Electro-Fenton treatment of imidazolium-based ionic liquids: Kinetics and degradation pathways

Article abstract of DOI:10.1039/c5ra24070k

In this study, the removal of five imidazolium-based ionic liquids (ILs) from water was accomplished by a heterogeneous electro-Fenton treatment. Different ILs were selected in order to study the effect of the alkyl chain length and the nature of the anion. Initially, the effect of the catalyst (iron alginate beads) dosage and current were evaluated. The results showed that the optimum conditions were attained when operating with 4.27 g of catalyst and 0.3 A, achieving degradation yields and TOC reductions higher than 95% after 90 min and 80% after 480 min, respectively. Regarding the ILs with common cations, lower degradation rates were obtained for those with longer alkyl chains, thus 1-ethyl-3-methylimidazolium dicyanamide, exhibits faster degradation than 1-hexyl-3-methylimidazolium dicyanamide and 1-butyl-3-methylimidazolium dicyanamide. In addition, the influence of the counter anions on the oxidation rate of different ILs under an electro-Fenton treatment was demonstrated, following the order: methylsulfate > dicyanamide > acetate. The Microtox ecotoxicity tests of the initial and treated samples revealed that none of the three studied ILs with shorter alkyl chain lengths showed toxicity and only 1-hexyl-3-methylimidazolium dicyanamide and 1-butyl-3-methylimidazolium dicyanamide presented toxic levels, with EC₅₀ values of 269.85 and 47.55 mg L^-1, respectively. Nonetheless, after 480 min of heterogeneous electro-Fenton treatment at 0.3 A, the toxicity of both was completely reduced. Finally, to confirm the mineralization of these compounds, the identification of several reaction intermediates in the heterogeneous electro-Fenton treatment of three ILs was assayed and a plausible degradation pathway was proposed.

Full text of DOI:10.1039/c5ra24070k

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Electro-Fenton treatment of imidazolium-based
ionic liquids: kinetics and degradation pathways
Cite this: RSC Adv., 2016, 6, 1958
*
Elvira Bocos, Marta Pazos and M. A´ngeles Sanroman
´
In this study, the removal of five imidazolium-based ionic liquids (ILs) from water was accomplished by
a heterogeneous electro-Fenton treatment. Different ILs were selected in order to study the effect of
the alkyl chain length and the nature of the anion. Initially, the effect of the catalyst (iron alginate beads)
dosage and current were evaluated. The results showed that the optimum conditions were attained
when operating with 4.27 g of catalyst and 0.3 A, achieving degradation yields and TOC reductions
higher than 95% after 90 min and 80% after 480 min, respectively. Regarding the ILs with common
cations, lower degradation rates were obtained for those with longer alkyl chains, thus 1-ethyl-3-
methylimidazolium dicyanamide, exhibits faster degradation than 1-hexyl-3-methylimidazolium
dicyanamide and 1-butyl-3-methylimidazolium dicyanamide. In addition, the influence of the counter
anions on the oxidation rate of different ILs under an electro-Fenton treatment was demonstrated,
following the order: methylsulfate > dicyanamide > acetate. The Microtox ecotoxicity tests of the initial
and treated samples revealed that none of the three studied ILs with shorter alkyl chain lengths showed
toxicity and only 1-hexyl-3-methylimidazolium dicyanamide and 1-butyl-3-methylimidazolium
dicyanamide presented toxic levels, with EC50 values of 269.85 and 47.55 mg Lꢀ1, respectively.
Nonetheless, after 480 min of heterogeneous electro-Fenton treatment at 0.3 A, the toxicity of both was
completely reduced. Finally, to confirm the mineralization of these compounds, the identification of
several reaction intermediates in the heterogeneous electro-Fenton treatment of three ILs was assayed
and a plausible degradation pathway was proposed.
Received 14th November 2015
Accepted 16th December 2015
DOI: 10.1039/c5ra24070k
www.rsc.org/advances
Furthermore, it has been demonstrated the strong inuence
of the alkyl chain length of the cation on the removal of ILs.8–10
1. Introduction
They found a signicant primary biodegradation for (eco)toxi-
cologically unfavourable compounds carrying long alkyl side
chains (C6 and C8). In contrast for (eco)toxicologically more
recommendable imidazolium ionic liquids with short alkyl
(#C6) and short functionalised side chains, no biological
degradation could be found.9
Over the past decade, the use of ionic liquids (ILs) has been
extended to a broad number of industrial applications such as
catalysts,1 biocatalysts,2 biotechnology, synthetic chemistry and
electrochemistry.3
Typical ILs are salts composed of an organic or inorganic
cation with delocalized charges and an inorganic or organic
ꢁ
At the present time, scarce studies have been conducted to
evaluate the degradation of ILs; however these pollutants are
being introduced into the environment. Thus, the development of
new technologies able to treat and ensure the complete removal
of these compounds from aquatic environments is required.
Recently, the so-called electrochemical advanced oxidation
processes (EAOPs) have been on the spotlight of the scientic
community. They are based on the electrochemical generation
of oxidants used for the degradation of the pollutants. Thus,
EAOPs have several advantages such as their high efficiency,
versatility and safety since they operate at mind conditions.
Among them, electro-Fenton is almost the most typical.11
The electro-Fenton treatment is based on the generation of
powerful oxidant radicals such as cOH, able to degrade a huge
number of hazardous compounds. In this process, many reac-
tions take place and had been described in detail by Zhao et al.12
anion, generally a liquid below 100 C. Because of their negli-
gible vapour pressure, low-ammability and high chemical and
thermal stability, they have been regarded as “green” solvents.4
However, due to their high stability certain amounts of ILs
would be found in a few years in the conventional wastewater
treatment plants.
Even though the abiotic and biotic effect of ILs on the
environment is still unknown, their environmental fate is being
carefully regarded. Recently, biological evaluations have
pointed out the negative effect that these “green” solvents can
provoke on microorganisms, even more harmful than the
conventional solvents.5–7
Department of Chemical Engineering, University of Vigo, Isaac Newton Building,
Campus As Lagoas, Marcosende 36310, Vigo, Spain. E-mail: sanroman@uvigo.es;
Fax: +34 986 812380; Tel: +34 986 812383
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Among them, the main reaction is the generation of hydroxyl for the degradation of different ILs studied were proposed based
radicals13 (eqn (1)) from Fenton's reaction between Fe2+ present on the identication of the degradation products and different
in the medium and the H2O2 electrochemically generated by the cations released along the electro-Fenton treatment and deter-
continuous air ow on a suitable cathode.
mined by GC/MS and ion-exclusion HPLC.
Fe2+ + H2O2 / Fe3+ + cOH + OHꢀ
(1)
2. Materials and methods
2.1. Chemicals
Some of the main advantages of this technique are the onsite
production of H2O2 and the continuous regeneration of Fe3+ to
Fe2+ at the cathode. However, it is difficult the operation in
continuous mode when the classic homogeneous electro-
Fenton process is used.
Five ILs from the imidazolium family were purchased to Iolitec
(GmbH, Germany). Their complete and abbreviated names, as
well as their chemical structures are depicted in Table 1. The
initial concentration of the ve ILs, used in all assays, was 0.5 g
Lꢀ1. Na2SO4 used as electrolyte was provided by Sigma-Aldrich
(Barcelona, Spain). All aqueous solutions were prepared with
Millipore ltered water and volumetric lab equipment.
Over the past years, a wide range of inexpensive heteroge-
neous iron catalysts, in which iron can be effectively binding
into the structure of several materials, have been developed
demonstrating their potential to address this problem.
Recently, Bocos et al.14 synthesized polyacrylamide iron
enriched hydrogels able to be used as heterogeneous catalysers
in the electro-Fenton treatment of dye polluted streams.
Furthermore, it has been extensively reported the high catalytic
activity of iron alginate beads (FeAB) on the elimination of
several organic compounds with minimal iron leaching.15,16
FeAB have demonstrated their high efficiency and stability
working in batch and continuous mode during the decontam-
ination of wastewater by an electro-Fenton process.16,17
Furthermore, their high versatility has made of them an inter-
esting alternative when treating different pollutants such as
dyes or pesticides.18,19
Up to now, only few studies have reported the application of
advanced oxidation processes (AOPs) to the elimination of ILs
from water. Stepnowski and Zaleska,20 studied the degradation
of ILs comparing different photodegradation methods at room
temperature. Fabianska et al.21 studied the elimination of ve
ILs using BDD anodes by anodic oxidation, demonstrating the
great inuence of the IL composition on the degradation rate of
the pollutant as well as the inuence of chlorine on this treat-
ment, which enhances the elimination of the parent compound.
Another interesting study was performed by Siedlecka et al.,22
who elucidate the degradation pathway of the IL of 1-butyl-3-
methylimidazolium chloride by anodic oxidation treatment
using 4 anode materials: IrPt, IrO2, PbO2 and BDD. As expected,
these authors determined that BDD anode is the most suitable
for the elimination of this IL from water. More recently, Munoz
et al.23 reported the elimination of different families of ILs by
the application of Fenton treatment at different temperatures
ranging from 70ꢁ to 90 ꢁC. However, to our best knowledge there
are not studies dealing with the elimination of ILs by an electro-
Fenton treatment.
2.2. Electrochemical experiments
All experiments were carried out in a 0.25 L cylindrical glass
reactor with 0.20 L of IL solution and Na2SO4 (0.05 M) at pH 3.
Besides, different FeAB dosage (3.20 g, 4.27 g and 5.33 g) were
added to the bulk as catalyst in order to test the effect of iron
concentration in the treatment. The catalyst was prepared
following the procedure optimized by Iglesias et al.24 The solu-
tion was continuously agitated with a magnetic stirrer to avoid
concentration gradients. H2O2 was in situ electrochemically
generated through the continuous bubbling of compressed air
(0.75 L minꢀ1) on the cathode surface. The electric parameters
were monitored with a multimeter (Fluke 175). A constant
current was applied (0.1, 0.2 or 0.3 A) by two electrodes con-
nected to a direct power supply (HP model 3662). Carbon felt
(Carbon Lorraine, France) and boron-doped diamond (BDD)
supplied by DIACHEM®, Germany (4–5 mm diamond lm
thickness and doping level around 2500 ppm) were selected as
cathode and anode, respectively. Carbon Felt was placed on the
inner wall of the cell (6 ꢂ 12 cm), covering the total internal
perimeter while BDD (3 ꢂ 6 ꢂ 0.2 cm) was centred in the reactor.
All the experiments were performed in duplicate at ambient
temperature. The reported results were the mean values with
a standard deviation lower than 5%.
Samples were drawn periodically from the reactor and
centrifuged at 10 000 rpm for 5 min. The supernatant was
separated to analyse pH, and IL concentration.
2.3. Kinetic studies
Kinetic studies were conducted to determine the model of
behaviour of the degradation each IL. ILs concentrations along
the time were tted to a suitable kinetic equation and the rate
constants were calculated using the soware Sigma plot. Based
on the Marquard–Levenberg algorithm, this soware uses an
iterative procedure that seeks the values of the parameters to
minimize the sum of the square differences between the pre-
dicted and the observed values.
On this work, the breakdown of ve imidazolium-based ILs by
electro-Fenton process using FeAB as heterogeneous catalyst was
evaluated and different key parameters were optimized. Thus,
the effect of FeAB dosage and current, on the degradation kinetic
of the ILs and their mineralization, were determined. Besides, it
was evaluated the contribution of the alkyl chain length as well
as the nature of the anion on their elimination. To determine the 2.4. Analytical method
toxicity of the solutions, ecotoxicity assays with the bacterium
Vibrio scheri were performed. Finally, mineralization pathways
HPLC analysis. The concentrations of the ve ILs were
monitored by an Agilent 1100 HPLC equipped with a Synergi 4u
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Table 1 Chemical name, abbreviated name and molecular structure of the five ILs studied
IL name
Abbreviated name
Chemical structure
Molecular structure
1-Hexyl-3-methylimidazolium
dicyanamide
[C6mim][N(CN)2]
C12H19N5
1-Butyl-3-methylimidazolium
dicyanamide
[C4mim][N(CN)2]
[C2mim][N(CN)2]
[C2mim][CH3CO2]
C10H15N5
1-Ethyl-3-methylimidazolium
dicyanamide
C8H11N5
1-Ethyl-3-methylimidazolium
acetate
C8H14N2O2
1-Ethyl-3-methylimidazolium
methylsulfate
[C2mim][CH3SO4]
C2H14N2O4S
Polar-RP 80A column. Prior to injection, the samples were 1.2 mL minꢀ1. For the GC separation, the GC injection port
ltered through a 0.45 mm Teon lter. The injection volume temperature was set at 280 ꢁC. The program temperature
was set at 1 mL, and the isocratic eluent acetonitrile/water (5 mM started at 50 ꢁC (held during 5 min).ꢀS1ubsequently, the
of Na2HPO4 + 7.5 mM H3PO4) was pumped at a rate of 1 mL temperature ramp was set at 5ꢁC min
to 280 ꢁC. The
minꢀ1. The ratio acetonitrile/water was adjusted for the deter- temperature was maintained at 280 ꢁC for 5 min The MS
mination of each IL. [C6mim][N(CN)2] and [C4mim][N(CN)2]: detector was operated in EI mode (70 eV).
25 : 75; and [C2mim][N(CN)2], [C2mim][CH3CO2] and [C2mim]
Total organic carbon analysis. Total organic carbon (TOC)
[CH3SO4]: 2 : 98. Detection was performed with a diode array was determined using a multi N/C 3100 equipment (Analytic
detector set at 212 nm, and the column temperature was Jena) coupled with a NDIR detector (C.A.C.T.I, University of
maintained at room temperature.
Vigo). From these results, the TOC percentages abatements
In order to identify the generated aliphatic carboxylic acids were calculated from the following equation (eqn (2)):
obtained in the ILs degradation several samples were analysed
DTOC
TOC reduction ð%Þ ¼
ꢂ 100
(2)
with an ion-exclusion HPLC (Agilent 1100) equipped with
a Rezex™ ROA-organic acid H+ (8%), column (300 ꢂ 7.8 mm
i.d., 8 mm). A 0.005 N H2SO4 solution at a ow rate of 0.5 mL
minꢀ1 was used as the mobile phase. Detection was performed
with a diode array detector at 210 nm, and the column
temperature was maintained at 55 ꢁC. The identication of
intermediates was made by comparison of retention time and
UV spectra with those of pure standards.
TOC0
where DTOC is the variation of TOC (mg Lꢀ1) during the treat-
ment and TOC0 (mg Lꢀ1) is the TOC value at the beginning of
the treatment.
2.5. Toxicity tests
GC/MS analysis. Extractions of 200 mL of the aqueous
sample were performed three times with 30 mL of ethyl
acetate each time. Aer extraction, samples were dried with
a rotary evaporator and taken up to 200 mL of ethyl acetate for
their analysis by GC-MS. Thus, the identication of the
degradation products formed during the electro-Fenton
treatment of the ILs was performed using a 6850 Agilent GC
equipped with a 5955C VLMSD equipped with a HP-5-MS
column. Hydrogen was the carrier gas at a ow rate of
The potential toxicity of the ve ILs and their reaction inter-
mediates was evaluated following the bioluminescence of the
marine bacterium Vibrio scheri (Lumistox LCK 487), by means
of the Microtox® method according to the international stan-
dard process 81.9% Basic Test. The value of the inhibition of the
bacterium luminescence (%) was measured aer 15 min of
exposition of Vibrio scheri to the studied solutions at 20 ꢁC. The
bioluminescence measurements were carried out in samples
extracted at initial time and aer 480 min of treatment by
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Table 2 Influence of FeAB dosage on the kinetic rate constant of the
five ILs studieda
electro-Fenton at constant current (0.3 A) and in control
samples without ILs.
IL
FeAB (g)
k (minꢀ1
)
R2
3. Results and discussion
[C6mim][N(CN)2]
3.20
4.27
5.33
3.20
4.27
5.33
3.20
4.27
5.33
3.20
4.27
5.33
3.20
4.27
5.33
0.0094
0.0154
0.0032
0.0129
0.0195
0.0102
0.0207
0.0356
0.0157
0.0198
0.0317
0.0158
0.0254
0.0370
0.0164
0.9973
0.9968
0.9957
0.9924
0.9990
0.9966
0.9996
0.9974
0.9937
0.9968
0.9971
0.9959
0.9975
0.9983
0.9991
There are numerous families of ILs, such as pyridinium, gua-
nidinium, phosphonium or imidazolium. Those with aromatic
cations, as for example imidazolium have showed to be more
toxic and to have antimicrobial properties. Furthermore, the
toxicity of these latter is variable, being especially increased for [C2mim][N(CN)2]
cations with long alkyl chain and anions with high lipophilicity
or difficult to be hydrolysed.9 Given the elevated use, in different
industrial elds, of imidazolium-based ILs, this study deals the
elimination from water of ve ILs from this family. Furthermore
the inuence of the cation cores and the anion nature will be
studied in depth.
[C4mim][N(CN)2]
[C2mim][CH3CO2]
[C2mim][CH3SO4]
a
k is the kinetic coefficient for the rst-order reaction.
3.1. Effect of FeAB dosage on the degradation kinetic of the
ILs
It is well known that iron concentration is one of the key
parameters controlling the electro-Fenton process, since it rules
the generation of hydroxyl radicals from Fenton's reaction. On
this way, high or low dosages can inhibit the efficiency of the
treatment25 by the increase or reduction on the rate of the
reaction described on eqn (1).
Initially, the electro-Fenton process was carried out at
constant current of 0.1 A and different FeAB dosages (3.20, 4.27
or 5.3 g) were added to the medium in order to follow the
degradation of the different ILs. Table 2 shows the obtained
results, which indicated that the degradation of the different ILs
could be quantitatively described by a rst-order kinetic equa-
tion with respect to the IL concentration. In addition, Fig. 1
represent the proles of the IL [C6mim][N(CN)2] degradation by
electro-Fenton treatment at different FeAB dosage. Notably, the
removal of the IL from the solution was faster with FeAB
dosages of 4.27 g. Besides, as it can be concluded from Table 2,
same trend was observed for the different ILs studied. The use
of higher FeAB dosage did not provide positive effects; on the
contrary, a delay in ILs oxidation rates was observed when the
FeAB dosage was increased in the medium. This fact could be
explained as a result of secondary reactions between hydroxyl
radical and the excess of iron present.26 Moreover, at low dosage
the reaction rate was reduced. Thus a dosage of 4.27 g of FeAB in
0.2 L of reaction medium seemed to be the optimal catalyst
concentration. These results are in accordance with the ob-
tained by Iglesias et al.24 They already pointed out, that working
with 4.27 g of FeAB dosage improves the efficiency of the electro-
Fenton treatment of the pesticide imidacloprid. Thus, aer
obtain similar results during degradation experiments; this
concentration was used henceforth in the following
experiments.
Fig. 1 Effect of the FeAB dosage on the degradation profile of the IL
[C6mim][N(CN)2] by electro-Fenton process at constant current of 0.1
A. Symbols: circles: 3.20 g of FeAB; triangles: 4.27 g of FeAB; crosses:
5.33 g of FeAB.
length such as [C6mim][N(CN)2], [C4mim][N(CN)2] and [C2mim]
[N(CN)2] and three ILs with different anion like [C2mim]
[N(CN)2], [C2mim][CH3CO2] and [C2mim][CH3SO4] were used as
model pollutants in this study. The values of rst-order kinetic
parameter and the statistical correlation parameters of the
different studied ILs aer 120 min of electro-Fenton treatment
are listed in Table 2. As expected, the lowest apparent rate
constants were obtained when the compounds of longest alkyl
chain were treated. Increasing the alkyl chain length, the imi-
dazolium ring is less targeted to be attacked by the oxidant
radicals. These results are in accordance with those previously
reported for the degradation of ILs by different systems such as
UV, UV/H2O2 or Fenton systems.20,27 Zhou et al.27 for example,
concluded that the degradation rate of ILs was much faster for
[C2mim]Br and [C4mim]Br than for [C6mim]Br and [C8mim]Br
and [C10mim]Br in a nZVI/H2O2 system.
3.2. Effect of the alkyl chain length and the counter anion on
the degradation kinetic
To determine the effect of the alkyl chain length and the counter
anion on the degradation of ILs, three different alkyl chain
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28
´
Recently, Domınguez et al. demonstrated the inuence of much lower than the found for the ILs with [C2mim] cation.
the counter anions on the oxidation rate of different ILs under Likely, the attack of the oxidants would begin by the cleavage of
a Fenton's treatment, following the order: methanesulfonate > the alkyl side chain from the N atom of the ring, which is known
methylsulfate > chloride > acetate. In our assays the degradation to be a less stable bond than others present on the ring.27,32
rate followed the order: methylsulfate > dicyanamide > acetate.
As can be seen in Fig. 2, the mineralization rates achieved by
28
´
Similarly to the results found by Domınguez et al., it was the ILs with [C2mim] cation followed the same order than those
detected the slower degradation of [C2mim][CH3CO2] which previously found during the degradation assays. Thus, highest
could be associated with the precipitation of iron species removal rates were obtained for the [C2mim][CH3SO4] while
detected in these experiments, with the resulting inhibition of [C2mim][N(CN)2] and [C2mim][CH3CO2] showed lowest miner-
its catalytic action.
alization rates. Moreover, in this assay the system attained the
maximum oxidation rate at 0.3 A.
3.3. Inuence of the current on the degradation and
mineralization yields of the ILs
3.4. Microtox toxicity tests
The toxicity tests were performed following the inhibition on
the luminescence of the bacterium Vibrio scheri due to its
exposure to the ILs. Initially, for all ILs concentration–response
curves were obtained and EC50 values were calculated following
the method described in Section 2. The obtained EC50 values of
ILs samples revealed that none of the ILs with the cation C2mim
showed elevated contribution in the toxicity levels. It was only
detected toxic effects for [C4mim][N(CN)2] and [C6mim][N(CN)2]
with EC50 values of 269.85 and 47.55 mg Lꢀ1, respectively. These
results are in agreement with those previously reported by
It has been extensively reported that the current plays an
important role on the pollutant degradation achieved under an
electrochemical treatment. Usually, as the current is incre-
mented, the degradation rates can be accelerated25 due to the
higher production of homogeneous cOH from the reaction of
the hydrogen peroxide formed at the bulk (eqn (1) and (3)). In
addition, the generation of heterogeneous radicals on the
electrode surface following eqn (4) permits the degradation of
organic matter (eqn (5)) and this fact can considerably enhance
the results attained during the treatment.
6
28
´
Cvetko Bubalo et al. and Domınguez et al., who pointed out
(3) that the lipophilic character of the IL is increased with the
length of the alkyl chain thus increasing the disruption into the
O2 + 2H+ + 2eꢀ / H2O2
BDD + H2O / BDD (cHO) + H+ + eꢀ
BDD(cOH) + R /BDD + CO2 + H2O
(4)
bacterium luminescence. On this way, the nature of the anion
did not show a signicant contribution on the toxicity assays.
However, since the initial values were below the EC50 this could
not be consider as a concluding result.
It has been shown that only [C4mim][N(CN)2] and [C6mim]
[N(CN)2] have toxicities in the bioassay used. For this reason,
aer 480 min of heterogeneous electro-Fenton treatment of
solution of [C6mim][N(CN)2] and [C4mim][N(CN)2] at 0.3 A,
samples were drawn and the inhibition of the Vibrio scheri
luminescence was determined. The assays showed negligible
inhibition values that revealed that the toxicity of these solutions
was completely reduced by the oxidation of these compounds.
(5)
where BDD is anode material; BDD(cOH) is heterogeneous cOH
radicals formed at the anode surface and R is organic matter.
Many authors have reported that higher current values can
provoke a decrease of the mineralization current efficiency
since more energy is spent in oxygen evolution and as conse-
quence there are less cOH species able to oxidize the organic
31
pollutants.29,30 In 2008, Ozcan et al. reported that the disap-
¨
pearance of the pesticide picloram become longer aer raise
the current at upper values than 0.3 A. As these organics
decrease at the bulk, there is less matter to degrade, thus
promoting parasitic reactions that can affect the efficiency of
the process.
3.5. Degradation pathway
The analysis of the degradation intermediates of the three ILs
In this work, current intensities ranging from 0.1 to 0.3 A with the same cation ([C6mim][N(CN)2], [C4mim][N(CN)2] and
were studied, following their inuence on the degradation and [C2mim][N(CN)2]) were performed by GC/MS at 0, 30 and 60
mineralization of the target compounds. In relation to the effect min of treatment. Similar intermediate products were detected
of current on degradation of IL, in Fig. 2 is showed the obtained in the samples obtained in the degradation of studied ILs and
results for all ILs studied. It is highlighted the high degradation at different times, therefore the degradation pattern was
levels, around 90%, were attained aer 120 min of treatment at established. In all cases, as previously reported by some
0.1 and 0.2 A of current; while the complete degradation of the authors23,27,32 the rst step was de hydroxylation of the ILs,
ILs was yielded aer the same treatment time using 0.3 A of resulting in the formation of higher molecular structure than
current. However, more time is required to obtain near the initial sample. In addition, aer oxidation of the imidazo-
complete mineralization. The mineralization level of all ILs was lium ring, it was opened and further degraded in new by-
measurement as TOC reduction percentage. The results depic- products. Subsequently, the hydroxyl radicals further attack
ted in Fig. 2 reveal the progressive mineralization decay, to these formed by-products, ending on the formation of
however the increase on the alkyl chain length has an important carboxylic acids at the end of the treatment. In addition, the
contribution on the TOC reduction. In this way, the rate of analysis of the carboxylic acids reveals the presence in the
elimination of [C6mim][N(CN)2] and [C4mim][N(CN)2] was samples of oxamic, oxalic, glycolic, acetic and formic acid. It is
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Fig. 2 Effect of the current on degradation (left) and mineralization (right) of [C6mim][N(CN)2], [C4mim][N(CN)2], [C2mim][N(CN)2], [C2mim]
[CH3SO4] and [C2mim][CH3CO2] by electro-Fenton process with 4.27 g of FeAB. Symbols on degradation: circles 0.1 A; crosses 0.2 A; squares 0.3
A. Symbols on mineralization: squares 0.1 A; crosses 0.2 A; circles 0.3 A.
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Fig.
3 Degradation pathway of the IL [C6mim][N(CN)2]: (a) 1-ethyl-3-hexyl-2,4,5 trioxoimidazolidine, (b) 1-ethyl-3-butyl-2,4,5-triox-
oimidazolidine, (c) N-formyl-N-methyl, (d) formamide, N-hexyl, (e) formamide, N-butyl-N-methyl, (f) formamide, N-pentyl, (g) pentanoic acid (h)
oxalic acid, (i) glycolic acid, (j) acetic acid, (k) formic acid, (l) oxamic acid.
known that these compounds are normally found aer electro-
Finally, the identication of carboxylic acids by ion-exclusion
Fenton processes, nally ending on the formation of CO2 and HPLC allowed the verication of the feasibility of the heteroge-
H2O. As example in the Fig. 3 a schema with the molecular neous electro-Fenton system on the treatment of the studied ILs.
structures of the intermediates generated in the degradation of
[C6mim][N(CN)2] is showed. The highest peaks of the
compounds a–g appeared at 30 min and then they gradually
Acknowledgements
This research has been nancially supported by the Spanish
Ministry of Economy and Competitiveness, Xunta de Galicia
and ERDF Funds (Projects CTM2014-52471-R and GRC 2013/
003). The authors are grateful to the Spanish Ministry of
Economy and Competitiveness for the nancial support of
Elvira Bocos under the FPI program and Marta Pazos under the
were diminished. Therefore, the different intermediates
determined in the samples conrm that the application of
a heterogeneous electro-Fenton system on the treatment of
different ILs it allows their complete mineralization.
4. Conclusions
´
Ramon y Cajal program.
In this work, the efficiency of a heterogeneous electro-Fenton
system on the treatment of different ILs was probed. It was
demonstrated the effect of FeAB dosage and current in the
degradation rate. Moreover, the effect of the current on the
degradation yields and the degradation kinetic was studied.
Regarding to the ILs, it was demonstrated that lowest degra-
dation rates were obtained for those with longest alkyl chain
and the inuence of the counter anions on the oxidation rate of
different ILs under electro-Fenton treatment, following the
order: methylsulfate > dicyanamide > acetate. The ecotoxicity
test of the initial and treated samples showed initial toxicity
upper than the EC50 only for longest alkyl chain length.
However, aer the heterogeneous electro-Fenton treatment the
toxicity was completely reduced.
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