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1536 © IWA Publishing 2013 Water Science & Technology | 68.7 | 2013

Pulsed corona discharge: the role of ozone and hydroxyl

radical in aqueous pollutants oxidation

S. Preis, I. C. Panorel, I. Kornev, H. Hatakka and J. Kallas

ABSTRACT

Ozone and hydroxyl radical are the most active oxidizing species in water treated with gas-phase

pulsed corona discharge (PCD). The ratio of the species dependent on the gas phase composition

and treated water contact surface was the objective for the experimental research undertaken for

aqueous phenol (fast reaction) and oxalic acid (slow reaction) solutions. The experiments were

carried out in the reactor, where aqueous solutions showered between electrodes were treated with

100-ns pulses of 20 kV voltage and 400 A current amplitude. The role of ozone increased with

increasing oxygen concentration and the oxidation reaction rate. The PCD treatment showed energy

efficiency surpassing that of conventional ozonation.

doi: 10.2166/wst.2013.399

S. Preis (corresponding author)I. C. PanorelH. HatakkaJ. KallasLaboratory of Separation Technology, LUT

Chemistry,Lappeenranta University of Technology,P.O. Box 20,Lappeenranta 53851,FinlandE-mail: sergei.preis@lut.fi

I. KornevHigh Technology Physics Institute,Tomsk Polytechnic University,2A Lenin Ave.,Tomsk,Russia

Key words | electric discharge, energy efficiency, oxalate, phenol, water treatment

INTRODUCTION

An interest towards intense cost-effective technologiesable to oxidize refractory pollutants in water/wastewater

treatment is growing due to accumulation and diversifica-tion of hazardous substances in the environment, andtightening standards for water supply and wastewater dis-

posal. Advanced oxidation processes (AOPs) arepowerful, human-friendly and effective means for watertreatment, although the excessively expensive character

of ozone production and application makes it a privilegeof industrially developed countries. The way to promoteAOPs in water treatment technology is the substantialimprovement of their cost efficiency. One of the AOPs is

the application of electric discharges to oxidation–reduction reactions with short-living powerful oxidants,such as hydroxyl radical (OH), ozone (O3) and atomic

oxygen (O), generated directly in the treated water or atthe gas–liquid interface. Previous studies (Yavorovskyet al. ; Chauhan et al. ; Hoeben et al. ;

Kornev et al. ) showed the concentration of gas-phase discharges in close vicinity of the gas–liquid inter-face, where the short-living oxidants generated in the

discharge react with pollutants in the boundary layer ofwater. Various discharge systems were proposed and arestill under study: spark discharge in gas bubbles (Anpilovet al. ), pulsed streamer discharges in liquid and gas-

bubbled reactors with pulses at micro-second diapason

(Shih & Locke ; Ruma et al. ), gas-phase dielectricbarrier discharge of various configurations (Yavorovsky

et al. ; Malik & Schoenbach ; Marotta et al.), plasmotrons utilizing gliding arc discharge for bom-bardment of treated surfaces with ionized gas (Locke &

Thagard ; Benstaali et al. ; Merouani et al. ),pulsed corona discharge (PCD) over the water surface(Grabowski et al. ; Magureanu et al. ), flash

corona over the water surface (Aristova & Piskarev), and PCD in water aerosol (Pokryvailo et al. ).The authors earlier proposed PCD (Panorel et al. ),where water was dispersed in gas in the form of droplets,

jets and films sized up to a few millimetres, forming a suf-ficient interface treated with PCD in a cost-effectivemanner.

The described method showed the energy efficiencyexceeding that of traditional ozonation by a few times(Panorel et al. ) using simple equipment: the pulsed dis-

charge, unlike ozone synthesis in conventional ozonegenerators, is insensitive towards gas humidity; the reactorclosed compartment, in which the treatment takes place,

makes the residual ozone destruction minor, if air is used,or unnecessary with an oxygen-enriched gas.

The PCD treatment of water is often offering an energy-efficient alternative to conventional ozonation forming

ozone and OH radicals in humid air (Ono & Oda )

1537 S. Preis et al. | Pulsed corona discharge: ozone and hydroxyl radical Water Science & Technology | 68.7 | 2013

and most likely on the treated water surface thus involving

water itself in the oxidation:

�eþH2O> �eþH� þOH� (1)

Hydroxyl radicals and ozone may directly oxidize aqu-

eous pollutants in the vicinity of the gas–liquid interface.In the bulk of water the dissolved ozone contributes to oxi-dation directly or decomposes via formation of OH radical

(von Gunten ). The ratio of ozone and direct OH radicalcontributions may determine the oxidation chemistry andkinetics and the choice of treatment parameters, i.e. pulse

repetition frequency and the gas composition, resulting ulti-mately in the treatment cost. The present research had anobjective of establishing the ratio of contributions of ozoneand OH radicals directly formed from water to oxidation

of rapidly reacting phenol and slowly reacting oxalate.

MATERIALS AND METHODS

The experimental system consists of a PCD reactor and ahigh voltage (HV) pulse generator shown in Figure 1. The

reactor utilizes wire-plate corona geometry: horizontal elec-trode wires are placed between vertical earthed plateelectrodes. The electrodes’ geometry parameters determin-ing the pulse characteristics (Briels et al. ) were

chosen for the maximum pulse energy; the electrodes weremade of stainless steel wire of 0.5 mm diameter, positioned

Figure 1 | Experimental setup outline.

at 17 mm from the vertical grounded plate electrodes with

the distance of 30 mm between the HV electrodes. Thetotal length of the HV electrodes was 32 m in 0.5-m sections,i.e. 64 electrodes were positioned between two plates sized

0.5 × 2.0 m. The volume of the discharge zone of the reactorthus was 34 L. Water is fed to the top of reactor, where it isdispersed through a perforated plate producing jets, dropletsand films. Water showers between electrodes to the zone of

gas-phase PCD formation, where the treatment with oxi-dants takes place. The power supply generates thedischarge pulses of voltage pulse amplitude of 18–20 kV,

current of 380–400 A, and 100 ns duration at pulse rep-etition frequency from 100 to 840 pulses per second (pps)(see Figure 2). The pulse parameters were registered with

the Agilent 54622D oscilloscope. The energy delivered tothe reactor, calculated as an integral product of voltageand current peak areas, was 0.30–0.33 J per pulse. Theenergy consumption efficiency of the pulse generator was

67%.The experiments were performed using 50-L samples of

circulating solutions, if not stated otherwise. Oxygen and

nitrogen were delivered to the reactor using the port in thetank cover. Oxygen content in the gas phase was measuredusing a Servomex 570A oxygen analyzer.

Phenol and oxalate were dissolved in water at ambienttemperature remaining at 18–20 WC in experiments; thetemperature of the solution stabilized at about 20–22 WC in

equilibrium with the ambient temperature. pH was adjustedwith sodium hydroxide. The operating parameters included

Figure 3 | Oxidation efficiency of phenol vs oxygen content in the gas phase: phenol

initial concentration 100 mg L�1, initial pH 11, efficiency assessed at 50%

phenol removal.

Figure 2 | Voltage and current oscillograms of the pulse.

1538 S. Preis et al. | Pulsed corona discharge: ozone and hydroxyl radical Water Science & Technology | 68.7 | 2013

the phenol content of 100 mg L�1 and oxalic acid content

from 100 to 1,000 mg L�1, oxygen concentration in airfrom 0 to 90% vol., circulating water flow rate 3, 5, 7, 10and 15 L min�1. The gas–liquid contact surface wasmeasured at corresponding water flow rates by the classical

method of sulphite oxidation with air oxygen in the presenceof cobalt sulphate catalyst.

The reactor was open to the atmosphere through a 5-mm

port for the pressure equalization. This port showednegligible impact when no reaction took place: water circu-lation in the reactor filled with 90% oxygen showed no

change in oxygen content for 6 h. The replacement ofoxygen consumed in reaction with ambient air allows pre-cise calculation of oxygen consumption using the reading

of an oxygen gas analyzer.Ozone concentration in the gas was measured iodo-

metrically by blowing the exact amount of the gas, 1 L,from the reactor through a Drexel trap filled with acidified

potassium iodide solution. The free iodine was titratedwith 0.1-N sodium thiosulphate solution. Ozone concen-tration in water was also measured iodometrically.

Attempts to measure the content of hydrogen peroxidewere also undertaken using the titanyl sulphate methoddescribed by Eisenberg (). The results of analysis,

however, showed negligible contents, in amounts smaller

than 1 mg L�1, of ozone and hydrogen peroxide in samplestaken immediately from the bottom of the reactor even inMillipore water.

Phenol was chosen as the reference substance for theenergy efficiency analysis as there is substantial literatureto support its use (Grabowski et al. ; Marotta et al.). Oxalate is known to be a refractory compound with

slow oxidation rate. Phenol concentration was determinedby the 4-nitroaniline method (Leithe ), oxalate concen-tration was matched with the total organic carbon (TOC)

determined using a Shimadzu 5050 TOC analyzer. Chemi-cal oxygen demand (COD) was measured by thepotassium dichromate closed reflux colorimetric method

(Standard Methods ).

RESULTS AND DISCUSSION

Energy efficiency in oxidation

The oxidation efficiency of phenol, zero in 100% nitrogen,increased with oxygen volumetric concentration, approxi-mating to a constant level at 60% vol. with further oxygen

concentration growth at about 120 g phenol per 1 kWh ofdelivered energy at 840 pps and 140 g kWh�1 at 100 pps inalkaline solutions with starting pH 11 (Figure 3). The effi-

ciency in air at oxygen concentration of 20.5% vol. rangedfrom 55 to 88 g kWh�1 at 840 and 100 pps respectively,noticeably surpassing conventional ozonation efficiency

(Krichevskaya et al. ). These numbers were obtainedfor 50% degradation of phenol in 100-ppm solutions. Theeffect of the reduced frequency diminishes with the

increased oxygen concentration indicating probably a

1539 S. Preis et al. | Pulsed corona discharge: ozone and hydroxyl radical Water Science & Technology | 68.7 | 2013

bigger contribution of ozone in oxygen-enriched gas. Ozone,

however, plays a more significant role in air observed in thenoticeable improvement in PCD efficiency at decreased fre-quency: lower pulse repetition frequency likely allows

longer-living ozone to participate in oxidation betweenpulses. Pulse parameters did not vary with the variation ofpulse repetition frequency.

Alkaline medium, as expected, was more beneficial

than acidic (initially neutral pH decreased during thecourse of the treatment from 6.5–7.0 to about 4.0) inphenol oxidation by a factor of about two. The efficiency

in mineralization of phenol, i.e. in TOC degradationranged from 5 g C kWh�1 in air to about 10 g C kWh�1 in90% oxygen in alkaline solutions.

Oxidation efficiency of oxalate in air under thedescribed conditions was independent of pH from 3 to 11showing the mineralization efficiency of about 5–10 g CkWh�1 dependent on the oxalate initial concentration

(100–800 mg L�1). The energy efficiency of oxalate oxi-dation was invariant with the pulse repetition frequency,indicating possibly a minor role of longer-living ozone in

the reaction.

Contact surface and efficiency correlation

The flow rate of treated phenolic solution was varied from 3

to 15 L min�1, which, relating to the cross-section of thePCD zone, corresponds to the surface velocity from 10.4to 52.9 m h�1. The contact surface determined by the sul-

phite method (Danckwerts ) was showing lineargrowth with the flow rate from 9 to 43 m�1 respectively.The phenol oxidation efficiency reaches its maximum at

20 m�1, remaining constant with further growth of circulat-ing water flow and the contact surface (Figure 4). This

Figure 4 | Oxidation efficiency of phenol vs gas–liquid contact surface: phenol initial

concentration 100 mg L�1, initial pH 11, efficiency assessed at 50% phenol

removal, pulse repetition frequency 840 pps.

indicates the discharge power, not the contact surface,

being a restraining factor in oxidation kinetics over the con-sidered surface limit.

The pulse parameters practically did not change with

variation of wetting density. The detailed study of theimpact of wetting with conductive solutions on the pulseparameters can be found in Kornev et al. ().

Surface reaction

The immediate mineralization of phenol, i.e. TOC degrada-tion, was observed from the start of treatment, witnessed

indirectly by the surface character of the oxidation reaction(Figure 5): immediate mineralization is not characteristic ofphenol oxidation in a bulk solution with ozone. The hypoth-

esis of the surface reaction was also supported by the role ofOH-radical scavenging agents added to the treated solution.The addition of the well-known OH-radical scavenger tert-butyl alcohol (TBA) to the treated solution did not have aneffect even at the TBA concentration exceeding that ofphenol by a factor of 10. At the same time, the addition of

a non-ionic surfactant, igepal C-630 (2-[2-(4-nonylphenoxy)ethoxy] ethanol, C19H32O3) in concentration equimolarwith phenol, resulted in a noticeable slowdown of oxidationin neutral–acidic solutions (Figure 6). This indicates the sur-

face character of radical attack screened by the surfactant.None of the used radical scavengers is oxidized by PCD.

The role of ozone and OH radical in oxidation

Phenol oxidation

Oxygen in the gas space of the reactor is mostly consumedin the synthesis of ozone with the further reaction of the

Figure 5 | Mineralization of phenol: initial pH 11, applied power 250 W.

Figure 6 | Phenol degradation in presence of igepal C-630 surfactant radical scavenger:

air; initial pH 6.0; legend gives concentration of surfactant; phenol initial

concentration 1 mM, applied power 150 W.

1540 S. Preis et al. | Pulsed corona discharge: ozone and hydroxyl radical Water Science & Technology | 68.7 | 2013

latter. If oxidation takes place on account of ozone only, theCOD degradation should be equal to oxygen consumptionfrom the reactor’s gas phase. The authors, however,

observed a misbalance between these two values: CODdegraded substantially faster than oxygen was consumed.

The misbalance between oxygen consumption in the reac-

tor and the decreasing COD showed a substantial contributionof OH radicals produced from water molecules. The exper-iment was carried out with 700 L of phenolic solution

treated in air (20.5% vol. of O2) and oxygen-enriched gas(78.5% vol. O2). The big volume of solution was applied tominimize the influence of changing phenol concentration,keeping it close to uniform for a longer time, and to minimize

the gas volume in the reactor, 50 L, for noticeable oxygen gasphase concentration change. The COD decrease, i.e. oxygenintroduction to the solution, surpassed the oxygen decrease

in the gas phase in the reactor 1.9 and 1.3 times in air andin the oxygen-enriched gas respectively. The contribution ofOH radicals at low concentrations of oxygen thus was substan-

tially bigger: the radicals’ share in phenol oxidation decreasedwith increasing oxygen concentration. Negligible oxidation ofphenol at zero oxygen concentration may be explained by the

absence of oxygen-scavenging H radicals formed simul-taneously with the OH radical at the interface with theirrecombination, or the OH radical formation reaction beingmore complex than the one described in Equation (1).

Oxalate oxidation

Slow oxidation of oxalic acid showed substantial contributionof OH-radicals to oxidation. The drastic difference in oxi-dation rate was observed in experiments, in which solutions

were treated with PCD and solely with ozone at the sameozone gas-phase concentrations. The experiments were

carried out in air atmosphere with the equilibrium gaseous

ozone concentration maintained at 5 mg/L for identicalperiods of time. The ozone concentration in the reactor with-out PCD application was maintained by ‘blinking’ switching

the pulse generator on for 5 s with 10-s intervals; the treatedsolution flow was turned off for the time PCD was applied.The oxidation rate with ozone alone was about five timeslower than with the PCD application. The oxidation reaction

with ozone thus contributed, under the experimental con-ditions, about 20% of the total oxidation rate.

Phenol oxidation kinetics: practical description

The degradation rate coefficients at the initial stage of reac-

tion (50% degradation), within which degradation followedthe linear pattern, were determined. The reaction rate coef-ficients were determined assuming that the combined effect

of the OH-radicals and ozone results in a second order reac-tion rate, the first order relative to the target pollutant andthe first order towards the oxidant. The second reaction

order is attributable to the majority of aqueous ozonationreactions, proceeding via ozone decomposition in water(Hoigné & Bader ; von Gunten ).

The description of the reaction kinetics is complicated

due to the unknown sum of oxidants’ concentration andtheir share in the reaction: the short-living OH-radicals pre-sumably formed at the surface of treated water are difficult

to quantify; ozone formed in the discharge is present indynamic equilibrium established between the formationand consumption rate. The indifference of the discharge treat-

ment efficiency towards the hydrodynamics ofwater flowandthe contact surface of treated solutions observed earlier(Panorel et al. ) and in this work indicates a constantamount of oxidants available momentarily in the discharge

zone. Therefore, the sum concentration of oxidants takingpart in the reaction could be characterized with an accuracysufficient for practical applications by the power P delivered

to the volume of the discharge zone V, and so used with thesecond order reaction rate constant k2:

dC=dt ¼ k2 � C � PV�1 (2)

where k2 is the second order reaction rate constant, m3 J�1; Cis the concentration of the pollutant, mol m�3; P is the pulsedpower delivered to the reactor, W; V is volume of the dis-charge zone, m3, in the experimental device: 0.034 m3.

The second order reaction rate coefficients calculated forthe initial period of treatment are given in Table 1. One can

Table 1 | The second order reaction rate coefficients for phenol degradation in alkaline

solutions

Second order reaction rate constant k2, m3 J�1

Frequency, pps Power, W Air 90% oxygen

100 30 36± 4 × 10–8 58± 4 × 10�8

200 60 34± 4 × 10�8 57± 5 × 10�8

400 120 31± 3 × 10�8 54± 3 × 10�8

600 180 27± 3 × 10�8 52± 3 × 10�8

840 250 23± 3 × 10�8 50± 3 × 10�8

1541 S. Preis et al. | Pulsed corona discharge: ozone and hydroxyl radical Water Science & Technology | 68.7 | 2013

see the oxidation rate coefficients expectedly growing withthe content of oxygen: ozone equilibrium concentration ishigher in oxygen-rich atmosphere. The second order reactionrate coefficient decreases with the pulse repetition frequency

due to the increased ozone impact at lower frequency.The proposed kinetic approach appears to be of practi-

cal value and the numerical values obtained for phenol

may be used for further references. The higher reactionrate in oxygen media shows the increasing role of ozonein oxidizing phenol.

CONCLUSIONS

The PCD method appeared to be highly effective in oxi-

dation of phenol, surpassing the closest competitor,conventional ozonation, in energy efficiency.

The short-living oxidants formed at the water surface

treated with the PCD contribute to oxidation of organic pol-lutants. The contribution of short-living oxidants decreaseswith the increased reaction rate and, for fast oxidation reac-

tions, the oxygen concentration in the gas phase.Oxidation of slowly oxidized refractory pollutants may

benefit from high pulse repetition frequencies, whereaswith rapidly reacting substances low frequency treatment

is beneficial for a larger contribution of ozone participatingin oxidation between pulses, although the oxidation ratealso decreases.

The mass transfer showed minor impact to the oxidationefficiency; the process rate is determined primarily by thepulsed power delivery rate.

ACKNOWLEDGEMENTS

This study was supported by Finnish Funding Agency for

Technology and Innovation (Tekes), projects 40418/06and 40131/08, the Academy of Finland, and the Ministry

of Education and Science of the Russian Federation, project

No. 14.B37.21.1244.

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