22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Removal of the pharmaceutical diclofenac by pulsed corona discharge
M. Magureanu1, D. Dobrin1, C. Bradu2, N.B. Mandache1 and V.I. Parvulescu2
1
2
National Institute for Lasers, Plasma and Radiation Physics, Magurele-Bucharest, Romania
University of Bucharest, Department of Organic Chemistry, Biochemistry and Catalysis, Bucharest, Romania
Abstract: This work investigates the degradation of the pharmaceutical compound
diclofenac (DCF) in water by pulsed corona discharge in contact with the liquid. Complete
removal of the initial DCF was obtained after 15 min plasma treatment. The detected
reaction products suggest that DCF degradation by plasma starts with the cleavage of the
C–N bond, followed by further oxidation, ring opening and finally mineralization.
Keywords: pulsed corona discharge, water treatment, pharmaceuticals, diclofenac
1. Introduction
The presence of pharmaceutical compounds in surface
water and ground water has been reported in numerous
works [1-3]. Some of these contaminants are persistent
and accumulate in water, others have shorter half-lives,
but their removal is compensated by the high continuous
input, leading to a pseudo-persistent character [3]. Even
at the very low concentrations found in the environment,
pharmaceuticals may cause adverse developmental effects
in aquatic organisms. In addition, some pharmaceuticals
cannot be removed in drinking water treatment plants and
can thus reach drinking water [3, 4]. This raises questions
regarding the effect on human health of long-term
exposure to trace amounts of these compounds.
Therefore, water pollution with pharmaceutical products
started to be regarded as a serious environmental problem.
Diclofenac (DCF) is a non-steroidal anti-inflammatory
drug (NSAID) commonly used as analgesic, antiarthritic,
antirheumatic and in other inflammatory disorders. It is
one of the pharmaceuticals frequently found in water [1,
5], as a result of high consumption and relatively poor
removal efficiency in wastewater treatment plants
(20 - 40%) [5].
Advanced oxidation processes (AOPs) have been
extensively investigated during the last years for the
removal of recalcitrant water pollutants, including
pharmaceuticals, with the aim of finding an alternative to
conventional waste water treatment [6]. AOPs generate
in situ strong oxidizers, mainly hydroxyl radicals, which
can efficiently degrade the organic contaminants. DCF
oxidative degradation has been studied using various
AOPs, such as ozonation [7, 8], TiO 2 photocatalysis [9,
10], UV/H 2 O 2 oxidation [11], photo-Fenton process [12]
and sonolysis [13, 14].
Electrical discharges in liquid and in contact with liquid
produce in-situ oxidizing species such as ozone, hydroxyl
radicals, hydrogen peroxide etc. [15] which can
decompose organic compounds.
Such non-thermal
plasmas have also been recently investigated for the
degradation of various pharmaceuticals in water [16-21].
These studies generally show that pharmaceutical
compounds are removed relatively fast by plasma, partly
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degraded, partly even mineralized, and thus, after
sufficiently long treatment time the final organic
by-products present in the solutions are mainly small
molecules in an advanced oxidation state.
In the present work the degradation of DCF in water is
investigated using non-thermal plasma generated in a
pulsed corona discharge above liquid. The reaction
intermediates and final products are detected and the
mineralization degree is evaluated. A comparison with
results obtained by other AOPs is attempted.
2. Experimental set-up
The experiments are carried out in an insulating vessel,
which contains the DCF aqueous solution, using a pulsed
corona discharge above liquid.
The high voltage
electrode is an array of 15 copper wires (100 µm
diameter, 36 mm length, 6 mm distance between adjacent
wires) placed above the solution, and the ground electrode
was an aluminium tape (length 122 mm, width 48 mm)
placed on the bottom of the vessel. The height of solution
layer is 7 mm and the discharge gap is 3.5 mm. Oxygen
was flown through the plasma reactor with a rate of
1 L/min.
The high voltage pulses are generated by discharging a
6 nF capacitor charged at high voltage using a spark-gap
switch in self-breakdown mode. The amplitude of the
voltage pulses is 18 kV and the rise rate is ~1 kV/ns.
A pulse frequency of 22 Hz is used. The energy per pulse
is in the range 1-1.1 J and the average power dissipated in
the discharge is ~24 W.
The chemical structure of DCF is shown in Fig. 1. DCF
solutions are prepared by dissolving the pharmaceutical
compound in tap water and have conductivity of
250 µS/cm and pH 6. In each experiment 55 mL solution
is used, with DCF initial concentration of 50 mg/L.
Fig. 1. Chemical structure of diclofenac (DCF).
1
The DCF concentration is measured by HPLC (High
Performance Liquid Chromatography), using a Varian
ProStar apparatus equipped with an UV detector (at
λ = 276 nm) and a Microsorb-MV 100–5 C18 column.
The degree of mineralization of DCF is evaluated by
Total Organic Carbon (TOC) analysis. TOC is calculated
by subtracting from the value of the Total Carbon (TC)
the value of the Total Inorganic Carbon (TIC). TC and
TIC measurements are performed using an analyser based
on carbon dioxide infrared absorption (HiPerTOC
Thermo Electron).
The by-products formed during DCF degradation are
identified by GC-MS (Gas Chromatography-Mass
Spectrometry) using Agilent 6890-5973 chromatograph
equipped with a SPME (Solid Phase Micro Extraction)
sampler and by HPLC with UV–Vis diode array detector.
3. Results and discussion
Fig. 2 shows the evolution of the DCF concentration in
solution (red), as well as the decrease of the TOC (green)
and TIC (blue) values as a function of plasma treatment
time.
DCF, TOC, TIC (mg/L)
50
DCF
target compound to CO 2 , H 2 O, and inorganic products.
Considerable reduction of TOC is obtained by plasma
treatment: TOC decreases almost linearly with treatment
time and reaches almost 50% after 30 minutes plasma
exposure. Extending the duration of the plasma treatment
would most likely increase the mineralization degree even
further.
The TIC (which consists in carbonate, bicarbonate, and
eventually CO 2 dissolved in water) also decreases to
about half of the initial value in the first 15 minutes of
treatment and later remains approximately constant.
Several by-products resulting from DCF degradation
were identified and are listed in Table 1.
Table 1. Identified by-products of DCF degradation by
plasma.
Identified
by-product
Chemical structure
Molecular mass
(g/mol)
Dichloroaniline
162.02
Oxalic acid
90.03
Maleic acid
116.07
Malonic acid
104.06
Succinic acid
118.09
Formic acid
46.03
Acetic acid
60.05
40
30
20
TOC
10
TIC
0
0
5
10
15
20
25
30
treatment time (min)
Fig. 2. Evolution of the concentration of DCF in solution
and of TOC and TIC values as a function of plasma
treatment time.
It is observed that the DCF concentration decreases fast
and the initial DCF is completely removed after only
15 min plasma treatment. This corresponds to an energy
efficiency of ~1 g/kWh for 50% DCF removal and
0.76 g/kWh for 90% removal.
The mineralization degree reflects the total degradation
of the organic compound to carbon dioxide, water and,
eventually, other inorganic products. Mineralization is
evaluated by the TOC value, which is the amount of
carbon bound in the organic compounds present in
solution, including the initial compound and its
degradation products.
As shown by the TOC decrease in Fig. 2, mineralization
occurs much slower than DCF removal. This is a natural
behaviour, since degradation is a gradual process, firstly
resulting in the formation of various organic intermediates
and finally leading to complete decomposition of the
2
Dichloroaniline is detected in the DCF solutions
exposed to plasma for a short time, but its concentration
starts to decrease after 5 minutes treatment and is absent
for treatment time longer than 15 minutes. While DCF,
dichloroaniline and other chlorinated intermediates are
degraded chlorine is released in the solution as chloride
ions. The chlorine balance confirms the absence of any
chlorinated by-product in the solutions treated for more
than 15 minutes.
Several carboxylic acids are detected as oxidation
by-products of DCF: oxalic, maleic, malonic, succinic,
formic and acetic acid. The chromatograms contain also
some unidentified peaks.
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Based on the detected reaction products, it can be
assumed that the first step in DCF degradation under nonthermal plasma conditions is the cleavage of the C–N
bond, with formation of 2,6-dichloroaniline, followed by
further oxidation, ring opening with formation of shortchain acids and finally mineralization.
A comparison between the results obtained in the
present work and those reported using other AOPs is
shown in Table 2, in terms of DCF removal,
mineralization and chlorine release (where available).
Ozone treatment removes the target compound from
water very fast, depending on experimental conditions
(ozone dose, initial DCF concentration, solution pH, etc.).
However, mineralization is rather slow in case of ozone
oxidation [7, 11, 22], and the chlorine balance indicates
the persistence of refractory chlorinated by-products in
the treated solutions [7]. Hydroxylated derivatives are
detected in these studies as the main intermediates of
DCF degradation, accompanied by C–N cleavage
products [7, 11]. Using a H 2 O 2 / UV system, Vogna et al.
suggested similar oxidation pathways and also indicated
that most of the intermediates still conserve Cl atoms in
their formula [11], since the chlorine release was only
51% after 90 min treatment.
The results obtained in photocatalytic experiments are
scattered over a wide range, depending on the radiation
intensity, catalyst type and loading [9, 23, 24]. However,
it is shown that with careful tuning of the experimental
parameters and by sufficiently extending the treatment
time the chlorinated by-products can also be degraded and
even complete mineralization can be achieved [9].
DCF degradation using sonolysis also resulted in rather
low mineralization, even in the presence of catalysts
[13, 14], and the persistence of chlorinated products in the
treated solutions is observed [14].
The results obtained in the present work using plasma
compare favourably with those achieved by other AOPs,
especially with respect to the chlorinated degradation
products, which are completely removed for treatment
times exceeding 15 minutes. The mineralization degree is
superior in plasma experiments as compared to the other
AOPs for similar treatment duration. However, a detailed
analysis should involve the energy efficiency of the
various techniques.
Table 2. Comparison between results on DCF degradation obtained with various AOPs.
Initial DCF
concentration
(mg/L)
Treatment time
(min)
DCF removal
(%)
Mineralization
(%)
Cl release
(%)
200
30
60
> 99
-
24
56
[7]
296
10
90
100
-
10
32
95
[11]
29.6
7
90
100
-
<5
45
-
[22]
296
90
95
39
51
[11]
10
240
85
-
-
[23]
15
30
60
120
75
95
-
40
85
100
70
100
-
[9]
8
15
60
> 95
-
40
-
[24]
Sonolysis
4.4
30
100
-
-
[13]
Sonolysis + Fe
8.9
90
-
43
-
[13]
Sonolysis + TiO 2
50
30
84
15
35
[14]
50
15
30
100
-
50
> 95
100
AOP
Ozone treatment
H 2 O 2 + UV
TiO 2 photocatalysis
Non-thermal plasma
4. Conclusions
The pharmaceutical compound diclofenac was degraded
using a pulsed corona discharge generated in oxygen
above the DCF solution. DCF was completely removed
after 15 minutes non-thermal plasma treatment. During
30 minutes plasma exposure approximately 50% of the
initial DCF was mineralized. The chlorine balance
indicated that no chlorinated by-products were present in
the solutions treated for more than 15 minutes.
Dichloroaniline was identified as one of the intermediate
products, suggesting that cleavage of the C–N bond is one
of the first steps in the degradation process. Ring opening
is evidenced by the presence of several carboxylic acids
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Reference
This work
in the treated solutions: formic, acetic, oxalic, malonic,
maleic and succinic.
5. Acknowledgment
This work was financially supported by the Ministry of
Education and Scientific Research – UEFISCDI,
Programme “Partnerships in priority areas – PN II”,
project number 141/2014.
6. References
[1] C. Tixier, et al. Environ. Sci. Technol., 37, 1061
(2003)
3
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
[10]
[11]
[12]
[13]
[14]
[15]
[16]
[17]
[18]
[19]
[20]
[21]
[22]
[23]
[24]
4
M. Rabiet, et al. Environ. Sci. Technol., 40, 5282
(2006)
S.K. Khetan and T.J. Collins. Chem. Rev., 107,
2319 (2007)
S. Mompelat, et al. Environ. Int., 35, 803 (2009)
Y. Zhang, et al. Chemosphere, 73, 1151 (2008)
M. Klavarioti, et al. Environ. Int., 35, 402 (2009)
A.D. Coelho, et al. Sci. Total Environ., 407, 3572
(2009)
M.M. Sein, et al. Environ. Sci. Technol., 42, 6656
(2008)
P. Calza, et al. Appl. Catal. B: Environ., 67, 197
(2006)
L. Rizzo, et al. Water Res., 43, 979 (2009)
D. Vogna, et al. Water Res., 38, 414 (2004)
L.A. Perez-Estrada, et al. Environ. Sci. Technol.,
39, 8300 (2005)
G.T. Güyer and N.H. Ince. Ultrason. Sonochem.,
18, 114 (2011)
J. Hartmann, et al. Chemosphere, 70, 453 (2008)
P. Lukes, et al. in: Plasma Chemistry and Catalysis
in Gases and Liquids. (Weinheim: Wiley-VCH)
(2012)
H. Krause, et al. Chemosphere, 75, 163 (2009)
D. Gerrity, et al. Water Res., 44, 493 (2010)
M. Magureanu, et al. Water Res., 44, 3445 (2010)
M. Magureanu, et al. Water Res., 45, 3407 (2011)
Y. Liu, et al. Chem. Engng. Process., 56, 10 (2012)
D. Dobrin, et al. Chem. Engng. J. 234, 389 (2013)
J.F. Garcia-Araya, et al.
J. Chem. Technol.
Biotechnol., 85, 798 (2010)
A. Achilleos, et al. Chem. Engng. J., 161, 53 (2010)
C. Martinez, et al. Appl. Catal. B: Environ., 107,
110 (2011)
P-III-9-22