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Peroxomonosulphate, an efficient oxidant for the photocatalysed degradation of a textile dye, acid red 88
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Solar Energy Materials & Solar Cells 90 (2006) 1875–1887 www.elsevier.com/locate/solmat
Peroxomonosulphate, an efficient oxidant for the photocatalysed degradation of a textile dye, acid red 88 J. Madhavan, B. Muthuraaman, S. Murugesan, S. Anandan1, P. Maruthamuthu,2 Department of Energy [Chemistry-Interdisciplinary], University of Madras, Guindy Campus, Chennai 600 025, India Received 1 October 2005; accepted 4 December 2005 Available online 23 January 2006
Abstract Visible light-assisted degradation of a mono azo textile dye acid red 88 (AR88) was carried out in presence of titanium dioxide photocatalyst. Various operational parameters such as variation of the initial dye concentration, photocatalyst and pH on the photocatalytic degradation rate were studied. Effect of the amount of oxidants such as peroxomonosulphate (PMS) and peroxodisulphate (PDS) and the ratio of concentration of oxidant to the concentration of dye (Coxidant/Cdye) on the photocatalysed degradation rate were also investigated. Though the rate of photodegradation of the dye decreased with increase in dye concentration, the rate increased with Coxidant/Cdye ratio. Total organic carbon (TOC) analysis revealed a rapid mineralisation of AR88 in the presence of PMS. A suitable mechanism explaining the observed enhanced decolorisation and mineralisation rate of the dye with PMS is presented. r 2005 Elsevier B.V. All rights reserved. Keywords: Photocatalysis; Acid red 88; Peroxomonosulphate; Peroxodisulphate; Visible light
Corresponding author. Tel.: +91 44 22301576; fax: +91 44 22352494.
E-mail addresses: [email protected] (S. Anandan), [email protected] (P. Maruthamuthu). Present Address: Central Electrochemical Research Institute—Madras Unit, CSIR Madras Complex, Taramani, Chennai 600 113, India. 2 Present Address: Vice-chancellor, Madurai Kamaraj University, Madurai 625 021, India. 1
0927-0248/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2005.12.001
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1. Introduction The rapid industrialisation, besides its benefit, has resulted in hazardous effects of chemicals on the environment. For example, effluents released from pulp and paper industries and petrochemical, textile and dyeing industries discharge different types of pollutants that are toxic, carcinogenic, and mutagenic to the aquatic lives as well as human beings. Several stringent legislations are being imposed world wide to protect the environment from pollution and the advanced oxidation processes (AOPs) are currently gaining significant importance [1–3] to meet the limitations of the current legislations. AOPs mainly involve the generation of very powerful and non-selective oxidising agent, the hydroxyl radical (dOH), [4–6] which is produced by different combinations of ozone, hydrogen peroxide, UV radiation and titanium dioxide and also by the combination of hydrogen peroxide with ferrous ions in the so-called Fenton’s reagent, for the destruction of hazardous pollutants in air and water [7,8]. AOPs are especially attractive because they hold out the promise of completely mineralising the target pollutant and hence are useful complement to the conventional treatment methods like flocculation, carbon adsorption, air stripping, reverse osmosis, aerobic biological oxidation, etc [9]. Heterogeneous photocatalysis is a viable method for waste water treatment as it completely mineralises the organic pollutants. Among the semiconductor photocatalysts available [10–12], TiO2 is widely used owing to its properties like resistance to photocorrosion, less expensive, nontoxic and the use of it at ambient conditions. Irradiation of TiO2 ðE g ¼ 3:2 eVÞ with light energy XEg, causes valence-band electrons to be excited to the conduction band creating holes in the valence band. The conduction band + electrons (e CB) and valence band holes (hVB) migrate to the surface and participate in interfacial oxidation–reduction reactions. hu
TiO2 ! TiO2 ðe ; hþ Þ TiO2 ðhþ Þ þ H2 O ! TiO2 þ OHd þ Hþ TiO2 ðe Þ þ O2 ! TiO2 þ O2 d The superoxide radical anion generated is ultimately converted into dOH [13] that degrades the dye further. + The recombination of the e CB and hVB is undesirable for the efficient degradation of the pollutants and the addition of oxidants like H2O2, K2S2O8, KHSO5, KBrO3 etc., avoids the recombination process by rapid reaction with the conduction band electrons generating very reactive oxidizing radicals, thus increasing the efficiency of photocatalysts. S2 O8 2 þ e ! SO4 d þ SO4 2 HSO5 þ e ! SO4 d þ OH For efficient degradation of pollutants these oxidants should (a) readily accept the conduction band electrons (b) rapidly dissociate into harmless products and (c) lead to the formation of dOH or other radicals which are highly reactive [14]. Although, many Refs. [15–18] are available for the photocatalysed degradation of organic pollutants using peroxodisulphate (PDS) as an oxidant, the literature involving peroxomonosulphate (PMS) as oxidant is limited. Al-Ekabi et al. [14] carried out the photocatalysed degradation
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of organic pollutants over TiO2 using H2O2, (NH4)2S2O8, KBrO3 and Oxone (commercial name of PMS) as oxidants and found that oxone is the most effective one. Similarly, Malato et al. [19] reported solar light-assisted degradation of pentachlorophenol (PCP) over TiO2 in the presence of PMS. Recently, Anipsitakis and Dionysiou [20,21] and Fernandez et al. [22,23] have reported the decomposition of organic water pollutants and the non-bio degradable textile azo-dye (Orange II) by transition metal-coupled oxonebased AOPs. Degradation of acid red 88 (AR88) by various methods [24–28] like biological degradation involving Spingomonas sp. strain 1CX, adsorption by neutral alumina, photocatalytic oxidation and ozonation are found to be less effective. So, the focus of our investigation is to apply peroxomonosulphate and peroxodisulphate as oxidants, thereby to enhance the photocatalytic degradation rates of AR88 under visible light and compare the efficiencies of these oxidants. 2. Experimental section 2.1. Materials Acid red 88, the textile dye with the molecular formula C20H13N2O4SNa which absorbs in the visible region ðlmax ¼ 506 nmÞ, is used as the substrate. TiO2 photocatalyst (Degussa P25, Germany) having a specific surface area of 57 m2 g1 is used. Potassium peroxomonosulphate, a triple salt with the composition 2KHSO5.KHSO4.K2SO4 from Janssen Chimica, Belgium and potassium peroxodisulphate (Merck) were used as received. When preparing the molar solutions, the fact that 1 mol of oxone gives 2 mol of peroxomonosulphate was considered as shown by its chemical formula. All the experiments were carried out at natural pH except for the experiments involving pH variation in which case the pH of the medium was adjusted using sodium hydroxide/ perchloric acid. 2.2. Photocatalytic reactor and light source A photochemical reactor of 100 ml capacity (borosilicate glass) having an opening at the top for sample removal is used. A continuous spectrum throughout the visible portion of light was provided by a 250 W tungsten–halogen lamp (Philips, India). 2.3. Experimental procedure and analysis A typical experimental procedure adopted in the present investigation is described below. By dissolving the appropriate amount of the dye in 70 ml of double distilled water in the photoreactor, the desired concentration of the dye is maintained. A known amount of the photocatalyst, viz., TiO2, was then added to the dye solution and prior to irradiation the aqueous suspension was mixed continuously in dark for 45 min to ensure adsorption/ desorption equilibrium. The concentration of the dye in bulk solution at this condition was used as the initial concentration (C0) for further kinetic analysis of the photodegradation process. During the irradiation, 5 ml aliquots are withdrawn at appropriate time intervals and the photocatalyst is removed immediately by centrifugation and filtration through a syringe filter (0.45 mm, Sartorius). The decrease in the concentration of the dye was
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determined spectrophotometrically using the UV–VIS spectrophotometer (Shimadzu, model: UV-1601) by following the absorbance of the clear dye solution at its lmax (506 nm). Total organic carbon (TOC) was analysed by direct injection of the filtered samples into a Thermo Euroglas, Model TOC 1200, total organic carbon analyser (Thermo Electron Corporation) calibrated with standard solutions of potassium hydrogen phthalate. TOC0 is the TOC measured before the addition of oxidants and TOC obtained at various irradiation times is denoted as TOCt.
3. Results and discussion Experiments were carried out to find out the extent of degradation of the dye in presence of (i) light and PDS, (ii) light and PMS, (iii) light and TiO2, and, (iv) light, TiO2 and PMS or PDS. Photodegradation of the dye takes place to some extent in the first and second case (ca. 18% with PDS and 16.6% with PMS after an hour of illumination), whereas higher photodegradation rates were observed in the combinations, dye+TiO2+light and dye+TiO2+light+PMS/PDS (Fig. 1). Experiments were carried out to optimise the amount of photocatalyst C TiO2 by varying the amount of TiO2 in the range of 0.286–1.428 g/L. It can be seen from Fig. 2 that as the amount of TiO2 increases, degradation rate constantly increases and reaches a maximum for 1.143 g/L above which the rate constant tend to decrease due to the scattering of the light on TiO2 particles. We have conveniently used 0.857 g/L (60 mg of TiO2 in 70 mL of the reaction mixture) of TiO2 for all the experiments, except for the variation of TiO2 amounts, carried out in this study. The photocatalytic degradation of AR88 is found to follow a pseudo-first-order kinetics. The plots of normalised dye concentration, Ct/C0, as a function of irradiation time (Fig. 3) 1.2 1.0
C/C0
0.8 0.6 0.4 0.2 0.0 0
10
20
30
40
50
60
Irradiation Time (min) Fig. 1. Preliminary experiments for the photodegradation of acid red 88. ’, AR88+PMS+Light; , AR88+PDS+Light; m, AR88+TiO2+Light; ., AR88+TiO2+PMS+Light; ~, AR88+TiO2+PDS+Light. C AR88 ¼ 5 105 M; C TiO2 ¼ 0:857 g=L; C PMS ¼ C PDS ¼ 1:25 mM.
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4.5 4.0
k' ×104S-1
3.5 3.0 2.5 2.0 1.5 0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
C TIO2, (g/L) Fig. 2. Effect of titanium dioxide amount on the degradation rate of acid red 88. C AR88 ¼ 5 105 M.
gives an exponential decay curve conforming first-order kinetics and the apparent rate constants (k0 ) were evaluated from the slopes of the plots of log (OD)t vs. irradiation time. 3.1. Variation of initial dye concentration 3.1.1. In the absence of oxidants Photocatalytic experiments were carried out with constant amount of titanium dioxide photocatalyst (0.857 g/L) in aqueous solution containing various initial concentrations of AR88 (3 105–9 105 M) at natural pH (6.2). The results showed that the rate constants decreased with increase in the initial dye concentrations as could be seen in the Fig. 4. The observed results are in accordance with the fact that as the initial concentration of the dye increases, it is the dye rather than TiO2 that absorbs the light which in turn result in the mitigated charge separation (e, h+) processes of the photocatalyst [29–31]. 3.1.2. In the presence of oxidants Photocatalytic experiments were also carried out with constant amount of titanium dioxide (0.857 g/L) and oxidant (1.25 mM) in aqueous solution containing various initial concentrations of AR88 (3 105–9 105 M) at natural pH. Fig. 4 shows the comparison of rate constants for dye degradation in the presence and absence of oxidants as a function of initial dye concentrations. In Fig. 4, it can be seen that the observed rate constant increases in the presence of oxidants (5 times in the presence of PMS) but decreases with increase in the concentration of the dye. It is found that the rate of the reaction depend on the ratio of the oxidant to the dye, viz., Coxidant/Cdye rather than the concentration of the dye and the results are presented in Table 1 and Fig. 5. The rate of the photocatalytic degradation of the dye is enhanced very much in the presence of PMS but only very small enhancement is seen in the presence of PDS. In order to verify the existence of the effect of Coxidant/Cdye factor, experiments were carried out with various initial dye concentrations maintaining the C PMS =C dye ¼ 10. As expected, the rate constants of all the photocatalytic
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1.0 Without any Oxidant
0.8
0.8
0.6
0.6
C/C0
C/C0
1.0
0.4
0.4
0.2
0.2
With PMS
0.0
0.0 0
10
20
30
40
50
60
0
Irradition Time (min)
10
20
30
40
50
60
Irradition Time (min)
1.0 With PDS
C/C0
0.8
0.6
0.4
0.2
0.0 0
5 10 15 20 25 30 35 40 45 50 55 60 65 Irradition Time (min)
Fig. 3. Normalized concentration vs time plot for the photocatalysed degradation of acid red 88 at different concentrations. ’, 3 105 M; , 5 105 M; m, 7 105M; ., 9 105 M. C TiO2 ¼ 0:857 g=L; C PMS ¼ C PDS ¼ 1:25 mM.
degradation were similar concluding that the degradation rate depends on the ratio of oxidant to the dye in the case of PMS and the results are given in Table 2. The observed enhancement in the rate with PMS in comparison to that with PDS may be attributed to + the involvement of both the e CB and hVB in the photocatalytic degradation [32,33]. 3.2. Variation of PMS PMS is a powerful oxidizing agent ½E1 ¼ 1:84 V, which undergoes radiolytic and photolytic reactions [33,34] but not photochemical decomposition unless it is irradiated in the UV region with the wavelength p260 nm. However, in the presence of photocatalysts efficient decomposition of PMS has been observed with light of wavelength X390 nm. For
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30
k' × 104 S-1
20
10
0 3
4
5
6
7
8
9
CAR88 × 105 M Fig. 4. Rate constants for the photcatalysed degradation of acid red 88 at different dye concentrations. ’, No oxidant; , with PMS; m, with PDS; C TiO2 ¼ 0:857 g=L; C PMS ¼ C PDS ¼ 1:25 mM.
Table 1 Apparent rate constants for the photocatalysed degradation of AR88 over TiO2 at different dye concentrations and with different oxidant concentrations C TiO2 ðg=LÞ
0.857 0.857 0.857 0.857 0.857 0.857 0.857
Cdye (Cdye 105 M)
3 5 5 5 5 7 9
Coxidant (mM)
1.25 0.5 1.25 1.75 2.5 1.25 1.25
Coxidant/Cdye
41.6 10 25 35 50 17.85 13.88
Apparent rate constant k0 104 (s1) Without oxidant
With PMS
With PDS
7.13 3.24 3.24 3.24 3.24 2.31 1.96
23.03 11.1 17.8 19.97 26.4 15.1 11.5
10.35 3.9 7.06 8.77 11.07 3.02 1.77
instance, Maruthamuthu et al. [35,36] studied effective photocatalytic decomposition of PMS with various semiconductors such as WO3, Bi2O3 and Fe2O3 using visible light irradiation, which otherwise could happen only by UV irradiation in the absence of these photocatalysts. PMS being acidic, it decreases the pH of the reaction solution when added. Anheden et al. [37] reported similar results for the addition of H2O2 to 5-fluorouracil. Ball and Edwards [38] observed a spontaneous decomposition of PMS in the pH range 6.0–11.65 and hence, no attempt was made to adjust the pH of the dye solution containing oxidants. If the pH is increased from the natural one, then the degradation of the dye would take place both by the self-decomposition and photocatalytic decomposition of PMS. Trial experiments carried out by altering the natural pH of PMS (2.8) to 5.5 showed a faster
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32 28
k ' × 103 S-1
24 20 16 12 8 4 10
20
30
40
50
C PMS/C dye Fig. 5. Effect of Coxidant/Cdye factor on the degradation rate of acid red 88 with constant amount of TiO2 (0.857 g/L) for the oxidant PMS.
Table 2 Apparent rate constants for the photocatalysed degradation of AR88 over TiO2 at constant CPMS/Cdye of 10 with constant amount of TiO2 (0.857 g/L) Cdye 105 (M)
CPMS (mM)
CPMS/Cdye
k0 103 (s1)
5 7 9
0.5 0.7 0.9
10 10 10
1.11 1.11 1.15
discolouration of the dye. Also, it should be noted that, it is not customary to adjust the pH of the medium when real wastewater is considered. In order to understand the effect of PMS on the photocatalysed degradation of AR88, the concentration of PMS (CPMS) was varied from 0.5 to 5 mM, keeping the concentration of dye (5 105 M) and amount of TiO2 (0.857 g/L) constant. The rate constants were found to increase with increase in the concentration of PMS from 0.5 to 2.5 mM. However, only a little change in the rate constant was observed if the concentration of PMS was increased further as depicted in the Fig. 6. 3.3. Variation of PDS PDS is a powerful oxidizing agent with a standard potential of E1 ¼ 2:01 V and can be decomposed to SO2 4 ion only by UV radiation (lp270 nm). However, Ashok kumar and Maruthamuthu [39] reported the decomposition of PDS by the visible light in the presence of doped and undoped WO3 semiconductors. The effect of PDS on the photocatalytic degradation of the dye was studied at various concentrations of PDS, CPDS, (0.5–5 mM) keeping the concentration of dye (5 105 M)
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3.0 2.8 2.6 2.4 k ' × 103 S-1
2.2 2.0 1.8 1.6 1.4 1.2 1.0 0
1
2
3
4
5
CPMS, mM Fig. 6. Effect of [PMS] on the degradation rate of acid red 88. C AR88 ¼ 5 105 M; C TiO2 ¼ 0:857 g=L.
16 14
k ' × 104 S-1
12 10 8 6 4 2 0
1
2
3
4
5
CPDS, mM Fig. 7. Effect of [PDS] on the degradation rate of acid red 88. C AR88 ¼ 5 105 M; C TiO2 ¼ 0:857 g=L.
and amount of TiO2 constant. A steady raise in the rate constant was observed with increase in the concentration of PDS from 0.5 to 5 mM as shown in Fig. 7. But the increase in the rate constant due to the addition of PDS is small. Saquib and Muneer [26] carried out the effect of oxidants such as hydrogen peroxide, bromate and persulphate ions on the photocatalysed degradation of AR88 and observed no marked influence on the degradation rate with any of the oxidants. The same trend was observed in our case also when persulphate was employed as the oxidant.
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1.1 1.0 0.9
TOCt/TOC0
0.8 0.7 0.6 0.5 0.4 0.3 0.2 0
50 100 150 200 250 300 350 400 Irradiation Time (min)
Fig. 8. Comparison of photocatalysed mineralisation of acid red 88 in the presence and absence of oxidants. ’, No oxidant; , with PDS; m, with PMS; C AR88 ¼ 5 105 M; C TiO2 ¼ 0:857 g=L; C PMS ¼ C PDS ¼ 1:25 mM.
3.4. TOC analysis A rather straightforward way of measuring oxidation progress of an organic compound is the determination of the carbon content of the oxidation product mixture. Fig. 8 shows the TOC reduction of an AR88 solution over TiO2 alone as well as over TiO2 in the presence of a fixed concentration of PMS or PDS (1.25 mM). It is clear from Fig. 8 that the rate of photocatalysed mineralisation is enhanced by the addition of oxidants. Interestingly, the rate of mineralisation of AR88 over TiO2 is increased by a factor of 1.7 by the addition of PMS. 3.5. PMS an effective oxidant for the photocatalysed degradation of AR88 A comparison of the efficiency of the oxidants (PMS and PDS) for the photocatalysed degradation of AR88 (5 105 M) was obtained by comparing the results of the experiments carried out under identical experimental conditions but with different oxidants (CPMS or C PDS ¼ 1:25 mM). A complete decolorisation of the dye was observed in the presence of PMS and 49.8% of decolorisation was achieved with PDS while only 37% of decolorisation was effected in the absence of any oxidants in 30 min, under the same experimental conditions as depicted in Fig. 9. In the photocatalytic degradation of the dye over TiO2, the addition of PMS enhances the degradation rate by 2.7 times whereas the addition of PDS increases the rate by a factor of 1.3. While 87% decolorisation of the dye in 30 min was achieved only when the concentration of PDS was increased 100 times (5 mM) as that of dye, a complete decolorisation could be achieved with 25 times of CPMS, which clearly demonstrates the efficiency of PMS over that of PDS. When the solution was irradiated for 6 h, 44% of TOC was removed in the absence of any oxidant, whereas an enhanced TOC removal, viz., 65% and 75% could be achieved in the presence of PDS and PMS, respectively as given in Fig. 9. Thus, TOC analysis further
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110
110 With PMS
100
90
With PMS
80 With PDS
70 60 40
With PDS No Oxidant
90 80 70 60
No Oxidant
50 40
30
30
20
20
10
10
0
0
Percentagae TOC removal
Percentage decolorisation
100
50
1885
Fig. 9. Comparison of photocatalysed decolorisation (30 min) and mineralisation (6 h) in the presence and absence of oxidants. C AR88 ¼ 5 105 M; C TiO2 ¼ 0:857 g=L; C PMS ¼ C PDS ¼ 1:25 mM.
confirms the efficiency of PMS over that of PDS in the photocatalysed degradation of the acid red 88. However, it has to be noted that the decolorisation rate of the dye solution is more rapid when compared to mineralisation rate since the latter takes longer times for oxidation. An enhanced efficiency of PMS over PDS can be rationalised since PMS gets + decomposed through both e CB and hVB of the semiconductor photocatalysts whereas PDS can be decomposed only by eCB [32,33] and the possible reactions are represented below. With PMS:
HOOSO3 þ hþ VB ! SO5 d þ Hþ H2 O
2ðSO5 d Þ ! 2HSO4 þ O2 With PDS: S2 O8 2 þ e CB ! SO4 2 þ SO4 d
3.6. Influence of pH The influence of pH on the photocatalysed degradation of AR88 was investigated at various pH levels (1.5–11.0) with constant dye concentration and photocatalyst amount. A maximum degradation of the dye was observed at pH closer to zero point charge ðpHzpc ¼ 6:5Þ and a minimum degradation of the dye was observed both at lower and higher pH values. The observed trend is in line with the results published earlier [26]. At pH 1.5 no
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photocatalytic degradation of AR88 was observed which might be due to the complete coverage of the dye on TiO2 surface leading to a red coloration of the photocatalyst. Similar results were observed in the earlier studies also for the photocatalysed degradation of orange II [40] and pararosaniline [41] at pH p1.5. At this pH, the dye completely gets adsorbed on the catalyst surface that blocks the irradiation and prevents the photoexcitation of the catalyst. During the photocatalytic processes when the reactant molecule is strongly bonded to surface atoms, the photocatalytic site becomes inactive and said to be poisoned i.e., no photocatalysis is possible [42]. 4. Conclusions The visible light-induced photocatalytic degradation of a textile dye, acid red 88 over TiO2 in the presence and absence of PMS and PDS indicated that the addition of PMS increases the photodegradation rate by 2.7 times. The results of the variation of initial dye concentration with and without oxidants (PMS/PDS) showed that the factor Coxidant/Cdye plays a significant role in the photodegradation reaction rate. A rapid decolorisation and TOC removal rates were observed with PMS indicating PMS as more efficient oxidant than PDS for the photocatalysed degradation of AR88. At pH 1.5 no photocatalysis was observed due to the poisoning of the photocatalyst surface. Acknowledgements The financial support received from the CSIR, New Delhi, in the form of senior research fellowship is duly acknowledged by one of the authors, J. Madhavan. References [1] A. Reife, H. Freeman, Environmental Chemistry of Dyes and Pigments, Wiley/Interscience, New York, 1996. [2] M. Hoffmann, M. Martin, W. Choi, D. Bahnemann, Chem. Rev. 95 (1995) 69. [3] M.R. Dhananjeyan, E. Fine, J. Kiwi, J. Photochem. Photobiol. A 136 (2000) 125. [4] D.F. Ollis, H. Al-Ekabi (Eds.), Photocatalytic Purification and Treatment of Water and Air, Elsevier, Amsterdam, 1993. [5] N. Serpone, E. Pelizzetti (Eds.), Photocatalysis, Fundamentals and Applications, Wiley, New York, 1989. [6] M. Schiavello, Photocatalysis and Environment, Kluwer, Dordecht, 1988. [7] R. Andreozzi, V. Caprio, A. Insola, R. Marotta, Catal. Today 53 (1999) 51. [8] J.M. Herrmann, Catal. Today 53 (1999) 115. [9] J. Cunningham, G. Al-Sayyed, P. Sedlak, J. Caffrey, Catal. Today 53 (1999) 145 and the references cited therein. [10] A. Mills, R.H. Davies, D. Worsley, Chem. Soc. Rev. (1993) 417. [11] O. Legrini, E. Oliveros, A.M. Braun, Chem. Rev. 93 (1993) 671. [12] A. Fujishima, T.N. Rao, D.A. Tryk, J. PhotoChem. Photobiol. C 1 (2000) 1. [13] T. Wu, T. Lin, J. Zhao, H. Hidaka, N. Serpone, Environ. Sci. Technol. 33 (1999) 1379. [14] H. Al-Ekabi, B. Butters, D. Delany, J. Ireland, N. Lewis, T. Powell, J. Story, in: D.F. Ollis, H. Al-Ekabi (Eds.), Photocatalytic Purification and Treatment of Water and Air, 1993, p. 321. [15] A.B. Prevot, C. Baiocchi, M.C. Brussino, E. Pramauro, P. Savarino, V. Augugliaro, G. Marci, L. Palmisano, Environ. Sci. Technol. 35 (2001) 971. [16] V. Augugliaro, C. Baiocchi, A.B. Prevot, E. Garcia-Lopez, V. Loddo, S. Malato, G. Marci, L. Palmisano, M. Pazzi, E. Pramauro, Chemosphere 49 (2002) 1223. [17] M. Saquib, M. Muneer, Dyes Pigments 53 (2002) 237. [18] M. Muruganandham, M. Swaminathan, Sol. Energy Mater. Sol. Cells 81 (2004) 439.
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Solar Energy Materials & Solar Cells 90 (2006) 1875–1887 www.elsevier.com/locate/solmat
Peroxomonosulphate, an efficient oxidant for the photocatalysed degradation of a textile dye, acid red 88 J. Madhavan, B. Muthuraaman, S. Murugesan, S. Anandan1, P. Maruthamuthu,2 Department of Energy [Chemistry-Interdisciplinary], University of Madras, Guindy Campus, Chennai 600 025, India Received 1 October 2005; accepted 4 December 2005 Available online 23 January 2006
Abstract Visible light-assisted degradation of a mono azo textile dye acid red 88 (AR88) was carried out in presence of titanium dioxide photocatalyst. Various operational parameters such as variation of the initial dye concentration, photocatalyst and pH on the photocatalytic degradation rate were studied. Effect of the amount of oxidants such as peroxomonosulphate (PMS) and peroxodisulphate (PDS) and the ratio of concentration of oxidant to the concentration of dye (Coxidant/Cdye) on the photocatalysed degradation rate were also investigated. Though the rate of photodegradation of the dye decreased with increase in dye concentration, the rate increased with Coxidant/Cdye ratio. Total organic carbon (TOC) analysis revealed a rapid mineralisation of AR88 in the presence of PMS. A suitable mechanism explaining the observed enhanced decolorisation and mineralisation rate of the dye with PMS is presented. r 2005 Elsevier B.V. All rights reserved. Keywords: Photocatalysis; Acid red 88; Peroxomonosulphate; Peroxodisulphate; Visible light
Corresponding author. Tel.: +91 44 22301576; fax: +91 44 22352494.
E-mail addresses: [email protected] (S. Anandan), [email protected] (P. Maruthamuthu). Present Address: Central Electrochemical Research Institute—Madras Unit, CSIR Madras Complex, Taramani, Chennai 600 113, India. 2 Present Address: Vice-chancellor, Madurai Kamaraj University, Madurai 625 021, India. 1
0927-0248/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2005.12.001
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1. Introduction The rapid industrialisation, besides its benefit, has resulted in hazardous effects of chemicals on the environment. For example, effluents released from pulp and paper industries and petrochemical, textile and dyeing industries discharge different types of pollutants that are toxic, carcinogenic, and mutagenic to the aquatic lives as well as human beings. Several stringent legislations are being imposed world wide to protect the environment from pollution and the advanced oxidation processes (AOPs) are currently gaining significant importance [1–3] to meet the limitations of the current legislations. AOPs mainly involve the generation of very powerful and non-selective oxidising agent, the hydroxyl radical (dOH), [4–6] which is produced by different combinations of ozone, hydrogen peroxide, UV radiation and titanium dioxide and also by the combination of hydrogen peroxide with ferrous ions in the so-called Fenton’s reagent, for the destruction of hazardous pollutants in air and water [7,8]. AOPs are especially attractive because they hold out the promise of completely mineralising the target pollutant and hence are useful complement to the conventional treatment methods like flocculation, carbon adsorption, air stripping, reverse osmosis, aerobic biological oxidation, etc [9]. Heterogeneous photocatalysis is a viable method for waste water treatment as it completely mineralises the organic pollutants. Among the semiconductor photocatalysts available [10–12], TiO2 is widely used owing to its properties like resistance to photocorrosion, less expensive, nontoxic and the use of it at ambient conditions. Irradiation of TiO2 ðE g ¼ 3:2 eVÞ with light energy XEg, causes valence-band electrons to be excited to the conduction band creating holes in the valence band. The conduction band + electrons (e CB) and valence band holes (hVB) migrate to the surface and participate in interfacial oxidation–reduction reactions. hu
TiO2 ! TiO2 ðe ; hþ Þ TiO2 ðhþ Þ þ H2 O ! TiO2 þ OHd þ Hþ TiO2 ðe Þ þ O2 ! TiO2 þ O2 d The superoxide radical anion generated is ultimately converted into dOH [13] that degrades the dye further. + The recombination of the e CB and hVB is undesirable for the efficient degradation of the pollutants and the addition of oxidants like H2O2, K2S2O8, KHSO5, KBrO3 etc., avoids the recombination process by rapid reaction with the conduction band electrons generating very reactive oxidizing radicals, thus increasing the efficiency of photocatalysts. S2 O8 2 þ e ! SO4 d þ SO4 2 HSO5 þ e ! SO4 d þ OH For efficient degradation of pollutants these oxidants should (a) readily accept the conduction band electrons (b) rapidly dissociate into harmless products and (c) lead to the formation of dOH or other radicals which are highly reactive [14]. Although, many Refs. [15–18] are available for the photocatalysed degradation of organic pollutants using peroxodisulphate (PDS) as an oxidant, the literature involving peroxomonosulphate (PMS) as oxidant is limited. Al-Ekabi et al. [14] carried out the photocatalysed degradation
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of organic pollutants over TiO2 using H2O2, (NH4)2S2O8, KBrO3 and Oxone (commercial name of PMS) as oxidants and found that oxone is the most effective one. Similarly, Malato et al. [19] reported solar light-assisted degradation of pentachlorophenol (PCP) over TiO2 in the presence of PMS. Recently, Anipsitakis and Dionysiou [20,21] and Fernandez et al. [22,23] have reported the decomposition of organic water pollutants and the non-bio degradable textile azo-dye (Orange II) by transition metal-coupled oxonebased AOPs. Degradation of acid red 88 (AR88) by various methods [24–28] like biological degradation involving Spingomonas sp. strain 1CX, adsorption by neutral alumina, photocatalytic oxidation and ozonation are found to be less effective. So, the focus of our investigation is to apply peroxomonosulphate and peroxodisulphate as oxidants, thereby to enhance the photocatalytic degradation rates of AR88 under visible light and compare the efficiencies of these oxidants. 2. Experimental section 2.1. Materials Acid red 88, the textile dye with the molecular formula C20H13N2O4SNa which absorbs in the visible region ðlmax ¼ 506 nmÞ, is used as the substrate. TiO2 photocatalyst (Degussa P25, Germany) having a specific surface area of 57 m2 g1 is used. Potassium peroxomonosulphate, a triple salt with the composition 2KHSO5.KHSO4.K2SO4 from Janssen Chimica, Belgium and potassium peroxodisulphate (Merck) were used as received. When preparing the molar solutions, the fact that 1 mol of oxone gives 2 mol of peroxomonosulphate was considered as shown by its chemical formula. All the experiments were carried out at natural pH except for the experiments involving pH variation in which case the pH of the medium was adjusted using sodium hydroxide/ perchloric acid. 2.2. Photocatalytic reactor and light source A photochemical reactor of 100 ml capacity (borosilicate glass) having an opening at the top for sample removal is used. A continuous spectrum throughout the visible portion of light was provided by a 250 W tungsten–halogen lamp (Philips, India). 2.3. Experimental procedure and analysis A typical experimental procedure adopted in the present investigation is described below. By dissolving the appropriate amount of the dye in 70 ml of double distilled water in the photoreactor, the desired concentration of the dye is maintained. A known amount of the photocatalyst, viz., TiO2, was then added to the dye solution and prior to irradiation the aqueous suspension was mixed continuously in dark for 45 min to ensure adsorption/ desorption equilibrium. The concentration of the dye in bulk solution at this condition was used as the initial concentration (C0) for further kinetic analysis of the photodegradation process. During the irradiation, 5 ml aliquots are withdrawn at appropriate time intervals and the photocatalyst is removed immediately by centrifugation and filtration through a syringe filter (0.45 mm, Sartorius). The decrease in the concentration of the dye was
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determined spectrophotometrically using the UV–VIS spectrophotometer (Shimadzu, model: UV-1601) by following the absorbance of the clear dye solution at its lmax (506 nm). Total organic carbon (TOC) was analysed by direct injection of the filtered samples into a Thermo Euroglas, Model TOC 1200, total organic carbon analyser (Thermo Electron Corporation) calibrated with standard solutions of potassium hydrogen phthalate. TOC0 is the TOC measured before the addition of oxidants and TOC obtained at various irradiation times is denoted as TOCt.
3. Results and discussion Experiments were carried out to find out the extent of degradation of the dye in presence of (i) light and PDS, (ii) light and PMS, (iii) light and TiO2, and, (iv) light, TiO2 and PMS or PDS. Photodegradation of the dye takes place to some extent in the first and second case (ca. 18% with PDS and 16.6% with PMS after an hour of illumination), whereas higher photodegradation rates were observed in the combinations, dye+TiO2+light and dye+TiO2+light+PMS/PDS (Fig. 1). Experiments were carried out to optimise the amount of photocatalyst C TiO2 by varying the amount of TiO2 in the range of 0.286–1.428 g/L. It can be seen from Fig. 2 that as the amount of TiO2 increases, degradation rate constantly increases and reaches a maximum for 1.143 g/L above which the rate constant tend to decrease due to the scattering of the light on TiO2 particles. We have conveniently used 0.857 g/L (60 mg of TiO2 in 70 mL of the reaction mixture) of TiO2 for all the experiments, except for the variation of TiO2 amounts, carried out in this study. The photocatalytic degradation of AR88 is found to follow a pseudo-first-order kinetics. The plots of normalised dye concentration, Ct/C0, as a function of irradiation time (Fig. 3) 1.2 1.0
C/C0
0.8 0.6 0.4 0.2 0.0 0
10
20
30
40
50
60
Irradiation Time (min) Fig. 1. Preliminary experiments for the photodegradation of acid red 88. ’, AR88+PMS+Light; , AR88+PDS+Light; m, AR88+TiO2+Light; ., AR88+TiO2+PMS+Light; ~, AR88+TiO2+PDS+Light. C AR88 ¼ 5 105 M; C TiO2 ¼ 0:857 g=L; C PMS ¼ C PDS ¼ 1:25 mM.
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4.5 4.0
k' ×104S-1
3.5 3.0 2.5 2.0 1.5 0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
C TIO2, (g/L) Fig. 2. Effect of titanium dioxide amount on the degradation rate of acid red 88. C AR88 ¼ 5 105 M.
gives an exponential decay curve conforming first-order kinetics and the apparent rate constants (k0 ) were evaluated from the slopes of the plots of log (OD)t vs. irradiation time. 3.1. Variation of initial dye concentration 3.1.1. In the absence of oxidants Photocatalytic experiments were carried out with constant amount of titanium dioxide photocatalyst (0.857 g/L) in aqueous solution containing various initial concentrations of AR88 (3 105–9 105 M) at natural pH (6.2). The results showed that the rate constants decreased with increase in the initial dye concentrations as could be seen in the Fig. 4. The observed results are in accordance with the fact that as the initial concentration of the dye increases, it is the dye rather than TiO2 that absorbs the light which in turn result in the mitigated charge separation (e, h+) processes of the photocatalyst [29–31]. 3.1.2. In the presence of oxidants Photocatalytic experiments were also carried out with constant amount of titanium dioxide (0.857 g/L) and oxidant (1.25 mM) in aqueous solution containing various initial concentrations of AR88 (3 105–9 105 M) at natural pH. Fig. 4 shows the comparison of rate constants for dye degradation in the presence and absence of oxidants as a function of initial dye concentrations. In Fig. 4, it can be seen that the observed rate constant increases in the presence of oxidants (5 times in the presence of PMS) but decreases with increase in the concentration of the dye. It is found that the rate of the reaction depend on the ratio of the oxidant to the dye, viz., Coxidant/Cdye rather than the concentration of the dye and the results are presented in Table 1 and Fig. 5. The rate of the photocatalytic degradation of the dye is enhanced very much in the presence of PMS but only very small enhancement is seen in the presence of PDS. In order to verify the existence of the effect of Coxidant/Cdye factor, experiments were carried out with various initial dye concentrations maintaining the C PMS =C dye ¼ 10. As expected, the rate constants of all the photocatalytic
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1.0 Without any Oxidant
0.8
0.8
0.6
0.6
C/C0
C/C0
1.0
0.4
0.4
0.2
0.2
With PMS
0.0
0.0 0
10
20
30
40
50
60
0
Irradition Time (min)
10
20
30
40
50
60
Irradition Time (min)
1.0 With PDS
C/C0
0.8
0.6
0.4
0.2
0.0 0
5 10 15 20 25 30 35 40 45 50 55 60 65 Irradition Time (min)
Fig. 3. Normalized concentration vs time plot for the photocatalysed degradation of acid red 88 at different concentrations. ’, 3 105 M; , 5 105 M; m, 7 105M; ., 9 105 M. C TiO2 ¼ 0:857 g=L; C PMS ¼ C PDS ¼ 1:25 mM.
degradation were similar concluding that the degradation rate depends on the ratio of oxidant to the dye in the case of PMS and the results are given in Table 2. The observed enhancement in the rate with PMS in comparison to that with PDS may be attributed to + the involvement of both the e CB and hVB in the photocatalytic degradation [32,33]. 3.2. Variation of PMS PMS is a powerful oxidizing agent ½E1 ¼ 1:84 V, which undergoes radiolytic and photolytic reactions [33,34] but not photochemical decomposition unless it is irradiated in the UV region with the wavelength p260 nm. However, in the presence of photocatalysts efficient decomposition of PMS has been observed with light of wavelength X390 nm. For
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30
k' × 104 S-1
20
10
0 3
4
5
6
7
8
9
CAR88 × 105 M Fig. 4. Rate constants for the photcatalysed degradation of acid red 88 at different dye concentrations. ’, No oxidant; , with PMS; m, with PDS; C TiO2 ¼ 0:857 g=L; C PMS ¼ C PDS ¼ 1:25 mM.
Table 1 Apparent rate constants for the photocatalysed degradation of AR88 over TiO2 at different dye concentrations and with different oxidant concentrations C TiO2 ðg=LÞ
0.857 0.857 0.857 0.857 0.857 0.857 0.857
Cdye (Cdye 105 M)
3 5 5 5 5 7 9
Coxidant (mM)
1.25 0.5 1.25 1.75 2.5 1.25 1.25
Coxidant/Cdye
41.6 10 25 35 50 17.85 13.88
Apparent rate constant k0 104 (s1) Without oxidant
With PMS
With PDS
7.13 3.24 3.24 3.24 3.24 2.31 1.96
23.03 11.1 17.8 19.97 26.4 15.1 11.5
10.35 3.9 7.06 8.77 11.07 3.02 1.77
instance, Maruthamuthu et al. [35,36] studied effective photocatalytic decomposition of PMS with various semiconductors such as WO3, Bi2O3 and Fe2O3 using visible light irradiation, which otherwise could happen only by UV irradiation in the absence of these photocatalysts. PMS being acidic, it decreases the pH of the reaction solution when added. Anheden et al. [37] reported similar results for the addition of H2O2 to 5-fluorouracil. Ball and Edwards [38] observed a spontaneous decomposition of PMS in the pH range 6.0–11.65 and hence, no attempt was made to adjust the pH of the dye solution containing oxidants. If the pH is increased from the natural one, then the degradation of the dye would take place both by the self-decomposition and photocatalytic decomposition of PMS. Trial experiments carried out by altering the natural pH of PMS (2.8) to 5.5 showed a faster
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k ' × 103 S-1
24 20 16 12 8 4 10
20
30
40
50
C PMS/C dye Fig. 5. Effect of Coxidant/Cdye factor on the degradation rate of acid red 88 with constant amount of TiO2 (0.857 g/L) for the oxidant PMS.
Table 2 Apparent rate constants for the photocatalysed degradation of AR88 over TiO2 at constant CPMS/Cdye of 10 with constant amount of TiO2 (0.857 g/L) Cdye 105 (M)
CPMS (mM)
CPMS/Cdye
k0 103 (s1)
5 7 9
0.5 0.7 0.9
10 10 10
1.11 1.11 1.15
discolouration of the dye. Also, it should be noted that, it is not customary to adjust the pH of the medium when real wastewater is considered. In order to understand the effect of PMS on the photocatalysed degradation of AR88, the concentration of PMS (CPMS) was varied from 0.5 to 5 mM, keeping the concentration of dye (5 105 M) and amount of TiO2 (0.857 g/L) constant. The rate constants were found to increase with increase in the concentration of PMS from 0.5 to 2.5 mM. However, only a little change in the rate constant was observed if the concentration of PMS was increased further as depicted in the Fig. 6. 3.3. Variation of PDS PDS is a powerful oxidizing agent with a standard potential of E1 ¼ 2:01 V and can be decomposed to SO2 4 ion only by UV radiation (lp270 nm). However, Ashok kumar and Maruthamuthu [39] reported the decomposition of PDS by the visible light in the presence of doped and undoped WO3 semiconductors. The effect of PDS on the photocatalytic degradation of the dye was studied at various concentrations of PDS, CPDS, (0.5–5 mM) keeping the concentration of dye (5 105 M)
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3.0 2.8 2.6 2.4 k ' × 103 S-1
2.2 2.0 1.8 1.6 1.4 1.2 1.0 0
1
2
3
4
5
CPMS, mM Fig. 6. Effect of [PMS] on the degradation rate of acid red 88. C AR88 ¼ 5 105 M; C TiO2 ¼ 0:857 g=L.
16 14
k ' × 104 S-1
12 10 8 6 4 2 0
1
2
3
4
5
CPDS, mM Fig. 7. Effect of [PDS] on the degradation rate of acid red 88. C AR88 ¼ 5 105 M; C TiO2 ¼ 0:857 g=L.
and amount of TiO2 constant. A steady raise in the rate constant was observed with increase in the concentration of PDS from 0.5 to 5 mM as shown in Fig. 7. But the increase in the rate constant due to the addition of PDS is small. Saquib and Muneer [26] carried out the effect of oxidants such as hydrogen peroxide, bromate and persulphate ions on the photocatalysed degradation of AR88 and observed no marked influence on the degradation rate with any of the oxidants. The same trend was observed in our case also when persulphate was employed as the oxidant.
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1.1 1.0 0.9
TOCt/TOC0
0.8 0.7 0.6 0.5 0.4 0.3 0.2 0
50 100 150 200 250 300 350 400 Irradiation Time (min)
Fig. 8. Comparison of photocatalysed mineralisation of acid red 88 in the presence and absence of oxidants. ’, No oxidant; , with PDS; m, with PMS; C AR88 ¼ 5 105 M; C TiO2 ¼ 0:857 g=L; C PMS ¼ C PDS ¼ 1:25 mM.
3.4. TOC analysis A rather straightforward way of measuring oxidation progress of an organic compound is the determination of the carbon content of the oxidation product mixture. Fig. 8 shows the TOC reduction of an AR88 solution over TiO2 alone as well as over TiO2 in the presence of a fixed concentration of PMS or PDS (1.25 mM). It is clear from Fig. 8 that the rate of photocatalysed mineralisation is enhanced by the addition of oxidants. Interestingly, the rate of mineralisation of AR88 over TiO2 is increased by a factor of 1.7 by the addition of PMS. 3.5. PMS an effective oxidant for the photocatalysed degradation of AR88 A comparison of the efficiency of the oxidants (PMS and PDS) for the photocatalysed degradation of AR88 (5 105 M) was obtained by comparing the results of the experiments carried out under identical experimental conditions but with different oxidants (CPMS or C PDS ¼ 1:25 mM). A complete decolorisation of the dye was observed in the presence of PMS and 49.8% of decolorisation was achieved with PDS while only 37% of decolorisation was effected in the absence of any oxidants in 30 min, under the same experimental conditions as depicted in Fig. 9. In the photocatalytic degradation of the dye over TiO2, the addition of PMS enhances the degradation rate by 2.7 times whereas the addition of PDS increases the rate by a factor of 1.3. While 87% decolorisation of the dye in 30 min was achieved only when the concentration of PDS was increased 100 times (5 mM) as that of dye, a complete decolorisation could be achieved with 25 times of CPMS, which clearly demonstrates the efficiency of PMS over that of PDS. When the solution was irradiated for 6 h, 44% of TOC was removed in the absence of any oxidant, whereas an enhanced TOC removal, viz., 65% and 75% could be achieved in the presence of PDS and PMS, respectively as given in Fig. 9. Thus, TOC analysis further
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110
110 With PMS
100
90
With PMS
80 With PDS
70 60 40
With PDS No Oxidant
90 80 70 60
No Oxidant
50 40
30
30
20
20
10
10
0
0
Percentagae TOC removal
Percentage decolorisation
100
50
1885
Fig. 9. Comparison of photocatalysed decolorisation (30 min) and mineralisation (6 h) in the presence and absence of oxidants. C AR88 ¼ 5 105 M; C TiO2 ¼ 0:857 g=L; C PMS ¼ C PDS ¼ 1:25 mM.
confirms the efficiency of PMS over that of PDS in the photocatalysed degradation of the acid red 88. However, it has to be noted that the decolorisation rate of the dye solution is more rapid when compared to mineralisation rate since the latter takes longer times for oxidation. An enhanced efficiency of PMS over PDS can be rationalised since PMS gets + decomposed through both e CB and hVB of the semiconductor photocatalysts whereas PDS can be decomposed only by eCB [32,33] and the possible reactions are represented below. With PMS:
HOOSO3 þ hþ VB ! SO5 d þ Hþ H2 O
2ðSO5 d Þ ! 2HSO4 þ O2 With PDS: S2 O8 2 þ e CB ! SO4 2 þ SO4 d
3.6. Influence of pH The influence of pH on the photocatalysed degradation of AR88 was investigated at various pH levels (1.5–11.0) with constant dye concentration and photocatalyst amount. A maximum degradation of the dye was observed at pH closer to zero point charge ðpHzpc ¼ 6:5Þ and a minimum degradation of the dye was observed both at lower and higher pH values. The observed trend is in line with the results published earlier [26]. At pH 1.5 no
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photocatalytic degradation of AR88 was observed which might be due to the complete coverage of the dye on TiO2 surface leading to a red coloration of the photocatalyst. Similar results were observed in the earlier studies also for the photocatalysed degradation of orange II [40] and pararosaniline [41] at pH p1.5. At this pH, the dye completely gets adsorbed on the catalyst surface that blocks the irradiation and prevents the photoexcitation of the catalyst. During the photocatalytic processes when the reactant molecule is strongly bonded to surface atoms, the photocatalytic site becomes inactive and said to be poisoned i.e., no photocatalysis is possible [42]. 4. Conclusions The visible light-induced photocatalytic degradation of a textile dye, acid red 88 over TiO2 in the presence and absence of PMS and PDS indicated that the addition of PMS increases the photodegradation rate by 2.7 times. The results of the variation of initial dye concentration with and without oxidants (PMS/PDS) showed that the factor Coxidant/Cdye plays a significant role in the photodegradation reaction rate. A rapid decolorisation and TOC removal rates were observed with PMS indicating PMS as more efficient oxidant than PDS for the photocatalysed degradation of AR88. At pH 1.5 no photocatalysis was observed due to the poisoning of the photocatalyst surface. Acknowledgements The financial support received from the CSIR, New Delhi, in the form of senior research fellowship is duly acknowledged by one of the authors, J. Madhavan. References [1] A. Reife, H. Freeman, Environmental Chemistry of Dyes and Pigments, Wiley/Interscience, New York, 1996. [2] M. Hoffmann, M. Martin, W. Choi, D. Bahnemann, Chem. Rev. 95 (1995) 69. [3] M.R. Dhananjeyan, E. Fine, J. Kiwi, J. Photochem. Photobiol. A 136 (2000) 125. [4] D.F. Ollis, H. Al-Ekabi (Eds.), Photocatalytic Purification and Treatment of Water and Air, Elsevier, Amsterdam, 1993. [5] N. Serpone, E. Pelizzetti (Eds.), Photocatalysis, Fundamentals and Applications, Wiley, New York, 1989. [6] M. Schiavello, Photocatalysis and Environment, Kluwer, Dordecht, 1988. [7] R. Andreozzi, V. Caprio, A. Insola, R. Marotta, Catal. Today 53 (1999) 51. [8] J.M. Herrmann, Catal. Today 53 (1999) 115. [9] J. Cunningham, G. Al-Sayyed, P. Sedlak, J. Caffrey, Catal. Today 53 (1999) 145 and the references cited therein. [10] A. Mills, R.H. Davies, D. Worsley, Chem. Soc. Rev. (1993) 417. [11] O. Legrini, E. Oliveros, A.M. Braun, Chem. Rev. 93 (1993) 671. [12] A. Fujishima, T.N. Rao, D.A. Tryk, J. PhotoChem. Photobiol. C 1 (2000) 1. [13] T. Wu, T. Lin, J. Zhao, H. Hidaka, N. Serpone, Environ. Sci. Technol. 33 (1999) 1379. [14] H. Al-Ekabi, B. Butters, D. Delany, J. Ireland, N. Lewis, T. Powell, J. Story, in: D.F. Ollis, H. Al-Ekabi (Eds.), Photocatalytic Purification and Treatment of Water and Air, 1993, p. 321. [15] A.B. Prevot, C. Baiocchi, M.C. Brussino, E. Pramauro, P. Savarino, V. Augugliaro, G. Marci, L. Palmisano, Environ. Sci. Technol. 35 (2001) 971. [16] V. Augugliaro, C. Baiocchi, A.B. Prevot, E. Garcia-Lopez, V. Loddo, S. Malato, G. Marci, L. Palmisano, M. Pazzi, E. Pramauro, Chemosphere 49 (2002) 1223. [17] M. Saquib, M. Muneer, Dyes Pigments 53 (2002) 237. [18] M. Muruganandham, M. Swaminathan, Sol. Energy Mater. Sol. Cells 81 (2004) 439.
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