Effect of hydrogen peroxide on the destruction of organic contaminants synergism and inhibition in a con

Applied Catalysis B: Environmental 50 (2004) 259–269
Effect of hydrogen peroxide on the destruction of organic
contaminants-synergism and inhibition in a continuous-mode
photocatalytic reactor
Dionysios D. Dionysiou
a,∗
, Makram T. Suidan
a
, Isabelle Baudin
b
, Jean-Michel La
ˆ
ıné
b
a
Department of Civil and Environmental Engineering, Drinking Water, Water Supply, Quality and Treatment Laboratories,
University of Cincinnati, 765 Baldwin Hall, Mail Stop #0071, Cincinnati, OH 45221-0071, USA
b
Centre International de Recherche Sur l’Eau et l’Environnement, CIRSEE, ONDEO Services, 38 rue du President Wilson, 78230 Le Pecq, France
Received 18 September 2003; received in revised form 16 January 2004; accepted 27 January 2004
Available online 2 April 2004
Abstract
The effect of hydrogen peroxide on the photocatalytic degradation of organic contaminants in water was investigated using a TiO
2
-rotating
disk photocatalytic reactor (RDPR) operated in a continuous-mode and at steady state. The experiments were performed at pH 3.0, in the
presence of near-UV radiation, and using 4-chlorobenzoic acid (4-CBA) as a model non-volatile organic contaminant at influent concentration
of 300
␮mol l
−1
. Experiments were performed at concentrations of hydrogen peroxide in the range 0–10.74 mmol l

−1
. Addition of hydrogen
peroxide at small concentrations (<2 mmoll
−1
) had a synergistic effect and increased considerably the rates of photocatalytic reactions. An
optimum influent hydrogen peroxide concentration wasobserved at 1.6mmoll
−1
, which caused an increased in the ratesof4-CBAdegradation
and total organic carbon (TOC) mineralization by 1.72 and 2.13 times, respectively. This corresponded to an optimum oxidant to contaminant
molar ratio of 5.33. At higher concentrations, hydrogen peroxide was found to cause an inhibiting effect on the photocatalytic reactions. The
synergistic and inhibiting effects of hydrogen peroxide were rationalized based on the reaction rate constants between relevant radical species.
© 2004 Elsevier B.V. All rights reserved.
Keywords: TiO
2
; Photocatalysis; Photocatalytic; Hydrogen peroxide; H
2
O
2
; Radicals; Hydroxyl; Superoxide; Perhydroxyl; Reaction rate constants; Water
treatment; Detoxification; Destruction; Organic; Contaminants; Rotating disk; Reactor; Continuous; Chlorobenzoic acid; Green engineering
1. Introduction
Considering the chemical components of various tech-
nologies for water treatment, including the so-called ad-
vanced oxidation technologies (AOTs), TiO
2
photocatalysis
can be viewed as a “green” technology. It is becoming more
popular as a detoxification technology because of several
reasons. First, TiO
2

photocatalytic systems that incorporate
the catalyst immobilized require only the addition of UV
radiation for the generation of the primary reactive species
(i.e., electrons and holes) and subsequently the hydroxyl
radicals, which are the primary oxidizing species in the
process. Second, TiO
2
photocatalysis can result in the com-
plete destruction of virtually all organic contaminants (i.e.,
mineralization) when the reactor set-up is optimized [1–12].
Third, TiO
2
catalyst is non-toxic, insoluble in water, photo-
∗
Corresponding author. Tel.: +1-513-556-0724;
fax: +1-513-556-2599.
E-mail address: (D.D. Dionysiou).
stable, and relatively inexpensive [1,5–9,13]. Forth, recent
advances in the manufacture of more efficient artificial UV
sources and the potential of using solar light to photoexcite
the TiO
2
catalyst [14–16] give additional positive perspec-
tives for TiO
2
photocatalysis. Fifth, rapid progress on the
preparation of TiO
2
nanostructured materials [17–20] and
new insights on the fundamental aspects of TiO

2
photo-
catalysis [11,20–23] have resulted in significant progress on
the development of more efficient photocatalytic systems.
Nevertheless, TiO
2
photocatalysis is limited, at some
extent, by significant radiation energy losses due to the
electron–hole recombination process [23–27]. While the
desirable pathway for the primary species (i.e., electrons
and holes) is to reach the surface and generate other ef-
fective reactive species (i.e., superoxide radical anion and
hydroxyl radical, respectively), the majority of electrons
and holes recombine in the volume or at the surface of the
catalyst [9]. This recombination effect is detrimental to the
photocatalytic process and the photon energy is lost as heat.
For this reason, the quantum yields (i.e., number of primary
0926-3373/$ – see front matter © 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcatb.2004.01.022
260 D.D. Dionysiou et al. /Applied Catalysis B: Environmental 50 (2004) 259–269
chemical reactions per photon absorbed) of photocatalytic
processes are relatively low [5,7]. One way to partially
overcome this problem is to use efficient electron acceptors
that will capture the electrons and inhibit the recombination
effect [27,28]. Oxygen is the most common and relatively
efficient electron acceptor. However, aiming at further in-
hibiting the electron–hole recombination effect, several
previous studies have investigated the role of alternative
electron acceptors, including hydrogen peroxide [29–38].
Hydrogen peroxide, the simplest of the peroxides, is

an important precursor in chemical synthesis [39,40].Itis
also an integral component of several chemical oxidation
technologies including Fenton, photo-Fenton, UV-based
chemical oxidation, polyoxometallate processes, and higher
valence transition metal-based oxidation [41–44]. It is con-
sidered environmentally friendly, since it is composed only
of hydrogen and oxygen atoms and under appropriate condi-
tions can yield to environmentally desirable final products,
such as water or hydroxyl ions. Because of its environmen-
tally friendly properties, hydrogen peroxide was recently of
great interest in several applications dealing with “green”
chemistry and “green” engineering [39]. Along with its
application in several other advanced oxidation technolo-
gies (AOTs), hydrogen peroxide is often used as an oxidant
additive in TiO
2
photocatalytic processes to enhance the
rates of photocatalytic reactions [29–38]. While most of
these applications focused on the destruction of organic
contaminants in water using either artificial UV [45–47] or
solar illumination [30–32], few studies dealt also with the
destruction of gaseous contaminants [48]. Persulfate, per-
oxymonosulfate, periodate and other oxidants are also used
[32,49] but hydrogen peroxide is more popular because of
its advantage as a “green” additive. In some cases, hydrogen
peroxide was also applied to reactivate the TiO
2
catalyst that
suffered deactivation in gas phase photocatalytic reactions
due to accumulation of less degradable reaction byproducts

[50].
The enhancement of the photocatalytic rates using hy-
drogen peroxide was attributed to several factors. First,
hydrogen peroxide is a better electron acceptor than oxy-
gen [28,29,32,51–53]. The potential for oxygen reduc-
tion is −0.13V while that of H
2
O
2
reduction is 0.72V
[52]. It has been reported that removal of photogenerated
electrons in the conduction band by oxygen reduction is
the rate-controlling step in the photocatalytic mechanism
[26,54]. Consequently, conditions that favor the removal of
conduction band electrons can have a positive effect on the
photocatalytic process. Such conditions include enhanced
concentrations of oxygen [54,55] and addition of other
efficient electron acceptors, such as hydrogen peroxide.
Second, addition of hydrogen peroxide can enhance the
rate of generation of hydroxyl radicals. This can be the con-
sequence of different mechanisms. One is the generation
of hydroxyl radicals by direct photolysis of hydrogen per-
oxide [28,56,57]. Based on the first law of photochemistry,
this requires that hydrogen peroxide absorbs the photons
of incident radiation and that the radiation energy is suf-
ficient to cause photocleavage of the molecule [58].In
most of the studies employing solar radiation or artificial
UV sources emitting near-UV radiation (i.e., UV-A), it is
unlikely that direct photolysis of hydrogen peroxide will
be important [31]. Although radiation energy with wave-

length of 561.6 nm or shorter is energetically sufficient to
split the O–O bond (213kJmol
−1
) of the H
2
O
2
molecule,
this will not practically become significant at wavelengths
longer than 300 nm [58]. This is because hydrogen perox-
ide absorbs radiation strongly only at the lower wavelength
range (λ<300nm). For example, the molar extinction
coefficient of H
2
O
2
is 6.6 × 10
−4
,1× 10
−2
, 1.0, 19.6,
and 140 l mol
−1
cm
−1
at 400, 360, 300, 253.7, and 200 nm,
respectively [59]. Comparing radiation wavelengths at
253.7 nm (germicidal UV radiation) and 360nm (UV-A
range), the ratio of H
2

O
2
molar extinction coefficient is
1960. Another pathway is the reaction of hydrogen peroxide
with superoxide radical [31,37,38,49,52,60,61]. In addition,
the reaction of hydrogen peroxide with photogenerated in-
termediates could be another possibility [49,62]. Hydrogen
peroxide can also be beneficial in situations where there is
limited availability of oxygen [28,62].
Several research groups have previously investigated the
effect of hydrogen peroxide on the photocatalytic degra-
dation of organic contaminants. Poulios et al. studied the
photocatalytic oxidation of eosin Y dye using TiO
2
(De-
gussa P-25) and ZnO in batch slurry reactors [49]. When
they investigated the effect of hydrogen peroxide on the
photocatalytic oxidation of the dye and the dissolved or-
ganic carbon (DOC), they observed a beneficial effect for
both catalysts, although for TiO
2
the degradation rates
were significantly faster than those of ZnO in the absence
and presence of hydrogen peroxide. The rate enhancement
factor was higher than 2 for both the parent contaminant
(i.e., decolorization) and the DOC. Madden et al. inves-
tigated the photocatalytic oxidation of EDTA and several
metal–EDTA complexes using Degussa P-25 slurry in batch
photocatalytic systems [29]. Among other factors, they also
studied the effect of continuous addition of hydrogen per-

oxide on the photocatalytic treatment of Ni(II)–EDTA and
Cu(II)–ETDA complexes. They observed that the addition
of hydrogen peroxide enhanced significantly the degrada-
tion of Ni(II)–EDTA and its mineralization. Positive but
less dramatic increase in the photocatalytic rates was also
observed for Cu(II)–ETDA, for which the degradation rate
was high even in the absence of hydrogen peroxide. The
authors suggested that this higher degradation rate could
be partially due to the dark Fenton reaction of Cu(II) [29].
Positive effects were also reported by Bandala et al. for
the photocatalytic degradation of the pesticide Aldrin using
non-concentrated and concentrated solar radiation [63]. Ben-
eficial effects of the addition of hydrogen peroxide were re-
ported for the photocatalytic treatment of dissolved organic
matter in the effluent of a cellulose and paper mill industry
[64], several pesticides and other organic contaminants.
D.D. Dionysiou et al. /Applied Catalysis B: Environmental 50 (2004) 259–269 261
On the other hand, Chun and Park reported that hydro-
gen peroxide (oxidant to contaminant molar ratio (OCMR)
of 5:1 and 10:1) caused a slight inhibition of the rates of
trichloroethylene degradation while periodate had a positive
effect and thiocyanate ions discontinued the degradation re-
action [45]. Similarly, Dillert et al. found that, in most cases,
the addition of H
2
O
2
caused a reduction in the degrada-
tion rates of 2,4,6-trinitrotoluene and 1,3,5-trinitrobenzene
at various initial pH values (3–11) compared to those of the

UV (λ>320nm)/TiO
2
system [35]. The authors hypoth-
esized that this effect could be due to the competition of
H
2
O
2
with the nitroaromatic contaminants for conduction
band electrons. They explained that these reactions gener-
ate
•
OH and the corresponding nitroaromatic radical an-
ion (reductive pathway), respectively. The latter is not an
efficient pathway for the photocatalytic degradation of ni-
troaromatics. In general the rates of the heterogeneous sys-
tem (UV/TiO
2
/H
2
O
2
) were higher than those of the homo-
geneous system (UV/H
2
O
2
) [35].
However, most of the previous photocatalytic studies on
the effect of hydrogen peroxide reported the existence of an

optimum concentration. Mengyue et al. studied the effect
of hydrogen peroxide on the photocatalytic degradation of
monocrotophos and parathion organophosphorus pesticides
[46]. They found that a concentration of 6 mM hydrogen per-
oxide enhanced the degradation efficiency (i.e., expressed
as the fraction of organophosphate that was mineralized as
soluble phosphate ion) by approximately four times for both
pesticides. The enhancement effect was slightly lower at hy-
drogen peroxide concentrations of 8 and 10 mM with a grad-
ually reducing trend (i.e., 3.76 and 3.54 for monocrotophos;
3.78 and 3.60 for parathion, respectively). Kumar and Davis
studied the effect of hydrogen peroxide on the photocatalytic
degradation of 2,4-dinitrotoluene (DNT) in batch slurry re-
actors [56]. They used initial 2,4-DNT concentration of
0.6 mM and varied the concentration of hydrogen peroxide
from zero to 100 mM. They observed a slight enhancement
of the degradation rate by 10% at hydrogen peroxide con-
centrations of 1–10 mM. At 100 mM of hydrogen peroxide,
the rate decreased to that in the absence of hydrogen perox-
ide. Haarstrick et al. studied the photocatalytic degradation
of 4-chlorophenol and p-toluenesulfonic acid in a fluidized
bed photocatalytic reactor operated in a batch mode [47].
In experiments dealing with the effect of hydrogen peroxide
concentration on the degradation rates, they used equimolar
mixtures of the two contaminants with total organic carbon
(TOC) of 140 mg l
−1
and hydrogen peroxide molar concen-
trations varied in the range 0–9mM. The degradation rates
increased with increasing hydrogen peroxide in this range.

The enhancement factor at hydrogen peroxide concentra-
tions of 2–3 mM was approximately 2. However, this factor
did not considerably increase at higher concentrations. For
this reason, and considering the cost of hydrogen peroxide,
the authors suggested that the optimum hydrogen peroxide
was at 2 mM. Cornish et al. reported an optimum concentra-
tion of hydrogen peroxide on the photocatalytic destruction
of microcystin-LR toxin with a maximum enhancement fac-
tor of approximately 2 [52]. The authors also observed that
during dark adsorption, hydrogen peroxide competed with
microcystin-LR for active sites and that hydrogen peroxide
at concentration of 0.6% (v/v) in water caused coagulation
of the suspension, an effect that was attributed to catalyst
surface charge modifications by adsorbed hydrogen perox-
ide molecules. This hypothesis was further supported by the
formation of a yellow color in the suspension solution, which
was attributed to the Ti(IV)–peroxo complexes [52].
In summary, previous studies dealing with the role of hy-
drogen peroxide on the photocatalytic degradation of organic
contaminants reported positive, neutral, or negative effect.
Most studies reported that hydrogen peroxide could increase
the reaction rates or cause inhibition effects depending on
its concentration in the reaction solution. The results of all
these studies suggest that the effect of hydrogen peroxide
is a function of many interrelated parameters including the
properties of radiation (i.e., wavelength, intensity), solution
pH, physicochemical properties of the contaminant, type of
catalyst (i.e., surface characteristics) and the oxidant to con-
taminant molar ratio.
However, in most previous studies, the effect of hydrogen

peroxide was investigated in batch or semi-batch photocat-
alytic systems in which both the concentrations of contami-
nant and hydrogen peroxide were changing with time. Few
studies dealt with addition of hydrogen peroxide at constant
rate [29] but again in batch or semi-batch systems. As ex-
plained above, synergistic or inhibitive effects are a func-
tion of the magnitude of hydrogen peroxide concentration
relative to other reaction conditions (i.e., contaminant con-
centration, total organic carbon, UV light flux). When such
conditions are time-dependent, the task to elucidate the ef-
fect of a certain parameter becomes more difficult.
Our approach was to investigate the effect of hydrogen
peroxide in a continuous-mode photocatalytic reactor oper-
ated at steady state. The reactor used in this study is the ro-
tating disk photocatalytic reactor (RDPR). In addition, the
RDPR has mixing characteristics similar to a continuously
stirred tank reactor (CSTR), meaning that the concentration
of chemical species and the conditions of the effluent are
equal to those inside the reactor vessel. This is very impor-
tant for assessing the effect of hydrogen peroxide concen-
tration in the reactor solution, during a process in which all
conditions in the reactor (i.e., concentrations, pH, tempera-
ture) remain stable with time when a steady state operation
is achieved.
2. Experimental procedures
2.1. Rotating disk photocatalytic reactor (RDPR)
Details on the development, characterization, evaluation,
and mechanism of operation of the RDPR for the destruction
of pesticides and other organic pollutants in water at small
262 D.D. Dionysiou et al. /Applied Catalysis B: Environmental 50 (2004) 259–269

concentrations (i.e., <100 mg l
−1
) were provided recently in
a series of articles [55,65,66]. In summary, the RDPR is a
thin film reactor that can be operated in a batch, semi-batch
or continuous-mode. The catalyst is immobilized on the ro-
tating disk and the reactions occur in a thin film of liquid car-
ried by the disk at the interface of the liquid film and the cata-
lyst. The maximum volumetric capacity of the reactor vessel
is 3.5 l. The catalyst is in the form of composite spherical ce-
ramic balls of ∼6mm in diameter (ST-B01, Ishihara Techno
Corp., Japan) that are immobilized on the disk as discussed
in previous publications [55,66]. These are composed of an
inner ceramic support (mainly SiO
2
/Al
2
O
3
) and an outer
thin layer (10–30 ␮m) of TiO
2
nanoparticles attached to the
surface of the support. The RDPR incorporates two UV ra-
diation sources (Spectronics Corporation, Westbury, New
York) that are located at either side of the rotating disk. Each
radiation source comprised of two 15W integrally filtered
low-pressure mercury UV tubes mounted in silver-anodizing
aluminum housings with UV reflectors. Each tube emits ra-
diation in the range of 300–400nm with a peak at 365 nm

(Technical Bulletin, Spectronics Corporation).
2.2. Photocatalytic degradation of 4-CBA
In this study, 4-chlorobenzoic acid (CBA) was used as
a model organic contaminant of aromatic structure. The
selection was based on several reasons including its prac-
tically zero volatility, its inertness towards oxidation by
ozone, and our familiarity for its measurements at very low
concentrations using HPLC techniques [65]. In addition,
aromatic compounds of similar chemical structure are used
in the dying industry and in the manufacture of fungicides
and pharmaceuticals and have bactericide properties at high
concentrations. As a result, they are resistant to biological
treatment.
The RDPR was operated in a continuous-mode at a flow
rate (Q) of 53mlmin
−1
, residence time (τ) of 66min, and
room temperature (18-21
◦
C). The reactor was closed at the
top for controlling the gas atmosphere and for gas analy-
sis. The experiments were performed at pH 3.0 (influent
and the solution inside the reaction vessel), disk angular
velocity (ω) of 6rpm, solution ionic strength as KNO
3
of
10 mM, solution volume in the reaction vessel (V
R
) of 3.5 l,
and influent concentration of 4-CBA (C

in
)of300␮moll
−1
.
The incident average light intensity (I) was approximately
896 ␮Wcm
−2
. Air was the gas atmosphere except when
otherwise specified. The experiments were performed at
different concentrations of hydrogen peroxide in the influent
in the range 0–10.74 mM. This corresponds to an OCMR in
the range 0–35.8. The experiments were performed using
solutions of 4-CBA in deionized water (18 M). HNO
3
was used to adjust the pH.
Prior to the irradiation phase of the experiment, the RDPR
was operated in the dark for 30 min to ensure that dark
equilibrium adsorption of 4-CBA was achieved. This was
verified by measuring the effluent concentration of 4-CBA
during the 30 min of dark adsorption phase. During prelim-
inary experiments, as well as from immediate analysis of
the samples obtained during this study, it was observed that
steady state was achieved in approximately three hours after
the beginning of the irradiation phase. During the irradiation
phase of the experiment, samples were taken every 30 min
for 4 h. To check for mixing in the reactor, samples were
also withdrawn from inside the reaction vessel (bottom of
the reactor vessel). Sampling from inside the reactor was
taken using a sampling/drainage valve. Sample filtration
and analysis was performed immediately after collection.

2.3. Analyses
Analysis of 4-CBA was performed using an Agilent
Series 1050 HPLC equipped with a C-18 column (J&W
Scientific). The mobile phase was 30% (v/v) acetonitrile
and 70% (v/v) sulfuric acid (0.01N) with a flow rate
of 1.5 ml min
−1
. Each run was 30 min. The concentra-
tion of chloride ion in the solution was measured using
an ion chromatograph (IC, DX-120 DIONEX) equipped
with a A514 4mm column and a ASRS
®
-ultra 4mm SRS
(self-regenerating suppressor). The flow rate and the pres-
sure of the pump were 1.23 ml min
−1
and 1400–1600 psi,
respectively. The mobile phase was a solution of 3.5mM
Na
2
CO
3
and 1.0 mM NaHCO
3
. The concentration of H
2
O
2
in the reaction solution was measured immediately after the
sample was withdrawn from the reactor. The concentration

of H
2
O
2
was measured using the LaMotte Octet Compara-
tor test kit HP-40 as well as using an HPLC equipped with
an LC-18, 15cm × 4.6mm, 5␮m HPLC column and a
UV-Vis diode array spectrophotometer. The mobile phase
was a mixture of acetonitrile (50% (v/v)) and water (50%
(v/v)) with a flow rate of 0.8 ml min
−1
. The column temper-
ature was 30
◦
C. The concentration of hydrogen peroxide
was measured at 206nm. Comparison of the two methods
(LaMotte HP-40 and HPLC) for hydrogen peroxide anal-
ysis in the overlapping range (1–40mgl
−1
) gave similar
results. Total organic carbon analysis was performed us-
ing a Shimadzu (Kyoto, Japan) TOC-5000 analyzer. Prior
to TOC analysis, samples were acidified to pH 2.0 using
HCl. The composition of the gas phase was analyzed for
CO
2
,O
2
and N
2

using a gas chromatograph with thermal
conductivity detector (GC/TCD). A one-point calibration
was employed using a standard provided by Matheson Gas
Products that contained the following components: carbon
dioxide 0.998%, oxygen 19.930%, and balance nitrogen.
The injection volume for standards or samples was 0.5 ml.
The details of the GC conditions for quantifying CO
2
have
been described in detail elsewhere [67].
2.4. Chemicals
The following chemicals were used as supplied: acetoni-
trile (CH
3
CN, FW = 41.05, HPLC grade, Fisher Scien-
tific, Pittsburgh, PA), 4-chlorobenzoic acid (ClC
6
H
4
CO
2
H,
D.D. Dionysiou et al. /Applied Catalysis B: Environmental 50 (2004) 259–269 263
FW = 156.57, 99%, Aldrich, Milwaukee, WI), nitric acid
(HNO
3
, 68–71% (w/w), trace metal grade, Fisher), potas-
sium nitrate (KNO
3
,FW= 101.11, 99.3%, Fisher) and

hydrogen peroxide solution (H
2
O
2
,FW= 34.01, 30.3%,
Fisher).
3. Results and discussion
During the continuous-mode operation of the RDPR, the
concentration of 4-CBA in the effluent decreased with time
during the first three hours of UV irradiation. Afterwards,
the reactor reached steady state and the concentration of
4-CBA remained constant. At steady state, the effluent con-
centration of 4-CBA was approximately 64% of the initial
value while TOC was reduced to 86.5% of the initial value.
It should also be noted that the concentration of 4-CBA in
samples obtained from the reactor vessel were the same as
those in samples taken from the effluent, as we expected
since the RDPR operates close to a CSTR as we previously
reported [65]. Higher removal efficiencies could also be ob-
tained at smaller flow rates. However, this flow rate was
selected in order to be able to observe differences between
various conditions applied in the process for both 4-CBA
concentration and TOC.
Fig. 1 shows the normalized (effluent/influent) 4-CBA and
TOC concentrations versus the influent (Fig. 1a) or effluent
(Fig. 1b) hydrogen peroxide concentrations at steady state
(4 h). It is clearly seen that the minimum effluent concentra-
tion for both 4-CBA and TOC is obtained at approximately
55 mg l
−1

(1.6 mmol l
−1
) of hydrogen peroxide in the influ-
ent, corresponding to OCMR
in
of 5.33. At these conditions,
the degradation of the parent contaminant was 63.1% and the
removal of TOC was 38.36%. These minima correspond to
effluent hydrogen peroxide concentration of approximately
3.9 mg l
−1
(0.11 mmol l
−1
) and an OCMR
eff
of 1.0 (based on
the effluent molar concentrations of both H
2
O
2
and 4-CBA).
Compared to the control experiment in the absence of hy-
drogen peroxide, these optimum conditions resulted in en-
hancement factors for 4-CBA degradation and TOC removal
of 1.72 and 2.13, respectively. At much higher concentra-
tions (10.74 mmol l
−1
) of hydrogen peroxide in the feed so-
lution, the rates decrease significantly but still remain higher
compare to those in the absence of hydrogen peroxide.

These results are supported by the data for the concen-
tration of free Cl
−
in the effluent at steady state as shown
in Fig. 2. Maximum concentrations of Cl
−
were obtained
at 55 mg l
−1
(1.6 mmol l
−1
) of hydrogen peroxide in the in-
fluent; this again corresponded to approximately 3.9 mg l
−1
(0.11 mmol l
−1
) of hydrogen peroxide in the effluent. Even
small concentrations (∼3mgl
−1
) of hydrogen peroxide in
the feed solution increased the rates by 30%. Fig. 3 reports
results for the concentration of hydrogen peroxide in the
effluent as a function of its concentration in the feed solu-
tion. It is clearly seen that when the concentration of hy-
drogen peroxide in the influent surpass a critical value (i.e.,
[H
2
O
2
]

effluent
, mg/L
0 20 40 60 80 100 120 140 160 180
C
eff
/C
in
or TOC
eff
/TOC
in
0.3
0.4
0.5
0.6
0.7
0.8
0.9
TOC
eff
/TOC
in
C
eff
/C
in
[H
2
O
2

]
influent
, mg/L
0 100 200 300 400
C
eff
/C
in
or TOC
eff
/TOC
in
0.3
0.4
0.5
0.6
0.7
0.8
0.9
C
eff
/C
i
n
TOC
eff
/TOC
in
(a)
(b)

Fig. 1. Effect of hydrogen peroxide on the degradation of 4-CBA in
the RDPR (continuous-mode operation) at ω = 6rpm, pH = 3.0,
I = 896 ␮Wcm
−2
, Q = 53 mlmin
−1
, V
R
= 3.5l, τ = 66 min,
C
influent
≈ 300 ␮mol l
−1
, [KNO
3
] = 10mM, and air as the gas phase.
Plots of normalized 4-CBA concentration (C
eff
/C
in
) and total organic car-
bon (TOC
eff
/TOC
in
) at steady state as a function of (a) influent hydrogen
peroxide concentration, [H
2
O
2

]
influent
and (b) effluent hydrogen peroxide
concentration, [H
2
O
2
]
effluent
.
[H
2
O
2
]
influent
, mmol/L
0 246810 12
[H
2
O
2
]
effluent
, mmol/L
0 12345
Effluent Cl
-
Concentration, mol/L
100

120
140
160
180
200
Fig. 2. Effect of hydrogen peroxide on the degradation of 4-CBA
in the RDPR (continuous-mode operation) at ω = 6 rpm, pH = 3.0,
I = 896 ␮Wcm
−2
, Q = 53 mlmin
−1
, V
R
= 3.5l, τ = 66 min,
C
influent
≈ 300 ␮mol l
−1
, [KNO
3
] = 10mM, and air as the gas phase.
Effluent Cl
−
concentration as steady state as a function of influent and
effluent hydrogen peroxide concentrations.
264 D.D. Dionysiou et al. /Applied Catalysis B: Environmental 50 (2004) 259–269
[H
2
O
2

]
influent
, mg/L
0 100 200 300 400
[H
2
O
2
]
effluent
, mg/L
0
40
80
120
160
Fig. 3. Effect of hydrogen peroxide on the degradation of 4-CBA in
the RDPR (continuous-mode operation) at ω = 6rpm, pH = 3.0,
I = 896 ␮Wcm
−2
, Q = 53 mlmin
−1
, V
R
= 3.5l, τ = 66 min,
C
influent
≈ 300 ␮mol l
−1
, [KNO

3
] = 10mM, and air as the gas phase.
Plot concentration of hydrogen peroxide in the effluent at steady state,
[H
2
O
2
]
effluent
, as a function of concentration of hydrogen peroxide in the
influent at steady state, [H
2
O
2
]
influent
.
55 mg l
−1
), then there is significant amount of excess hydro-
gen peroxide (i.e., unreacted) in the effluent. As presented in
Fig. 4, similar results were also obtained for the evolution of
CO
2
. The results for mineralization to CO
2
are also in good
agreement with the results obtained from TOC analysis.
Results from the comparison of the extent of reaction in
the presence of hydrogen peroxide at a feed concentration

(∼50 mg l
−1
) close to its optimum and two different oxygen
concentrations in the gas phase (21 and 82%) are presented
in Fig. 5. The data show that, in the presence of hydrogen
peroxide, a raise in oxygen concentration by almost 4 times
did not have any effect on the degradation rates, which were
measured with respect to 4-CBA transformation (%), min-
eralization to CO
2
(%) and amount of Cl
−
released in the
solution. On the other hand, a raise of oxygen concentration
in the gas phase, and in the absence of hydrogen peroxide
in the liquid phase, resulted in a significant enhancement of
the degradation rates, as it was shown in another study using
the RDPR [55].
The results obtained in this study clearly show that by the
continuous addition of small amounts (i.e., 40–60mgl
−1
)of
hydrogen peroxide in the feed solution, both the degradation
and mineralization rates increase significantly. The data also
suggest that small amounts of hydrogen peroxide in the re-
action solution favor the degradation rates whereas an inhi-
bition effect is encountered as the concentration of hydrogen
peroxide in the reaction solution surpasses a certain value.
Considering that the RDPR operates similar to a CSTR and
based on the results obtained in this work, it is suggested that

hydrogen peroxide is required only at small concentrations.
Addition of hydrogen peroxide at larger concentrations may
be detrimental to the process, since it results in rates similar
to those of the control and increases operational costs.
The results provide convincing evidence for the enhance-
ment effect of photocatalytic reactions in the presence of
[H
2
O
2
]
influent
, mg/L
0 100 200 300 400
% Conversion
10
20
30
40
50
60
70
80
TOC Degradation (%)
4-CBA Conversion (%)
Mineralization to CO
2
(%)
[H
2

O
2
]
effluent
, mg/L
0 20 40 60 80 100 120 140 160 180
% Conversion
10
20
30
40
50
60
70
80
TOC Degradation (%)
4-CBA Conversion (%)
Mineralization to CO
2
(%)
(a)
(b)
Fig. 4. Effect of hydrogen peroxide on the percentage conversion of
4-CBA, TOC and CO
2
formation in the RDPR (continuous-mode oper-
ation) at ω = 6 rpm, pH = 3.0, I = 896 ␮Wcm
−2
, Q = 53 ml min
−1

,
V
R
= 3.5l, τ = 66 min, C
influent
≈ 300 ␮moll
−2
, [KNO
3
] = 10mM, and
air as the gas phase. Plots of percentage conversion of 4-CBA, TOC and
(%) CO
2
formation based on influent 4-CBA concentration as a function
of (a) influent hydrogen peroxide concentration, [H
2
O
2
]
influent
and (b)
effluent hydrogen peroxide concentration, [H
2
O
2
]
effluent
.
[O
2

] in the Gas Phase, %
Extent of Reaction
0
20
40
60
160
180
200
82
21
4-CBA Transformation, %
Cl
-
Released, µmol/L
Mineralization to CO
2
, %
Fig. 5. Degradation of 4-CBA in the RDPR (continuous-mode operation)
in the presence of hydrogen peroxide and different oxygen concentra-
tions in the gas phase at ω = 6rpm, pH = 3.0, I = 896␮Wcm
−2
,
Q = 53 mlmin
−1
, V
R
= 3.5l, τ = 66min, C
influent
≈ 300 ␮moll

−1
,
[KNO
3
] = 10 mM, and [H
2
O
2
]
influent
≈ 48 mgl
−1
. Concentration of oxy-
gen in the gas phase: 21 and 82%.
D.D. Dionysiou et al. /Applied Catalysis B: Environmental 50 (2004) 259–269 265
hydrogen peroxide at optimum concentrations and are in
agreement in general terms with those of most of previ-
ous photocatalytic studies. For example, the existence of a
maximum enhancement factor at optimum H
2
O
2
concen-
trations was reported for the photocatalytic degradation of
microcystin-LR [52], several organophosphorus pesticides
(methamidophos, malathion, diazion, phorate, and EPN)
[34], 4-chlorophenol and p-toluenesulfonic acid [47],sev-
eral organochlorine compounds (i.e., 1,2-dichloroethylene,
trichloroethylene, 1,2-dichlorobenzene) [30,68,69], salicylic
acid [31], chlorinated aromatics [70], various dyes (i.e., acid

orange 8, gentian violet) [62,71] and several proteins (bovine
serum albumin, hen egg-white ovalbumin and bovine serum
gamma-globulin) [72]. In many cases, the OCMR was con-
sidered to be important for optimizing the degradation rates.
Maximum enhancement was found to occur in the range
5–200 depending on the type of organic contaminant and
the conditions of the photocatalytic reactions. It should be
noted, however, that most of these studies were performed
in batch or semi-batch systems and the referred OCMRs
are those of the initial concentrations. These concentrations
vary with time since both reactants are consumed gener-
ating other species of again varying concentration with
time. In essence, the OCMR does not remain constant with
time. In this study, because the RDPR was operated as a
continuous-mode system in one-pass process and at steady
state, the effect of hydrogen peroxide can be rationalized
under more controlled conditions. Malato et al. studied this
effect using an alternative but also interesting approach [32].
They investigated the effect of hydrogen peroxide on the
photocatalytic degradation of pentachlorophenol (PCP) us-
ing a batch solar reactor and suspensions of Degussa P-25.
They observed an optimum at 10 mM, which corresponded
toa[H
2
O
2
]/[PCP]
o
molar ratio of approximately 106. The
authors also compared the extent of mineralization of PCP

in two cases: one with 10mM [H
2
O
2
] added once at the
beginning of the experiment and one at which [H
2
O
2
]was
maintained at 10 mM throughout the reaction. The mineral-
ization rates were similar in both cases, suggesting that in
batch studies the use of constant concentration of hydrogen
peroxide in the reaction solution may not be beneficial.
This is because the concentration of organic contaminants
decreases but that of hydrogen peroxide remains the same,
resulting in continually increasing [H
2
O
2
]/organic molar
ratios.
The enhancement of the reaction rates with the addition
of hydrogen peroxide and the existence of an optimum
was attributed to various factors including increase in the
formation of hydroxyl radicals and other reactive species.
Sun and Bolton found that adding hydrogen peroxide in the
range 0–26 mM caused a Langmuirian-type increase of the
quantum yield for the generation of hydroxyl radicals [54].
The quantum yield increased almost linearly at the small

concentrations range (0–2 mM) and reached a plateau at
the higher concentration range (18–26mM). The quantum
yield was 0.04 in the absence of hydrogen peroxide and
0.22 at the maximum values, an enhancement factor of
5.5. More recently, Hirakawa and Nosaka performed a very
interesting photocatalytic study on the effect of hydrogen
peroxide on the formation of O
2
•
−
and
•
OH using UV-A
radiation (387 nm) at pH 11.5 [73]. They measured the
concentration of O
2
•
−
using luminol chemiluminescence
and that of
•
OH using terephthalic acid fluorescence tech-
niques. They found that the concentration of both O
2
•
−
and
•
OH increased with the addition of hydrogen peroxide at
the optimum conditions by approximately 3 and 3.6 times,

respectively. They also observed that the concentration of
O
2
•
−
reached a maximum at an optimum concentration
of hydrogen peroxide of 0.2mM while the evolution of
•
OH followed a Langmuir-type relationship. At higher
concentrations of hydrogen peroxide (up to 0.5 mM), the
enhancement factor for the formation of O
2
•
−
decreased
gradually reaching a plateau at 0.4 mM hydrogen peroxide
with an enhancement factor of approximately 2. They at-
tributed the enhancement effect for the generation of
•
OH
mainly to the effectiveness of H
2
O
2
in capturing the pho-
toinduced e
−
(i.e., Fenton-like reaction), preventing thus
the recombination effect, and producing additional
•

OH
(i.e., to those generated during the oxidation of OH
−
by
valence band holes). The decrease of the concentration
of O
2
•
−
at higher concentrations of H
2
O
2
was attributed
to the competition for adsorption between H
2
O
2
and O
2
[73].
At higher concentrations of hydrogen peroxide, hydrogen
peroxide may compete with the organic contaminants for
adsorption at catalytic actives sites. Such an effect was ob-
served by Bandala et al. for the photocatalytic degradation of
the pesticide Aldrin using concentrated solar systems [63].
Addition of hydrogen peroxide caused desorption of pread-
sorbed pesticide molecules resulting in a “chromatographic
peaking effect” of pesticide concentration in solution during
the initial stages of the photocatalytic process. This was at-

tributed to the stronger interactions with the catalysts active
sites (i.e., oxygen for the hydrogen peroxide and chlorine
for Aldrin). Competitive adsorption by hydrogen peroxide
at higher concentrations was also considered as one of the
reasons for the reduction in the degradation rates for safira
HELX, an anionic reactive azo dye [74] and microcystin-LR
[52].
It has been reported that chemisorption of H
2
O
2
, gaseous
or dissolved in water, at the surface of rutile or anatase TiO
2
results in a complex with yellow color and that the color
is stronger for rutile [75,76]. Under UV illumination the
color disappears leading to the production of O
2
[75,76].
Boonstra and Mutsaers proposed that the yellow color was
due to the formation of Ti
s–
O–O–H complexes on the sur-
face [75]. Jenny and Pichat observed that the quantity of
oxygen produced during photocatalytic decomposition of
hydrogen peroxide was less than the stoichiometric (i.e.,
formation of 1H
2
O and 1/2O
2

), suggesting internal hy-
droxylation of the TiO
2
surface layers or the formation of
stable peroxo species [76]. The authors explained their data
266 D.D. Dionysiou et al. /Applied Catalysis B: Environmental 50 (2004) 259–269
using the Langmuir–Hinshelwood mechanisms and sug-
gested a non-dissociative adsorption of hydrogen peroxide
at the catalyst surface during the photocatalytic process.
More recently, Ohno et al. performed studies on the effect
of hydrogen peroxide on the photocatalytic transformation
of 1-decene to 1,2-epoxydecane in a mixed solvent (wa-
ter, acetonitrile and butyronitrile) using TiO
2
powders and
molecular oxygen as a source of oxygen [53]. They found
that the type of crystal phase of the catalyst was an impor-
tant parameter. Addition of hydrogen peroxide had no effect
in systems utilizing anatase TiO
2
, whereas it had a dramatic
increase in the reaction rates in systems utilizing rutile
TiO
2
. Moreover, in the latter case the reaction could pro-
ceed even in the presence of visible light. Diffuse reflection
spectra of TiO
2
particles treated with hydrogen peroxide
indicated strong absorption in the visible range and for-

mation of peroxide species at the catalyst surface for both
rutile and anatase crystal forms. Additional FT-IR analysis
showed that these Ti-peroxo species were very reactive for
both anatase and rutile since they practically disappeared
after 20 min of their formation. However, the same analysis
showed that different Ti-peroxo species were formed in
each case: Ti-␩
2
-peroxide for rutile and Ti-␮-peroxide for
anatase. The authors hypothesized that the Ti-␩
2
-peroxide
can interact with 1-decene forming a transient species
through one of the oxygen atoms of the peroxide and the
carbons of the double bond. Then the oxygen could be
finally transferred to 1-decane to form 1,2-epoxydecane
[53].
Another aspect that deserves further discussion is the
role of UV wavelength. In this work, UV-A of 365nm
was the radiation used to photoexcite the catalyst. In our
study, it was difficult to perform the experiment only with
UV/H
2
O
2
since the disk already incorporated the catalyst.
As explained in a previous section, hydrogen peroxide does
not absorb appreciably at wavelengths in this range and
it is not expected to undergo significant direct photoly-
sis, especially in the thin (few mm) liquid film that forms

on the disk during the rotation. In some previous studies,
especially when using UV-C radiation, the enhancement
effect of hydrogen peroxide was attributed mainly to the
UV/H
2
O
2
homogeneous process [36,77]. Wang and Hong
studied the role of hydrogen peroxide and other inorganic
oxidants on the photocatalytic degradation of chlorobiphenyl
in aqueous TiO
2
suspensions using UVA-340 light tubes
(λ>295 nm) [33]. They compared three different sys-
tems: UV/TiO
2
, UV/oxidant, and UV/TiO
2
/oxidant. Their
study showed that addition of 10 mM H
2
O
2
significantly in-
creased the reaction rate compared to the UV/TiO
2
system.
However, the degradation rate was somewhat slower to that
of UV/H
2

O
2
system. Similar results were observed for the
other oxidants. The authors suggested that the enhancement
effect of the oxidants could be due to homogeneous photol-
ysis. However, they also pointed out that shading and scat-
tering effects in the heterogeneous system will result in less
actual photon flux input in this system [33].
Positive effects of hydrogen peroxide were observed by
Suárez-Parra et al. in the visible light (λ>400nm)-induced
photodegradation of a blue azo dye and using composite
TiO
2
/CdO–ZnO nanoporous film as the catalyst and acidic
pH = 3.0 [78]. The authors suggested that hydrogen perox-
ide plays a significant role in this process since it scavenges
the photoinjected electrons from the dye, preventing thus the
recombination of the electrons with the cation radical of the
dye.
Several previous studies have reported on several re-
actions that take place in a photocatalytic process and
involve H
2
O
2
,
•
OH and O
2
•

−
. The following pertinent
reactions have appeared in the literature dealing with
TiO
2
photocatalysis and the effect of hydrogen peroxide
([46,47,73,74,76,77] and references therein):
TiO
2
+ hv → TiO
2
(h
+
+ e
−
) (1)
H
2
O
2
hv
−→ 2
•
OH (2)
H
2
O
(ads)
+ h
+

→
•
OH + H
+
(3)
OH
−
(ads)
+ h
+
→ HO
•
(4)
O
2
+ e
−
→ O
2
•
−
(5)
O
2(ads)
+ e
−
+ H
+
→ HO
2

•
(6)
HO
2
•
+ HO
2
•
→ H
2
O
2
+ O
2
(7)
O
2
•
−
+ HO
2
•
→ HO
2
−
+ O
2
(8)
HO
2

−
+ H
+
→ H
2
O
2
(9)
H
2
O
2(ads)
+ e
−
→
•
OH + OH
−
(10)
H
2
O
2
+ O
2
•
−
→
•
OH + OH

−
+ O
2
(11)
H
2
O
2(ads)
+ 2h
+
→ O
2
+ 2H
+
(12)
H
2
O
2(ads)
+ 2H
+
+ 2e
−
→ 2H
2
O (13)
H
2
O
2(ads)

+ h
+
→ HO
2
•
+ H
+
(14)
O
2(ads)
+ 2e
−
+ 2H
+
→ H
2
O
2(ads)
(15)
O
2
•
−
+ H
+
⇔ HO
2
•
(16)
O

2
•
−
+ h
tr
+
→ O
2
(17)
H
2
O
2
+
•
OH → H
2
O + HO
2
•
(18)
HO
2
•
+
•
OH → H
2
O
2

+ O
2
(19)
•
OH +
•
OH → H
2
O
2
(20)
HO
2
•
+ H
2
O
2
→
•
OH + H
2
O + O
2
(21)
In these reactions e
−
and h
+
refer to conduction band elec-

tron and valence band hole, respectively, generated during
the photoexcitation process (reaction (1)). On the other hand,
h
tr
+
(see reaction (17)) refers to a trapped hole and has redox
potential of 1.5 V [73]. Although several of these reactions
D.D. Dionysiou et al. /Applied Catalysis B: Environmental 50 (2004) 259–269 267
Table 1
Reaction rate constants of radicals in aqueous solutions (obtained from
[81])
k Value (l mol
−1
s
−1
)pH T (K) Original
reference
k
7
1 × 10
6
≤2.0 [82]
k
8/9
9.7 × 10
7
[83]
k
11
1 × 10

−4
–2.3 5.4–9.9 [85–88]
k
18
2.7 × 10
7
(average)
∗
Neutral
to basic
[94]
2.7 × 10
7
7.8 293 [82]
2.0 × 10
7
7.0 [103]
3.8 × 10
7
7.7–11.0 [104]
2.4 × 10
7
7.0 [98]
k
19
1.0 × 10
10
2.0 298 [97]
k
20

4.2 × 10
9
298 [99]
5.2 × 10
9
3.7 [98]
k
21
1 × 10
−2
–5 0.5–3.5 [85]
[89]
[90]
[91]
[92]
are considered to take place at the surface of the catalyst, a
short discussion on some relevant reaction rate constants in
aqueous solutions will be helpful to further rationalize the
existence of an optimum concentration of hydrogen peroxide
and its inhibition effect at higher concentrations. A summary
of the reaction rate constants of certain of these reactions is
provided in Table 1. Redox potentials of many of these re-
actions in homogeneous solutions and at various pH ranges
(acidic, neutral, basic) are provided elsewhere [79,80].
As explained in a previous section, reaction (2) occurs
mainly at wavelengths lower than 300 nm, where H
2
O
2
ab-

sorbs more strongly and it is unlikely to have a significant
effect at the wavelengths employed in this study. Dimer-
ization of perhydroxyl radical (reaction (7)) has k value of
1 × 10
6
l mol
−1
s
−1
at pH ≤ 2.0 [82] while reaction be-
tween HO
2
•
and O
2
•
−
(reactions (8) and (9)) has a k value
of 9.7 × 10
7
l mol
−1
s
−1
[83]. Reaction between H
2
O
2
with
HO

2
•
/O
2
•
−
(reactions (11) and (16)) has been reported to
have a very small reaction rate constant, k = 1.1 l mol
−1
s
−1
(T = 273K) and 3.7 l mol
−1
s
−1
(T = 298K) [84]. Small k
value was reported for reaction (11) (1 × 10
−4
l mol
−1
s
−1
to 2.3 l mol
−1
s
−1
) at high pH (5.4–9.9) [85–88] and for
reaction (21) (1 × 10
−2
l mol

−1
s
−1
to <5lmol
−1
s
−1
)at
low pH (0.5–3.5) [85,89–92]. Similarly, a small k value
(<2 l mol
−1
s
−1
) was reported for the reaction between
HO
2
−
and O
2
•
−
at basic pH (8.9–12.7) [93]. The reaction
rate constant between hydroxyl radical and hydroperoxide
ion is 7.5 × 10
9
l mol
−1
s
−1
[82]. However, at acidic pH,

the speciation of HO
2
−
/H
2
O
2
favors the formation of H
2
O
2
(pK = 11.95) which leads to reaction (18) with somewhat
lower k value. Reaction of hydroxyl radical with hydrogen
peroxide (reaction (18)), which leads to the formation of
superoxide radical anion and water (see reactions (16) and
(18)), has a reaction rate constant of 2.7 × 10
7
l mol
−1
s
−1
[94]. The pK
a
value of reaction (16) is at 4.88, so it is
expected that HO
2
•
is the predominant species between
O
2

•
−
/HO
2
•
at the pH of the experiments in our study (3.0).
Reaction between hydroxyl radical with hydroxyl ions has
a high k value (1.2–1.3 × 10
10
l mol
−1
s
−1
) [95,96] butitis
not expected to be significant at such acidic pH. Reaction
(19) has a k value of 1 × 10
10
l mol
−1
s
−1
at pH = 2.0 [97]
while reaction (20) has a k value of 5.2 × 10
9
l mol
−1
s
−1
at
pH = 3.7 [98] and it is expected to be more significant as

the concentration of hydroxyl radical increases. The value
of k
20
was reported to be in the range from 3.6 × 10
9
to
6.2 × 10
9
l mol
−1
s
−1
at neutral pH [94,99–102].
In the presence of H
2
O
2
, reactions (10)–(14), (18), and
(21) take place. Some of these reactions yield species that
can be beneficial to the process. In particular, reactions (10),
(11) and (21) result in the formation of
•
OH. However, as
seen in reactions (12) and (14), H
2
O
2
competes with hy-
droxyl and other electron donors for reaction with the pho-
togenerated hole. Reactions (11) and (21) are unlikely to be

important due to their low k values. When it is added at low
or moderate concentrations (i.e., optimum values), H
2
O
2
as-
sists O
2
for electron removal and thus inhibiting the e
−
–h
+
recombination process. In addition, in reaction (10), H
2
O
2
forms
•
OH and thus further enhances the degradation re-
actions. At high concentrations (i.e., beyond the optimum),
H
2
O
2
competes with various species for adsorption includ-
ing O
2
and organic contaminants. This will result in the
reduction of the concentration of O
2

•
−
as reported by Hi-
rakawa and Nosaka [73]. In addition, the reaction of H
2
O
2
with
•
OH can be important due to its relatively high k. In
turn, reaction (19), which consumes both HO
2
•
and
•
OH,
may also become important due to its high k value. Reaction
(20) has a high k value but its overall rate will be a func-
tion of the concentration of
•
OH, which is expected to be
much less than that of H
2
O
2
at concentrations beyond the
optimum.
Quantitative analysis of the actual process and determin-
ing the contribution of each reactive species can provide
additional insights but it requires knowledge of the reaction

rate constants and the concentrations of all the species in-
volved at the surface of the catalyst and in solution. Such
analysis is difficult to be performed but it certainly war-
rants further investigation in the future. Nevertheless, the
results obtained in this study provide additional support to
the beneficial effect of hydrogen peroxide when added at
optimum concentrations on the rates of photocatalytic re-
actions. By performing the reactions in a continuous-mode
photocatalytic reactor at steady state, it is possible to in-
vestigate the effect of a particular parameter under more
controlled conditions. Based on the specific features of the
RDPR and the results obtained in this study, it is suggested
that higher degradation rates are achieved at relatively
low concentrations of hydrogen peroxide in the reaction
solution, which also helps to minimize the costs of the
oxidant.
268 D.D. Dionysiou et al. /Applied Catalysis B: Environmental 50 (2004) 259–269
4. Conclusions
Photocatalytic studies using the RDPR in a continuous-
mode operation at steady state demonstrated that hydrogen
peroxide, when added at small to moderate concentra-
tions, has a beneficial effect on the degradation rate of
4-chlorobenzoic acid, which was used as a model aro-
matic contaminant at feed concentration of 300 ␮mol l
−1
.
The results revealed the existence of an optimum con-
centration of hydrogen peroxide at oxidant to contami-
nant molar ratio of 5.33 (based on feed concentrations).
This was observed for the degradation of the parent con-

taminant, the total organic carbon and its mineraliza-
tion to CO
2
. At higher than the optimum concentrations
(up to 10.74 mM), hydrogen peroxide decreased the re-
action rates but not below those of the control experi-
ments (absence of hydrogen peroxide). This synergism
was attributed to the beneficial effect of hydrogen perox-
ide as an electron acceptor, which is better than molec-
ular oxygen as well as to the generation of additional
hydroxyl radicals by the corresponding reaction. Conse-
quently, addition of hydrogen peroxide will result in a
dual positive effect: (a) increased concentration of avail-
able holes for oxidation (enhanced generation of hydroxyl
radicals); and (b) direct formation of additional hydroxyl
radicals due to H
2
O
2
reduction by the conduction band
electron. In turn, higher rates of hydroxyl radical forma-
tion will result in higher degradation rates of the organic
contaminants.
On the other hand, addition of large amounts of hydrogen
peroxide in the feed solution (continuous-mode operation)
or at the beginning of the photocatalytic process (i.e., batch
reactor) will diminish the effectiveness of the process due to
favorable inhibiting reactions that scavenge hydroxyl radi-
cals as well as due to competitive adsorption between hydro-
gen peroxide, oxygen and organic contaminants. It was also

demonstrated that addition of hydrogen peroxide in the reac-
tion solution that is oxygenated by air as a source of oxygen
results in rates similar to those obtained in solution that is
oxygenated with a gas stream containing much higher con-
centrations of oxygen (82%). This, along with the fact that
hydrogen peroxide is a better electron acceptor than oxygen,
suggests that hydrogen peroxide may be beneficial in cases
where there is oxygen starvation during the reaction. It is be-
lieved that in a certain photocatalytic process, the optimum
hydrogen peroxide loading will be a function of the charac-
teristics of the feed solution (type and concentration of or-
ganics, pH, presence of inorganic ions), the concentration of
oxygen in the reaction solution, the magnitude of the inten-
sity and wavelength of the UV light, the desirable extent of
treatment, and the cost of hydrogen peroxide. Nevertheless,
the fact that this optimum occurs at small concentrations
of hydrogen peroxide in the reaction solution is promising
for the development of more efficient and cost-effective
green technologies for the remediation of polluted
water.
Acknowledgements
The authors are grateful to the Center of International Re-
search for Water and the Environment (Centre International
de Recherche Sur l’Eau et l’Environnement) of ONDEO
Services for providing financial support to this study.
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