- Email: [email protected]
magnetite nanoparticles@polypyrrole composite
Accepted Manuscript Title: Coextraction of acidic, basic and amphiprotic pollutants using multiwalled carbon nanotubes/Fe3 O4 @polypyrrole composite Author: Ali Akbar Asgharinezhad Homeira Ebrahimzadeh PII: DOI: Reference:
S0021-9673(15)01075-4 http://dx.doi.org/doi:10.1016/j.chroma.2015.07.087 CHROMA 356714
To appear in:
Journal of Chromatography A
Received date: Revised date: Accepted date:
24-5-2015 13-7-2015 22-7-2015
Please cite this article as: A.A. Asgharinezhad, H. Ebrahimzadeh, Coextraction of acidic, basic and amphiprotic pollutants using multiwalled carbon nanotubes/Fe3 O4 @polypyrrole composite, Journal of Chromatography A (2015), http://dx.doi.org/10.1016/j.chroma.2015.07.087 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Coextraction of acidic, basic and amphiprotic pollutants using multiwalled carbon
2
nanotubes/Fe3O4@polypyrrole composite
3
Ali Akbar Asgharinezhad, Homeira Ebrahimzadeh*
4 5 6
Faculty of Chemistry, Shahid Beheshti University, G.C., Evin, Tehran, Iran
ip t
1
cr
7
us
8 9
an
10
M
11
ed
12 13
ce pt
14 15
Ac
16 17 18 19 20 21 22 23
∗
Corresponding author. Tel.: +98 21 29902891; fax: +98 21 22403041. E-mail address: [email protected] (H. Ebrahimzadeh)
1 Page 1 of 36
Abstract
25
The simultaneous extraction of acidic, basic and amphiprotic pollutants from various samples
26
is a considerable and disputable concept in sample preparation strategies. In this study, for the
27
first time, coextraction of acidic, basic and amphiprotic pollutants (polar and apolar) with
28
multiwalled carbon nanotubes/Fe3O4@polypyrrole (MWCNTs/Fe3O4@PPy) composite based
29
dispersive
30
chromatography-photo diode array detection was introduced. Firstly, the extraction efficiency
31
of various magnetic nanosorbents including Fe3O4, MWCNTs/Fe3O4, graphen oxide/Fe3O4
32
(GO/Fe3O4), Fe3O4@PPy, MWCNTs/Fe3O4 @PPy and GO/Fe3O4@PPy were compared. The
33
results revealed that MWCNTs/Fe3O4 @PPy nanocomposite has higher extraction efficiency
34
for five selected model analytes (4-nitrophenol, 3-nitroaniline, 2,4-dichloroaniline, 3,4-
35
dichloroaniline and 1-amino-2-naphthol). Box-Behnken design methodology combined with
36
desirability function approach was applied to find out the optimal experimental conditions.
37
The opted conditions were: pH of the sample, 8.2; sorbent amount, 12 mg; sorption time, 5.5
38
min; salt concentration, 14% w/w; type and volume of the eluent, 120 µL acetonitrile; elution
39
time; 2 min. Under the optimum conditions detection limits and linear dynamic ranges were
40
achieved in the range of 0.1-0.25 µg L-1 and 0.5-600 µg L-1, respectively. The percent of
41
extraction recovery and relative standard deviations (n = 5) were in the range of 45.6-82.2
42
and 4.0-8.5, respectively. Ultimately, the applicability of this method was successfully
43
confirmed by analyzing rain, snow and river water samples and satisfactory results were
44
obtained.
45
Keywords: Dispersive micro-solid phase extraction; Coextraction; Multiwalled carbon
46
nanotubes/Fe3O4@polypyrrole; High performance liquid chromatography; Pollutants.
phase
extraction
followed
by
high
performance
liquid
Ac
ce pt
ed
M
an
us
cr
micro-solid
ip t
24
47
2 Page 2 of 36
48
Highlights: A novel strategy for coextraction of acidic, basic and amphiprotic pollutants.
50
The method is based on dispersive micro solid phase extraction.
51
The extraction efficiency of various magnetic nanosorbents was compared.
52
MWCNTs/Fe3O4@PPy composite depicted higher extraction efficiency than the
54
cr
others.
The method is fast and simple, and reduces the consumption of sorbent and solvent.
us
53
ip t
49
an
55
M
56 57
ed
58
1. Introduction
60
It has been reported that aniline, phenol and their derivatives are serious environmental
61
pollutants and they are classified as the hazardous wastes and priority toxic pollutants by
62
Environmental Protection Agency of America [1,2]. Moreover, they have been suspected to
63
be carcinogenic agents [2,3]. They are used in divers manufacturing processes such as
64
pesticides and herbicides, pharmaceuticals, plastics, dyestuff, pigments, wood preservatives,
65
rubber chemicals, and explosives [4,5]. Anilines and phenols can easily permeate through soil
66
and contaminate ground water due to their high solubility in water [2]. Herein, coextraction
67
of these pollutants is in a point of view.
Ac
ce pt
59
3 Page 3 of 36
Nitrophenols are some of the most important pollutants present in the environment.
69
Nitrophenols are formed in the atmosphere from the photochemical reaction of benzene with
70
nitrogen monoxide in highly polluted air [2,6]. For example, 4-nitrophenol (4-NP) is one of
71
the 129 organic pollutants listed by the United States Environmental Protection Agency as
72
carcinogens and hazardous to human beings as well as the environment [7]. Furthermore, 4-
73
NP damages mitochondria and inhibits energy metabolism in human and animals [4,7].
74
Hence, exploring a simple, rapid, sensitive, environmentally friendly and cost effective
75
method for 4-NP determination is crucial.
76
Azo dyes are synthetic organic colorants generally produced by coupling a diazonium
77
compound with an aromatic amine or a phenol and they are used in various areas such as
78
nutrition, cosmetics, and the paper, pharmaceutical, printing ink, textile and tanning
79
industries, among others [8]. Several azo dyes used as colorants for food, drugs and
80
cosmetics can be reduced by cell suspensions of predominant intestinal anaerobes [9],
81
therefore, it can be assumed that the ingestion of certain azo dyes is a risk for human health
82
indeed. In this sense, 1-amino-2-naphthol (1-A2N), produced by the reduction of Acid
83
Orange 7, has been reported to induce bladder tumors [10]. The high toxicity of 1-A2N (EC50
84
0.1±0.03 mg L-1) is probably due to its high lipid solubility [11].
85
Aromatic amines such as chloro and nitro-substituted derivatives (CAs, NAs), have become
86
more and more significant in environmental science due to their high toxicity and their
87
suspected carcinogenic properties [12,13]. These pollutants are mainly used as intermediates
88
in the synthesis of dyes, pharmaceuticals, pesticides, herbicides and cosmetics [13], and they
89
are liberated into the environment directly as industrial waste or indirectly as breakdown
90
products of pesticides and herbicides [13,14].
Ac
ce pt
ed
M
an
us
cr
ip t
68
4 Page 4 of 36
Several analytical methods for the determination of phenol, aniline and their derivatives such
92
as high-performance liquid chromatographic (HPLC) method with ultra violet [2,6], mass
93
spectrometry [15] or electrochemical detection [16], gas chromatography with flame
94
ionization [17], or mass spectrometry detection [18], and capillary zone electrophoresis
95
[19,20] have been utilized. All the named methods have been successfully applied for routine
96
analysis of each category, but none of them afford simultaneous quantification of the
97
mentioned acidic, basic and amphiprotic pollutants in a single step.
98
Sample preparation procedures play a dominant role in chemical analyses. Extensive sample
99
cleanup procedures are usually required to remove matrix components which may interfere
100
with the analysis [21]. Liquid-liquid extraction and solid phase extraction were exceedingly
101
applied as two sample pretreatment techniques in analytical chemistry [13,22,23]. Solid phase
102
extraction has distinguished from many other extraction techniques due to the advantages
103
such as lower cost, higher enrichment factor and less consumption of organic solvents [23].
104
Though SPE has gained a lot of popularity for extraction processes, it suffers from some
105
shortcomings such as solvent loss, large secondary wastes, time and labor consuming
106
procedure, and need for complex equipment [24]. Dispersive micro-solid phase extraction
107
(D-μ-SPE) is a miniaturized alternative to SPE technique. The D-µ-SPE exhibits some
108
advantages over traditional SPE, such as high efficiency, simplicity, rapidity and less
109
consumption of organic solvent [24,25]. Moreover, this method is economic and consumes a
110
small amount of sorbent [26,27]. Various sorbents can be employed with D-µ-SPE.
111
Compared to traditional SPE sorbents, nanomaterials offer large surface area to volume ratio
112
and short diffusion route, which may result in high extraction efficiency and rapid extraction
113
kinetic. To counter the shortcomings of using nanomaterials packed into a cartridge, such as
114
high back pressure and long sample loading time; magnetic solid-phase extraction (MSPE),
115
as a novel SPE method, has been developed based on magnetic nanoparticles (MNPs) [28].
Ac
ce pt
ed
M
an
us
cr
ip t
91
5 Page 5 of 36
Magnetic nanoparticles are interesting and technologically substantial objects of physical and
117
chemical researches and are widely used in separation science [29,30]. MNPs have large
118
constant magnetic moments and can be easily gathered by applying an external magnetic field
119
placed outside the extraction container without additional time-consuming centrifugation or
120
filtration step, which makes sampling and collection easier and faster [31]. MNPs such as
121
Fe3O4 are good candidates for magnetic carrier technology by considering the main
122
advantages: (1) MNPs can be synthesized in large quantity using a simple and economic
123
method; (2) it can be expected that their sorption capacity is high due to their large surface
124
area; (3) they have strong magnetic properties and low toxicity [32,33]; and (4) these
125
particles are super paramagnetic, that means metal-loaded sorbent can be easily separated
126
from the treated sample by means of an external magnetic field [24,27]. However, the
127
disadvantages of using MNPs for sample preparation are their low selectivity toward target
128
analytes, low stability in strong acidic aqueous media and low dispersibility in various sample
129
matrices [21]. Hence, the modification of MNPs core with a suitable protective coating has
130
been proven to be one of the most efficient approaches. There has been an increasing interest
131
in the investigation of new coating materials for MSPE [31,34]. Among different types of
132
coating sorbents used for the extraction of organic compounds, conductive polymers have
133
attracted a great deal of attention due to their multifunctional properties such as
134
hydrophobicity, acid-base character, π-π interaction, polar functional groups, ion exchange
135
property, hydrogen bonding and electro-activity [35,36].
136
An alternative to solve the aggregation problem of MNPs is utilizing a support and spacer
137
such as carbon nano tubes (CNTs) and graphen sheets [37,38]. Nanotubes have become
138
attractive candidates for the sorption of diverse compounds at trace level because of their
139
large surface area. Their large surface area makes them a promising solid sorbent for SPE
140
procedure [38,39]. However, the poor dispersibility of CNTs and difficulty in collecting them
Ac
ce pt
ed
M
an
us
cr
ip t
116
6 Page 6 of 36
from the extraction media causes much labor for their practical application. Hence, by
142
combining the attractive surface properties, nanoscaled and tubular structures of CNTs with
143
magnetic properties of Fe3O4, magnetic carbon nanotubes (MCNTs) are expected to be
144
excellent sorbents. The ability of being easily dispersed and isolated from the extraction
145
container by an external magnetic field is the result of combining and utilizing diverse
146
properties in MCNTs [40].
147
In this context, the aim is to develop a magnetic d-µ-SPE method for the coextraction and
148
determination of some priority acidic, basic and amphiprotic model pollutants in various
149
samples for the first time. In a previous work, a polypyrrole/multiwalled carbon nanotubes
150
composite decorated with Fe3O4 NPs was chemically synthesized and applied as an adsorbent
151
for the extraction of methocarbamol from human plasma [41]. However the synthesis
152
procedure that used in this work is different from the former work basically. In another work,
153
a nanocomposite fabricated from monodispersed 4-nm Fe3O4 coated on the surface of
154
carboxylic acid containing multi-walled carbon nanotube and polypyrrole was synthesized
155
but any analytical application in terms of extraction methods was not described [42]. Yang et
156
al. used MWCNTs for aqueous adsorption of aniline, phenol, and their substitutes, without
157
involving preconcentration and elution steps [43]. To the best of our knowledge, there is no
158
report on the coextraction of acidic (4-nitrophenol), basic (3-nitroaniline, 2,4-dicoloroaniline
159
and
160
MWCNTs/Fe3O4@PPy based d-µ-SPE method. The special properties of this nanocomposite
161
made the coextraction of the analytes of interest efficient. The nanosorbent were isolated
162
using a supermagnet; which makes it particularly suitable for sample preparation since no
163
centrifugation or filtration is needed. Afterward, Box-Behnken design methodology
164
combined with desirability function approach was applied to find out the optimal
ce pt
ed
M
an
us
cr
ip t
141
Ac
3,4-dicoloroaniline)
and
amphiprotic
(1-amino-2-naphthol)
pollutants
using
7 Page 7 of 36
165
experimental conditions. Finally, the opted procedure was applied to determine the model
166
analytes in various matrices satisfactorily.
167
2. Experimental
169
2.1. Chemicals and reagents
170
4-NP, 3-NA, 2,4-DCA, 3,4-DCA and 1-A2N were purchased from Sigma-Aldrich
171
(Milwaukee, WI, USA). NaCl, HCl, ferric chloride (FeCl3), ammonium ferrosulphate
172
((NH4)2Fe(SO4)2.6H2O), ammonium hydroxide (28% w/v), NaOH, acetone, acetic acid
173
(HOAc), dimethylformamide (DMF), ethanol and 2-propanol which all were of analytical
174
grade were supplied by Merck (Darmstadt, Germany). HPLC grade acetonitrile (ACN) and
175
methanol (MeOH) were purchased from Caledon (George-town, Ont., Canada). Graphene
176
oxide was obtained from Nano Sakhtare Avizhe (Tehran, Iran). The oxidized multiwalled
177
carbon nanotube (MWCNTs) with 40-60 nm diameters, 1-25 μm length and 5-10 nm core
178
diameters was purchased from Research Institute of the Petroleum Industry (Tehran, Iran).
179
Ultrapure water was prepared using a milli-Q system from millipore (Bedford, MA, USA).
180
Rain and snow water samples were collected during April 2013 and February 2014,
181
respectively. River water sample was collected from Karaj River (Karaj, Iran).
ce pt
ed
M
an
us
cr
ip t
168
Ac
182 183
2.2. Equipment
184
Analysis of the standard and test samples was performed by Shimadzu SCL-10AVP HPLC
185
instrument from Shimadzu Company (Tokyo, Japan) combined with an LC-10AVP pump,
186
SPD-M10AVP diode array detector (DAD), a Rheodyne 7725i (PerkinElmer, USA) injector,
187
along with a 100 µL sample loop. The LC-solution program for LC was used to perform data
188
processing. A capital HPLC column (Scotland, UK) ODS-H C18 (250 mm × 4.6 mm, i.d. 5
189
µm) was employed for all separations. The mobile phase was a mixture of deionized water 8 Page 8 of 36
and acetonitrile (40:60, v/v) in isocratic elution mode at the flow rate of 1 mL min −1 with the
191
detector wavelength set at 230 nm for 3-NA, 240 nm for 2,3-DCA, 2,4,DCA and 1-A2N and
192
315 nm for 4-NP. The pH of solutions was adjusted by using a methrohm digital pH meter
193
827 equipped with a glass calomel electrode. In the extraction procedure, a 25 mL sample
194
vial, and a MR 3001 heating-magnetic stirrer from Heidolph Company (Kelheim, Germany)
195
and a MS3 digital vortex agitator from IKA Company (Staufen, Germany) were used to mix
196
the MNPs with the sample. EBA 20 Hettich centrifuge (Oxford, England) and a 50 and 500
197
µL Hamilton HPLC syringe (Reno, NV, USA) were applied, too. The morphology and
198
dimension of the nanocomposites were explored by a scanning electron microscope (SEM)
199
model VEGA TESCAN (Brno, Czech Republic). IR spectra were recorded by a Bruker IFS-
200
66 FT-IR Spectrophotometer (Bruker Optics, Karlsruhe, Germany). The energy dispersive X-
201
ray spectroscopy (EDX) was performed on a VEGAII TESCAN.
M
an
us
cr
ip t
190
202
2.3. Preparation of standard solutions and real samples
204
Stock standard solution of pollutants (1000 mg L -1) was prepared in HPLC grade methanol
205
and stored in a fridge at 4 ºC and brought to ambient temperature just prior to use. Mixed
206
working solutions of the analytes at different concentrations were prepared by dilution with
207
ultra-pure water or deionized water containing various NaCl concentrations. The water
208
samples were filtered through a Millipore 0.22-μm cellulous acetate filter before the
209
extraction process. Then 20 mL spiked/non-spiked of each sample was used without any
210
dilution.
Ac
ce pt
ed
203
211 212
2.4. Synthesis of the nanosorbents
213
2.4.1. Synthesis of the MWCNTs/Fe3O4@PPy nanocomposite
9 Page 9 of 36
The MWCNTs/Fe3O4 was synthesized according to the literature [38]. At first, MWCNTs
215
were purified with 1 mol L-1 HNO3 solution for 6 h at room temperature and then were
216
washed several times with distilled water and dried at 100 °C, then 0.5 g MWCNTs was
217
suspended in 250 mL of solution containing 0.85 g (NH4)2Fe(SO4)2 .6 H2O and 0.422 g FeCl3
218
at 50 °C under nitrogen atmosphere. Afterward, the suspension was sonicated for 20 min, and
219
then 20 mL of 8 mol L-1 NH4OH aqueous solution was added drop wise to precipitate the
220
Fe3O4 NPs on the wall of MWCNTs while the mixture solution was being sonicated. The pH
221
of the final mixture was controlled in the range of 10-11. To promote the complete growth of
222
the nanoparticles crystals, the reaction was done at 50 °C for 30 min. The dispersion was
223
cooled to room temperature and magnetic MWCNT (MMWCNT) was collected from the
224
mixture with the help of a strong permanent magnet. Separated MMWCNT was washed
225
thrice with deionized water followed by ethanol. The synthesis of MMWCNT was confirmed
226
by IR spectroscopy.
227
To synthesis MWCNTs/Fe3O4@PPy nanocomposite 0.6 g of the dried MWCNTs/Fe3O4 was
228
added to 250 mL deionized water at pH 9 under stirring for 5 min and then, 0.4 mL pyrrole
229
was added and stirred for 10 min. Subsequently, 0.8 g sodium perchlorate was added to the
230
mixture and stirred for 5 min. In the end, 0.56 g of FeCl3 was dissolved in 50 mL deionized
231
water and added dropwise to the mixture under stirring. Subsequently, polymerization
232
reaction was continued for 15 h and finally, the nanocomposite was obtained. The precipitate
233
was washed several times by double distilled water and methanol, respectively. The washing
234
procedure was continued until the filtrate become colorless, and then it was dried. The
235
synthesis of MWCNTs/Fe3O4@PPy was confirmed by IR spectroscopy, SEM and EDX
236
analysis.
Ac
ce pt
ed
M
an
us
cr
ip t
214
237 238
2.4.2. Synthesis of the GO/Fe3O4@PPy nanocomposite
10 Page 10 of 36
239
The GO/Fe3O4 was synthesized according to the literature. [45]. At first, 0.5 g GO was
240
suspended in 500 mL water and ultrasonicated for 3 h to obtain a clear dispersion. Then, 0.85
241
g (NH4)2Fe(SO4)2 .6 H2O and 0.422 g FeCl3 was added to the mixture at 50 °C under nitrogen
242
atmosphere. Afterward, the suspension was sonicated for 10 min, and then 20 mL of 8 mol L -
243
1
244
while the mixture being sonicated. To promote the complete growth of the nanoparticles
245
crystals, the reaction was done at 50 °C for 30 min. The dispersion was cooled to room
246
temperature and magnetic GO (MGO) was collected from the mixture with the help of a
247
strong permanent magnet. Separated MGO was washed five times with deionized water
248
followed by ethanol. The synthesis of MGO was confirmed by IR spectroscopy.
249
To synthesis GO/Fe3O4@PPy nanocomposite, 0.6 g of the dried GO/Fe3O4 was added to 250
250
mL deionized water at pH 9 under ultrasonication for 60 min and then, 0.5 mL pyrrole
251
monomers was added and the mixture was stirred for 10 min. Subsequently, 0.8 g of sodium
252
perchlorate was added to the mixture and stirred for 10 min. In the end, 0.56 g of FeCl3 was
253
dissolved in 50 mL deionized water and added dropwise to the mixture under stirring.
254
Subsequently, polymerization reaction was continued for 15 h and finally, the nanocomposite
255
was formed. The precipitate was washed several times by double distilled water and
256
methanol, respectively. The washing procedure was continued until the filtrate become
257
colorless, and then it was dried. The synthesis of GO/Fe 3O4 @PPy was confirmed by IR
258
spectroscopy and SEM analysis.
ip t
cr
us
an
M
ed
ce pt
Ac
259
NH4OH aqueous solution was added drop wise to precipitate the Fe 3O4 NPs on GO sheets
260
2.4.3. Synthesis of Fe3O4@PPy NPs
261
The Fe3O4 NPs were prepared by the chemical co-precipitation method [21]. Briefly, 7.2 g of
262
FeCl3, 7.84 g of ((NH4)2Fe(SO4)2.6H2O) were dissolved in 400 mL of deionized water in a
263
beaker which was degassed with nitrogen gas for 10 min. Then, this stock solution was added
11 Page 11 of 36
to a reactor vessel and heated to 80 °C. Afterwards, 20 mL of ammonia solution (25% w/w)
265
was added to the reactor under the nitrogen gas protection and vigorous stirring (1000 rpm).
266
During the whole process, temperature of the solution was maintained at 80 °C and nitrogen
267
gas was used to prevent the intrusion of oxygen.
268
Fe3O4@PPy NPs were synthesized according to Tahmasebi et al. with some modifications.
269
[45]. One gram of the dried Fe3O4 NPs was added to 400 mL deionized water at pH 9 under
270
stirring for 5 min and then, 0.5 mL of pyrrole monomers was added and stirred for 10 min.
271
Subsequently, 1 g of sodium perchlorate was added to the mixture and stirred for 10 min.
272
Then, 0.9 g of FeCl3 was dissolved in 50 mL deionized water and added dropwise to the
273
mixture under stirring. Subsequently, polymerization reaction was continued for 15 h and
274
finally, a precipitate (Fe3O4@PPy) was obtained. The precipitate was washed several times
275
by double distilled water and methanol, respectively. The washing procedure was continued
276
until the filtrate become colorless, and then it was dried. The synthesis of GO/Fe 3O4@PPy
277
was confirmed by IR spectroscopy and SEM analysis.
ed
M
an
us
cr
ip t
264
278
2.5. MWCNTs/Fe3O4@PPy based d-µ-SPE procedure
280
Extraction
281
MWCNTs/Fe3O4@PPy (12 mg) was put into a 25 mL sample vial and 20 mL aqueous solution
282
of the analytes (0.5 mg L-1) was added to the vial, (2) pH of the solution was adjusted to 8.2
283
with 2 mol L-1 NaOH, (3) the solution was stirred at a constant rate of 1250 rpm to facilitate
284
mass transfer and sorption of the model analytes onto the sorbent, (4) afterward, the sorbent
285
was separated quickly from the sample solutions, by the use of the supermagnet, (5) 15 mL
286
of the supernatant was decantated and the remained 5 mL solution was transferred to a 6 mL
287
conical tube and after decantation, the sorbent was eluted with 120 μL of ACN by fierce
ce pt
279
was
performed
according
to
the
following
steps:
(1)
Ac
procedure
12 Page 12 of 36
288
vortex for 2.0 min. (6) Finally, the eluate was isolated from the sorbent by the magnet and
289
100 μL of eluate was injected into the HPLC instrument for subsequent analysis.
290
2.6. Experimental design methodology and desirability function approach
292
A principal motivation for improving a new separation and quantification method is reducing
293
the required time and number of trials that ends in total required costs, therefore in order to
294
optimize the preconcentration of the model analytes by the proposed method, a Box-Behnken
295
design (BBD) in combination with desirability function (DF) was employed. It’s worth to
296
note that for an experimental design involving five variables expressed by BBD, linear,
297
quadratic and cross terms can be involved. The precise optimum point can be obtained by the
298
aid of response surface methodologies exhibiting relationships between variables and
299
responses graphically [46,47].
300
In the case of multiple response optimization, the Derringer function or desirability function
301
(DF) can be applied, since it’s the most critical and most widely used multi-criteria
302
methodology in analytical procedures [48]. At first, in DF approach, each predicted response
303
is transformed to a dimensionless desirability value (d) and then all transformed responses are
304
combined into one particular response. The scale of the individual DF ranges between 0-1,
305
while for the most desirable response d is equal to 1 and for a completely undesired response
306
d is 0 [49]. Different transformations on data may be implemented depending on whether the
307
response is optimum when it is maximized, minimized, or at a predefined value [50].
308
In this work, the experimental design matrix and data analysis were carried out by the
309
Design-Expert statistical software program (7.0.0 trial version).
Ac
ce pt
ed
M
an
us
cr
ip t
291
310 311
3. Results and discussion 13 Page 13 of 36
3.1. Characterization of the nanosorbents
313
The morphology and dimension of the prepared Fe3O4@PPy NPs, MWCNTs/Fe3O4 @PPy
314
and GO/Fe3O4@PPy nanocomposite were explored by SEM analysis. As depicted in Fig. 1a,
315
the prepared Fe3O4@PPy NPs are nearly spherical in shape with an average diameter of about
316
40-50 nm Fe3O4 @PPy NPs. Fe3O4 NPs tended to aggregate to form larger particles which is
317
related to their large specific surface area, high surface energy and magnetization. The SEM
318
image of prepared Fe3O4@PPy NPs exhibited less agglomeration than bare Fe3O4 that is
319
attributed to PPy coating layer [21]. As illustrated in Fig. 1b, Fe 3O4 NPs were successfully
320
coated on the surface of GO to form a GO/Fe3O4 @PPy nanocomposite. The average size of
321
the Fe3O4 NPs in this nanocomposite was about 45 nm. The SEM image of
322
MWCNTs/Fe3O4@PPy nanocomposite (Fig. 1c and 1d) exhibits that the surfaces of
323
MWCNTs were densely coated with Fe3O4 NPs. Moreover, it is indicated that Fe3O4 NPs
324
have grown on the surface of MWCNTs and well distributed, hence the aggregation problem
325
of MNPs is solved by utilizing MWCNTs as a support and spacer [37]. Also the diameters of
326
MWCNTs were 50-60 nm with Fe3O4 NPs coated on their surfaces.
cr
us
an
M
ed
ce pt
327
ip t
312
The energy dispersive x-ray (EDX) spectrometry analysis of MWCNTs/Fe3O4 @PPy
329
nanocomposite (Fig. 1S, Electronic Supplementary Information) demonstrated the peaks
330
were associated with Fe, C, N, O, and Cl atoms with molar ratio of 53.53%, 32.56%, 2.93%,
331
10.48% and 0.50%, respectively. These results confirmed the existence of polypyrrole,
332
perchlorate as a dopant, and Fe3O4 NPs on the surface of MWCNTs with no impurity.
333
The FT-IR spectra of Fe3O4 and Fe3O4@PPy NPs, MWCNTs/Fe3O4 @PPy and
334
GO/Fe3O4 @PPy nanocomposites were recorded using KBr pellet method (Fig. 2S, Electronic
335
Supplementary Information). In the IR spectrum of Fe3O4, absorptions peaks of Fe-O were
336
observed at 584 and 3435 cm-1 which demonstrates the successful synthesis of Fe3O4
Ac
328
14 Page 14 of 36
337
nanoparticles. On the other hand, the absorption peaks appeared at 1196, 1201, and 1198 cm-1
338
(C-N),
339
GO/Fe3O4 @PPy nanocomposites respectively, confirmed the presence of polypyrrole on the
340
surface of these sorbents. Furthermore, the peaks at 3411, 3374 and 3441 cm−1 in the FT-IR
341
spectra of Fe3O4 @PPy NPs, MWCNTs/Fe3O4 @PPy and GO/Fe3O4@PPy nanocomposites
342
respectively are attributed to N-H band of PPy [51].
FT-IR
spectra of Fe3O4 @PPy NPs,
MWCNTs/Fe3O4@PPy and
ip t
in the
cr
343
3.2. Optimization of MWCNTs/Fe3O4@PPy based d-μ-SPE parameters
345
The factors affecting the extraction efficiency of the proposed method such as sorbent type,
346
pH of the sample, amount of sorbent, sorption time, salt content of sample solution and
347
elution conditions (eluent type and volume) were explored and opted. Out of these seven
348
factors, sorbent and eluent types were optimized using one variable at a time method. The
349
optimization of the five other factors was performed using a response surface methodology
350
approach.
an
M
ed ce pt
351
us
344
3.2.1. Sorbent type
353
The sorbent type is a crucial variable affecting the extraction efficiency. In this context,
354
extraction abilities of bare Fe3O4 NPs, Fe3O4@PPy, MWCNTs/Fe3O4, GO/Fe3O4,
355
MWCNTs/Fe3O4@PPy and GO/Fe3O4@PPy were investigated comprehensively (Fig. 3a).
356
The results revealed that MWCNTs/Fe3O4 @PPy and GO/Fe3O4 @PPy can act as the best
357
sorbents due to the presence of PPy coating, higher surface area and higher dispersibility of
358
these sorbents compared to Fe3O4 and Fe3O4@PPy NPs. Although the existence of Fe3O4 NPs
359
on the surface of MWCNTs and GO can decrease the surface active site, PPy can compensate
360
this reduction by enhancing the active surface sites on the sorbent through ion-pair and
361
hydrogen bonding formation, electrostatic and hydrophobic interactions with the target
Ac
352
15 Page 15 of 36
analytes [37]. The anionic form of 4-NP and 1-A2N can interact with positive charged site of
363
PPy (ion-pair formation, electrostatic interaction) and basic analytes can interact through
364
hydrophobic interaction with pyrrole ring as well as GO and MWCNTs skeleton. Moreover,
365
π-π interaction is a possible driving force for sorption of all the analytes on carbon skeleton.
366
As depicted in Fig. 2a the performance of MWCNTs/Fe3O4@PPy is slightly higher than
367
GO/Fe3O4 @PPy. Moreover, the required time for gathering MWCNTs/Fe 3O4 @PPy was less
368
than GO/Fe3O4@PPy. Thus MWCNTs/Fe3O4 @PPy was selected as the most appropriate
369
sorbent for the rest of the studies.
us
cr
ip t
362
370
3.2.2. Selection of eluent
372
After sorption, the analytes should be eluted from the sorbent using an appropriate eluent
373
solvent prior to HPLC analysis. The elution of the target analytes was explored using
374
different water miscible organic solvents, i.e. methanol, acetone, acetonitrile, 2-propanol,
375
DMF, 5% (v/v) and 10% HOAc in methanol, 5% and 10% (v/v) HOAc in ACN, 0.05 mol L -1
376
HCl in ACN and 0.05 mol L-1 HCl in methanol since they are HPLC compatible solvents.
377
Among the different eluents, desorption ability of ACN was found to be higher than the other
378
solvents (Fig. 2b). This can be explained by suitable solubility of the model analytes in ACN.
379
M
ed
ce pt
Ac
380
an
371
381
3.2.3. Box-Behnken design and desirability function
382
In the next step, the affecting factors were selected based on preliminary experiments and
383
opted by a BBD experiment. In other words, BBD was applied to optimize the effect of five
384
factors (pH of sample, sorption time, sorbent amount, salt content of sample solution and
385
eluent volume). According to the experiment equation obeying BBD; N = 2K (K-1) + C0,
386
where K is the number of variables and Co is the number of center points, K and C0 were set 16 Page 16 of 36
at 5 and 6, respectively, which mean that 46 experiments should be performed [52]. By
388
applying Derringer’s desirability function, in which responses are simultaneously optimized,
389
the position of the optimum extraction condition was determined; predicting the design space
390
and the response and from the polynomial equation. A unique function named Global
391
Desirability function (D or DF) or geometric mean (Geo mean) is obtained when n variables
392
(factor and responses levels) are combined after transformation in desirability functions.
393
For the best joint response acquisition, the following equation can be implemented:
us
cr
ip t
387
an
394
Eq. 1
where ri is the importance of each variable relative to the others. A matter of the utmost
396
importance is maximization of DF in the optimization procedure, i.e. when DF (ranging from
397
0 to 1) is a non-zero value, all the variables which are simultaneously opted can be supposed
398
to have a desirable value [47]. Obtaining an appropriate set of conditions that will meet all
399
the determined criteria is the main goal of an optimization procedure and attaining a DF=1 is
400
not an objective. In order to obtain the best extraction conditions in this study, the results of
401
BBD were investigated according to the criteria assigned based on desirable levels of factors
402
and response (Table 1). To get desired extraction efficiency as objective function, Geo mean
403
as an indicator of extraction efficiency was maximized. An initial data preprocessing, i.e.,
404
normalizing the related response of each analytes is necessary. Subsequently, the obtained DF
405
would be an input value for BBD. In this way, firstly the responses of each analyte are
406
normalized, i.e. each predicted response is transformed to a dimensionless desirability value
407
(d) and then all transformed responses are combined into one particular response (Geo mean).
408
S0021-9673(15)01075-4 http://dx.doi.org/doi:10.1016/j.chroma.2015.07.087 CHROMA 356714
To appear in:
Journal of Chromatography A
Received date: Revised date: Accepted date:
24-5-2015 13-7-2015 22-7-2015
Please cite this article as: A.A. Asgharinezhad, H. Ebrahimzadeh, Coextraction of acidic, basic and amphiprotic pollutants using multiwalled carbon nanotubes/Fe3 O4 @polypyrrole composite, Journal of Chromatography A (2015), http://dx.doi.org/10.1016/j.chroma.2015.07.087 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Coextraction of acidic, basic and amphiprotic pollutants using multiwalled carbon
2
nanotubes/Fe3O4@polypyrrole composite
3
Ali Akbar Asgharinezhad, Homeira Ebrahimzadeh*
4 5 6
Faculty of Chemistry, Shahid Beheshti University, G.C., Evin, Tehran, Iran
ip t
1
cr
7
us
8 9
an
10
M
11
ed
12 13
ce pt
14 15
Ac
16 17 18 19 20 21 22 23
∗
Corresponding author. Tel.: +98 21 29902891; fax: +98 21 22403041. E-mail address: [email protected] (H. Ebrahimzadeh)
1 Page 1 of 36
Abstract
25
The simultaneous extraction of acidic, basic and amphiprotic pollutants from various samples
26
is a considerable and disputable concept in sample preparation strategies. In this study, for the
27
first time, coextraction of acidic, basic and amphiprotic pollutants (polar and apolar) with
28
multiwalled carbon nanotubes/Fe3O4@polypyrrole (MWCNTs/Fe3O4@PPy) composite based
29
dispersive
30
chromatography-photo diode array detection was introduced. Firstly, the extraction efficiency
31
of various magnetic nanosorbents including Fe3O4, MWCNTs/Fe3O4, graphen oxide/Fe3O4
32
(GO/Fe3O4), Fe3O4@PPy, MWCNTs/Fe3O4 @PPy and GO/Fe3O4@PPy were compared. The
33
results revealed that MWCNTs/Fe3O4 @PPy nanocomposite has higher extraction efficiency
34
for five selected model analytes (4-nitrophenol, 3-nitroaniline, 2,4-dichloroaniline, 3,4-
35
dichloroaniline and 1-amino-2-naphthol). Box-Behnken design methodology combined with
36
desirability function approach was applied to find out the optimal experimental conditions.
37
The opted conditions were: pH of the sample, 8.2; sorbent amount, 12 mg; sorption time, 5.5
38
min; salt concentration, 14% w/w; type and volume of the eluent, 120 µL acetonitrile; elution
39
time; 2 min. Under the optimum conditions detection limits and linear dynamic ranges were
40
achieved in the range of 0.1-0.25 µg L-1 and 0.5-600 µg L-1, respectively. The percent of
41
extraction recovery and relative standard deviations (n = 5) were in the range of 45.6-82.2
42
and 4.0-8.5, respectively. Ultimately, the applicability of this method was successfully
43
confirmed by analyzing rain, snow and river water samples and satisfactory results were
44
obtained.
45
Keywords: Dispersive micro-solid phase extraction; Coextraction; Multiwalled carbon
46
nanotubes/Fe3O4@polypyrrole; High performance liquid chromatography; Pollutants.
phase
extraction
followed
by
high
performance
liquid
Ac
ce pt
ed
M
an
us
cr
micro-solid
ip t
24
47
2 Page 2 of 36
48
Highlights: A novel strategy for coextraction of acidic, basic and amphiprotic pollutants.
50
The method is based on dispersive micro solid phase extraction.
51
The extraction efficiency of various magnetic nanosorbents was compared.
52
MWCNTs/Fe3O4@PPy composite depicted higher extraction efficiency than the
54
cr
others.
The method is fast and simple, and reduces the consumption of sorbent and solvent.
us
53
ip t
49
an
55
M
56 57
ed
58
1. Introduction
60
It has been reported that aniline, phenol and their derivatives are serious environmental
61
pollutants and they are classified as the hazardous wastes and priority toxic pollutants by
62
Environmental Protection Agency of America [1,2]. Moreover, they have been suspected to
63
be carcinogenic agents [2,3]. They are used in divers manufacturing processes such as
64
pesticides and herbicides, pharmaceuticals, plastics, dyestuff, pigments, wood preservatives,
65
rubber chemicals, and explosives [4,5]. Anilines and phenols can easily permeate through soil
66
and contaminate ground water due to their high solubility in water [2]. Herein, coextraction
67
of these pollutants is in a point of view.
Ac
ce pt
59
3 Page 3 of 36
Nitrophenols are some of the most important pollutants present in the environment.
69
Nitrophenols are formed in the atmosphere from the photochemical reaction of benzene with
70
nitrogen monoxide in highly polluted air [2,6]. For example, 4-nitrophenol (4-NP) is one of
71
the 129 organic pollutants listed by the United States Environmental Protection Agency as
72
carcinogens and hazardous to human beings as well as the environment [7]. Furthermore, 4-
73
NP damages mitochondria and inhibits energy metabolism in human and animals [4,7].
74
Hence, exploring a simple, rapid, sensitive, environmentally friendly and cost effective
75
method for 4-NP determination is crucial.
76
Azo dyes are synthetic organic colorants generally produced by coupling a diazonium
77
compound with an aromatic amine or a phenol and they are used in various areas such as
78
nutrition, cosmetics, and the paper, pharmaceutical, printing ink, textile and tanning
79
industries, among others [8]. Several azo dyes used as colorants for food, drugs and
80
cosmetics can be reduced by cell suspensions of predominant intestinal anaerobes [9],
81
therefore, it can be assumed that the ingestion of certain azo dyes is a risk for human health
82
indeed. In this sense, 1-amino-2-naphthol (1-A2N), produced by the reduction of Acid
83
Orange 7, has been reported to induce bladder tumors [10]. The high toxicity of 1-A2N (EC50
84
0.1±0.03 mg L-1) is probably due to its high lipid solubility [11].
85
Aromatic amines such as chloro and nitro-substituted derivatives (CAs, NAs), have become
86
more and more significant in environmental science due to their high toxicity and their
87
suspected carcinogenic properties [12,13]. These pollutants are mainly used as intermediates
88
in the synthesis of dyes, pharmaceuticals, pesticides, herbicides and cosmetics [13], and they
89
are liberated into the environment directly as industrial waste or indirectly as breakdown
90
products of pesticides and herbicides [13,14].
Ac
ce pt
ed
M
an
us
cr
ip t
68
4 Page 4 of 36
Several analytical methods for the determination of phenol, aniline and their derivatives such
92
as high-performance liquid chromatographic (HPLC) method with ultra violet [2,6], mass
93
spectrometry [15] or electrochemical detection [16], gas chromatography with flame
94
ionization [17], or mass spectrometry detection [18], and capillary zone electrophoresis
95
[19,20] have been utilized. All the named methods have been successfully applied for routine
96
analysis of each category, but none of them afford simultaneous quantification of the
97
mentioned acidic, basic and amphiprotic pollutants in a single step.
98
Sample preparation procedures play a dominant role in chemical analyses. Extensive sample
99
cleanup procedures are usually required to remove matrix components which may interfere
100
with the analysis [21]. Liquid-liquid extraction and solid phase extraction were exceedingly
101
applied as two sample pretreatment techniques in analytical chemistry [13,22,23]. Solid phase
102
extraction has distinguished from many other extraction techniques due to the advantages
103
such as lower cost, higher enrichment factor and less consumption of organic solvents [23].
104
Though SPE has gained a lot of popularity for extraction processes, it suffers from some
105
shortcomings such as solvent loss, large secondary wastes, time and labor consuming
106
procedure, and need for complex equipment [24]. Dispersive micro-solid phase extraction
107
(D-μ-SPE) is a miniaturized alternative to SPE technique. The D-µ-SPE exhibits some
108
advantages over traditional SPE, such as high efficiency, simplicity, rapidity and less
109
consumption of organic solvent [24,25]. Moreover, this method is economic and consumes a
110
small amount of sorbent [26,27]. Various sorbents can be employed with D-µ-SPE.
111
Compared to traditional SPE sorbents, nanomaterials offer large surface area to volume ratio
112
and short diffusion route, which may result in high extraction efficiency and rapid extraction
113
kinetic. To counter the shortcomings of using nanomaterials packed into a cartridge, such as
114
high back pressure and long sample loading time; magnetic solid-phase extraction (MSPE),
115
as a novel SPE method, has been developed based on magnetic nanoparticles (MNPs) [28].
Ac
ce pt
ed
M
an
us
cr
ip t
91
5 Page 5 of 36
Magnetic nanoparticles are interesting and technologically substantial objects of physical and
117
chemical researches and are widely used in separation science [29,30]. MNPs have large
118
constant magnetic moments and can be easily gathered by applying an external magnetic field
119
placed outside the extraction container without additional time-consuming centrifugation or
120
filtration step, which makes sampling and collection easier and faster [31]. MNPs such as
121
Fe3O4 are good candidates for magnetic carrier technology by considering the main
122
advantages: (1) MNPs can be synthesized in large quantity using a simple and economic
123
method; (2) it can be expected that their sorption capacity is high due to their large surface
124
area; (3) they have strong magnetic properties and low toxicity [32,33]; and (4) these
125
particles are super paramagnetic, that means metal-loaded sorbent can be easily separated
126
from the treated sample by means of an external magnetic field [24,27]. However, the
127
disadvantages of using MNPs for sample preparation are their low selectivity toward target
128
analytes, low stability in strong acidic aqueous media and low dispersibility in various sample
129
matrices [21]. Hence, the modification of MNPs core with a suitable protective coating has
130
been proven to be one of the most efficient approaches. There has been an increasing interest
131
in the investigation of new coating materials for MSPE [31,34]. Among different types of
132
coating sorbents used for the extraction of organic compounds, conductive polymers have
133
attracted a great deal of attention due to their multifunctional properties such as
134
hydrophobicity, acid-base character, π-π interaction, polar functional groups, ion exchange
135
property, hydrogen bonding and electro-activity [35,36].
136
An alternative to solve the aggregation problem of MNPs is utilizing a support and spacer
137
such as carbon nano tubes (CNTs) and graphen sheets [37,38]. Nanotubes have become
138
attractive candidates for the sorption of diverse compounds at trace level because of their
139
large surface area. Their large surface area makes them a promising solid sorbent for SPE
140
procedure [38,39]. However, the poor dispersibility of CNTs and difficulty in collecting them
Ac
ce pt
ed
M
an
us
cr
ip t
116
6 Page 6 of 36
from the extraction media causes much labor for their practical application. Hence, by
142
combining the attractive surface properties, nanoscaled and tubular structures of CNTs with
143
magnetic properties of Fe3O4, magnetic carbon nanotubes (MCNTs) are expected to be
144
excellent sorbents. The ability of being easily dispersed and isolated from the extraction
145
container by an external magnetic field is the result of combining and utilizing diverse
146
properties in MCNTs [40].
147
In this context, the aim is to develop a magnetic d-µ-SPE method for the coextraction and
148
determination of some priority acidic, basic and amphiprotic model pollutants in various
149
samples for the first time. In a previous work, a polypyrrole/multiwalled carbon nanotubes
150
composite decorated with Fe3O4 NPs was chemically synthesized and applied as an adsorbent
151
for the extraction of methocarbamol from human plasma [41]. However the synthesis
152
procedure that used in this work is different from the former work basically. In another work,
153
a nanocomposite fabricated from monodispersed 4-nm Fe3O4 coated on the surface of
154
carboxylic acid containing multi-walled carbon nanotube and polypyrrole was synthesized
155
but any analytical application in terms of extraction methods was not described [42]. Yang et
156
al. used MWCNTs for aqueous adsorption of aniline, phenol, and their substitutes, without
157
involving preconcentration and elution steps [43]. To the best of our knowledge, there is no
158
report on the coextraction of acidic (4-nitrophenol), basic (3-nitroaniline, 2,4-dicoloroaniline
159
and
160
MWCNTs/Fe3O4@PPy based d-µ-SPE method. The special properties of this nanocomposite
161
made the coextraction of the analytes of interest efficient. The nanosorbent were isolated
162
using a supermagnet; which makes it particularly suitable for sample preparation since no
163
centrifugation or filtration is needed. Afterward, Box-Behnken design methodology
164
combined with desirability function approach was applied to find out the optimal
ce pt
ed
M
an
us
cr
ip t
141
Ac
3,4-dicoloroaniline)
and
amphiprotic
(1-amino-2-naphthol)
pollutants
using
7 Page 7 of 36
165
experimental conditions. Finally, the opted procedure was applied to determine the model
166
analytes in various matrices satisfactorily.
167
2. Experimental
169
2.1. Chemicals and reagents
170
4-NP, 3-NA, 2,4-DCA, 3,4-DCA and 1-A2N were purchased from Sigma-Aldrich
171
(Milwaukee, WI, USA). NaCl, HCl, ferric chloride (FeCl3), ammonium ferrosulphate
172
((NH4)2Fe(SO4)2.6H2O), ammonium hydroxide (28% w/v), NaOH, acetone, acetic acid
173
(HOAc), dimethylformamide (DMF), ethanol and 2-propanol which all were of analytical
174
grade were supplied by Merck (Darmstadt, Germany). HPLC grade acetonitrile (ACN) and
175
methanol (MeOH) were purchased from Caledon (George-town, Ont., Canada). Graphene
176
oxide was obtained from Nano Sakhtare Avizhe (Tehran, Iran). The oxidized multiwalled
177
carbon nanotube (MWCNTs) with 40-60 nm diameters, 1-25 μm length and 5-10 nm core
178
diameters was purchased from Research Institute of the Petroleum Industry (Tehran, Iran).
179
Ultrapure water was prepared using a milli-Q system from millipore (Bedford, MA, USA).
180
Rain and snow water samples were collected during April 2013 and February 2014,
181
respectively. River water sample was collected from Karaj River (Karaj, Iran).
ce pt
ed
M
an
us
cr
ip t
168
Ac
182 183
2.2. Equipment
184
Analysis of the standard and test samples was performed by Shimadzu SCL-10AVP HPLC
185
instrument from Shimadzu Company (Tokyo, Japan) combined with an LC-10AVP pump,
186
SPD-M10AVP diode array detector (DAD), a Rheodyne 7725i (PerkinElmer, USA) injector,
187
along with a 100 µL sample loop. The LC-solution program for LC was used to perform data
188
processing. A capital HPLC column (Scotland, UK) ODS-H C18 (250 mm × 4.6 mm, i.d. 5
189
µm) was employed for all separations. The mobile phase was a mixture of deionized water 8 Page 8 of 36
and acetonitrile (40:60, v/v) in isocratic elution mode at the flow rate of 1 mL min −1 with the
191
detector wavelength set at 230 nm for 3-NA, 240 nm for 2,3-DCA, 2,4,DCA and 1-A2N and
192
315 nm for 4-NP. The pH of solutions was adjusted by using a methrohm digital pH meter
193
827 equipped with a glass calomel electrode. In the extraction procedure, a 25 mL sample
194
vial, and a MR 3001 heating-magnetic stirrer from Heidolph Company (Kelheim, Germany)
195
and a MS3 digital vortex agitator from IKA Company (Staufen, Germany) were used to mix
196
the MNPs with the sample. EBA 20 Hettich centrifuge (Oxford, England) and a 50 and 500
197
µL Hamilton HPLC syringe (Reno, NV, USA) were applied, too. The morphology and
198
dimension of the nanocomposites were explored by a scanning electron microscope (SEM)
199
model VEGA TESCAN (Brno, Czech Republic). IR spectra were recorded by a Bruker IFS-
200
66 FT-IR Spectrophotometer (Bruker Optics, Karlsruhe, Germany). The energy dispersive X-
201
ray spectroscopy (EDX) was performed on a VEGAII TESCAN.
M
an
us
cr
ip t
190
202
2.3. Preparation of standard solutions and real samples
204
Stock standard solution of pollutants (1000 mg L -1) was prepared in HPLC grade methanol
205
and stored in a fridge at 4 ºC and brought to ambient temperature just prior to use. Mixed
206
working solutions of the analytes at different concentrations were prepared by dilution with
207
ultra-pure water or deionized water containing various NaCl concentrations. The water
208
samples were filtered through a Millipore 0.22-μm cellulous acetate filter before the
209
extraction process. Then 20 mL spiked/non-spiked of each sample was used without any
210
dilution.
Ac
ce pt
ed
203
211 212
2.4. Synthesis of the nanosorbents
213
2.4.1. Synthesis of the MWCNTs/Fe3O4@PPy nanocomposite
9 Page 9 of 36
The MWCNTs/Fe3O4 was synthesized according to the literature [38]. At first, MWCNTs
215
were purified with 1 mol L-1 HNO3 solution for 6 h at room temperature and then were
216
washed several times with distilled water and dried at 100 °C, then 0.5 g MWCNTs was
217
suspended in 250 mL of solution containing 0.85 g (NH4)2Fe(SO4)2 .6 H2O and 0.422 g FeCl3
218
at 50 °C under nitrogen atmosphere. Afterward, the suspension was sonicated for 20 min, and
219
then 20 mL of 8 mol L-1 NH4OH aqueous solution was added drop wise to precipitate the
220
Fe3O4 NPs on the wall of MWCNTs while the mixture solution was being sonicated. The pH
221
of the final mixture was controlled in the range of 10-11. To promote the complete growth of
222
the nanoparticles crystals, the reaction was done at 50 °C for 30 min. The dispersion was
223
cooled to room temperature and magnetic MWCNT (MMWCNT) was collected from the
224
mixture with the help of a strong permanent magnet. Separated MMWCNT was washed
225
thrice with deionized water followed by ethanol. The synthesis of MMWCNT was confirmed
226
by IR spectroscopy.
227
To synthesis MWCNTs/Fe3O4@PPy nanocomposite 0.6 g of the dried MWCNTs/Fe3O4 was
228
added to 250 mL deionized water at pH 9 under stirring for 5 min and then, 0.4 mL pyrrole
229
was added and stirred for 10 min. Subsequently, 0.8 g sodium perchlorate was added to the
230
mixture and stirred for 5 min. In the end, 0.56 g of FeCl3 was dissolved in 50 mL deionized
231
water and added dropwise to the mixture under stirring. Subsequently, polymerization
232
reaction was continued for 15 h and finally, the nanocomposite was obtained. The precipitate
233
was washed several times by double distilled water and methanol, respectively. The washing
234
procedure was continued until the filtrate become colorless, and then it was dried. The
235
synthesis of MWCNTs/Fe3O4@PPy was confirmed by IR spectroscopy, SEM and EDX
236
analysis.
Ac
ce pt
ed
M
an
us
cr
ip t
214
237 238
2.4.2. Synthesis of the GO/Fe3O4@PPy nanocomposite
10 Page 10 of 36
239
The GO/Fe3O4 was synthesized according to the literature. [45]. At first, 0.5 g GO was
240
suspended in 500 mL water and ultrasonicated for 3 h to obtain a clear dispersion. Then, 0.85
241
g (NH4)2Fe(SO4)2 .6 H2O and 0.422 g FeCl3 was added to the mixture at 50 °C under nitrogen
242
atmosphere. Afterward, the suspension was sonicated for 10 min, and then 20 mL of 8 mol L -
243
1
244
while the mixture being sonicated. To promote the complete growth of the nanoparticles
245
crystals, the reaction was done at 50 °C for 30 min. The dispersion was cooled to room
246
temperature and magnetic GO (MGO) was collected from the mixture with the help of a
247
strong permanent magnet. Separated MGO was washed five times with deionized water
248
followed by ethanol. The synthesis of MGO was confirmed by IR spectroscopy.
249
To synthesis GO/Fe3O4@PPy nanocomposite, 0.6 g of the dried GO/Fe3O4 was added to 250
250
mL deionized water at pH 9 under ultrasonication for 60 min and then, 0.5 mL pyrrole
251
monomers was added and the mixture was stirred for 10 min. Subsequently, 0.8 g of sodium
252
perchlorate was added to the mixture and stirred for 10 min. In the end, 0.56 g of FeCl3 was
253
dissolved in 50 mL deionized water and added dropwise to the mixture under stirring.
254
Subsequently, polymerization reaction was continued for 15 h and finally, the nanocomposite
255
was formed. The precipitate was washed several times by double distilled water and
256
methanol, respectively. The washing procedure was continued until the filtrate become
257
colorless, and then it was dried. The synthesis of GO/Fe 3O4 @PPy was confirmed by IR
258
spectroscopy and SEM analysis.
ip t
cr
us
an
M
ed
ce pt
Ac
259
NH4OH aqueous solution was added drop wise to precipitate the Fe 3O4 NPs on GO sheets
260
2.4.3. Synthesis of Fe3O4@PPy NPs
261
The Fe3O4 NPs were prepared by the chemical co-precipitation method [21]. Briefly, 7.2 g of
262
FeCl3, 7.84 g of ((NH4)2Fe(SO4)2.6H2O) were dissolved in 400 mL of deionized water in a
263
beaker which was degassed with nitrogen gas for 10 min. Then, this stock solution was added
11 Page 11 of 36
to a reactor vessel and heated to 80 °C. Afterwards, 20 mL of ammonia solution (25% w/w)
265
was added to the reactor under the nitrogen gas protection and vigorous stirring (1000 rpm).
266
During the whole process, temperature of the solution was maintained at 80 °C and nitrogen
267
gas was used to prevent the intrusion of oxygen.
268
Fe3O4@PPy NPs were synthesized according to Tahmasebi et al. with some modifications.
269
[45]. One gram of the dried Fe3O4 NPs was added to 400 mL deionized water at pH 9 under
270
stirring for 5 min and then, 0.5 mL of pyrrole monomers was added and stirred for 10 min.
271
Subsequently, 1 g of sodium perchlorate was added to the mixture and stirred for 10 min.
272
Then, 0.9 g of FeCl3 was dissolved in 50 mL deionized water and added dropwise to the
273
mixture under stirring. Subsequently, polymerization reaction was continued for 15 h and
274
finally, a precipitate (Fe3O4@PPy) was obtained. The precipitate was washed several times
275
by double distilled water and methanol, respectively. The washing procedure was continued
276
until the filtrate become colorless, and then it was dried. The synthesis of GO/Fe 3O4@PPy
277
was confirmed by IR spectroscopy and SEM analysis.
ed
M
an
us
cr
ip t
264
278
2.5. MWCNTs/Fe3O4@PPy based d-µ-SPE procedure
280
Extraction
281
MWCNTs/Fe3O4@PPy (12 mg) was put into a 25 mL sample vial and 20 mL aqueous solution
282
of the analytes (0.5 mg L-1) was added to the vial, (2) pH of the solution was adjusted to 8.2
283
with 2 mol L-1 NaOH, (3) the solution was stirred at a constant rate of 1250 rpm to facilitate
284
mass transfer and sorption of the model analytes onto the sorbent, (4) afterward, the sorbent
285
was separated quickly from the sample solutions, by the use of the supermagnet, (5) 15 mL
286
of the supernatant was decantated and the remained 5 mL solution was transferred to a 6 mL
287
conical tube and after decantation, the sorbent was eluted with 120 μL of ACN by fierce
ce pt
279
was
performed
according
to
the
following
steps:
(1)
Ac
procedure
12 Page 12 of 36
288
vortex for 2.0 min. (6) Finally, the eluate was isolated from the sorbent by the magnet and
289
100 μL of eluate was injected into the HPLC instrument for subsequent analysis.
290
2.6. Experimental design methodology and desirability function approach
292
A principal motivation for improving a new separation and quantification method is reducing
293
the required time and number of trials that ends in total required costs, therefore in order to
294
optimize the preconcentration of the model analytes by the proposed method, a Box-Behnken
295
design (BBD) in combination with desirability function (DF) was employed. It’s worth to
296
note that for an experimental design involving five variables expressed by BBD, linear,
297
quadratic and cross terms can be involved. The precise optimum point can be obtained by the
298
aid of response surface methodologies exhibiting relationships between variables and
299
responses graphically [46,47].
300
In the case of multiple response optimization, the Derringer function or desirability function
301
(DF) can be applied, since it’s the most critical and most widely used multi-criteria
302
methodology in analytical procedures [48]. At first, in DF approach, each predicted response
303
is transformed to a dimensionless desirability value (d) and then all transformed responses are
304
combined into one particular response. The scale of the individual DF ranges between 0-1,
305
while for the most desirable response d is equal to 1 and for a completely undesired response
306
d is 0 [49]. Different transformations on data may be implemented depending on whether the
307
response is optimum when it is maximized, minimized, or at a predefined value [50].
308
In this work, the experimental design matrix and data analysis were carried out by the
309
Design-Expert statistical software program (7.0.0 trial version).
Ac
ce pt
ed
M
an
us
cr
ip t
291
310 311
3. Results and discussion 13 Page 13 of 36
3.1. Characterization of the nanosorbents
313
The morphology and dimension of the prepared Fe3O4@PPy NPs, MWCNTs/Fe3O4 @PPy
314
and GO/Fe3O4@PPy nanocomposite were explored by SEM analysis. As depicted in Fig. 1a,
315
the prepared Fe3O4@PPy NPs are nearly spherical in shape with an average diameter of about
316
40-50 nm Fe3O4 @PPy NPs. Fe3O4 NPs tended to aggregate to form larger particles which is
317
related to their large specific surface area, high surface energy and magnetization. The SEM
318
image of prepared Fe3O4@PPy NPs exhibited less agglomeration than bare Fe3O4 that is
319
attributed to PPy coating layer [21]. As illustrated in Fig. 1b, Fe 3O4 NPs were successfully
320
coated on the surface of GO to form a GO/Fe3O4 @PPy nanocomposite. The average size of
321
the Fe3O4 NPs in this nanocomposite was about 45 nm. The SEM image of
322
MWCNTs/Fe3O4@PPy nanocomposite (Fig. 1c and 1d) exhibits that the surfaces of
323
MWCNTs were densely coated with Fe3O4 NPs. Moreover, it is indicated that Fe3O4 NPs
324
have grown on the surface of MWCNTs and well distributed, hence the aggregation problem
325
of MNPs is solved by utilizing MWCNTs as a support and spacer [37]. Also the diameters of
326
MWCNTs were 50-60 nm with Fe3O4 NPs coated on their surfaces.
cr
us
an
M
ed
ce pt
327
ip t
312
The energy dispersive x-ray (EDX) spectrometry analysis of MWCNTs/Fe3O4 @PPy
329
nanocomposite (Fig. 1S, Electronic Supplementary Information) demonstrated the peaks
330
were associated with Fe, C, N, O, and Cl atoms with molar ratio of 53.53%, 32.56%, 2.93%,
331
10.48% and 0.50%, respectively. These results confirmed the existence of polypyrrole,
332
perchlorate as a dopant, and Fe3O4 NPs on the surface of MWCNTs with no impurity.
333
The FT-IR spectra of Fe3O4 and Fe3O4@PPy NPs, MWCNTs/Fe3O4 @PPy and
334
GO/Fe3O4 @PPy nanocomposites were recorded using KBr pellet method (Fig. 2S, Electronic
335
Supplementary Information). In the IR spectrum of Fe3O4, absorptions peaks of Fe-O were
336
observed at 584 and 3435 cm-1 which demonstrates the successful synthesis of Fe3O4
Ac
328
14 Page 14 of 36
337
nanoparticles. On the other hand, the absorption peaks appeared at 1196, 1201, and 1198 cm-1
338
(C-N),
339
GO/Fe3O4 @PPy nanocomposites respectively, confirmed the presence of polypyrrole on the
340
surface of these sorbents. Furthermore, the peaks at 3411, 3374 and 3441 cm−1 in the FT-IR
341
spectra of Fe3O4 @PPy NPs, MWCNTs/Fe3O4 @PPy and GO/Fe3O4@PPy nanocomposites
342
respectively are attributed to N-H band of PPy [51].
FT-IR
spectra of Fe3O4 @PPy NPs,
MWCNTs/Fe3O4@PPy and
ip t
in the
cr
343
3.2. Optimization of MWCNTs/Fe3O4@PPy based d-μ-SPE parameters
345
The factors affecting the extraction efficiency of the proposed method such as sorbent type,
346
pH of the sample, amount of sorbent, sorption time, salt content of sample solution and
347
elution conditions (eluent type and volume) were explored and opted. Out of these seven
348
factors, sorbent and eluent types were optimized using one variable at a time method. The
349
optimization of the five other factors was performed using a response surface methodology
350
approach.
an
M
ed ce pt
351
us
344
3.2.1. Sorbent type
353
The sorbent type is a crucial variable affecting the extraction efficiency. In this context,
354
extraction abilities of bare Fe3O4 NPs, Fe3O4@PPy, MWCNTs/Fe3O4, GO/Fe3O4,
355
MWCNTs/Fe3O4@PPy and GO/Fe3O4@PPy were investigated comprehensively (Fig. 3a).
356
The results revealed that MWCNTs/Fe3O4 @PPy and GO/Fe3O4 @PPy can act as the best
357
sorbents due to the presence of PPy coating, higher surface area and higher dispersibility of
358
these sorbents compared to Fe3O4 and Fe3O4@PPy NPs. Although the existence of Fe3O4 NPs
359
on the surface of MWCNTs and GO can decrease the surface active site, PPy can compensate
360
this reduction by enhancing the active surface sites on the sorbent through ion-pair and
361
hydrogen bonding formation, electrostatic and hydrophobic interactions with the target
Ac
352
15 Page 15 of 36
analytes [37]. The anionic form of 4-NP and 1-A2N can interact with positive charged site of
363
PPy (ion-pair formation, electrostatic interaction) and basic analytes can interact through
364
hydrophobic interaction with pyrrole ring as well as GO and MWCNTs skeleton. Moreover,
365
π-π interaction is a possible driving force for sorption of all the analytes on carbon skeleton.
366
As depicted in Fig. 2a the performance of MWCNTs/Fe3O4@PPy is slightly higher than
367
GO/Fe3O4 @PPy. Moreover, the required time for gathering MWCNTs/Fe 3O4 @PPy was less
368
than GO/Fe3O4@PPy. Thus MWCNTs/Fe3O4 @PPy was selected as the most appropriate
369
sorbent for the rest of the studies.
us
cr
ip t
362
370
3.2.2. Selection of eluent
372
After sorption, the analytes should be eluted from the sorbent using an appropriate eluent
373
solvent prior to HPLC analysis. The elution of the target analytes was explored using
374
different water miscible organic solvents, i.e. methanol, acetone, acetonitrile, 2-propanol,
375
DMF, 5% (v/v) and 10% HOAc in methanol, 5% and 10% (v/v) HOAc in ACN, 0.05 mol L -1
376
HCl in ACN and 0.05 mol L-1 HCl in methanol since they are HPLC compatible solvents.
377
Among the different eluents, desorption ability of ACN was found to be higher than the other
378
solvents (Fig. 2b). This can be explained by suitable solubility of the model analytes in ACN.
379
M
ed
ce pt
Ac
380
an
371
381
3.2.3. Box-Behnken design and desirability function
382
In the next step, the affecting factors were selected based on preliminary experiments and
383
opted by a BBD experiment. In other words, BBD was applied to optimize the effect of five
384
factors (pH of sample, sorption time, sorbent amount, salt content of sample solution and
385
eluent volume). According to the experiment equation obeying BBD; N = 2K (K-1) + C0,
386
where K is the number of variables and Co is the number of center points, K and C0 were set 16 Page 16 of 36
at 5 and 6, respectively, which mean that 46 experiments should be performed [52]. By
388
applying Derringer’s desirability function, in which responses are simultaneously optimized,
389
the position of the optimum extraction condition was determined; predicting the design space
390
and the response and from the polynomial equation. A unique function named Global
391
Desirability function (D or DF) or geometric mean (Geo mean) is obtained when n variables
392
(factor and responses levels) are combined after transformation in desirability functions.
393
For the best joint response acquisition, the following equation can be implemented:
us
cr
ip t
387
an
394
Eq. 1
where ri is the importance of each variable relative to the others. A matter of the utmost
396
importance is maximization of DF in the optimization procedure, i.e. when DF (ranging from
397
0 to 1) is a non-zero value, all the variables which are simultaneously opted can be supposed
398
to have a desirable value [47]. Obtaining an appropriate set of conditions that will meet all
399
the determined criteria is the main goal of an optimization procedure and attaining a DF=1 is
400
not an objective. In order to obtain the best extraction conditions in this study, the results of
401
BBD were investigated according to the criteria assigned based on desirable levels of factors
402
and response (Table 1). To get desired extraction efficiency as objective function, Geo mean
403
as an indicator of extraction efficiency was maximized. An initial data preprocessing, i.e.,
404
normalizing the related response of each analytes is necessary. Subsequently, the obtained DF
405
would be an input value for BBD. In this way, firstly the responses of each analyte are
406
normalized, i.e. each predicted response is transformed to a dimensionless desirability value
407
(d) and then all transformed responses are combined into one particular response (Geo mean).
408