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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)
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 . 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 .
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
. 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 . 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
. 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 À1968
C (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 . 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 . 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 .
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  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 . 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 . 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.
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  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 . 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 . 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 , 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
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 .
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
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