Water purification by electrical discharges

INSTITUTE OF PHYSICS PUBLISHING PLASMA SOURCES SCIENCE AND TECHNOLOGY

Plasma Sources Sci. Technol. 10 (2001) 82–91 www.iop.org/Journals/ps PII: S0963-0252(01)20244-8

Water purification by electrical dischargesMuhammad Arif Malik1,3, Abdul Ghaffar1 andSalman Akbar Malik2

1 Applied Chemistry Division, PINSTECH, PO Nilore, Islamabad, Pakistan2 Department of Biological Sciences, Quaid-i-Azam University, Islamabad, Pakistan

E-mail: [email protected]

Received 28 June 2000, in final form 14 December 2000

AbstractThere is a continuing need for the development of effective, cheap andenvironmentally friendly processes for the disinfection and degradation oforganic pollutants from water. Ozonation processes are now replacingconventional chlorination processes because ozone is a stronger oxidizingagent and a more effective disinfectant without any side effects. However,the fact that the cost of ozonation processes is higher than chlorinationprocesses is their main disadvantage. In this paper recent developmentstargeted to make ozonation processes cheaper by improving the efficiency ofozone generation, for example, by incorporation of catalytic packing in theozone generator, better dispersion of ozone in water and faster conversion ofdissolved ozone to free radicals are described. The synthesis of ozone inelectrical discharges is discussed. Furthermore, the generation and plasmachemical reactions of several chemically active species, such as H2O2, O•,OH•, HO•

2, O∗3, N∗

2, e−, O−2 , O−, O+

2, etc, which are produced in the electricaldischarges are described. Most of these species are stronger oxidizers thanozone. Therefore, water treatment by direct electrical discharges mayprovide a means to utilize these species in addition to ozone. Much researchand development activity has been devoted to achieve these targets in therecent past. An overview of these techniques and important developmentsthat have taken place in this area are discussed. In particular, pulsed coronadischarge, dielectric barrier discharge and contact glow dischargeelectrolysis techniques are being studied for the purpose of cleaning water.The units based on electrical discharges in water or close to the water levelare being tested at industrial-scale water treatment plants.

1. Introduction

Much work has been carried out on the application ofelectrical discharges for air pollution control [1, 2]. Thisincludes the abatement of acidic gases (SOx , NOx) [3, 4],green house gases (CH4, CO2, etc) [5], toxic volatile organiccompounds (VOCs) [4, 6, 7], hazardous particulates [8] andharmful microorganisms [9]. Electrical discharge reactorsare being tested on the semi-industrial and industrial scalefor the purpose of air purification [10, 11]. In the area ofwater purification, ozone synthesis is an industrially acceptedapplication of electrical discharges [12, 13]. Ozone is requiredin huge quantities for drinking water and wastewater treatment[14–16]. The major advantages of the ozonation process overconventional chlorination processes for water treatment arelisted below.3 Author to whom all correspondence should be addressed.

• There is no need to store and handle toxic chemicals.• By-products of ozonation do not have any known adverse

effects on health or the environment.• Ozone is a stronger and faster-acting oxidizer.• Ozone can safely destroy a broader range of organic

contaminants.• Ozone helps in removal of colour, odour and suspended

solid materials.• Ozone is far more efficient in killing bacteria, viruses,

spores and cysts.

The ozonation process can be made more competitive by(a) improving the energy efficiency and ozone yield of theozone generator, (b) developing better ozone–water contactors,and/or (c) catalyzing the chemical reactions of ozone. Theimportant developments made in recent years to achieve thesetargets are presented in this paper.

0963-0252/01/010082+10$30.00 © 2001 IOP Publishing Ltd Printed in the UK 82

Water purification by electrical discharges

Table 1. Oxidation potential of common oxidants.

Oxidation potentialSpecies (V)

F2 3.03OH• 2.80O• 2.42O3 2.07H2O2 1.78O2H• 1.70Cl2 1.36

Electrical discharges taking place in an air or oxygenenvironment convert oxygen into ozone. In addition to ozoneelectrical discharges in air produce a variety of chemicallyactive species, such as O•, OH•, N•, O∗

3, N∗2, N∗, OH−, O−

2 ,O−, O+

2, N+2 , N+, O+, etc [17–20]. These species are short

lived and decay before ozone enriched air/oxygen gets intowater. However, if the reactor is designed so that the electricaldischarges take place in close proximately to the water surface,i.e. just above the water level, some of these species may getinto water and destroy the pollutants. In this way, the watertreatment unit becomes simpler as there will be no need fora separate electrical discharge reactor for ozone synthesis andtubing to carry ozone enriched air/oxygen.

Electrical discharges in aerated water are also possibleand they produce OH•, H•, O•, O3, H2O2, etc [21]. Mostof these species are among the strongest oxidizing agents(table 1). Therefore, instead of ex situ electrical dischargesfor ozone production, the in situ electrical discharges in watermay provide a means to utilize most of these chemicallyactive species for water cleaning. Furthermore, the intenseelectric fields necessary for electrical discharges are alsolethal to several kinds of microorganisms found in water [22]and show a synergistic lethal effect when combined withconventional disinfectants such as O3 and H2O2 [23]. Theelectrical discharges in water may also produce ultraviolet(UV) radiation [24] and shock waves [25, 26], which helpin the destruction of pollutants [27]. For these reasonsdirect electrical discharges in water are clearly the bestnext-generation technologies for water treatment—they areenvironment friendly and may prove far more effective thanconventional oxidants and disinfectants. Techniques of directelectrical discharges in water [21] and the electrical dischargesin close proximity to the water surface [28] are being rapidlydeveloped and tested on the industrial scale [29] for water andwastewater treatments. Important developments in these areasare reviewed in this paper.

2. Electrical discharges and the production ofchemically active species

Several types of electrical discharges can take place in air[17, 18] and are being studied for the abatement of air pollution[1, 30]. In the case of water purification the three followingtypes of electrical discharges are often reported:

(i) contact glow discharge electrolysis,(ii) dielectric barrier discharges (also called silent discharges)

and(iii) pulsed corona discharges.

In contact glow discharge electrolysis a continuous dcvoltage of around 0.5 kV is applied to a thin wire anode incontact with the water surface while the cathode is dipped inwater and isolated from anode through porous glass [31, 32].A sheath of vapour forms around the anode through whichcurrent flows as a glow discharge. Charged species in theplasma (present in the discharge gap or sheath of vapour aroundthe anode) are accelerated due to the steep potential gradientand enter the liquid phase with an energy that may be as highas 100 eV [33]. In the case of contact glow discharges almostall the species in the discharge zone, i.e. anions, cations andneutrals, heat up, so the plasma generated in the reactors canbe called a hot plasma [33]. In silent discharges and pulsedcorona discharges, described below, only free electrons gainhigh energy and the rest of the heavier charges and neutralsremain close to room temperature and the plasma so generatedis called a cold plasma or a non-equilibrium plasma.

In a dielectric barrier discharge reactor the electricaldischarges take place between electrodes where at least one ofthe electrodes is covered with a thin layer of dielectric material,such as glass or quartz [12]. In the case of the water treatmentapplication of dielectric barrier discharge reactors a layer ofwater around one of the electrodes acts as a dielectric [34].Usually an ac voltage of around 15 kV is applied acrossthe electrodes. Ions in the discharge gap, particularly freeelectrons (being the lightest charged species), accelerate underthe influence of the applied electric field. Upon inelasticcollision the free electron may ionize an ambient gas molecule,thus producing more free electrons. The free electrons mayrepeat the process and thus produce an electron avalanche(streamer). Multiple streamers are produced, distributedin space and time. In a streamer the electron density isaround 1014 cm−3 and they may have energy in the rangeof 1–10 eV [17]. The discharge-generated ions traverse thespace and accumulate on the dielectric, where they produce areverse electric field and stop current flow in few nanoseconds.Due to the short duration of the micro-discharge only electrons,being the lightest charged particles, can gain high energy whilethe rest of the heavier charges and neutrals remain close toroom temperature. The energetic electrons, in turn, initiatethe plasma chemical reactions that are responsible for theproduction of free radicals and ions, which ultimately destroythe pollutants.

In both cases of contact glow discharge electrolysis anddielectric barrier discharge reactors the electrical dischargestake place in the gas phase in close proximity to the watersurface. They require an intense electric field of the order of1 MV cm−1 for electrical discharge to take place in water.Such a high electric field is possible by applying high-voltagepulses of 15–100 kV, usually of positive polarity, with asharp rise time (a few nanoseconds) and short duration (nano-to microseconds) in a pulsed corona discharge reactor [35].Furthermore, the pulsed corona discharges are effectivedisinfectants [23] and they can also take place in the gas phasein close proximity to the water surface [28]. This is whymost of the studies on water treatment are carried out usingpulsed corona discharge reactors and the available industrial-scale units are also based on this technique.

A pulsed corona discharge reactor requires a pulsegenerator and a reactor as illustrated in figure 1. The pulse

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Figure 1. Pulse corona discharge reactors: (a) pulse generator and wire–cylinder type reactor and (b) needle–plate type reactor.

generator is commonly based on the discharge of a capacitor ona low-inductance circuit through a spark gap switch [36, 37].The reactor is comprized of metallic electrodes and fittingsmade of some insulating material. The electrodes are usuallyin a needle–plate arrangement where a needle is connected tothe high-voltage terminal and the plate is earthed. The needleis covered with an insulator, for example Teflon, and only itstip is exposed, so that an intense electric field may develop atthe needle tip.

In a wire–cylinder (or coaxial) arrangement of electrodes,a larger volume of water can be exposed to electricaldischarges, but it is more difficult to develop the intense electricfield necessary to initiate corona discharges than in the needle–plate arrangement. However, if the wire electrode is coveredwith a thin ceramic layer the electric field strength may beenhanced by a factor of εw/εc ∼ 10 (εw and εc are thepermittivity of water and ceramic, respectively) as comparedwith a bare metallic wire in the case low water conductivity(σw < 10 µS cm−1) and short pulses (τ < 1 µs) [38, 39].An upper estimate of the electric field in this case is up toE ∼ U/dc (dc is the thickness of the ceramic layer) in thecase of high water conductivity and a long pulse. Due to theinhomogeneous nature of the ceramic layer, the electric fieldat some points may be higher than that estimated, which mayinitiate streamers from those spots. Pulsed corona dischargesin water using a ceramic-coated stainless-steel wire electrodein a wire–cylinder type reactor have been reported [38, 39].Such a system will be easier to scale-up and the porous ceramicmaterial may also be explored for its catalytic activity or as asupport for a suitable catalyst of the plasma chemical reactionsin future studies.

The electrode material may also have a catalytic effect onthe reactions taking place during water cleaning. In the caseof gas phase plasma chemical reactions, the electrodes madeof copper perform better than stainless-steel [40–42] probablydue to a catalytic effect of copper. It has been observed thatthe electrode material is excited during electrical discharges inwater [33, 43]. However, the catalytic effect of the electrodematerial on the plasma chemical reactions in water has notyet been reported. Among the electrode materials, titaniumreleases a minimum amount into water while non-metals andbrittle metals such as tungsten fail in this application [44].Electromagnetic compatibility should be considered duringthe designing of pulsed corona discharge reactors as otherwisethey may interfere with neighbouring instruments due to theirelectromagnetic emissions [45].

Free electrons in the discharge gap are accelerated underthe influence of the high electric field applied to the needle (inneedle–plate reactor) or the wire (in wire–cylinder reactor).The accelerating free electrons may ionize the ambient gasmolecules that come into their path, thus producing morefree electrons. The free electrons may repeat the processand initiate electron avalanche (streamer or plasma channel).Usually a positive dc voltage is applied and in this case thefree electrons are attracted towards the high-voltage electrode.The drift of free electrons leaves behind a positive charge atthe streamer head, which enhances the applied electric fieldeffect and attracts the electrons of any secondary avalanche.When secondary avalanche electrons intermix with primaryavalanche ions they leave behind a positive charge, whichenhances the electric field effect. The whole process isrepeated and in this way the streamer propagates. Freeelectrons in streamer head have energy of up to 15 eV, whilein rest of the streamer the free electrons have energy of upto 4 eV. The average electron energy in a streamer is around5 eV and the electron density is around 1013 cm−3 [17].Multiple streamers distributed in space and time, originatefrom the high-voltage electrode and propagate towards thecounter electrode. The values of the electron energy anddensity stated above have been taken from reports describingpulsed corona discharges in air.

The detailed mechanism of the initiation of coronadischarges in water is still not fully understood. Two typesof theories, i.e. electronic theories and thermal breakdown(bubble) theories, have been proposed to explain the initiationof corona discharges in water [35, 46, 47]. According tothe electronic theories the free electrons accelerate underthe applied electric field and may collide with and ionizethe ambient molecules, thus producing more free electrons(electron avalanche) and leading to breakdown in water.According to the thermal breakdown (bubble) theories thecurrent in the high-field region causes heating and vaporizationof the liquid, forming bubbles. Gas breakdown occurs withineach bubble, causing further heating and growth of the bubbleuntil complete breakdown of the gap occurs.

A single streamer has a fraction of a millimetre diameterand can propagate to a distance of more than a centimetrein water [35]. The electron density in the case of streamersin water increase with the solution conductivity and is of theorder of 1018 cm−3 when the conductivity is 210 S cm−1 [39].To the best of our knowledge there are no published data onthe electron temperature in streamer coronas in water.

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If the duration of the high-voltage pulse is long enough,one of the streamers may bridge the gap between theelectrodes. In this case a high-intensity current flows throughthe conductive plasma channel and streamer discharge changesto a spark discharge [27]. The temperature in the plasmachannel may reach 14 000–50 000 K and the plasma emitsUV radiation and intense shock waves. As the high-voltagepulse ends, the plasma channel cools and transfers its thermalenergy to the surrounding water, resulting in the formationof steam bubbles. In the steam bubbles the temperatureand pressure are high enough to form transient supercriticalwater. A spark discharge provides a more reactive environmentthan a streamer corona because high-energy particles, UVradiation, shockwaves and supercritical water simultaneouslycause pyrolytic and free radical reactions in and around theplasma channel.

The high-energy electrons produced in electricaldischarges ultimately face inelastic collisions with ambientmolecules which result in either the excitation, dissociation,electron capture or ionization of the target molecules, asillustrated with some examples in table 2. A generalizedexample of such a reaction is given below:

e−∗ + X → Y + e−

where ∗ indicates a high-energy state, X is a reactant and Y isa product. The concentration of product can be calculated bythe rate equation

dY/dt = sKnePX

where s is a stoichiometric coefficient, K is a reaction rateconstant, ne is the number density of energetic electrons andPX is the number density of the reactant. Air comprises ofaround 80% N2, around 20% O2 and a small amount of H2O;these three types of molecules are usually the reactants andtheir main reactions responsible for free radical production arelisted in table 3. The rate constants of the reactions are takenfrom [19] and presented in the form

K = A exp(−B/(E/N))

where A is in cm3 s−1, B and E/N are in townsend (1 Td ≡10−17 V cm2), E is the electric field strength and N is thenumber density of molecules. The values of A and B aregiven in table 3.

The average electron energy in pulsed corona dischargeslies in the range of 3–6 eV [48], which suits well for theexcitation of N2 and dissociation of O2 into 2O•. Excitednitrogen (N2(A 3�)) primarily reacts with O2 through thefollowing reaction [49]:

N2(A3�) + O2 → N2 + O• + O•. (R8)

Therefore, a G-value (the number of radicals produced per100 eV of input energy) of 3–4 for O•is higher compared withthe G-value of 0.3–0.4 for N• [20]. O• is a strong oxidizingagent and can oxidize organic compounds (pollutants). Inthe absence of pollutants the O• is consumed in a number ofreactions, such as the following [12]:

O• + O2 + M → O3 + M (R9)

Table 2. Some examples of excitation, dissociation, ionization andelectron capture reactions of high-energy electrons in electricaldischarges in air.

Excitatione−∗ + N2 → N∗

2 + e−

Dissociatione−∗ + N2 → N• + N• + e−

e−∗ + O2 → O• + O• + e−

e−∗ + H2O → OH• + H• + e−

Ionizatione−∗ + O2 → O+

2 + 2e−

Electron capturea

e−∗ + O2 + M → O−2 + M

a M is a third collision partner,which may be O2, N2, etc.

where M is a third collision partner, which may be O2, N2, etc.In the presence of a suitable adsorbent/catalyst in the dischargegap, some of the plasma chemical reactions may also take placeon solid surfaces, see for example [50]:

O• + O2 (adsorbed) → O3 (adsorbed) → O3. (R10)

In the case of electrical discharges in water the primary reactantis H2O. The main reactions responsible for free radicalproduction and free radical termination in water are listed intable 4, along with their respective rate constants taken from[51]. The free radicals, particularly O• and OH• in the caseof air and OH• in the case of water, play a major role in thedestruction of pollutants. Ion–molecular reactions may playsome role in the destruction of organic compounds in air [52],but their role in the destruction of pollutants in water duringelectrical discharges is not reported in the literature. H2O2 andO3 can oxidize some organic compounds, but their rates ofreaction are much slower than that of OH•. O3 (in ozonationprocesses) or a mixture of O3 and H2O2 in the presence ofUV radiation (in advanced oxidation processes) converts intoOH•, which, in turn, destroys the pollutants as discussed in thefollowing section [53].

3. Ozone and water treatment

Commercial ozone generators are usually based on di-electric barrier discharges [12, 13], which can give around200 g O3 kW h−1 (up to 6 wt%) from an oxygen feed and around90 g O3 kW h−1 (0.5–2.0 wt%) from an air feed. Pulsed coronadischarges have been found to produce up to 240 g O3 kW h−1

from an oxygen feed, which makes them good alternative di-electric barrier discharges for ozone generation [54].

The ozone is produced primarily by reaction (R9) and,simultaneously, a fraction of the produced ozone is destroyedby several possible reactions. Ozone synthesis has been wellreviewed in the literature [12, 13]. The ozone concentrationat the reactor outlet may be increased by increasing theprobability of ozone generation reactions and/or by decreasingthe probability of ozone destruction reactions. By employinga double discharge surfaces reactor [55] or a hybrid of silentand surface discharges [56], which may increase the number ofsites for ozone generation reactions within the given dischargevolume, the efficiency of up to 274 g O3 kW h−1 from anoxygen feed has been achieved. Porous silica gel packing in

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Table 3. Main reactions of N2, O2 and H2O responsible for the production of free radicals and parameters of their rate constants in the caseof electrical discharges in air.

Reaction A (cm3 s−1) B (Td)

(R1) e−∗ + O2 → O2(a 1�) + e− 1.0 × 10−9, E/N � 40 120, E/N � 406.3 × 10−11, E/N > 40 8.1, E/N > 40

(R2) e−∗ + O2 → O• + O• + e− 1.3 × 10−8 309(R3) e−∗ + O2 → O•(1D) + O• + e− 1 × 10−8 338(R4) e−∗ + N2 → N2(A 3�) + e− 1 × 10−8 336(R5) e−∗ + N2 → N• + N• + e− 6.3 × 10−9 949(R6) e−∗ + N2 → N2(C 3H) + e− 6.3 × 10−9 486(R7) e−∗ + H2O → OH• + H• + e− 2 × 10−11 322

the discharge gap of the reactor has been found to improvethe ozone generation efficiency to up to 300 g O3 kW h−1

(up to 11 wt%) as compared to 200 g O3 kW h−1 when nosuch packing was present [50]. The probable reasons for theenhancement of ozone generation are the spread of an activeplasma zone due to the micro-discharges at contact pointsbetween particles (packing material) and the surface mediatedreaction (R10). In the case of the surface mediated reaction(R10), the adsorbent may take away the excess energy fromthe newly born ozone molecule so that its chances of entryinto ozone destruction reactions are reduced [50]. Anotherclue to support the role of surface mediated reactions was theobservation that the pore size in the case of alumina packingis a critical factor in its ozone generation activity [57]. Furtherwork in this direction is required to find new catalytic materialsand to understand the role of the properties of the packingmaterials, such as their dielectric constant, porous texture,crystalline phase, etc, on their ozone generation activity.

The ozone so produced is in contact with water as finebubbles formed through a porous disc or by propellers/turbines,etc. Recently, an impinging zone reactor has been reported[58] in which the gas enters the reactor through two nozzlesin two separate water streams at high velocity. The two liquidstreams impinge in a central tube and then enter into a tank. Theimpinging zone reactor may improve the safety of the system,reduce the size of the reactor, lower the ozone requirementand lower the energy cost compared to a conventional system.Pulsed corona discharges have the ability to break up large gasbubbles (flowing through the hypodermic needle electrode)into many very fine bubbles [35, 59]. This happens becausethe electric field passing from the water, with large dielectricconstant (ε = 80), to the air, with small dielectric constant(ε = 1), causes the water to exert a force on the air, squeezingit and ultimately breaking it into small bubbles. Another reasonmay be the columbic forces due to the charged bubble–liquidinterface. So, pulsed corona discharges during ozonation mayachieve the benefits observed in the case of the impingingzone reactor and may also offer some additional benefits asdiscussed later in this paper.

The dissolved ozone dissociates into OH• through a cyclicchain mechanisms (figure 2) [14]. The OH•, which is primarilyresponsible for the oxidation of aqueous pollutants [60], reactswith organic contaminants much faster (107–109 M−1 s−1)

than ozone itself (10−1–107 M−1 s−1) [14]. Therefore, thefaster the rate of ozone conversion into hydroxyl radicals thefaster it destroys the pollutants. UV radiation, H2O2 [61–63],activated carbon [64], etc, catalyze the O3 to OH• conversion inadvanced oxidation processes (AOPs). Similarly, the catalytic

Table 4. The main reactions responsible for free radical productionand termination in the case of electrical discharges in water and theirrespective rate constants (k).

Reaction k

(R11) H2O → OH• + H• 9.25 × 10−10 M s−1

(R12) H2O → 12 H2O2 + 1

2 H2 1.2 × 10−6 M s−1

(R13) 2H2O → H3O+ + e−eq + OH• 2.35 × 10−9 M s−1

(R14) H• + O2 → HO•2 1.0 × 1010 M−1 s−1

(R15) H• + H2O2 → H2O + OH• 1.0 × 1010 M−1 s−1

(R16) OH• + H2O2 → H2O + HO•2 5 × 107 M−1 s−1

(R17) e−eq + OH• → OH− 3 × 1010 M−1 s−1

(R18) e−eq + H• + H2O → OH− + H2 2.5 × 1010 M−1 s−1

(R19) e−eq + H2O2 → OH• + OH− 1.2 × 1010 M−1 s−1

(R20) H• + OH• → H2O 2.4 × 1010 M−1 s−1

(R21) 2OH• → H2O2 4.0 × 109 M−1 s−1

(R22) 2HO•2 → H2O2 + O2 2.0 × 106 M−1 s−1

(R23) H• + HO•2 → H2O2 1.0 × 1010 M−1 s−1

(R24) 2H• → H2 1.0 × 1010 M−1 s−1

(R25) HO•2 + OH• → H2O + O2 1.0 × 1010 M−1 s−1

(R26) H3O+ + OH− → 2H2O 3.0 × 1010 M−1 s−1

conversion of ozone into O• has been reported, which canimprove the efficiency of ozonation [65, 66]. Pulsed coronadischarges during ozonation also offer the following potentialadvantages:

(i) dispersed ozone enriched air/oxygen [35, 59] increase therate of ozone dissolution in water,

(ii) enhanced dissociation of O3 into free radicals [35] and(iii) generation of additional free radicals such as OH• and O•

[35, 43].

Because of the advantages mentioned above, the technique ofpulsed corona discharges in water during ozonation needs tobe investigated. Since electrical discharges can generate freeradicals and neutral active species in water or near water level,they may eliminate the need for a separate reactor for ozonegeneration. The developments in the area of AOPs based onelectrical discharges in water are presented in the next section.

4. Water treatment by direct electrical discharges

Contact glow discharge electrolysis, silent discharges andpulsed corona discharges can take place in close proximityof water surfaces and may be utilized for water purification.In contact glow discharge electrolysis H2 is formed in thegas phase and H2O2 in the aqueous phase [32]. Similarly,a bubble has been reported to develop upon application of

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a 2.5–5 kV dc voltage to a needle electrode in water under110 Torr pressure. Pulsed discharges were observed insidethe bubble with the formation of H•, OH• and H2O2 [67]. AH2O2 concentration of up to 50 ppm was reached under theseconditions. The formation of these species from gaseous watercan be accounted for by reaction (R7) (table 3) followed byreaction (R21) (table 4).

The application of an ac high voltage to electrodes in aparallel plate arrangement with one electrode covered witha layer of water resulted in the formation of cones of water[34]; corona discharges originated from the tips of the cones.Ozone of up to 6000 ppm and with an energy efficiency of up to110 g O3 kW h−1 was formed in oxygen above the water layer.The application of a dc high voltage to a needle placed abovewater resulted in electrical discharges in the air gap, but ozonewas only produced in very small amounts [68]. Pulsed dc ofpositive polarity, 25 kV and 100 Hz, applied to multiple needleelectrodes (30 steel needles at a 5 mm mutual distance) at adistance of 1 cm from water resulted in electrical discharges inair with the formation of ozone [28]. The energy efficiencyfor ozone generation was 40 g O3 kW h−1. O3 producedin close proximity to a water surface can easily dissolve inwater and later dissociate, resulting in the generation of OH•,as illustrated in figure 2. OH• radicals were also detectedby OH-specific molecular probes in this study [28]. In asimilar study, a pulsed dc voltage (up to 30 kV) was applied toplate–plate type electrodes through which air bubbles flowed[69, 70]. Light from the electrical discharges in the bubbleswas observed. Around 300 ppm ozone was formed in thebubbles with an efficiency of about 40 g O3 kW h−1. Despitethe fact that the efficiencies described above are lower thanthose of commercial ozone generators, these figures are stillsignificant because an appreciable amount of OH• (precursorof H2O2) is also produced from humid air or humid oxygen,along with O• (precursor of ozone). Furthermore, O• maybe consumed in several competing reactions, such as thefollowing:

O• + H2O → 2OH•. (R27)

In the case of electrical discharges in air, nitrogen may oxidizeto nitrate [71] and dissolve in water as nitric acid. In the caseof corona discharges above water level, the nitrate yield inthe water was found to be approximately 0.5 mole per moleof electrons discharged [68]. The nitric acid product slightlyreduces the pH of the solution. N2O, which is also a product ofcorona discharges in air, has been detected in a water sampletreated with corona discharge in air [28]. These results indicatethat nitrates are minor, though detectable, products in the caseof electrical discharges in air above water level.

Unlike the remaining types of electrical discharges, pulsedcorona discharges can also take place in water and producechemically active species, such as the free radicals OH•, H•

and O•. The main chemically active molecules produced incorona discharges in water with air or oxygen bubbling areH2O2 and O3. The results of studies on the production of thesereactive species by pulsed corona discharges in water can besummarized as follows [35, 39, 43, 72–75].

• In general the density of the chemically active speciesincreases with an increase in the applied voltage [43],a decrease in the radius of curvature of the high-voltage needle electrode [43], positive polarity rather than

Figure 2. The mechanism of ozone conversion to free radicals inthe ozonation process.

negative polarity of dc voltage [35], bubbling some gasduring the discharges [43] and using argon instead ofoxygen for gas bubbling [75].

• OH•, H• and H2O2 are produced without gas bubbling[73]. The pH of the solution increases slightly due to theaccumulation of H2O2 with time [73]. OH•, H• and O• areproduced with gas bubbling and their intensity increaseswith an increase in the rate of gas bubbling [43, 75].

• The relative density of OH•, H• and O• is different in thecase of argon and oxygen bubbling [35, 43, 75]. The H•

intensity is strongest when argon is bubbled and lowestwhen oxygen is bubbled. During oxygen bubbling the O•

intensity is the highest.• The OH• density is higher in neutral or alkaline media

than in an acidic medium under the same experimentalconditions [43].

• O3 is produced when oxygen is bubbled during a discharge[35, 43, 75]. The rate of O3 production is optimum atmoderate applied voltages (15–25 kV) [35].

• The concentrations of H2O2 and O3 increase with timeuntil they reach a saturation level [39].

• The maximum emission intensity for the OH• andO• signals and the maximum concentration of H2O2

was obtained at a water conductivity in the range of10–80 µS cm−1 [39, 43, 73].

• Spark discharges produce UV radiation and shock waves;they also produce OH• and H2O2 with efficiencies betterthan those in streamer discharges [72].

The observations described above can be explained on thebasis of the streamer formation and propagation mechanismin water described in earlier sections. The discharge becomesstronger and streamer length increases with an increase in theapplied voltage and a decrease in the radius of curvature ofthe high-voltage needle electrode, resulting in an increase inthe chemically active species. An electron avalanche or astreamer initiates in the pre-existing bubbles or the bubblesgenerated by the local heating of the liquid [35]. In the case ofa positive-polarity high-voltage pulse, the positive charge at thestreamer head enhances the overall electric field effects causingintense ionization and the production of more free radicals.With gas bubbling, there are more initial bubbles in the waternear the high-voltage electrode and mean free path available

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to free electrons in gas phase is longer than in the liquidphase. Therefore, the free electrons are easier to energizeinside the bubbles, resulting in more intense ionization and theproduction of more radicals and, consequently, more activemolecules such as O3 and H2O2 from the free radicals. Alongwith the physical effect, the gas may also have chemical effect;for example oxygen may react with H•, which may explainthe lower intensity of H• during oxygen bubbling than duringargon bubbling. The production of O3 with oxygen bubblingindicates that O3 synthesis takes place from oxygen in thebubbles.

Water conductivity plays an important role in thegeneration of corona discharges and on the production ofchemically active species [35, 39, 43]. In deionized water thedischarge is relatively weak [35]. A certain concentration ofions (conductivity in the range of 10–80 µS cm−1) enhancesconduction, resulting in a stronger discharge, higher currentflow, longer streamer length and an increase in the productionof chemically active species [43]. As the corona dischargeis partial (not contacting with the counter electrode), thedischarge current should be transferred by ions present in thesolution. Thus, an increase in the water conductivity (furtherfrom the optimum value of 10–80 µS cm−1) results in a fastercompensation of the space charge electric field on the streamerhead (shorter streamer channel length) [39] and a decreasein rate of production of chemically active species [43]. Itshould be kept in mind that there is a large difference in thedischarge in deionized water and in water with a conductivityof 0.5 mS cm−1, which should be expected for wastewater.

Compared to streamer discharges, the peak voltages andcurrent densities are higher in spark discharges, which causeintense excitation and ionization, so that more OH• and,consequently, more of H2O2 (through reaction (R21), table 4)are produced. Although thermal effects and UV radiation maydecompose a fraction of the H2O2 its overall concentration stillremains higher than that in streamer discharges.

The O3 and H2O2 produced by electrical dischargesdissociate into free radicals through reactions similar to thosein other AOPs [21, 76] as illustrated in figures 2 and 3. The freeradicals, in turn, oxidize organic compounds. OH• is primarilyresponsible for the oxidation of organic compounds, both inthe case of ozonation and in the case of electrical discharges inwater or above water level. Therefore, the conclusions drawnfrom studies in the case of electrical discharges close to watersurfaces remain valid for the case of electrical discharges inwater and vice versa, as far as the mechanism of pollutantoxidation is concerned. As an example, the oxidation ofphenol and other aromatic compounds may start with thehydroxylation of the benzene ring by OH• in the case ofelectrical discharges in water [51, 74, 75, 77–79] as well as inthe case of electrical discharge close to water level [28, 80–82].The –OH group on the benzene ring directs the incoming –OHgroup to a para or ortho position [80]. The –COOH group on abenzene ring directs the incoming –OH group to the para andmeta positions [82]. In the case of chlorophenols, the –OHgroup directs an incoming –OH group to its para or orthoposition while the –Cl group may eliminate or be substitutedby the –OH group under these conditions [81]. Consecutivehydroxylation may take place, resulting in intermediates suchas hydroxyhydroquinone, pyrogallol, etc. Further oxidation of

Figure 3. The mechanism of the conversion of oxidizers to freeradicals in AOPs.

Table 5. The main reactions responsible for oxidation of phenol andits byproducts in the case of electrical discharges in water and theirrespective rate constants (k).

Reaction k

(R28) phenol+OH• →hydroquinone+H• 6.5×109 M−1 s−1

(R29) phenol+OH• →catechol+H• 8.0×109 M−1 s−1

(R30) phenol+OH• → resorcinol+H• 1.0×109 M−1 s−1

(R31) hydroquinone+OH• →products 1.0×1011 M−1 s−1

(R32) catechol+OH• →products 1.0×1010 M−1 s−1

(R33) resorcinol+OH• →products 1.0×1010 M−1 s−1

these intermediates results in oxidative ring cleavage, whichproduces new intermediates such as oxalic acid, formic acid,malonic acid, tartaric acid, malic acid, etc. These partialoxidation products further oxidize to simple molecules suchas carbon dioxide and water under these conditions. Theoxidation of the contaminants follows first-order kinetics[80, 82], which indicates that the first attack of OH• (or someother free radical, e.g. O•) on the organic contaminants shouldbe the rate-determining step. The main reactions responsiblefor the oxidation of phenol in the case of electrical dischargesin water are listed in table 5 along with their respective rateconstants taken from [51]. The decomposition of differentorganic compounds is initiated by different reactions in thecase of air purification by electrical discharges [6]. Furtherstudies are needed to clarify the reaction mechanism of otherorganic compounds and the role of reactive species other thanOH•, i.e. cations, anions and the other free radicals in the caseof water purification by electrical discharges.

The destruction of organic compounds by electricaldischarges in water was demonstrated by decolorization ofanthraquinone dye [35]. Phenol is usually selected as a modelorganic compound for such studies [83] and its decompositionbehaviour has been predicted through mathematical simulationstudies [74]. The conclusions drawn from the studies onphenol decomposition by pulsed corona discharges in waterare summarized below.

• Bubbling oxygen during the electrical discharges greatlyenhances the rate of oxidation of phenol [21].

• Bubbling argon also enhances the rate of oxidation ofphenol, but less so than oxygen [75].

• The presence of iron significantly enhances the rate ofdecomposition of phenol [21, 77].

• Iron in the ferrous state is more efficient than in the ferricstate for the oxidation of phenol [78].

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• An optimum concentration of ferrous ions, which showsthe maximum enhancement in the rate of oxidation ofphenol, has to be explored experimentally [78].

• The rate of the oxidation of phenol in the presenceof different anions follows the following order:chloride>phosphate>chlorate [78].

• Under alkaline conditions the rate of oxidation of phenolis almost twice that under acidic conditions [78].

• The energy efficiency for the decomposition of phenol ina spark discharge is more than two times higher than thatin a streamer discharge [79].

• Additives such as H2O2 [79] and O3 [27] during electricaldischarges show a synergistic effect on the destruction oforganic compounds, i.e. they enhance the overall rate ofoxidation of pollutants.

A higher density of chemically active species withoxygen or argon bubbling explains the first two observations.The addition of iron increases the OH• concentration bydecomposing H2O2 in Fenton or Fenton-like reactions [51, 78].Furthermore, ferric ions may also oxidize phenol or itsdecomposition products [78]. Phenol oxidation takes placefaster in the presence of Cl− than H2PO−

4 or ClO−4 because of

the dichlorine ion radical reaction. In alkaline medium, phenolexists in the phenoxide form, which reacts faster with OH•andis also auto-oxidized by dissolved oxygen.

The electrohydraulic discharge technique, which is aspark discharge technique, has been tested to study thedecomposition of several pollutants, such as 4-chlorophenol,3,4-dichloroaniline and 2,4,6-trinitrotoluene [27]. Inelectrohydraulic discharges the destruction of pollutants maybe initiated simultaneously by pyrolysis in a hot plasmachannel, free radical oxidation, UV radiation, supercriticalwater oxidation, etc. This hypothesis explains the higher rateof pollutant destruction in spark discharges than in streamerdischarges. The addition of O3 or H2O2 accelerates theoxidation of pollutants during electrical discharges becausethese compounds decompose into free radicals by streamer orspark discharges.

Air stripping is sometimes applied to remove volatileorganic compounds from water. The application of pulsedcorona discharges in water during the air stripping ofchloroform, trichloroethylene, benzene and toluene from waterhas been studied and found to remove the pollutants at afaster rate than air stripping or pulsed corona discharges alone[84, 85].

The incorporation of a suitable catalyst/adsorbent in thedischarge gap has been proved to be fruitful in terms ofthe improved efficiency of plasma chemical reactions duringelectrical discharges in the gas phase [6, 86]. In the case ofpulsed corona discharges in water, the addition of activatedcarbon resulted in a faster decomposition of phenol fromwater [51]. The probable mechanism in the presence ofactivated carbon is the adsorption of pollutants followed bysurface mediated reactions with active species and, finally, therelease of the products, resulting in regeneration of the activesite. Further work on the combination of catalyst/adsorbentwith electrical discharges for water purification is mandatorybecause it holds promise in making the processes moreeffective and cheaper.

The electrical discharges in gas consume many timesless energy than electrical discharges in water. The high-energy ions and free radicals produced in gas may easilygo into water and oxidize the aqueous pollutants in such asystem. One example is contact glow discharge electrolysiswhere the discharges take place in water vapours aroundthe electrodes. Complete oxidation of phenols [80], vanillicacid [80], chlorophenols [81], and benzoic acid [82] hasbeen observed in contact glow discharge electrolysis. Thedestruction of methylene blue and carbon tetrachloride by dccorona discharges between a needle in air and a water layer hasbeen demonstrated [68]. Pulsed corona discharges betweenmultiple needle electrodes placed in air above water have beenfound to decompose phenol with an energy efficiency muchhigher than that in direct electrical discharges in water [28].The oxidation of phenol was also observed when air wasreplaced with argon, which indicates that species other thanozone are also involved in the destruction of pollutants inthis case. The studies presented above were for the case ofan atmospheric corona discharge. However, less energy isrequired to generate a corona discharge below atmosphericpressure. The destruction of pentachlorophenol in waterby corona discharge between needle and water layer underreduced pressure (50 Torr) has been studied [87]. The energyefficiency for the destruction of aqueous pentachlorophenolwas found to be higher in the case of a corona discharge underreduced pressure than in the case of a corona discharge atatmospheric pressure. However, a corona discharge underreduced pressure make the system technologically complexand requires additional equipment to evacuate the reactionvessel.

5. Sterilization by electrical discharges

Electrical discharges are usually associated with intenseelectric fields, shock waves, UV radiation, O3, H2O2, etc,each of which can kill microorganisms. Therefore, acombination of all or most of these, as in the case ofelectrical discharges in water or air, may form a veryeffective sterilization medium [9, 37, 88]. The high-intensitypulsed electric fields without corona or spark dischargeactivities are being considered as a technology of thefuture for the sterilization of food products [22]. Sincethe electric fields kill microorganisms but do not denaturefood constituents, such as proteins, vitamins, etc, thetaste and nutritive value of the food is likely to remainintact.

The sterilization effect of a pulsed electric field dependson many factors, such as the duration and the peak value of thevoltage, the shape of the electrodes, etc [89]. The needle–plate or wire–cylinder electrode systems are more efficientthan the plate–plate system because an intense electric fieldis developed on a needle tip or on a thin wire [23, 90]. Theinsertion of insulating plates with small holes between theelectrodes can also concentrate the electric fields and havebeen reported to improve sterilization efficiency [23]. Smallamounts of bactericides, such as O3 or H2O2, improve thesterilization efficiency of pulsed electric fields [23]. Coronaor spark discharges during the application of a strong electricfield improve the sterilization process [73].

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6. Conclusions

From the above discussion it can be inferred that electricaldischarge techniques may prove to be more effective, cheaperand environmentally friendly than conventional water treat-ment techniques. There is a need for further developments inthe area of water treatment by electrical discharges. In particu-lar, there is a need to find new and more efficient materials thatmay be used as catalysts for ozone generation. Further studiesare required to clarify the role of the physical and chemicalproperties of catalyst materials on ozone generation.

The application of pulsed high voltages during ozonationmay result in better dispersion of ozone in water and fasterconversion of ozone into free radicals, which may lowerthe cost of the ozonation processes. Water treatment bydirect electrical discharges is being tested on an industrialscale. Further study on the destruction behaviour of possiblewater pollutants, including microorganisms and toxic organiccompounds from industrial effluents, is mandatory. Theidentification of breakdown products needs to be carried out toclarify the mechanism of plasma chemical reactions involved.In particular, the studies on the application of suitable catalystsin an electrical discharge reactor have great possibilities tomake the process more effective, cheaper and competitivewith conventional methods. Both the electrical discharges inwater and above water level have their merits and demerits.Electrical discharges above water level, that is in gas phase,require less energy for the discharge to take place whileelectrical discharges in water make a simpler system andproduce the chemically active species in water, which candirectly attack the aqueous pollutants. Among the types ofelectrical discharges, pulsed corona discharges are the mostoften studied and seem to be the most promising for waterpurification. The growing demand for the cleaning up of rawwater and industrial waste water without the use of hazardouschemicals or the generation of secondary pollutants togetherwith the rapid pace of development in the area of electricaldischarges for water purification suggest that these techniquesmay play a major role in the water treatment industry in thefuture.

Acknowledgments

The authors are thankful to Professor N Hershkowitz(University of Wisconsin) for his valuable suggestions, whichwere helpful in explaining more clearly the plasma state,electron energies and densities in the electrical discharges. Theauthors are also thankful to the reviewers for their valuablesuggestions, which were helpful in improving the manuscript.

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Water purification by electrical discharges