Ionic liquids breakdown by Fenton oxidation

Article abstract of DOI:10.1016/j.cattod.2014.03.028

Fenton oxidation has proved to be an efficient treatment for the degradation of ionic liquids (ILs) of different families viz. imidazolium, pyridinium, ammonium and phosphonium, in water. The intensification of the process, defined as the improvement on the efficiency of H₂O₂ consumption, by increasing the temperature is necessary to avoid high reaction times and the need of large excess of H₂O₂. In this work, temperatures within the range of 70-90°C have been used, which allowed an effective breakdown of the ILs tested (1 g L^-1 initial concentration) with the stoichiometric amount of H₂O₂ and a relatively low Fe³⁺dose (50 mg L^-1). Under these conditions conversion of the ILs was achieved in less than 10 min, with TOC reductions higher than 60% upon 4 h reaction time, except for the phosphonium IL. The remaining TOC corresponded mainly to short-chain organic acids. The treatment reduced substantially the ecotoxicity up to final values below 0.01 TU in most cases and a significant improvement of the biodegradability was achieved. Upon Fenton oxidation of the four ILs tested hydroxylated compounds of higher molecular weight than the starting ILs, fragments of ILs partially oxidized and short-chain organic acids were identified as reaction by-products. Reaction pathways are proposed.

Full text of DOI:10.1016/j.cattod.2014.03.028

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Contents lists available at ScienceDirect
Catalysis Today
journal homepage: www.elsevier.com/locate/cattod
Ionic liquids breakdown by Fenton oxidation
Macarena Munoz∗, Carmen M. Domínguez, Zahara M. de Pedro, Asunción Quintanilla,
Jose A. Casas, Juan J. Rodriguez
Ingenieria Quimica, Universidad Autonoma de Madrid, Crta. Colmenar km 15, 28049 Madrid, Spain
a r t i c l e i n f o
a b s t r a c t
Article history:
Fenton oxidation has proved to be an efficient treatment for the degradation of ionic liquids (ILs) of dif-
ferent families viz. imidazolium, pyridinium, ammonium and phosphonium, in water. The intensification
of the process, defined as the improvement on the efficiency of H2O2 consumption, by increasing the
temperature is necessary to avoid high reaction times and the need of large excess of H2O2. In this work,
temperatures within the range of 70–90 ◦C have been used, which allowed an effective breakdown of the
ILs tested (1 g L−1 initial concentration) with the stoichiometric amount of H2O2 and a relatively low Fe3+
dose (50 mg L−1). Under these conditions conversion of the ILs was achieved in less than 10 min, with TOC
reductions higher than 60% upon 4 h reaction time, except for the phosphonium IL. The remaining TOC
corresponded mainly to short-chain organic acids. The treatment reduced substantially the ecotoxicity
up to final values below 0.01 TU in most cases and a significant improvement of the biodegradability
was achieved. Upon Fenton oxidation of the four ILs tested hydroxylated compounds of higher molec-
ular weight than the starting ILs, fragments of ILs partially oxidized and short-chain organic acids were
identified as reaction by-products. Reaction pathways are proposed.
Received 15 January 2014
Received in revised form 6 March 2014
Accepted 10 March 2014
Available online xxx
Keywords:
Advanced oxidation processes
Fenton
Ionic liquid
Ecotoxicity
Biodegradability
© 2014 Elsevier B.V. All rights reserved.
1. Introduction
Although the low vapor pressure of ILs may reduce the costs
and environmental risks of air pollution, release of these com-
Room temperature ionic liquids (ILs) are a novel, broad class
of semi-organic salts or salt mixtures that are liquid below 100 ◦C.
They are gaining increasing attention due to their unique prop-
erties, such as extremely low vapor pressure, high thermal and
chemical stabilities, non-flammability and high solvent capacity
[1,2]. Thus, these materials are often branded as “environmentally
friendly” or “green solvents”, and have been suggested as ideal
replacements for volatile organic solvents [3–5].
A huge number of different ILs can be synthesized by the com-
bination of different anion and cation cores, or by modifying their
alkyl chain length or introducing oxygenated groups. Thus, ILs have
been termed “designer solvents” since polarity, solvent miscibil-
ity and hydrophobicity can be tuned by a suitable combination of
anion and cation [6]. In this sense, they have been found suitable for
catalysis, biocatalytic processing, extraction, electrochemistry or
separation [1,7], and the number of their commercial applications
as well as patents on IL technology has been rising exponentially
for the last years [8]. Questions regarding the potential impact of
ILs on the environment have been raised recently.
pounds into aquatic environment may lead to water pollution
because of their high solubility in aqueous phase. In fact, indus-
trial application of ILs usually involves water streams in the process
and the synthesis routes of ILs frequently include aqueous media.
Thus it can be expected the presence of varying amounts of ionic
liquids in effluents related with their manufacture and use. This
represents an important concern due to their high stability and tox-
icity [3–5,9,10]. Romero et al. [11] pointed out that the low vapor
pressure of ILs is not by itself sufficient to consider them “green
solvents” since ILs are poorly biodegradable and can be more toxic
than conventional solvents.
The relatively high solubility of ILs in water has important envi-
ronmental consequences and should be taken into account for
the design of processes involving ILs. In this sense, it is essen-
tial to improve those processes, minimizing the discharge of ILs
to the aquatic media. On the other hand, downstream separation
or treatment steps must be required to remove the ILs from the
wastewater streams at the end of these processes. Although adsorp-
tion on activated carbon has proved to be an effective method for
the uptake of ILs [12], current studies have also evidenced that
steric effects restrict the adsorption of those with large molecu-
lar volumes [13]. Moreover, the recovery of ILs from the exhausted
adsorbent is essential and implies the use of conventional volatile
∗
Corresponding author. Tel.: +34 91 497 3991; fax: +34 91 497 3516.
E-mail address: macarena.munnoz@uam.es (M. Munoz).
http://dx.doi.org/10.1016/j.cattod.2014.03.028
0920-5861/© 2014 Elsevier B.V. All rights reserved.
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(3.9, 5.0, 5.5 and 6.0 g L−1 for [emim][Cl], [bmpyr][Cl], [tbN][Cl] and
[tbP][Cl], respectively). The Fe3+ dose was varied within the range
of 10 to 125 mg L−1 and temperatures between 50 and 90 ◦C were
tested. Temperature above the ambient is used in order to increase
the efficiency of hydrogen peroxide consumption and reduce the
reaction time [20]. As recently demonstrated, increasing the tem-
perature does not imply an extra cost since heat can be recovered
from the exit stream and it has also to be considered the exothermic
character of the oxidation process [24].
Blank experiments in the absence of catalyst were carried out
at all the temperatures tested and negligible conversions of all ILs
(<5%) were always observed. All the experiments were performed
by duplicate being the standard deviation lower than 5% in all cases.
solvents [13]. On the other hand, destructive methods allow the
degradation of ILs to less harmful intermediates and partial min-
eralization. The application of biological treatments is seriously
limited due to the high toxicity and low biodegradability of ILs
[11,14–17]. Wells and Coombe [16] studied the ecotoxicity and the
microbial degradation of a wide number of IL families (ammonium,
imidazolium, phosphonium and pyridinium), concluding that ILs
display high ecotoxicity and are strongly resistant to biodegrada-
tion. These authors claimed that those ILs are far from displaying
the green image that is often accepted in the literature. Due to
the high stability and resistance to biodegradation, chemical treat-
ments would represent the best alternative for the removal of ILs.
In this context, Advanced Oxidation Processes (AOPs), based on
the action of hydroxyl radicals at near ambient temperature and
pressure, can be regarded as a potential solution. Among them,
Fenton oxidation using H2O2 and iron salts (Fe2+/Fe3+) is recog-
nized as one of the most cost-effective treatments [18]. It has been
successfully applied in the treatment of a wide range of organic
pollutants (phenols, chlorophenols, nitrophenols, formaldehydes,
toluenes, chlorobenzenes, amines or halomethanes [19]) as well
as real industrial wastewaters [19,20]. However, so far only few
works have been reported dealing with the degradation of ILs by
Fenton oxidation [21–23]. Those studies have been focused only on
the destruction of imidazolium ILs at ambient conditions. Complete
conversion of the IL ([emim][Cl]) was achieved within 90 min reac-
tion time using large excess of H2O2 ([IL]0 = 1 mM; [Fe3+]0 = 1 mM;
[H2O2]0 = 400 mM, which represents 24 times the theoretical stoi-
chiometric amount for the complete oxidation of IL) [21]. Those
authors did not provide information about the evolution of total
organic carbon (TOC) or ecotoxicity of the effluents, which is crucial
in order to evaluate the potential application of Fenton oxidation.
The aim of this work is to analyze the capability of the Fenton
process for the degradation of different IL families viz. imidaz-
olium, pyridinium, ammonium and phosphonium. Following this
objective, the effect of temperature and catalyst load has been
investigated. The identification of the reaction by-products has
been accomplished for the sake of elucidating the oxidation path-
ways. Besides, the biodegradability and ecotoxicity of the ILs and
the resulting Fenton effluents have been evaluated.
2.3. Analytical methods
The progress of the reactions was followed by periodically
withdrawing, cooling and immediately analyzing liquid sam-
ples from the reactor. Ionic liquids were quantified by liquid
chromatoghraphy–mass spectrometry (LC/MS SQ Agilent 6120)
equipped with a quadrupole detector using a Synergy 4 mm Polar-
´
˚
RP 80 A column (15 cm length, 4.6 mm diameter) (Phenomenex)
as the stationary phase. The analyses were carried out at ambient
temperature and a flow rate of 0.5 mL min−1. The elution profiles
were monitored at 230 nm. The mobile phase was a mixture of
water (0.1% formic acid) and acetonitrile in gradient elution. The
MS analysis was performed with electrospray ionization (ESI) inter-
face in the positive mode with a capillary voltage of 2000 V. The
nebulizer gas (N2) pressure was set to 60 psi and the drying gas
flow was 5 L min−1. The drying gas temperature was set at 250 ◦C.
All MS data acquisition and processing were carried out using
the software package LC/MSD ChemStation. External mass cali-
bration for the positive ESI mode was conducted prior to analysis
in the mass range of m/z 100–900. The quantification of the four
ionic liquids studied in this work was carried out using external
standard calibration. Five levels of calibration were used: 1, 2.5, 5,
10 and 20 mg L−1 (r2 ([emim][Cl]) = 0.987; r2 ([bmpyr][Cl]) = 0.994;
r2 ([tbN][Cl]) = 0.991; r2 ([tbP][Cl]) = 0.989, being r2 linear correla-
tion coefficient). Three replicates of each calibration point were
carried out being the standard deviation less than 5% in all cases.
This method achieves detection limits at or below 0.5 mg L−1
,
2. Materials and methods
whereas the quantification ones were 1.0 mg L−1. Those analyses
were also used for the tentative identification of the reaction by-
products. Total organic carbon (TOC) was measured with a TOC
analyzer (Shimadzu, mod. TOC, VSCH) and the H2O2 concentra-
tion was determined by the titanium sulfate method [25] using
an UV 1603 Shimazdu UV/vis spectrophotometer. Chemical oxy-
gen demand (COD) measurements were performed by the Moore
method [26]. Short-chain organic acids were analyzed by ion chro-
matography with chemical suppression (Metrohm 790 IC) using
a conductivity detector. A Metrosep A supp 5–250 column (25 cm
length, 4 mm internal diameter) was used as stationary phase and
a 3.2 mM Na2CO3 aqueous solution as the mobile phase.
2.1. Chemicals
All the chemicals were analytical grade reagents and
were used without further purification. The four ILs tested,
1-ethyl-3-methylimidazolium chloride ([emim][Cl]), 1-butyl-4-
methypyridinium chloride ([bmpyr][Cl]), tetrabutylammonium
chloride ([tbN][Cl]) and tetrabutylphosphonium chloride
([tbP][Cl])), were purchased from Iolitec, Fluka and Sigma-Aldrich.
Hydrogen peroxide solution (30% w/w) in stable form, nitric acid
(65%) and formic acid (95%) were purchased from Sigma-Aldrich.
Iron (III) nitrate nonahydrate (98%), sodium chloride (99.5%) and
sodium hydroxide (>98%) were purchased from Panreac. Titanium
oxysulfate (>99%) was purchased from Fluka. Acetonitrile (99.8%)
was purchased from Scharlau.
2.4. Ecotoxicity tests
The ecotoxicity of the IL solutions and the samples from reaction
was determined by the Microtox toxicity test (ISO 11348-3, 1998).
The bioluminisence was measured in a M500 Microtox Analyzer
(Azur Environmental). The toxicity tests were performed using a
range of concentrations (from 0 to 100%) of each IL solution or
Fenton sample. The light output of the luminescent bacteria was
measured and compared with that of a blank negative control sam-
2.2. Fenton experiments
The oxidation runs were carried out at atmospheric pressure
in a 500 mL glass batch reactor equipped with a magnetic stir-
rer (700 rpm) and temperature control. The initial pH value was
adjusted to 3 with nitric acid. The starting concentration of IL
was always 1 g L−1 and the theoretical stoichiometric amount of
H2O2 for complete mineralization was used in all the experiments
ple. We used as positive control a solution of phenol (100 mg L−1
whose EC50 value is 16 mg L−1). The test was conducted at 15 ◦C,
,
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Fe3+ (mg·L-1)
50
H2O2
COD
TOC
100
80
60
40
20
0
10
125
100
80
60
40
20
100
80
[emim][Cl]
[bmpyr][Cl]
[tbN][Cl]
[tbP][Cl]
Fig. 3. H2O2 conversion, COD and TOC reduction upon Fenton oxidation of
the ILs tested after 4 h reaction time ([IL]0 = 1000 mg L−1
; ] ;
[Fe3+ 0 = 50 mg L−1
[H2O2]0 = stoichimetric dose; pH0 = 3; T = 70 ◦C).
40
20
out in a liquid static respirometer provided with an air dif-
fuser and a mechanical stirrer. The specific oxygen uptake rate
(SOUR, mg O2 g−1 VSS h−1) profiles were directly monitored using
an oxygen meter (Model 407510, EXTECH). The temperature was
controlled by means of a thermostatic bath at 25 ◦C. An initial
biomass (VSS) of 350 mg L−1 was used. The original inoculum was
taken from a municipal wastewater treatment plant and was main-
tained under aeration, with acetate and glucose as carbon sources.
The respirometric lasted 48 h and were carried out by triplicate. The
data reproducibility was better than 5% in all cases.
0
50
70
90
Temperature (ºC)
Fig. 1. Effect of temperature on H2O2 conversion and TOC reduction in Fenton oxi-
dation of [emim][Cl] (1000 mg L−1) at different Fe3+ doses. ([H2O2]0 = stoichiometric
dose: 4000 mg L−1; pH0 = 3; t = 4 h).
3. Results and discussion
3.1. Analysis of the operating conditions
adjusting the osmotic pressure close to 2% NaCl and pH between
6 and 8. The EC50 is defined as the effective nominal concentra-
tion (mg L−1) that reduces the intensity of light emission by 50%
after 15 min contact time. For complex samples, like the Fenton
oxidation effluents, IC50 is used, defined as the reciprocal of the
dilution percentage giving rise to 50% reduction of light emission.
The IC50 values are inversely proportional to the biological toxicity
expressed as toxicity units (TU).
Previous works in the literature dealing with Fenton oxidation of
ILs have been carried out with large excess of H2O2 [21–23]. How-
ever, since H2O2 consumption is a critical issue for the feasibility of
the process it is important to learn more in depth on the true H2O2
needs. In this sense, increasing the temperature has demonstrated
to allow a more efficient use of the reagent [20,24,27].Thus, experi-
ments were performed at different temperatures and catalyst (Fe3+
)
concentrations maintaining the dose of H2O2 at the stoichiometric
amount.
2.5. Biodegradability assays
Fig. 1 shows the results obtained with [emim][Cl] at different
temperatures and Fe3+ doses, in terms of H2O2 conversion (decom-
position) and TOC reduction after 4 h reaction time. As observed,
The biodegradability of the initial IL solutions and the reac-
tion effluents was evaluated by biological batch runs carried
Fig. 2. Evolution of [emim][Cl] (a), H2O2 and TOC (b) upon Fenton oxidation at different temperatures. ([Fe3+ 0 = 50 mg L−1; [H2O2]0 = stoichiometric dose: 4000 mg L−1).
]
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[bmpyr][Cl]
CH3
[emim][Cl]
H3C
N+
N
CH3
m/z = 150
m/z = 111
N+
Hydroxylation
O
CH3
Hydroxylation
O
O
OH
CH3
N+
H3C
N
C
H3C
N
CH3
OH
m/z = 226
N
N
CH3
m/z = 166
OH
O
C
OH
OH
m/z = 210
m/z = 126
N+
m/z = 142
O
Ring-opening
CH3
C
N+
O
OH
O
O
m/z = 85
m/z = 61
C
H3C
N
CH
NH2
NH2
OH
m/z = 130
CH2
C
m/z = 116
Ring-opening
H3C
m/z = 102
O
OH
O
HO
HO
O
C
+
C
NH2
OH
CH3
HO
CH3
[tbP][Cl]
[tbN][Cl]
H3C
H3C
H3C
H3C
N+
CH3
CH3
P+
CH3
CH3
m/z = 259
m/z = 291
m/z = 242
Hydroxylation
Hydroxylation
OH
m/z = 258
m/z = 274
OH
m/z = 275
OH
OH
OH
H3C
H3C
H3C
H3C
H3C
H3C
H3C
H3C
P+
N+
N+
CH3
CH3
P+
CH3
CH3
CH3
CH3
CH3
OH
CH3
OH
Breakdown
Breakdown
H3C
N+
m/z = 146
CH3
OH
O
O
O
C
O
O
HO
P+
C
P+
CH3
NH+
m/z = 102
C
CH3
O
HO
+
HO
OH
HO
OH
HO
N+
CH3
m/z = 160
m/z = 175
m/z = 88
m/z = 237
Short-chain organic acids
HOOC
COOH
m/z = 116
COOH
COOH
m/z = 104
O
O
O
O
O
m/z = 46
H
HO
OH
HO
OH
OH
m/z = 90
CO2 + H2O
O
CH3
m/z = 60
OH
2CO2 + H2O
Fig. 4. Proposed reaction pathways for Fenton oxidation of the ILs tested.
or 125 mg L−1). Working at iron doses as low as possible improves
the efficiency of H2O2 consumption by minimizing competitive
scavenging reactions [26,28,29]. In addition, it has the obvious ben-
efit of reducing the catalyst consumption, which is lost with the
increasing the temperature improved the conversion of both H2O2
and TOC. This effect is more pronounced when using moderate Fe
concentrations (50 mg L−1), since at or above 70 ◦C practically the
same TOC reduction was obtained regardless the iron dose used (50
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[emim][Cl]
[bmpyr][Cl]
80
60
40
20
0
80
60
40
20
0
100
80
100
80
60
40
20
0
60
40
20
0
0
10
20
time (h)
[tbN][Cl]
30
40
50
0
10
20
30
40
50
time (h)
[tbP][Cl]
80
60
40
20
0
80
60
40
20
0
100
80
100
80
60
40
20
0
60
40
20
0
0
10
20
30
40
50
0
10
20
30
40
50
time (h)
time (h)
Fig. 5. Respirometric profiles (circles) and evolution of TOC (squares) in the biological batch runs with the starting ILs solutions (solid symbols) and the effluents from Fenton
oxidation in the conditions of Fig. 3 (open symbols).
effluent requiring further disposal of the Fe(OH)3 sludge resulting
upon neutralization. However, the use of very low iron concen-
trations (10 mg L−1) leads to increased reaction times to achieve
acceptable efficiencies. Therefore, we selected 50 mg L−1 as the iron
dose for further experiments.
fairly different percentages of mineralization were achieved, vary-
ing from almost 65% for the imidazolium IL to less than 30% for the
phosphonium one.
3.2.1. Reaction pathways
Fig. 2 shows the evolution of [emim][Cl], H2O2 and TOC upon
reaction time, in the oxidation of the IL at different temperatures. It
is noteworthy that complete conversion of the IL was achieved at all
the temperatures tested in very short reaction times (5 min, 3 min
and 10 s were required at 50, 70 and 90 ◦C, respectively), in contrast
with the much higher reaction time (90 min) previously reported
in the literature at 25 ◦C [21]. Similarly, the oxidation rate increased
significantly with temperature but beyond 70 ◦C the residual TOC
became quite similar. The efficiency on the use of H2O2, namely
To the best of our knowledge, so far the only work dealing with
the identification of the by-products from Fenton oxidation of ILs
was carried out by Siedlecka et al. [22] who studied the oxidation
of [bmim][Cl]. These authors reported the formation of different
hydroxylated by-products upon the early stages of the process.
In the current work, the reaction samples from the experiments
performed at all the temperatures tested were analyzed by LC/MS.
From the results obtained, the reaction pathways depicted in Fig. 4
are proposed on the basis of the m/z of the detected compounds.
In all the cases, the first step of the oxidation process consisted
in the hydroxylation of the IL giving rise to different hydroxy-
lated by-products of higher molecular weight than the starting
ILs. Then, ring opening of the imidazolium and pyridinium ILs
and breakdown of the ammonium and phosphonium ones took
place leading to partially oxidized fragments. Further attack of
OH radicals to the aforementioned by-products resulted in the
formation of organic acids, primarily maleic and fumaric which
evolved into malonic, acetic, oxalic and formic. In fact, these are
the compounds typically found as final reaction products upon
Fenton oxidation of most organic species. These acids are par-
tially oxidized up to CO2 and H2O at slow rate, being oxalic and
acetic the most resistant [30–32]. It has to be highlighted that all
of the aforementioned by-products with the exception of short-
chain organic acids were completely degraded upon 4 h at 70 ◦C
and 90 ◦C. The organic acids represented 60% of the remaining TOC.
Their concentrations are collected in Table S1 of Supplementary
data.
the TOC reduction per unit of H2O2 consumption (XTOC /XH 2 , %)
O
was considerably improved from 39% at 50 ◦C to 57% at 702◦C. No
further improvement of that ratio was observed beyond 70 ◦C (55%
at 90 ◦C). Therefore, the temperature for further experiments was
set at 70 ◦C. Nevertheless, that complete conversion of the starting
IL did not imply more than about 60% mineralization, as can be seen
in Fig. 2.
3.2. Fenton oxidation of ILs from different families
Four ILs were tested corresponding to different families, whose
cations were imidazolium, pyridinium, ammonium and phospho-
nium with chloride as anion in all the cases. Complete conversion
of all the ILs was achieved in less than 10 min in the operating con-
ditions of Fig. 3. That figure depicts the results obtained after 4 h
reaction time, in terms of H2O2 conversion, COD and TOC reduc-
tion, where significant differences can be seen among the four ILs
tested. Although almost complete decomposition of H2O2 occurred
in all the cases except for the oxidation of [tbP][Cl] (XH
= 77%),
O
2
2
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3.2.2. Evolution of ecotoxicity
and moderately for the ammonium and phosphonium ones. Reac-
tion pathways have been proposed from the identification of the
by-products from Fenton oxidation.
None of the four ILs tested showed high ecotoxicity. In fact,
the measured EC50 values were fairly high: 4000, 294, 476 and
172 mg L−1 for the imidazolium, pyridinium, ammonium and phos-
phonium ILs, respectively. The differences on the ecotoxicity
values observed between the ring ILs, imidazolium and pyri-
dinium, are consistent with the results reported by Docherty
et al. [33], who established that the toxicities of imidazolium
and pyrinidium were not directly comparable, being [bmim][Br]
one order of magnitude less toxic than [bmpyr][Br] upon Vibrio
fischeri ecotoxicity test. On the other hand, the higher ecotox-
icity value displayed by the phosphonium IL is in agreement
with the results obtained by Ventura et al. [34] who estab-
lished that phosphonium-based ILs are more toxic than the analog
imidazolium-based ILs (with the same anion and equivalent alkyl
chains).
Acknowledgments
This research has been supported by the Spanish Ministry of Sci-
ence in Spain (MICINN) through the projects CTQ2008-03988 and
CTQ2010-14807 and by the Community of Madrid (CM) through
the project S-2009/AMB-1588.
Appendix A. Supplementary data
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.cattod.2014.
03.028.
Oxidation at 50 ◦C led to a moderate increase of the ecotoxicity
but at 70 and 90 ◦C, negligible values (<0.01 TU) were measured. The
higher toxicity observed when working at 50 ◦C seems to be directly
related with the resulting oxidation by-products. In fact, mineral-
References
[1] T. Welton, Chem. Rev. 99 (1999) 2071.
[2] R.D. Rogers, K.R. Seddon, ACS Symposium Series, American Chemical Society,
Washington, DC, 2003, p. 856.
[3] J. Pernak, K. Sobaszkiewicz, I. Mirska, Green Chem. 5 (2003) 52.
[4] R.J. Bernot, M.A. Brueseke, M.A. Evans-White, G.A. Lamberty, Environ. Toxicol.
Chem. 1 (2005) 87.
[5] S.P.M. Ventura, A.M.M. Gonc¸ alves, T. Sintra, J.L. Pereira, F. Gonc¸ alves, J.A.P.
Coutinho, Ecotoxicology 22 (2013) 1.
ization was really low (<20% in all cases) and low concentration
ꢀ
of non-toxic short-chain organic acids ( Corganic acids/TOC < 20%)
were obtained, being the hydroxylated compounds the main reac-
tion intermediates.
3.2.3. Biodegradability
[6] R.A. Sheldon, Green Chem. 7 (2005) 267.
[7] C.M. Gordon, Appl. Catal., A: Gen. 222 (2001) 101.
[8] N.V. Plechkova, K.R. Seddon, Chem. Soc. Rev. 37 (2008) 123.
[9] J. Ranke, K. Molter, F. Stock, U. Bottin-Weber, J. Poczobutt, J. Hoffmann, B.
Ondruschka, J. Filser, B. Jastorff, Ecotoxicol. Environ. Saf. 58 (2004) 396.
[10] A. Latala, P. Stepnowski, M. Nedzi, W. Mrozik, Aquat. Toxicol. 73 (2005) 91.
[11] A. Romero, A. Santos, J. Tojo, A. Rodríguez, J. Hazard. Mater. 151 (2008) 268.
[12] J. Palomar, J. Lemus, M.A. Gilarranz, J.J. Rodriguez, Carbon 47 (2009) 1846.
[13] J. Lemus, J. Palomar, F. Heras, M.A. Gilarranz, J.J. Rodriguez, Sep. Purif. Technol.
97 (2012) 11.
[14] N. Gathergood, M.T. Garcia, P.J. Scammells, Green Chem. 3 (2004) 166.
[15] N. Gathergood, P.J. Scammells, M.T. Garcia, Green Chem. 8 (2006) 156.
[16] A.S. Wells, V.T. Coombe, Org. Process Res. Dev. 10 (2006) 794.
[17] K.M. Docherty, J.K. Dixon, C.F. Kulpa, Biodegradation 18 (2007) 481.
[18] S. Esplugas, J. Giménez, S. Contreras, E. Pascual, M. Rodríguez, Water Res. 36
(2002) 1034.
[19] P. Bautista, A.F. Mohedano, J.A. Casas, J.A. Zazo, J.J. Rodriguez, J. Chem. Technol.
Biotechnol. 83 (2008) 1323.
[20] G. Pliego, J.A. Zazo, S. Blasco, J.A. Casas, J.J. Rodriguez, Ind. Eng. Chem. Res. 51
(2012) 2888.
[21] E.M. Siedlecka, W. Mrozik, Z. Kaczynski, P. Stepnowski, J. Hazard. Mater. 154
(2008) 893.
[22] E.M. Siedlecka, M. Goêbiowski, J. Kumiska, P. Stepnowski, 53, Chem. Anal. (War-
saw) (2008) 943.
Fig. 5 collects the respirometric profiles and the TOC evolu-
tion in the biological tests with the starting IL solutions and the
corresponding effluents from Fenton oxidation. None of the four
ILs tested were degraded upon 48 h biodegradation runs (<5%
TOC reduction in all cases). On the opposite, the Fenton effluents
(4 h) showed highly susceptible to biodegradation in the cases of
the imidazolium and pyridinium ILs and moderately in the two
other cases. Docherty et al. [17] studied the biodegradability of six
imidazolium and pyridinium ILs by an activated sludge microbial
community, concluding that imidazolium and pyridinium rings are
not biodegradable and cannot be mineralized. They observed par-
tial degradation of the side chain but the rings remained intact and
were not used as carbon source by the microbial community. Thus,
the ring opening accomplished upon Fenton oxidation must be
determining for the observed high biodegradability of the resulting
effluents.
The higher resistance to biodegradation of the effluents from
the oxidation of aliphatic ILs must be associated to the lower COD
and TOC removal achieved as well as to the nature of the reaction
by-products.
[23] E.M. Siedlecka, M. Goêbiowski, Z. Kaczynski, J. Czupryniak, T. Ossowski, P. Ste-
penowski, Appl. Catal., B: Environ. 91 (2009) 573.
[24] J.A. Zazo, G. Pliego, S. Blasco, J.A. Casas, J.J. Rodriguez, Ind. Eng. Chem. Res. 50
(2011) 866.
[25] G.M. Eisenberg, Ind. Eng. Chem. Res. 15 (1943) 327.
[26] APHA, Standard Methods for the Examination of Water and Wastewater, 22nd
edition, American Public Association, Washington, DC, 2012.
[27] M. Munoz, Z.M. de Pedro, J.A. Casas, J.J. Rodriguez, Chem. Eng. J. 228 (2013) 646.
[28] N. Kang, D.S. Lee, J. Yoon, Chemosphere 47 (2002) 915.
[29] J.J. Pignatello, E. Oliveros, A. Mackay, Crit. Rev. Environ. Sci. Technol. 36 (2006)
1.
[30] E. Chamarro, A. Marco, S. Esplugas, Water Res. 35 (2001) 1047.
[31] J. Carriazo, E. Guélou, J. Barrault, J.M. Tatibouët, R. Molina, S. Moreno, Water
Res. 39 (2005) 3891.
[32] J.A. Zazo, J.A. Casas, A.F. Mohedano, M.A. Gilarranz, J.J. Rodriguez, Environ. Sci.
Technol. 39 (2005) 9295.
[33] K.M. Docherty, C.F. Kulpa, Green Chem. (2005) 185.
[34] S.P.M. Ventura, C.S. Marques, A.A. Rosatella, C.A.M. Afonso, F. Gonc¸ alves, J.A.P.
Coutinho, Ecotoxicol. Environ. Saf. 76 (2012) 162.
4. Conclusions
Fenton oxidation at 70 ◦C has proved to be efficient for the break-
down of ILs of four different families, viz. imidazolium, pyridinium,
ammonium and phosphonium. At that temperature, using the stoi-
chiometric amount of H2O2 and a Fe3+ dose of 50 mg L−1 allows
complete conversion of the starting ILs in fairly short reaction time
(>10 min) with more than 60% TOC reduction after 4 h, except for
the phosphonium IL. The resulting effluents showed negligible eco-
toxicity values. The scarce biodegradability of the ILs (>5%) was
highly improved in the cases of the imidazolium and pyridinium ILs
Please cite this article in press as: M. Munoz, et al., Ionic liquids breakdown by Fenton oxidation, Catal. Today (2014),
http://dx.doi.org/10.1016/j.cattod.2014.03.028