Review Jur Nal 2012

8/16/2019 Review Jur Nal 2012

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502 X. Wang et al./ Electrochimica Acta 83 (2012) 501–512

1. Introduction

Duringlastfew years,large numberof paperspublished on glowdischarge plasma (GDP) in and in contact with liquids due to theincreasing interest for its application. Nowadays, it has providedmanypossible applications,suchas hydrogenproduction [1,2] , syn-thesis [3–8] as well as wastewater treatment [9–15] . The study of glow discharge couldtrace back tomorethana century ago. Itis alsotermed as electrode effect in some references [16–18] dependingon which electrode glow discharge takes place. Plasma electrol-ysis [1] , glow discharge electrolysis (GDE) [19–21] , contact glowdischarge electrolysis (CGDE) [22–28] and diaphragm glow dis-charge [29–32] are also used in literatures according to differentreactors. Early studies suggested that glow discharge had roots inradiation [19,22] . Indeed, glow discharge in and in contact withliquids can dissociate water molecule into hydroxyl radical ( • OH)and hydrogen radical ( • H) [33–35] . These radicals candiffuse in thesurrounding liquid and be used for the removal of dissolved com-pounds. Especially, • OH are able to oxidize any organic moleculeinto “harmless” carbon dioxide in a non-selective way. This makesGDP particularly suitable for decontamination and sterilizationproposes. This is the main reasonwhy GDP hasbeen studied exten-sively as effective method for the removal of hazardous chemicalsin aqueous solution.

In recent years, some researchers have reviewed electricaldischarge and non-thermal plasmas for water treatment and men-tioned GDP briey [36–40] . However, GDP has its own uniquecharacteristics. Althoughtwo reviews about its chemical effect andearly application of GDP have published [41,42] , large numbers of papers published during the last few years. Both the onset of GDPand its application for the wastewater treatment has been system-atically investigated. So far, GDP has been utilized for not only thedegradation of organic pollutants [9–15] , but also the reduction of Cr(VI) [43–45] and the inactivation of algae [46] . Therefore, in thepresent contribution, we review current state of research focus-ingon the application of GDP on wastewater treatment. This paperalso reviews generation mechanisms and unique physical charac-teristics during the onset of glow discharge. The emphasis in thispaper is on the mechanism of the removal of hazardous chemicalsinduced by GDP and the combined process. What’s more, reactorscommonly used for the generation of GDP are discussed here indetails. In addition, GDP is evaluated and compared with otherAOPs in energy efciency, the formation rate constants of activespecies, removal efciency of substrateand TOC. Theoutlook of thisemerging technology for the upcoming future is nally envisaged.It should be noted that this paper only reviewed GDP on sustainedby direct current in and in contact with liquids.

2. Fundamentals

2.1. The onset of glow discharge

2.1.1. I–V curve characteristic Experimentally, GDP is sustained by direct current around the

pointed metallic anode or a small hole on diaphragm which sepa-rate reactor into two compartments. When the applied voltage issufciently high, conventionalelectrolysis develops spontaneouslyto generate GDP [22,23,47–49] . I –V curve (current vs. voltage) is atypicalcharacteristic during theonset of glowdischarge. Fig.1 is thetypical I –V curve during the onset of GDP. Several different regionsare usually identied:

0 < V < V B: ohmic region; current linearly increases with theincrease of the applied voltage. Conventional electrolysis occurs

and electrolysis of water takes place with small bubbles of gas

Fig. 1. I –V characteristic during the onset of glow discharge.

leaving the pointed anode or the small hole. V B was termed as thebreakdown voltage in Refs. [23,47] .

V B < V < V D: transition region; the current startsto decrease withthe applied voltage due to the vapor sheath forming around the

pointed anode or the small hole. The gas bubbles in vapor sheathundergo the process of formation and collapse, so the oscillationof the current occurs. V D was the critical voltage (also termed asmidpoint voltage in Refs. [22,23,28] ). Sometimes, CD region is ahorizontal line during the onset of glow discharge when the anodeis submersed underwater, as reported in a few Refs. [50,51] .

V > V D: glow discharge region; current transport happensthrough glow discharge. The current begins to increase steadilywith the rising voltage and the glow becomes more luminous. Insection DE, the color of glow discharge is similar to the ame of metal ion in electrolyte solution.

2.1.2. Critical voltageIn I –V curve, V B and V D are two signicant points marking the

breakdown of normal electrolysis and the onset of glow discharge.Initially, Senguptaand Singh studied theonsetand locationof CGDEand found that V B was signicantly sensitive to variation in theanolyte surface tension, the anode wire dimension and the anodematerial [23] . V B was 125 V with equi-conducting electrolyteswhen platinum wire diameter was 0.35mm and temperature is70 ± 2 ◦ C. V D was 420 V in anodic CGDE and was unaffected by theelectrolyte’s composition, concentration, temperature and surfacetension [23,28] .

Recently, it has been reported that initial conductivity of thesolution is one of the most important parameters for the onsetof glow discharge. V B and V D decrease with the increasing con-ductivity and then maintain a certain value [48,49] . What’s more,much attention has been paid to V D in CGDE, since glow discharge

takes place and large amount of active species form only when theapplied voltage is above V D. It was found that V D decreased from542V to 438V when the conductivity of Na 2 SO4 solution was inthe range of 1.00–5.99 mS/cm. When the conductivity of Na 2 SO4solution was 7.00, 8.20, 9.25, 10.48 and 11.38mS/cm, the values of V D were 432, 431, 425, 420 and 425V, which were close to 420 V,obtained from Sengupta’s experimental result. That is, when theconductivity of solution was in the range of 7.00–11.38mS/cm, theeffect of conductivity on V D was largely insignicant. In addition,CGDE was also conducted in different electrolyte with the sameconductivity (5.00 mS/cm). It was also found that all values of V Dwere very close in different electrolytes with the same initial con-ductivity of solution. That is, V D was unaffected by the electrolyte’scomposition if the conductivity of solution was given a specic

value in a certain range.


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X. Wang et al. / Electrochimica Acta 83 (2012) 501–512 503

Primary Reaction Zone

⋅H ⋅OH H2 H2O2

Discharge

Secondary Reaction Zone

S S SBulk Solution

Fig. 2. Hickling’s theoryabout GDP(S denotes substratein thesolution).

The critical voltage in multi-anode CGDE was also investigated.It wasnotedthat critical voltages with multi-anode were similar tothat with single anode, since multi-anode was parallel connectedin the circuit after all [13] .

2.2. Energetic species in glow discharge plasma

Initially, Hickling and Ingram thought that GDP was analogousto alpha particles in radiation chemistry [19,22] . As shown in Fig.2 ,within the primary reaction zone, water was ionized or activated,and then bombard each other to break up by charge transfer. Theultimate result was the production of free • OH radicals and some-times • H. They may interact each other to form H 2 O2 , HO2 • andH2 . Thus, a mixture of • OH, • H, H2 O2 and H 2 diffused out of theprimary zone (that was called plasma later) and interacted eachother or wasscavenged by substratein thesecondary reactionzone.They thought that the primary reaction zone may be likened to aspur in radiationchemistry. Thereactionzone might be consideredas a single spur, which was located in a xed position and con-stantly renewed. Later, the production of • OH radical in the liquidwas proved by an ESR study using 5,5-dimethylpyrroline-1-oxide(DMPO) as the spin trap [33,34] . Emissions of • H and • OH radicalswere also observed using a photo-multichannel analyzer with awavelength range from 200 to 900 nm [35] .

Sengupta et al. paid much attention to nonfaradaic yields of anodic CGDE. They found that nonfaradaic yields of anodic CGDEoriginated in two different reaction zones: the plasma around theanode and the liquid phase near the plasma–anolyte interface[24,28] . In the reaction zone within the plasma around the anode,H2 O vapor molecules dissociated into H 2 and O 2 E (O2 in excess of the Faradaic yield), according to the following mechanism.

H2 O → H• + • OH (1)

• H + • H → H2 (2)

• OH + • OH → H2 O + 1 / 2O2 (3)

• H + • OH → H2 O (4)

In theliquid-phasereactionzone nearthe plasma–anolyte inter-face, several liquid H 2 O molecules broke up into H 2 and H 2 O2 plusO2 E by being bombarded by each H 2 O+gas from the anodic plasmafollowing Hickling’s radiolytic mechanism.

H2 O+ gas + nH2 O → n • OH + n • H (5)

• H + • H → H2

(6)

• OH + • OH → H2 O2 (7)

• OH + H2 O2 → HO2 • + H2 O (8)

• OH + HO2 • → H2 O + O2 (9)

• OH + • H → H2 O (10)

Recently, the dissociation of water molecules in GDP has often

been written as following equation [43,44] :

H2 O+ + nH2 O → H3 O+ + n • OH + (n − 1) • H (11)

The value of n was estimated by using ferrous ion and cerousion as • OH scavenger and the results were 7.9–10.6mol [22] and12mol [27] • OH radicals for the passage of each mol electron of electricity, respectively. Using iso-propanol, n-propanol, n-butanolandacetoneas • H scavenger, the value of n isestimatedtobeashighas 9.8 [52] . Hydroxyl radical is a short lived species, and they canrecombineinto H 2 O2 . Therefore, • OHandH 2 O2 canbe continuouslyproduced in GDP in and in contact with liquids. • OH and H 2 O2 areall oxidative agents, especially • OH withhigher oxidation potential(2.8 V), which could mineralize nearly most of organic pollutants.

3. Experimental features

3.1. Glow discharge reactors

A variety of different types of reactors have been studied for thegeneration of GDP in and in contact with liquids. The reactors canbe divided into two groups according to the location of the glowdischarge. The rst group is around the pointed metallic anode inand in contact with liquid. Another is around the small hole ondiaphragm which separate reactor into two compartments.

With anode in contact with liquids, GDP takes place around thepointed anode and interface between gaseous and liquid phase,which is often termed as CGDE and the most commonly used

conguration listed in Fig. 3(a), where the anodic compartmentand cathodic compartment are separated by a sinter glass disk of medium porosity [9,12,53–70] . Pointedplatinum wire is often usedas the anode. The cathode such as platinum wire, carbon rod andstainless steel stick are usually preferred. H type reactor with twoequal volume compartments in references [10,71–73] also belongsto this kind of reactor. Multiple anodes are used to generate largevolume GDP in contact with liquids. An improved reactor has beendeveloped by replacing platinum wires with stainless steel wires[13] .

When the anode is placed in liquid shown in Fig. 3(b), it hasever been termed as direct glow discharge electrolysis [11,74]or submersed glow discharge [75] , glow discharge electrolysis[50,51,76,77] in literatures. Some researchers believe that reactor

(b) could prolong the lifetime of platinum anode when the anodewas placed underwater. But, sometimes, the anode wire still meltsdue to excess heat. The anode is still a pointed platinum wire andthe cathode can be graphite stick. Similarly, multiple anodes areused to generate large volume GDP underwater [78] .

When GDP occurs around a small hole on diaphragm shownin Fig. 3(c), it is termed as diaphragm discharge or diaphragmglow discharge (DGD) [29–32] . A quartz (1.0mm in thickness) andPET diaphragm (thickness of 0.25mm) are used as diaphragm.The diameter of small hole is in the range of 0.25–2.0mm. Twographite electrodes act as the same role in normal electrolysis andare placed in each compartment, respectively. GDP occurs aroundthe small hole which can induce chemical reactions in both anodicand cathodic compartment. The role of small hole in reactor (c) issimilar to the pointed needle anode in other reactor.


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Fig. 3. Overview of experimental setups developed for GDP.

3.2. Inuence of operating parameters

3.2.1. Applied voltageThe chemical reactivity of GDP depends on operating param-

eters such as applied voltage, current, conductivity, pH value of solution and electrolyte constituents.Most of these parameters areoptimizedfor achieving thehigherefciency andlower energy cost.Applied voltage is keyparameter forthe chemicalreactivityof GDP,since glow discharge takes place only when the applied voltage isabove the critical voltage [22–28,48,49] . The applied voltage canbea certainvalue above thecritical voltage.Some experimental resultsshowed that increasing applied voltage facilitated the treatmentprocess [79] , since the rise in applied voltage accelerate the forma-tionof energeticspecies [77] . However, it was also found that whenthe applied voltage increased to a certain value, the concentrationof H2 O2 and the degradation of substrate in both compartment of reactor (c) increased little [30] . Furthermore, the applied voltagecannot betoo high intermsof energy efciency. Inaddition, erosion

of theanode andthe small hole appeared when applied voltage wastoo high. Table 1 lists the voltage usually applied for different reac-tors. In practice, 500 V is usually employed as the applied voltagefor the generation of GDP in contact with liquid [9,12,13,53–68] ,600V for GDP in liquid [11,50,51,74,75,78] , 750V for DGD [30] ,respectively.

3.2.2. Current The effect of current on the degradation of substrates was only

investigated with reactor (a). The current was increased by theincreasing depth of the anode. The concentration of H 2 O2 andremoval rate of substrate increased with the increase of current[63] . As an example, when the current was 100mA, about 60% of nitrobenzene could be removed and 2.3mmol of hydrogen perox-

ide produced after 50min

treatment. However, when the currentincreased to 400mA, more than 90% of nitrobenzene could beremoved and 4.0mmol of hydrogen peroxide produced within25min.Accordingly,the rst-order rateconstantlinearlyincreasedwith the increase of current [15,55,62] . Liu et al. thought that the

Table 1Summary for different reactors.

Reactor a b c

Location of GDParound

Pointed anode gas/liquid interface

Pointed anodein liquid

Small hole inliquid

Voltage usuallyapplied (V)

500 600 750

Supportingelectrolyte

2g/LNa 2 SO4 2g/LNa 2 SO4 4g/LNa 2 SO4

increase in the plasma zone in contact with the solution explainedthe increase in the degradation rate of chlorobenzene [62] . But Jiang et al. believed plasma zone was so small and could be neg-ligible in comparison with that in the bulk zone. The increase of

current actually increased the concentration of •

OH and H 2 O2 inbulk solution [15] . In practice, current uctuates due to the changeof solution conductivity. Therefore,muchattention was paid to theinitial conductivity of electrolyte solution.

3.2.3. Initial conductivity of solutionSince the glow discharge is transferred by ions, it is evident that

the conductivity of the solution has an inuence on the onset of GDP. Therefore, the initial conductivity of solution is an importantparameter for GDP. It has been reported that the critical voltage inCGDE decreased with the increasing conductivity of solution, andthenmaintaineda certainvalue [49] . If the initialconductivityis toolow, there areonly fewcharge carriers in system, and subsequentlythe resistance is so high that the applied voltage must be high

enough to initiate for GDPgeneration.For example,whenthe initialconductivity of solution in reactor (c) is 0.1 mS/cm, only when theapplied voltage is 2.2kV, the discharge phenomenon appears [80] .However, when the initial conductivity increased to 6.4 mS/cm,750 V is enoughfor the onset of DGD [30] . Furthermore, it has beenfound that the concentration of active species such as • OH, • H andH2 O2 increased with the increasing initial conductivity of solution[77] . The discoloration rate of crystal violet increased from 20% toabout 95% when the concentration of Na 2 SO4 increased from 1 g/L to4 g/L after 20min GDP treatment with reactor (b) [79] . However,the erosion of the platinum anode is evident when the concentra-tion of Na 2 SO4 was 4 g/L. Therefore, too higher initial conductivityshould be avoided.In addition, theconductivityof solutionchangesduring thetreatment. Usually,conductivity increasesdue to thefor-

mation of organicacids withorganic pollutants as substrate [68,69] .Therefore,too highor too lowinitialconductivityshouldbe avoidedin terms of energyefciency. Usually, 3–7mS/cm is selected for theinitial conductivity of electrolyte solution. Therefore, GDP treat-ment has the advantage for real wastewater containing salts.

3.2.4. Initial pH value of solutionThe initial pH value is an important factor in wastewater treat-

ment.The effectof initial pHof the solutionhasbeeninvestigated bymany researchers. For different substrates, pH value shows differ-enteffects. Theremoval rate of some substrates was not inuencedby pH [62,71] . For example, pH had little effect on the rst-orderrate constants of chlorobenzene and the rate constants were allabout1.96 × 10 − 2 min − 1 wheninitialpH valuewere3.0,5.0,6.5 and

9.0 [62] . However, many authors think that pH of thesolutionplays


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a signicant role in theoxidationpowerof GDP.Some experimentalresults showed that the faster removal rate could be obtained at arelatively lower pH value, since more • OH and H 2 O2 are generated[77] and the oxidative potential of • OH is stronger in acid solution(the oxidative potential of the hydroxyl radical is 2.70V at pH 3.0and 2.34V at pH 9.0) [15,29,30,74,75,81] . Especially for GDP com-bined with Fenton process, the degradation of substrate proceededrapidly in acid condition. But for some dyes such as alizarin red Sand crystal violet, it has been reported that higher discolorationrate is achieved under alkalinity condition [51,79] . In fact, pH valueof electrolyte solution changed duringGDP treatment. The value of pH usually decreased because of the formation of carbonic acid. Inpractice, the initial pH value of wastewater often varies in a largerange. Usually, pH adjustment was unnecessary before treatmentif substrate was not a given organic pollutant and the wastewaterwas acidic or neutral. Otherwise, the optimal of pH is necessary.

3.2.5. Electrolyte constituentsUsually, wastewater often contains various organics and inor-

ganic substances. Hydroxyl radical scavenger can consume theradicals and decrease the degradation process. Any organics thatcan compete with target pollutant for the available hydroxylradicals, consequently, decrease the degradation rate of the tar-get pollutant. Methanol and n-butanol have been reported ashydroxyl radical scavengers [29,64,66] . For example, the presenceof methanol decreases the degradation rate of substrate accordingto the following reaction.• OH + CH3 OH → H2 O + • CH2 OH k12 = 8 .4 × 10 8 M− 1 s− 1 (12)

Similarly, the carbonate ions and bicarbonate ions consume theavailable hydroxyl radicals by the following reactions [15,29]

CO3 2− + • OH → • CO3 − + OH− (13)

HCO3 − + • OH → • CO3 − + H2 O (14)

Although • CO3 − is an oxidizing agent, it reacts much moreslowly with substrates as compared with hydroxyl radicals. As aresult, carbonate ions also decelerate the decomposition of sub-strates. The presence of a small quantity of Na 2 CO3 and NaHCO 3 insolution has slight inuence on the degradation of substrates.

It have been reported that the presence of Cl − has enhancingeffect on the degradation rateof pollutants [64,66,70] . Cl− can reactwith ·OH and produce chlorine radical. Two chlorine radicals reactwith each other to form one molecular of chlorine. Then, HClO isformed and it also can react with H 2 O2 to produce singlet oxygen.• OH + Cl− → Cl• + OH− (15)

Cl• + Cl• → Cl2 (16)

Cl2 + H2 O → HCl + HClO (17)

2HClO + H2 O2 → 2Cl− + 1 O2 + 2H+ (18)

The singlet oxygen is strongly oxidizing and can oxidize sub-strates. As a result, the presence of Cl − accelerates the degradationprocess. Similar results are achieved in solutions containing otherhalogen anions [31] .

However, another study shows that the presence of low con-centrations of Cl − ions in the solution may have inhibitory effectson dyedegradation, and that only higherconcentrations of Cl − ions(>0.02 M) can signicantly enhance the dye degradation efciencyduringGDP treatmentbut notincrease dye mineralization. In addi-tion, the presence of Cl − results in the formation of adsorbableorganic halogens (AOX) [81] . Although the results indicated thatAOX could be abated with prolonged treatment, this observation issignicant for the assessment of the environmental impact of GDP

in the presence of Cl−

.

MetalionssuchasFe 2+ , Fe3+ ,Cr(VI),Co2+ , Mn 2+ andCu 2+ displaysenhancing effects on the degradation rate of organic pollutants[10,15,64,70,71] . All of these metal ions except Cr(VI) can decom-pose H 2 O2 into • OH, which accelerate the degradation of substrate.The enhancing effects of Fe 2+ , Fe3+ and Cr(VI) are more apparentthan Co 2+ , Cu2+ and Mn 2+ , whichis discussedin the following parts,accordingly.

3.3. Combined process

3.3.1. Fenton processSince a lot of H 2 O2 form during GDP treatment, it is useful to

add iron salts to convert H 2 O2 into • OH and enhance the removalof pollutants through Fenton’s reactions. Manyresearchers employferric and ferrous ions as catalyst in GDP and both of them showapparent catalytic effect on the removal of pollutants, which areshown in Table 2 . The additionof Fe 2+ produces additionalhydroxylradicals through the following reaction:

H2 O2 + Fe2+ → Fe3+ + • OH + OH− k19 = 76 M − 1 s− 1 (19)

As shown in Table 2 , Fe2+ shows apparent catalytic effect onthe degradation of dyes, phenols and other organic pollutants

[10,11,29,30,50,51,62–71,75,79,82] . Atthe same time,the optimumdosage of Fe 2+ is also investigated, since too higher concentrationof Fe2+ also consumes • OH due to the following reaction:

• OH + Fe2+ → Fe3+ + OH− k20 = 4.3 × 10 8 M− 1 s− 1 (20)

The effect of Fe 3+ is also investigated. It seems that, for dyes,the catalytic effect of Fe 2+ is more evident than that of Fe 3+ [68,70] .It has been reported that only when the concentration of Fe 3+ ionis over 120mg/L, its catalytic effect is better than that of Fe 2+ ionduring GDP treatment for polar brilliant B [64] . Many researchersbelieve that the catalytic effect of Fe 3+ is not apparent than that of Fe2+ , since the rate constant of Fe 2+ ion with H 2 O2 (76M − 1 s− 1 )is almost four orders of magnitude larger than that of Fe 3+ ionwith H 2 O2 (0.01M − 1 s− 1 ). And the oxidativeability of HO 2 • is much

lower than that of •

OH.Fe3+ + H2 O2 → Fe2+ + HO2 • + H+ k21 = 0.01 M − 1 s− 1 (21)

HO2 • + H2 O2 → • OH + H2 O + O2 k22 = 3 M − 1 s− 1 (22)

But Wang et al. found that Fe 3+ showed better catalytic effectthan Fe 2+ during the degradation of aromatic organic substrate[29,30,62,63,65–67] . The possiblemechanism may be thefollowingreactions besides the aforementioned reactions:

RH + • OH → R • + H2 O k23 = 10 7 –10 10 M− 1 s− 1 (23)

R • + Fe3+ → R + + Fe2+ k24 k23 (24)

H• + Fe3+ → H+ + Fe2+ k25 = 2.0 × 10 6 M− 1 s− 1 (25)

The hydroxyl radicals in GDP can both react with RH to produceR • radicals and with Fe 2+ ions to produce Fe 3+ ions. As for k24 k23 ,Fe3+ ion can be quickly transformed to Fe 2+ ion by R • . As a result,alot of Fe 2+ ions were formed in the solution and the newly formedFe2+ ions further react with H 2 O2 in the solution to produce morehydroxyl radicals. On the other hand, Fe 3+ has the scavenging abil-ity for H • shown in Eq. (25) , which makes more hydroxyl radicalavailable for the oxidation of substrate. The fact that H 2 producedin the anodic compartment is reduced with the addition of Fe 3+

veried this [45] .

3.3.2. TiO 2 lmAs GDP enables the generation of numerous high energy par-

ticles integrated with light, electricity and chemical oxidation,

functioning as a multiple energy eld, there exist the conditions


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forthe combination of TiO 2 and GDP [14] . Initially, TiO 2 was immo-bilized on glass by using sol–gel method to couple with GDP withreactor (a). The results showed that the TiO 2 lm with 5 layershad high catalytic activity and good durability. Using the couplingsystem, the degradation efciency of methylene blue ( C 0 =5mg/L)reached up to 95% after 15min treatment [83] .

Recently, TiO 2 lm has been developed to newly generatein situ with Ti electrode as the anode [14] . TiO2 lm was formedon the Ti anode, which then broken down when GDP occurred.The system realizes the process of electrode catalysis, as well aselectrolyte catalysis. Under optimum conditions (550 V; electrodedepth: 2mm, supporting electrolyte: 5 g/L Na 2 SO4 ; pH = 2 ), themaximum decolorization ratio of 100 mL of 20mg/L brilliant redB solution was 97.8% after 40min treatment. GDP integrated withTiO2 catalysis system on the electrode creates a mutually couplingand self-exciting system showed the synergic function of the mul-tiple energy elds, which had promising prospects for practicalapplication.

4. Environmental applications of glow discharge plasma

GDP has been applied for the degradation of wastewater con-

taining dyes, phenols, benzene derivatives, algae and so on. Most of these studies are to demonstrate the viability of GDP for wastew-ater treatment. In addition, much attention has been paid to thedetection of intermediates, degradation mechanism and kineticbehavior.

4.1. Environmental applications

4.1.1. DyesEfuents from textile dyeing industry cause severe environ-

mental problems due to the characteristics of strong color, highchemical oxygen demand (COD) and low biodegradability. Beingtoxic to microbes, dyes-containing wastewater cannot be treatedwellby biological processes.In addition, aquaticmetabolism is sup-pressed due to the shortage of sunlight, which in turn lowers theself-purication ability of thewastewater. Therefore,the treatmentof dyeing wastewater is becoming a matter of great concern. GDPhas proven effective for the discoloration of dyeing wastewater.

The discoloration of dyeing wastewater has been car-ried by different reactors shown in Fig. 3 , as well combinedwith Fenton process. Different kinds of dyes have beenemployed as target pollutants for the viability of GDP[10,13,14,31,32,50,51,64,68–71,73,78,79,81,83] . Most of researchers employ GDP combined with Fe 2+ for dye discol-oration in short time, which is displayed in Table 2 . For example,wastewater containing cationic blue dye (25.0 mg/L) underwentefcient degradation with reactor (a) assisted by Fe 2+ . The degra-dation ratio was about 99.7% after 3 min treatment under 500 V

when the concentration of Fe 2+ was 20mg/L. The removal of CODreached up to 58.7% after 10min treatment, which was prior to55.7% after 90min treatment without Fe 2+ [68] .

Gao et al. investigated the discoloration of Brilliant Green,with reactor (b). COD removal reached more than 95% within10min with Fe 2+ (2.0 × 10 − 4 mol/L). GC–MS was used to identifythe intermediates and found that benzoic acid, 1,2,3,4,5,6-cyclohexanehexaol and organic acid formed after 5 min treatment.Then possible pathway was assumed that Brilliant Green moleculewas cleaved into benzoic acid and 1,2,3,4,5,6-cyclohexanehexaolat rst, and further producing lots of smaller molecule of organicacids after opening the ring [50] .

Reactor (c) is also used for dye discoloration such as Direct Red79. It was found that higher degradation efciency was obtained

in the anodic compartment of the diaphragm reactor where nal

decomposition efciency reached 80% after 30min treatment. Thedecomposition products were also analyzed by HPLC–MS duringthe GDP treatment. It was found that ve main compounds in theanolyte and another three ones in the catholyte [32] .

4.1.2. Phenol and phenolic compoundsPhenol and phenolic compounds are widely used in the

manufacture of antioxidants, disinfectants, pesticides, and othersynthetic materials. Therefore, such contaminants are commonlyfound in industrial wastewater. Reactor (a), (b) and (c) are used forphenol degradation and GDP has proven efcient for the remedi-ation of efuents containing these pollutants. Especially, by GDPcombined with Fenton process, phenols can be degraded in shorttime shown in Table 2 . Furthermore, more attention is paid tothe degradation pathway of phenol, kinetics and energy efciency,besides the study of the optimum conditionwith different reactors.

The degradation of phenol has been systematically investigatedwith GDP without catalysts and in the presence of Fe 2+ and Fe 3+ .Without catalysts, the degradation of phenol proceeded smoothlyinduced by GDP [9,11] . The concentration of phenol decreasedexponentially with the discharge time and approximately 80%phenol disappeared after 2h with reactor (a). Three isomericdihydroxylbenzenes and hydroxyhydroquinone were identiedand the distribution of dihydroxylbenzene intermediates wascatechol> hydroquinone resorcinol. In addition,catechol,hydro-quinone and hydroxyhydroquinone did not disappear after 2htreatment. It was also found that both oxalic and formic acidswere detected as major products. Other dicarboxylic acids, suchas tartaric acid, malic acid and malonic acid were also detectedduring GDP treatment. Basedon the intermediate products,Tezukabelieved that thecollapse of phenolproceeded in a successive man-ner as follows: (1) hydroxylation of benzene ring, (2) oxidativering cleavage leading to the formation of carboxylic acids and ulti-mately, (3) oxidation to inorganic carbon. The reaction pathway of phenol without catalysts was shown in Fig. 4(a).

Initially, much attention has been paid to the addition of Fe 2+ ,since Fe 2+ can converthydrogenperoxide in solution to • OH, whichapparently accelerates the degradation of phenol. When the con-centration of Fe 2+ was 60 mg/L, thedegradationefciency of phenolwas upto 55% after 30min treatment under 600V with reactor (b).The intermediate products were determined by HPLC. It was foundthat benzonquinone (not detected without Fe 2+) was detected inconsiderable amounts besides o-dihydroxybenze, hydroxyhydro-quinone, p-dihydroxybenze, and carboxylicacids [11] . Accordingtothe intermediate products aforementioned, the reaction pathwayof phenol in the presence of Fe 2+ was proposed, which is shown inFig. 4(b).

Wang and Jiang found that both Fe 2+ and Fe 3+ showed the cat-alytic effect on the degradation of phenol [29,65] . COD and TOCsharply decreased in the presence of iron salts. In addition, the

catalytic effect Fe 3+ was better than that of Fe 2+ . 100mg/L phe-nol solution was treated with reactor (a) assisted by Fe 2+ and Fe 3+ ,respectively. The phenol removal was 84% with Fe 3+ and 75% withFe2+ after 6min treatment when their concentrations were both37mg/L. Possible mechanism is shown in Fig. 4(c). When phe-nol underwent GDP treatment, dihydroxycyclohexadienyl radicalsformed due to the attack of the hydroxyl radicals on benzene ring.Possessingreducing nature, thedihydroxycyclohexadienyl radicalscan rapidly reduce Fe 3+ to Fe 2+ . A lot of Fe 2+ were formed in thesolution and further reacted with H 2 O2 to produce more hydroxylradicals. The concentration of H 2 O2 in solution veried this. Therewas a lot of H 2 O2 accumulated in the solution in the presence of Fe2+ only, Fe 3+ only, Fe 2+ plus phenol. However, no H 2 O2 accumu-latedinthepresenceofFe 3+ plusphenol.The intermediate products

such as benzoquinone, hydroquinone and catechol formed during


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OH

OH

OH

OH

OH

OH

OH

OH

OH

OHOH

carboxylicacids

(a)

OH

OH

OH

OH

OH

O

O

O

O

carboxylicacids

(b)

OH

OH

OH

OH

OH

OH

OH

OH

OH

O

O carboxylicacids

Fe 3+

Fe 3+

(c)

Fig. 4. Reaction pathway forphenol oxidationin GDP(a) without catalyst (b)in thepresence Fe 2+ (c) in the presence of Fe 3+ .

the rst 10min GDP treatmentwhen theconcentration of Fe 3+ was37mg/L.

In addition, Tezuka investigated kinetics of phenol degradationwith reactor (a). The experimental results showed that the initialrate of phenol degradation might be governed by the rst-orderrate law, when the initial concentration was sufciently low and,

vice versa, the zero-order rate law [58] .Liu and Jiang also investigated the degradation of phenol under

acidic, neutral, and basic conditions with reactor (c) [29] . In a basicmedium, the phenol degradation rate was slower and the byprod-ucts were less abundant than those in acidic medium. The productdistributionwas also differentin basic mediumas compared to thatin neutral or acidic medium. In basic medium, the phenols existin phenolate ion, which makes the benzene ring more suscepti-ble to hydroxyl radical attack, owing to the electrophilic nature of the hydroxyl radicals and the electron-donating properties of phe-nolate ions. As a result, the hydroxylation reaction prevailed, andthe hydroquinone and pyrocatechol were the major intermediateproducts. In acidic medium, phenol was present in its undissoci-ated form, making the hydrogen abstraction of the phenolic group

by the hydroxyl radical prevail, producing 1,4-benzoquinone.

Table 3Hydroxylation products of different benzenic derivatives.

No. Substracts Products Ref.

1

OHOH

OH

OH

OH

[9,11]

2

COOH

OH

COOHCOOH

HO

[54]

3

NH 2

NH 2

OH

NH 2

OH

NH2

OH

[55]

4

ClCl

OH

Cl

OH

Cl

OH

[62]

5

NO 2

NO 2

OH

NO 2

HO

NO 2

OH

[63]

Others also investigated the degradation of nitrophenol [67,75]and chlorophenols [30,53,59–61] induced by GDP. Most abundantintermediate productswere formed byattacking theortho-or para-positionof OHdueto the highelectrophilicnature ofthehydroxyl

radicals. Other intermediate products were similar to those of phe-nol. Chlorine atoms in the organics were liberated as chloride ions.CODand TOC sharply decreased in the presence of Fe 2+ and Fe 3+ . Inaddition, Fe 3+ showed a better catalytic effect than Fe 2+ .

4.1.3. Other benzenic and heteroaromatic derivativesOther benzenic and heteroaromatic derivatives are also inves-

tigated by GDP treatment with different reactors. For benzenicderivatives, carboxylic acids during ringcleavage are similar to thatof phenol. Oxalic, formic and malonic acids are also found amongthe product mixture. The difference results from the hydroxylationof benzene ring. The hydroxylation products of different benzenicderivatives are listed in Table 3 .

As shown in Table 3 , the hydroxylation products of phenol

are catechol and hydroquinone [9,11] . However, when aniline issubject to GDP, p-aminophenol is predominant product, whilethe minor isomers, o-aminophenol and m -aminophenols areyielded in comparable amounts [55] . For benzoic acid, both m -hydroxybenzoic acid and p-hydroxybenzoic acid are yielded incomparable amounts, while o-hydroxybenzoic acid cannot bedetected [54] . With nitrobenzene as substrate, the distributionof the intermediates of nitrophenol during GDP treatment fol-lows the order of o-nitrophenol> p-nitrophenol> m -nitrophenolandwith the ratio of 1.0:0.85:0.70 at 40min [63] . When chloroben-zene is underwent GDP treatment, the chlorophenol intermediatesare found to be in the following order: p-chlorophenol > o-chlorophenol> m -chlorophenol [62] . Steric hindrance, conjugatingeffect accounts for the discrepancy of the isomeric ratio caused

by hydroxyl radicals. Since hydroxyl radical has the electrophilic


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reactivity, with phenol as substrate, the hydroxyl group is smallerin size and ortho, para-directing rather than meta-directing arefavorable in the hydroxylation of phenol. However, for benzoicacid, the size of carboxyl group is large enough to cause sterichindrance to the ortho position, so para- and meta-directing prop-erties are almost equal. For aniline and nitrobenzene, the sterichindrance of ortho-position can be ignored compared with car-boxyl group and this will increase the chance of hydroxyl radicalto attack on ortho- and para-position of nitro group and aminogroup. Since nitro group has the electron-withdrawing effect onthe benzene ring, o-nitrophenol is more stable than the p- or m -nitrophenol. As a consequence, the o-nitrophenol is the majorhydroxylating products in the degradation of nitrobenzene. How-ever, amino-group hasthe electron-donatingeffecton benzeneringand p-aminophenol is more stable than o- or m -aminophenols.Therefore, p-aminophenol is predominant product, with anilineas substrate. For chlorobenzene, although Cl group has electron-withdrawing effect on benzene ring, it is also the donator of lonepair of electrons to the ortho- and para-positions of the benzenering by conjugation. Therefore, p-chlorophenol is more predomi-nant in the intermediate products.

Ontheother hand, thehydroxylationof benzenering alsoresultsin the formation of phenol and inorganic ions [62] . With benzenicderivatives containing Cl group as substrate, the concentration of Cl− increases with the treatment time. With benzenic derivativescontaining NO 2 as substrate, nitrate ion increases at a much higherrateduring the whole process, whereas nitrite ion decreases slowlyfrom the beginningof GDP treatment [63] . This maybe the fact thatthe nitrite produced is not stable and is rapidly oxidized to nitrateby hydrogen peroxide or hydroxyl radical in GDP. Similarly, ben-zenesulfonates are exhaustively destroyed by GDP to liberate sulfogroup as sulfate ion [12,56] .

In most cases, many researchers believe that most of intermedi-ate products results from the attack of • OH. However, appreciableamount of aniline produced in the degradation of azobenzene [84]veries the role of • H in GDP. Therefore, combined effect of activespecies in GDP should be considered in the mechanism of degrada-tion.

For heteroaromatic derivatives [72,82,84] , intermediate prod-ucts are more complex than those of benzenic derivatives. GDP canmakecleavage of compounds occur at aromaticring andother bondsuch as C C, C N and N N. For example, 2-naphthylamine solu-tion is subjectedto GDP treatmentwith reactor (b)and three majorintermediate products are o-phthalic acid, o-hydroxylbenzoic acidand catechol. The attack of • OH occurs at aromatic ring and leadsto the cleavage of aromatic ring [82] . For azobenzene, it seems thatGDP leads to the cleavage of N N bond and aniline is the majorproduct [84] .

4.1.4. Reduction of Cr(VI)Initially, the application of GDP focuses on the utilization of

oxidative species in GDP such as •

OH and H 2 O2 . And it seems thatthe existence of • H would be insignicant. Wang lei et al. utilized• H in GDP for the reduction of hexavalent chromium [Cr(VI)] forthe rst time [43] . Experimental results showed that Cr(VI) couldbe smoothly reduced to Cr(III) byreactor (a) with 500 V as the opti-mum operating voltage. The reduction rate of Cr(VI) was enhancedby either decreasing thesolutionpH or adding radicalscavengers tothe solution such as ethanol and phenol. At initial pH 2.0, 100 mg/L Cr(VI) was completely reduced within 10min treatment in thepresence of 100mg/L phenol. Therefore, simultaneous oxidation of phenol and reduction of Cr(VI) could be achieved in GDP treatment[44] . Experimental results also showed that the phenol oxidationincreased evidently in the presence of Cr(VI) and the Cr(VI) reduc-tionincreased withthe additionof phenol.The possiblemechanism

can be explained as follows.

When GDP occurred, water molecules near the plasma/solutioninterface dissociated to • OH, • H and H 2 O2 . • H was captured byCr(VI) according to the following reaction.

• H + Cr(VI) → Cr(III) + 3H+ (26)

Since • OH in GDP can combine with • H to form water, there-fore, the addition of phenol reduced the concentration of • OH andmore • H radicals were available forreduction of Cr(VI). Onthe otherhand, the addition of phenol inhibited the oxidationof Cr(III), since• OH can oxidize Cr(III) and Cr(V) to Cr(IV) and Cr(VI), respectively.Based on the above reasons, both the reductionof Cr(VI)and oxida-tion of phenol were efciently accelerated in GDP treatment. Thendings would be helpful for the removal of organic and inorganicpollutants simultaneously in the complex wastewater.

4.1.5. AlgaeRecently, GDP has been utilized for the inactivation of algae in

the aqueous solution. The inactivation of Microcystis aeruginosa(MA)was investigatedwith reactor(a) [46] . Theresultsshowed thatrelatively higher applied voltage, current and treatment time ben-eted MA removal and the inactivation ratio reached 90% within5 days after 20min treatment under optimal conditions (volt-age: 530V; current: 30mA). Furthermore, the concentration of Chlorophyll-a and dehydrogenase decreased after 20min treat-ment, which demonstrated GDP treatmentcould effectively inhibitthe growth of MA and cause the decrease of MA activity and dete-riorating of cell integrity. Although the underlying mechanism of inactivation of algae with GDP remained unclear, this study stillprovided a kind of possible application of GDP for the inactivationof algae.

4.1.6. Polymer GDP was also used for the degradation of polymers. 0.1g

poly(acrylamide) was dissolvedin 50mL of 1.7 × 10 − 3 M NaCl solu-tion and underwent GDP treatment with reactor (a). The voltage

was 700 ± 10V and the current varied from 60 to 120mA.

Theanalytical results of the degradation products showed that thedegradation took place stepwise caused by the cleavages of themain and also the side chains of the polymer [85] .

Recently, GDP has been explored to enhance the adsorptionproperties of chitosan for dye removal. The results showed thatthe GDP treatment changed the morphology and crystallinity of chitosan particles, and that the number of CH 2 and CH 3 groupin chitosan samples increased. Various pretreatment parameters,including discharge current and pretreatment time, played sig-nicant roles on the chitosan modication. Dye uptake in GDPmodiedchitosanwas faster thanadsorptionin untreatedchitosan.Thus, GDP is an attractive pretreatment method for environmentaladsorption materials [86] .

4.2. Comparison with other AOPs

As a kind of AOPs, it is instructive to assess GDP by comparingwithother AOPs. Althoughthere are not systematical experimentalresults for each reactor, some results available in literatures areenough to demonstrate the advantage of GDP. Removal efciency,energy efciency and formation rate constants of active species inGDP have been compared with other AOPs in some papers.

With regard to the removal of either substrate or TOC in thedegradation of benzenesulfonate and p-toluenesulfonate, experi-mental results show that GDP with reactor (a) are comparable tothose with UV/TiO 2 and ozone [12] , which is listed in Table 4 . Incase of pentachlorophenolate, GDP seems somewhat superior to

UV/TiO2 .


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Table 4Comparison of substrate and TOC removal efciency of GDPwith other AOPs.

Substrate C 0 (mM) Method (1) (2) Ref.

Pentachlorophenolate 2.1 GDP 35 W with reactor (a) 100/120 90/240 [61]0.65 UV/TiO2 (0.1 g/L) 125 W 97/360 91/360

Benzenesulfonate 1.00 GDP 45 W with reactor (a) 100/150 89/240 [12]UV/TiO2 (500W) 100/120 90/300

p-Toluenesulfonate 1.00 GDP 45 W with reactor (a) 100/150 91/240 [12]Ozone 24mg O3 /min pH = 3 100/180 28/180

Note: (1) denotes substrate removal/reaction time (%/min); (2) denotes TOC removal/reaction time (%/min).

Besides the removal of substrate and TOC, energy efciency isanother important parameter. Generally, the energy efciency iscalculated using the following equation:

=(1 / 2)C 0V

UIt 1/ 2(27)

where C 0 is the initial substrate concentration, V is the liquid vol-ume, U is the applied voltage, I is the current, and t 1/2 is the reactiontime required for decomposing half of initial substrate.

Previous study has reported energy efciency of GDP with dif-ferent pollutants as substrate. As shown in Table 5 , with phenolas substrate, all values of energy efciency are in the magnitude of 10 − 9 mol/J, which is comparable to those in pulsed corona streamerdischarge (PCD) and spark discharge [29,65] . Similarly, the degra-dationof MTBEand bisphenolA (BPA) inducedby GDPconrms thatenergyefciency in GDPtreatment with reactor (a)is in thesimilarorder of magnitude to those in high-density plasma and ultra-sonic degradation [15,66] . With MTBE, Cr(VI) and BPA as substrate,energy efciency of GDP is higher than that with UV/TiO 2 , espe-cially in GDP combined with Fenton process [15,43,66] . Recently,Malik calculates energy yields ( G50 ) and relative energy efciency(REE) with different GDP reactors with dyes as substrate, some of which is shown in Table 6 . With methyl orange as substrate, G50with reactor (a) is 0.024 g/kWh, and REE is 1. It also demonstratesthat GDPin contact with liquid is comparable to that in PCD, whichis consistent with that with phenol as substrate aforementioned.At the same time, it was found that GDP reactor is less efcientcomparing with other plasma reactors such as dielectric barrierdischarges (DBD) and microwave discharges (MWD) according tothe value of REE [37] .

Gao et al. also compared the formation rate constants of activespecies in GDP withthose inPCD, which was shown in Table 7 [77] .It can be seen that the formation rate of H 2 O2 and • OH with reactor(d) was higher than that in PCD. What’s more, the experimentalequipment in GDP is much simpler than that of PCD. Further-more, the PCD process seems to perform best in water with lower

Table 6Comparison of energy yields ( G50 ) and relative energy efciencies (REE) of GDPreactors with other plasma reactors [37] .

Nameof dye C 0 (mg/L) Reactor G50 (g/kWh) REE

Eosin 12 b 0.029 0.07Methylene blue 12 c 0.042 0.7Methyl orange 13 a 0.024 1Eosin 12 c 0.307 0.7Methyl orange 10 Pulsed-DBD 9.38 400Methylene blue 10 MWD 0.155 3

Note : REE= (REEr)G50 /G50r , where ‘G 50r ’ and ‘REEr’ are energy yield and relativeenergy efciencyof the referencereactor. REE is beingcalculatedpulsed corona dis-charge(PCD)in water without bubbling gas, which is taken as thereference reactorand assigned REEvalue 1.

conductivity, which is notsuitablefor realwastewater thatcontainssalts. However, GDP seems to have advantage in this respect.

5. Conclusion and outlook

This manuscript provided a holistic view on theprogress of GDPin wastewater treatment, as well as mechanisms of GDP generationand the removal of hazardous pollutants, with the aim of drawingmore attention to this emerging technique, which exhibit uniquecharacteristic such as simpler equipment, easier operation, higherenergy efciency and environmental compatibility. What’s more,GDP was suitable for the degradation of efuents containing salts.This review also summarizes reactors commonly used for the gen-eration of GDP,coveringvarious kinds of experimental parameters,electrode materials.

As GDP enables the generation of numerous high energy par-ticles integrated with light, electricity and chemical oxidation,functioning as a multiple energyeld,thereexist the conditions forthe combination of catalysis and GDP. The synergistic combinationof GDP and catalysis suchas TiO 2 and Fentonprocess canmore ef-ciently utilize active species and energies generated in GDP, which

Table 5Comparison of energy efciencyof GDPwith other AOPs.

Substrate C 0 Method Energy input Ref.

Phenol 100 mg/L GDP with reactor (a) 50 W 0.6 × 10 − 9 mol/J [65]GDP with reactor (a)+ Fe 2+ 37 mg/L 50 W 7.4 × 10 − 9 mol/JGDP with reactor (a)+ Fe 3+ 37 mg/L 50 W 9.5 × 10 − 9 mol/JGDP with reactor (c) 99 W 3.1 × 10 − 9 mol/J [29]

489 M PCD 40 kW 1.04 × 10 − 9 mol/J532 M Spark discharge 2 MW 2.28 × 10 − 9 mol/J

MTBE 50 mg/L GDP with reactor (a) 53 W 0.76 × 10 − 9 mol/J [15]50 mg/L High-density plasma 200 W 2.65 × 10 − 9 mol/J200 mg/L GDP with reactor (a) 53 W 3.00 × 10 − 9 mol/J200 mg/L UV/TiO 2 1 kW 1.89 × 10 − 11 mol/J

Cr(VI) 50 mg/L GDP with reactor (a) pH = 2 50 W 5.0 × 10 − 4 mg/J [43]36 mg/L UV/TiO 2 (0.5 g/L) pH = 1 125 W 1.0 × 10 − 5 mg/J

BPA 100 mg/L GDP with reactor (a) Na 2 SO4 50 W 6.1 × 10 − 5 mg/J [66]100 mg/L GDP with reactor (a) NaCl 50 W 43.8 × 10 − 5 mg/J100 mg/L GDP with reactor (a) Na 2 SO4 +0.33mM Fe 3+ 50 W 207 × 10 − 5 mg/J40 mg/L UV/TiO 2 (1.0 g/L) 200 W 5.0 × 10 − 7 mg/J26.9 mg/L Ultrasonic degradation 80 W 4.2 × 10 − 5 mg/J


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