Chemical Engineering Journal 245 (2014) 41–46

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Chemical Engineering Journal

journal homepage: www.elsevier .com/locate /cej

Removal of p-chlorophenol in mist by DC corona discharge plasma

http://dx.doi.org/10.1016/j.cej.2014.01.0891385-8947/� 2014 Elsevier B.V. All rights reserved.

⇑ Corresponding author. Tel./fax: +86 10 82314215.E-mail address: zhutl@buaa.edu.cn (T. Zhu).

Xiao Yan a, Tianle Zhu a,⇑, Xing Fan b, Yifei Sun a

a School of Chemistry and Environment, Beihang University, Beijing 100191, Chinab College of Environmental and Energy Engineering, Beijing University of Technology, Beijing 100124, China

h i g h l i g h t s

� The removal of 4-CP and TOC improves as input power increases.� 4-Chlorocatechol is the dominating hydroxylated intermediate.� Most of the chlorine atoms are released from the removed 4-CP to form Cl�.� Yields of ring-opening and ring-containing products take up less than 50% and 1%.� Both short-living and long-living plasma species contribute to 4-CP degradation.

a r t i c l e i n f o

Article history:Received 29 November 2013Received in revised form 25 January 2014Accepted 28 January 2014Available online 5 February 2014

Keywords:4-ChlorophenolDC corona discharge plasmaMistOzoneDFT

a b s t r a c t

Volatile organic pollutants in mist are becoming of great concern around the world. In the present study,removal of p-chlorophenol (4-CP) in mist by corona discharge plasma was investigated using a link toothwheel-cylinder plasma reactor energized by a positive DC power supply. The results indicate that the cor-ona discharge is effective in removal of 4-CP in mist. The removal of 4-CP and TOC (total organic carbon)improves as input power increases. With the help of Gaussian03 package program combined with fron-tier orbital theory and density functional theory, it was speculated that 4-chlorocatechol was the domi-nating hydroxylated intermediate and C–Cl would be initially attacked by the plasma species to form Cl�

due to its longest bond length, which is verified by HPLC and IC analysis results of 4-CP degradation prod-ucts. Most of the chlorine atoms are released from the removed 4-CP to form Cl�. The Cl� takes up morethan 93% while the 4-chlorocatechol accounts for less than 1% of the reacted chlorine. The ring-opening(maleic and oxalic acids) and ring-containing products (mainly 4-chlorocatechol) account for less than50% and 1% of the converted carbon respectively under the investigated conditions, with the other con-verted carbon being contained in inorganic products such as CO2 and CO. Both short-living and long-liv-ing plasma species contribute to 4-CP degradation. Based on the analysis of 4-CP degradation productsand the roles of active species in 4-CP degradation, possible pathways of 4-CP degradation in mist wereproposed.

� 2014 Elsevier B.V. All rights reserved.

1. Introduction

Volatile organic compounds (VOCs) in mist are becoming ofgreat concern around the world [1]. Conventional technologies,such as adsorption [2,3], catalytic oxidation [4] and biological pro-cess [5,6], have been widely employed to degrade VOCs in gasstream. However, the adsorption performance and catalytic activ-ity readily deteriorate in the presence of mist [2–4]. On the otherhand, chlorinated organic compounds are found to be resistant tobiochemical degradation, and their inherent toxicities to microor-ganisms inhibit their complete mineralization in biological process

[5,6]. Thus, it is highly recommended to find an alternative methodto treat VOCs in mist.

Non-thermal plasma, especially corona discharge plasma, hasreceived great attention in pollution control field because it isappropriate for simultaneous treatment of various refractory pol-lutants by a large amount of oxidative species even if under com-plex atmosphere [7–10]. Previous studies have demonstrated thatorganic compounds in exhaust gas and wastewater can be satisfac-torily degraded by corona discharge plasma [11–14]. Therefore, itis suggested that corona discharge plasma may be powerful en-ough to treat organic pollutants in mist atmosphere. When mistcontaining VOCs passes through the discharge zone, the gas mole-cules can be excited, ionized and dissociated by high energy elec-trons, producing chemically active species to react with VOCs. In

42 X. Yan et al. / Chemical Engineering Journal 245 (2014) 41–46

fact, our previous research has proved that the phenol in mist canbe degraded efficiently by corona discharge plasma [1]. However,the underlying mechanisms involved in VOCs degradation in mistare not well understood yet.

In the present study, p-chlorophenol (4-CP) was chosen as thetarget pollutant due to its toxicity and refractory to biochemicaldegradation. The removal of 4-CP in mist by corona discharge plas-ma was investigated using a plasma system with a well applicationperspective because of both its simple internal structure and lowpressure drop. In order to clarify the mechanisms (pathway) of4-CP degradation, the degradation products were analyzed boththeoretically and experimentally, and the roles of active speciesin 4-CP degradation were investigated.

2. Experimental

The experimental set-up is illustrated in Fig. 1. It consists of ageneration unit of mist containing 4-CP, a reaction gas supply unitwith a set of mass flow controllers, a plasma reactor energized by a25 kV/5 mA positive DC high-voltage power supply and a series ofanalytical instruments.

A stainless steel cylinder with an inner diameter of 42 mm andlength of 450 mm served as the ground electrode of the plasmareactor, while a stainless steel rod with an outer diameter of6 mm through which 25 discharge teeth wheels were linked witha space interval of 15 mm was used as the discharge electrode.There are eight teeth per discharge tooth wheel and the angle ofdischarge tip is 30�. The picture of corona discharge generated ina link tooth wheel-cylinder has been reported in our previousstudy [15], which showed a brilliant and uniform streamer coronaacross the entire interelectrode space of the plasma reactor.

The mist was produced by the ultrasonic nebulizer with anaverage nebulization rate of 0.75 mL/min for 4-CP solution(100 mg/L). The mist was introduced by passing dry air throughthe ultrasonic nebulizer at a flow rate of 2.0 L/min. In order toinvestigate the role of plasma species, 4-CP removal experimentsby discharge plasma and by ozonation were separately performed.Ozone (O3) produced by an ozone generator was introduced to thereactor and the plasma was turned off during the ozonation exper-iment, while the ozone generator was turned off and the plasmawas turned on during the discharge plasma experiment. All thetests were carried out at room temperature and atmospheric pres-sure. All the experiments were conducted in triplicate under iden-

DC power supp

Air

MFC

Buffer flask

MFC

Ozone generator

Fig. 1. Schematic diagram of the experime

tical conditions and the results were presented as the averagevalue.

When 4-CP mist passes through the discharge zone, some of themist droplets can be trapped by the reactor and gathered by thecollecting tank as trapped solution. 4-CP and products depositedon the inner wall and electrodes of the plasma reactor were col-lected using ultrapure water, named as collected solution. Beforethe mist was exhausted, it was absorbed by NaOH solution(0.5 mol/L, 100 mL) and called as absorbed solution.

4-CP was analyzed by high performance liquid chromatography(HPLC, LC-20AT Prominence, Shimadzu, Japan), equipped with anInertsil ODS C18 column (Ø4.6 mm � 150 mm) and a prominenceUV/VIS detector. The mobile phase was prepared by the H3PO4

solution (0.001, v/v) and methanol. A total organic carbon (TOC)analyzer (TC-VCPH, Shimadzu, Japan) was used to determine theTOC concentration. Qualitative and quantitative analysis of organicacids and Cl� in the products was carried out by ion chromatogra-phy (IC, ICS2000, Dionex Corporation, USA). O3 was sampled at theexit of the plasma reactor and then analyzed by indigo disulpho-nate spectrophotometric method [17]. H2O2 was collected throughfritted-glass impinger containing 10 mL titanium tetrachloride inacid solution at the exit of the reactor, followed by colorimetricanalysis at 415 nm with a spectrophotometer [18].

The 4-CP and TOC removal efficiency (g4-CP, gTOC) can be calcu-lated as follows:

g4-CP ¼mðOÞ �mðRÞ �mðTÞ �mðCÞ �mðAÞ

mðOÞ �mðRÞ� 100% ð1Þ

gTOC ¼mTOCðOÞ �mTOCðRÞ �mTOCðTÞ �mTOCðCÞ �mTOCðAÞ

mTOCðOÞ �mTOCðRÞ� 100% ð2Þ

where m(O), m(R), m(T), m(C) and m(A) represent the mass of 4-CP inthe initial solution, residual solution, trapped solution, collectedsolution and absorbed solution (mg), respectively; mTOC(O), mTOC(R),mTOC(T), mTOC(C) and mTOC(A) represent the mass of TOC in the initialsolution, residual solution, trapped solution, collected solution andabsorbed solution (mg), respectively.

The mass and yields of the organic acids or Cl� can be calculatedas follows:

Morganic acid or Cl� ¼ MðTÞ þMðCÞ þMðAÞ ð3Þ

Yorganic acid or Cl� ¼Morganic acid or Cl�

g4-CP � ðmðOÞ �mðRÞÞ� 100% ð4Þ

ly

Plasmareactor

Vent

Ultrasonic nebulizer

10 Ω

Discharge tooth wheel

D 6

10 mm

Φ8

Collecting solution

NaOH solutionV

Oscillograph

Voltagesensor

ntal set-up for 4-CP removal in mist.

0 2 4 6 8 10 12

0.00

0.25

0.50

0.75

1.00

(b)

air air with water mist air with 4-CP mist

Dis

char

ge c

urre

nt (m

A)

Applied voltage (kV)

(a)

100

200

300

400 O3- air O3- air with water mist O3- air with 4-CP mist H2O2- air H2O2- air with water mist H2O2- air with 4-CP mist

2O2 c

once

ntra

tion

(ppm

)

X. Yan et al. / Chemical Engineering Journal 245 (2014) 41–46 43

where Morganic acid or Cl� (lg) and Yorganic acid or Cl� are the mass andyields of organic acids or Cl�, respectively; M(T), M(C) and M(A) arethe mass of organic acids or Cl� in trapped solution, collected solu-tion and absorbed solution (mg), respectively.

The input power (W) and energy efficiency (g/kW h) were se-lected to value the power input to the reactor and the decomposedamount of 4-CP per 1 kW h of energy consumption, respectively.Input energy and energy efficiency can be calculated using the fol-lowing equations:

Input power ¼ UI ð5Þ

Energy efficiency ¼ g4-CP � ðmðOÞ �mðRÞÞInput power� 2

ð6Þ

where U and I are the applied voltage (kV) and discharge cur-rent (mA), respectively. The voltage was recorded by an oscillo-graph (DPO3012, Tektronix, USA) with a voltage sensor (P6139A,Tektronix, USA) whereas the current was calculated by measuringthe voltage across a 10 O resistor with a multimeter; 2 is the dura-tion time of discharge (h).

In order to clarify the mechanism of 4-CP degradation by coronadischarge plasma, the intermediates were analyzed theoreticallywith the help of Gaussian03 package program [16] combined withfrontier orbital theory and density functional theory. The 4-CP geo-metric structure was optimized using DFT (B3LYP) method with 6-31G + (d) basis set. The corresponding HOMO coefficients werecomputed at the same level with Gaussian03 package program.

0 2 4 6 8 10

0O3 a

nd H

Input power (W)

Fig. 2. The discharge characteristics (a) and active species formation (b).

0 2 4 6 8 100

20

40

60

80

100 4-CP removalTOC removal

Energy efficiency

Input power (W)

Rem

oval

eff

icie

ncy

(%)

0.25

0.30

0.35

0.40

0.45

0.50

0.55

Ene

rgy

effic

ienc

y (g

·kW

h-1)

Fig. 3. Removal efficiency and energy efficiency as functions of input power.

3. Results and discussion

3.1. Discharge characteristics and active species formation

The discharge characteristics of the plasma reactor under dry air,water mist and 4-CP mist atmosphere were first investigated, as de-picted in Fig. 2(a) (Error bars represent standard deviations based onthe replicates). The results indicate that the introduction of watermist to air resulted in a decrease of discharge current under the sameapplied voltage, which may be attributed to the fact that ion attach-ment to mist gives rise to the formation of charged mist, the driftvelocity of which is lower than that of ions. The results also indicatedthat the presence of low-concentration 4-CP, compared with water,did not basically affect the discharge characteristics.

To explore the mechanisms of 4-CP degradation in mist by dis-charge plasma, the outlet concentrations of long-living plasma spe-cies, O3 and H2O2, were analyzed under dry air, water mist and 4-CP mist atmosphere, respectively, and the results were presentedin Fig. 2(b). In Fig. 2(b), ‘‘O3-air’’, ‘‘O3-air with water mist’’ and ‘‘O3-air with 4-CP mist’’ are used to indicate the ozone production inair, in air containing water mist and in air containing 4-CP mist,respectively. It is obvious that O3 was detected but the H2O2 concen-tration was below the minimum detectable concentration (0.4 mg/m3) when the sampling volume was 30 L for all atmosphere.

Compared with dry air atmosphere, the presence of water mistresults in a decrease of O3 concentration, which could be primarilyattributed to the removal of �O by H2O [9]. For the same inputpower, the outlet concentration of O3 under 4-CP mist atmospherewas slightly lower than that under water mist atmosphere, proba-bly due to the reaction of O3 with 4-CP.

3.2. Removal efficiency and energy efficiency

Fig. 3 gives the removal efficiency of 4-CP and TOC, and energyefficiency as functions of input power.

It can be seen that the removal efficiency of 4-CP and TOCimproved as input power increased. The increase in input power

contributes to the boost in active species such as �OH and �Oradicals and O3 in both gaseous and liquid phase, resulting in high-er removal efficiency and mineralization of 4-CP. For a given inputpower, the removal efficiency of TOC was much lower than that of4-CP, indicating the formation of large amounts of organicby-products [7]. Furthermore, there is a remarkable influence ofinput power on 4-CP removal when input power is less than3 W, which gradually weakens with further increase of inputpower. This phenomenon could be due to the competition for input

Table 1Main bond lengths and atomic charges of 4-CP.

Bond name Bond length (Å) Atom Atomic charge (e)

C(2)–C(3) 1.395 C(1) 0.298C(1)–C(2) 1.397 C(2) �0.261C(2)–H(7) 1.086 C(3) �0.241C(3)–C(4) 1.395 C(4) �0.055C(3)–H(8) 1.085 C(5) �0.241C(4)–C(5) 1.395 C(6) �0.260C(4)–Cl(13) 1.760 H(7) 0.250C(5)–C(6) 1.395 H(8) 0.254C(5)–H(9) 1.085 H(9) 0.254C(1)–C(6) 1.397 H(10) 0.250C(6)–H(10) 1.086 O(11) �0.730C(1)–O(11) 1.387 H(12) 0.491O(11)–H(12) 0.970 Cl(13) �0.009

The bold in Table 1 indicate the bonds with relatively long bond length and atomswith relatively strong electronegativities.

44 X. Yan et al. / Chemical Engineering Journal 245 (2014) 41–46

energy of intermediates oxidation, leading to the decrease of inputenergy consumed by 4-CP degradation. In addition, the energy effi-ciency exhibits maxima at 0.47 g/kW h as a function of inputpower.

3.3. Degradation products of 4-CP

3.3.1. Theoretical analysis of intermediatesThe degradation of organic pollutants takes place through the

electrophilic reactions of organic molecules with highly active oxy-gen species such as �OH and O3 [19]. In order to predict the primaryintermediates, the attack of active species towards 4-CP was ana-lyzed by theoretical calculation.

There are several theoretical methods to predict the attack po-sition of �OH radicals and O3 [19–21]. Among them, ‘‘frontier orbi-tal theory’’ is one of the most successful theories, which states thatthe attack point of electrophilic reaction is at the position of thegreatest electron density in the highest occupied molecular orbital(HOMO) of the molecule [19,20]. In other words, the higher HOMOcoefficient is, the greater the possibility of the electrophilic reac-tion takes place. 4-CP geometry was optimized and the corre-sponding HOMO coefficients were calculated with the help ofGaussian03 package program, as shown in Fig. 4. It can be seen thatthere is a high preference for the two-ortho positions with respectto the functional –OH group for electrophilic attack by �OH radicalsor O3. This result is consistent with the analysis of the atomiccharge, which indicates the electronegativity of atom in quantumchemistry based on the density functional theory [19]. As shownin Table 1, the electronegativities of C (3) and C (5) were weakerthan those of C (2) and C (6), and thus C (2) and C (6) would ini-tially be attacked by electrophilic radicals. Therefore, the formationof intermediates related with the two-ortho positions with respectto the functional –OH group is more likely than that of the two-meta positions. In other words, 4-chlorocatechol may be the dom-inant hydroxylated intermediate in 4-CP degradation by coronadischarge plasma.

On the other hand, as seen from Table 1, the C (4)–Cl (13) bondhas the longest length (1.760 Å) in 4-CP molecule, and therefore itwould initially be attacked by active species such as �OH radicals.After the –Cl was removed from the aromatic ring via the cleavageof C (4)–Cl (13), intermediates such as phenol, hydroquinone andcatechol would be generated. Meanwhile, the ring-opening prod-ucts could also be formed by the direct breakage of the C–C bondsof 4-CP, preferentially the C (1)–C (2) and C (1)–C (6) bonds due totheir slightly longer bond length (1.397 Å) than other C–C bonds(1.395 Å).

C(6) C(5)

C(3)C(2)

Cl(13)C(4)C(1)O

(11)

H(7) H(8)

H(10) H(9)

H(12)

(-0.1551) (-0.2554)

(0.2604)

(-0.5009) (0.4422)

(0.3159)(-0.3187)

(0.0117) (0.0023)

(0.0489)

(-0.2929)

(0.0116) (0.0025)

Fig. 4. The optimized 4-CP structure and corresponding HOMO coefficients.

3.3.2. Experimental analysis of productsProducts from the 4-CP degradation by discharge plasma,

including 4-chlorocatechol, oxalic acid, maleic acid and Cl� wereidentified and quantified by HPLC and IC. Being consistent withthe theoretical analysis results, 4-chlorocatechol was the dominanthydroxylated product, while oxalic acid and maleic acid repre-sented the ring-opening products. In addition, the formation ofCl� confirmed the cleavage of the C (4)–Cl (13) bond during thedegradation process. Therefore, it is feasible to predict the interme-diates of 4-CP degradation by corona discharge plasma usingGaussian03 package program.

The effect of input power on mass and yields of the above-men-tioned main products under input power of 1.0–9.5 W was de-picted in Fig. 5. It is seen that the yield of Cl� could reach above93% and 4-chlorocatechol accounted for less than 1% of all removedchlorine, indicating that most of the chlorine atoms were releasedfrom the removed 4-CP. The mass of oxalic acid and maleic acid in-creased persistently with the increase of input power due to theproduction of more oxidative radicals for 4-CP degradation at high-er input power. On the other hand, the yield of maleic acid in-creased while that of oxalic acid remained invariable as the inputpower increased, suggesting that the percentage of carbon con-verted into maleic acid improved while that into oxalic acid leveledoff with the increase in input power. The yield of 4-chlorocatecholin carbon was less than 1%, which suggests that 4-chlorocatecholcan be easily oxidized to organic acids or inorganic carbon in theplasma atmosphere. The ring-opening products (maleic and oxalicacids) take up less than 50% and the ring-containing products

Fig. 5. The mass and yields of products.

Table 2Removal of 4-CP in mist by discharge plasma and ozonation.

Discharge plasma Ozonation

O3 concentrationunder water mistatmosphere(ppm)

Inputpower

4-CPdegradationefficiency(%)

O3 concentrationunder water mistatmosphere(ppm)

4-CPdegradationefficiency(%)

55 2.06 27.7 57 17.8133 4.336 56.4 134 33.8200 9.456 71.1 198 46.3

X. Yan et al. / Chemical Engineering Journal 245 (2014) 41–46 45

(mainly 4-chlorocatechol) account for less than 1% of the convertedcarbon in 4-CP under the investigated conditions. The missing car-bon fraction can be found in inorganic products, namely carbondioxide (CO2) and carbon monoxide (CO) in the outlet mist stream.

3.4. Roles of active species in 4-CP degradation

Generally, both short-living (energetic electrons, ions and radi-cals) and long-living plasma species (O3, H2O2, etc.) contribute toVOCs removal by discharge plasma [8,22]. H2O2 was not detectedin gas or liquid phase in our study, but substantial O3 was indeeddetected in the exhaust mist stream (Fig. 2(b)). In order to clarifythe role of O3 in 4-CP degradation under mist atmosphere, 4-CP re-moval by ozonation was investigated under the similar O3 concen-trations to those detected in the discharge plasma system, and theresults are compared with those obtained for the discharge plasmasystem, as shown in Table 2. It can be seen that the 4-CP removalby ozonation increased with the increase of ozone concentration,indicating that ozone can oxidize 4-CP under mist atmosphere.However, 4-CP removal efficiency by ozonation was lower thanthat by discharge plasma under the similar O3 concentration, andthe discrepancy increased with the increase of O3 concentration.These results indicate that besides O3, short-living species also playan important role in 4-CP degradation in mist by discharge plasma,especially for higher input power.

3.5. Pathway of 4-CP degradation

Based on the analysis of 4-CP degradation products and theroles of active species in 4-CP degradation, possible degradationpathways of 4-CP in mist by discharge plasma were proposed. As

Fig. 6. Possible degradation pathways o

shown in Fig. 6, 4-CP firstly undergoes hydroxylation to form 4-chlorocatechol, followed by the cleavage of C–Cl bond to form cat-echol [23,24]. On the other hand, the substitution of Cl in 4-CP byOH radical may also occur with hydroquinone produced [25]. Andbenquinone can be generated by the further oxidation of hydroqui-none [26]. The �OH and O3 can also attack the Cl group in 4-CP toyield phenol, which suffers further oxidization to form catecholand hydroquinone [27,28]. Further attacks to the ring-retainingproducts mentioned above by plasma species would lead to ring-cleavage of the aromatic ring to form organic acids (namely maleicacid and oxalic acid), which will undergo further oxidation to yieldfinal products including CO2 and CO [29].

Besides the above-mentioned electrophilic reactions, O3 canalso react with 4-CP through cyclo-addition reactions causing di-rect ring cleavage to produce (2E, 4E)-hexa-2, 4-dienedial, whichwill undergo further oxidation to yield organic acids under theplasma atmosphere [26].

4. Conclusions

The behavior of 4-CP degradation in mist by corona dischargeplasma was investigated using a link tooth wheel-cylinder plasmareactor energized by positive DC power supply. The main findingscan be summarized as follows:

(1) The introduction of water mist to air resulted in a decreaseof discharge current under the same applied voltage. Coronadischarge plasma is effective in removal of 4-CP in mist. Theremoval of 4-CP and TOC improves as input power increases.

(2) According to the theoretical analysis, 4-chlorocatechol is thedominating hydroxylated intermediate, and C–Cl would ini-tially be attacked by the plasma species to form Cl� due to itslongest bond length, which is verified by HPLC and IC anal-ysis results of 4-CP degradation products

(3) Most of the chlorine atoms are released from the removed 4-CP to form Cl�. The Cl� takes up more than 93% while the 4-chlorocatechol accounts for less than 1% of the reactedchlorine.

(4) The ring-opening (maleic and oxalic acids) and ring-contain-ing products (mainly 4-chlorocatechol) account for less than50% and 1% of the converted carbon respectively under theinvestigated conditions, with the other converted carbonbeing contained in inorganic products such as CO2 and CO.

f 4-CP in mist by discharge plasma.

46 X. Yan et al. / Chemical Engineering Journal 245 (2014) 41–46

(5) Both short-living and long-living plasma species contributeto 4-CP degradation. 4-CP degradation may take placethrough the electrophilic reactions between 4-CP and highlyactive oxygen species and cyclo-addition reactions between4-CP and O3.

Acknowledgments

The authors would like to thank the National Natural ScienceFoundation of China (No. 21177006), Beijing Natural Science Foun-dation (No. 8122029) and NSFC-Guangdong Joint Fund (No.U1201231) for their financial support.

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