Dechlorination of chlorophenols found in pulp bleach plant E-1 effluents by advanced oxidation processes

Bioresource Technology 96 (2005) 897–906

Dechlorination of chlorophenols found in pulp bleach plantE-1 effluents by advanced oxidation processes

Rui Wang a,b, Chen-Loung Chen a,*, Josef S. Gratzl a

a Department of Wood and Paper Science, North Carolina State University, Raleigh, North Carolina 27695-8005, USAb Mead-Westvaco Corporation, Covington mill, Covington, Virginia 24426, USA

Received 28 April 2003; received in revised form 26 August 2004; accepted 26 August 2004

Available online 26 October 2004

Abstract

Studies were conducted on the response of 2,4,6-trichlorophenol (1), 2,3,4,5-tetrachloro-phenol (2) and 4,5-dichloroguaiacol (3)

toward advanced oxidation processes, such as UV-, O2/UV-, H2O2/UV-, O3/UV- and O3–H2O2/UV-photolyses with irradiation of

254nm photons. The compounds 1–3 are among the chlorophenols found in the Kraft-pulp bleach plant E-1 effluents. The studies

were extended to treatment of these compounds with ozonation and O3–H2O2 oxidation systems in alkaline aqueous solution.

Except for the O2/UV-photolysis of 1 and H2O2/UV-photolysis of 2, the dechlorination of 1–3 by O2/UV- and H2O2/UV-potolyses

were less effective than the corresponding N2/UV-potolysis of 1–3. Guaiacol-type chlorophenols were more readily able to undergo

dechlorination than non-guaiacol type chlorophenols by N2/UV-, O2/UV- and H2O2/UV-potolyses. In addition, the efficiency

for the dechlorination of 1–3 by N2/UV-, O2/UV- and H2O2/UV-potolyses appeared to be dependent upon the inductive and

resonance effects of substituents as well as number and position of chlorine substituent in the aromatic ring of the compounds.

The dechlorination of 2 by treatment with O3 alone is slightly more effective than the corresponding the O3/UV-photlysis, whereas

the dechlorination of 2 by treatment with the combination of O3 and H2O2 was slightly less effective than the corresponding

O3–H2O2/UV-photolysis. In contrast, the dechlorination of 3 on treatment with O3 alone was slightly less effective than the corre-

sponding the O3/UV-photolysis, whereas the dechlorination of 3 on treatment with the combination of O3 and H2O2 was slightly

more effective than the corresponding the O3–H2O2/UV-photolysis. In the dechlorination of 2 and 3, chemical species derived from

ozone and hydrogen peroxide in alkaline solution were dominant reactions in the O3/UV- and O3–H2O2/UV-photolysis systems as in

the O3 and O3–H2O2 oxidation systems. Possible dechlorination mechanisms involved were discussed on the basis of kinetic data.

� 2004 Elsevier Ltd. All rights reserved.

Keywords: Chlorinated phenols; Advanced oxidation processes; UV-photolysis; Dechlorination; Kinetics; Oxygen; Hydrogen peroxide; Ozone

1. Introduction

In the kraft pulping, the resulting crude pulp usuallycontains approximately 3% of residual lignin (Marton,

1971). The crude kraft pulp is then treated successively

with elemental chlorine under acidic condition (C stage),

extracted with an alkaline solution (E-1 stage), hydrogen

peroxide (P stage), then twice with chlorine dioxide (DD

0960-8524/$ - see front matter � 2004 Elsevier Ltd. All rights reserved.

doi:10.1016/j.biortech.2004.08.011

* Corresponding author. Tel.: +1 919 515 5749; fax: +1 919 515

6302.

E-mail address: [email protected] (C.-L. Chen).

stage), i.e., bleaching sequence CEPDD (Dence and

Annergren, 1979; Gellerstedt and Zhang, 2001; Rajan

et al., 1996) to remove the bulk of the residual ligninto enhance rightness. In the previous investigation,

bleaching plant E-1 effluent from bleaching of Loblolly

pine kraft pulp with a bleaching sequence CEPDD

was found to contain considerable color carriers con-

tribute to its color, 13,000 Co–Pt units (Wang et al.,

2003). The total solid of the effluent was 5.6g/l, of which

3.5 and 2.1g/l are organic and inorganic materials,

respectively. The total chlorine content of the effluentwas 957mg/l, of which 689 and 268mg/l are chloride

mailto:[email protected]

OH

Cl

ClCl

OH

Cl

Cl

ClCl

OH

Cl

OCH3

Cl

1 2 3

Fig. 1. Structure of 2,4,6-trichlorophenols (1), 2,3,4,5-phenol (2) and

4,5-dichloroguaiacol (3).

898 R. Wang et al. / Bioresource Technology 96 (2005) 897–906

(Cl�) and organically bound chlorine (OCl), respec-

tively. Approximately 5% of the OCl were ether-extract-

able. Thus, it is evident that the major part of the OCl is

present in the high relative molecular mass fractions that

are not extractable by ether.

Furthermore, it was found that the N2/UV-photolysis

did not be appreciable effectiveness in color reduction

and removal of OCl in the E-1 effluent. Addition ofhydrogen peroxide promoted the reduction of color car-

riers and degradation of polychlorinated oxylignins

(PCOLs), but did not improve appreciably in the

dechlorination. However, introduction of ozone stream

into the UV-photolysis system resulted in appreciable

improvement in both the decolorization and dechlorina-

tion of E-1 effluent. Up to approximately 40% of the

total organically bound chlorine (TOCl) in these sub-strates were converted into chloride ion within reaction

time of 5min. Therefore, the objective of such as UV-,

O2/UV-, O3/UV- and O3–H2O2/UV-photolyses with

irradiation of 254nm photons. This investigation is to

elucidate possible reaction mechanisms by studying the

kinetics for dechlorination of chlorophenols identified

in the E-1 effluent by advanced oxidation processes,

Because 2,3,4,5-tetrachlorophenol (2) and 4,5-dichloro-guaiacol (3) are identified as the major components of

chlorinated phenols in the E-1 effluent, they were

selected as model compounds in this study (see Fig. 1

for structures). In addition, 2,4,6-trichlorophenol (1)

was also selected since it is shown to be a potential pre-

cursor for the formation of dioxin in the conventional

chlorine bleaching (Hruford and Negri, 1992; Rajan

et al., 1994, 1996), although the E-1 effluent contains itin rather small amount.

2. Methods

2.1. Chlorophenols

2,4,6-Trichlorphenol (mp 79–80 �C) and 2,3,4,5-tetrachlorophenol (mp 116–7 �C) were purchased from

Aldrich Chemical Inc., Milwaukee, WI, USA and 4.5-

dichloroguaiacol (mp 68–70 �C) from Helix Biotech.

Corp., Toronto, Ontario, Canada. These phenols were

recrystallized from CHCl3–petroleum ether to con-

stant mp.

2.2. Ozone

Ozone was prepared by passing oxygen through an

ozone generator (Model T-816, Polymetric, Inc. San

Jose, CA, USA) at a constant power. The concentration

of ozone produced was controlled at the range of either2.0–2.5% or 4.0–4.5% depending on the requirement of

particular studies. The concentration of the ozone

stream was constantly monitored with an ozone monitor

(Model HC, PCI Ozone and Control System, Inc. West

Caldwell, NJ, USA) before sending to the reactor.

2.3. UV-photolysis

Thin film reactor (Ace Glass Laboratories, Vineland,

NJ, USA) used in this study was made of borosilicate

glass and equipped with a weir arrangement to provide

a flow of reactant solution over a knife-edge and down

the inner wall of the reactor. An impeller type fluid

pump with adjustable pumping capacity was provided

to circulate the effluent through the reactor. A low

pressure Hg lamp, 12W with energy profile of approxi-mately 3.4W at 254nm was held vertically and sur-

rounded by a water-cooled jacket. In this study, the

volume of the E-1 effluent, and the solution of selected

model compounds with concentration of 0.5mM in

0.01M NaOH were 400ml with a circulation rate of

250ml/min. The flow rate of both O2 and O3 streams

were either 50 or 100ml/min when applied to the system.

Introduction of 4.5% O3 stream with flow rate of 50ml/min for 1min corresponds to approximately 0.1mmol of

O3. The amount of substrate in 400ml of the 0.5mM

solution is 0.2mmol. The 3.5% O3 stream with flow rate

of 400ml/min for one min corresponds to O3 charge of

30mg/min. In the case of studies under N2 atmosphere,

N2 was introduced into the reactor at 1 l/min for at least

1h to assure completion of air displacement. The pH of

the solution was monitored through the entire reactionperiod. Samples were withdraw at certain intervals from

the sample port and injected into ion chromatography,

Dionex 2010i (Dionex Corp. Sunnyvale, CA, USA),

equipped with an anion separator column AS-4 to ana-

lyze and quantify the chloride anion formed during

dechlorination.

3. Results and discussion

Alkaline solutions of 2,4,6-trichlorophenol (1),

2,3,4,5-tetrachlorophenol (2) and 4,5-dichloroguaiacol

(3) (0.5mM in 0.01M NaOH solution) were initially

irradiated with 254nm photons under nitrogen atmos-

phere with an initial pH of 10.34. The dechlorination

was monitored by quantitative determination of chlo-ride anion formed, the results of which were used as

standards. The UV-photolyses were then carried out

y = 100e

y = 100e

y = 100e

-0.0027x

R2 = 0.9806

-0.0036x

R2 = 0.9938

-0.0055x

R2 = 0.956

0

20

40

60

80

100

120

0 20 40 60 80 100 120 140

UV-Irradiation Time (min)

Org

anic

ally

Bou

nd C

hlor

ine

Con

tent

(Mol

% p

er O

rigi

nal)

Fig. 3. Effect of oxidants on dechlorination of 2,4,6-trichlorophenol

(1) on UV-photolysis with 254nm photons. Under nitrogen atmos-

phere: (-j-j-); under oxygen atmosphere: (-m-m-); with H2O2 under

nitrogen atmosphere (H2O2 charge: 100% on substrate by weight).

Initial pH: 10.34; temperature: ambient; UV source: 12W low pressure

Hg lamp (254nm photons).

y = 100e-0.0015x

R2 = 0.9823

y = 100e-0.0031x

R2 = 0.9665

y = 100e-0.0033x

R2 = 0.9445

y = 100e-0.2084x

R = 0.9852

20

40

60

80

100

120

Org

anic

ally

Bou

nd C

hlor

ine

Con

tent

(Mol

% p

er O

rigi

nal)

R. Wang et al. / Bioresource Technology 96 (2005) 897–906 899

under oxygen atmosphere and with hydrogen peroxide

(H2O2 charge of 2% on substrate by weight) under nitro-

gen atmosphere. In addition, comparative studies were

conducted on the dechlorination of 2 and 3 in alkaline

solution with ozone (O3 charge of 0.1mol/min) and the

combination of ozone and hydrogen peroxide (O3

charge of 0.1mol/min and H2O2 charge of 2% on sub-

strate by weight, respectively). The O3/UV- and O3–

H2O2/UV-photolyses of 2 and 3 were carried under the

same condition as the treatment of these compounds

with ozone, and with combination of ozone and hydro-

gen peroxide under aerial atmosphere. Based on the re-

sults it appeared that the responses of 1–3 towards the

UV-photolysis systems with 254nm photons and theoxidant systems alone were different (Figs. 2–5 and 8

and 9, Table 1). Nevertheless, the kinetics of the dechlo-

rination for the chlorophenols investigated were all first

order with respect to the organically bound chlorine

(OCl) in the substrate. The dechlorination rate constants

of these reactions are summarized in Table 1.

3.1. UV-photolysis under nitrogen atmosphere

Compound 3 readily undergoes dechlorination in

alkaline solution with the first order reaction rate con-

stant of 1.8 · 10�4 s�1 by N2/UV-photolysis (Fig. 2,

Table 1). By contrast, the dechlorination rate constant

of 2 is the slowest among the compounds investigated

with the rate constant of 0.43 · 10�4 s�1 under the same

reaction condition. Legrini et al. (1993) postulated thathydroxyl radicals (HO�) are responsible for oxidation

of organic compounds, such as dechlorination of chloro-

phenols, by UV-photolysis with 254nm photons in the

presence of added oxidants. The hydroxyl radical is a

y = 100e-0.0026x

R2 = 0.9331

y = 100e-0.0036x

R2 = 0.995

y = 100e-0.0108x

R2 = 0.9967

0

20

40

60

80

100

120

0 20 40 60 80 100 120 140


Org

anic

ally

Bou

nded

Chl

orin

e C

onte

nt(M

ol %

per

Ori

gina

l)

Fig. 2. Dechlorination of chlorophenolics on UV-photolysis with

254nm photons under nitrogen atmosphere. 2,4,6-Trichlorophenol (1)

(-�-�-); 2,3,4,5-tetrachlorophenol (2) (-j-j-), and 4,5-dichloroguaia-

col (3) (-m-m-). Initial pH: 10.34; temperature: ambient; UV source:

12W low pressure Hg lamp (254nm photons).

00 50 100 150 200


Fig. 4. Effect of oxidants on dechlorination of 2,3,4,5-tetrachlorophe-

nol (2) on UV-photolysis with 254nm photons. Under nitrogen

atmosphere: (-j-j-); under oxygen atmosphere: (-m-m-); with H2O2

under nitrogen atmosphere (H2O2 charge: 2% on substrate by weight);

with O3 under aerial atmosphere: (-d-d-) (O3 charge: 0.1mmol/min).

Initial pH: 10.34; temperature: ambient; UV source: 12W low pressure

Hg lamp (254nm photons).

very strong oxidant with reox potential of 2.80V. Since

water does not absorb 254nm photons, it is not a source

of hydroxyl radicals in the N2/UV-photolysis of 1, 2 and

3 without added oxidants. Thus, as demonstrated in our

previous work (Thomas et al., 1995), the only direct

pathway to dechlorinate the substrates is the homolytic

cleavage of Ar–Cl bond in the phenoxide anion, Clx-(ph-

O�), of the substrates in alkaline solution, producing thecorresponding p-and o-aryl radical species of the type

Cl(x�1)-(�O-ph�) and chlorine radicals (Cl�) from 1, p-,

m- and o-aryl radical species and chlorine radicals from

y = 100e-0.1672x

R = 0.99012

y = 100e -0.0108x

R = 0.99672

y = 100e-0.0099x

R = 0.99812

y = 100e-0.009x

R2 = 0.9987

0

20

40

60

80

100

120

0 20 40 60 80 100 120 140

Reaction Time (min)

Org

anic

ally

Bou

nd C

hlor

ine

Con

tent

(Mol

% p

er O

rigi

nal)

Fig. 5. Effect of oxidants on dechlorination of 4,5-dichloroguaiacol (3)

on UV-photolysis with 254nm photons. Under nitrogen atmosphere:

(-j-j-); under oxygen atmosphere: (-m-m-); with H2O2 under

nitrogen atmosphere (H2O2 charge: 2% on substrate by weight); with

O3 under aerial atmosphere: (-d-d-) (O3 charge: 0.1mmol/min). Initial

pH: 10.34; temperature: ambient; UV source: 12W low pressure Hg

lamp (254nm photons).


2, and p-, and m-aryl radical species and chlorine radi-

cals from 3. These aryl radical species derived from 1,2 and 3 would behave chemically in several ways in alka-

line solutions under nitrogen atmosphere. For example,

these radical species could undergo recombination with

chlorine radicals to regenerate the starting substrates

and polymerization to produce the corresponding bi-

phenyl derivatives (Thomas et al., 1995). In addition,

according to McElroy (1990), the chlorine radicals pro-

duced can oxidize water molecules to produce hydroxylradicals (HO�) because of the large molarity of H2O (liq)

present in the reaction system. The chlorine radicals

(Cl�) are in turn reduced to chloride anions (Cl�).

On the basis of observed dechlorination rate con-

stants and the structures of chlorophenols 1, 2 and 3,

Table 1

Rate constants for dechlorination of 2,4,6-trichlorophenols (1), 2,3,4,5-ph

photons with and without added oxidants, and by hydrogen peroxide, ozon

Oxidation system and chemicals

added to the reaction mixtureaDechlorination r

2,4,6-Trichloroph

UV-photolysis under nitrogen atmosphere 0.6 · 10�4 s�1

UV-photolysis under oxygen atmosphere 0.92 · 10�4 s�1

H2O2/UV-photolysisb under nitrogen atmosphere 0.45 · 10�4 s�1c

Oxidation with O3d under aerial atmosphere NDe

O3/UV-photolysisd under aerial atmosphere NDe

Oxidation with O3d and H2O2

b under aerial atmosphere NDe

O3–H2O2/UV-photolysis b,d under aerial atmosphere NDe

a Substrate solution: 400ml with concentration of 0.5mM in 0.1M NaOHb Hydrogen peroxide charge: 2% on substrate in weight.c Hydrogen peroxide charge: 100% on substrate in weight.d Ozone stream of with ozone concentration of 4.5% with flow rate of 50e ND = not determined.

the dechlorination rate seems to be dependent upon

the inductive and resonance effects of substituents in

addition to the initial concentration of the substrate.

In terms of an inductive effect, phenolic hydroxyl group

and aromatic methoxyl groups are electron releasing

groups, which activate the aromatic ring. In contrast,chlorine group is the electron attracting group that

deactivates the aromatic ring. Furthermore, the dechlo-

rination rate is also affected by the number and orienta-

tion of chlorine substituents with respect to phenolic

hydroxyl group in the aromatic ring of the compounds.

3.2. UV-photolysis under oxygen atmosphere

The UV-photolysis of chlorophenols 1, 2 and 3 with

254nm photons was carried out in alkaline solution

under oxygen atmosphere. Except for the oxygen atmos-

phere, the reaction condition is the same as the corre-

sponding N2/UV-photolysis. The first order reaction

rate constant for dechlorination of 1 is increased as com-

pared to that by N2/UV-photolysis, from 0.6 · 10�4 s�1

to 0.92 · 10�4 s�1 (Fig. 3, Table 1). In contrast, thedechlorination rate constants for 2 and 3 are decreased,

from 0.43 · 10�4 s�1 to 0.23 · 10�4 s�1 (Fig. 4, Table 1)

and from 1.8 · 10�4 s�1 to 1.5 · 10�4 s�1 (Fig. 5, Table

1), respectively.

Conceivably, in the O2/UV-photolysis of 1, 2 and 3 in

alkaline solution, the resulting p-, m- and o-aryl radical

species of the type Cl(x�1)-(�O-phÆ) derived from the

homolytic cleavage of Ar–Cl bonds in the correspondingphenoxide anions of the type Clx-(ph-O

�) react with the

dissolved molecular oxygen to form the corresponding

unstable polychlorinated p-, m- and o-phenoxide anion

peroxyl radical species of the type Cl(x�1)-(�O-ph-O-

O�) in addition to the reactions involving in the dechlo-

rination of these compounds in alkaline solution by N2/

photolysis as discussed in the previous section. Since the

O2 concentration in the O2-saturated solution (1.29mM

enol (2) and 4,5-dichloroguaiacol (3) by UV-photolysis with 254nm

e and hydrogen peroxide-ozone

ate constants (j)

enol (1) 2,3,4,5-Tetrachlorophenol (2) 4,5-Dichloroguaiacol (3)

0.43 · 10�4 s�1 1.8 · 10�4 s�1

0.23 · 10�4 s�1 1.5 · 10�4 s�1

0.43 · 10�4 s�1b 1.65 · 10�4 s�1b

3.74 · 10�3 s�1 2.72 · 10�3 s�1

3.47 · 10�3 s�1 2.8 · 10�3 s�1

3.47 · 10�3 s�1 2.95 · 10�3 s�1

3.91 · 10�3 s�1 2.8 · 10�3 s�1

, corresponding to 0.2mmol substrate.

ml/min, corresponding to ozone charge of 0.1mmol/min.


at 25 �C) is greater than the initial substrate concentra-

tion (0.5mM) (Thomas et al., 1995), the reactions lead-

ing to the formation of the polychlorinated phenoxide

anion peroxyl radical species is a dominant reaction in

the UV-photolysis under oxygen atmosphere. The

resulting p- and o-phenoxide anion peroxyl radical spe-cies, 4 and 6 derived from 1 would immediately undergo

hydrolysis to the corresponding p-and o-benzoquinone

derivatives, 5 and 7, with concomitant formation of hyd-

roxyl radicals (HO�), respectively (Fig. 6). In contrast,

the m-phenoxide anion peroxyl radical species derived

from 2 and 3 undergo hydrolysis to produce resorcinol

derivatives, such as 9 from 2d via the corresponding

and o-phenoxide anion peroxyl radical species 8, withconcomitant formation of hydroxyl radicals (HO�)

(Schuchmann and Sonntag, 1987). Moreover, 5 and 7

would further undergo dechlorination by nucleophilic

attack of hydroxide anion (HO�) at C-b of a,b-unsatu-

Fig. 6. Possible reaction mechanisms for dechlorination of 2,4,6-trichloroph

photolysis with 254nm photons under oxygen atmosphere.

rated ketone moiety in these products to give the corre-

sponding chlorohydrol intermediates 10 and 12,

followed by b-elimination of chloride ion to give the cor-

responding phenoxide anion of 4-hydroxy-6-chloro-p-

quninone (11) and 2-hydroxy-6-chloro-o-quninone

(13), respectively (Fig. 6). It is noteworthy that 11 and13 are interconverting via tautomerism. The resulting

o- and p-quinone derivatives would further undergo oxi-

dative degradation under the reaction condition.

3.3. UV-photolysis with hydrogen peroxide under

nitrogen atmosphere

The dechlorination rates for compounds 1, 2 and 3varied when their alkaline solutions were irradiated with

254nm photons in the presence of hydrogen peroxide

under nitrogen atmosphere. In the dechlorination of

1, when hydrogen peroxide charge of 100% on the

enol (1) and 2,3,4,5-tetrachloropehnol (2) in alkaline solution by UV-


substrate (by weight) was added to the UV-photolysis

system under nitrogen atmosphere, the dechlorination

rate slowed appreciably as compared to that under

nitrogen atmosphere alone (Fig. 3, Table 1). The first

order reaction rate constant for dechlorination of 1

decreased from 0.6 · 10�4 s�1 to 0.45 · 10�4 s�1. Theaddition of 2% hydrogen peroxide on substrate to the

UV-photolysis 2 under nitrogen atmosphere, the dechlo-

rination rate of 2 is the same with that under nitrogen

atmosphere along; the rate constant of 0.43 · 10�4 s�1

for both (Fig. 4, Table 1). However, in the UV-photoly-

sis of 3, the addition of 2% hydrogen peroxide on sub-

strate under nitrogen atmosphere resulted in slowing

down slightly the dechlorination rate; the rate constantdecreased from 1.8 · 10�4 s�1 to 1.65 · 10�4 s�1 (Fig. 5,

Table 1).

It is well established that the rate of UV-photolysis

for hydrogen peroxide to produce hydroxyl radicals

(HO�) in aqueous solution is, in general, pH dependent

and increases with increasing concentration of alkaline

solution (Eq. (1)) (Legrini et al., 1993). In addition,

hydrogen peroxide undergoes decomposition by a dis-mutation reaction with a maximum rate at the pH of

its pKa value, i.e., pH11.6 (Eq. (2)) (Legrini et al.,

1993). The hydroxyl radicals thus produced are readily

recombined back to hydrogen peroxide (Eq. (3)). More-

over, when an excess of hydrogen peroxide is used, hyd-

roxyl radicals will oxidize the excess hydrogen peroxides

Fig. 7. Possible reaction mechanisms dechlorination of 2,4,6-trichloropheno

under nitrogen atmosphere.

to produce hydroperoxyl radicals via abstraction of

hydrogen atoms (Eq. (4)).

H2O2 !hm2HO� ð1Þ

H2O2 þHOO� ! H2OþO2 þHO� ð2Þ

2HO� ! H2O2 ð3Þ

H2O2 þHO� ! H2OþHOO� ð4Þ

The hydroperoxyl radical (Reox potential,

E0 = 1.70V) was much less reactive as oxidant than hyd-

roxyl radical (Reox potential, E0 = 2.80V). In addition,the concentration of hydroperoxyl radical was control-

led by the pH of the reaction system. The rate constant

for dechlorination of chlorophenols decreased, in gen-

eral, with increasing concentration of hydrogen peroxide

(Table 1). Thus, the lower dechlorination rate constant

in the H2O2/UV-photolysis of 1 was attributable to the

presence of excess hydrogen peroxide (Fig. 3, Table 1).

In the homolytic cleavage of Ar–Cl bonds of 2,4,6-tri-chlorophenol (1) in the alkaline solution on irradiation

of 254nm photons in the presence of hydrogen peroxide,

the substrate was present in the reaction medium as the

corresponding phenoxide anion 1a. Thus, in addition to

the cleavage of Ar–Cl bonds, 1a would undergo single-

electron abstraction by hydroxyl radical (HO�) to give

2,4,6-trichlorophenoxyl radical (1d) that was resonating

l (1) in alkaline solution by H2O2/UV-photolysis with 254nm photons

y = 100e-0.2241x

R2 = 0.9472 y = 100e-0.2084x

R2 = 0.985

y = 100e-0.2346x

R2 = 0.9852

y = 100e-0.2083x

R2 = 0.9721

0

20

40

60

80

100

120

0 1 2

Reaction Time (min)

Org

anic

ally

Bou

nd C

hlor

ine

Con

tent

(Mol

% p

er O

rigi

nal)

3 4 5 6 7 8 9

Fig. 8. Dechlorination of 2,3,4,5-tetrachloropehnol (2) on ozone-

involved oxidation systems. With O3 alone: (-�-�-); with O3/

UV-photolysis: (-j-j-); with O3–H2O2: (-m-m-); with O3–H2O2/

UV-photolysis: (-d-d-). Initial pH: 10.34; temperature: ambient; O3

charge: 0.1mmol/min; H2O2 charge: 2% on substrate by weight; UV

source: 12W low pressure Hg lamp (254nm photons).

y = 100e-0.1634x

R2 = 0.9966

y = 100e-0.1672x

R2 = 0.9901

y = 100e-0.1675x

R = 0.98272

y = 100e-0.1772x

R2 = 0.99140

20

40

60

80

100

120

0 2 4 6 8 10 12 14Reaction Time (min)

Org

anic

ally

Bou

nd C

hlor

ine

Con

tent

(Mol

% p

er O

rigi

nal)

Fig. 9. Dechlorination of 4,5-dichloroguaiacol (3) on ozone-involved

oxidation systems. With O3 alone: (-�-�-); with O3/UV-photolysis:

(-j-j-); O3–H2O2: (-m-m-); O3–H2O2 /UV-photolysis: (-d-d-). Initial

pH: 10.34; temperature: ambient; O3 charge: 0.1mmol/min; H2O2

charge: 2% on substrate by weight; UV source: 12W low pressure Hg

lamp (254nm photons).


among radical species 1e and 1f (Fig. 7). Since 1 was

symmetric with respect to C1–C4 axis in the aromatic

ring, the chance of having radical species 1e was twice

that of radical species 1f. Addition of hydroxyl radical

to C-2 of 1e and C-4 of 1f produced the corresponding

chlorohydrol intermediates 14 and 15, which undergob-elimination to give chloride anion and 4,6-dichloro-

o- and 2,6-dichloro-p-quinones (7 and 5), respectively

(Fig. 7). As shown in Fig. 6, the resulting quinones 5

and 7 would further undergo dechlorination to the cor-

responding hydroxyquinones 11 and 13, respectively.

The dechlorination rate for the H2O2/UV-photolysis

of 4,5-dichlorguaiacol (3) in alkaline solution under

nitrogen atmosphere with hydrogen peroxide charge of2% on substrate was slightly slower than that by N2/

photolysis. Conceivably, the slower dechlorination rate

was attributable to the fact that 3 has a methoxyl group

substituted ortho to the phenolic hydroxyl group, and a

chlorine atom substituted meta to the phenolic hydroxyl

group but para to the methoxyl group. Consequently,

the compound would undergo oxidation by hydro-

peroxyl radical to give 2-methoxy-5-chloro-p-quinone,5-hydroxy-2-methoxy-p-quinone and 4,5-dichloro-o-

quinone in the O2/UV- and H2O2/UV-photolyses, the

formation of which was analogous to the formation of

5, 11 and 7 from 1 in the O2/UV- and H2O2/UV-photo-

lyses of 1, respectively. In addition, the Ar–Cl bond of 3

at C-5 undergoes initially only homolysis to give the cor-

responding m-aryl radical species and chlorine radicals

(ClÆ) under the reaction condition. It follows that thenature of substituents and number of chlorine substitu-

ents in phenol affect appreciably the rate constant for

dechlorination of chlorophenemols.

3.4. UV-photolyses with ozone and combination of

ozone–hydrogen peroxide under aerial atmosphere,

and oxidation with ozone and combination of ozone–

hydrogen peroxide

Ozone is one of the strongest and yet environment be-

nign oxidants. It was, therefore, applied to the UV-

photolysis systems to see its effect on the UV-photolysis

of chlorophenols with 254nm photons. The lignin model

compounds 2 and 3 in 0.01M NaOH solution (pH10.34)

were treated with O3/UV- and O3–H2O2/UV-photolyses

with 254nm photons, and with ozone alone (ozonation)and combination of ozone and hydrogen peroxide as

reference. Although some differences have been ob-

served, the dechorination rate constants for 2 and 3 were

significantly increased in the O3/UV- and O3–H2O2/UV-

photolyses, as compared to the N2/UV photolysis of 2

and 3 (Figs. 8 and 9, Table 1).

In alkaline solution, ozone reacts with hydroxide

anions by transferring an oxyl anion radical (�O�), con-jugated base of hydroxyl radical (HO�), from the ozone

to the hydroxide anion to give superoxide (�O�2) and

hydroperoxyl (HO2�) radicals. Alternatively, this could

be accomplished when an oxygen atom was transferred

from the ozone to the hydroxide anion with a concomi-

tant single-electron transfer from the hydroxide anion to

the ozone (Eq. (5)) (Forni et al., 1982). The products,

hydroperoxyl radicals and superoxide, then react further

to give hydroperoxyl anion (HOO�) and molecular oxy-

gen (O2) (Eq. (6)). Since reaction rate constant for reac-tion 6 was much larger than that for reaction 5 (second

order reaction rate constant 8.7 · 107M�1 s�1 versus

48M�1 s�1), combination of these two reactions could

be simplified as an overall reaction (Eq. (7)). This

reaction was determined to be second order with the


reaction rate constant of (40 ± 2) · 106M�1 s�1 (Tomi-

yasu et al., 1985; Grasso, 1987; Wang, 1993).

O3 þHO� ! HOO� þ �O–O� ð5Þ

HOO� þ �O–O� ! HOO� þO2 ð6Þ

O3 þHO� ! HOO� þO2 ð7ÞThe hydroperoxyl anion subsequently undergoes sin-

gle-electron-transferring oxidation by ozone to give

hydroperoxyl radical (HOO�) and ozone anion radical

(�O�3) with second order reaction rate constant of

2.2 · 106M�1 s�1 (Eq. (8)), which is the initiation reac-

tion (Tomiyasu et al., 1985; Grasso, 1987; Wang,1993). The resulting hydroperoxyl radical dissociates to

give its conjugated base, superoxide anion (�O�2) in alka-

line solution with equilibrium constant (ja) of 10�4.8 (Eq.

(9)). The superoxide anion then undergoes single-

electron-transferring oxidation by ozone to producing

ozone anion radical (�O�3) and molecular oxygen with

second reaction rate constant of 1.6 · 109M�1 s�1 (Eq.

(10)) (Sehested et al., 1983). The ozone anion radicalundergoes hydrolysis to hydroxyl radical (HO�), molecu-

lar oxygen (O2) and hydroxide anion (HO�) with second

order reaction rate of approximately 30M�1 s�1 (Eq.

(11)) (Tomiyasu et al., 1985; Grasso, 1987; Wang,

1993). In addition, the cross radical combination be-

tween the ozone anion radical and hydroxyl radical, fol-

lowed by homolytic cleavage of the �O–O–O–O–H

intermediate produces supper oxide anion and hydrope-roxyl radical with second reaction rate constant of

6 · 109M�1 s�1 (Eq. (12)). Alternatively the ozone anion

radical undergoes oxidation by hydroxyl radical to pro-

duce ozone and hydroxide anion with second reaction

rate constant of 2.5 · 109M�1 s�1 (Eq. (13)) which re-

turns to the initiation reaction state (Eq. (8)) after series

of reactions.

HOO� þO3 ! HOO� þ �O�3 ð8Þ

HOO� þHO��

�O–O� þH2O ð9Þ�O–O� þO3 ! �O�

3 þO2 ð10Þ

�O�3 þH2O ! HO� þO2 þHO� ð11Þ

�O�3 þHO� ! �O–O� þHOO� ð12Þ

�O�3 þHO� ! O3 þHO� ð13ÞThus, the ozone dissolved in alkaline solution is a

rather complex chemical system that contains several

reactive chemical species. Conceivably, these chemicalspecies contribute to degradation of chlorophenols in

alkaline solution saturated with ozone although it is

not well established the effectiveness of ozone molecules

under the reaction condition.

On the basis of numerous investigations (Legrini

et al., 1993), a two step process was proposed to involve

the UV light-induced homolysis of ozone in aqueous

solution. It has been postulated that ozone in aqueous

solution undergoes homolysis on irradiation of UV light

with wavelength smaller than 310nm to give molecular

oxygen and O(1D) species (Eq. (14)). The latter then re-

acts with water to produce hydroxyl radicals (HO�) (Eq.(15)) (Legrini et al., 1993). However, it has been ob-

served that UV-photolysis of ozone dissolved in water

leads to the formation of hydrogen peroxide (Eq. (16))

in a sequence of reactions, where hydroxyl radicals,

if formed at all, can not escape from the solvent cage

(Legrini et al., 1993).

O3 !hm<310 nmO2 þOð1DÞ ð14Þ

Oð1DÞ þH2O ! HO� þHO� ð15Þ

O3 þH2O!hm ! H2O2 þO2 ð16Þ

Conceivably, the hydrogen peroxide produced in the

O3/UV-photolysis under aerial atmosphere undergoes

dissociation to give superoxide anion. The hydroperoxyl

anion was subsequently oxidized by ozone (Eq. (8)) to

give hydroperoxyl radical (HOO�) and ozone anion radi-

cal (�O�3), which initiate reactions 9–13. Therefore, the

reactive chemical species in the O3/UV-photolysis in

alkaline solution were almost the same with those with

ozone dissolved in water, except for the generation of

hydroxyl radical by UV-photolysis of ozone (Eqs. (11),

(14) and (15)). Thus, addition of hydrogen peroxide to

the O3/UV-photolysis would not promote the dechlorin-

ation of chlorinated phenols appreciably, which was in

good agreement with the kinetics of the O3/UV- andO3–H2O2/UV-photolyses of 2 and 3 (Figs. 8 and 9,

Table 1).

It is noteworthy that the reactions of chlorophenols

with ozone and the chemical species derived from ozone

dissolved in alkaline solution are mostly electrophilic

reactions that prefer phenols substituted with electron-

attracting groups. Although aliphatic hydroxyl and

methoxyl groups are electron-attracting groups, botharomatic hydroxyl and methoxyl groups are strong elec-

tron-releasing groups in term of inductive effect. This re-

sults in activating the corresponding aromatic ring. In

addition, the resonance effect of their lone electron-pairs

result in electron-rich centers at the carbon atoms ortho

and para to these groups, which readily undergo electro-

philic attack by ozone and most of the chemical species

derived from ozone in alkaline solution. In contrast,aromatic halogen is an electron-attracting group and

the resulting inductive effect deactivates the ring. How-

ever, resonance of lone electron pairs in the aromatic

halogen also establishes electron-rich centers at the car-

bon atoms ortho and para to the halogen group. Thus,

the aromatic ring with more chlorine substituents is

likely to undergo less electrophilic attack by ozone and


the chemical species derived from ozone because of the

greater deactivation of the aromatic ring. Consequently,

4,5-dichlorguiacohol (3) should be easier to undergo

oxidative degradation by ozone with concomitant

dechlorination than 2,3,4,5-tetrachlorophenol (2). How-

ever, the opposite is true (Figs. 8 and 9, Table 1). Whenalkaline solutions of 2 and 3 were treated with ozone

alone, the dechlorination rate of 2 was much faster than

that of 3, the rate constant of 3.74 · 10�4 s�1 versus

2.72 · 10�4 s�1 (Table 1). Conceivably, 3 is susceptible

to electrophilic attack by ozone and the chemical species

derived from ozone at C-1 and C-2 leading to an oxida-

tive ring cleavage between these two carbons with oxy-

gen-containing functional groups because of the higherelectron density in these carbon atoms (Kratzl et al.,

1976). A ring cleavage of this nature will not convert

organically bound chlorine (OCl) directly to chloride

anion (Cl�). However, the resulting a,b-unsaturatedcarbonyl intermediates containing a b-chlorine sub-

stituent could undergo further nucleophilic attack by

hydroxide anion, followed by b-elimination of Cl�. In

contrast, 2 is less susceptible to the oxidative ringcleavage than the aromatic ring of 3 does because the

aromatic ring of 2 is deactivated more by chlorine-sub-

stituents. However, when 2 undergoes oxidative ring

cleavage, it produces immediately Cl� from OCl. As a

result, the dechlorination rate of 3 is slower than that

of 2. Nevertheless, over 85% of OCl could be removed

from these chlorophenols in less than reaction time of

15min by treatment with ozone alone. When 2 wastreated with the O3/UV- and O3–H2O2/UV-photolysis

systems, the dechlorination rate constant for the

O3/UV-photolysis was slightly decreased as compared

to the treatment with O3 alone, 3.47 · 10�3 s�1 vs

3.74 · 10�3 s�1 (Fig. 8, Table 1). In contrast, the dechlo-

rination rate constant for the O3–H2O2/UV-photolysis

was slightly increased as compared to the treatment with

the combination of O3 and H2O2, 3.91 · 10�3 s�1 vs3.74 · 10�3 s�1 (Fig. 8, Table 1). However, the dechlo-

rination rate constant for the O3/UV-photolysis was

the same with that in the treatment of 2 with the combi-

nation of O3 and H2O2. When 3 was treated by O3/UV-

and O3–H2O2/UV-photolysis systems, no appreciable

differences in the dechlorination rates were observed;

dechlorination rate constant for both is 2.8 · 10�3 s�1

(Fig. 9, Table 1). These results clearly indicate that deg-radation reactions by chemical species derived from

ozone and hydrogen peroxide in alkaline solution are

dominant reactions in the dechlorination of 2 and 3 by

the O3/UV- and O3–H2O2/UV-photolysis systems.

4. Conclusions

On the basis of kinetic data, the dechlorination of

2,4,6-trichlorophenol (1), 2,3,4,5-tetrachlorophenol (2)

and 4.5-dichloroguaiacol (3) in alkaline solution by

N2/UV-, O2/UV-H2O2/UV-, O3/UV- and O3–H2O2/

UV-photolyses is first order reaction with respect to

the organically bound chlorine in the substrate. The

dechlorination of 1–3 in alkaline solution by O2/UV-

and H2O2/UV-photolyses is less effective than the corre-sponding N2/UV-photolysis of 1–3, except for the O2/

UV-photolysis of 1 and H2O2/UV-photolysis of 2. Gua-

iacol-type chlorophenols are more likely to undergo

dechlorination than non-guaiacol type chlorophenols

by N2/UV-, O2/UV- and H2O2/UV-photolyses. In addi-

tion, the efficiency for the dechlorination of 1–3 by N2/

UV-, O2/UV- and H2O2/UV-photolyses seems to be

dependent upon the inductive and resonance effects ofsubstituents as well as number and position of chlorine

substituent in the aromatic ring of the compounds.

Introduction of ozone and combination ozone and

hydrogen peroxide into the UV-photolysis system result

in appreciable improvement in dechlorination of 2 and

3, which were effectively degraded. Up to �40% of the

total organically bound chlorine in these compounds

was converted into chloride ion within reaction time of5min. However, the dechlorination of 2 by treatment

with O3 alone is slightly more effective than the corre-

sponding the O3/UV-photlysis, whereas the dechlorina-

tion of 2 by treatment with the combination of O3 and

H2O2 is slightly less effective than the corresponding

O3–H2O2/UV-photolysis. In contrast, the dechlorination

of 3 on treatment with O3 alone is slightly less effective

than the corresponding the O3/UV-photolysis, whereasthe dechlorination of 3 on treatment with the combina-

tion of O3 and H2O2 is slightly more effective than the

corresponding the O3–H2O2/UV-photolysis. Thus, in

the dechlorination of 2 and 3, chemical species derived

from ozone and hydrogen peroxide in alkaline solution

are dominant reactions in the O3/UV- and O3–H2O2/

UV-photolysis systems.

Acknowledgements

This project is supported by the USDA competitive

research grant under cooperative agreement no. 88-

33521-4084, for which the authors are thankful. The

authors are also grateful to Mead-Westvaco Corpora-

tion for supplying the E-1 effluent for this study.

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Dechlorination of chlorophenols found in pulp bleach plant E-1 effluents by advanced oxidation processes