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

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

Effect of hydrogen peroxide on the destruction of organic

contaminants-synergism and inhibition in a continuous-mode

photocatalytic reactor Dionysios D Dionysioua,∗, Makram T Suidana, Isabelle Baudinb, Jean-Michel Laˆınéb

aDepartment 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

bCentre 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 TiO2-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 mmol l−1) had a synergistic effect and increased considerably the rates of photocatalytic reactions An optimum influent hydrogen peroxide concentration was observed at 1.6 mmol l−1, which caused an increased in the rates of 4-CBA degradation 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: TiO2; Photocatalysis; Photocatalytic; Hydrogen peroxide; H2O2; 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), TiO2photocatalysis

can be viewed as a “green” technology It is becoming more

popular as a detoxification technology because of several

reasons First, TiO2photocatalytic 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, TiO2photocatalysis 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, TiO2catalyst is non-toxic, insoluble in water,

photo-∗Corresponding author Tel.:+1-513-556-0724;

fax: +1-513-556-2599.

E-mail address: dionysios.d.dionysiou@uc.edu (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 TiO2catalyst[14–16]give additional positive perspec-tives for TiO2 photocatalysis Fifth, rapid progress on the preparation of TiO2 nanostructured materials [17–20] and new insights on the fundamental aspects of TiO2 photo-catalysis[11,20–23]have resulted in significant progress on the development of more efficient photocatalytic systems Nevertheless, TiO2 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

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] It is

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 TiO2 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 TiO2catalyst 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 −0.13 V while that of H2O2 reduction is 0.72 V

[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 (213 kJ mol−1) of the H

2O2molecule, 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 < 300 nm) For example, the molar extinction

coefficient of H2O2 is 6.6 × 10−4, 1× 10−2, 1.0, 19.6, and 140 l mol−1cm−1at 400, 360, 300, 253.7, and 200 nm, respectively [59] Comparing radiation wavelengths at 253.7 nm (germicidal UV radiation) and 360 nm (UV-A range), the ratio of H2O2 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 TiO2 (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 TiO2 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 mineralizadegrada-tion 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

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 H2O2 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 > 320 nm)/TiO2 system[35] The authors

hypoth-esized that this effect could be due to the competition of

H2O2 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/TiO2/H2O2) were higher than those of the

homo-geneous system (UV/H2O2)[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−1and hydrogen peroxide molar

concen-trations varied in the range 0–9 mM 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 destrucconcentra-tion

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 concentrafunc-tion 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

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∼6 mm 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 SiO2/Al2O3) and an outer

thin layer (10–30␮m) of TiO2nanoparticles 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 15 W 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 300–400 nm 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 53 ml min−1, residence time (τ) of 66 min, 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 6 rpm, solution ionic strength as KNO3 of

10 mM, solution volume in the reaction vessel (VR) of 3.5 l,

and influent concentration of 4-CBA (Cin) of 300␮mol l−1.

The incident average light intensity (I) was approximately

896␮W cm−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) HNO3

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 soluconcentra-tion was measured using

an ion chromatograph (IC, DX-120 DIONEX) equipped with a A514 4 mm column and a ASRS®-ultra 4 mm 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 3.5 mM

Na2CO3and 1.0 mM NaHCO3 The concentration of H2O2

in the reaction solution was measured immediately after the sample was withdrawn from the reactor The concentration

of H2O2was measured using the LaMotte Octet Compara-tor test kit HP-40 as well as using an HPLC equipped with

an LC-18, 15 cm × 4.6 mm, 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 206 nm Comparison of the two methods (LaMotte HP-40 and HPLC) for hydrogen peroxide anal-ysis in the overlapping range (1–40 mg l−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 CO2, O2 and N2 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 CO2have been described in detail elsewhere[67]

2.4 Chemicals

The following chemicals were used as supplied: acetoni-trile (CH3CN, FW = 41.05, HPLC grade, Fisher

Scien-tific, Pittsburgh, PA), 4-chlorobenzoic acid (ClC H CO H,

FW = 156.57, 99%, Aldrich, Milwaukee, WI), nitric acid

(HNO3, 68–71% (w/w), trace metal grade, Fisher),

potas-sium nitrate (KNO3, FW = 101.11, 99.3%, Fisher) and

hydrogen peroxide solution (H2O2, 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 1shows 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 OCMRinof 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

effof 1.0 (based on the effluent molar concentrations of both H2O2and 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 (∼3 mg l−1) of hydrogen peroxide in

the feed solution increased the rates by 30%.Fig 3reports

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

C eff

0.3 0.4 0.5 0.6 0.7 0.8

0.9

[H 2 O 2 ] influent , mg/L

C eff

0.3 0.4 0.5 0.6 0.7 0.8 0.9

Ceff/Cin TOCeff/TOCin

(a)

(b)

Fig 1 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 ␮W cm−2, Q = 53 ml min−1, VR = 3.5 l, τ = 66 min,

Cinfluent ≈ 300 ␮mol l −1, [KNO3]= 10 mM, and air as the gas phase.

Plots of normalized 4-CBA concentration (Ceff/Cin) and total organic

car-bon (TOCeff /TOCin) at steady state as a function of (a) influent hydrogen peroxide concentration, [H2 O2]influent and (b) effluent hydrogen peroxide concentration, [H2O2]effluent.

[H 2 O 2 ] influent , mmol/L

0 2 4 6 8 10 12

[H 2 O 2 ] effluent , mmol/L

- Concentration,

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 ␮W cm−2, Q = 53 ml min−1, VR = 3.5 l, τ = 66 min,

Cinfluent ≈ 300 ␮mol l −1, [KNO3]= 10 mM, and air as the gas phase Effluent Cl −concentration as steady state as a function of influent and

effluent hydrogen peroxide concentrations.

[H 2 O 2 ] influent , mg/L

O 2

] effl

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 = 6 rpm, pH = 3.0,

I = 896 ␮W cm−2, Q = 53 ml min−1, VR = 3.5 l, τ = 66 min,

Cinfluent ≈ 300 ␮mol l −1, [KNO3]= 10 mM, and air as the gas phase.

Plot concentration of hydrogen peroxide in the effluent at steady state,

[H2 O2]effluent, as a function of concentration of hydrogen peroxide in the

influent at steady state, [H2O2]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

CO2 The results for mineralization to CO2are 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

inFig 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 CO2(%) 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., 40–60 mg l−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

10 20 30 40 50 60 70 80

TOC Degradation (%) 4-CBA Conversion (%)

[H 2 O 2 ] effluent , mg/L

10 20 30 40 50 60 70 80

TOC Degradation (%) 4-CBA Conversion (%)

(a)

(b)

Fig 4 Effect of hydrogen peroxide on the percentage conversion of 4-CBA, TOC and CO2 formation in the RDPR (continuous-mode oper-ation) at ω = 6 rpm, pH = 3.0, I = 896 ␮W cm−2, Q = 53 ml min−1,

VR= 3.5 l, τ = 66 min, Cinfluent≈ 300 ␮mol l −2, [KNO3]= 10 mM, and air as the gas phase Plots of percentage conversion of 4-CBA, TOC and (%) CO2 formation based on influent 4-CBA concentration as a function

of (a) influent hydrogen peroxide concentration, [H2O2]influent and (b) effluent hydrogen peroxide concentration, [H2O2]effluent.

0 20 40 60 160 180 200

82 21

4-CBA Transformation, %

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 = 6 rpm, pH = 3.0, I = 896 ␮W cm−2,

Q = 53 ml min−1, VR = 3.5 l, τ = 66 min, Cinfluent ≈ 300 ␮mol l −1,

[KNO3] = 10 mM, and [H2O2]influent ≈ 48 mg l −1 Concentration of

oxy-gen in the gas phase: 21 and 82%.

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 H2O2

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

to a [H2O2]/[PCP]omolar ratio of approximately 106 The

authors also compared the extent of mineralization of PCP

in two cases: one with 10 mM [H2O2] added once at the

beginning of the experiment and one at which [H2O2] 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 [H2O2]/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 (18–26 mM) 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 O2•− and•OH using UV-A radiation (387 nm) at pH 11.5 [73] They measured the concentration of O2•− using luminol chemiluminescence and that of•OH using terephthalic acid fluorescence tech-niques They found that the concentration of both O2•−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 O2•− reached a maximum at an optimum concentration

of hydrogen peroxide of 0.2 mM 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 O2•− 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 H2O2 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 O2•− at higher concentrations of H

2O2 was attributed

to the competition for adsorption between H2O2 and O2

[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 H2O2, gaseous

or dissolved in water, at the surface of rutile or anatase TiO2 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 O2 [75,76] Boonstra and Mutsaers proposed that the yellow color was due to the formation of Tis–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 1H2O and 1/2O2), suggesting internal hy-droxylation of the TiO2 surface layers or the formation of stable peroxo species[76] The authors explained their data

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 TiO2 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 TiO2, whereas it had a dramatic

increase in the reaction rates in systems utilizing rutile

TiO2 Moreover, in the latter case the reaction could

pro-ceed even in the presence of visible light Diffuse reflection

spectra of TiO2 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 365 nm

was the radiation used to photoexcite the catalyst In our

study, it was difficult to perform the experiment only with

UV/H2O2 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/H2O2 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 TiO2 suspensions using UVA-340 light tubes

(λ > 295 nm) [33] They compared three different

sys-tems: UV/TiO2, UV/oxidant, and UV/TiO2/oxidant Their

study showed that addition of 10 mM H2O2significantly

in-creased the reaction rate compared to the UV/TiO2system

However, the degradation rate was somewhat slower to that

of UV/H2O2system 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 > 400 nm)-induced

photodegradation of a blue azo dye and using composite TiO2/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 H2O2, •OH and O2•− The following pertinent reactions have appeared in the literature dealing with TiO2 photocatalysis and the effect of hydrogen peroxide ([46,47,73,74,76,77]and references therein):

OH−

H2O2(ads)+ e−→•OH+ OH− (10) H2O2+ O2 •−→•OH+ OH−+ O2 (11)

H2O2(ads)+ 2h+→ O2+ 2H+ (12)

H2O2(ads)+ h+→ HO2 •+ H+ (14)

O2(ads)+ 2e−+ 2H+→ H2O2(ads) (15)

HO2•+ H2O2→•OH+ H2O+ O2 (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,

htr+(see reaction (17)) refers to a trapped hole and has redox potential of 1.5 V[73] Although several of these reactions

Table 1

Reaction rate constants of radicals in aqueous solutions (obtained from

[81])

k Value (l mol −1s−1) pH T (K) Original

reference

k18 2.7 × 10 7 (average) ∗ Neutral

to basic

[94]

[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 H2O2

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 × 106l mol−1s−1 at pH ≤ 2.0 [82] while reaction

be-tween HO2•and O

2 •−(reactions (8) and (9)) has a k value

of 9.7 × 107l mol−1s−1[83] Reaction between H

2O2with

HO2•/O2 •− (reactions (11) and (16)) has been reported to

have a very small reaction rate constant,k = 1.1 l mol−1s−1

(T = 273 K) and 3.7 l mol−1s−1(T = 298 K)[84] Small k

value was reported for reaction (11) (1× 10−4l mol−1s−1

to 2.3 l mol−1s−1) at high pH (5.4–9.9) [85–88] and for

reaction (21) (1 × 10−2l mol−1s−1 to <5 l mol−1s−1) at

low pH (0.5–3.5) [85,89–92] Similarly, a small k value

(<2 l mol−1s−1) was reported for the reaction between

HO2−and O

2 •− at basic pH (8.9–12.7)[93] The reaction

rate constant between hydroxyl radical and hydroperoxide

ion is 7.5 × 109l mol−1s−1 [82] However, at acidic pH,

the speciation of HO2−/H

2O2favors the formation of H2O2 (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 × 107l mol−1s−1

[94] The pKa value of reaction (16) is at 4.88, so it is expected that HO2• is the predominant species between

O2•−/HO2 •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 × 1010l mol−1s−1)[95,96]but it is not expected to be significant at such acidic pH Reaction

(19) has a k value of 1× 1010l mol−1s−1at pH= 2.0[97]

while reaction (20) has a k value of 5 2 × 109l mol−1s−1at

pH = 3.7[98]and it is expected to be more significant as the concentration of hydroxyl radical increases The value

of k20 was reported to be in the range from 3.6 × 109 to

6.2 × 109l mol−1s−1at neutral pH[94,99–102].

In the presence of H2O2, 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), H2O2 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), H2O2 as-sists O2for electron removal and thus inhibiting the e−–h+ recombination process In addition, in reaction (10), H2O2 forms •OH and thus further enhances the degradation re-actions At high concentrations (i.e., beyond the optimum), H2O2competes with various species for adsorption includ-ing O2 and organic contaminants This will result in the reduction of the concentration of O2•− as reported by Hi-rakawa and Nosaka[73] In addition, the reaction of H2O2 with•OH can be important due to its relatively high k In turn, reaction (19), which consumes both HO2• 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 concentrafunc-tion of •OH, which is expected to be much less than that of H2O2 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

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 CO2 At higher than the optimum concentramineraliza-tions

(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 H2O2 reduction by the conduction band

electron In turn, higher rates of hydroxyl radical

forma-tion will result in higher degradaforma-tion 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, oxyhydro-gen and organic contaminants It was also

demonstrated that addition of hydrogen peroxide in the

reac-tion solureac-tion 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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