Advanced oxidation processes for the removal of [bmim][Sal] third generation ionic liquids: Effect of water matrices and intermediates identification

Article abstract of DOI:10.1039/c6ra04416f

Unique properties of ionic liquids make them green alternatives for conventional volatile organic compounds. Due to increased production and the high stability of these substances, they could be classified as persistent pollutants and could break through classical treatment systems into natural waters. A preliminary ionic liquid hydrolysis study demonstrated a pH dependent degradation profile with a significant decrease in hydrolysis efficiency as pH lowered from 10.0 to 2.8. In order to examine future prospects for ionic liquid removal, different advanced oxidation processes (TiO₂ Degussa P25/H₂O₂, TiO₂ Degussa P25, 7.2Fe/TiO₂/H₂O₂, and H₂O₂) were studied for their applicability in the degradation of imidazolium-based ionic liquids in aqueous solution. These processes were conducted in the dark as well as in the presence of UVA and simulated sunlight (SS) radiation. Among the investigated dark processes, the 7.2Fe/TiO₂/H₂O₂ system showed the highest efficiency, which can be attributed to a dark heterogeneous Fenton process. Otherwise, the most efficient among all the studied degradation processes was the UVA/TiO₂ Degussa P25/H₂O₂ process. In order to make degradation processes more similar to that of the practical process SS radiation was used. Among studied processes, the 7.2Fe/TiO₂/H₂O₂ system showed the greatest potential for the removal of ionic liquids. Also, it was observed that the impact of anions on the cation degradation efficiency was much more pronounced. Due to the possible fate of ionic liquids in the environment, for five different waters (pond, rain, tap, river, and condensate) degradations in the dark and under simulated sunlight were studied. For all processes, and all water types in the presence of SS radiation a remarkable positive effect of naturally dissolved organic matter on the degradation efficiency was observed. Also, in all experiments, the anion was less stable than the cation. The major photodegradation products identified using liquid chromatography-mass spectrometry (HPLC-MS/MS) techniques were hydroxylated compounds.

Full text of DOI:10.1039/c6ra04416f

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PAPER
Аdvanced oxidation processes for the removal of [bmim][Sal]
third generation ionic liquid: Effect of water matrices and
intermediates identification
Received 00th January 20xx,
Accepted 00th January 20xx
Nemanja Banić, Biljana Abramović*, Filip Šibul, Dejan Orčić, Malcolm Watson, Milan Vraneš and
Slobodan Gadžurić
DOI: 10.1039/x0xx00000x
www.rsc.org/
Unique properties of ionic liquids make them today green alternatives for conventional volatile organic compounds. Due
to increased production and high stability of these substances, they could be classified as persistent pollutants and could
break through classical treatment systems into natural waters. A preliminary ionic liquid hydrolysis study demonstrated a
pH dependent degradation profile with a significant decrease in hydrolysis efficiency as pH lowered from 10.0 to 2.8. In
order to examine future prospects for ionic liquid removal, different advanced oxidation processes (TiO2 Degussa
P25/H2O2, TiO2 Degussa P25, 7.2Fe/TiO2/H2O2, and H2O2) were studied for their applicability in the degradation of
imidazolium-based ionic liquid in aqueous solution. These processes were conducted in the dark as well as in the presence
of UVA and simulated sunlight (SS) radiation. Among the investigated dark processes, 7.2Fe/TiO2/H2O2 system showed the
highest efficiency, which can be attributed to a dark heterogeneous Fenton process. Otherwise, the most efficient among
all studied degradation processes was UVA/TiO2 Degussa P25/H2O2 process. In order to make degradation processes more
similar to that of the practical process SS radiation was used. Among studied processes, 7.2Fe/TiO2/H2O2 system showed
the greatest potential for the removal of ionic liquid. Also, it was observed that the impact of anion on the cation
degradation efficiency was much more pronounced. Due to the possible fate of ionic liquids in the environment, for five
different waters (pond, rain, tap, river, and condensate) degradations in the dark and under simulated sunlight were
studied. For all processes, and all water types in the presence of SS radiation a remarkable positive effect of naturally
dissolved organic matter on the degradation efficiency was observed. Also, in all experiments, anion was less stable than
cation. The major photodegradation products identified using liquid chromatography–mass spectrometry (HPLC–MS/MS)
techniques were hydroxylated compounds.
batteries,5 gas separation, compression and handling,6,7 metal
plating,5,8,9 dyesensitized solar cells, and as paint or polymer
additives.10,11
1. Introduction
Room-temperature ionic liquids are receiving considerable
attention as green, high-tech reaction media of the future.1
This can be explained by the fact that ionic liquids possess
negligible vapor pressure, so that they can be a good
alternative to the emissions of toxic vapors from conventional
molecular organic solvents.2 In addition to these features, ionic
liquids have other good properties, such as good thermal
stability, wide electrochemical potential window, high electric
conductivity and miscibility with water or organic solvents
which make them interesting for application in different
fields.3,4 Up to now the ionic liquids were used in lithium ion
Furthermore, the third generation of ionic liquids includes
salts with
melting point below 100 oC that contain
biologically active cations and/or anions in the structure.12,13
a
A
special attention of the scientific community in the recent
years has been paid to ionic liquids with salicylate anion, since
they show possibility of a wide range of applications in the
field of pharmaceutical industry,14 the synthesis of
nanoparticles,15 heat storage,16 electrodeposition of transition
metals17 etc. Complexing (chelating) ability of salicylate anions
was used for the synthesis of the hydrophobic ionic liquids for
selective metal (metalloid) extraction from the industrial and
the communal wastewaters.18-21
On the other hand a broad implementation of ionic liquids
leads us to the following question: what will be the outcome
when these substances get through the environment? It can
be assumed that their low vapor pressure provides air
protection from the emission of toxic vapors, but on the other
hand their stability makes them hazardous to water and soil.22
Due to its high stability in the near future it can be expected
University of Novi Sad, Faculty of Sciences, Department of Chemistry,
Biochemistry and Environmental Protection, Trg D. Obradovića 3, 21000 Novi Sad
(Serbia)
Tel: (+381) 21 4852753
Fax: (+381) 21 454065
E-mail: biljana.abramovic@dh.uns.ac.rs
†Electronic Supplementary Information (ESI) available: Proposed structures of
intermediates, ICP-MS scan signal intensities, and photodegradation of different
ionic liquids. See DOI: 10.1039/x0xx00000x
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the presence of significant amounts of ionic liquids in
technological wastewaters and their break through classical
treatment systems into natural waters.1 Therefore,
information concerning their fate in the aquatic environment
is very important.
2. Experimental section
2.1 Chemicals and solutions
All chemicals were of reagent grade and were used without
purification. Studied 1-butyl-3-methylimidazolium salicylate
([bmim][Sal]) ionic liquid was synthesized according to the
procedure mentioned in the literature.17 The Ionic liquid 1-
benzyl-3-methylimidazolium chloride ([Bzmim][Cl]) was
manufactured by Iolitec-Ionic Liquids Technologies GmbH,
Heilbronn (Germany) and 1-butyl-3-methylimidazolium
chloride ([bmim]Cl) by Merck. Other ionic liquids selected for
these studies were obtained from the ionic liquids collection of
the working group of Dr. Milan Vraneš. They were
Also, some studies showed that ionic liquids are more toxic
to the cells in comparison to conventional solvents.1 These
studies are in good agreement with other studies in which the
most common imidazolium based compounds showed high
resistance toward microbial degradation.22
It can be also assumed that among the various
transformation processes (biotic and abiotic), photostability is
an important factor influencing the fate of ionic liquids in the
environment. Thus, it is very important to find an efficient
method for a complete and safe removal of ionic liquids from
the environment.23-25 The advanced oxidation processes
(AOPs), for example, UVA/H2O2, UVA/Fe2+/H2O2, and UVA/TiO2
are potentially useful for wastewater treating because they
generate hydroxyl radicals,26-28 a powerful nonspecific oxidant,
and can provide an almost total degradation.29 Among
numerous AOPs, Fenton’s process is one of the promising
technologies because of the high oxidation potential and
relatively low cost.30 This process can be enhanced by visible
light due to the decomposition of the photo-active Fe(OH)2+
species producing additional OH-radicals in solution.31 In these
processes participation of iron oxide minerals (hematite α-
Fe2O3, goethite α-FeOOH, magnetite Fe3O4 and ferrihydrite) as
the catalysts in the presence of hydrogen peroxide was also
effective in the past, because uniform rates of hydroxyl radical
generation at high concentration of hydrogen peroxide used.
Moreover α-FeOOH/H2O2 system has a number of advantages
as compared to a homogenous Fenton’s system with respect
to reaction pH range and the removal of iron.32
tetrabutylammonium
salicylate
([TBA][Sal]),
1-(3-
hydroxypropyl)-3-methylimidazolium salicylate ([Hpmim][Sal])
and imidazolium salicylate ([im][Sal]). Sodium salicylate
([Sal]Na) was produced by Merck. 30% H2O2, was obtained
from Centrohem (Stara Pazova, Serbia); 99.8% acetonitrile (J.T.
Baker); 85% H3PO4 (Lachema, Neratovice, Czech Republic);
H2SO4, formic acid, ammonium acetate (Merck); and NaOH
(ZorkaPharm, Šabac, Serbia). All solutions were prepared using
doubly distilled water. The concentration of ([bmim][Sal])
stock solution used in photodegradation experiments was 0.38
1
mmol L− . Photocatalyst with 7.24% of Fe (w/w, denoted as
7.2Fe/TiO2) synthesized by a simple deposition–precipitation
method,27 TiO2 support obtained from Molar Chemicals KFT
(Hungary) and TiO2 P25 manufactured by Evonik Corporation
(USA) (75% anatase and 25% rutile form, surface area of 50 m2
g−1 according to the manufacturer’s specification, with average
particle size about 20 nm,37 were used as photocatalysts.
2.2 Degradation experiments
All experiments were carried out in a batch reactor made of
Pyrex glass (total volume of cca 100 mL, solution depth 46
mm). The UVA radiation was provided from a 125 W high-
In this study efficiency of ionic liquid degradation was
investigated and compared by different AOPs, including direct
and H2O2 indirect photolysis, as well as heterogeneous
photocatalysis using TiO2 and 7.2Fe/TiO2 catalysts in the
presence of UVA/simulated sunlight (SS) radiation. In contrast
to previously studied ionic liquids that contain an inorganic
anion ie. chloride1,22,33-35 or bromide36, in this paper an ionic
liquid in which both components were organic ions was
investigated. Also, mutual influence of anion and cation was
studied in detail. For broaden the application of used AOPs,
the photocatalytic degradation of different kind of ionic liquids
were also studied. Because there is no information in the
available literature on the photodegradation of ionic liquid
under natural aquatic conditions, the present study deals with
the influence of water matrixes on the ionic liquid
photodegradation efficiency. In this study five different types
pressure mercury lamp (λ
> 290 nm, Philips HPL-N,
Netherlands, emission bands in UV region at 304, 314, 335,
and 366 nm, with the maximum emission at 366 nm). Intensity
of UVA radiation was 3.57 × 10− W cm−2. Irradiation with SS
3
light was performed using a set of four 35 W halogen lamps (λ
> 300 nm, VITO, MR 16, China), placed in a symmetrical way
around the reactor. UVA and Vis radiation intensities for
applied SS light were 1.8 × 10− W cm− and 116.2 × 10−
cm−2, respectively.
4
2
3
W
1
A solution of 30 mL [bmim][Sal] containing 1.67 g L− of
catalyst and/or 0.45 mmol L−1 H2O2 with successive addition of
H2O2 (70 μL each hour during the degradation) was used
(except for the study of hydrolysis and direct photolysis). All
experiments were performed at pH 2.8 (except in the case of
hydrolysis study). A value of 2.8 was chosen because it was
optimal value for 7.2Fe/TiO2/H2O2 system.27 The H2SO4 and
NaOH solutions were used to adjust the pH of reaction
solutions. The lamps were turned on for 15 min prior to
degradation to obtain a constant light intensity output. At the
same time reaction solution was stirred continuously in the
dark in order to ensure equilibrium adsorption of [bmim][Sal]
on the photocatalyst.
of waters was used HPLC−MS/MS technique was used to track
the 23 intermediate products formed during the course of
ionic liquid degradation.
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To ensure a constant O2 the reaction solution was
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ARTICLE
Table 1 Characteristics of selected waters.
continuously purged to the bottom of the reactor with
molecular oxygen (99.99% purity) at a constant flow rate of 5
mL min−1. Furthermore the solution was homogenized with the
aid of a stirring bar, to ensure completely mixed batch
conditions. The temperature of the reaction solution was kept
Origin of water
samples
pH
Conductivity
[μS cm− ]
1.7
849.0
90.4
DOC[a]
[mg L− ]
not detected
2.22
1
1
Distilled
6.5
7.9
6.4
6.0
6.0
5.0
Tap
Condensate
Pond
17.16
3.59
o
at 25 ± 0.5 C throughout the experiment by water circulation
151.1
746.0
19.6
system. Dark experiments were conducted under identical
conditions but without radiation.
River
7.91
Rain
0.48
The matrix effects of five water types on the efficiency of
ionic liquid degradation were evaluated. For this purpose we
used the following samples: river water, rain water, pond
water, tap water, and condensate from air conditioner. The
river water sample was collected from the Danube (Novi Sad,
Serbia) in July 2014. Of the same year in the urban area of Novi
Sad, rain water and pond water samples were collected during
the rainy periods between the ninth and fourteenth of July. In
the same month tap water sample was collected from public
drinking water supply system of Bački Jarak (Serbia) while the
condensate water from the air conditioner harvested in one
flat that is also located at the territory of Novi Sad. To avoid
microbial growth, water samples were sterilized by filtration
and all glassware was sterilized by autoclaving for 30 min at
140 oC.
[a] Dissolved organic carbon (limit of detection 0.05 mg L−1
)
The conductivity measurements were performed using a
conductivity meter type Cond 3210 (WTW, Weilheim,
Germany) equipped with a standard conductivity measuring
cell TetraCon 325.
The pH measurements were made using
a Hanna
Instruments combined glass electrode (Kehl, Gemany), on a
previously calibrated pH-meter (Iskra, Kranj, SFRJ).
In addition, in order to gain insight into the relationship
between the elements present in the different water matrixes
and the efficiency of ionic liquid degradation, the signal
intensities obtained by scanning the waters by inductively
coupled plasma mass spectrometry (ICP
Table S1, ESI†. The ICP MS analysis was carried out on an
Agilent 7700e ICP MS (Hachioji, Japan), with a collision cell
−MS) are given in
2.3 Analytical methods
−
For the kinetic studies of ionic liquid removal, 0.5 mL of the
reaction mixture were sampled at the beginning of the
experiment and at regular time intervals, followed by filtration
through Millipore (Millex-GV, 0.22 μm) membrane filter (in the
presence of catalyst). The lack of ionic liquid adsorption onto
the filters was confirmed by a preliminary test. The HPLC–DAD
analyses were performed on a Shimadzu 20A series ultra-fast
liquid chromatograph (UFLC, Shimadzu Cooperation, Kyoto,
Japan), equipped with UV–vis diode array detection and polar
−
operated in helium mode. General purpose plasma conditions
were applied, with a power of 1550 W and a sampling depth of
10 mm. A full mass scan was carried out, from Li 7 to Pb 208,
and the resulting spectra analysed using the Agilent
MassHunter software.
For the HPLC−ESI−MS/MS identification of intermediates,
1
0.38 mmol L− of ionic liquid solution was prepared, and
UVA/TiO2 P25/H2O2 system was used. Filtration was carried
out to separate the photocatalyst particles as previously
described. Then, a 20 µL of sample was injected and analyzed
by an Agilent Technologies 1200 series HPLC with Agilent
Technologies 6410A series electrospray ionization triple-
quadrupole MS/MS, using the afore mentioned polar CN
column held at 25oC. The mobile phase, consisting of 0.1%
CN column (Inertsil CN-3 column; 150 mm
× 4.6 mm i.d.,
particle size 3 μm). The injection volume was 20 μL. Column
temperature was held at 25 oC, mobile phase was a mixture of
0.1% H3PO4−ACN (6:4, v/v), pH 2.56, and the total flow rate
1
was 0.8 mL min− . Ionic liquid elution was monitored at 210
nm. The retention time of ionic liquid’s cation was 1.44 min
while the same for the anion was 6.58 min.
aqueous formic acid and ACN, was delivered at flow rate 0.8
1
mL min−
.
For
collected
water
samples
physicochemical
characteristics are given in Table 1.
In negative mode, isocratic elution (20% ACN) was used. In
positive mode, 100 mmol L−1 ammonium acetate was added to
aqueous phase to enhance retention of charged quaternary
nitrogen compounds (that otherwise elute at dead time), and
gradient mode (0 min 20% ACN, 4 min 20% ACN, 6 min 100%,
post time 3 min) was employed to facilitate separation.
Analytes were ionized using the electrospray ion source, with
Dissolved organic carbon (DOC) of different water samples
(filtered through 0.22 micrometer filter) was determined on an
Elementar Liqui TOC II (Germany) in accordance with Standard
US EPA Method 9060A. Before measurements samples were
acidified.
The radiation fluxes were measured using a Delta Ohm HD
2102.2 (Padova, Italy) radiometer which was fitted with the LP
471 UVA (spectral range 315–400 nm) and LP 471 RAD
(spectral range 400–1050 nm) sensors.
o
nitrogen as the drying gas (temperature 350 C, flow 10 dm3
min−1) and nebulizer gas (50 psi), and a capillary voltage of 4.0
kV. High purity nitrogen was used as the collision gas. Full scan
mode (m/z range 50–500, scan time 100 ms, fragmentor
voltage 100 V), using both polarities, was used to detect
degradation intermediates, determine their molecular weight
and select precursor ions for MS2 experiments. To obtain
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structural information and identify the detected compounds,
Every degradation study was performed with both ions of
product ion scan analysis (MS2 experiment) was used, with ionic liquid (except where otherwise stated), but for the clarity
[M+H]+ and [M–H]– ions as precursors, and collision energies in reasons the results for [bmim]+ and [Sal]− were in each figure
10–40 V interval (10 V increments).
separately shown.
3.2 Preliminary dark experiments
3. Results and discussion
Since the suspension was stirred continuously in the dark for
15 min prior to the degradation, it was examined whether this
contact time was sufficient for [bmim][Sal] to reach
adsorption–desorption equilibrium on the photocatalyst. Even
after 180 min, the removal of [bmim][Sal] by dark adsorption
at natural pH and pH 2.8 was almost negligible (Fig. 2).
Furthermore, obtained results for the experiments
conducted in dark showed that the ionic liquid was stable in
the presence of H2O2 alone (Fig. 3, curve 1), as well as in a
combination of the TiO2 photocatalysts (TiO2 P25 or TiO2
support) and H2O2 (Fig. 3, curves 3 and 4). Also, using
7.2Fe/TiO2 did not cause the ionic liquid degradation (Fig. 3,
curve 5).
3.1 Effect of pH on ionic liquid hydrolysis
The degradation of ionic liquid in aqueous solution in the
absence of light at ambient temperature of 25 oC was
monitored at different pH values. As can be seen from Fig. 1a
after 93 days in pH range between pH 2.8 and 10.0, [bmim]+
(i.e. cation of the ionic liquid) was stable to hydrolysis. Unlike
the [bmim]+, [Sal]− (i.e. anion of the ionic liquid) was less stable
as the pH increased (Fig. 1b). Namely, at pH 7.0 and 10.0 after
51 days 41.5, and 100% of the [Sal]− was hydrolyzed,
respectively. Also, at pH 2.8 no noticeable hydrolysis of [Sal]−
occurred. Finally, it can be concluded that the ionic liquid was
stable at pH 2.8 toward hydrolysis. Therefore, all subsequent
experiments were carried out at pH 2.8.
However, in the presence of H2O2 and 7.2Fe/TiO2 after 180
minutes 27% of [bmim]+ (Fig. 3a, curve 2), and 92% of [Sal]−
(Fig. 3b, curve 2) has been degraded. This behavior can be
attributed to the Fenton process.
According to our previous examinations and since all of the
experiments were conducted at pH 2.8, for 7.2Fe/TiO2/H2O2
system is expected that the concentration of iron in solution
will not exceed 8 ppb. In compliance with this, the degradation
efficiency was studied in the presence of 8 ppb dissolved
iron(III) and appropriate amounts of hydrogen peroxide. After
180 min it has been found that the ionic liquid degradation
efficiency at this level of dissolved iron concentration
(homogeneous Fenton processes) was negligible.
The same conclusion was derived from the experiment
with 60 ppb of Fe(III). This experimental evidence implies that
the overall degradation efficiency originates from the
heterogeneous Fenton process (reactions 1
−
5).38
≡FeIII−OH + H2O2
H2O2
(1)
(2)
(3)
ꢀ
ꢁs
H2O2
≡FeII + H2O + HO•2
ꢀ
ꢁs
≡FeII + H2O2
HO•2
≡FeIII−OH + HO•
H+ + O•2ꢀ
(4)
(5)
≡FeIII−OH + HO2• /O•2ꢀ
≡FeII + H2O/OHꢀ + O2
Finally, it can be concluded that within this process
existence of surface bonded iron oxide was crucial. This
statement is based on our previous experiments27 which were
carried out with PrecFe (iron particles contained in 7.2Fe/TiO2
photocatalyst) in the presence of H2O2, and it was found that
the degradation efficiency was practically the same to that in
its absence. This was explained with the fact that the solution
was not practically contain Fe3+ ions. However, the presence of
H2O2 was increased the catalytic efficiency of 7.2Fe/TiO2 to a
higher extent than of the TiO2 support and PrecFe, which
suggests that H2O2 plays a different role, i.e., in the case of
1
Fig. 1 Effect of pH on ionic liquid (c0 = 0.38 mmol L− ) hydrolysis: a) [bmim]+; b) [Sal]−.
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7.2Fe/TiO2/H2O2, in addition to the electron-acceptor role,
After 180 minutes of radiation, no significant change in the
H2O2 is also involved in the heterogeneous photo-Fenton [bmim]+ concentration could be observed (Fig. 4a, curve 1).
process.
Following the same radiation period 7% [Sal]− was degraded
(Fig. 4b, curve 1).
Fig. 3 Evolution of ionic liquid (c0 = 0.38 mmol L−1) dark conversion along the time: a)
[bmim]+ and b) [Sal]−. Reaction conditions: (1) 45 mmol L−1 H2O2 + 70 μL h−1 H2O2 at pH
Fig. 2 Evolution of ionic liquid (c0 = 0.38 mmol L−1) adsorption along the time: a)
1
[bmim]+ and b) [Sal]−. Applied conditions: (1) 7.2Fe/TiO2 (1.67 g L− ) at natural pH; (2)
1
1
2.8; (2) 7.2Fe/TiO2 (1.67 g L− ) + 45 mmol L− H2O2 + 70 μL h-1 H2O2 at pH 2.8; (3) TiO2
1
7.2Fe/TiO2 (1.67 g L− ) at pH 2.8.
1
1
support (1.67 g L− ) + 45 mmol L− H2O2 + 70 μL h-1 H2O2 at pH 2.8; (4) TiO2 P25 (1.67 g
1
L− ) + 45 mmol L−1 H2O2 + 70 μL h-1 H2O2 at pH 2.8; (5) 7.2Fe/TiO2 (1.67 g L−1) at pH 2.8.
3.3 UVA photodegradation studies
The [bmim]+ strongly absorbs UV radiation over a range of
wavelengths, from 200 to 235 nm, with the maximum
absorption band at 210 nm. On the other hand, the [Sal]−
showed UV absorption at a broader range between 200 and
335 nm, with three absorption maxima at 205, 236, and the
lowest at 303 nm. Since the UV radiation of used lamp below
of closely 275 nm cannot pass through Pyrex glass,25 no direct
photolysis of [bmim]+ could be expected. Also, observing the
radiation of lamp which passed through the wall of the reactor
and the absorption spectrum of absorbed radiation by [Sal]− in
the range of 275 to 335 nm, a direct photolysis of the [Sal]−
could be possible.
Besides direct photolysis in Fig. 4 were presented the
efficiency of other AOPs that can be potentially used for the
ionic liquid degradation. This includes the results obtained
using indirect photolysis by H2O2 (Fig. 4, curve 2), and
photocatalysis with TiO2 P25 (with and without H2O2, Fig. 4,
curves 3 and 5), as well as with 7.2Fe/TiO2/H2O2 system (Fig. 4,
curve 4). In experiments in which H2O2 was employed, because
of its depletion, H2O2 was added successively (70 μL each hour
during the radiation). By comparing the results presented in
Figs. 3 and 4 it can be noticed that the presence of UVA
radiation for all investigated processes significantly increased
efficiency of ionic liquid degradation. The contribution of UVA
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radiation on the studied [bmim]+ degradation decreases in the Also increased efficiency of indirect photolysis can be
following order: UVA/TiO2 P25/H2O2
>
UVA/TiO2 P25
>
explained by the fact that radiation with wavelengths less than
UVA/7.2Fe/TiO2/H2O2 > UVA/H2O2 > UVA. For [Sal]− the above- 300 nm can cause photolysis of hydrogen peroxide.39
mentioned contribution sequence would be as follows:
3.4 Simulated sunlight photodegradation studies
UVA/TiO2 P25/H2O2
> UVA/TiO2 P25 > UVA/H2O2 >
UVA/7.2Fe/TiO2/H2O2 > UVA.
Further tests were conducted to examine the performance of
different AOPs through degradation of ionic liquid under the
SS radiation for the first time. Using this kind of the light
source, the degradation process was more similar to that of
the practical process which proceeds under sunlight.
Comparing obtained results under SS radiation (Fig. 5) with
those obtained for the same processes which were carried out
in the dark (Fig. 3), in all cases increasing efficiency can be
observed.
Fig. 4 Evolution of ionic liquid (c0 = 0.38 mmol L−1) degradation along the time using
UVA radiation: a) [bmim]+ and b) [Sal]−. Applied AOPs: (1) direct photolysis at pH 2.8;
2
1
1
1
(2) 4.5 × 10− mol L− H2O2 + 70 μL h− H2O2 at pH 2.8; (3) TiO2 P25 (1.67 g L− ); (4)
7.2Fe/TiO2 (1.67 g L− ) + 4.5 × 10− mol L− H2O2+ 70 μL h−1 H2O2 at pH 2.8; (5) TiO2 P25
1
2
1
1
2
(1.67 g L− ) + 4.5 × 10− mol L−1 H2O2 + 70 μL h−1 H2O2.
The most efficient degradation process included H2O2
coupled with catalytic degradation and the complete removal
of [bmim]+ and [Sal]− required only 120, and 45 min,
respectively. Somewhat lowered, but still high efficiency of
TiO2 P25 photocatalysis in the absence of H2O2 resulting from
the absorption spectrum of TiO2 P25.39 After 180 min of
radiation, increased degradation efficiency for heterogeneous
7.2Fe/TiO2/H2O2 Fenton system which comes from UVA
radiation (heterogeneous photo-Fenton) was 58.1% in the case
of [bmim]+, and only 7.3% in the case of the [Sal]−.
1
Fig. 5 Evolution of ionic liquid (c0 = 0.38 mmol L− ) degradation along the time using SS
radiation: a) [bmim]+ and b) [Sal]−. Applied AOPs: (1) 45 mmol L−1 H2O2 + 70 μL h−1 H2O2
at pH 2.8; (2) TiO2 P25 (1.67 g L−1); (3) 7.2Fe/TiO2 (1.67 g L− ) + 45 mmol L−1 H2O2 + 70 μL
1
h−1 H2O2 at pH 2.8.
For the studied SS/H2O2, SS/TiO2 P25 and
SS/7.2Fe/TiO2/H2O2 processes, these increases of efficiency
6 | RSC Adv., 2016, 00, 1-3
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were different for cation and anion. Namely, for the [bmim]+ likely that these substances will get into the different types of
(Fig. 5a) were 17%, 9%, and 4.5%, respectively, while the same water that can be often found in the environment.
in the case of [Sal]− (Fig. 5b) were 74%, 52%, and 5.3%,
The
studied
degradation
processes
included
respectively. In this case, the spectral energy distribution of heterogeneous Fenton and photo-Fenton processes. Fig. 7
used SS source was more suited for conducting indirect depicts the degradation curves of the ionic liquid in different
photolysis in relation to the photocatalysis. On the other hand, waters by applying dark heterogeneous Fenton process. In the
for the SS/7.2Fe/TiO2/H2O2 process it was noticed that the case of [bmim]+ (Fig. 7a), experimental data obtained after 180
contribution of the heterogeneous photo-Fenton oxidation min indicated that efficiency of degradation decreases in the
was negligible compared to the dark heterogeneous Fenton following order: pond < rain < tap < river < condensate water,
oxidation.
showing a strong dependence on the constitution of the
Also, if compared Fig. 5 and Fig. 4, for all of the studied radiated media.
processes, ionic liquid photodegradation efficiency was
significantly lower using SS radiation, compared to that in the
presence of UVA radiation.
Further research has been focused on studying mutual
facilitation of [bmim]+ and [Sal]− on the photodegradation
efficiency in the presence of the SS radiation. For this purpose,
the SS/TiO2 P25 process was chosen. By applying this process
the degradation efficiency in the presence of only [bmim]+ or
only [Sal]− has been studied (Fig. 6) and compared with those
obtained when they are in a mixture (referring to the ionic
liquid, Fig. 5). When the degradation efficiency of individual
ions was studied, [bmim]+ was present in the form of
chlorides, while the [Sal]− was present in the form of the
sodium salt. Comparing results presented in Figs. 5 and 6 it can
be seen that after 180 min of only [Sal]− degradation,
efficiency decreased by 5.4%, while in the case of [bmim]+,
efficiency was increased for 36%. Based on previous
conclusions it can be established that the impact of [Sal]− on
the [bmim]+ degradation efficiency was much more
pronounced. It was also noted that in all experiments when
both ions are present in the mixture, the [Sal]− showed
significantly higher degradation tendency in comparison with
[bmim]+.
Fig. 7 Evolution of ionic liquid (c0 = 0.38 mmol L−1) dark conversion along the time: a)
[bmim]+ and b) [Sal]−. Water matrix: (1) tap water; (2) condensate water; (3) pond
1
water; (4) river water; (5) rain water. Other reaction conditions: 7.2Fe/TiO2 (1.67 g L−
+ 45 mmol L−1 H2O2 + 70 μL h−1 H2O2 at pH 2.8.
)
1
Fig. 6 Photodegradation of [bmim]Cl and [Sal]Na (c0 = 0.38 mmol L− ) using SS radiation
1
and TiO2 P25 (1.67 g L− ) along the time.
During the [bmim]+ degradation in acidic media (pH 2.8)
the surface of 7.2Fe/TiO2 particles was positively charged, so
that the extent of adsorption of metal ions was relatively
insignificant. From the literature it is known that Na+, K+, Ca2+,
and Mg2+ ions are the common cations in aquatic
3.5 Degradation in different water types
It was very important to study the stability properties of ionic
liquids toward AOPs in different types of water, since it is very
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environment. They are all in the highest and stable oxidation end it can also be concluded that the amount range and
state and cannot capture electrons or holes in solution.40 In composition of DOC (Table 1) in different types of water was
the case of dark Fenton process it can be assumed that the such that the ionic liquid degradation efficiency in presence SS
[bmim]+ degradation efficiency mostly depended on the P, S, were nearly the same.
and Cl content (Table S1, ESI†). The highest degradation
efficiency was found in pond water and it is probably due to
the lowest total content of P, S, and Cl. By comparison
composite of pond water with rainwater in which the
efficiency of the [bmim]+ degradation is some lower, it can be
seen that in rainwater was 4 times higher content of P, 1.5
times higher S content, but 1.3 times lower Cl content. In the
case of tap water also in comparison with pond water, the S
content content was 8 times lower, but the contents of P and
Cl were eight and two times higher, respectively.
Furthermore, the Danube water had the highest content of
P, S and Cl, so its degradation efficiency was the lowest. It can
be assumed that the impact of content on the efficiency of
degradation process decreases in the following order: P
> Cl >
S. Bali et al. came to a similar conclusion of negligible effect of
−
−
−
SO24 and Cl , in comparison with the effect of PO34 which was
remarkable.41 Condensate water has the lowest P content,
followed by pond water. The lower efficiency of degradation
observed in condensate water can be attributed to the highest
content of copper which leads to complexation with humic
acids.42 On the other hand, in the case of [Sal]− after 180 min
of degradation approximately equal efficacy was observed for
all types of used water (Fig. 7b).
By applying a dark Fenton process, for all types of
investigated water, a higher degradation efficiency of [Sal]−
compared to [bmim]+ was observed (Fig. 7b vs. Fig. 7a). This
can be explained by the fact that salicylic acid could present in
different forms - non-dissociated at pH < 3.0, and dissociated
at pH 6.0−
12 (the pKa for salicylic acid is 2.97).43 It was also
found that the adsorption of salicylate was the highest at pH 3
on iron containing surface.44 Besides, higher degradation
efficiency as recorded in our case, can be explained by strong
ability of salicylic acid to complex with surface iron atoms of
photocatalyst under acidic conditions.32
1
Further studies were focused on examining of ionic liquids
degradation efficiency in different types of water by using
heterogeneous photo-Fenton process (Fig. 8). For both
[bmim]+ and [Sal]− a significant increase in the degradation
efficiency by SS/7.2Fe/TiO2/H2O2 process was observed (Fig. 5
vs. Fig. 8). The small contribution to degradation efficiency
comes from the heterogeneous photo-Fenton process (see
simulated sunlight photodegradation studies). However,
significantly greater contribution to degradation efficiency
comes from presence of SS radiation due of dissolved organic
carbon (DOC). By absorption of sunlight radiation DOC could
provide a rich variety of photochemical reactions.45 The
resulting excited states of the DOC could participate in energy
transfer, electron transfer, and free radical reaction, which
affect the fate of aquatic pollutants.46
Fig. 8 Evolution of ionic liquid (c0 = 0.38 mmol L− ) degradation along the time using SS
radiation: a) [bmim]+ and b) [Sal]−. Water matrix: (1) tap water; (2) condensate water;
(3) pond water; (4) river water; (5) rain water. Other reaction conditions: 7.2Fe/TiO2
(1.67 g L− ) + 45 mmol L−1 H2O2 + 70 μL h−1 H2O2 at pH 2.8.
1
3.6 Identification of intermediates
Identification of the intermediates was carried out using the
HPLC−ESI−MS/MS technique. Based on their MS/MS
fragmentation data, twenty-two intermediates originating
from [bmim]+ were identified in positive mode, and one
intermediate originating from [Sal]− was identified in negative
mode (Table 2; Table S2, ESI†).
P1 is a compound with Mmi = 137, which is 2 mass units
lower than the molecular mass of 3-butyl-1-methylimidazolium
(PI), indicating loss of two hydrogen atoms, most likely from
butyl-chain, indicating that P1 is 3-(2-but-2-en-1-yl)-1-methyl-
In this case, it can be concluded that the present DOC
together with SS radiation provided a powerful sensitization
effect (DOC was significant source of OH radicals production)
and thus increase the ionic liquid degradation efficiency. At the
1H-imidazol-3-ium. P2 and P3 are ionic compounds with Mmi
=
153. ∆Mmi of 14 indicates the presence of oxo group in both
compounds, thus it seems likely that P2 is 3-butanoyl-1-
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methyl-1
H-imidazol-3-ium and P3 is 1-methyl-3-(2-oxobutyl)-
P6
P7
2.55
2.76
171
171
C8H15N2O2
1H-imidazol-3-ium. P4 and P5 are ionic compounds with Mmi =
155. Mass difference of 16 units suggests hydroxylation, with
compound P4 likely standing for 3-(1-hydroxybutyl)-1-methyl-
C8H15N2O2
1H-imidazol-3-ium, and compound P5 being 3-(4-
hydroxybutyl)-1-methyl-1H-imidazol-3-ium. P6 and P7 are
isobaric compounds with Mmi = 171, which is 32 units higher
than the monoisotopic mass of 3-butyl-1-methylimidazolium
(PI), indicating the presence of two hydroxyl groups. P6 could
P8
P9
2.82
2.92
173
173
C8H17N2O2
C8H17N2O2
possibly
represent
3-(3,4-dihydroxybutyl)-1-methyl-1
H-
-
imidazol-3-ium. and P7 is 3-(2,3-dihydroxybutyl)-1-methyl-1
H
imidazol-3-ium. Compounds P8 and P9 both have Mmi = 173.
The monoisotopic mass, 34 mass units greater than that of 3-
butyl-1-methylimidazolium, points out to the presence of two
hydroxyl groups and either reduction of one double bond in
imidazolium ring or ring cleavage. P8 likely stands for
P10
P11
2.30
2.41
187
187
C8H15N2O3
C8H15N2O3
[formyl(methyl)amino]-
N
-(hydroxybutyl)-
N-
methylmethaniminium
and P9
is
likely
N-
butyl[formyl(methyl)amino]-
N-
P12
2.73
187
C8H15N2O3
(hydroxymethyl)methaniminium. Although compounds P10
,
P11 and P12 have identical monoisotopic masses of Mmi = 187,
their fragmentation patterns suggest that they have various
distribution of three hydroxyl groups (Δm/z = 48) present in
them. P10 likely stands for 3-(dihydroxybutyl)-1-
-
P13
P14
2.36
3.23
189
189
C8H17N2O3
C8H17N2O3
(hydroxymethyl)-1
identified as 1-methyl-3-(trihydroxybutyl)-1
and P12 is 3-(dihydroxybutyl)-1-(hydroxymethyl)-1
H
-imidazol-3-ium, intermediate P11 was
-imidazol-3-ium
-imidazol-
H
H
P15
2.13
203
C8H15N2O4
3-ium. P13 and P14 both correspond to compounds with
monoisotopic mass Mmi = 189, 50 mass units higher than the
molecular mass of 3-butyl-1-methylimidazolium. Based on
their PI MS2 spectra and specific patterns of fragmentation, it
was concluded that these compounds could have either three
P16
P17
P18
2.20
2.47
2.57
203
203
203
C8H15N2O4
C8H15N2O4
C8H15N2O4
Table
2 Proposed structures of intermediates for the photolitic degradation of
[bmim][Sal].
Molecular
formula
Peak
tR (min)
Mmi
Structure
[bmim]+
2.88
139
C8H15N2
P1
P2
2.58
2.41
137
153
C8H13N2
P19
2.67
203
C8H15N2O4
ꢀ
ꢀ
ꢀ
P20
P21
P22
[Sal]−
2.49
2.63
2.79
9.60
205
205
205
137
C8H17N2O4
C8H17N2O4
C8H17N2O4
C7H5O3
C8H13N2O
P3
P4
2.54
2.37
153
155
C8H13N2O
C8H15N2O
P23
7.75
153
C7H5O4
P5
2.47
155
C8H15N2O
hydroxyl groups and reduction of one double bond in
imidazolium ring, or cleavage of imidazolium ring with
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hydroxyl and oxo group attached, along with another hydroxyl contains two aromatic ring. This points to the significant
on butyl or N-methyl moiety. It was not possible to elucidate impact of the anion structure on the cation degradation
the structure of P13, while P14 most likely stands for hydroxy- efficiency, and vice versa. Similarly, (Section 3.4), the
N
-(3-hydroxybutyl)[methyl(2-oxoethyl)amino]methaniminium. degradation efficiency of the [bmim]+ in the form of chloride
P15 P16 P17
P18 and P19 all have monoisotopic masses of was even 36% higher in comparison with [bmim]+ in the form
,
,
,
Mmi = 203, indicating that they either have four additional of salicylate.
hydroxyl groups in the 3-butyl-1-methylimidazolium structure,
or cleaved imidazolium ring with two hydroxyl and two oxo
4. Conclusions
groups attached. P15 corresponds either to
dihydroxybutyl)-
formyl[formyl(methyl)amino]methaniminium,
dihydroxybutyl)-4,5-dihydroxy-1-methyl-1 -imidazol-3-ium.
N
-(1,3-
N
-
After 93 days in studied pH range between pH 2.8 and 10.0,
[bmim]+ was stable to hydrolysis. In contrast to cation, [Sal]−
was less stable as the pH value increased. The following dark
experiments showed that the ionic liquid was stable in the
or
3-(1,3-
H
P16 and P17 are both most likely tetrahydroxy derivatives of 3-
butyl-1-methylimidazolium. Fragmentation patterns of P18
and P19 are very similar, with the only difference in the
positions of the four hydroxyls, which cannot be determined
presence of hydrogen peroxide alone as well as in
a
combination of the TiO2 photocatalysts (TiO2 P25 or TiO2
support) and the peroxide. However, in the presence of
hydrogen peroxide and 7.2Fe/TiO2 after 180 minutes 27% of
[bmim]+ and 92% of [Sal]− has been degraded. This behavior
can be attributed to the heterogeneous Fenton process, and
within this process existence of surface bonded iron oxide was
crucial. UVA photolysis experiments showed that no significant
change in the [bmim]+ and [Sal]− concentration could be
observed. Besides direct photolysis, the contribution of UVA
radiation on the studied AOPs was decreased in the following
with certainty. P20
with monoisotopic mass Mmi = 205, which is 2 units higher
than compounds P15 P19. This leads to an assumption that
, P21 and P22 all correspond to compounds
-
these compounds have four hydroxyls and one reduced double
bond in imidazolium ring, or two hydroxyls in butyl moiety and
hydroxyl and oxo group bound to a cleaved imidazolium ring.
PI MS2 spectra of P20 and P21 are almost identical, thus it
was not possible to further elucidate their structures. There is
also limited information from the fragmentation pattern for
order:
UVA/TiO2
P25/H2O2
>
UVA/TiO2
P25
>
identification of the intermediate P22. P23 is a compound with
UVA/7.2Fe/TiO2/H2O2 > UVA/H2O2 > UVA. For the first time,
the performance of different AOPs through degradation of
ionic liquid under the simulated sunlight radiation was
examined. Comparing obtained results, with those obtained
for the same processes which were carried out in the dark, in
all cases increasing efficiency can be observed. Also, for all of
the studied processes, ionic liquid photodegradation efficiency
was significantly lower in the presence SS radiation, compared
to that in the presence of UVA radiation. By studying mutual
facilitation of [bmim]+ and [Sal]− on the photodegradation
efficiency, the impact of [Sal]− on the [bmim]+ degradation
efficiency was much more pronounced.
m/z = 153, identified in the NI mode. Its mass is 16 units higher
than salicylate, indicating that one hydroxyl more is present in
this molecule. However, it is not possible to determine
whether P23 stands for 3-hydroxysalicylate or 5-
hydroxysalicylate.
3.7 Photocatalytic degradation of various ionic liquids by the
SS/7.2Fe/TiO2/H2O2 process
Bearing in mind that the photo-Fenton process involving solar
radiation has significant potential applications, we investigated
the efficiency of the 7.2Fe/TiO2/H2O2 process in the presence
of SS radiation on five various types of ionic liquids. Based on
the obtained results, it may be concluded that all the ionic
liquids were decomposed under these experimental conditions
(Fig. S1, ESI†). For all studied ionic liquids it can be noted that
the stability of salicylate anion was lower compared to their
corresponding cation. Also, it can be observed that salicylate
photodegradability in relation to the cation increased in the
following order: [bmim]+ < [TBA]+ < [im]+ < [Hpmim]+. From the
same figure we can also conclude that [im]+ cation in a smaller
extent slows the degradation of salicylate compared with
acyclic tetrabutylammonium cation 95.2% vs. 92.3%
degradated, respectively. On the other hand, the introduction
of methyl and butyl groups at the positions N-3 and N-1 of
imidazolium ion decreases the efficiency of degradation for
32.3%, compared to that with a [im]+ cation alone. One might
also note that the salicylate completely decomposed for 60
min, if only a terminal methyl group of the side alkyl chain is
replaced by a hydroxyl group. Compared with other studied
cations, maximum efficiency degradation has been reached in
the case of [Bzmim]+ cation (68.4%), even though this cation
From a practical point of view, it was very important to
study the stability properties of ionic liquids toward AOPs in
different types of water. Applying dark heterogeneous Fenton
process on [bmim]+ indicated that efficiency of degradation
decreases in the following order: pond < rain < tap < river <
condensate water showing a strong dependence on the
constitution of the radiated media. On the other hand, in the
case of [Sal]− approximately equal efficacy was observed for all
types of used water. Also, for all types of water, a higher
degradation efficiency of [Sal]− compared to [bmim]+ was
observed. Furthermore, in presence of SS radiation the present
DOC provided
increases the ionic liquid degradation efficiency.
LC MS/MS analysis indicated that intermediates with one
a powerful sensitization effect and thus
−
or more OH groups were produced during the photocatalytic
degradation process. In addition, for the most efficient
UVA/TiO2 P25/H2O2 system structure of nineteen
intermediates are assumed.
10 | RSC Adv., 2016, 00, 1-3
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27 N. Banić, B. Abramović, J. Krstić, D. Šojić, D. Lončarević, Z.
Cherkezova-Zheleva and V. Guzsvány, Appl. Catal. B-Eviron.,
2011, 107, 363.
28 V. Guzsvány, N. Banić, Z. Papp, F. Gaál and B. Abramović,
Reac. Kinet. Mech. Cat., 2010, 99, 225.
29 M. Pera-Titus, V. García-Molina, M.A. Baños, J. Giménez and
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30 D.F. Laine and I.F. Cheng, Microchem. J., 2007, 85, 183.
31 S. Sabhi and J. Kiwi, Water Res., 2001, 35, 1994.
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Acknowledgements
This article was financially supported by the Ministry of
Education, Science and Technological Development of Republic
of Serbia under project contracts ON172042 and ON172012.
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43 INCHEM
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