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Contribution of free radicals to chlorophenols decomposition

by several advanced oxidation processes

F. Javier Benitez *, Jesus Beltran-Heredia, Juan L. Acero, F. Javier Rubio

Departamento de Ingenieria Quimica y Energetica, Universidad de Extremadura, 06071, Badajoz, Spain

Received 18 June 1999; accepted 29 October 1999

Abstract

The chemical decomposition of aqueous solutions of various chlorophenols (4-chlorophenol(4-CP), 2,4-dichlor-

ophenol (2-DCP), 2,4,6-trichlorophenol (2,4,6-TCP) and 2,3,4,6-tetrachlorophenol (2,3,4,6-TeCP)), which are envi-

ronmental priority pollutants, is studied by means of single oxidants (hydrogen peroxide, UV radiation, Fenton's

reagent and ozone at pH 2 and 9), and by the Advanced Oxidation Processes (AOPs) constituted by combinations of

these oxidants (UV/H2O2, UV/Fenton's reagent and O3/UV). For all these reactions the degradation rates are evaluated

by determining their rst-order rate constants and the half-life times. Ozone is more reactive with higher substituted

CPs while OH radicals react faster with those chlorophenols having lower number of chlorine atoms. The improvement

in the decomposition levels reached by the combined processes, due to the generation of the very reactive hydroxyl

radicals, in relation to the single oxidants is clearly demonstrated and evaluated by kinetic modeling. 2000 Elsevier

Science Ltd. All rights reserved.

Keywords: Chlorophenols; Hydroxyl radicals; Advanced oxidation processes; UV radiation/hydrogen peroxide; UV radiation/Fenton's

reagent; Ozone/UV radiation

1. Introduction

The chemical oxidation of toxic and hazardous or-

ganic pollutants, which frequently are present in surface

waters and wastewaters, is often carried out by using

single oxidants such as chlorine, ozone, UV radiation,hydrogen peroxide, etc. However, sometimes this de-

composition by conventional treatments may be dicult

if these pollutants are present at low concentrations or if

they are especially refractory to the oxidants. For these

situations, it has been necessary to develop more eec-

tive processes for the destruction of such contaminants.

Among them, some systems based on the generation

of very reactive and oxidizing free radicals, especially

hydroxyl radicals, have experimented an increasing in-

terest due to their high oxidant power. These systems are

commonly named Advanced Oxidation Processes

(AOPs), and the production of those radicals is achieved

by the combinations of ozone, hydrogen peroxide andUV radiation (Glaze et al., 1987; Glaze and Kang, 1989;

Masten and Davies, 1994); and also, with the combi-

nation of hydrogen peroxide with ferrous ions in the so-

called Fenton's reagent (Walling, 1975).

Chlorophenols (CPs) constitute a group of organic

substances that are introduced into the environment as a

result of several man-made activities, such as water

disinfection, waste incineration, uncontrolled used of

pesticides and herbicides, etc., and also as byproducts in

the bleaching of paper pulp with chlorine (Ahlborg and

Thunberg, 1980). Because of their numerous origins,

they can be found in industrial wastewaters, soils and

surface waters, and several of them have been listed

Chemosphere 41 (2000) 12711277

*Corresponding author. Tel.: +34-924-289384; fax: +34-

924-271304.E-mail address: [email protected] (F.J. Benitez).

0045-6535/00/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved.

PII: S 0 0 4 5 - 6 5 3 5 ( 9 9 ) 0 0 5 3 6 - 6

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among the 65 priority pollutants by the US EPA (Keith

and Telliard, 1979).

Their conventional destructive technologies include

biological treatments, incineration, adsorption over ac-

tivated carbon, air stripping, etc. (Jardin et al., 1997).

Some of these treatments, like the biological treatments,can be aected by the toxicity of some substances; oth-

ers, like incineration, present considerable emission of

other hazardous compounds; and adsorption or air

stripping requires a post-treatment to remove the pol-

lutants from the newly contaminated environment.

Therefore, a promising technology could be the use of

single chemical oxidants, or the more eective destruc-

tion by the AOPs, which can provide an almost total

degradation as has been reported by several authors in

the decomposition of a wide variety of organic con-

taminants (Peyton et al., 1982; Guittonneau et al., 1988;

Legrini et al., 1993).According to these considerations, a research pro-

gram was designed which focused on the oxidation of

several chlorophenols (4-chlorophenol (4-CP), 2,4-di-

chlorophenol (2,4-DCP), 2,4,6-trichlorophenol (2,4,6-

TCP) and 2,3,4,6-tetrachlorophenol (2,3,4,6-TeCP)) by

single oxidants and by dierent AOPs. The objectives

were to provide data about the removal obtained, to

report values of the rate constants for the global pro-

cesses, and to compare the eciency of the dierent

AOPs tested. Also, the enhancements reached in the

degradation levels in these combined oxidations com-

pared to the single oxidation processes are determined,

and the partial contributions of the OH radical pathwayto the global oxidation process are calculated for the

dierent AOPs studied.

2. Materials and methods

Analytical grade 4-CP, 2,4-DCP, 2,4,6-TCP and

2,3,4,6-TeCP were obtained from Sigma, hydrogen

peroxide (33% v/v) was purchased from Panreac and

ferrous sulfate heptahydrate was from Merck. Ozone

was produced from an oxygen stream in an ozone gen-

erator (Yemar, mod. HPA), and the radiation sourcewas a Heraeus TQ150 high pressure mercury vapor lamp

which emitted a polychromatic radiation in the range

from 185 to 436 nm.

The reactor used in all the experiments consisted of a

500 ml cylindrical glass reactor provided with the nec-

essary elements for the development of the dierent

processes: photodecomposition, Fenton's reagent oxi-

dation, ozonation, and the dierent combinations of

these oxidants. In the photochemical experiments, the

radiation source was located in the reactor in axial po-

sition. The reactor was lled with 350 ml of an aqueous

solution of the selected CP (initial concentration of

3 104 M in all cases), and the selected pH (2 or 9) was

obtained by adding orthophosphoric acid and sodium

hydroxide. The required amounts of ferrous sulfate and

hydrogen peroxide were added to the reactor in Fenton's

reagent oxidation experiments and in the photo-Fenton

experiments, and only the required amounts of hydro-

gen peroxide in the UV/H2O2 experiments. In theozonation experiments and combined UV/ozone exper-

iments, the ozoneoxygen mixture was fed to the reactor

through a porous plate gas sparger located at the bot-

tom of the reactor. The temperature was kept constant

at 25C.

The CPs concentrations in the samples withdrawn

from the reactor at regular reaction times were analyzed

by HPLC using a Waters Chromatograph equipped with

a 996 Photodiode Array Detector and a Nova-Pak C18

Column. The detection was made at 290 nm with a

mobile phase composed of a mixture methanolwater

acetic acid (65/33/2 in volume) and with a ow rate of 1ml/min. Ozone concentration was measured in the gas

stream iodometrically by bubbling the gas in a potassi-

um iodide solution. The concentration of H2O2 was

determined by the colorimetric method proposed by

Bader et al. (1988).

3. Results and discussion

3.1. Decomposition by single oxidants

In a rst stage, the decomposition of all four selected

CPs was performed by the action of the following singleoxidants: UV radiation, Fenton's reagent and ozone. A

previous set of degradation experiments by using hy-

drogen peroxide alone was also conducted, but no sig-

nicant CPs degradation was obtained with this oxidant.

Therefore, it can be concluded that H2O2 does not oxi-

dize the studied organic compounds, this eect being

already observed by several authors with some refrac-

tory pollutants (Masten and Davies, 1994; Benitez et al.,

1995).

3.1.1. UV radiation

The photooxidation of CPs by a polychromatic UVradiation was conducted at pH 2. Fig. 1 shows thedecrease in concentration as a function of irradiation

time: As can be observed, 4-CP was rapidly degraded;

decreasing rates are obtained for 2,4-DCP, 2,4,6-TCP,

and for 2,3,4,6-TeCP. It suggests that the addition of

substituent chlorine atoms decreases the susceptibility of

the aromatic ring to be attacked by the photons gener-

ated by the UV radiation. It can be explained by taking

into account that during photochemical treatments,

electronically excited states of polychlorinated phenols

are generated (Skurlatov et al., 1998). In these excited

states, the CP molecules undergo intramolecular trans-

formations and stabilize states with dierent electron

1272 F.J. Benitez et al. / Chemosphere 41 (2000) 12711277

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distributions, followed by decomposition to radical or

molecular products. A higher level of chlorine substi-

tution must bother the formation of the excited state or

the stabilization of the intermediate state.

Since reaction mechanisms are complex for thephotodegradation of organic compounds, a rigorous

kinetic study cannot be performed. However, the shape

of the lines in Fig. 1 looks like a rst-order reaction with

respect to CPs degradation. Therefore, an approach to

this kinetic study can be performed by assuming that the

photochemical decomposition reaction follows a rst-

order kinetics, and can be represented by the simple

expression

P hm3kpPoxidY 1

where Poxid symbolizes intermediates and nal products,and kp is the rst-order rate constant. This approach is

frequently used by several authors in similar studies

(Sundstron et al., 1989; Shen et al., 1995).

In order to evaluate these rate constants, the terms ln

(CP0/CP) are plotted versus reaction time, and after lin-

ear regression analysis, the rst-order rate constants are

determined and showed in Table 1 with the correlation

coecients. Obviously, they follow the trend already

commented for the degradation rates: 4-CP >2,4-DCP

>2,4,6-TCP >2,3,4,6-TeCP. Table 1 also summarizes

another interesting kinetic parameter, the half-life times

t1a2 or time necessary to reduce the initial concentrationof the CPs in solution by a factor of two. As can be

observed, these t1a2 increase when the substituent chlo-

rine atoms also increase, corroborating the sequence

obtained for the decomposition rates.

3.1.2. Fenton's reagent

In a second step, the decomposition of the four se-

lected CPs was explored by means of the very reactive

and oxidizing hydroxyl radicals, which are generated by

Fenton's reagent, a mixture of hydrogen peroxide and

ferrous ions, according to the reaction (Walling and

Kato, 1971; Walling, 1975)

H2O2 Fe2 3 Fe3 OH OHX 2

The hydroxyl radicals formed attack any organic

compound P, and thus, cause its chemical decomposi-

tion

POH3krPoxidY 3

where Poxid again symbolizes dierent intermediates and

nal products of the chain degradation reactions that

take place, and kr is the rst-order rate constant for the

reaction of CPs with OH.

In these experiments, the initial concentrations of

Fe2 and H2O2 were respectively 1 104 and

7X5 103 M, and the pH 2. The degradation curvesobtained versus reaction time again suggest that a rst-

order kinetics perfectly approaches the real kinetics of

this Fenton's reagent oxidation process. Fig. 2 shows the

plot of ln (CP0/CP) against reaction time: as can be seenpoints lie satisfactorily around straight lines. After re-

gression analysis, the results obtained for kr and for the

half-life times are depicted in Table 1. The sequence of

oxidation rates is the same as in the photodegradation

process, that is: 4-CP >2,4-DCP >2,4,6-TCP >2,3,4,6-

TeCP. Hydroxyl radicals usually attack the aromatic

ring at the sites which are not occupied by chlorine at-

oms, and therefore, hydroxylation is the rst elementary

step which precedes the dissociation of chlorine atoms

(Tang and Huang, 1996). So, the increase in the chlorine

atoms number in the aromatic ring decreases the reac-

tivity towards the hydroxyl radicals, and subsequently,the trend in the t1a2 values is inverse: they increase when

the substituent chlorine atoms also increase.

Table 1

Rate constants and half-life times for the decomposition of CPs by single oxidants

Compound UV Fenton O3 pH 2 O3 pH 9

kp 103

(min1)

t1a2(min)

kr 103

(min1)

t1a2(min)

kO3 103

(min1)

t1a2(min)

kO3 103

(min1)

t1a2(min)

4-CP 564 (0.99) 1.1 1877 (0.99) 0.4 17 (0.99) 38.5 239 (0.99) 3.4

2,4-DCP 38 (0.99) 17.5 209 (0.99) 2.4 24 (0.99) 30.4 315 (0.98) 3.3

2,4,6-TCP 26 (0.99) 25.2 98 (0.99) 5.1 44 (0.98) 20.6 314 (0.98) 3.1

2,3,4,6-TeCP 21 (0.99) 30.6 9 (0.98) 49.5 94 (0.98) 10.6 415 (0.99) 1.9

Fig. 1. CPs decomposition curves by direct photolysis.

F.J. Benitez et al. / Chemosphere 41 (2000) 12711277 1273

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3.1.3. Ozone

Finally, the CPs were oxidized by ozone (with an

ozone partial pressure in the ozoneoxygen gas streamof 90 Pa) at 25C and in acidic media pH 2. At thispH the decomposition of ozone, which is initiated by the

action of hydroxide ions (Staehelin and Hoigne, 1982), is

too low and consequently the formation of hydroxyl

radicals is limited. This decomposition of organic com-

pounds by ozone can be represented by a single global

reaction which follows a rst-order kinetics:

PO33KO3

PoxidX 4

When the four CPs were ozonated, the rate constant kO3for the reaction represented by Eq. (4) was in the fol-lowing decreasing rate of magnitude: 2,3,4,6-TeCP

>2,4,6-TCP >2,4-DCP >4-CP as can be observed in

Table 1. Similar results were obtained by Trapido et al.

(1997) in the ozonation of several CPs. Therefore, it can

be concluded that the sequence of degradation is the

inverse to that found in the photodegradation and

Fenton's reagent processes; that is, the increase in the

number of chlorine atoms in the aromatic ring provides

an increase in the degradation rate. The pathways pro-

posed by Chen and Ni (1998) to describe the complete

ozonation of 2,4-dichlorophenol were hydroxylation,

dechlorination and ring-cleavage. According to thismechanism, the presence of more atoms of chlorine

enhances the dechlorination step, and therefore, the

degradation is faster.

Another group of CPs ozonation experiments were

conducted in basic media (pH 9). In this case, thedegradation curves were very close for all the CPs

studied, with similar conversions for every CP. Table 1

also shows the values obtained for the rst-order rate

constants kO3 and the half-life times which conrm thesmall dierences in the removal of the CPs at this pH.

However, the most important fact that can be observed

when comparing the results at pH 2 and pH 9 is thefurther increase in the rate constant (and decrease in the

half-life time) with increasing the pH. This nding can

be explained by the faster production of OH radicals at

basic pH and the dissociation of phenols to phenolate

ions that are able to react with ozone faster than the

non-dissociated species (Hoigne and Bader, 1983).

3.2. Decomposition by advanced oxidation processes

In the second part of this research, the Advanced

Oxidation Processes constituted by the combinations of

UV radiation plus H2O2, UV radiation plus Fenton's

reagent, and UV radiation plus ozone, have been used

for the study of CPs decomposition. As was mentioned

in Section 1, these processes are characterized by the

generation of free radicals, mainly hydroxyl radicals

(Glaze et al., 1987; Glaze and Kang, 1989; Peyton et al.,

1982). Therefore, the objectives of this study are focused

on the evaluation of the enhancements caused in the

oxidation reactions by these radicals in comparison to

the reduction levels reached by the single oxidants at

similar operating conditions.

3.2.1. UV/H2O2 system

Photodegradation experiments of the four selected

CPs in the presence of hydrogen peroxide (with a H2O2initial concentration of 5 104 M in these experiments)were conducted at 25C and pH 2, and the sequenceof degradation rates achieved was just the same as that

obtained for the single photochemical process (showed

in Fig. 1): the degradation rates increase when the

number of chlorine substituents decreases. This se-

quence can be observed in Table 2, where the conver-

sions obtained at three selected reaction times (2.5, 5 and30 min) are summarized, as well as the rst-order rate

constants for this combined process kt and the half life

times t1a2. It is seen that these rate constants present a

Table 2

Conversions reached, rate constants and half-life times in the decomposition of CPs by UV/H2O2

Compound X2X5 (%) X5 (%) X30 (%) kt 103 (min1) t1a2 (min) kr 10

3 (min1)

4-CP 70 89 a 601 1.0 36

2,4-DCP 15 25 74 44 14.5 6.9

2,4,6-TCP 11 18 66 33 19.2 7.2

2,3,4,6-TeCP 10 21 63 29 20.1 7.9

a Total conversion reached at 9 min.

Fig. 2. Determination of rst-order rate constants kr in CPs

decomposition experiments by Fenton's reagent.


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moderate higher values than those obtained in the single

photodecomposition process (see Table 1, values of kp).

These ndings demonstrate the additional contribution

to the single photoreaction of the hydroxyl radicals

generated by the presence of hydrogen peroxide.

This supplementary contribution can be determinedby considering the mechanism of the combined process.

Thus, in addition to the direct photoreaction (1), the

following reactions must be taken into account:

Direct photolysis of hydrogen peroxide with the for-

mation of hydroxyl radicals:

H2O2 hm3 2 OH 5

Radical reaction between the organic compound and

the hydroxyl radicals generated in (5), which is given

by Eq. (3).

According to this mechanism, the reaction rate for

the global photodecomposition rT can be proposed as

the addition of the direct rP (reaction (1)) and radical rR(reaction (3)) reaction rates in the form

rT

dCP

dt

! rP rR kp krCP kt CPX

6

According to Eq. (6), the rate constants for the radical

reaction kr are easily deduced by subtracting the previ-

ously determined kp from kt, and Table 2 also depicts the

kr obtained. When comparing these values to those of kp

in Table 1, it is shown that the direct photolysis provides

a contribution to the total reaction higher than that of

the radical reaction.

In order to validate the proposed mechanism which

allows to calculate the rate constants, Fig. 3 presents the

theoretically calculated degradation curves of 2,4,6-TCP

(taken as example, similar results for other CPs) for thesingle radical and photochemical reactions, and for the

combined UV/H2O2 reaction. These theoretical curves

were calculated by means of computer simulations by

using the program ACUCHEM, which solves compli-

cated systems of chemical reactions (Braun et al., 1988).

For these calculations, the evaluated rate constants kpand kr have been used, and the global degradation is

determined as the sum of both contributions. In the

combined reaction, the experimental concentrations

obtained are also plotted. The excellent agreement be-

tween model calculations and experimental data sup-

ports the proposed mechanism.

3.2.2. Photo-Fenton system

Decomposition experiments of the CPs were carried

out by the simultaneous action of UV radiation and

Fenton's reagent (with initial concentrations of

Fe2 1 105 M and H2O2 5 104 M), and the

same sequence of degradation rates as in the single

photodecomposition or Fenton's reagent processes, or

as in the combined UV/H2O2 system was obtained.

Table 3 depicts the conversions obtained at three

reaction times, as well as the rst-order rate constants ktand the half-life times. When compared these rate con-

stants kt to the single photodecomposition rate con-

stants (kp in Table 1), higher values for kt can be seen

which again conrm the additional contribution of the

radical reaction due to the generation of the hydroxyl

radicals by Fenton's reagent and by the H2O2.

To evaluate the contribution of OH radical in this

process, in addition to reactions (1) and (2), corre-

sponding to the direct photodecomposition and the

generation of hydroxyl radicals by Fenton's reagent re-

spectively, another hydroxyl radicals generation reaction

must be considered due to the photolysis of H2O2 which

is represented by Eq. (5). According to this mechanism

described by Eqs. (1), (2), (5) and (3), Eq. (6) can be usedto determine the hydroxyl radicals contribution to the

global reaction. Table 3 depicts the kr values obtained

after using Eq. (6) and higher values ofkr for this system

Fig. 3. Decomposition curves for 2,4,6-TCP in the UV/H2O2system. ( ) theoretical values. (e) experimental values.

Table 3

Conversions reached, rate constants and half-life times in the decomposition of CPs by the photo-Fenton system

Compound X2X5 (%) X5 (%) X20 (%) kt 103 (min1) t1a2 (min) kr 10

3 (min1)

4-CP 81 96 a 642 0.9 79

2,4-DCP 24 39 84 88 7.4 50

2,4,6-TCP 21 37 79 78 8.5 52

2,3,4,6-TeCP 16 28 67 58 12.0 37



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in comparison to the AOP UV/H2O2 are observed.

Furthermore, the contribution to the global reaction of

the radical reaction is higher than that of the direct

photodecomposition (comparison between kr of Table 3

and kp of Table 1), except for 4-CP whose direct pho-

tolysis is really fast.Similar to the former UV/H2O2 system, Fig. 4 shows

the theoretical degradation curves for 2,3,4,6-TeCP

(taken as example), calculated as described above, and

the experimental concentrations obtained in this com-

bined system: again a perfect agreement is observed

between experimental results and model calculations. In

addition, is also seen the higher degradation rate cor-

responding to the radical reaction in comparison to the

direct photolysis.

3.2.3. O3/UV system

Finally, degradation experiments of the four CPs bythe combined process UV/O3 were conducted at 25C

and pH 2. The results obtained for the conversions atthree reaction times (2.5, 5 and 20 min), the global rst-

order rate constants kt (with correlation coecients

higher than 0.99 in all cases) and the half-life times t1a2are depicted in Table 4. When comparing these results to

those of the single photodecomposition or pH 2 oz-onation processes (Table 1), it is observed that this

combination accelerates the decomposition rate as could

be expected, with an extremely high rate constant kt for

4-CP (as a consequence of its extremely high rate for the

single photolysis), and lower rates for the rest of CPs.

These remaining CPs (2,4-DCP, 2,4,6-TCP and 2,3,4,6-

TeCP) present slightly increasing rate constants when

the substituent chlorine atoms increase, which was the

sequence observed in the single ozonation.

As Peyton et al. pointed out (Peyton et al., 1982),

ozone absorbs UV radiation and produces hydrogenperoxide

O3 hm3 H2O2 7

and then, there is a photolysis of hydrogen peroxide to

generate hydroxyl radicals in the form described by re-

action (5). In addition, H2O2 accelerates O3 decompo-

sition into OH radicals (Staehelin and Hoigne, 1982)

O3 H2O2 3 OHX 8

These radicals generated by reactions (5) and (8) con-

stitute the principal active species in the photolytic oz-

onation. Thus, in the O3/UV process, there is a

synergistic eect of several individual reactions: direct

ozonation (reaction (4)), direct photolysis (reaction (1))

and hydroxyl radical decomposition (reaction (3)).

However, this synergism cannot be accounted for on the

basis of an additive eect as in the former cases (UV/

H2O2 and photo-Fenton), and its magnitude varies from

substrate to substrate.

4. Conclusions

Several single oxidants and combined systems have

been used for the decomposition of some CPs, like 4-CP,

2,4-DCP, 2,4,6-TCP and 2,3,4,6-TeCP, being the de-

gradation rates evaluated by means of rst-order rate

constants. In the decomposition by single UV radiation

and Fenton's reagent, 4-CP is the most rapidly degraded

and 2,3,4,6-TeCP presents the lowest rate, while in the

single ozonation process the sequence of degradation is

inverse: 2,3,4,6-TeCP >2,4,6-TCP >2,4-DCP >4-CP,

with a clear increase in the degradation rate when the

pH is increased from 2 to 9. Therefore, ozone attacks to

these CPs with a dierent mechanism than OH radicals,

being this study the goal of current investigations.In the decomposition experiments by the advanced

oxidation processes, an enhancement in the degradation

is observed due to the generation of the hydroxyl radi-

Fig. 4. Decomposition curves for 2,3,4,6-TeCP in the photo-

Fenton system. ( ) theoretical values. (h) experimental values.

Table 4

Conversions reached, rate constants and half-life times in the decomposition of CPs by O3/UV

Compound X2X5 (%) X5 (%) X20 (%) kt 103 (min1) t1a2 (min)

4-CP 78 96 a 644 1.3

2,4-DCP 3 14 64 65 15.6

2,4,6-TCP 16 28 73 68 11.8

2,3,4,6-TeCP 22 37 77 71 9.7



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cals. This improvement is moderate in the UV/H2O2system and signicant in the UV/Fenton's reagent sys-

tem. In the last one, the contribution of the radical re-

action to the global reaction is higher than that of the

direct photolysis. In the O3/UV system an improvement

of the degradation rate is also observed, but in this casethe synergistic eect is not possible to be determined as a

result of an additive eect.

Acknowledgements

Authors wish to thank CICYT of Spain for its -

nancial support under Project AMB97-339, and Junta de

Extremadura for its Project IPR98A014. F. Javier Rubio

also thanks Junta de Extremadura for being granted with

a PhD Grant.

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