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21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
Impact of Pulsed Discharge Plasma on the Decoloration of Dyes
Wahyudiono1, Y. Hayashi1, S. Machmudah1,2, M. Goto1
1
Department of Chemical Engineering, Nagoya University, Nagoya, Japan
Department of Chemical Engineering, Sepuluh Nopember Institute of Technology, Surabaya, Indonesia
2
Abstract: Recently, plasma-water interaction has attracted growing interest as it may provide
experimental chemists with a quite unique reaction medium. Pulsed discharge plasma is typical oxidation technologies for disposing organic compounds in aqueous solutions. In this
work, electric field produced by pulsed high-voltage discharge plasma over water surface has
been performed under argon atmosphere to degrade dye compounds. The effects of various
parameters with pulsed high-voltage arc discharge plasma are observed for decoloration of
Orange G, Orange II, Congo Red, Naphthol Blue Black.
Keywords: Pulse discharge plasma; Decoloration; Dyes; Wastewater; Conversion.
1. Introduction
Amongst the many chemicals used in various industrial
processes are the organic synthetic dyes. They possess
color because they absorb light with wavelength in visible
range (350-700 nm), have at least one chromophore electron systems with conjugated double bonds and
auxochromes, and exhibit resonance of electrons, which is
a stabilizing force in organic compounds [1,2]. The color
will be lost if one of these features was released from the
molecular structure. Due to the use of these chemicals in
industries extensively, they also become an integral part
of industrial effluent. Most of the dyes are potentially
toxic and carcinogenic in nature; therefore if they are
discharged directly into the environment, they persist as
environmental pollutant as well as traverse through the
entire food chains, leading to biomagnifications. Many
dyes are visible in water at concentration as low as 1
mg/L. Therefore a major environmental challenge is their
removal from the industrial effluents.
A wide range of methods has been developed for the
removal of synthetic dyes from industrial effluents or
wastewaters to decrease their impact on the environment.
In general, they can be treated in two ways [3]: (i) by
chemical or physical methods of dye removal, which refers to the process called decoloration and (ii) by a biodegradation process. Traditional wastewater treatment
technologies have proven to be markedly ineffective for
handling wastewater of synthetic textile dyes because of
the chemical stability of these pollutants. Several advanced oxidation technologies have also been utilized for
the purpose of degrading organic pollutants in wastewater,
such as UV photolysis, photocatalysis, sonochemistry,
supercritical water oxidation, electrical discharge plasma
technology and electron-beam irradiation [4-10]. However, due to the variety of different organic compounds contained in dyes, there were some difficulties to specify the
universal methods for a removal of dye from wastewater.
Consequently, the presence of intermediates arising as a
new and different risk of pollution is faced and they could
be more harmful than the pollutant itself.
In this work, the pulsed discharge plasma over a water
surface is considered to be an applicable method for removal of organic pollutants from wastewater. Several
kinds of inorganic salts or other chemicals were added on
the decolorization of dyes using advanced oxidation processes [11-13]. Here, the synthetic wastewater was directly fed as a starting material without chemical reagents.
It is simple and versatile technique that has gained much
attention because of its capability and feasibility in the
decomposition of large quantities of wastewater from
industries. The utilization of pulsed discharge plasma has
demonstrated to be effective at degrading aromatic compounds in aqueous solutions as it not only produces a hydroxyl radical but also atomic species with high oxidation
potential [14-16]. Orange G, Orange II, Congo Red, and
Naphthol Blue Black were selected as the model organic
contaminants, which were dissolved in distilled water to
form a synthetic wastewater. These aqueous solutions of
dye was employed as the grounding water electrode, so
that, the plasma could directly attack to the surface of
aqueous solution between electrodes.
2. Experimental
The dyes Orange G (C16H10N2Na2O7S2; 80%), Orange
II
(C16H11N2NaO4S;
92%),
Congo
Red
(C13H22N6Na2O6S2; 85%), and Naphthol Blue Black
(C22H14N6Na2O9S2; 85%) commercial products purchased
from Sigma Chemical Co., and used without further purification. The synthetic wastewater was prepared by dissolving of each dye with distilled water at 1000 mg/L as
an initial concentration. The conductivity and pH of the
sample liquid was measured using a conductivity meter
(ECTestr 11, EUTECH Instruments, Singapore) and pH
meter (Compact pH Meter AS-211, Horiba. Ltd, Japan).
At 1000 mg/L, the conductivity of Orange G, Orange II,
Congo Red, and Naphthol Blue Black were 280, 240, 310,
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21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
Fig. 1 Schematic diagram of experimental apparatus.
Fig. 1 shows a schematic diagram for discharge plasma
production over a water surface under argon atmosphere.
The reactor contained two inspection windows to monitor
the performance of plasma production and was made of
stainless steel (SUS-316, AKICO, Japan) with a volume
of 31 mL and a maximum temperature of 423 K and
pressure of 30 MPa. The discharge plasma over the water
surface was initiated by a cylindrical anode composed of
copper wire 0.1 cm in diameter which was set at a distance of 0.3 cm from the water surface. A power lead was
introduced through the center of a long bushing made of
polyether ether ketone (PEEK), and the annular space was
sealed with an O-ring placed around its outer surface to
prevent gas leakage. The distance of inside wall reactor as
a cathode was 3 cm from the water surface. To begin the
experiment, feed containing 1000 mg/L of synthetic dye
(7 mL) was loaded to the reactor manually. The reactor
was repeatedly (three times) purged of air using argon gas
with a pressure of about 0.2 MPa; it was then heated up to
313 K. A thermocouple and needle valve were used to
control the temperature and the pressure of the reactor,
respectively. After the temperature achieved the desired
operating values, pulse power was applied to produce
electric discharge. A voltage (18.6 kV) was applied to the
positive electrode using an MPC (Magnetic Pulse Compressor, MPC2000S, Suematsu Electronics Co. Ltd., Japan) charged by a high-voltage stabilization DC power
supply. The breakdown voltage and the current were respectively measured using a high-voltage probe (EP50K,
Pulse Electronic Engineering Co. Ltd., Japan) and a current transformer (4997, Pearson Electronics, Inc., USA)
that were connected to a digital oscilloscope. The distribution of optical emission intensity was captured using a
6
200
4
Current
100
2
0
0
-100
Ampere [A]
Liquid surface
3. Results and Discussion
Fig. 2 shows the waveform profile of pulsed voltage (5
kV) and current under argon atmosphere. The principal
parameters of the source are: peak voltage (0-31 kV adjustable) and repetition-rate of pulse (0-250 pps adjustable). In this work, the repetition rate of pulse discharge
plasma applied was 4 pps in all experiments. The time
period from when the voltage was applied to when the
streamer channel reaches the aqueous surface as an electrode is called the streamer arc discharge time. In general,
this time changes with the peak voltage and the distance
of an electrode. It should be noted that the various of an
electrode distance was not modified during the course of
an experiment. As shown in Fig. 2, peak following the
primary pulse is due to parasitic capacitive and inductive
elements from the electrode feed through into the reactor
as well as from the voltage and current probes. This peak
occurs after the primary pulse and does not affect the
measurements of its voltage amplitude.
3
Electrode
charge-coupled device camera with a gated image intensifier (ICCD camera, Princetone Instruments, inc.).
Voltage [V; x 10 ]
and 290 S/cm, respectively. On average, the initial pH of
them was 9.15, 7.60, 8.18, and 9.40, respectively. No pH
variation was observed during the course of these experiments.
Voltage
-200
-2
-4.0x10
-7
-2.0x10
-7
0.0
2.0x10
-7
4.0x10
-7
Time [s]
Fig. 2 Typical waveform of voltage and current.
In order to understand the physical mechanism of pulse
discharge plasma, it is important to investigate their characteristics. In this work, ICCD camera was used to determine spatial-temporal characteristics of the streamer discharge plasma propagation for both polarities of the
high-voltage pulse. Generally, electrical breakdown can
be defined as the moment when a conductive plasma
channel forms an electrical connection between two electrodes inside the medium. This leads to the formation of a
spark or arc. Fig. 3 shows typical images of discharge
emission dynamics from pulse discharge plasma generated by copper electrode with 5 kV applied pulse voltage.
Each image shown in this figure was selected from a set
of different discharge observation at the same conditions.
The streamer discharge plasma emission, initially observed near the high voltage electrode, propagates along
the dielectric surface toward the grounded electrode (sur-
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21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
face of water). It might be explained as follows. The free
electrons in the discharge gap are accelerated under the
influence of the high electric field applied to the cylinder
electrode. The accelerating free electrons may collide
with and ionize the ambient (environment) molecules,
thus producing more free electrons causing electron avalanche. In this work (DC voltage) the free electron are
attracted towards the high voltage electrode. The drift of
free electrons leaves behind a positive charge at the
streamer head, which enhances the applied electric field
effect and attracts the electron of any secondary avalanche.
When secondary avalanche electrons intermix with primary avalanche ions, positive space charge remains in the
head of the streamer and greatly enhances the electric
field strength at the end of the streamer. By extension of
this way, the streamers propagation occurred. As shown in
Fig. 3, during the rise period of the applied voltage, the
streamer propagated from needle (copper) to water surface. After the peak of the applied voltage, a part of
streamers propagated from needle to water surface with
keeping velocity and other streamers disappeared. Finally,
all streamers disappear. When the applied voltage was at
the peak, plasma reached its greatest width.
a. 0 s.
b. 6.7 s.
c. 6.8 s.
d. 6.9 s.
In this work, the decoloration rate was calculated based
on the following equation:
Decoloration rate (%) = [ 1 – ( Ct/C0 ) ] x 100
(1)
where C0 (mg/L) is the initial concentration of dye and Ct
(mg/L) is the concentration of dye after treatment by
pulsed discharge plasma. The liquid products were directly identified by UV-Vis spectrophotometer V-550
(Jasco Corporation, Japan). They were recorded from 200
to 800 nm with a spectrometric quartz cell (1 cm path
length). The maximum absorbance wavelength (λmax) of
Orange G, Orange II, Congo Red, and Naphthol Blue
Black are 476, 486, 499, and 620 nm, respectively. The
residual concentrations of the dye in the mixture and
withdrawn at different reaction times, were determined by
measuring the absorption intensity at each max (maximum absorbance wavelength) of dyes and by the use of a
calibration curve. Chemically, chromophore and/or
auxochrome groups in the dyes molecule had responsible
for the dye color. For example, they are azo (-N=N-),
carbonyl (-C=O), methine (-CH=), nitro (-NO2), amine
(-NH2), carboxyl (-COOH), sulfonate (-SO3H) and
quinoid groups. As an explained above, the color will be
lost if one of these features was released from the molecular structure.
Fig. 4 Conversion of dyes after treatment by pulse
discharge plasma.
e. 7.5 s.
f. 7.7 s.
Fig. 3 The pulsed streamer generated by a single
discharge on the surface of water.
Fig. 4 showed the results for dyes conversion when the
pulse discharge plasma was applied at 2 kV peak voltage.
The figure depicted that the dyes degradation efficiency
increases with increasing the discharge numbers of plasma at the same of peak pulse voltage applied. When pulse
discharge plasma occurs, the following individual effects
occurred simultaneously: over pressure shock wave,
strong electrical field, production of various free radicals,
and intense ultraviolet radiation [17]. Localized effects
occur in the immediate vicinity of the plasma channel,
whereas extended effects primarily result from UV radiation and the intense shock wave, which radiates out into
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21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
the bulk of the solution. In this case, the arc discharge
directly contacted over the water surface where the
streamer channel propagates along the surface of the water. They will cleavage active side on the dye structure
group (chromophore and auxochrome) which promoted
by the various reactive chemical species such as radicals
(OH, H, O, HO2) and molecular species (H2O2, H2, O2).
Next, the opening of this ring leads to formation of its
derived compounds with lower molecular weight as degradation products. In addition, the pH and the conductivity of the liquid products are decrease and increase dramatically after pulse discharge plasma treatment.
4. Conclusions
Decoloration of dyes by pulsed high-voltage discharge
plasma over water surface at 313 K, 0.2 MPa argon atmospheres in a batch-type reactor has been performed.
Dyes were degraded into its derived compounds with the
appearance of light color. UV-Vis spectrophotometer analyzed that the intermediate compounds from the degradation of dyes consist primarily of aromatic compounds
which contain nitrogen functional groups. At each reaction conditions, the decoloration rate of Orange G, Orange
II, Congo Red, and Naphthol Blue Black significantly
increased with increasing discharge numbers at the same
applied peak voltage. The maximum degradation of them
was 50.05%, 41.64%, 44.98%, and 53.25%, respectively.
With increasing the pulse discharge times, the pH and the
conductivity of dye solutions changed clearly.
5. Acknowledgement
This work was supported by the Grants-in-Aid for Scientific Research by the Ministry of Education, Culture,
Sports, Science and Technology, Japan
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