Degradation of Organic Contaminants in Water by Plasma

8/12/2019 Degradation of Organic Contaminants in Water by Plasma

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I. INTRODUCTION

Recently, contamination of environmental waterbecomes a big problem. Groundwater, river, and lakesare polluted by human activities and industrial wastes.Chemical and/or biological treatments have been widelyused to purify the water, but there are no suitabletechnologies to decompose recalcitrant organic matter.The hydroxyl radical is the most powerful oxidant todecompose organic materials, and is produced easily due

to underwater discharge plasma or gaseous plasma.Underwater plasma is produced due to the electrical

breakdown of water, when the electric field concentratesto the needle tip [1]. Many kinds of active species were

detected from the streamer discharge by emissionspectrum [2,3]. The active species decompose organicmaterials in water, finally into carbon dioxide and waterthrough intermediate materials. Formation mechanism ofthe underwater plasma was considered that the initialdischarge could start in a small bubble on the needleelectrode surface, and then could propagate into waterphase. Introducing gas bubbles into the plasma regioncould decrease discharge voltage [4]. Energy efficiencyto decompose organic materials is depended on the

reactor design, i.e., (a) gas bubbling through metal tubeelectrode [5], (b) gas bubbling through porous ceramictube [6,7], (c) discharge through pinhole [8,9], (d) water

surface plasma and wetted-wall plasma reactor [10,11],or hybrid type reactor [12], and so on [13]. Electricalbreakdown or disruption of a biological cell by pulsedelectric field (PEF) is well understood to occur byelectromechanical compression of the cell membrane,which results in inactivation of microorganisms. Theeffective sterilization by using PEF-induced irreversibledisruption of biological membranes was investigated andreported. The treatment temperature, growth temperature,

and the shape of the reactor were found to have a great

effect on PEF sterilization. The PEF-induced reversibledisruption of the membrane could be utilized for

selective release of intracellular proteins from a certaingene-engineered E. coli. The secretion of periplasmicprotein from E. coliwas achieved during cultivation. Inaddition to the cell inactivation, chromosomal DNA andRNA molecules were decomposed, and activation andinactivation of enzymes would be possible by PEFtreatment. The outlines of some environmentalapplications using high voltage pulsed plasma in water orin gas phase are mentioned briefly in the present paper.

Degradation of organic contaminants in water by plasma

M. Sato

II. PURIFICATION OF ENVIRONMENTAL WATER BYPLASMA

A. Underwater plasma

Using underwater electrical discharges in point-to-plane electrode geometry, magenta colored streamer wasobserved. The streamer length was varied with varyingelectrical conductivity of water, having a plateau at about0.01 mS/cm of the water conductivity. The streamer

length increased as the pulse width increased. When thestreamer touched to the ground electrode, the discharge

mode changed to spark discharge. Emission spectra fromH, OH, O radicals were detected as shown in Fig. 1. Fig.2 shows time profile of voltage and current in the case of

Professor Emeritus, Gunma University, Japan

AbstractAtmospheric pressure plasma in water or in gaseous media is applicable to decompose organic materials inenvironmental water and kill bacteria in waste water. Plasma has a characteristic feature that all kinds of organic

materials including recalcitrant matter are decomposed due to active species produced by plasma. The plasma is generated

by pulsed discharge in water, on the water surface, and in the bubbles in water. For applying to industrial use, the energy

efficiency for generating plasma will be the most important factor. To increase the total energy efficiency (decomposed

molecules / input energy), a hybrid system such as plasma-biotechnology or plasma-chemical reaction would be one of the

solutions.

Keywordsunderwater plasma, water surface plasma, contaminant degradation, water purification

Corresponding author: Masayuki Sato

e-mail address: [email protected]

Accepted; March 25, 2009

Fig. 1. Emission spectrum from pulsed discharge in water withpoint-to-plane electrode configuration [5].

8 International Journal of Plasma Environmental Science and Technology Vol.3, No.1, MARCH 2009


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spark discharge. The streamer discharge started after thefast rising pulse voltage was applied to the electrode,showing a small current. When the streamer channelapproached to the opposite ground electrode, the sparkdischarge occurred, where the discharge currentincreased rapidly up to 320 A. After that the currentbecame zero, because the pulse power source was acapacitor discharge type.

Hydrogen peroxide was produced by the streamerdischarge in water [2,14-17]. Variation of hydrogenperoxide concentration due to the streamer discharge in

distilled water with and without addition of catalase isshown in Fig. 3. In this measurement, the total amount of

oxidizing agents, such as ozone, hydrogen peroxide, andothers was measured after pulse treatment by abiochemical test method using enzymes.

Discharge mode changed with changing the distancebetween point-to-plane electrode; with 6 mm for sparkdischarge, 15 mm for spark with streamer dischargecombined, and 45 mm for streamer discharge [18]. Theapplied voltage was 20 kV with a frequency of 50 Hz. Alittle KCl was added into the solution in order to change

the liquid conductivity. Illustrations of three dischargemodes are shown in Fig. 4. In the case of streamerdischarge (a), there are many magenta colored plasma

channels formed in the liquid. In the case of spark

discharge (c), a single plasma channel is formed in theliquid, and their energy is very strong, which seems to bea high intensity ultraviolet light source without any wallto interrupt the penetration into the water. During theformation of plasma channels, a strong shock wave is

generated in liquid. The high energy electrons in theplasma channels, strong ultraviolet radiation and shock

waves are very effective in exciting and ionizing thewater molecules, and therefore, more radicals are formedin the spark discharge compared to the streamerdischarge. On the other hand, in the case of spark withstreamer discharge mode, there are many plasmachannels produced. Their energy seems to be strong, and

the channels are more stretched than in the other cases.Since the distribution of active species occurred mainlyaround the plasma channels [19], a large number ofchannels with a strong energy would be very effective togenerate a large number of active species. As shown inFig. 5, decoloration of Rhodamine B was very effectivein the case of streamer with spark discharge mode than

the other cases. In the spark mode, the addition ofhydrogen peroxide is effective to raise energy efficiencyof decoloration. The electrical energy of 360 J/mL wasnecessary for 80 % decoloration without hydrogenperoxide addition, but the energy efficiency wasimproved to 70 to 80 J/mL with the addition of hydrogen

peroxide. In the presence of hydrogen peroxide, thedecomposition rate of phenol was very high in the case of

Fig. 4. Illustration of three discharge modes in water, where (a)

streamer (electrode distanceL: 45 mm), (b) spark with streamercombined (L: 15 mm), (c) spark (L: 6 mm) [18].

Fig. 5. Decoloration of aqueous Rhodamine B solution by three

discharge modes with and without addition of hydrogen peroxide,where concentration of Rhodamine B and hydrogen peroxide: 0.01

g/L and 8.8x10-3mol/L [26].

Fig. 2. Wave patterns of voltage and current in the case of spark

discharge in water using point-to-plane electrode configuration [19].

Fig. 3. Varying hydrogen peroxide concentration by applied pulse

energy with and without catalase addition, where pulse voltage: 19kV, in distilled water [14].

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spark discharge. This is because the spark dischargeradiates very strong ultraviolet light compared to theother discharge modes. Approximately 30 % of theplasma energy is radiated in the UV spectrum accordingto Robinson [20], and then a large number of hydroxylradicals can be produced in the reactor (Eq. 1).

H2O2+ h= OH + OH (1)

Phenol decomposition was the same tendency in reactionrate of the decoloration.

B. Simultaneous discharge in gas bubbles and in water

As shown in Fig. 6, pulsed discharge plasma wasgenerated in the gas phase, and the produced plasma waspermeated into the water phase through the pinhole [6].Water (upper) and gas (lower) were separated by aninsulating plate with a pinhole, through which gas was

bubbled into the water phase. In the gas phase, the high-voltage pulse was applied between the needle electrodeand the ground electrode (immersed in the water phase).The pulsed discharge plasma was generated in the gasphase; simultaneously, the plasma channels were

permeated into the water phase accompanied by the gasbubbles. Fig. 7 shows an example of simultaneous

discharge in liquid phase at 20 kV applying pulse voltagewhen oxygen gas was bubbled through the pinhole at therate of 100 mL/min. The streamer channels are stretched

into the water phase accompanying with the gas bubbles.To increase the processing fluid volume, a porous

ceramic tube (average pore size: 15 m, wall thickness: 4mm) was used as a gas disperser instead of a pinhole asshown in Fig. 8. The discharges spread over the ceramictube surface and formed many streamers in the water

phase. The streamer and the active species decomposeorganic materials. The aqueous Chicago sky bluesolution with 10 ppm initial concentration was decolored

by about 95 % in 10 min treatment (using pinholesystem). The decoloration rate increased with increasing

electrical conductivity of the solution. The decolorationrate became higher with increasing applied voltage,because the discharge intensity was getting stronger withthe higher voltage. However, as shown in Fig. 9, theenergy efficiency for decoloration in each appliedvoltage (plots of decoration rate versus pulse energy)falls on almost the same line.

The effect of electrical conductivity of the solution to

the decoloration rate is shown in Fig. 10, where theoxygen gas was bubbled with 150 mL/min, applied

pulsed voltage was 15 kV, and initial dye concentrationwas 10 ppm. As shown clearly in the figure, the

decoloration rate increased with increasing conductivityof the aqueous dye solution. It was reported in theprevious papers, in the case of producing pulsed

stainless steel mesh

porous ceramic tube

ground

to HVP

gas inlet

water inlet

gas and water mixture

Fig. 8. Porous ceramic tube reactor, where treatment cell: 60 mm

height, 50 mm inside diameter; average pore size: 15 m, wall

thickness of ceramic tube: 4 mm.

0 100 200 300 400 500 600

0

20

40

60

80

100

Energy [J/mL]

Decolorationrate[%]

+10 kV +25 kV +15 kV +20 kV +8 kV

Fig. 9 Percentage decoloration of Chicago sky blue with varyinginput pulse energy and applied voltage (100 mL/min oxygen

bubbling).

HV pulse

PVC thin plate

ground

Fig. 6. Extended view of the treatment cell of simultaneousdischarge in gas bubbles and in water, where treatment cell: 60 mm

height, 50 mm inside diameter; PVC thin plate: 2 mm thickness with0.5 mm diameter pinhole [6].

Fig. 7 Photograph of discharge state in water phase with oxygen gasbubbling, where gas flow rate: 100 mL/min, applied voltage: +20kV.



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discharge directly in water using needle-plate electrodeconfiguration, the electrical current flowing through the

electrode increased greatly with increasing theconductivity of water [5]. The streamer length decreased

with increasing conductivity, which resulted in thedecoloration rate decreased. From the figure, thedecoloration rate reached to almost 100 % at 20 minutes

treatment in the case of 500 - 1000 S/cm of the solutionconductivity. Therefore, when we apply to treat tap wateror sewage water having conductivity of 200 to 2000

S/cm, this system is considered to have a characteristicfeature to raise energy efficiency for degradation of

organic contaminants in water.

C. Water surface plasma

For decomposition of organic contaminants in water,the gas phase corona or streamer discharge on the watersurface have been reported [11,21]. The corona dischargeon the water surface produces some kinds of activespecies that effectively react with organic materials inwater. The streamer discharge on the water surface isalso effective for decomposing organic materials in water

due to their active species and UV light, and high-energyelectrons hit the water surface. As illustrated in Fig. 11

and shown photo in Fig. 12, filamentary discharge fromthe needle tip in the gas phase on the water surface wasobserved. Ozone and active species produced by thedischarge may dissolve into water through the watersurface, and then they were decomposed to some kinds ofradicals or hydrogen peroxide. High-energy electronsmay dissociate water molecules then produce radicals.Many chemical reactions may occur by water surfaceplasma, which may decompose organic contaminants inwater. When electrode distance between the needle tip

and the water surface was adjusted to 5 mm, the watersurface plasma discharge could be obtained in both argon

and oxygen gases. In argon gas, bright blue colored

discharge streamers were seen and were accompanied bypopping sounds. On the other hand, rather weak watersurface plasma was obtained in oxygen gas. The percent

decomposition increased with increasing treatment timeand was higher for oxygen than for air, because theelectrical discharge produced much more ozone inoxygen than in air. The generated ozone probablydissolves in the water through the surface layer and then

reacts with the phenol by hydroxyl radicals or otheractive species converted from the dissolved ozone.

Fig. 11. Illustration of water surface plasma.

Fig. 12. Discharge state (Argon: 1 L/min, Pulse voltage: +25 kV,distance between needle tip and water surface: 8 mm).

0 10 20 30 40 50 60

0

20

40

60

80

100

Treatment time [min]

Decoloratio

nrate[%]

S/cm1000

50010010

Fig. 10 The effect of electrical conductivity of aqueous dye solution,where initial concentration of Chicago sky blue: 0.01 g/L, applied

voltage: 15 kV, oxygen gas bubbling with 150 mL/min.

Among the various discharge modes for

decomposing phenol in water, Fig. 13(a) shows thepercent phenol decomposition using oxygen as a functionof elapsed treatment time for three different dischargemodes obtained by changing the electrode distance: 1)water surface plasma (WSP), 2) streamer, and 3) corona

discharge. The phenol decomposition increased withincreasing treatment time, particularly in the water

surface plasma mode. On the other hand, when thedecomposition was plotted against the consumed energy,as shown in Fig. 13(b), the points for the three modeswere all on the same curve. Each discharge mode such ascorona, streamer, and water surface plasma may havedifferent decomposition mechanisms. The corona and

streamer discharge modes can decompose organicmaterials with low energy, but a long treatment time isrequired. On the other hand, in the water surface plasmamode, the decomposition rate is higher than the other

discharge modes, but it requires more energy for watertreatment.

The water surface plasma on the horizontal watersurface was applied to the vertical water film in wetted-wall reactor [10]. As shown in Fig. 14, it is expected thatthe reaction area could be larger, liquid volume fortreatment could be larger, and the processing time couldbe shorter in the wetted-wall reactor. The pulseddischarge plasma occurred between the center electrode

and ground electrode, where the positive pulsed highvoltage was applied to the center electrode. The sample

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liquid was circulated by peristaltic pump, in which thewater fell down along with the inner surface of thecylinder forming wetted-wall. Gas was introduced fromthe upper part of the reactor and out from the bottom as

gas-water mixture.Typical types of reactors are shown in Fig. 15(a) and

(b). Reactor (a) is made of aluminum cylinder and centerelectrode with 1 x 1 mm tungsten square wire. Innersurface of the cylinder was treated by corona dischargebefore starting the experiment to increase contact anglebetween aluminum and water. In reactor (d), the disk

electrode is set at the middle of the cylinder to makeplasma easier in the oxygen gas, and grounded electrodesare glued at the top and the bottom of the Plexiglas

cylinder. Reactor (a) shows the basic structure of ourstudy, in which square wire are allocated at the center of

cylindrical ground electrode. Using a reactor (a), whenthe gas content was varied, discharge mode changed as:(1) in the case of argon, streamer-like discharge occurredfrom the center wire electrode to the water surface, anddecomposition rate of phenol was high (85 %decomposition at 60 min treatment). However, thestreamer mode easily shifted to sparking mode, becausethe streamer discharge paths could touch to the surface ofthe aluminum cylinder through the thin water film. (2) inthe case of air or oxygen, the discharge mode was corona,

and the decomposition rate in the case of oxygen was alittle less than the above case (1). Using a disk plate with

sharp edge as a discharge electrode (d), it was possible tokeep streamer mode even in the case of air, oxygen, andargon gases. In the streamer mode, the decomposition

rate of phenol was higher in argon (40 % after 60 min)than the case of oxygen (25 %). It was because thedischarge channels strike the water surface and produceactive species by direct dissociation of water molecules.In order to check the effect of electrical conductivity ofthe liquid on the decomposition of phenol, theconductivity was varied with adding NaCl to the liquid.

Effective decomposition of dissolved phenol could beachieved independently with the solution conductivity, so

that the present wetted-wall reactor is applicable to treattap or wastewaters.

0 10 20 30 40 50 60

20

40

60

80

100

Phenoldecomposition

[%]

Treatment time [min]

(a) (b)

Fig. 15. Two types of wetted wall reactor.

Fig. 16. Variation of phenol decomposition rate with some wettedwall reactor types and gas phase.

0

: WSP: Streamer: Corona

Oxygen : 1 [L/min]

0.1 1 10 100

20

40

60

80

100

: WSP: Streamer: Corona

Oxygen : 1 [L/min]

Input energy [J/mL]

Phenoldeco

mposition

[%]

0

Fig. 13. Percent phenol decomposition for three kinds of discharge

modes with an oxygen gas flow, where oxygen gas flow rate: 1L/min, pulse frequency: 100 Hz for water surface plasma (WSP) and

600 Hz for streamer and corona discharges, electrode distance: 7.5mm for WSP, 15 mm for streamer, and 20 mm for corona.

High voltage pulse

Gas in

Water in

Wetted-wall

Gas and

water out

High voltage pulse

Gas in

Water in

Wetted-wall

Gas and

water out

Fig. 14. Illustration of wetted-wall plasma reactor [10].

Fig. 16 shows comparison of decompositionefficiency of phenol with varying reactor types (a) to (d)

in the case of argon and oxygen, respectively. Dischargemodes were corona (oxygen gas in reactor (a)) andstreamer (other cases). In the case of reactor (a), the



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decomposition rates in argon and oxygen were almost thesame, though the strength of plasma was quite differentbetween corona in oxygen and streamer in argon. Coronadischarge in oxygen would generate ozone efficiently,and the ozone seemed to contribute the decomposition ofphenol in water. Reactor (a) was most effective for

phenol decomposition in the cases of argon and oxygenas the environmental gases. These systems can beutilized to not only the water treatment but alsosimultaneous treatment of water and gas at the same time.

III. BIOTECHNOLOGICAL APPLICATIONS OF HIGHVOLTAGE PULSE

It is well known that the high voltage pulsed electric

field can kill bacteria in liquid by its field effect on the

cell membrane disruption. However, energy efficiency of

these technologies has been low as aiming for the

industrial applications to food industry. The inactivationrate was considerably improved by convergence of

electric field. Among some kinds of electrode systems,

i.e., plate-plate, insulated plate-plate, needle-plate, ring-

cylinder, coiled wire-cylinder, and double spiral wire, the

double spiral wire was more effective than the others,

which was about four orders of magnitude higher than

the plate-plate system in the survival ratio reduction

[22,23]. Some factors that influence to the pulsed electric

field inactivation have been investigated such as: (a)

liquid temperature, (b) liquid conductivity, (c) liquid

flow rate (flow velocity and residence time), (d)

electrode shape and configuration, (e) electric field

strength and input pulse energy, (f) additives, and others.

The author reported that the intracellular proteins are

released from recombinant biological cell selectively by

using high voltage electric field [24]. The mechanism of

the selective release is a controlled disintegration of the

cell membrane by pulsed electric field. Extracellular

release of recombinant -amylase from recombinant E.

coli was also possible by applying high voltage pulsed

electric field during fed-batch cultivation [25]. When the

pulsed electric field was applied intermittently (12 kV, 3

Hz, applying 30 min with an interval of 30 min) from the

beginning of the stationary phase of cultivation, the

amount of -amylase released was about 30 % of the

total amount of -amylase produced in the cells. The

release ratio and the total amount of -amylase extracted

from the periplasm were higher than that of batch

cultivation system.

IV. CONCLUSION

The pulsed plasma produced in water and/or in

gaseous phase to purify environmental water has

characteristic feature that organic contaminants are

decomposed and finally converted into carbon dioxideand water in a relatively short period. Bacteria are

inactivated under normal temperature by pulsed electric

field, which is applicable to fresh liquid food sterilization.

The energy efficiency to decompose organic materials in

water is still not high, but it will be much better in the

near future. The author proposes a combination of

plasma and other methods i.e., biological or chemical

methods, where the organic molecules are decomposedinto an intermediate state due to plasma and then

degraded into carbon dioxide and water by bacteria as an

environment-friendly technology.

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