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Oxidative Decoloration of Dyes by Pulsed Discharge Plasma in Water

Anto Tri Sugiartoa, Shunsuke Itoa, Takayuki Ohshimaa, Masayuki Satoa and Jan D. Skalnyb

aDepartment of Biological and Chemical Engineering, Gunma University,

1-5-1 Tenjin-cho, Kiryu-shi, Gunma 376-8515, Japan

bDepartment of Plasma Physics, Comenius University Bratislava 84215, Slovakia

Abstract

Degradation of organic dyes by the pulsed discharge plasma between needle-to-plane

electrodes in contaminated water has been investigated in three discharge modes: (i) streamer,

(ii) spark, (iii) spark-streamer mixed mode. The process of the decoloration has been found to

be most effective if the discharge operates in the spark-streamer mixed mode in dye solutions.

The decoloration rate during the pulsed discharge plasma treatment was depended on the initial

pH values. The decoloration rate was increased when more acidic condition was used,

especially in the case of streamer discharge mode. The decoloration rate at the pH value of 3.5

was found to be approximately three times higher than that at the pH value of 10.3. A small

effect of initial pH during the decoloration process by spark and spark-streamer discharge

mode means that the physical effects, such as shock-wave and ultraviolet radiation, may play

an important role in the oxidation process. It was found that the decoloration rates in the case of

spark and spark-streamer mixed discharge modes, which are characterized by high intensity

ultraviolet radiation, were found to be much higher than that in the case of streamer discharge

that is characterized by low intensity ultraviolet radiation. In addition, the considerable

increase in the decoloration efficiency of H2O2 containing solutions can be attributed to the

increase in hydroxyl radicals’ concentration. These are produced by ultraviolet light

photo-dissociation of H2O2 molecules in water surrounding the plasma channel.

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Keywords: Pulsed Discharge Plasma; Advanced Oxidation Process; Ultraviolet radiation; Dye

Decoloration

1. Introduction

A variety of new synthetic organic dyes are frequently used by modern textile technologies.

Therefore, the removal of these dyes from effluents becomes the major environmental problem

of the textile industry, not only because of the potential toxicity of certain dyes, but often due to

their visibility in wastewater. Although the dye concentration in wastewater is usually lower

than that of many other chemical compounds, these dyes are visible even at very low

concentrations. In general, dye-containing wastewater can be treated in two ways: (i) by

chemical or physical process and (ii) by biodegradation process. Due to the variety of different

organic compounds, containing various substituted aromatic nuclei, there is no universal

chemical method for a removal of dye from wastewater.

Recent experimental investigations have revealed that reactive dyes can be decolorized by

advanced oxidation processes (AOPs). For example, the ultraviolet light induced degradation

combined with H2O2/O3 or Fenton process alone was utilized for such processing [1-5]. More

recently, among the AOPs, the pulsed discharge plasma in water is considered to be an

applicable method for removal of organic pollutants from wastewater [6,7]. Pulsed discharge

plasma in water is efficient in the formation of chemically active species such as ·OH, ·H, ·O, O3,

H2O2, etc [8,9]. Most of these species are among the strongest oxidizing agents. The major

active species involved in the degradation of organic pollutants are hydroxyl radical and

hydrogen peroxide [8]. The hydroxyl radicals can directly attack organic pollutants contained

in water due to their high oxidation potential, and the hydrogen peroxide can effectively be

decomposed by ultraviolet radiation into hydroxyl radical [10]. In addition, depending upon

the solution conductivity and the magnitude of the discharge energy, shock-waves and

ultraviolet light may also be formed [11]. These effects also play an important role in

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destroying harmful organic pollutants in wastewater [11].

The pulsed discharge in water has several modes such as streamer, spark, and spark-streamer

mixed mode. It has been reported earlier that the pulsed discharge mode can affect the removal

efficiency of phenol in water solutions [12]. The differences in removal efficiency were

considered to be caused by the differences in the physical and chemical processes of each

pulsed discharge mode in the water.

The objective of this paper is to present the experimental data on the oxidative decoloration

of dyes in water using three pulsed discharge modes. The effect of the initial pH, the emission

intensity of ultraviolet light from the pulsed discharge plasma, and the effect of hydrogen

peroxide additives on the decoloration efficiency were also examined.

2. Experimental apparatus and method

The schematic diagram of experimental apparatus is shown in Fig. 1. The pulsed electric

discharge was generated in the electrode system of the needle-to-plane electrode geometry

located in the centre of plexiglas cylinder (50 mm inner diameter) reactor having the volume

100 cm3. Stainless steel tube needle (0.5 mm inner and 1.5 outer diameter) protruded 1 mm

from silicone insulator and was placed on axis of the reactor opposite to the stainless steel plane

electrode (diameter of 30 mm). Three distances between the needle and plane electrodes were

fixed at: 30 mm for the streamer, 15 mm for the spark-streamer mixed, and 7 mm for the spark

discharge mode [12], respectively. In experiment the spark-streamer mixed mode was in the

time course with the mixing degree was 50:50.

The pulse power supply with a rotating spark-gap switch was used to generate high voltage

pulse. The pulse voltage amplitude, pulse frequency, and the capacitance of the storage

capacitor: 20 kV, 25 Hz, and 6 nF, respectively, were kept constant in all experiments.

The total volume of 300 mL of solution was circulated through the reactor and temperature

controller by peristaltic pump at the solution flow rate of100 mL/min. Rhodamine B (basic dye),

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Methyl Orange (acid dye), and Chicago Sky Blue (direct dye) were treated in experiments.

Their concentrations were 0.01 g/L and 0.05 g/L. The conductivity of the solution was changed

by adding KCl and was adjusted to 100 µS/cm. Since the conductivity greatly affect the

formation of plasma discharge in water, the initial conductivity should be kept constant in all

experiment. Therefore, the initial solution pH was adjusted by added a little amount of HCl

and/or KOH into the solution until the conductivity becomes 100 µS/cm.

Spectrophotometer (Shimadzu, UV-1200) was used for measuring dye absorption. The

decoloration factor FD was calculated by the following formula.

( ) ( )( )initialabsorption

treatedabsoptioninitialabsorptionFD

!= (1)

The temporal development of FD factor was measured.

In order to investigate the emission intensity of ultraviolet light produced by pulsed

discharge plasma, the light emitted from the discharge was collected by the optical fiber and

transmitted to the entrance slit of a monochromator (McPherson 2035) equipped with a

photomultiplier tube (Hamamatsu R764). The end of the optical cable was located 10 mm from

the point electrode and 10 mm from the axis of the discharge gap. The temporal variation of the

light emission in the wavelength of ultraviolet region (200 < < 300 nm) was recorded on

oscilloscope (Tektronix TDS3032). Ten peak values of successive light pulses were averaged

with fluctuation less than 20% of the average value.

3. Results and discussion

3.1 Decoloration of dyes

Due to differences in the chemical composition of dyes, there is no universally applicable

chemical technology for a removal of dyes from wastewater. Therefore, it is necessary to find a

nonselective method in order to simplify the decoloration process. Using the pulsed discharge

plasma in water, decoloration of several kinds of dyes was tested. Results obtained in the

spark-streamer mixed mode of discharge are shown in Fig. 2. The decoloration factor reached

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the value of 95% after approximately 100 minutes of treatment if the initial concentration of all

three substances was 0.01 g/L. The rate of decoloration was considerably affected by initial

concentration of dye in solution. The decrease in the rate of decoloration for Rhodamine B is

evident from Fig. 2.

It can be easily surmised from the presented results that the pulsed discharge plasma in water

is an effective method for the decoloration of dyes from water solutions. The process of

decoloration is complex including more mechanisms that act simultaneously. Among them,

the break-up of chromosphores bounds, due to the activity of hydroxyl radicals, is the dominant

process in the dye solution.

3.2 Effect of various pulsed discharge modes

The efficiency of the decoloration process is affected by the mode of the pulsed discharge as

it follows from plots shown in Fig. 3. The differences in decoloration rate of Rhodamine B by

various pulsed discharge modes are remarkable. The spark-streamer mixed mode was found to

be the most effective for the decoloration of dye in water solutions. The mentioned regime is

characterized by great amount of plasma channels distributed bush like in great volume of the

discharge gap and also by relatively high discharge currents.

Processes acting in the plasma channel determine the differences in the decoloration rate. In

the case of the spark discharge, a single plasma channel is formed in the liquid, but the channel

has the high peak current of several hundred amperes compared to the streamer discharge

characterized by peak currents below of 100 A [13]. The concentration of electrons producing

the hydroxyl radicals by electron impact dissociation of water molecules and the gas

temperature in the plasma channel of the spark mode, are higher than that of corresponding

parameters in the streamer mode. Therefore, more radicals are formed in the spark mode

compared to the streamer mode. For these reasons the decoloration rate becomes higher in the

case of the spark mode than in the case of the streamer mode. Moreover, dyes can be

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decolorized even directly by ultraviolet light. It can be considered that the intense radiation of

ultraviolet light emitted from the spark plasma channel is active in radical reaction of the dye

decoloration.

The spark-streamer mixed discharge mode is characterised by an appearance of many

plasma channels [12]. The conditions are favourable for the generation of hydroxyl radicals by

electron impact dissociation in the channels distributed over a large water volume that can

explain the highest efficiency of such mode in decoloration of dyes. Moreover the direct

photo-dissociation of dyes can be also active.

3.3 Effect of pH

It is well known that the oxidation processes are very sensitive to the pH of the aqueous

solutions. It was reported that the decoloration of dyes using photolysis process is more

effective at low pH values than high pH values [14]. Kang et al. [15] reported that the optimum

pH for both the formation of hydroxyl radicals and dye removal in photo Fenton process ranges

from 3 to 5.

Using various pulsed discharge plasma modes, the efficiency of decoloration of aqueous

Chicago Sky Blue with different initial pH was tested. The obtained results after 30 minutes

treatment are shown in Fig. 4. The initial pH of each plasma discharge mode was 3.5, 7.5 and

10.3. The initial conductivity of each plasma discharge was 100 µS/cm. The decoloration rates

of Chicago Sky Blue were different for various modes, and depended on the initial pH of dye

solutions. It was found that pH and conductivity after plasma treatment was different for each

plasma discharge mode, e.g. in the case of streamer discharge, the pH was changed from 7.5 to

6.2, and conductivity was changed from 100 to 120 µS/cm after 30 minutes treatment. In the

case of spark discharge, the pH was changed from 7.5 to 5.5, and conductivity was changed

from 100 to 155 µS/cm after 30 minutes treatment. These results are in accordance with

decoloration of dye using ozonation process [14], which shows that the dye molecules

decompose into organic acids, aldehydes, resulting in a decrease in pH values and increase in

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water conductivity. The decoloration rates were increased when more acidic conditions were

used, especially when the streamer discharge mode was used. The decoloration rate at the initial

pH of 3.5 is approximately three times faster than that at the initial pH of 10.3.

Joshi et al [8] reported that the major active species involved in the degradation of organic

pollutants using pulsed streamer corona discharge are hydroxyl radicals and hydrogen peroxide.

However, in the case of pH values higher than 7, the hydroxyl radicals are unselective and react

readily with the carbonate ions, substantially reducing the efficiency of the oxidation process

[16]. Carbonates are generated through the breakdown of the organic materials during the

oxidation processes. For these reasons, it can be considered that the decoloration by hydroxyl

radical is the dominant process during the streamer discharge mode treatment.

On the other hand, in the case of spark and spark-streamer mixed modes, the effect of initial

pH on the decoloration rate was small. It means that the other physical effects such as shock

wave and ultraviolet radiation play an important role in the decoloration process during the

spark and spark-streamer mixed mode of the discharge treatment.

3.4 Ultraviolet radiation

One of the physical effects produced by pulsed discharge plasma in water is ultraviolet

radiation. In the case of the pulsed arc discharge reactor, the temperature in the plasma channel,

which forms during an electrohydraulic discharge, can reach values of 14.000 – 15.000 K and

thus functions as a blackbody radiation source. A maximum of emittance of such source is in

the vacuum ultraviolet (VUV) region of the spectrum (=75-185 nm) [14]. The vacuum

ultraviolet light emitted from the hot plasma is absorbed immediately in the water layer

surrounding the plasma channel [11], and the ultraviolet light with >185 nm penetrates into the

bulk of the solution.

The emission intensity of ultraviolet light produced by three pulsed discharge plasma

modes in distilled water is shown in Fig. 5. The intensity of ultraviolet light radiation in the

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case of spark discharge was much higher than that of the other discharge modes. In the case of

spark discharge, a single plasma channel is formed in the liquid. The plasma channel has the

high peak current of several hundred amperes, and according to Bårmann et al. [17] the electron

density in the propagated leader channel is high, close to the values typical for a thermal arc.

This condition makes the plasma channel that becomes a source of light with high intensity of

ultraviolet radiation.

However, the intensity of ultraviolet light in the case of streamer discharge was found to be

very low. This is because streamer discharge in the distilled water is characterised with the

bush like-streamer discharge channel, and a moderate amount of bubbles. Therefore, the

plasma channels emit very low intensity of ultraviolet light as shown in Fig. 5. It should be

noted that an increase in the solution conductivity might increase the emission intensity of

ultraviolet light of the pulsed discharge plasma in water [18].

3.5 Effect of hydrogen peroxide

The use of chemical additives for enhancement of the energy efficiency of the depletion of

dyes is one of the recommended methods. In this study, the effect of hydrogen peroxide on the

removal of Rhodamine B by various pulsed discharge modes has been investigated. Results are

shown in Fig. 6. The initial concentration of hydrogen peroxide of 8.8"10-3 mol/L was used for

all experiments. The initial pH of each plasma discharge mode was 7.5. The increase by

addition of hydrogen peroxide in the rate and in FD values is apparent for all three modes of the

discharge.

The dramatic increase of decoloration rate most likely appears due to the reactions of dyes

with hydroxyl radicals formed by photo-dissociation of hydrogen peroxide in the gap

surrounding discharge channels. The low threshold energy for photo-dissociation of H2O2 in

comparison with water makes this process strictly dependent on the light intensity. The spark

discharge mode is characterised with high light intensity as shown in Fig. 5. Therefore, the

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process of dye removal from water is most effective in the case of spark discharge mode. The

production of hydroxyl radicals by electron impact dissociation, which normally dominates in

streamer or spark-streamer mixed modes, becomes less important. The changes in the

dominant mechanism of hydroxyl radical production can be explained by the high efficiency of

the single spark mode of the discharge. The intensity of UV radiation from spark-streamer

mixed mode of discharge and especially pure streamer mode are evidently lower.

3.6 Mechanism and intermediates

Decoloration process using plasma discharge in water is complex including more

mechanisms that act simultaneously. Our experimental results showed that hydroxyl radical

and ultraviolet radiation were dominant on decoloration process during plasma discharge. This

process is similar like in photooxidation of dye using combination of ultraviolet radiation and

hydrogen peroxide. As reported by several authors, the schematic mechanism of

photooxidation on decoloration process are proposed as follows [14, 19],

RH + h R• + ·H (2)

H2O2 + h ·OH + ·OH (3)

·OH + RH R• + H2O (4)

R• + O2 RO2• (5)

A direct photooxidation of dye with ultraviolet light alone can lead to the decoloration of dye.

However, direct photooxidation of dye in water is very limited since water absorbs significantly

in the vacuum UV region. Therefore, ultraviolet light is practically used in combination with

oxidant (e.g. hydrogen peroxide) or catalysts (e.g. titanium oxides). The combination of

ultraviolet and hydrogen peroxide can form hydroxyl radicals. The hydroxyl radicals react with

dye (RH) to form dye+• (R•), and then dye+• could hydrolize or react with some oxidizer such as

dissolved oxygen (19).

If the decoloration process by plasma discharge is as same as the photooxidation process by

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ultraviolet with hydrogen peroxide, the decoloration of methyl orange occurs due to the break

up of the azo bond (NN) by hydroxyl radicals, generating 4-methylamino aniline, and then

underwent a further opening of phenyl-rings to form small molecular compounds such as oxalic

acid and butenedioic acid [20]. In the case of plasma discharge with strong ultraviolet radiation

(i.e. spark and/or spark–streamer mixed discharges), ultraviolet radiation will take part in the

break up of the azo bond.

3.7 Energy efficiency

The above investigation was focused on the reaction mechanisms and improvement of the

removal rate. However, in practice, energy efficiency is an important factor. The energy

efficiencies of three pulsed discharge modes for Rhodamine B (see Figure 6) were 25 mg/kWh

for streamer discharge, 80 mg/kWh for spark discharge, and 160 mg/kWh for spark-streamer

mixed discharge modes. When the additive (8.8"10-3 mol/L hydrogen peroxide) was injected

into the reactor, the energy efficiency became 320 mg/kWh for spark-streamer mixed

discharge.

4. Conclusions

The process of dye decoloration from water solutions has been investigated using three

pulsed discharge modes in water. The decoloration process was complex and active oxidation

mechanisms were most likely hydroxyl radicals and ultraviolet light radiation. In the case of

streamer discharge, the decoloration rate was dependent on the initial pH solution. Hence, the

oxidation process by hydroxyl radicals is considered to be the dominant dye removal process.

However, a small effect of the initial pH solution on the decoloration rates by spark discharge

and spark-streamer mixed discharge mode mean that the other physical effects such as

shock-waves and ultraviolet radiation play an important role in the decoloration process.

In water solution hydroxyl radical generated mostly in the discharge channel by the electron

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impact dissociation of water molecules. Therefore, the spark-streamer mixed discharge mode

was the most effective. In addition, the intensity of ultraviolet light radiation generated by the

plasma channels was high enough for direct photo-dissociation of dyes.

In solutions containing hydrogen peroxide, hydroxyl radicals formed in the water by

ultraviolet light stimulated photolysis of hydrogen peroxide. Therefore the highest efficiency

of spark mode characterised by the highest intensity of emitted light has been found in

experiments.

Acknowledgements Part of this work was supported by a Grant-in Aid for Scientific Research of the Ministry of Education, Science, Sport and Culture, Japan, #11555205 and 12895021. References

[1] N.H. Ince, D.T. Gonenc, Treatability of textile azo dye by UV/H2O2, Environ. Technol.18

(1997) 179-185.

[2] S.F. Kang, H.M. Chang, Coagulation of textile secondary effluents with fenton’s reagent,

Wat Sci. Technol. 36 (1997) 215-222.

[3] C.H. Liao, M.D. Gurol, Chemical oxidation by photolytical decomposition of hydrogen

peroxide, Environ. Sci. & Technol. 29 (1995) 3007-3014.

[4] W.G. Kuo, Decolorizing dye wastewater with fenton’s reagent, Wat. Res. 26 (1992)

881-886.

[5] E.G. Solozhenko, N.M. Soboleva, V.V. Goncharuk, Decolourization of azodye solutions by

Fenton’s oxidation, Wat. Res. 29 (1995) 2206-2210.

[6] A.K. Sharma, B.R. Locke, P. Arce, W.C. Finney, A preliminary study of pulsed streamer

corona discharge for degradation of phenol in aqueous solution, Hazard. Was. Hazard.

Mater. 10 (1993) 209-219.

[7] B. Sun, M. Sato, J.S. Clements, Environ. Sci. & Technol. 34 (2000) 509-513.

[8] A.A. Joshi, B.R. Locke, P. Arce, W.C. Finney, Formation of hydroxyl radicals, hydrogen

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peroxide and aqueous electrons by pulsed streamer corona discharge in aqueous solution, J.

Hazard. Materials 41 (1995) 3-30.

[9] B. Sun, M. Sato, J. S. Clements, Optical study of active species produced by a pulsed

streamer corona discharge in water, J. Electrostatics 39 (1997) 189-202.

[10] O. Legrini, E. Oliveros, A.M. Braun, Photochemical processes for water treatment, Chem.

Rev. 93 (1993) 671-698.

[11] D.M. Willberg, P.S. Lang, R.H. Hochemer, A. Kratel, M.R. Hoffmann, Degradation of

4-chlorophenol, 3,4-dichloroanilin, and 2,4,6-trinitrotoluene in an electrohydraulic

discharge, Environ. Sci. Technol. 30 (1996) 2526-2534.

[12] Anto Tri Sugiarto, M. Sato, Thin Solid Films, 386 (2001) 295-299

[13] J.S. Chang, P.C. Looy, K. Urashima, A.D. Bryden, K. Yoshimura, Proceedings of

Asia-Pacific Workshop on Water and Air Treatment by Advanced Oxidation Technology,

Tsukuba, Japan, 1998, 148-153.

[14] W.S. Perkins, Oxidative decoloration of dyes in aqueous medium, Textile Chemist and

Colorist 1(4) (1999) 33.

[15] F.S. Kang, C.H. Liao, S.T. Po, Chemosphere, 41 (2000) 1287-1294.

[16] J.M. Winterbottom, Z.A. Khan, P. Boyes, S. Ramahasay, Environmental Progress 16 (2)

(1997) 125.

[17] P. Bårmann, S. Kröll, A.J. Sunnesson, Spectroscopic measurements of streamer filaments in

electric breakdown in a dielectric liquid, J. Phys. D: Appl. Phys 29 (1996) 1188.

[18] P. Lukes, P. Sunka, M. Clupek, V. Babicky, V. Janda, J. Novak, J.D. Skalny, Proceedings of

the Sixth International Conference on Advanced Technologies for Water and Air

Remediation, London, Ontario, Canada, June 26-30, 2000.

[19] Uygur A., An overview of oxidative and photooxidative decolorisation treatments of textile

waste waters, J. Soc. Dyes Colourists 113 (1997) 211-217.

[20] Chen F., Xie Y., He J., Zhao J., Photo-Fenton degradation of dye in methanolic solution

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under both UV and visible irradiation, J. Photochemistry and Photobiology A: Chemistry

138 (2001) 139-146.

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Figure Captions

Fig. 1 Schematic diagram of the experimental apparatus

Fig. 2 Decoloration of aqueous dye solutions by pulsed discharge treatment

Fig. 3 Decoloration of aqueous Rhodamine B solution by various pulsed discharge

modes

Fig. 4 Decoloration efficiency of aqueous Chicago sky blue solution by various

pulsed discharge modes as a function of initial pH

(Applied voltage = 20 kV, Treatment time = 30 min, Dye concentration = 0.01 g/L)

Fig. 5 Spectrum of ultraviolet emission of various pulsed discharge modes in distilled

water

Fig. 6 Decoloration of aqueous Rhodamine B solution by various pulsed discharge

modes in the presence of hydrogen peroxide

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Fig. 1 Schematic diagram of the experimental apparatus

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Fig. 2 Decoloration of aqueous dye solutions by pulsed discharge treatment

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Fig. 3 Decoloration of aqueous Rhodamine B solution by various pulsed discharge

modes

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Fig. 4 Decoloration efficiency of aqueous Chicago sky blue solution by various

pulsed discharge modes as a function of initial pH

(Applied voltage = 20 kV, Treatment time = 30 min, Dye concentration = 0.01 g/L)

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Fig. 5 Spectrum of ultraviolet emission of various pulsed discharge modes in distilled

water

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Fig. 6 Decoloration of aqueous Rhodamine B solution by various pulsed discharge

modes in the presence of hydrogen peroxide