Removal of ciprofloxacin from aqueous solution using long TiO2 nanotubes with a high specific surface area

Article abstract of DOI:10.1039/c5ra17956d

Long TiO₂ nanotubes (TNs) were successfully prepared by the reaction of TiO₂ and NaOH. The raw materials were treated by stirring, ion exchange, centrifugation, and freeze-drying, and then the target TNs was synthesized. Anatase TNs were obtained by calcinating the TNs at 823 K for 4.5 h. The TNs were characterized by Brunauer-Emmett-Teller surface area analysis, X-ray diffraction analysis, scanning electron microscopy, transmission electron microscopy, and X-ray photoelectron spectrometry. The results indicated that the TNs had a larger specific surface area (ca. 160 m² g^-1) and pore volume (ca. 0.6 cm³ g^-1) than the commercial product P25. The adsorption of ciprofloxacin onto the TNs was compared with their adsorption onto P25. The adsorption isotherm, kinetics, and regeneration performance were investigated. The experimental results indicated that the maximum adsorption capacity of the TNs and P25 was 26.38 and 5.32 mg g^-1, respectively, and their adsorption behavior was better fitted by the Langmuir model than by the Freundlich model. The kinetic regression results showed that the adsorption kinetics were more accurately represented by a pseudo-second-order model than by a pseudo-first-order model; the rate of the pseudo-second-order reactions on P25 and the anatase TNs were 0.0442 and 0.27463 min^-1, respectively. After adsorption, the TNs had better regeneration properties than P25 under UV irradiation at 500 W for 3 h in 5 mL of aqueous solution. These results show that long TNs have a better adsorption capacity and regeneration properties than P25. This study provides a green method for the removal of organic pollutants by combining enrichment by adsorption with photocatalytic degradation.

Full text of DOI:10.1039/c5ra17956d

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Removal of ciprofloxacin from aqueous solution
using long TiO2 nanotubes with a high specific
surface area†
Cite this: RSC Adv., 2016, 6, 3625
a
a
c
b
*
*
Kai Zheng, Xingye Zheng, Fei Yu and Jie Ma
Long TiO2 nanotubes (TNs) were successfully prepared by the reaction of TiO2 and NaOH. The raw
materials were treated by stirring, ion exchange, centrifugation, and freeze-drying, and then the target
TNs was synthesized. Anatase TNs were obtained by calcinating the TNs at 823 K for 4.5 h. The TNs
were characterized by Brunauer–Emmett–Teller surface area analysis, X-ray diffraction analysis, scanning
electron microscopy, transmission electron microscopy, and X-ray photoelectron spectrometry. The
results indicated that the TNs had a larger specific surface area (ca. 160 m2
g
ꢀ1) and pore volume (ca. 0.6
cm3 ꢀ1) than the commercial product P25. The adsorption of ciprofloxacin onto the TNs was compared
g
with their adsorption onto P25. The adsorption isotherm, kinetics, and regeneration performance were
investigated. The experimental results indicated that the maximum adsorption capacity of the TNs and
P25 was 26.38 and 5.32 mg gꢀ1, respectively, and their adsorption behavior was better fitted by the
Langmuir model than by the Freundlich model. The kinetic regression results showed that the adsorption
kinetics were more accurately represented by a pseudo-second-order model than by a pseudo-first-
order model; the rate of the pseudo-second-order reactions on P25 and the anatase TNs were 0.0442
and 0.27463 minꢀ1, respectively. After adsorption, the TNs had better regeneration properties than P25
under UV irradiation at 500 W for 3 h in 5 mL of aqueous solution. These results show that long TNs
have a better adsorption capacity and regeneration properties than P25. This study provides a green
method for the removal of organic pollutants by combining enrichment by adsorption with
photocatalytic degradation.
Received 3rd September 2015
Accepted 4th December 2015
DOI: 10.1039/c5ra17956d
www.rsc.org/advances
The detection of CIP in the atmosphere, water and soil
presents some health concerns. The discharge of wastewater
1. Introduction
containing CIP into aquatic environments may lead to antibi-
otic resistance within native bacterial populations. Fluo-
roquinolones are one of the most important types of antibiotic
with respect to annual sales and the variability of the drugs.13,14
They are effective in treating infections caused by all kinds of
bacteria and are regarded as the antibiotic of last resort when
other treatments are ineffective. CIP is a uoroquinolone
derivative and is a principal metabolite of enrooxacin. CIP is
present in different forms depending on the solution pH. The
main species of CIP include negative ions, positive ions, and
amphoteric ions. The form of CIP can affect many of its prop-
erties, such as adsorption in soils, photolysis, and its activity in
the target species.15–17
Adsorption and photocatalytic technology have been widely
utilized to remove CIP from aqueous solutions. One of the
advantages of adsorption is its low cost. However, the adsorp-
tion process is incomplete as a result of the heterogeneity of the
adsorbents and the difficulty in completely decomposing them.
Adsorbed CIP has been widely found in treated water bodies.18
CIP on adsorbents applied to the land is not efficiently
decomposed and may later be released into water bodies. As
Ciprooxacin (CIP) is a broad-spectrum quinolone antibiotic
and is widely used in human and veterinary medicine.1 CIP is
added to poultry feed to guard against disease during the growth
period; however, the uorinated functional groups in CIP are not
completely metabolized.2,3 More than 10 000 tons of antibiotics
were used in Europe in 1999, with farm animals and therapeutic
drugs accounting for 35 and 29%, respectively; this represents
a decrease of 50% since 1997. On release into the environment,
CIP is difficult to degrade4 and has been detected in surface
waters and effluent-dominated systems in the USA,5 Canada,6–8
and Europe at concentrations from ng Lꢀ1 to mg Lꢀ1
.
9–12
aDepartment of Environmental Engineering, Nanjing Institute of Technology, Nanjing,
211167, P. R. of China
bState Key Laboratory of Pollution Control and Resource Reuse, School of
Environmental Science and Engineering, Tongji University, 1239 Siping Road,
Shanghai 200092, P. R. of China. E-mail: fyu@vip.163.com; Tel: +86 21 6598 1831
cCollege of Chemistry and Environmental Engineering, Shanghai Institute of
Technology, Shanghai 2001418, P. R. of China. E-mail: jma@tongji.edu.cn; Tel: +86
21 6087 3182
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c5ra17956d
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adsorbents have a nite capacity for pollutant molecules, it is adsorption as a result of their larger specic surface area (SSA)
necessary either to regenerate or dispose of the adsorbents aer and could be used to efficiently adsorb and remove pollutants.
use.
The TNs also had an excellent catalyzing performance and could
Organic pollutants can be removed or destroyed by photo- be used to both mineralize pollutants and regenerate the
catalytic oxidation. The photocatalytic activity depends on the adsorbent. This method provides an effective approach for the
ability of the catalyst to create electron–hole pairs, which efficient removal of pollutants from contaminated waters.
generate free radicals (e.g. hydroxyl radicals, cOH) that undergo
secondary reactions. However, photocatalytic oxidation has not
been widely applied in practical wastewater treatment. The
removal of pollutants by the photocatalytic oxidation of TiO2
2. Experimental
2.1 Materials and chemicals
has the disadvantage that the existing particles and chromi-
nance from the pollutants may cause the refraction, reection
and scattering of light. In addition, the rate of catalysis of the
photocatalyst is much greater than the adsorption and diffusion
speed, which may lead to further electron–hole pair recombi-
nation during photocatalysis. Traditional TiO2 photocatalysts
and existing photocatalytic technology have a low catalytic
efficiency, high energy consumption, and high operational
costs.19 It is still challenging to extend the application of pho-
tocatalytic oxidation technology to the removal of organic
pollutants.
We divided traditional adsorption–catalysis into two sepa-
rate processes. The pollutants were rst enriched on the
adsorbent and were then placed in clear water to catalyze aer
preferential adsorption. This concentrated the pollutants and
facilitated their mineralization so that the catalyst or adsor-
bents were regenerated and the contaminants were removed via
mineralization.
All chemicals were of analytical-reagent grade and were
purchased from Sinopharm Chemical Reagent Co. Ltd
(Shanghai, China) and used without further purication. All
solutions were prepared using deionized water.
2.2 Preparation of TiO2 and TNs
TiCl4 (25 g, purity 99%) was added to 2 L of deionized water with
2 mL of concentrated sulfuric acid. Aer the TiCl4 had been
completely hydrolyzed, 120–140 mL of 17% ammonia were
added dropwise to give a pH between 7.0 and 7.5. The raw
mixture was ltered and rinsed with deionized water to remove
NH4Cl until the leaching liquid did not produce a white
precipitate with 0.2 mmol Lꢀ1 Ag2SO4. The ltered residue was
successively desiccated at 373 K for 2 h and vacuum-dried at 353
K at a pressure of 10ꢀ1 Pa for 3 h to give Ti(OH)4. The anatase
crystal form of TiO2 was obtained by heating Ti(OH)4 at 823 K
for 4.5 h.
A 3 g mass of anatase titanium dioxide was added to 350 mL
of 10 mol Lꢀ1 NaOH solution and the mixture was stirred
ultrasonically for 2 h and then magnetically at 200 rpm for 2 h.
The solution and a magnetic rotator were then placed in a 500
mL Teon reactor. The reactor was placed into a heater with
a magnetic stirring apparatus (RCT Basic, IKA Corporation,
Germany). The reactor was operated at about 600 rpm at 403 K
for 20–24 h.
Sodium titanate was obtained by centrifugation and then
rinsed ve times until the pH of washing solution was 11.
Hydrochloric acid (0.5 mol Lꢀ1) was then added in batches to
adjust the pH of the solution to 2; this was maintained for 3–4 h
to exchange hydrogen ions and with the sodium ions in solu-
tion. The mixture was then rinsed ve times until the pH of the
washing solution was 6.8–7.0 and it was then ltered to give the
hydrogen titanate nanotubes. The residue was dried under
vacuum. Crystals of long TNs were obtained at 823 K aer 4.5 h.
Many synthetic routes for the various morphologies and
crystallite phases of TiO2 have been reported previously. Among
the most commonly used methods are sol–gel methods,20
hydrothermal reactions,21 solvothermal reactions,22 anodic
oxidation,23 the use of a hard template,24 and direct oxidation.25
Titanium dioxide has been used for the degradation of CIP via
its photocatalysis properties under UV irradiation.26 However,
the surface area and pore volume of commercial TiO2 are only
about 50 m2 gꢀ1 and 0.1 cm3 gꢀ1, respectively. This low surface
area and pore volume restrict its adsorption and limit its
performance as a catalyst. To overcome these shortcomings, the
structure of TiO2 needs to be improved to enhance its adsorp-
tion capacity and catalyzing efficiency.
The use of TiO2 nanotubes improves the catalytic perfor-
mance. The most commonly used preparation methods are:
crystallization of amorphous anodized TiO2 nanotubes at low
temperature;27 electrochemical etching and hydrothermal
synthesis;28 and the treatment of TiO2 particles with NaOH.29
However, these procedures are time-consuming, complicated,
2.3 Batch adsorption experiments
and expensive and are not viable on an industrial scale. A cost- The concentration of CIP was determined by colorimetry at the
effective, large-scale preparation method for a stable, high maximum absorbance (lmax ¼ 275 nm). A calibration graph was
surface area, mesoporous form of TiO2 still needs to be plotted to obtain the absorbance–concentration prole of CIP.
developed.
At high concentrations of CIP, the dye samples were diluted
We report here the synthesis of long TiO2 nanotubes (TNs) before the absorbance measurements. The concentration of the
with a high surface area (160 m2 gꢀ1) using a hydrothermal CIP in the solution was determined by the Beer–Lambert law.30
method with stirring. The mechanical force-driven stirring Batch adsorption experiments were conducted in 50 mL glass
process synchronously improved the diffusion and surface bottles with 20 mg of the adsorbent and 40 mL of a CIP solution
reaction rate of titanate nanocrystal growth in the solution with different initial concentrations of 5–50 mg Lꢀ1. The pH of
phase, producing long TNs. The TNs had outstanding the solution was adjusted to ca. 7.0 with HCl or NaOH solutions.
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The timing of the adsorption period started as soon as the
solution was poured into the bottle. Sample bottles were
agitated on a shaker (TS-2102C, Shanghai Tensuclab Instru-
ments Manufacturing Co. Ltd, China) operated at a constant
temperature of 25 ꢁC and 150 rpm for 24 h to achieve adsorption
equilibrium. All adsorption experiments were conducted in
duplicate and only the mean values are reported. The maximum
deviation for the duplicates was usually <5%. Aer adsorption
equilibrium had been achieved, the CIP concentrations of the
solutions were measured using a spectrophotometer (UV759UV-
VIS, Shanghai Precision & Scientic Instrument Co. Ltd).
Kinetic studies were performed at a constant temperature of 25
ꢁC and 150 rpm with 25 (P25) and 50 (TN) mg Lꢀ1 initial
concentration of the CIP solutions.
The amount of CIP adsorbed (qt mg gꢀ1) was calculated as:
V
qt ¼ ðC0 ꢀ CtÞ ꢂ
(1)
m
where C0 and Ct were the CIP concentrations at the beginning
and aer a period of time (mg Lꢀ1), V was the initial solution
volume (L), and m was the adsorbent weight (g).
Fig. 1 Typical (a and b) SEM and (c–f) TEM images of TNs.
2.4 Regeneration studies
closely related to the centrifugal force and the length of the TNs
was closely related to the shearing force. Thus the structure of
the TNs can be effectively controlled by optimizing the experi-
mental parameters (temperature, time, mixing speed, post-
treatment of raw materials). It can be seen from Fig. 1e and f
that the length of prepared anatase TNs ranged from 1.0 to 2
mm, which is longer than that in previous reports (Table S1†)
and the TNs had the shape of a hollow tube.
Aer adsorption, the TN and P25 adsorbents were placed in
a quartz pot in a black box under UV irradiation from a 500 W
ultraviolet lamp for 3 h. The samples were rinsed with distilled
water and then dehydrated by freeze-drying. The adsorption
experiments were conducted on the adsorbent from the rst
regeneration cycle. The process was repeated six times to study
the ability of the photocatalytic technology to regenerate the
adsorbents.
Fig. 2a shows that the prepared TNs gave a single strong peak
at maximum diffraction angles of 25.3 and 48.2ꢁ; triple peaks
were seen at 36.9, 37.8 and 38.61ꢁ and a medium double peaks
at 54 and 55.3ꢁ. The characteristics of the TNs were almost the
same as those of standard anatase TiO2 (PDF#00-002-0387).
These results further indicated that the prepared anatase TNs
had the form of anatase crystals. Fig. S1† shows that sodium
titanate exhibited strong peaks attributed to titanate at about
10ꢁ. The results showed that peaks at about 10ꢁ were weaker
when the sodium titanate had been exchanged with hydrogen
ions, conrming that sodium titanate had been converted into
titanium dioxide. The results from Fig. S1† indicate that the
crystalline form of the TN heated at 823 K for 4.5 h was anatase.
The experimental results from Fig. 2b and c show that the
product had a maximum SSA (ca. 164 m2 gꢀ1) and pore volume
(ca. 0.598 cm3 gꢀ1) when the mixing speed was maintained at
500 rpm at 403 K for 18 h.
2.5 Characterization
The surface morphologies of the samples were characterized
using eld-emission scanning electron microscopy (FE-SEM,
Hitachi S-4800) operating at a typical accelerating voltage of
10 kV. The microstructure and morphology of the samples were
analyzed using high-resolution transmission electron micros-
copy (TEM, JEOL JEM 2100F, accelerating voltage 200 kV). To
determine the existing state of atoms, X-ray photoelectron
spectroscopy (XPS) was carried out on a Kratos Axis Ultra DLD
spectrometer using monochromatic Al Ka X-rays at a pressure of
1 ꢂ 10ꢀ9 Torr. A Unico UV2100 spectrophotometer was used to
determine the CIP concentration at a wavelength of 275 nm.
3. Results and discussion
3.1 Characterization of TNs
As can be seen from Fig. 3b and c, the binding energy of O1s
was 529.5 eV and the binding energies of Ti2p3 were 458.5 and
464.5 eV, identical to those reported previously,31,32 and indi-
cating that TNs retained the same structure as the TiO2.
Fig. 1 shows that the TiO2 powder was transformed into elon-
gated nanotubes. During the reaction, the TiO2 dissolved in the
NaOH solution was converted into sodium titanate. Saturated
sodium titanate can form crystals. The saturated sodium tita-
nate was vigorously stirred, which produced both centrifugal
and shearing forces; the centrifugal force and the stirring speed
3.2 Adsorption isotherms
had linear relationship, whereas the shearing force and stirring The isotherms in Fig. 4 show that the adsorption capacities of
speed had a curved relationship. The diameter of the TNs was the TNs and P25 were 26.38 and 5.32 mg gꢀ1, respectively. The
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Fig. 2 (a) XRD patterns, (b) N2 adsorption and desorption isotherms, and (c) distribution of pore diameters of the TNs.
Fig. 3 (a) XPS scan and (b) the Ti2p3 deconvolution and (c) the O1s deconvolution of TNs.
higher adsorption capacity of the TNs was a result of the higher model tted the adsorption behavior better than the Freundlich
SSA and pore volume (Fig. 2c) (ca. 50 m2 gꢀ1 and 0.1 cm3 gꢀ1 model. Table 1 gives R2 values for the Langmuir model of CIP
respectively). The tted curves in Fig. 4 show that the Langmuir adsorption onto P25 and TN as 0.97735 and 0.94364,
,
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Fig. 4 Adsorption isotherms of CIP on (a) TNs and (b) P25.
respectively. This was attributed to fewer functional groups in
the structures of P25 and TN. Their adsorption was mainly
a result of physical adsorption onto the monomolecular layer.33
Table 1 Langmuir and Freundlich isotherms parameters of CIP on P25
and TN
Langmuir model
Freundlich model
3.3 Adsorption kinetics of CIP
KL
qm
Adsorbent (L mgꢀ1
)
(mg gꢀ1
)
R2
KF
1/n
R2
The equilibrium times for the adsorption of CIP onto P25 and
TN are shown in Fig. 5a and were 300 and 200 min, respectively.
The physical and chemical effects of all the adsorption
processes involve the mass transfer of a substance from the
liquid phase to the adsorbent surface to form a complex
between CIP and the adsorbent. CIP solutions with an initial
concentration of 50 mg Lꢀ1 were used to investigate the
P25a
TNb
0.3830
0.1801
5.597
29.34
0.9774 2.101 0.3083 0.8996
0.9436 9.782 0.2714 0.8780
CIP concentration ¼ 10 mg Lꢀ1, P25 concentration ¼ 0.5 g Lꢀ1
.
CIP
a
b
concentration ¼ 25 mg Lꢀ1, TNs ¼ 0.5 g Lꢀ1
.
Fig. 5 Kinetic curves of (a) adsorption of CIP onto P25 and TN (initial concentrations of CIP 25 and 50 mg Lꢀ1, respectively); (b) pseudo-first-
order model (R2 ¼ 0.9489, 0.8288); and (c) pseudo-second-order model (R2 ¼ 0.9965, 0.9473).
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Table 2 Kinetic parameters of pseudo-first-order and pseudo-second-order adsorption kinetic models for CIP on the anatase TNs and P25a
Pseudo-rst-order model Pseudo-second-order model
Initial conc.
qe,exp
k1
qe,cal
k2
qe,cal
Adsorbent
(mg Lꢀ1
)
(mg gꢀ1
)
(minꢀ1
)
(mg gꢀ1
)
R2
(minꢀ1
)
(mg gꢀ1
)
R2
P25
TN
5.0
25
26.28
5.32
0.0029
0.0070
4.14
22.2
0.9489
0.8288
0.0442
0.2746
4.14
22.2
0.9965
0.9473
a
CIP concentration ¼ 10 or 25 mg Lꢀ1; TN or P25 ¼ 0.5 g Lꢀ1. CIP concentration ¼ 10 or 25 mg Lꢀ1; TN or P25 ¼ 0.5 g Lꢀ1
.
Fig. 6 Regeneration of (a) P25 (c0 ¼ 10 mg Lꢀ1) and (b) anatase TN (c0 ¼ 25 mg Lꢀ1).
adsorption kinetics of CIP on the TNs. The adsorption of CIP solution, as shown in Fig. 6. This self-regeneration process
onto the TNs was rapid and reached equilibrium in ca. 180 min. could be recycled more than six times with no decrease in
The concentration reduced stepwise due to the reduction of adsorption capacity. The saturated sorption capacity was ca. 16
adsorption sites on the TNs during the adsorption process. mg gꢀ1 aer the sixth cycle of regeneration with a UV lamp,
However, the removal of CIP in solution by adsorption on P25 indicating that the effectiveness of the TN adsorbent did not
was slow and required ca. 300 min to reach equilibrium. A signicantly change from the rst to the sixth cycle.
longer equilibrium time was required for the adsorption of CIP
onto P25 than onto the TNs, which may be attributed to the
different pore structures of the TNs and P25.
4. Conclusions
Pseudo-rst-order
and
pseudo-second-order
kinetic
models34,35 were used to further investigate the CIP adsorption
process and the kinetic parameters are given in Table 2. The
adsorption of CIP onto TN and P25 were better tted with the
pseudo-second-order model based on the correlation coefficient
(R2). The k2 value for the adsorption of CIP onto the TNs was
larger than the value for P25, which showed that it was adsorbed
faster onto the TNs than onto P25. The calculated q values
(qe,cal) derived from the pseudo-second model were more reli-
able than those from the pseudo-rst-order model.
As shown in Fig. 5a, CIP was adsorbed faster and to a greater
extent on the TNs than on P25, which was attributed to the
larger SSA and pore volume of the TNs (Fig. 2b). The meso-
porous character of the TNs allowed CIP to diffuse into the pore
canals of the TNs at a faster speed. The mesoporous charac-
teristics of TN are helpful in enhancing the photocatalytic
process.
Long TNs were successfully prepared by the reaction of TiO2 and
NaOH by a hydrothermal method with stirring. The TNs showed
outstanding adsorption as a result of their large SSA (ca. 160 m2
gꢀ1) and pore volume (ca. 0.6 cm3 gꢀ1), which was larger than
that of commercial P25, and were used to efficiently adsorb and
remove CIP from aqueous solution. The experimental results
indicated that the TNs have excellent adsorption of 26.38 mg
gꢀ1. The TNs also had better regeneration properties aer the
adsorption and mineralization of pollutants. This method
provides a green route for the removal of organic pollutants by
combining adsorption enrichment with photocatalytic
degradation.
Acknowledgements
This research was supported by the Fundamental Research
Funds for the Central Universities and Undergraduate Innova-
tion Fund Project (N20151201). We are also grateful to the
3.4 Regeneration properties of TN and P25
P25 and TN had good regeneration capacities under UV irradi- anonymous reviewers for their valuable comments to improve
ation with a 500 W ultraviolet lamp for 3 h in 5 mL of aqueous this manuscript.
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19 T. Paul, P. L. Miller and T. J. Strathmann, Environ. Sci.
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