Accepted Manuscript Removal of pesticides from wastewater by ion pair centrifugal partition extraction using betaine-derived ionic liquids as extractants Yannick De Gaetano, Jane Hubert, Aminou Mohamadou, Stéphanie Boudesocque, Richard Plantier-Royon, Jean-Hugues Renault, Laurent Dupont PII: DOI: Reference:
S1385-8947(15)01406-0 http://dx.doi.org/10.1016/j.cej.2015.10.012 CEJ 14281
To appear in:
Chemical Engineering Journal
Received Date: Revised Date: Accepted Date:
10 July 2015 30 September 2015 4 October 2015
Please cite this article as: Y. De Gaetano, J. Hubert, A. Mohamadou, S. Boudesocque, R. Plantier-Royon, J-H. Renault, L. Dupont, Removal of pesticides from wastewater by ion pair centrifugal partition extraction using betainederived ionic liquids as extractants, Chemical Engineering Journal (2015), doi: http://dx.doi.org/10.1016/j.cej. 2015.10.012
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1
Removal of pesticides from wastewater by ion pair centrifugal partition extraction using
2
betaine-derived ionic liquids as extractants
3
Yannick De Gaetano, Jane Hubert*, Aminou Mohamadou, Stéphanie Boudesocque, Richard
4
Plantier-Royon, Jean-Hugues Renault, Laurent Dupont*.
5
Université de Reims Champagne-Ardenne, Institut de Chimie Moléculaire de Reims (ICMR),
6
CNRS UMR 7312, UFR des Sciences Exactes et Naturelles, Bâtiment 18 Europol’Agro, BP
7
1039, F-51687 Reims Cedex 2, France.
8 9
Abstract
10
The extraction of pesticides from aqueous solutions using new ionic liquids (ILs) derived
11
from glycine betaine as extractants was investigated. These ILs incorporate cationic esters of
12
trimethyl(2-alkoxy-2-oxoethyl) ammonium (GBOCn+) associated with inorganic ClO4- or BF4-
13
anions. First, batch extraction experiments were performed by using the liquid-liquid biphasic
14
system IL/ethyl acetate/water (1:1:1; v/v) for four commonly used pesticides: 4-
15
chlorophenoxyacetic acid (4-CPA), 2,4-dichlorophenoxyacetic acid (2,4-D), 2-[(4-methyl-5-
16
oxo-3-propoxy-1,2,4-triazolin-1-yl)carbamidosulfonyl]benzoic acid methyl ester sodium salt
17
(propoxycarbazone) and 2-(4-isopropyl-4-methyl-5-oxo-2-imidazolin-2-yl)-5-methoxymethyl
18
nicotinic acid (imazamox). Then, the liquid-liquid extraction unit operation was intensified by
19
transposing the system into a Centrifugal Partition Extraction (CPE) device, using ethyl
20
acetate/n-butanol/water (1:4:5; v/v) as biphasic solvent system and potassium iodide, sodium
21
iodide or sodium perchlorate as potential displacers. The use of a lab-scale CPE column with
22
a capacity of 300 mL allowed the intensification of the extraction procedure. The extraction
23
and back-extraction of individual or mixture of pesticides were studied, with a particular
24
focus on the potential separation of individual pesticides and on the recyclability of the CPE
25
method. In optimal CPE conditions, a quantitative extraction for three of the four pesticides
1
26
was obtained, with recovery percentages of 95.6 %, 98.7 %, 99.0 %, and 100.0% for
27
imazamox, propoxycarbazone, 2,4-D, and 4-CPA, respectively. After the back-extraction
28
step, separated pesticides were recovered in fresh aqueous mobile phase. The recyclability
29
studies showed that the extraction/back-extraction process can be performed at least four
30
times while maintaining a quantitative extraction.
31 32
Keywords: Ionic liquid, pesticide, centrifugal partition extraction, water treatment,
33
decontamination
34
(*) Corresponding author. Address: ICMR (Institut de Chimie Moléculaire de Reims), Université de Reims Champagne-Ardenne, BP 1039,
35
51687 Reims Cedex 2, France. Tel.: +33 (0) 3 26 91 33 36; fax: +33 (0) 3 26 91 32 43. E-mail address:
[email protected] (L.
36
DUPONT.
[email protected] (J. HUBERT).
37 38 39 40
2
41
1. Introduction
42
The extensive use of pesticides in modern agriculture has become a major concern due to the
43
potential hazard that these compounds can cause to the environment and their known or
44
supposed toxic effects on human health, such as mutation, cellular degradation, and
45
interruption of hormone functions [1-2].
46
For this reason, the analytical control of pesticide concentrations is mandatory in different
47
water bodies within the EU including drinking waters [3]. Thus the development of efficient
48
purification strategies focused on their elimination remains a critical issue. Industrially, the
49
most common removal techniques of pesticides from water rely on chemical oxidation,
50
filtration on activated charcoal or inverse osmosis. However these processes are still
51
expensive and research efforts are currently made to find other alternatives. To date, the most
52
common laboratory-scale methods to extract pesticides from aqueous media have been based
53
on liquid-liquid extraction [4, 5] and solid-phase extraction (SPE) [6, 7], with typical
54
extraction volumes ranging from around 10 to a maximum of 100 mL. These techniques
55
usually involve the use of chlorinated solvents that exhibit toxicity (such as tetrachloroethane,
56
chlorobenzene, carbon tetrachloride) or n-hexane [8-10]. ILs have been recently considered
57
as interesting ionic species to remove pesticides from water via extraction mechanisms. ILs
58
are non-flammable liquids, exhibiting a negligible vapour pressure and a high thermal
59
stability. Some of them are liquid at room temperature, able to solubilize a wide range of
60
organic and inorganic compounds [11].
61
The utilization of pure ILs for the extraction of pesticides has been already investigated in
62
liquid-liquid extraction [12-14], solid-liquid extraction [15], magnetic solid-phase extraction
63
[16], micro-solid phase extraction [17], dispersive micro-solid phase extraction [18], pipette
64
tip-solid phase extraction [19] and hollow fiber-solid phase microextraction procedures [20].
3
65
However these methods remain time consuming and still display some drawbacks such as low
66
extraction yields, multiple operation steps [21], and high IL consumption. The present work
67
aimed to develop a novel efficient method for the extraction of pesticides that combines the
68
use of ILs as extractants with Centrifugal Partition Extraction (CPE) as liquid-liquid efficient
69
contactor.
70
CPE is a solid support-free liquid-liquid extraction technique involving transfer and
71
distribution of solutes between at least two immiscible liquid phases according to their
72
distribution and mass transfer coefficients.
73
A CPE column (Figure 1) consists in a series series of partition cells connected in cascade by ducts
74
and subjected to a centrifugal acceleration field [22]. One liquid phase is maintained inside
75
the partition cells (the stationary phase) while the other liquid phase (the mobile phase) is
76
pumped through the stationary phase. The separation process is based on the interfacial mass
77
transfer of solutes between the two liquid phases in each cell [23].
78 79
Fig. 1. (a) Centrifugal Partition Extractor FCPE300®, (b) CPE column containing 7 circular
80
partition disks, (c) Scheme of a circular partition disk
81
4
82
The design of CPE partition cells inside the column allows the loading of sample solutions
83
continuously with flow rates ranging from 10 to 100 mL/min at the laboratory-scale, which is
84
of particular interest for the extraction of micropollutants such as pesticides present at low
85
concentrations in aqueous media.
86
Considering the ionic nature of ILs (cation/anion association), it can be expected that the ion-
87
pair displacement mode in CPE [24] would provide interesting results to extract pesticides
88
from water and possibly to separate pesticides from each other at the end of the process. The
89
ion pair displacement mode in CPE consists in diluting an anionic or cationic species (here
90
ILs) in the organic stationary phase. By this way ionisable analytes (such as most pesticides)
91
contained in the inlet aqueous solution are captured by the IL inside the CPE column. Then
92
during the back-extraction step, a displacer agent that presents a high affinity for the
93
extractant (i.e. the IL) is introduced into the aqueous mobile phase to force the analytes to
94
competitively progress along the column. As a result the analytes (here the pesticides) initially
95
introduced into the CPE column as a mixture of compounds in a liquid phase are not only
96
extracted from their initial media over the extraction step, but also displaced out of the
97
column during the back-extraction and possibly recovered in a fresh aqueous solution as
98
individual compounds. This method has been successfully applied for the separation and
99
purification of ionizable natural products [25].
100
To date only a few ILs have been synthesized and tested for the extraction of organic
101
compounds. The most common families of ILs employed for this purpose were the 1,3-
102
dialkylimidazolium salts with hexafluorophosphate or bis(trifluoromethylsulfonyl)imide
103
anion, but these ILs exhibited some toxicity [26].
104 105
2. Materials and methods
106
5
107
In the present work, we report the study of the extraction of some pesticides by six news ILs
108
obtained from esterified glycine betaine (Scheme 1) using Centrifugal Partition Extraction.
109
The effects of the nature of both cation and anion forms of the ILs, as well as CPE parameters
110
(e.g.volume of treated solutions, flow rate, nature of the displacer) on the pesticide extraction
111
percentage have been investigated. Results related to the back-extraction of pesticides are also
112
presented.
113
114 115
Scheme 1. Ionic liquids based on Glycine Betaine esters.
116 117
The CPE extraction method was developed by using four systemic herbicides commonly used
118
worldwide for the control of broadleaf weeds. Two members of the phenoxy family of
119
herbicides: the 4-chlorophenoxyacetic acid (4-CPA) and the 2,4-dichlorophenoxyacetic acid
120
(2,4-D), as well as propoxycarbazone and imazamox were selected as models (Scheme 2).
121 122
6
123 124
Scheme 2. Chemical structures of the four pesticides studied.
125 126
2.1. Reagents and solvents
127
All chemicals were of analytical grade. Ethyl acetate (EtOAc) and n-butanol (n-BuOH) were
128
purchased from Carlo Erba Reactifs SDS (Val de Reuil, France). 4-chlorophenoxyacetic acid
129
(4-CPA), 2,4-dichlorophenoxyacetic acid (2,4-D), propoxycarbazone salt and imazamox were
130
purchased from Sigma Aldrich (Saint Quentin, France). Sodium perchlorate and sodium
131
iodide were obtained from Acros (Illkirch, France) and potassium iodide from Alfa Aesar
132
(Schiltigheim, France). All aqueous solutions were prepared in distilled water and stored safe
133
from the light at 4°C.
134 135
2.2. Apparatus: Fast Centrifugal Partition Extractor FCPE300®
136
Extraction experiments were developed on a lab-scale Centrifugal Partition Extractor
137
(FCPE300®, Rousselet Robatel Kromaton, Annonay, France) containing a rotor of 7 circular
138
partition disks engraved with a total of 231 twin partition cells. The stationary phase was
139
maintained inside the CPE column by a centrifugal force field generated by rotation around a
140
single central axis. The total column capacity is 303.5 mL [37]. The rotation speed can be
141
adjusted from 200 to 2000 rpm, producing a relative centrifugal acceleration in the partition
7
142
cell up to 437 g. The mobile phase was pumped through the stationary phase either in the
143
ascending or in the descending mode with low residual pulsation through a KNAUER
144
Preparative Pump 1800® V7115 (Berlin, Germany). Fractions were collected by a Pharmacia
145
Superfrac collector (Uppsala, Sweden). All experiments were conducted at room temperature
146
(20 ± 2 °C). The flow rate was fixed at 10 mL/min and the rotation speed at 1000 rpm for all
147
experiments.
148 149
2.3. HPLC analyses
150
All CPE fractions were analyzed by HPLC on an Ultimate® 3000 HPLC system (Dionex)
151
equipped with a Dionex Ultimate pump (model 3000), a WPS-3000 (SL) autosampler, and a
152
diode array detector DAD-3000(RS). The chromatographic column (Myrsine, 250 × 4.6 mm,
153
5 µm particule size) was maintained at 21 °C. The mobile phases, 0.5% acetic acid in water
154
(solvent A) and acetonitrile (solvent B), were pumped isocratically at 1.5 mL/min with a ratio
155
60/40 (v/v). The injection volume was 20 µL. Data acquisition was controlled by the
156
Chromeleon Software and the chromatograms were recorded for 15 min.
157
UV detection was performed at λ = 254 nm for imazamox (λmax = 250 nm) and
158
propoxycarbazone (λmax = 253 nm) and λ = 280 nm for 4-CPA (λmax = 279 nm) and (λmax =
159
283 nm). Calibration curves were established by serial dilution of two independent stock
160
solutions of each pure pesticide (1 g/L) and by plotting the peak area recorded from HPLC
161
chromatograms as a function of pesticide concentration. The correlation coefficient (R2)
162
calculated from the calibration curves of standard solutions were higher than 0.9996 for all
163
compounds. The four pesticides were identified over CPE experiments by comparison with
164
the HPLC retention time of their corresponding standard molecules. The retention times of
165
imazamox, 4-CPA, 2,4-D and propoxycarbazone were 3.0, 6.8, 9.1 and 13.2 min,
166
respectively. 8
167 168
2.4. Batch tests
169
2.4.1. Optimization of extraction conditions
170
Extraction conditions were optimized in batch tests by using the couple GBOC14-ClO4 and 4-
171
CPA as an IL/analyte test system. Experiments were performed using different molar ratios (
172
nIL n4−CPA
) ranging from 10 to 25 (corresponding to 8, 12, 16 and 20 mg of GBOC14-ClO4). An
173
aqueous solution containing 4-CPA at a concentration of 10-3 mol.L-1 (2 mL) was mixed with
174
an equal volume of IL diluted in ethyl acetate (2 mL). The mixture was stirred for 1 hour at
175
room temperature.
176
analyzed by HPLC. The efficiency of the extraction process was evaluated by determining the
177
extraction percentage (%E) using the following equation:
The liquid phases were then separated and the aqueous phase was
%E =
178
(Cin − C fin ) Cin
x100
179
where Cin and Cfin (mol. L-1) represent the concentrations of the pesticide in the initial and in
180
the final aqueous solutions.
181 182
2.4.2. Choice of the base for the extraction
183
The first goal was to find the best pH conditions to ensure the ionization state of pesticides
184
(COO-, N-) while maintaining their chemical integrity. HPLC analyses of the four pesticides
185
revealed two different peaks for propoxycarbazone at pH > 9, suggesting that this compound
186
is stable only up to this value. The other pesticides were also stable up to pH 9. Therefore the
187
pH was fixed at 9 in all extraction experiments. The pKa values for 4-CPA, 2,4-D and
188
propoxycarbazone are respectively 3.6, 2.7 and 2.1. Imazamox has three protonation sites
189
corresponding to pKa values of 2.3, 3.3 and 10.8 [27]. The first two pKas correspond
190
respectively to the deprotonation of pyridinium and carboxylic groups, whereas the third pKa 9
191
is ascribed to the neutralization of the protonated amidic nitrogen. Consequently, at pH 9,
192
Imazamox, 4-CPA, 2,4-D are present as their carboxylate salt and propoxycarbazone as its
193
anionic form (-N-). These observations are summarized in Table 1.
194
Several alkaline agents including ammoniac buffer (NH4OH), potassium hydroxide (KOH)
195
and sodium hydroxide (NaOH) were tested to adjust the pH of the aqueous solutions. For
196
experiments with NaOH and KOH, 18.6 mg of 4-CPA (corresponding to 0.1 mmol) were
197
firstly diluted in approximatively 100 mL of distilled water and then the pH was adjusted to 9
198
by adding few drops of concentrated solution of the base (1 mol.L-1), in order to obtain a
199
solution of 4-CPA at a concentration of 10-3 mol.L-1. For extraction experiments with
200
ammoniac buffer, 18.6 mg of 4-CPA corresponding to 0.1 mmol were directly diluted in
201
100 mL of ammoniac buffer at a concentration of 0.1 mol.L-1.
202 203
2.4.3. Extractant selection
204
GBOC12-ClO4, GBOC14-ClO4, GBOC16-ClO4, GBOC12-BF4, GBOC14-BF4 and GBOC16-BF4
205
were investigated as ILs for the extraction of 4-CPA. Experiments were performed at pH 9
206
using a molar ratio (
207
temperature.
nIL n4−CPA
) equal to 25. All experiments were carried out at room
208 209
2.4.4. Extraction of individual pesticides with GBOC14-ClO4
210
The fresh pesticides solutions ([C] = 5.10-4 mol.L-1) were independently prepared by
211
dissolving 9.3 mg of 4-CPA; 11.0 mg of 2,4-D; 15.2 mg of imazamox or 21.0 mg of
212
propoxycarbazone in 100 mL of distilled water. Then, 2 mL of each solution containing
213
individual pesticide at a concentration of 5.10-4 mol.L-1 (pH = 9) were mixed with 2 mL of
214
ethyl acetate containing GBOC14-ClO4 at concentrations ranging from 0 to 25 equivalents of
215
pesticide (from 0 to 10.4 mg) for 4-CPA and 2,4-D and from 0 to 50 equivalents of pesticides 10
216
(0 to 20.7 mg) for imazamox and propoxycarbazone. The solutions were stirred for 1 h. The
217
liquid phases were then separated and the aqueous phases were all analyzed by HPLC to
218
determine the extraction percentage of individual pesticides. Extraction experiments were
219
repeated 5 times for each experimental condition.
220 221
2.5. Centrifugal Partition Extraction (CPE)
222
2.5.1. Optimization of the extraction and back-extraction steps with individual pesticides
223
Extraction and back-extraction of individual pesticides were investigated at the preparative
224
scale by using CPE. The extraction step consists in the capture of ionic species (among which
225
pesticides) by the ionic liquid inside the column through the formation of ion pairs, while the
226
other non-ionic compounds are eluted out of the column. In a second step, the back-extraction
227
consists in introducing in the mobile phase a displacer agent (here iodides) which exhibits a
228
higher affinity for the ionic liquid than the captured analytes. By this way a competitive
229
process takes place and the target ionic compounds are eluted selectively in the order of their
230
affinity for the ionic liquid.
231
A biphasic solvent system (2 L) was prepared by mixing EtOAc/n-BuOH/water in the
232
proportions 4:1:5 (v/v/v). The column was filled at 200 rpm with the organic phase used as
233
the stationary phase containing the IL extractant GBOC14-ClO4 at a molar ratio
234
25. Individual pesticides were dissolved in the aqueous phase of the solvent system at a
235
concentration of 62.5 mg/L. The pH was adjusted to 9 by adding KOH (1 mol.L-1) and the
236
aqueous phase containing the pesticide was pumped through the stationary phase at
237
10 mL/min.
238
After pumping slightly more than one column volume (320 mL), aqueous solutions of
239
potassium iodide (KI) or sodium iodide (NaI) were tested for the back-extraction step. In this
nIL n pesticides
of
11
240
process, the pesticide previously extracted, were stripped off the stationary phase, following
241
this equation:
242 243 244
where R-COO- corresponds to the pesticide under his anionic form.
245 246
Fractions of 20 mL were collected over the whole experiments and analyzed by HPLC. The
247
back-extraction step was also studied using different molar ratios:
248 249
ndisplacer nIL
= 5 and
ndisplacer nIL
ndisplacer nIL
= 1,
ndisplacer nIL
= 2,
= 10. Finally, NaI and KI were investigated as displacers in order to
assess the influence of the cation on the back-extraction efficiency.
250 251
2.5.2. Extraction and back-extraction process description with mixtures of pesticides
252
Extraction and back-extraction of an equimolar mixture of the two phenoxyacetic acids 4-
253
CPA and 2,4-D were firstly investigated at a concentration of 2.8 x 10-4 mol.L-1 (21 mg of 4-
254
CPA and 25 mg of 2,4-D; 1.13 x 10-4 mol) were dissolved in 400 mL of distilled water. The
255
preparation of both the mobile and stationary phases is described above. The back-extraction
256
was carried out using a ratio
257
For the preparation of the mixture of the pesticides at a concentration of 2.0 x 10-4 mol.L-1,
258
14.9 mg of 4-CPA, 17.7 mg of 2,4-D, 24.4 mg of imazamox and 33.7 mg of
259
propoxycarbazone (8.0 x 10-5 mol) were dissolved in 400 mL of distilled water.
nKI = 10. nIL
260 261
2.5.3. Back-extraction with sodium perchlorate as displacer 12
262
This extraction was performed on the same pesticide mixture as described in § 2.5.2. Sodium
263
perchlorate was used as displacer. The back-extraction process was performed using a ratio
264
nNaClO4 n4−CPA
= 10.
265 266
2.5.4. Recyclability of the process
267
In order to investigate the recyclability of the whole CPE procedure, 18.6 mg of 4-CPA (10-4
268
mol) were dissolved in 400 mL of distilled water to obtain a solution at a concentration of 2.5
269
x 10-4 mol.L-1. The preparation of the mobile and stationary phases are described in § 2.5.1.
270
The back-extraction process was performed using sodium perchlorate as displacer and a ratio
271
nNaClO4 n4−CPA
= 10. After one cycle of extraction/back-extraction, 18.6 mg of 4-CPA were dissolved
272
in 400 mL of the fresh aqueous phase of the CPE biphasic solvent system and loaded again
273
into the column to perform a second extraction/back-extraction cycle. In total, four
274
consecutive cycles were performed.
275 276
2.5.5. Resolution (Rs) and selectivity factor (α)
277
Resolution (Rs) and selectivity factor (α) were calculated for two consecutive peaks. The Rs
278
is calculated using this following equation:
279
Rs = 2
(t r ( B) − t r ( A)) wb + wa
280
where tr(A) and tr(B) represent the retention time for solutes A and B, with B the more
281
retained solute; wa and wb represent the curve width of solute A and solute B respectively.
282
Selectivity factor is given by this following equation:
283
α=
k ' ( B) k ' ( A)
13
284
where k’(A) and k’(B) represent the capacity factor. These capacity factors are deduced by
285
these two equations:
k ' ( A) =
286 287
(t r ( A) − t 0 ) t0
k ' ( B) =
(t r ( B) − t0 ) t0
with t0 the time for the dead volume.
288 289
3. Results
290
All GBOCn-X ILs were obtained from the esterification reaction of betaine with n-alkyl
291
alcohols using methanesulfonate acid as catalyser, follow by anionic metathesis from the
292
methanesulfonate derivative using sodium perchlorate or sodium tetrafluoroborate [28].
293 294
3.1. Optimization of the extraction conditions in batch tests with 4-CPA as pesticide model
295
The first goal was to evaluate the influence of the pH of the aqueous solution and the nature
296
of the base used to adjust the pH on the pesticide extraction percentage. percentage. For these
297
experiments, 4-CPA and GBOC14-ClO4 were selected as models. Indeed, alkaline conditions
298
are necessary to obtain pesticides in their anionic form and therefore facilitate the formation
299
of ion pairs with ILs. Due to the poor stability of pesticides, especially propoxycarbazone
300
which hydrolyses above pH 9, the pH was limited to this value (Table 1).
301 302
Table 1
303
Chemical structure, molar mass (g.mol-1), stability under alkaline conditions and pKa values
304
of each pesticide. Chemical Structures of
Molar Mass Name
Stability
pKa values
stable
3.56
(g.mol-1)
Pesticides 4-chlorophenoxyacetic acid
186.59 (4-CPA)
14
2,4-dichlorophenoxyacetic acid 221.04
stable
2.73
(2,4-D) 2-[(4-Methyl-5-oxo-3-propoxy1,2,4-triazolin-1Stable at 420.37
yl)carbamidosulfonyl]benzoic
2.10 pH < 9
acid methyl ester sodium salt
(propoxycarbazone) 2-(4 (4-Isopropyl-4-methyl-5-oxo-
2.3(Npyridinium) 2-imidazolin-2-yl)-5305.33
stable
3.3 (COO-)
methoxymethyl nicotinic acid
10.8 (Namide) (imazamox)
305
nIL
306
For the same experimental conditions with a ratio (
307
Table 2 demonstrate that the extraction of 4-CPA using NH4OH buffer or NaOH was less
308
effective (%E = 27.0 - 63.0%) than with KOH (%E (%E = 69.0%). The extraction of 4-CPA under
309
its potassium salt form with GBOC14-ClO4 IL is thus more efficient than under sodium or
310
ammonium salt forms.
n4−CPA
) = 10, the results presented in
311 312
Table 2
313
Extraction percentage (%E) of 4-CPA with three different bases (NH3, NaOH and KOH) and
314
two different ratios
n IL n 4−CPA
.
315
Ratio
Extraction percentage
nIL n4−CPA
10
NH3
NaOH
KOH
27.0
63.0
69.00 15
316
25
47.3
89.5
99.1
317 318
nIL
319
This tendency was confirmed by the results obtained with a ratio (
320
base lead to a nearly quantitative extraction of 4-CPA (99.1%) was observed. However, the
321
extraction percentage was below 90% when using NaOH and 50% with NH4OH buffer. The
322
best extraction conditions were thus fixed at pH 9 with KOH and using 25 equivalents of ILs.
323
To determine the most efficient IL, these conditions were applied for a range of glycine
324
betaine-derived ILs differing from each other by their anionic moiety (ClO4- or BF4-) and
325
alkyl chain length (n=12, 14, 16). The results are summarized in Table 3.
n4−CPA
) = 25 using KOH as
326 327
Table 3.
328
Extraction percentage (%E) of 4-CPA, Experimental conditions: [4-CPA] = 10-3 mol.L-1, pH
329
9 (KOH), ratio (
nIL n4−CPA
) = 25.
ILs
GBOC12-ClO4
GBOC14-ClO4
GBOC16-ClO4
GBOC12-BF4
GBOC14-BF4
GBOC16-BF4
(% E)
92.1
99.1
96.7
59.6
65.5
71.4
330 331
Extraction yields obtained with ILs containing a tetrafluoroborate anion were comprised
332
between 59.6% and 71.4%, while all the extraction yields obtained with ILs containing a
333
perchlorate anion were higher than 90%. This effect is somewhat due to the ability of
334
perchlorate anions to precipitate in the presence of potassium as shown below.
335 336
Indeed, over the course of extraction, an anionic exchange occurs and the insoluble potassium
337
perchlorate salt is formed in the organic phase, thus enhancing the formation of GBOCn+ 416
338
CPA- and leading to a better extraction yield as compared to ILs with other anions. We also
339
observed that extraction yields were improved with ILs containing a longer alkyl chain as
340
observed in previous works [29]. For instance the extraction yields obtained with the
341
tetrafluoroborate series were 59.6. 65.5 and 71.4% for n = 12, 14, and 16, respectively. For
342
the perchlorate series, extraction yields were 92.1, 99.1, and 96.7% for n = 12, 14, and 16,
343
respectively. The lower extraction percentage obtained with GBOC16-ClO4 could be explained
344
by its lower solubility in EtOAc, which limits its efficiency as an extractant. GBOC14-ClO4
345
was therefore selected to perform all others extraction experiments.
346 347
3.2. Influence of IL concentration on the extraction profile of pesticides from aqueous
348
solutions
349
The results depicted in Figure 2 represent the extraction trends for the four pesticides using
350
GBOC14-ClO4 as the extractant.
Extraction percentage
100 80 60 40
4-CPA 2,4D Propoxycarbazone Imazamox
20 0 0
351
10
20
30
40
50
IL equivalents
352
Fig. 2. Extraction behaviors for 4-CPA, 2,4-D, propoxycarbazone and imazamox with
353
GBOC14-ClO4 IL (with a confidence interval +/- 0.3%).
354
17
355
Without IL, only 7.8% and 2.1% of the two chlorophenoxyacetic acids 4-CPA and 2,4-D were
356
transferred into the organic phase. For the two other pesticides, imazamox and
357
propoxycarbazone which are more soluble in water (4 g/L and 2.9 g/L, respectively), only 0.8
358
and 0.2 % of the pesticides were transferred into the organic phase.
359
With 25 equivalents of GBOC14-ClO4, the extraction percentages were 99.1 and 95.1% for 4-
360
CPA and 2,4-D, respectively. For imazamox and propoxycarbazone, the extraction
361
percentages were only 53.5 and 76.0 %, respectively. These lower values could be explained
362
by the higher hydrophilicity of these molecules as compared to 4-CPA and 2,4-D. With 50
363
equivalents of GBOC14-ClO4 ([C] = 2.5 x 10-2 mol.L-1), the extraction yields increased up to
364
61.0% for imazamox and up to 81.3% for propoxycarbazone. The extraction percentage
365
remained lower than those obtained for the two phenoxy herbicides. However, we expected
366
that the transposition of extraction experiments from batch test conditions to CPE by
367
exploiting the length (231 interconnecting partition cells) of the CPE column should provide
368
higher extraction percentages.
369
Each extraction experiment was replicated five times and the extraction percentage variations
370
were less than 1.5% between experiments, indicating a very good reproducibility (Figure 2).
371 372
3.3. Pesticide extraction by Centrifugal Partition Extraction (CPE)
373
Nowadays, ILs are mainly used in micro-extraction for the treatment of very low volumes of
374
aqueous solutions, typically from 2 to 10 mL, with a low amount of pure IL. Micro-extraction
375
is a very efficient process to obtain high enrichment factors (80-500) and extraction yields
376
higher than 90 % [30]. But to our knowledge, no literature deals with the use of ILs for the
377
treatment of large volumes of aqueous solution contaminated with pesticides. The main goal
378
of this work was to develop a preparative-scale process for the extraction process of pesticides
379
by combining ILs as extractants and CPE. Our objective was to extract quantitatively the
18
380
pesticides from aqueous solutions and to investigate the selectivity of the process (i.e. the
381
separation of the different pesticides) and its recyclability.
382 383
3.3.1. CPE: Extraction and back-extraction results on individual pesticides
384
Extraction and back-extraction steps were firstly performed by CPE with each individual
385
pesticide. Experiments were performed at a flow rate of 10 mL/min by using a molar ratio
386
nIL/npesticides = 25 with a pesticide concentration of 62.5 mg/L in the aqueous mobile phase.
387
After pumping slightly more than one column volume (320 mL), no pesticide was detected for
388
4-CPA, 2,4-D and propoxycarbazone in the fractions collected during the extraction step
389
indicating that these compounds were quantitatively extracted from the aqueous mobile phase.
390
However under the same experimental conditions, the extraction of imazamox was less
391
effective with an extraction percentage of 58.0 %. In order to improve the extraction
392
percentage of imazamox, another experiment using twice more extractant GBOC14-ClO4 was
393
carried out, but the extraction percentage remained limited to 63.0 %.
394
For the back-extraction step, KI was first used as a displacer with
395
ranging from 1 to 10. The recovery of 4-CPA during this back-extraction step increased from
396
86 % to 100 % when the molar ratio
397
that a minimum of 10 equivalents of KI are required to obtain a complete back-extraction of
398
4-CPA. Under the exact same conditions, high recovery percentages were obtained for the
399
other pesticides as indicated in table 4. The use of NaI as a displacer instead of KI was also
400
investigated. The results showed that the nature of the cation on the displacer agent did not
401
significantly influence the back-extraction performance, except for 2,4-D for which the back-
402
extraction was slightly improved.
nKI nGBOC14 −ClO4
nKI nGBOC14 −ClO4
molar ratios
increased from 1 to 10. These results showed
403 19
404
Table 4
405
Extraction percentage (%E), Recovery percentage (%R) of different pesticides; Experimental
406
conditions: mobile phase 1 (extraction process): mass of each pesticide = 25 mg, pH = 9
407
(KOH), ratio (
408
NaI (10 equivalents)
n IL n pesticide
) = 25; mobile phase 2 (back-extraction process): aqueous phase KI or
409 410 4-CPA
2,4-D
Propoxycarbazone
Imazamox
%E
quantitative
quantitative
quantitative
58.0
%R
100.0
99.0
98.7
95.6
411 412 413
3.3.2. CPE: Extraction and Back-extraction results on mixture of pesticides
414
Support-free liquid-liquid separation techniques such as CPC (including CPE type devices) or
415
couter-current chromatography (CCC) are well-known for their large sample loading capacity
416
and scaling-up ability, but are also very attractive in terms of selectivity [31,32], especially
417
when the displacement mode (ion-exchange, pH-zone refining) is performed. Here the
418
selective recovery of pesticides was investigated.
419
Firstly, extraction and back-extraction were carried out for a binary mixture of the two
420
phenoxy-herbicides 4-CPA and 2,4-D. During the extraction step, 320 mL of the aqueous
421
mobile phase pumped at a flow rate of 10 mL/min and a rotation speed of 1000 rpm. The two
422
pesticides were quantitatively retained in the organic stationary phase. The extraction/back-
423
extraction profile of these pesticides when using NaI as a displacer (
424
Figure 3. This profile can be divided in three distinct zones: zone 1 covered the range between
425
160 mL and 240 mL (fraction 8-12, recovered mass = 17.0 mg, 9.1 x 10-5 mol), 4-CPA was
nNaI = 10) is shown in nIL
20
426
obtained as a pure compound; zone 2 covered the range between 260 mL and 320 mL
427
(fraction 13-16, m (CPA) = 4.1 mg, 2.1 x 10-5 mol and m (2,4-D) = 5.7 mg, 2.6 x 10-5 mol) in
428
which both compounds were obtained as a mixture, and zone 3 ranged above 320 mL
429
(fraction 17-30, recovery mass = 18.8 mg, 8.5 x 10-5 mol) only 2,4-D was recovered. These
430
results indicate a quite good pesticide separation. Pure fractions of 4-CPA represent 81% of
431
the total injected mass of 4-CPA, and 76% for 2,4-D. The total recovery percentage was
432
quantitative for 4-CPA and 2,4-D, respectively (Figure 3). For this experiment, the selectivity
433
factor (α) calculated was equal to 2.2 with a Rs of 0.86. Zone 1
quantity of pesticides (mol %)
35
Zone 2
Zone 3
30
4-CPA 2,4-D
25 20 15 10 5 0 0
434
100
200 300 400 Volume of displacer (mL)
500
600
435 436
Fig. 3. Back-extraction of the equimolar mixture of 4-CPA and 2,4-D with NaI as displacer,
437
Experimental conditions: ratio nNaI/nIL = 10, flow rate= 10 mL/min, rotation speed 1000 rpm,
438
descending mode
439 440
The second experiment was the extraction and back-extraction of an equimolar mixture of the
441
four herbicides (8.0 x 10-5 mol). During the extraction step (
442
aqueous mobile phase pumped at a flow rate of 10 mL/min and a rotation speed of 1000 rpm),
nIL n pesticides
= 25, 320 mL of the
21
443
4-CPA, 2,4-D and propoxycarbazone were quantitatively retained in the organic stationary
444
phase. Only imazamox was partially extracted (%E = 68.7%). The back-extraction profile
445
obtained with NaI as a displacer (
446
between 80 mL and 120 mL (fraction 4-6, recovered mass = 17.0 mg, 5.6 x 10-5 mol), pure
447
fractions of imazamox were isolated. In zone 2, between 140 mL and 180 mL, a mixture of 4-
448
CPA, imazamox and propoxycarbazone was collected. Zone 3 (200–280mL) corresponds to a
449
mixture of 4-CPA and propoxycarbazone. In zone 4, between 300 and 380 mL, 4-CPA was
450
recovered as a pure compound. Finally zone 5, (400 mL- 600 mL) 2,4-D was isolated as a
451
pure compound. The total recovery percentages were 95.5% for 4-CPA, 98.7% for 2,4-D,
452
96.5% for imazamox and 96.0% for propoxycarbazone. (Figure 4)
nNaI = 10) can be divided in five distinct zones. In zone 1, nIL
453 Zone 5
Zone 4
Zone 3
Zone 2
Zone 1
454 455
Fig. 4. Back-extraction of the equimolar mixture of the four pesticides with NaI as displacer.
456 457
The selectivity factors α and Rs considering two consecutive peaks, between imazamox –
458
propoxycarbazone (peak 1 – peak 2), propoxycarbazone – 4-CPA (peak 2 – peak 3) and 4-
459
CPA – 2,4-D (peak 3 – peak 4) are summarized in Table 5.
22
460
Table 5
461
Selectivity factor α and Resolution Rs calculated for the mixture of the four pesticides.
462
Selectivity factor (α) Resolution (Rs)
peak 1 – peak 2
peak 2 – peak 3
peak 3 – peak 4
220 0.7
1.18 0.14
3.31 1.4
463 464
During this back-extraction step, imazamox and 2,4-D were well separated with a selectivity
465
factor superior to 2.2, but the back-extraction profiles of 4-CPA and propoxycarbazone were
466
rather similar and their separation was less effective.
467 468
3.4. Back-extraction with sodium perchlorate as displacer
469
The regeneration, recyclability and reuse of ILs without significant loss remains a critical
470
issue to make the process economically viable and reduce the potential environmental burden.
471
The recyclability of the method was thus studied.
472
Back-extraction with NaClO4 as a displacer (
473
to regenerate the IL GBOC14-ClO4 directly in the organic stationary phase, and to investigate
474
the possibility to recycle the system (Figure 5).
nNaClO4 nIL
= 10) was performed, the main goal being
23
475 476
Fig. 5. Extraction/Back-extraction cycle of 4-CPA ([C] = 2.5 x 10-4 mol.L-1) with GBOC14-
477
ClO4 IL as extractant and NaClO4 as displacer.
478 479
For this study, the extraction of an equimolar mixture of 4-CPA and 2,4-D, was investigated
480
at a concentration of 2.5 x 10-4 mol.L-1. During the extraction (
481
pesticides were quantitatively retained in the organic stationary phase. It was observed that
482
the back-extraction step was more efficient using NaClO4 as a displacer than with NaI.
483
Indeed, the back-extraction of 4-CPA was completed in only four fractions (80 mL) instead of
484
eight (160 mL) with NaI. In the same manner, the back-extraction of 2,4-D was realized in
485
180 mL instead of 320 mL. But in this latter case, the separation was less marked. The
486
selectivity factor was equal to 1.4 with a Rs value of 0.4. In the other hand, a selectivity factor
487
of 2.2 with a Rs of 0.9 were obtained for the back-extraction with NaI.
488
Only one fraction of pure 4-CPA was isolated (fraction 8). 2,4-D was isolated in fractions 12-
489
16. The total recovery percentage was 100% and 91% for 4-CPA and 2,4-D, respectively
490
(Figure 6).
nIL n pesticides
= 25), the two
491
24
492 493
Fig. 6. Back-extraction of the equimolar mixture of 4-CPA and 2,4-D ([C] = 2.8 x 10-4 mol.L-
494
1
) with NaClO4 as displacer.
495 496
3.5. Process recyclability
497
With regard to the efficiency of NaClO4 as a displacer, the recyclability of the process was
498
studied for four cycles with alternative extraction (0 - 350 mL, 700 - 1050 mL, 1400 - 1750
499
mL and 2100 - 2450 mL) and back-extraction (350 - 700 mL, 1050 – 1400 mL, 17500 – 2100
500
mL and 2450 – 2700 ml) steps. The quantitative extraction of 4-CPA for the two first cycles
501
proves that the regeneration of GBOC14-ClO4 was effective (Figure 7).
502
The high extraction percentages (98.3% and 97.1% for the third and the fourth cycle) showed
503
that IL maintained its extraction capacity over multiple extraction/back extraction cycles.
25
Back-extraction
504 505
(a)
506 507
(b)
508
Fig. 7. (a) Profile curve for four cycles of Extraction/Back-extraction of 4-CPA with NaClO4
509
as displacer. (b) Percentage of 4-CPA recovered during extraction and back-extraction
510
process of 4-CPA with NaClO4 as displacer
511 512
4. Conclusion
513
The main focus of this work was to demonstrate the efficiency of Centrifugal Partition
514
Extraction to remove pesticides from water samples and optimize this extraction process. 26
515
The extraction potential of six new ILs derived from glycine betaine was investigated in batch
516
tests and then transposed to CPE. Batch tests have shown that GBOC14-ClO4 is the most
517
efficient IL for the extraction of the four model pesticides (4-CPA, 2,4-D, propoxycarbazone
518
and imazamox), and the transposition to a larger scale was successfully achieved by CPE.
519
Good extraction yields were obtained by CPE with a quantitative extraction for three of the
520
four pesticides and high recovery rate over the back-extraction step (up to 96%). Selective
521
back-extractions were also obtained for the separation of the two phenoxy herbicides. The
522
recyclability of the process has also demonstrated that the extractant can be regenerated and
523
the extraction/back-extraction cycle can be repeated at least four times. However, it seems
524
worthwhile to extend this work by replacing the perchlorate anion by other anions such as
525
amino-acid which are renewable and convenient, allowing the extension of this method to
526
larger effluent volumes.
527 528
Acknowledgements
529
We are grateful to Dr. Yamina Belabassi (UMR 7312, University of Reims Champagne-
530
Ardenne) for linguistic improvement of this manuscript. We thank the Conseil Régional de la
531
Marne for financial support.
532 533
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32
Highlights
662 663 664
•
Pesticides are extracted from aqueous media using new bio-sourced ionic liquids
665
•
Extraction conditions were optimized in batch tests and transposed to Centrifugal
666 667
Partition Extraction •
668
Four model pesticides were successfully extracted by CPE in the ion-pair displacement mode
669
•
Pesticide mixtures were well separated and enriched over the back-extraction step
670
•
The recyclability of the process was demonstrated
671 672
34