Anal. Method Environ. Chem. J. 3 (3) (2020) 44-53
Removal of Metronidazole residues from aqueous solutions
based on magnetic multiwalled carbon nanotubes by
response surface methodology and isotherm study
Mohammad Reza Rezaei Kahkha
*,a
, Gholamreza Ebrahimzadeh
a
and Ahmad Salarifar
b
a
Department of Environmental Health Engineering, Faculty of Health, Zabol University Of Medical Sciences,Zabol.Iran
b
Environmental Engineering, Faculty of Natural Resources, Islamic Azad University, Bandar Abbas Branch, Bandar Abbas, Iran
ABSTRACT
Antibiotics and pharmaceutical products cannot remove by
conventional sewage treatment. In this work, an effective adsorbent
magnetic multiwalled carbon nanotube (Fe
3
O
4
@MWCNTs;
MMWCNTs) was synthesized by co-precipitation of MWCNTs with
Fe
3
O
4
and used for removal of Metronidazole from aqueous solutions.
Response surface methodology on central composition design (CCD)
was applied for designing of experiments and building of models for
Metronidazole removal before determination by HPLC. Four factors
including pH, the adsorbent dose, time, and temperature were
studied and used for the quadratic equation model to the prediction
of optimal points. By solvent the equation and considering the
regression coefficient (R
2
=0.9997), the optimal points obtained
as follows: pH =2.98; adsorbent dosage =2.16 g; time =22 min
and temperature = 37.9
o
C. The isotherm study of adsorption showed
that the metronidazole adsorption on Fe
3
O
4
@MWCNTs follows the
Langmuir model. The maximum adsorption capacity (AC) is 215
mg g
-1
obtained from Langmuir isotherm. The results showed that
three factors including pH, amount of adsorbent, and temperature
are signicant on removal efciency and an experimental point was
found to agree satisfactorily with the predicted values. The proposed
methods coupled to HPLC were used to analysis of metronidazole in
six real samples. The results showed the best removal efciency was
obtained at optimal points. Moreover, the reusability of adsorbent
showed that the Fe
3
O
4
@MWCNTs can be efciently removed
the Metronidazole from aqueous solutions as compared to other
Keywords:
Magnetic multiwalled carbon
Nanotubes,
Metronidazole,
Adsorption,
Response surface methodology,
Central composition design,
ARTICLE INFO:
Received 11 Jun 2020
Revised form 5 Aug 2020
Accepted 28 Aug 2020
Available online 29 Sep 2020
*Corresponding Author: Mohammad Reza Rezaei Kahkha
Email: m.r.rezaei.k@gmail.com
h
ttps://doi.org/10.24200/amecj.v3.i03.110
------------------------
Research Article, Issue 3
Analytical Methods in Environmental Chemistry Journal
Journal home page: www.amecj.com/ir
AMECJ
1. Introduction
Antibiotic residual in environmental ecosystems
is a serious concern for human’s health. The
conventional sewage treatment cannot remove
antibiotics and pharmaceutical products and hence,
a serious ecological risk that occurs by discharging
of these efuent in environmental ecosystems and
aquatic [1, 2]. Metronidazole (C
6
H
9
N
3
O
3
) is an
antibiotic that used to treat a wide variety bacterial
infections [3]. Recently, researchers reported that
average concentration of metronidazole in river
water and wastewater was approximately about 0.5
45
Removal of Metronidazole by Fe3O4@MWCNTs Mohammad Reza Rezaei Kahkha et al
and 1.3 ng L
-1
,
respectively [4]. In addition, many
pharmaceutical industries discharge this antibiotic
in environmental water by higher dosage [5]. Several
methods such as fenton process, ltration and
adsorption were used for removal of metronidazole
from different matrix and determined with HPLC [6].
Due to cost and simplicity methods, the adsorption
techniques are favorite method for removal of
metronidazole in water and biological samples.
Many techniques based on metal nanoparticles,
the polymer structures, the nanosheets, MWCNTs
modied with and neural network-genetic
algorithm were used and developed for separation
and determination metronidazole and other drug
in different matrixes [7]. Adsorption is simple,
effective and economic way with high recovery,
easy operation and low cost technique for removal
of contaminants such as antibiotic within water or
wastewater even at large concentration. The type
and size of adsorbent is a key factor for adsorption
process that inuences on removal efciency of
pollutant [8]. Carbon nanotubes (SWCNTs and
MWCNTs) and functionalized of CNTs are widely
used as an adsorbent in the removal, extraction
and preconcentration of many contaminants
including medicinal compounds, pesticides, and
other molecules [9]. High surface area, high
permeability, good mechanical and thermal stability
and repeatability are some of the unique properties
of nanotubes. Also, the absorption capacity is could
be increased by modifying the surface of CNTs by
NH
2
, COO, SH, C
6
H
5
groups and adsorption of
contaminants would be more specic [10]. Response
surface methodology (RSM) is a most applicable
method used in many elds such as antibiotics and
pharmaceutical products [11]. RSM is a technique
that used for statistical analysis of complicated
processes and can be utilized for investigating of
relative signicance of important factors even in
the presence of complex interactions [12].
High performance liquid chromatography (HPLC)
is a suitable method for isolation, identication and
measurement of many drugs. Combining liquid
chromatography with mass spectrometer(LC-MS,
HPLC-MS) is the most appropriate method for
identifying different species of organic compounds
in complex matrixes. Examples of these
compound are amino acids, proteins, nucleic acids,
hydrocarbons, carbohydrates, drugs, terpenoids
and pesticides, antibiotics, steroids, any organic or
inorganic metal and a group of various materials.
Experimental data points were obtained during
optimization based on Fe
3
O
4
@MWCNTs and a
model for central composition design (CCD). It is
good method for the consecutive experimentation
and illustrate accurate information for testing
several parameters while not involving an unusually
large number of data points. The adsorption
Metronidazole based on Fe
3
O
4
@MWCNTs was
determination by HPLC instrument.
2. Experimental
2.1. Reagent and material
All reagent and solutions were analytical grade
and purchased from Merck (Darmstadt, Germany).
Metronidazole (CASN: 443-48-1) was obtained
from Aldrich chemical Co. (Germany). HPLC
grade of acetonitrile (ACN) and DW purchased
from Sharloa (Spain). Carbon nanotubes with outer
diameter of 3–20 nm, length between 1–10 nm,
number of walls 3–15 and surface area of around
350 m
2
g
-1
were prepared from Plasma Chem.
GmbH (Berlin, Germany). Also, The pristine
MWCNTs (308068, 98% carbon base, O.D= 10 nm,
L=5-20 μm) was purchased from Sigma Aldrich.
A standard solution of 1000 mg L
-1
metronidazole
prepared by dissolving of 1 gr metronidazole
in 1liter deionized water. All standard working
solutions prepared daily by dilution of DW. ) The
shacking and centrifuging of blood samples were
used based on 300 rpm and 3500 rpm speeds by
vortex mixer (Thermo, USA) and Falcon centrifuge
(20 mL of polypropylene conical tubes, Thermo,
USA), respectively. The pH was adjusted pH by
0.25 mol L
-1
of sodium phosphate buffer solution
(Merck, Germany) for pH of 5.5 to 8.2 (Na
2
HPO
4
/
NaH
2
PO
4
).
2.2. Synthesis of MMWCNTs
Synthesis of magnetic carbon nanotube was
46
Anal. Method Environ. Chem. J. 3 (3) (2020) 44-53
performed by co-precipitation methods that
reported previously [13]. Briey, 10 mg of
pristine MWCNTs were added to 2 ml solution
composed of 4.33 mmol Fe
+2
and 8.66 mmol Fe
+3
solution was stirred in ultrasonic bath for 10
min at 50°C while 10 ml concentrated ammonia
(8 M) was added drop by drop to the solution. The
pH of nal solution should be alkaline in order
to deposition of Fe
3
O
4
on multi-walled carbon
nanotubes. The adsorbent was washed for 7 times
with distilled water and separated by a permanent
magnet.
2.3. Metronidazole removal by MMWCNTs
Batch adsorption experiments were carried out
as per the design developed with the central
composite design methodology. Experiments
were performed at a batch reactor in 500 ml beaker
that containing 50 ml of given concentration
of metronidazole. Beakers were shaken during
that shaked for the specied time period in a
temperature controlled incubation shaker at 200
rpm. The pH was adjusted by addition of 0.1 M
NaOH or HCl. After completion of experiments
adsorbent was removed by an external magnet
and remaining metronidazole was measured. The
measurement of metronidazole was performed
using Cecil HPLC (CECIL Corporation,
England) equipped ACE C
18
column and UV-
VIS detector at 230 nm. The mobile phase is
ACN: WATER (60:40). The removal percentage
of Metronidazole (%removal) was calculated as
follow by equation 1:
(Eq.1)
Where C
f
and C
0
are initial and nal metronidazole
concentration (mg L
-1
) of solution, respectively.
2.4. Experimental design
CCD was applied in this work to investigation
of variables for adsorption of metronidazole on
to MMWCNTs. The CCD for four variables (pH,
adsorbent dosage, time and the temperature), with
two levels (minimum and maximum), was used for
experimental design model. In the experimental
design model, pH (2-10), adsorbent dosage (0.5-
2.5 g), time (5-30 min) and temperature (20-60
o
C) were taken as input variables. Percentage
removal of (30 mg L
-1
) of metronidazole was
selected as response of the system. The quadratic
equation model for prediction of optimal point
was expressed by Equation 2.
(Eq. 2)
Where Y is the response of the system and X
i
and X
J
are the variables of action, β
0,
β
i,
β
ii,
β
ij
are constant coefcient, linear effects, quadratic
effects and interaction effects, respectively. The
coefcient of determination, namely, R
2
and
Adj-R
2
were used for the explanation of quality
of the model. The statistical signicance was
expressed with adequate precision ratio and the
F-test. Design expert (version 8) program was
used for regression and graphical analysis. A total
of 31 experiments were necessary to estimate of
the full model (Table 1)
3. Result and discussion
In this work, removal of metronidazole by a nano-
composite made of multi-walled carbon nanotubes
and iron nanoparticles were studied. Design of
Experiments were conducted using the RSM as
well as factors affecting on absorption process of
metronidazole such as pH, adsorbent dosage, time,
and the temperature were optimized. Finally, the
data obtained from experiments compared with
model output to optimize and predict the results.
3.1. Regression model and statistical analysis
The CCD has been successfully used for
optimizing conditions of Metronidazole removal.
A second-order polynomial regression model
equations relating the removal efciency and
process variables are given in Equation 3.
47
Removal of Metronidazole by Fe3O4@MWCNTs Mohammad Reza Rezaei Kahkha et al
%Removal=+41.66308+2.39902 (pH)+9.57629
(adsorbent dose)+0.7686
(temperature)+1.00897 (time)-4.68750×
10
-3
(pH.×adsorbent dose)+0.011172 (pH×
temperature)+7. 5×10
-3
( pH×time) -0.034687
(adsorbent×temperature)+0.077000 ( adsorbent
×time) -3.65×10
-3
(temperature×time)-0.27659
(pH
2
)-1.99484(adsorbent
2
)-9.07506×
10
-3
(temperature
2
)-0.025495(time
2
)
(Eq.3)
Design-Expert 8 software was applied for
determination of the coefcients in Equation 3.
The optimal points were as follows: pH = 2.98;
adsorbent dosage = 2.16 g; time = 22.2 min and
temperature = 37.88
o
C. The model prediction
of the metronidazole removal recovery is 82.8%
while the experimental amount of removal
efciency is 83.4%. These results conrmed
that RSM effectively used for the investigation
of parameters in complex process could be
utilized to optimize the process parameters.
Column1 Factor 1 Factor 2 Factor 3 Factor 4 Actual Predicted
Run
A:pH B:adsorbent dose C:temperature D:time
Actual
Value
Predicted Value
1
2 2.5 60 5 65 65.79284266
2
6 1.5 40 10 59 60.50063264
3
6 1.5 40 10 74 72.34013804
4
10 0.5 60 5 66 66.97292803
5
2 0.5 60 15 67 68.94958105
6
2 0.5 60 5 68.7 66.96949568
7
2 2.5 20 15 70.8 70.74179107
8
10 2.5 20 15 70 68.94958105
9
2 2.5 60 15 69.1 68.22151495
10
6 1.5 40 10 65 64.42930494
11
2 0.5 20 5 73 74.75160032
12
10 2.5 60 15 63 65.53095796
13
2 0.5 20 15 74 73.37046337
14
2 0.5 60 5 73 73.07825335
15
10 2.5 60 5 67 65.74816798
16
10 2.5 20 15 74 73.07825335
17
6 1.5 40 15 63 63.19805057
18
6 1.5 40 15 80.5 79.9594382
19
10 0.5 60 15 81 82.86325088
20
10 2.5 60 15 83.4 82.86325088
21
2 2.5 60 5 63 62.62408038
22
2 2.5 20 5 62 62.0334084
23
10 0.5 60 15 56 56.42972396
24
6 1.5 40 10 65 64.22776481
25
10 2.5 60 5 83.5 82.86325088
26
2 2.5 60 5 83.2 82.86325088
27
10 0.5 20 5 82.9 82.86325088
28
10 0.5 20 15 82.4 82.86325088
29
2 0.5 60 15 83.1 82.86325088
30
2 2.5 20 15 83.2 82.86325088
31
6 1.5 40 10 83 82.86325088
Table 1. Central composite design matrix with experimental and predicted values
48
Anal. Method Environ. Chem. J. 3 (3) (2020) 44-53
The mathematical expressions of relationship
between the independent parameters and
response of system are given in terms of encoded
factors. The results of regression analysis on
quadratic model are given in Table 3. The
signicance of each coefcient was expressed
by F-values and p-values (Table 2). The larger
of the F-values and the smaller of the p-values,
indicated more signicant of the corresponding
coefcients. Values of “prob > F” less than
0.0500 also indicated high signicant regression
at 95 percent condence level. According to the
F- and p-values, temperature, time and adsorbent
dose were found more effective on the adsorption
process. The “Lack of Fit F-value” of 0.26 implies
the Lack of Fit is not signicant relative to the
pure error. There is a 98.40% chance that a “Lack
of Fit F-value” this large could occur dueto noise.
The t of the model was checked by the
determination coefcient (R
2
).The “Pred
R-quared” of 0.9660 is in satisfactory accordance
with the “Adj R-Squared” of 0.9768.”Adeq
Precision” measures the signal to noise ratio. A
ratio greater than 4 is desirable. In this case “Adeq
Precision” of 42.257 indicates an adequate signal.
Thus, as a result of the statistical analysis, quadratic
model was found satisfactory for describing
the process and useful for developing empirical
relation. Metronidazole removal showed to be
very sensitive to changes in the adsorbent dosage
and time of adsorption. Magnitude of F-value in
Table 2 was expressed in comparison of these
two factors adsorbent dosage was more effective
on removal efciency of metronidazole than time
of experiments. Figure 1 showed 3D plots of
interaction effects of all parameters on removal
efciency. As can be seen in Figure 1 when the
pH increased from 2 to 6, the removal efciency
increased about 4%. Also (Figure 1), the results
of the study showed that removal efciency
decreased in alkaline solutions. Adsorption
time has more effect than pH on metronidazole
Mean P-value
Source
Sum of square Df Square F-Value Prob > F
Model 3120.63835 14 222.902739 148.517383 < 0.0001
A-pH 84.4231475 1 84.4231475 56.2501161 < 0.0001
B-adsorbent
dose
537.787036 1 537.787036 358.320959 < 0.0001
C-temperature 0.66785645 1 0.66785645 0.44498463 0.5091
D-time 116.402257 1 116.402257 77.5574075 < 0.0001
AB 0.01125 1 0.01125 0.00749574 0.9315
AC 25.56125 1 25.56125 17.0311499 0.0002
AD 4.5 1 4.5 2.99829525 0.0922
BC 15.40125 1 15.40125 10.2616655 0.0029
BD 29.645 1 29.645 19.7521028 < 0.0001
CD 26.645 1 26.645 17.7532393 0.0002
A^2 1107.54922 1 1107.54922 737.946571 < 0.0001
B^2 225.03502 1 225.03502 149.938096 < 0.0001
C^2 745.166816 1 745.166816 496.495584 < 0.0001
D^2 897.389678 1 897.389678 597.919825 < 0.0001
Residual 52.52985 35 1.50085286
Lack of Fit 5.00385003 10 0.500385 0.26321645 0.9840
Pure Error 47.526 25 1.90104
Table 2. ANOVA analysis for removal of metronidazole
49
Removal of Metronidazole by Fe3O4@MWCNTs Mohammad Reza Rezaei Kahkha et al
removal in this investigation. It was found that
nearly 22.2minute was enough to obtain highest
yield.
In addition, Figuer 2 showed the parity plot of
obtained results and predicted results that explains
a satisfactory correlation between the observed
results and tted values. In this work, the plotted
residuals indicate normal distribution; the data
points form an approximately straight line. The data
points farther from the line, expressed departure
from normality [14, 15]. In this study, the residuals
are approximately plotted along straight line for
response, indicating no evidence of non-normality
or unidentied variables.
Fig.1. Response surface modeling obtained by CCD.
50
Anal. Method Environ. Chem. J. 3 (3) (2020) 44-53
3.2. Isotherm study
The adsorption isotherms for metronidazole
adsorption on MMWCNTs were obtained with
different metronidazole concentrations (1–30 mg
L
-1
). The Freundlich and the Langmuir adsorption
isotherm models were used for evaluation
experimental data. The Langmuir model and
Freundlich model are given in Equation 4 and 5
as follows:
(Eq. 4)
(Eq. 5)
Where C
f
(mg L
-1
) is the equilibrium concentration
of metronidazole, q
f
(mg g
-1
) is adsorption capacity
at equilibrium, q
m
(mg g
-1
) is the maximum
adsorption capacity, b (L mg
-1
) is a constant related
to the adsorption energy, K
f
and n are Freundlich
constants which characterize a particular adsorption
isotherm. All the constants obtained according to the
slope and intercept of the related lines and they are
listed in Table 3. As shown in Table 3, the Langmuir
isotherm plot ts better to the experimental
adsorption data with higher correlation coefcient
(R
2
= 0.9994), which expressed that the adsorption
of metronidazole ions onto MMWCNTs follows
the Langmuir model (Fig. 3).
The q
m
and b calculated from the slope and
intercept of the regression line are 215.4 mg g
-1
and 0.52 L mg
-1
, respectively (Table 3). Langmuir
model depends on the acceptation of homogeneous
distribution of metronidazole molecules on to
surface of adsorbent.
Fig.2 The normal probability plot of the residuals and parity plot show the correlation
between the observed and predicted values
Table 3. Adsorption isotherm parameters of Langmuir and Freundlich models for adsorption
of the metronidazole on the MMWCNTs
Langmuir isotherm Ferundlich isotherm
q
m
(mg g
-1
) b(L mg
-1
) R
2
K
f
(l g
-1
) n R
2
215.14 0.52 0.991 65.815 2.063 0.879
51
Removal of Metronidazole by Fe3O4@MWCNTs Mohammad Reza Rezaei Kahkha et al
Fig.3. Fitting results of Langmuir and Ferundlich
models with experimental data for adsorption of the
metronidazole on the MMWCNTs
3.3. Desorption and reusability study
Desorption studies of metronidazole were carried
out by using different volume of methanol
containing NaOH 1% as eluent solvent.
For desorption studies, when adsorption of
metronidazole was completed, the adsorbent was
magnetically separated and washed with deionized
water. Then, (0.5-5) mL of the eluent was added
to the adsorbent. After 30 min samples were
collected to evaluate the metronidazole recoveries.
Based on results, the best volume of eluent with
high recovery of metronidazole was obtained by 2
mL. To assess the reusability and stability of the
adsorbent, the adsorption–desorption experiment
with eluent (methanol containing NaOH 1%) was
repeated with 30 mg L
-1
metronidazole several
cycles. After 15 cycles, the adsorption capacity of
the MMWCNTs decreases from 163 to 87 mg g
-1
.
This result shows that the adsorbent can be applied
effectively in a real process such as pharmaceutical
industries wastewater treatment. Moreover, in
order to investigate the inter-day validation of the
results, the method was used for the determination
of metronidazole (30.0 mg l
-1
) in three consecutive
days by HPLC, and the Relative Standard Deviation
percent was found to be 4.1%. These results
indicate that MMWCNTs are usable and stable for
the extraction of metronidazole and the method
has high reproducibility and repeatability for the
determination and extraction of the antibiotics by
HPLC.
3.4. Application of proposed methods to real
sample
To verify the potential application of proposed
method to real samples, the adsorption performance
within real wastewater that spiked with different
amount of metronidazole in optimum condition
that obtained from model is also provided in
Table 4. As expected, magnetic carbon nanotubes
therefore show high removal efciency (95-102
%) for the tested concentrations. It must be noted;
the adsorbent cannot completely removed the
metronidazole due to the competition between
other substances which is also present in the
wastewater.
Table 4. Adsorption of metronidazole in wastewater samples
by spiking of metronidazole at different concentration by HPLC (mg L
-1
)
Sample Added Initial concentration *Found %Removal Efciency
Sample A
----- 100 3.7 ± 0.2 96.3
50 150 5.2 ± 0.3 96.5
Sample B
200 10.8 ± 0.5 94.6
100 300 14.2 ± 0.7 95.3
Sample C
30 0.5 ± 0.03 98.3
30 60 1.9 ± 0.9 96.8
*Mean of three determinations of samples ± condence interval (P = 0.95, n =8)
52
Anal. Method Environ. Chem. J. 3 (3) (2020) 44-53
4. Conclusions
In this study, a central composition design (CCD) was
used for evaluation of four variables of adsorption
(time, temperature, initial ion concentration and
amount of adsorbent) for Metronidazole by Fe
3
O
4
@
MWCNTs. Quadratic model was developed to
correlate the variables to the response. Through
the analysis of response surfaces, adsorbent
dose, pH and adsorption time were found to have
signicant effects on removal efciency, whereas
adsorbent dose showed that most signicant. All
removal analysis of Metronidazole were done
based on Fe
3
O
4
@MWCNTs by HPLC in water and
wastewater samples. Optimization was carried out
and the experimental values were found to agree
satisfactorily with the predicted values. Isotherm
study of process showed maximum adsorption
capacity of Fe
3
O
4
@MWCNTs for removal of
metronidazole (215 mg g
-1
). Also, application
of proposed method for real wastewater sample
showed high removal efciency of proposed
sorbent.
5. Acknowledgement
Authors hereby appreciate the staffs of health
laboratory of Zabol University of medical sciences
for their cooperation to perform this research. The
research was funded by the Zabol University of
medical sciences.
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