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Degradation of tetracycline in aqueous medium by electrochemical method.

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ASIA-PACIFIC JOURNAL OF CHEMICAL ENGINEERING
Asia-Pac. J. Chem. Eng. 2009; 4: 568–573
Published online 26 May 2009 in Wiley InterScience
(www.interscience.wiley.com) DOI:10.1002/apj.286
Special Theme Research Article
Degradation of tetracycline in aqueous medium by
electrochemical method
Hui Zhang,* Fang Liu, Xiaogang Wu, Jianhua Zhang and Daobin Zhang
Department of Environmental Engineering, Wuhan University, Wuhan 430079, China
Received 31 October 2008; Revised 4 January 2009; Accepted 7 February 2009
ABSTRACT: The degradation of tetracycline by anode oxidation with Ti/RuO2 –IrO2 electrode was carried out in
an electrochemical cell. The effect of operating conditions such as electrical current density, initial pH, antibiotic
concentration, electrolyte concentration and hydroxyl radical scavenger on the oxidation of tetracycline was investigated.
The results showed that the degradation of tetracycline followed apparent pseudo-first-order kinetics. The rate constant
increased linearly with the current density, but the oxidation curves displayed the same dependence on the amount
of the specific charge passed. The degradation rate decreased with the initial antibiotic concentration. Either initial
pH or electrolyte concentration had little effect on the electrochemical oxidation of tetracycline. The presence of tertbutanol did not hinder the degradation rate, indicating the radical contribution to the oxidation of tetracycline could be
neglected.  2009 Curtin University of Technology and John Wiley & Sons, Ltd.
KEYWORDS: electrochemical; anode oxidation; tetracycline; degradation
INTRODUCTION
The occurrence of a large number of pharmaceuticals and personal care products (PPCPs) residues in
the environment has been frequently reported in recent
literature, which is received increasing attention as
emerging contaminants.[1] Among various pharmaceutical compounds, antibiotics are of special concern
because of their extensive use in human and veterinary medicine and their potential to promote growth
of resistant bacteria and pose adverse health effects to
humans.[1,2] As a result of their antibacterial nature,
antibiotic residues cannot be effectively destructed by
traditional biological methods.[3] Although antibiotics
could be removed by physical processes such as granular activated carbon filtration,[4] they were just transferred to another medium (carbon), which requires further treatment and disposal.[5] Chemical oxidation could
effectively destroy antibiotics and overcome most of
the limitations posed by the other conventional processes. Therefore, a variety of chemical oxidation methods including ozonation have been used to eliminate
antibiotics.[6] Apart from chemical oxidations, the electrochemical oxidation of recalcitrant organic contaminants such as antibiotics seems to be a new way,[7]
which attracted increasing interest in recent years.[8 – 12]
*Correspondence to: Hui Zhang, Department of Environmental
Engineering, Wuhan University, P.O. Box C319, Luoyu Road 129#,
Wuhan 430079, China. E-mail: eeng@whu.edu.cn
 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
Vedenyapina et al .[12] used an electrochemical cell with
separated cathodic and anodic compartments to oxidize
tetracycline with the initial concentration ranging from
50 to 500 mg l−1 . Weichgrebe et al .[8] used three different microbiological tests to observe the influence of
the biocide effect during electrochemical oxidation of
tetracycline in water when the initial concentration was
1 mg l−1 . However, the effect of various operation conditions on the electrochemical oxidation of tetracycline
was not systematically investigated. Therefore, in this
study, the electrochemical oxidation was used for the
elimination of antibiotics. Tetracycline was selected as
a model antibiotic because it is one of the most frequently prescribed groups of antibiotics that have been
used in human and veterinary medicine to treat and prevent bacterial infections, as an additive to animal feeds
(poultry, cattle and swine), in aquaculture, and to inhibit
fungal growth in fruit trees.[3,13] The effects of operating
conditions such as current density (i ), initial pH, initial
antibiotic concentration, electrolyte concentration and
hydroxyl radical scavenger on the elimination of tetracycline in electrochemical process were investigated.
MATERIALS AND METHODS
The tetracycline hydrochloride (C22 H24 O8 N2 · HCl)
(Fig. 1) used in electrochemical oxidation experiments
was obtained from Wuhan Yuancheng Technology
Asia-Pacific Journal of Chemical Engineering
DEGRADATION OF TETRACYCLINE BY ELECTROCHEMICAL METHOD
Figure 1. Chemical structure of tetracycline hydrochloride.
Batch experiments were performed in a rectangular
electrolytic reactor (plexy glass) containing 200 ml
solution. Electrolyses were conducted under constant
current conditions using a direct current (DC) power
supply (Model WYK-305) from Yangzhou Jintong
Source Co. Ltd. (China). One 5 cm × 11.9 cm plate
anode (Ti/RuO2 –IrO2 ) and one plate cathode (stainless
steel) of the same dimension were arranged parallel to
each other at a distance of 3.8 cm. The working surface
area of the electrode was 31.5 cm2 . A magnetic stirrer
(Model 78-1, Hangzhou Instrument Motors Factory,
China) provided the mixing of the solution in the
rector. At pre-selected time intervals, samples were
withdrawn from the electrolytic cell, and the residual
tetracycline concentration was determined by reversed
phase high-performance liquid chromatograph (HPLC)
using a Shimadzu SPD-M20A chromatograph equipped
with a C-18 column (Shim-Pack VP-ODS 4.6 µm-C-18,
250 mm × 4.6 mm). The detection was performed by
ultraviolet (UV) absorption at a wavelength of 365 nm
using a diode array detector. A mixture of acetonitrile
and an aqueous solution (20 mmol l−1 ) of potassium
dihydrogen phosphate (35 : 65 v/v) was used as the
eluent at a flow rate of 0.01 ml l−1 .
RESULTS AND DISCUSSION
Degradation of tetracycline
Figure 2. (a) UV–vis spectral changes with electrolysis
time; (b) HPLC–UV chromatograms at different times of
electrochemical oxidation (C0 = 100 mg l−1 , [Na2 SO4 ] =
0.1 mol l−1 , pH0 3.9).
Development Co., Ltd. (China). All other reagents
were of analytical grade. A stock solution of tetracycline hydrochloride was freshly prepared with deionized
water before each run. Sodium sulphate was added as
electrolytes, and sulfuric acid or sodium hydroxide was
used to adjust the initial pH (pH0 ) of the antibiotic
solution.
 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
To clarify the degradation of tetracycline as a result
of electrochemical oxidation, representative UV–visible
(vis) spectra changes in the antibiotic solution as a
function of electrolysis time were observed, and the
corresponding spectra are shown in Fig. 2(a). The tetracycline molecule has conjugated double-bond structures
with two carbonyl groups and enolic groups (Fig. 1).
This results in the fact that the absorption spectrum of
tetracycline in water was characterized by two major
absorption bands at 276 and 358 nm. The peaks at 276
and 358 nm decreased slowly with electrolysis time.
The HPLC–UV chromatograms obtained for aliquots
collected at different electrolysis times are shown in
Fig. 2(b). It can be observed that the intensity of the
tetracycline peak (retention time of 3.35 min) decreases
as the electrochemical oxidation proceeds. The new
peak was detected during the HPLC monitoring with
retention time shorter than that of tetracycline. This
result indicates that the degradation product has higher
polarity than that of tetracycline because it elute faster
from the non-polar (C-18) chromatographic column.[13]
Effect of current density
Current density is a very important parameter for the
oxidation of organic compounds. Therefore, the effect
Asia-Pac. J. Chem. Eng. 2009; 4: 568–573
DOI: 10.1002/apj
569
570
H. ZHANG ET AL.
Asia-Pacific Journal of Chemical Engineering
Figure 4. The effect of initial pH on the degradation of tetracycline (C0 = 100 mg l−1 , i = 47.6 mA cm−2 ,
[Na2 SO4 ] = 0.1 mol l−1 ).
removal follows apparent pseudo-first-order kinetics
according to the following rate equation:
−
dC
= kC
dt
(1)
where C is tetracycline concentration at time t and k is
the pseudo-first-order rate constant. This is in agreement
with the result reported by Vedenyapina et al .[12] The
pseudo-first-order rate constants calculated are 0.018,
0.028, 0.035 and 0.046 min−1 for 15.9, 31.7, 47.6
and 63.5 mA cm−2 , respectively, which indicates the
rate constants almost linearly increase with increasing
current density (Fig. 3(b)).
Generally, the oxidation of organics on the anode
occurs by two ways: direct electron transfer from
organics to the anode and oxidation by the adsorbed
hydroxyl radical (• OH) formed at the surface of a high
oxygen overvoltage anode from oxidation of water in
acid and neutral media[14,15] :
H2 O −−→
•
OHads + H+ + e−
(2)
or hydroxide ion at pH ≥10:
Figure 3. (a) The effect of current density on the
degradation of tetracycline. (b) Plot of pseudo-firstorder rate constant vs current density. (c) The evolution
of tetracycline concentration with specific charge at
different current densities (C0 = 100 mg l−1 , [Na2 SO4 ] =
0.1 mol l−1 , pH0 3.9).
of current density on the degradation of tetracycline
was investigated at different current densities when
antibiotic concentration was 100 mg l−1 , electrolyte
concentration was 0.1 mol l−1 and the initial pH value
was 3.9. As can be seen in Fig. 3(a), tetracycline
 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
OH− −−→
•
OHads + e−
(3)
The linear relationship between rate constant and current density is consistent with the fact that degradation
process by • OH is a bimolecular reaction between tetracycline and • OH with a constant generation of • OH that
is approximately proportional to the current density.[16]
In the meantime, the elimination via direct electron
transfer from tetracycline to the anode is also approximately proportional to the current density. When the
antibiotic concentration data are represented as a function of the specific charge, the oxidation curves show
Asia-Pac. J. Chem. Eng. 2009; 4: 568–573
DOI: 10.1002/apj
Asia-Pacific Journal of Chemical Engineering
DEGRADATION OF TETRACYCLINE BY ELECTROCHEMICAL METHOD
Figure 6. The effect of electrolyte concentration on
the degradation of tetracycline (C0 = 100 mg l−1 , i =
47.6 mA cm−2 , pH0 3.9).
Figure 5. (a) The effect of initial antibiotic concentration
on the degradation of tetracycline. (b) Plot of pseudofirst-order rate constant vs initial antibiotic concentration
(i = 47.6 mA cm−2 , [Na2 SO4 ] = 0.1 mol l−1 , pH0 3.9).
the same dependence on the amount of charge whatever
the value of current density used (Fig. 3(c)).[17]
Effect of initial pH
The pH of electrolyses medium is the other important
variable for the electrochemical oxidation of organics. However, many contradictory results were observed
from investigating pH effect on the anodic oxidation
process. For example, Lissens et al .[18] reported that
the electrochemical oxidation process was more efficient in alkaline media. In contrast, Scialdone et al .[19]
indicated low pH favoured the efficiency of the process. The effect of pH may strongly depend on the
nature of the investigated organics and the electrodes
used.[14,20] In order to know if tetracycline could be oxidized effectively in a wide pH range, the pH effect was
investigated when antibiotic concentration was 100 mg
 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
Figure 7. The effect of hydroxyl radical scavenger
on the degradation of tetracycline (C0 = 100 mg l−1 ,
i = 47.6 mA cm−2 , [Na2 SO4 ] = 0.1 mol l−1 , pH0 3.9).
l−1 , current density was 47.6 mA cm−2 and electrolyte
concentration was 0.1 mol l−1 . Three pH values were
selected around three pK a values of tetracycline[21]
(pK a1 = 3.3, pK a2 = 7.7, pK a3 = 9.7), and the oxidation result is shown in Fig. 4. It is found that the
effect of initial pH on the oxidation of tetracycline
at the anode used in this study is not pronounced.
There is no significant difference for the calculated rate
constant (0.038 ± 0.001 min−1 ) values at different pH
values. This situation indicates that the degradation of
tetracycline can be performed at any initial pH value
between 3.9 and 10.0 without any significant loss in
oxidation efficiency of the system. The similar result
was observed when clofibric acid, one pharmaceutical compound, was oxidized by anodic oxidation with
boron-doped diamond (BDD) electrode.[15]
Asia-Pac. J. Chem. Eng. 2009; 4: 568–573
DOI: 10.1002/apj
571
572
H. ZHANG ET AL.
Effect of initial antibiotic concentration
Figure 5(a) illustrates the degradation of tetracycline at
different initial antibiotic concentrations when current
density is 47.6 mA cm−2 , electrolyte concentration is
0.1 mol l−1 and initial pH value is 3.9. The degradation
rate of tetracycline was found to be decreased and
having no proportional variations with increase in the
initial concentration (Fig. 5(b)). The electrochemical
oxidation of tetracycline would lead to the production
of intermediates, which may also simultaneously be
degraded on the anode by direct electron transfer
and by hydroxyl radicals according to the reaction[2] .
The decrease in rate of tetracycline degradation may
be due to the competitive reaction of the daughter
compounds with the parent compound on the anode.
Although the removal efficiency of tetracycline is
lower at the higher initial concentration, the total
amount of degraded tetracycline is increased. After
60 min reaction, a tetracycline removal efficiency of
89.1% was achieved at the initial concentration of
50 mg l−1 compared with 82.2% at 200 mg l−1 initial
concentration, corresponding to 44.6 and 164.5 mg l−1
of degraded tetracycline, respectively.
Effect of electrolyte concentration
Figure 6 compares the degradation of tetracycline under
different electrolyte concentrations when antibiotic concentration was 100 mg l−1 , current density is 47.6 mA
cm−2 and initial pH value is 3.9. There was little effect
of electrolyte concentration on the oxidation efficiency
in the investigated range of 0.05–0.20 mol l−1 Na2 SO4 .
The difference for the calculated rate constant (0.037 ±
0.001 min−1 ) values at different electrolyte concentrations is insignificant. This is consistent with the result
of Chen and Chen when Orange II was oxidized on
Ti/BDD electrode.[20] The electrolysis voltage would
decrease with the electrolyte concentration. Therefore,
the increase of the electrolyte concentration is helpful
in saving energy consumption.[20]
Effect of hydroxyl radical scavenger
Anode oxidation would occur via direct electron transfer from tetracycline to the anode and oxidation by the
adsorbed hydroxyl radical (• OH) formed at the surface of a high oxygen overvoltage anode from water
oxidation.[14,15] To investigate which pathway is dominant, the degradation of tetracycline was carried out
in the presence of tert-butanol, a kind of hydroxyl
radical. Figure 7 displays the presence of tert-butanol
has no significant effect on the degradation rate when
tert-butanol ranged from 0.42 to 4.20 mmol l−1 , corresponding to 2–20 of tert-butanol to tetracycline mole
 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
Asia-Pacific Journal of Chemical Engineering
ratio. There is no significant difference for the calculated rate constant (0.034 ± 0.001 min−1 ) values at
different tert-butanol concentrations. Therefore, the radical contribution to the oxidation of tetracycline could
be neglected.
CONCLUSION
The electrochemical method can effectively degrade
tetracycline in aqueous solution. The oxidation of tetracycline followed apparent pseudo-first-order kinetics.
The pseudo-first-order rate constant increased linearly
with the current density, but the oxidation curves
showed the same dependence on the amount of the
specific charge passed. The degradation rate decreased
with the increasing initial antibiotic concentration. Initial pH and electrolyte concentration had little effect on
the electrochemical oxidation of tetracycline. The presence of tert-butanol did not slow down the degradation
rate, indicating anode oxidation of tetracycline mainly
proceeded through direct electron transfer from organics
to anode.
Acknowledgments
This study was supported by Hubei Provincial Science
and Technology Department through ‘The Gongguan
Project’ (Grant No. 2003AA307B01).
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573
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