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Dyes and Pigments 101 (2014) 280e285

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Dyes and Pigments

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

Does hydrogen peroxide really accelerate TiO2 UV-C photocatalyzeddecolouration of azo-dyes such as Reactive Orange 16?

Terry A. Egerton*, Herry Purnama 1

School of Chemical Engineering & Advanced Materials, Newcastle University, NE1 7RU, UK

a r t i c l e i n f o

Article history:Received 5 July 2013Received in revised form10 October 2013Accepted 11 October 2013Available online 19 October 2013

Keywords:Dye decolourationHydrogen peroxideTitanium dioxideUV-CPhotocatalysisHydroxyl radicals

* Corresponding author. Tel.: þ44 (0) 191 222 5618E-mail addresses: Terry.Egerton@ncl.ac.uk, tpj.eger

1 Present address: Muhammadiyah University of Su

0143-7208/$ e see front matter � 2013 Published byhttp://dx.doi.org/10.1016/j.dyepig.2013.10.019

a b s t r a c t

A comparison of UV-C initiated decolourations of the anionic azo-dye Reactive Orange 16 by H2O2, byanatase TiO2 (PC500), and by H2O2/TiO2 is reported. The decolouration rates induced by H2O2, by TiO2

and by H2O2/TiO2, were compared in order to determine whether H2O2/TiO2 was more beneficial thaneither H2O2 or TiO2 alone. UV-C photolyses H2O2 to form hydroxyl radicals. UV-C can also initiate hy-droxyl radical formation at the surface of TiO2, and in this study the photocatalytic conditions werechosen to enhance hydroxyl radical formation. In both cases the hydroxyl radicals lead to decolourationof the azo-dye Reactive Orange 16. Decolouration by irradiation of H2O2/TiO2 was w5 times faster thanby the TiO2 photocatalyzed reaction but slower than that caused by irradiation of H2O2 alone. Thus, therelative order of decolouration rates is: UV-C/TiO2 < UV-C/TiO2/H2O2 < UV-C/H2O2. Estimates of theabsorption of UV by both H2O2 and H2O2/Reactive Orange 16 solutions are used to show that this order isa consequence of the photonic efficiency of H2O2 photolysis being ten to a hundred times larger than thephotonic efficiency of hydroxyl radical generation at the surface of TiO2.

� 2013 Published by Elsevier Ltd.

1. Introduction

The potential of Advanced Oxidation Processes, particularlyphotocatalysis by TiO2, for the removal of waste dyes from waterhas been widely studied [1e9]. However, currently, low quantumyields limit the practical application of photocatalytic processes[3]. Within a programme of work, by the group of the corre-sponding author, to improve the photonic efficiency of theseprocesses, the decolouration of a representative anionic azo-dyeReactive Orange 16 (RO16) (C.I. 17757), and of other dyes by TiO2

photocatalysis and TiO2 photoelectrocatalysis have beencompared [10e13]. The effects of platinum on the photocatalyticdecolouration of Reactive Orange 16 and of the triphenyl methanedye Malachite Green by rutile, by a commercial anatase, PC 500and by anatase/rutile (Evonik P25, whose activity for decoloura-tion of Reactive Orange 16 and other dyes is comparable to that ofPC 500 [4]) have also been investigated [14]. The present study ofthe decolouration of RO16 by PC 500 in the presence of H2O2 wasinitiated because addition of H2O2 has been proposed as a furtherway of supplementing the low photonic efficiency of conventionalUV photocatalysis.

, þ44 (0) 1642 645732.ton@virgin.net (T.A. Egerton).rakarta, Indonesia.

Elsevier Ltd.

Photocatalysis can be by direct charge transfer of UV-generated charge carriers to or from the adsorbed reactant [3].Alternatively, the UV-generated hole may react with an OH� toform a hydroxyl radical [2,3,15]. Since hydroxyl radicals can alsobe formed by the photolysis of hydrogen peroxide, [16], thepossibility of using H2O2 to supplement photocatalytic hydroxylradical formation is explored in this paper. Conditions thatminimize dye adsorption on the TiO2 and so limit the possibilityof direct charge transfer from the TiO2 to the dye [17] have beenchosen. Thus, hydroxyl radicals are expected to be the activeintermediates in both the hydrogen peroxide photolysis and theTiO2 photocatalysis. The importance of H2O2 as a source of hy-droxyl radicals arises because, in contrast to the very low pho-tonic efficiencies of heterogeneous generation of hydroxylradicals at the surface of TiO2 [3,18] the quantum efficiency forthe homogeneous photolysis of H2O2

H2O2/2OH� (1)

can approach unity [16,19]; H2O2 photolysis degrades many in-dustrial pollutants including azo dyes [19]. However, because UVabsorption by H2O2 is negligible above 280 nm, the process re-quires UV-C irradiation. The absorption of H2O2 is greatest near220 nm, and for 254 nm irradiation, which is conveniently pro-vided by low pressure mercury lamps, the molar absorption

T.A. Egerton, H. Purnama / Dyes and Pigments 101 (2014) 280e285 281

coefficient is 19.6 M�1 cm�1. Commonly, dye-decolouration ratesincrease with increasing H2O2 concentration, but must reach alimit when all the UV is absorbed. 90% of 254 nm light isabsorbed by 2 cm of 25 mM H2O2 solution. In practice a ratherbroad rate-maximum at ca. 25 mM [4,20,21] is followed by a fallin the rate which is frequently attributed [4,20e23] to thescavenging of hydroxyl radicals by H2O2

H2O2 þ OH�/H2Oþ O2� þHþ (2)

By contrast the rates of azo-dye decolouration by UV-A/H2O2are much smaller than those for UV-A/TiO2, because anatase TiO2absorbs up to w380 nm, whereas H2O2 absorption is negligibleabove 300 nm. Thus Muruganandham, Selvam, and Swaminathanreport that UV-A/H2O2 decolouration is w10% (reactive yellow)or w15% (reactive orange) of the UV-A/TiO2 [24] and reportsimilar results for solar decolouration of reactive yellow [25]. Anysignificant enhancement of the rate of UV-A or solar initiatedTiO2-photocatalysed decolouration caused by H2O2 additioncannot be due to the mechanism described by equation (1) butmust be due to additional processes, such as dye sensitization.Such processes may contribute to high rates of solar decoloura-tion by H2O2 at low pH [25]. By contrast, addition of H2O2 to TiO2is expected to increase decolouration rates induced by 254 nmradiation. It has been shown that H2O2 addition to P25 acceler-ates the UV-C decolouration of the azo-dyes acid red 14 [26], acidorange 7 [27], reactive red 198 [28] and Safira HEXL [29].Accelerated decolouration of Tropaeoline 000 on addition ofH2O2 to anatase has also been reported [30].

2. Experimental

2.1. Materials

Reactive Orange 16 (RO16), an anionic mono-azo dye of mo-lecular mass 617.5, and formula C20H17N3Na2O11S3, (C.I.17757) withthe structure shown in Fig. 1 was supplied by Sigma Aldrich andwas used without further treatment. The TiO2 was Millenium (nowCrystal Global) PC500 with a specific area ofw320m2 g�1. No rutilediffraction peaks were detected in its powder diffractogram. Its100% anatase composition avoids complications associated withthe mixed anatase/rutile composition of the more commonly usedEvonik (formerly Degussa) P25 grade. However, preliminary studieson the effect of catalyst loading on photocatalytic activity were alsocarried out with Evonik P25 in addition to Millenium PC 500. TiO2catalysts were used as received without the sand-milling used inother studies [14].

2.2. Adsorption

In a preliminary survey of dye adsorption onto the TiO2,suspensions of PC500 in dye solution were equilibrated at differentpH’s in the absence of UV. The TiO2 was then removed by

Fig. 1. The molecular structure of the anionic dye Reactive Orange 16 (RO16).

centrifugation and the concentration of residual dye monitoredspectrophotometrically (Shimadzu UV mini 1240).

2.3. Photoactivity measurement

Preliminary, UV-A photocatalytic measurements of RO 16decolouration by both Millenium PC 500 and Evonik P25 grades ofTiO2 showed (Table 1) that the highest reaction rates were obtainedat catalyst loadings in the region of 1.0 and 2.0 g dm�3 TiO2.Therefore, catalyst loadings of 2 g dm�3 were used in this study.

The preliminary UV-A experiments were carried out in a cylin-drical pyrex-glass reactor [14]. However, because pyrex-glass ab-sorbs below 360 nm, the simple photoreactor used for this UV-Cperoxide study was an open pyrex-glass dish filled with 600 cm3

of 0.05 mM dye solution and irradiated from above, as shown inFig. 2, by two 8W UVC germicidal tubes (L ¼ 302 mm, D ¼ 16 mm).The reactant mixture was stirred by a magnetic chuck and spargedby a slow stream of oxygen via a sintered glass diffuser.

The stirred suspension of TiO2 in 0.05 mM dye solution at thenatural pH of the suspension, w6.5, was equilibrated for30 min without UV irradiation to allow adsorption equilibrium tobe achieved. The absorbance at this time was taken as the initialabsorbance, A0, in rate constant determinations. Samples of thesuspension were taken at fixed time, t, and centrifuged to removeTiO2 prior to spectrophotometric measurement of their absorbance,At. Studies of RO16 photocatalysis [10,11] showed that the 493 nmabsorption decreased at a slightly, w20%, faster rate than the other3 absorptions. Muruganandham and Swaminathan, reportedsimilar results for the H2O2/TiO2 degradation of the related azo-dyeReactive Orange 4; they used GC-MS to show that the decolourationrate was faster than the total degradation, and attributed this to theinitial cleavage of the chromophoric azo group [31]. This isconsistent with the arguments of Herrmann, Guillard and collab-orators that, during the TiO2 photocatalyzed degradation ofamaranth and congo red, nitrogen formation, proceeded by asimple two stage reaction in which hydroxyl radicals reacted withthe ReN ¼ NeR0 azo group by successive breakage of the ReN andNeR0 links. By contrast both aliphatics and aromatic rings were firstsubjected to successive attacks by photogenerated OH� which ledfirst to various hydroxylated intermediates followed by ringopening and, then, repeated subsequent “photo-Kolbe” reactions[3,32]. Therefore, the change in the 493 nm absorption was used tofollow the decolouration.

3. Results

3.1. RO16 spectra and RO16 adsorption on PC 500

As shown in Fig. 3(a), the spectrum of 0.05 mM RO16 has fourmain absorptions at 254, 297, 388 and 493 nm with absorbances

Table 1Rate constants, as a function of TiO2 loading, for the reduction of 493 nm absorbanceduring decolouration of 0.05 mM RO16 dye solution by two different TiO2 catalysts(Millenium PC 500 and Evonik P25 grades) irradiated by UV-A.

TiO2 loading g dm�3 Rate constant (k � 103 min�1)For PC 500 grade catalyst.

Rate constant(k � 102 min�1)For P25 grade catalyst.

0.2 3.1 � 0.4 3.3 � 0.70.5 2.8 � 0.4 4.7 � 0.21.0 3.9 � 0.7 5.9 � 0.22.0 7.2 � 0.5 8.4 � 0.52.0 (milled catalyst) e 6.4 � 0.54.0 6.4 � 1.5 e

2 × 8W UV-C tubes

O2 sparge

Magnetic stirrer

10cm

20cm

Fig. 2. Schematic diagram of photoreactor with UVC irradiation.

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Fig. 3. a. Spectra of 0.05 mM solutions of RO16 after equilibration with PC500 TiO2atpH’s increasing from 3 to 10. b. The 493 nm absorbance of 0.05 mM RO16 solution afterequilibration with PC500 TiO2 over the pH range 3e10. c. UV-A photocatalyzeddecolouration of RO16 at pHw6 and pHw4.

T.A. Egerton, H. Purnama / Dyes and Pigments 101 (2014) 280e285282

of 1.36, 1.16, 0.70 and 1.16 respectively. In the absence of UV,equilibration of 0.05 mM RO16 solutions with 2 g dm�3 TiO2caused similar proportional changes in all four absorptions. Aplot of the 493 nm absorbance as a function of pH, Fig. 3(b),shows that the absorption is approximately constant from pH 10to 6 but falls significantly below pH 6. In principle changes couldbe attributed to pH-dependent tautomerism, bond cleavage orprotonation. However, although such changes would be expectedto induce significant changes in the dye absorption spectrum, thespectra shown in Fig. 3(a) show little evidence of changed formover the pH range studied. Therefore it is considered that thechanges in adsorption are primarily a consequence of changes inthe charge on the PC500 surface. The isoelectric point of TiO2, itspoint of zero charge (p.z.c.), is generally taken to be in the pHrange 6e7 and above the p.z.c. the increasingly negative chargeon the TiO2 surface would minimize adsorption of the anionicdye. This implication of very limited adsorption of RO16 onPC500 at pH � 6 was tested by equilibrating a series of dye so-lutions (0.1; 0.05; 0.025; 0.0125; 0.00625; and 0.003125 mM)with PC 500) and estimating the adsorption from the decrease indye absorption. A Langmuir analysis of the resulting adsorptionisotherm at pH w6 indicated that the adsorption was indeed verysmall with a footprint greater than w2000 nm2 molecule�1. Asthe pH is lowered below pH w6, the corresponding increasedadsorption of anionic RO16 molecules on increasingly positivelycharged TiO2 would be expected to increase decolouration rate.As shown in Fig. 3(c), this was found to be the case; decreasingthe pH from w6 to w4 increased the decolouration rate by afactor of greater than 10, despite the hundredfold decrease in thesolution concentration of hydroxyl ions (precursors to hydroxylradicals). Therefore, with the aim of minimizing dye-adsorptionand consequently limiting any role of direct charge transferfrom the TiO2 during photocatalysis, the subsequent photo-catalytic experiments were carried out at pH 6e7.

3.2. Decolourization of Reactive Orange by hydrogen peroxide

In the absence of TiO2, the results of dye decolourization of0.05 mM RO16 by hydrogen peroxide under UV irradiation aresatisfactorily represented by a linear decrease of ln(At/A0) withtime, as shown in Fig. 4; the gradient of these lines were taken asthe pseudo-first order rate constants, k1. In the absence of H2O2,direct photochemical degradation of RO16 by UV-C was negligible,k1 � 0.03 � 10�2 min�1. For 0.02 mM H2O2, k1 was0.36 � 10�2 min�1, for 2 mM H2O2 it was 3.4 � 10�2 and for 20 mMH2O2 it was 10.1�10�2 min�1. Thus, under UV-C, the decolourationrates increased by a factor of 9.4 for a tenfold increase in H2O2 from

0.2 to 2 mM but increased by a further factor of only 3 when theH2O2 concentration was increased further from 2 to 20 mM.

The decolouration induced by hydrogen peroxide was verymuch faster under UV-C (k1 ¼ 10.1 � 10�2 min�1 for 20 mM H2O2)than under UV-A (k1 � 0.1 � 10�2 min�1) i.e. the results areconsistent with H2O2’s negligible absorption at l > 280 nm. Theeffect of added electrolyte was briefly investigated. Decolourationrates by 20 mM H2O2 in water, in 0.1 M NaCl and 0.1 Na2SO4 weresimilar, all within 20% of the average value.

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ln

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Fig. 4. The influence of H2O2 concentration (C 0 mM; D 0.2 mM;, 2 mM; > 20 mM)on UV-C decolouration of RO16 in the absence of TiO2.

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Fig. 5. UV-C decolouration of 0.05 mM RO16 in the presence of TiO2. (C UV-C only::2 g dm�3 TiO2: A2 g dm�3 TiO2, 20 mM H2O2: > No TiO2, 20 mM H2O2).

T.A. Egerton, H. Purnama / Dyes and Pigments 101 (2014) 280e285 283

3.3. The effect of H2O2 in combination with TiO2

The corresponding decolouration results measured in thepresence of 2 g dm�3 TiO2 are shown in Fig. 5. Although UV-C byitself does not photolyze R016, it does induce photocatalyticdecolouration of the dye by TiO2. In the absence of addedhydrogen peroxide the decolouration follows the pseudo-firstorder kinetics, k1 ¼ 0.5 � 0.1 � 10�2 min�1, that are typical ofthe initial stages of photocatalytic decolouration of azo dyes[3,27,28]. Addition of 20 mM H2O2 increased the decolourationrate to 5.6 � 10�2 min�1. Thus, as in many other studies [26e30],hydrogen peroxide addition increases the rates of photocatalyticdecolouration of dyes by TiO2. However, the increased rate ofdecolouration caused by the addition of 20 mM H2O2 to thesuspension of TiO2 in RO16 is significantly less than the figure of10.1 �10�2 min�1, reported in Section 3.1, for the effect of 20 mMH2O2 by itself.

The experiments were then repeated in 0.1 M NaCl and 0.1 MNa2SO4 solutions with the results shown in Table 2. For both TiO2and 20 mM H2O2/TiO2 the decolouration rates followed thesequence kNaCl � kwater > kSulphate. In the absence of H2O2 all therates were too small to merit further comment. For 20 mM H2O2/TiO2 the rates were kwater ¼ 5.6 � 10�2, kNaCl ¼ 7.5 � 10�2 andkSulphate ¼ 4.7 � 10�2 min�1.

Table 2The effect of 0.1 M NaCl and 0.1 M Na2SO4 electrolyte on the decolouration rateconstants (102 � k1/min�1). The concentration of H2O2 was 20 mM. The TiO2 loadingwas 2 g dm�3.

Reaction conditions k1 for H2O k1 for 0.1M NaCl

k1 for 0.1M Na2SO4

UV-C only w0 w0 w0UV-C þ TiO2 0.5 0.5 0.3UV-C þ TiO2 þ H2O2 5.6 7.5 4.7UV-C þ H2O2 10.1 13.6 13.2Ratio of

UV� Cþ TiO2 þ H2O2=UV� Cþ H2O2

0.55 0.57 0.36

4. Discussion

The UV-C decolouration of the azo dye RO16 by eitherhydrogen peroxide or TiO2 follows the general pattern reportedby papers cited in the introduction, e.g. Refs. [4,8,21] and con-firms the suitability of RO16 as a representative azo dye for acritical analysis of the synergetic effect of H2O2 on photocatalyticdecolouration of dyes by TiO2. Significantly, the results in Fig. 5and Table 2 show that the rate of decolouration by H2O2/TiO2is less than the rate by H2O2 alone, so there is no evidence of anypositive synergy associated with TiO2/H2O2 in combination. Tothe best of our knowledge only Gupta et al. have reported asimilar result, but they appear not to have realized its signifi-cance and made no comment on it [30]. To explore the possi-bility that the slower decolouration by H2O2/TiO2 is due tocompetition between TiO2 (with a low photonic efficiency forhydroxyl radical production) and H2O2 (with a high photonicefficiency for photolysis) for the UV-C, a fuller analysis of theoptics was made.

4.1. Decolouration of RO16 by hydrogen peroxide

The fraction of radiation absorbed by the composite solution ofhydrogen peroxide and dye is given by

IA=I0 ¼ 1� It=I0 ¼ 1� exp� ðð19:6cþ εdÞLÞ

Where It is the intensity of the transmitted beam, c is the concen-tration of H2O2 (extinction coefficient 19.6 mol�1 dm3 cm�1) andd is the concentration of the dye solution (extinction coefficient ε)and the path length is L. Plots of this function, shown in Fig. 6,demonstrate that 89.5% of incident 254 nm radiation is absorbed in2.5 cm of 20 mM H2O2 and the fraction of 254 nm radiationabsorbed increases from 0.022 to 0.202 to 0.895 for a 2.5 cm pathlength (from 0.044 to 0.363 to 0.989 for 5 cm path length) forsuccessive tenfold increases in hydrogen peroxide concentrationfrom an initial 0.2 mM.

However, at 254 nm the 0.05 mM RO16 absorbs much morestrongly than the H2O2 (When d ¼ is 0.05 mM, εd ¼ 1.34.).Consequently more than 95% of the incident radiation is absorbedwithin the first w1 cm of the H2O2/dye solution. The extra ab-sorption due to H2O2 is small and, as shown in Fig 7 (obtained bysubtracting the absorption for dye only from the absorption fordye þ H2O2) peaks at distances of w0.3 cm. At this distance the20 mM peroxide solution absorbs w9 times as much as the 2 mMsolution, whereas the decolouration rate increased by a factor ofonly 3. Thus, this fuller analysis, taking RO16 absorption into ac-count, indicates that the Beers Law dependence of absorption onconcentration does not, by itself, explain the dependence ofdecolouration rate on H2O2 absorbance. Therefore, the calculationsindirectly support the view that increasing H2O2 concentrations,

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Fig. 6. The fraction of 254 nm radiation absorbed by 0.2 mM , 2 mM , & 20 mM D,H2O2 solutions and by 0.05 mM RO16 solutions containing 0.2 mM , 2 mM , and20 mM : H2O2.

T.A. Egerton, H. Purnama / Dyes and Pigments 101 (2014) 280e285284

from 2 mM to 20 mM is less effective than increasing it from0.2 mM to 2 mM because hydroxyl radicals are scavenged by re-actions of the kind described by equation (2) [23]. These reactionshave been postulated to account for the observed maxima indecolouration rates of the azo dyes Remazol Black B [20] methylorange [21] and other dyes [21] including various simulated reac-tive dye-bath effluents [4,22]. Similar effects have also beenobserved in the degradation of the herbicide butachlor [33]and the effect of H2O2 on the photocatalytic degradation of chlor-fenapyr [34].

4.2. Decolouration of RO16 photocatalyzed by TiO2 and the effect ofH2O2 on this decolouration

The results reported in Section 3.1 confirmed that at the pH of6e7 used for the photocatalytic decolouration experiments, thepossibility of direct charge transfer between the TiO2 and thedye [17] was very small because the adsorption of dye onto theTiO2 was small. Instead, the dominant mechanism of RO16

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so

rb

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xid

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Fig. 7. The 254 nm absorption of 2 mM , and 20 mM - H2O2components of a mixedreactive orange/hydrogen peroxide solution.

photocatalytic decolouration of RO16 is considered to be via re-action with photogenerated hydroxyl radicals. Heterogeneouslyphotocatalyzed decolouration of RO16 can be initiated by both UV-A [10,11,13] and UV-C. The UV-C rate constant, 0.5 � 10�2 min�1,measured is much less than the 9.7 � 10�2 measured previouslyunder UV-A radiation [13]. Without careful actinometry it is notpossible to say whether this is because of intrinsic differences orbecause of experimental differences (reactor geometries, in-tensities, or strong absorption of 254 nm radiation by RO16(Figs. 3a and 7)). As this study aimed to investigate the effect ofH2O2 on photocatalytic decolouration and since UV-C is necessaryfor significant decolouration of RO16 by H2O2, only the UV-C re-sults are considered here. UV-A TiO2 photocatalyzed decolourationof RO16 has been reported elsewhere [14].

As shown in Table 2, addition of 20 mMH2O2 to the suspensionsof the anatase PC 500 in RO16 increased the rate of UV-C decol-ouration by a factor of w10. The 10 fold acceleration is within therange reported by others. Thus, H2O2 addition to anatase has beenreported to accelerate UV-C photocatalyzed decolouration of Tro-paeoline 000 (�2.6) [30] and of a xanthene dye (�w2.3) [35]. Forthe predominantly anatase P25, the addition of H2O2 has been re-ported to increase the decolouration rate of Safira HEXL anionic azodye (�4.5 at pH 7;�9 at pH 5) [29], of Acid Red 14 (�20) [26], ofAcid Orange 7 (�7) [27],and Reactive Red 198 (�1.7) [28]. Theseincreases have led to the impression that H2O2 addition acceleratesthe photocatalytic decolouration of dyes by TiO2.

However, for RO16 decolouration, the decolouration rate ofTiO2/H2O2 (5.6 � 10�2 min�1) is less than the rate induced by20mMH2O2 (10.1�10�2 min�1) in the absence of TiO2. Further, theresults in Table 2 show that the pattern remains the samewhen thedecolouration is in NaCl or Na2SO4 solution. With 2 mM as with20 mMH2O2 inwater adding TiO2 reduces photoactivity, by a factorof w3. Thus the order of activity is: e UV-C/H2O2 > UV-C/TiO2/H2O2 > UV-C/TiO2.

As demonstrated by the analysis in Section 4.1, and summarizedin Figs. 6 and 7, the optical path-length in the H2O2/RO16 is suffi-cient to absorb the entire incident UV-C. Photons absorbed by theH2O2 produce hydroxyl radicals with a photonic efficiency close to100% [16,19]. This efficient generation of hydroxyl radicals causesthe effective decolouration of RO16 when H2O2/RO16 solutions areirradiated. TiO2 also absorbs UV-C photons, and this absorption byTiO2 would necessarily shorten further the depth of suspension inwhich UV-C is attenuated. This absorption also leads to the for-mation of hydroxyl radicals, but because most photogeneratedcharge carriers recombine within the TiO2, the photonic efficiencyis low <5% [18]. This analysis suggests that the proper interpreta-tion of the effect of UV-C on TiO2/H2O2/dye mixtures is that TiO2lessens the activity of the H2O2, not that H2O2 increases the activityof the TiO2.

This interpretation appears to contradict that of many previousstudies of UV-C decolouration of azo dyes (although a definitivejudgement would require an analysis of the optics in individualsystems). It is probable that the different emphasis is drawnbecause previous studies focused on H2O2’s role as an electronacceptor and did not compare the results for TiO2/H2O2 with thosefor H2O2 alone [26,27,29,31,33]. Exceptionally, Wu found TiO2/H2O2

to be more effective than H2O2 by itself, but in those studies theH2O2 concentration was very low, 0.33 mM [28]. Although thepresent interpretation appears to contradict that from mostother studies of dye decolouration, it does reflect aspects of thewider literature. For example, it is well known that if intermediatereaction products can be degraded photochemically, as with thehydroquinone intermediates formed during photocatalytic degra-dation of chlorobenzene [36] theymay influence the photocatalyticdegradation kinetics, as with the effect of benzene oxidation

T.A. Egerton, H. Purnama / Dyes and Pigments 101 (2014) 280e285 285

products on the photocatalytic degradation of perchloroethylene[37]. Dillert and Bahnemann reported that in studies of disinfectionof municipal wastewater, addition of P25 decreased the disinfectionby a lamp which emitted a small but significant photon flux below280 nm [38], most probably because, as in the present study, ab-sorption by the TiO2 removed the highly effective UV-C photons. (Ifphotons below 280 nmwere filtered out by a different lamp sheathof different glass the disinfectionwas enhanced by P25). Analogouseffects also occur in other systems in which a TiO2 photocatalyst isadded to a substrate which is susceptible to degradation by directphotochemical processes. Thus the oxidation of polyethylene filmsto CO2 is reduced by pigmentationwith a TiO2 of low photocatalyticactivity (caused by the formation of a surface coating) becausereduction of direct UV-induced photochemical oxidation, as a resultof UV screening, by TiO2 is more important than the photocatalyticoxidation by the TiO2 [39].

5. Conclusions

There are many claims that H2O2 addition to suspensions of TiO2in azo-dye solution accelerates the photocatalytic decolouration ofthe dye, but few are supported by measurements of decolourationby H2O2 by itself. The rate of RO16 decolouration by H2O2/TiO2 isless than the decolouration rate by H2O2 alone. There is no evidenceof any positive synergy associated with TiO2/H2O2 in combination.Therefore it is inappropriate to claim that hydrogen peroxide reallyaccelerates TiO2 UV-C photocatalyzed decolouration of a repre-sentative azo-dye, Reactive Orange 16.

It is suggested that UV-C photon absorption by the TiO2 reducesthe number of photons available to photolyse H2O2 and thatdecolouration rates are reduced because the photonic efficiency ofhydroxyl radical production at the TiO2 surface is less than thephotonic efficiency of H2O2 photolysis.

It is recommended that future studies of decolouration by H2O2/TiO2 should includemeasurements of the decolouration induced byH2O2 by itself.

Acknowledgements

HP thanks the Professional Skills Development Sector Project ofthe Muhammadiyah University of Sukarto Indonesia for thefinancial support of his Ph.D. Studies at Newcastle.

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