents blocks certain surface sites and/or narrows the tetracycline molecules. More research is needed to verify the c Graphite
Fig. 5. Effect of ionic strength (NaCl, CaCl2) on distribution coefficient (Kd) for single-point adsorption at equilibrium pH of approximately 5.0. (a) Single-walledcarbon nanotubes (SWNT). (b) Multi-walled carbon nanotubes (MWNT). (c) Graphite. Tetracycline was spiked at 0.10 mmol/L for SWNT, 0.027 mmol/L forMWNT, and 0.0059 mmol/L for graphite. Error bars represent standard deviations from triplicate samples.
underlying mechanisms for the different Cu2þ effects on tetra- ionic strength effects observed on SWNT than on MWNT can cycline adsorption between SWNT and MWNT.
be attributed to the higher surface charge of SWNT at the Effect of ionic strength. Figure 5 displays the effect of ionic tested pH (see the Zeta potential results presented in Fig. 3).
strength (NaCl, CaCl2) on tetracycline adsorption on the three However, when compared with other adsorptive interactions carbonaceous adsorbents. Several trends are evident for SWNT (van der Waals forces, p-p EDA and cation-p bonding), the and MWNT. First, tetracycline adsorption decreases with the electrostatic forces should be considered only a minor cause for ionic strength (up to 4.5 times, as measured by changes in Kd).
tetracycline adsorption on carbon nanotubes. This is corrobo- Second, given the same ionic strength, tetracycline adsorption is rated by the fact that, after normalization to adsorbent surface stronger with NaCl than with CaCl2. Third, the above-men- area, SWNT exhibits even slightly lower adsorption than tioned effects are more pronounced on SWNT than on MWNT.
charge-free graphite as shown in our previous study [21].
An increase in ionic strength would interfere with the electro-static interactions between the cationized amino group oftetracycline zwitterions (predominated under the tested pH conditions) and the deprotonated carboxyl groups of carbon Previous studies show that carbon nanotubes are promising nanotubes, due to electronic screening of the surface charge special adsorbents for the removal of pharmaceutical antibi- sites by the added cation (Naþ and Ca2þ). On the other hand, no otics, including tetracycline from water. The present study clear trend of ionic strength effects is observed for graphite indicates that aqueous solution chemistry (dissolved humic because it has no net surface charge. In previous studies [16,37], acids, Cu2þ ion, and ionic strength) plays an important role a similar mechanism that Naþ or Ca2þ ions compete for in tetracycline adsorption on carbon nanotubes. The dissolved negatively charged sites has been proposed to explain the ionic humic acids (50 mg/L) inhibit tetracycline adsorption on strength effects observed on sorption of antibiotics (dodecyl- MWNT prominently, but only slightly affect tetracycline piridinium, sulfachloropyridazine, tylosin, and oxytetracycline) adsorption on SWNT; the difference could be attributed to to minerals and soils. In contrast with tetracycline, much the accessibility of adsorption sites for humic acids regulated by smaller and inconsistent ionic strength effects have been shown adsorbent porosity. Compared with SWNT, tetracycline adsorp- for adsorption of nonionic/anionic compounds such as naph- tion on MWNT is more significantly enhanced by the presence thalene and sulfamethoxazole on carbon nanotubes [22,38].
of Cu2þ. The larger mesoporous interstices of MWNT could The results are understandable because the mechanism of make the Cu2þ ions complexed on the surface more accessible attractive electrostatic forces is not applicable for the adsorption to the relatively bulky tetracycline molecule. Increasing the of these compounds.Compared with the monovalent Naþ ion, ionic strength from 0.01 M to 0.1 M (NaCl or CaCl2) consis- the bivalent Ca2þ ion causes larger electronic screening effect tently decreased tetracycline adsorption on SWNT and MWNT, and, hence, greater suppressed tetracycline adsorption on car- due to electronic shielding of the negatively charged surface bon nanotubes. The cation bridging mechanism proposed for the Cu2þ-enhanced adsorption is negligible for Ca2þ, because In addition to the potential application of carbon nanotubes of its much lower complexing ability than Cu2þ. The stronger as special adsorbents in water treatment, the present study Tetracycline adsorption on carbon nanotubes implies that the accidental or incidental release of carbon 17. Chen W, Duan L, Zhu D. 2007. Adsorption of polar and nonpolar organic nanotubes to the environment could greatly affect the bioavail- chemicals to carbon nanotubes. Environ Sci Technol 41:8295–8300.
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