- Email: [email protected]
Removal of methylene blue in a photocatalytic reactor using polymethylmethacrylate supported TiO2 nanofilm
Desalination 211 (2007) 1–9
Removal of methylene blue in a photocatalytic reactor using polymethylmethacrylate supported TiO2 nanofilm L. Rizzoa*, J. Kochb, V. Belgiornoa, M.A. Andersonb a
University of Salerno, Department of Civil Engineering, Fisciano (SA), Italy Tel. +39 (089) 96-4076; Fax +39 (089) 96-4100; email: [email protected] b University of Wisconsin-Madison, Environ. Chem. & Tech. Prog. Madison, WI, USA
Received 11 November 2005; revised 28 February 2006; accepted 28 February 2006
Abstract In this study, methylene blue (MB) was chosen as a model dye to test a novel photocatalytic reactor using titanium dioxide (TiO2) nanofilm. TiO2 photocatalysis is a light-activated process that has been successfully applied to remove organic and inorganic pollutants from water and wastewater. The reactor, made of stacked polymethylmethacrylate (PMMA) rings coated with a thin-film of TiO2 produced using a sol–gel method, operated in cycles from 2 to 77 h in a recirculation mode (by means of a pump and 2 L reservoir) under various conditions (with/without catalyst, with/without UV radiation, different pH conditions, after reactor reactivation by UV radiation). The MB removal efficiency was evaluated using UV absorbance at 664 nm. MB removal efficiency results (80% after 10 min at pH 9) show that TiO2 photocatalysis provides a promising technology to improve the quality of effluent from textile wastewater treatment plants. However, fouling and reactivation of photocatalyst are issues to be considered in order to evaluate the possibility in using the photocatalytic process for wastewater treatment. Keywords: Photocatalysis; Immobilized TiO2 nanofilm; Methylene blue; Polymethylmethacrylate reactor; UV activation of TiO2
1. Introduction The presence of organic dyes in textile wastewaters may result in poor water quality if proper *Corresponding author.
treatment methods are not applied. Due to the synthetic nature of organic dyes, biological treatment of wastewater alone is usually not effective for the removal of these chemical species. To meet increasingly stringent regulations, wastewater
Presented at the 9th Environmental Science and Technology Symposium, September 1–3, 2005, Rhodes, Greece. Organized by the Global NEST organization and prepared with the editorial help of the University of Aegean, Mytilene, Greece and the University of Salerno, Fisciano (SA), Italy. 0011-9164/07/$– See front matter © 2007 Published by Elsevier B.V. doi:10.1016/j.desal.2006.02.081
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L. Rizzo et al. / Desalination 211 (2007) 1–9
treatment plants have applied additional processes, like coagulation, membranes, or adsorption for the removal of these contaminants [1–3]. However, these processes simply transfer pollutants from their water matrix into biosolids, rather than completely eliminating these chemicals. For this reason, photocatalysis using titanium dioxide (TiO2) as the semiconducting photocatalyst has been studied as an alternative to conventional processes. Heterogeneous photocatalysis is a process by which the irradiation of a metal oxide semiconductor produces photo-excited electrons (e) and positively charged holes (h+). The photo-excitation of semiconductor particles, by means of light with a higher energy than the electronic band gap energy of the semiconductor, generates excess electrons in the conduction band (ecb) and an electron vacancy in the valence band (h+vb). Although several semiconductors exist, TiO2 is the most widely used catalyst, mainly because of its photostability, non-toxicity, low cost and water insolubility under most environmental conditions [4]. In recent years, TiO2 photocatalysis has been successfully applied to remove organic and inorganic pollutants [5], to inactivate microorganisms [6], and to control disinfection by-product formation [7]. The most commonly proposed mechanism for the mineralization of most organic pollutants is the following [8,9]: 1. Absorption of efficient photons TiO2 + hν → ecb + h+vb; 2. Oxygen adsorption leading to O2•– free radicals (O2)ads + ecb → O2•–; 3. Formation of OH• radical by photo-holes (H2O ↔ H+ + OH–)ads + h+vb → H+ + OH•; 4. Oxidation of organic (R) by OH• radical or holes R + OH• → R’• + H2O; R + h+ → R•+ → degradation products Although TiO2 photocatalysis was found to be effective for the destruction of a wide variety of
environmental contaminants present in water and wastewater, this technology has not yet been successfully commercialized in part because of the costs and problems connected to the separation of TiO2 particles from the suspension after treatment. In order to solve this problem, supported photocatalysts have been developed [10–12]; in particular, titania powder has been immobilized on supports transparent to UV radiation. Unfortunately, the surface area of active catalyst exposed to solution is lower in supported systems than in suspended systems, reducing the catalytic activity. In this study, methylene blue (MB) was chosen as a model dye to test a novel photocatalytic reactor. The reactor, containing polymethylmethacrylate waveguides coated with TiO2 nanofilm produced using a sol–gel method, operated in cycles from 2 to 77 hours in a recirculation mode, under various conditions (with/without catalyst, with/without UV radiation, different pH conditions, after reactor reactivation by UV radiation). MB degradation was tracked by measuring UV absorbance. We also measured pH, dissolved oxygen, and temperature. 2. Materials and methods 2.1. Materials The methylene blue (MB) from Alfa Aesar was used as a model dye. We added 20 mg of MB to 3 L of deionized water to reach a concentration of 18 µM/L. 2.2. TiO2 sol preparation The active TiO2 nanofilm (5 nm average diameter of TiO2 particles) was prepared by a sol– gel method [13]. Briefly, Ti(IV)-tetraisopropoxide (33 mL) was added to 200 mL of ultrapure water with 1.43 mL of concentrated HNO3 as a catalyst. The mixture was peptized for five days and stored under 4°C, dark conditions. Upon use, the sol was dialyzed using 3500 molecular weight cut off di-
L. Rizzo et al. / Desalination 211 (2007) 1–9
alysis tubing in ultrapure water overnight to pH ~2.5. For coating purposes, 25 mL TiO2 sol was diluted in 75 mL methanol (5.8 g TiO2/L). 2.3. The photocatalytic system An annular photocatalytic reactor (PCR) under recycle mode was used to study the degradation of MB. The system consisted of the PCR, a 1 L open reservoir equipped with a stirrer to ensure complete mixing, and a peristaltic pump to force the solution from the reservoir to the reactor (Fig. 1). The PCR was made of 35 stacked polymethylmethacrylate (PMMA) rings to form a reactor height of 24.5 cm. PMMA was choose because this kind of plastic is UV transparent and easy moldable. Each PMMA ring had an inner diameter of 33 mm, an outer diameter of 70 mm, and a height of 7 mm. Water was divided among the 35 rings and forced to follow a fixed route inside each ring before leaving the reactor, resulting in increased irradiation time. The UV lamp (335 nm), housed vertically in the inner reactor center, had an intensity of 3.0 mW/cm2 at the inner ring diameter and 1.1 mW/cm2 at the outer ring diameter. All experiments were performed at room temperature.
3
2.4. Reactor coating PMMA rings were ultrasonically spray-coated with TiO2 sol; 25 ml TiO2 sol was diluted in 75 ml ethanol and sprayed at a constant rate of 76.2 cm/min to achieve a spray coating of 0.011 ml/cm2. Then the rings were air-dried for 2 h. Finally they were put in the oven at around 70°C for 45 min. 2.5. Reactivation of the reactor by UV radiation PMMA rings cannot be heated past 80°C, but the TiO2 photocatalyst must be reactivated over time. In this study, the TiO2 coated PMMA rings were reactivated with UV light 6 months later the PMMA rings were spray-coated with TiO2 sol. In particular, the rings were put in a black box and left under radiation of full-spectrum UV lamp for 45 min. 2.6. Analysis Absorbance measurements were performed using a UV-Vis spectrophotometer [Hewlett Packard 8452A and Shimadzu Corporation UV2401 (PC)]. Dissolved oxygen and temperature (YSI 550A), as well as pH (Fisher Scientific Accumet 50) were monitored during the experiments. Irradiation intensity was measured using a photometer (IL 1400A). 3. Results 3.1. MB photocatalytic degradation and absorbance behavior
Fig. 1. Schematic diagram of photocatalytic system.
Photocatalytic destruction of the MB solution was measured using UV absorbance at 664 nm (UV664). The 8-h photocatalytic experiment at pH 7 without adjustment resulted in reduction of UV664 as shown in Fig. 2. The UV664 decreased by 24, 40, 48 and 70% after 1, 2, 3, and 8 h, respectively. During this experiment, the absorbance peak shifted from 664 nm to 654 nm after 480 min as shown in Figs. 2 and 3. The reduction in absorbance is likely due to the degradation of the MB
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L. Rizzo et al. / Desalination 211 (2007) 1–9 1.4 0h
absorbance (1/cm)
1.2 1 0.8 0.6 0.4
8h 0.2 0 200
300
400
500
600
700
wavelength (nm)
Fig. 2. MB absorbance spectra resulting from 8-h photocatalytic experiment.
1.4
664
662 1.2
abs at 664 nm max abs
660
1 656 0.8
654
0.6 0.4 0.2 0 0
100
200
300
400
500
time (min.)
Fig. 3. Absorbance peak shift during 8-h photocatalytic experiment.
chromophore, and the peak shift is due to de-methylation occurring at the catalyst surface [14]. 3.2. Influence of pH during MB photocatalytic degradation Since pH affects both the surface charge of the TiO2 particles comprising the thin-film sup-
ported on the PMMA waveguides, and the ionization state of ionizable organic molecules, the pH of the solution is an important variable in this process. By adjusting the pH of the solution, one can significantly improve upon the photocatalytic removal efficiency due to increased adsorption of the MB. The photocatalyst surface becomes negatively charged for pH values higher than the point
L. Rizzo et al. / Desalination 211 (2007) 1–9
of zero charge (pzc) and positively charged for pH values lower than the pzc, according to the following equilibria [15]:
pH < pzc: Ti-OH + H + ⇔ TiOH +2
(1)
pH > pzc: Ti-OH + OH − ⇔ TiO− + H 2 O
(2)
Since MB is a cationic dye, increased MB removal for higher pH values was expected. In a second experiment, the system operated continuously for 77 h under different conditions. The temperature was almost constant during the experiments (23–24°C), while the dissolved oxygen concentration ranged from 6.0 to 6.9 mg/L (71–82% saturation). The lowest value was recorded at pH 9 (10 min after pH adjustment), which corresponded to the highest MB removal rate. Before the 77-h experiment, the reactor operated with uncoated waveguides and the UV light on. During this test, no variation in UV absorbance was detected. Next, the TiO2 thin-film was immobilized on the waveguides and the reactor operated with the UV light off for 24 h. During this time, no significant variation in UV664 was observed (Fig. 4). Thus, the removal of MB detected after the lamp was turned on (pH 7) can
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only be a result of photocatalysis and not a result of adsorption on the TiO2 surface. After 38 and 72 h, the pH was adjusted to 4 and 9, respectively. The MB removal was completely inhibited at pH 4. Conversely, at pH 9, an almost instantaneous decrease in UV664 peak indicated removal up to 80% in the first 10 min. This represents a higher removal efficiency than reported in previous studies with MB utilizing both UV irradiated aqueous suspensions of TiO2 [16] and in a photocatalytic reactor using immobilized TiO2 films [14]. According to the stoichiometry of the overall oxidation reaction [Eq. (3)] the pH of the solution is expected to decrease, although pH value depends on several conditions: (i) water dissociation equilibrium, (ii) the surface charge of titania with respect to its pzc [Eqs. (1) and (2)] and (iii) the ionization state of the organic reactants and of their metabolites [16]. C16 H18 N 3S+ + 51/ 2O 2 → 16CO 2 + 3NO3− + 6H + + 6H 2 O
Compared to the pH 9 solution, the pH 7 solution showed lower removal efficiency, likely because this pH value is closer to the pH of the pzc.
1.6 UV on (pH 7)
1.4
UV664 (1/cm)
1.2 1 0.8 pH 4
0.6
pH 9
0.4 0.2 0 10
(3)
100
1000 time (min.)
Fig. 4. Behavior of UV664 in the different experimental conditions.
10000
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No significant pH variation was detected in pH 4 experiment, as compared to the pH 9 experiment where the final pH value (after 180 min) was 6.9 (Fig. 5). In order to evaluate the contribution of the adsorption mechanism in UV664 reduction at pH 9, a test was performed using an initial concentration of 1.4 mg MB/L. In the first 60 min, the
reactor was operated without UV light and 15% UV664 removal was detected; then, with the UV light turned on, after 60 min UV664 decreased by 40% (Fig. 6). However, 35% was removed in the first 10 min showing that the photocatalytic oxidation of MB mainly occurs early, while adsorption develops more gradually.
UV664 removal (%)
100 90 80
8.8
70 60
7.4
7.8
8.6
6.9
50 40
6.9
30 20 10
6.9
6.9 4.1
0
4.4
4.4
0
50
100
150
200
time (min.) initial pH 7
initial pH 4
initial pH 9
Fig. 5. Initial pH influence on UV664 removal.
45 UV on
40 UV664 removal (%)
35 30 25 20 15
UV off
10 5 0 0
10
20
30 time (min)
Fig. 6. Evaluation of adsorption contribution in UV664 removal at pH 9.
40
50
60
L. Rizzo et al. / Desalination 211 (2007) 1–9
3.3. Effect of UV reactivation Since 6 months after the first set of experiments the photocatlytic system was not able to remove MB, the same reactor was reactivated according to the procedure described in the “Materials and methods” section. The UV reactivated reactor was less effective in removing MB compared to the performances obtained by the same reactor immediately after the first activation at low temperature (Fig. 7). At pH 7, only 8% was removed after 60 min, and 18% after 180 min in the reactivated test. In the first two experiments without UV reactivation 24%/16%, and 48%/34% were removed after 60 min and 180 min respectively. The decrease in MB removal is likely due to the progressive fouling of the catalyst rather than the activation process (either thermal or UV radiation). Indeed, after the first test (no UV activation_1, Fig. 7) the reactor appeared bluecoloured due to MB adsorption; the blue colour of the reactor was stronger at the end of the second test (no UV activation_2, Fig. 7), thus less sites were available for MB adsorption in the third test (after UV activation, Fig. 7). 3.4. Discussion In this study we showed that positive results
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were achieved in MB removal using an immobilized TiO2 nanofilm photoreactor. The MB removal amounted to 24 and 48% after 1 and 3 h, respectively at pH 7, and 80% after 10 min at pH 9. The MB removal was due to both photocatalytic degradation and adsorption. In particular, due to the cationic nature of MB, the adsorption mechanism provided significant contribution at pH 9 (higher than the pzc of TiO2), when the titania surface is negatively charged. However, experimental tests showed that the efficiency may significantly decrease in long term use due to both fouling of catalyst and reducing of catalytic activity. The exposure of the reactor to UV radiation may be useful to both mineralize organic compounds adsorbed on the catalyst and to activate the catalyst. The results obtained using the photoreactor after UV radiation exposure suggest that longer exposure time (>45 min) may improve both fouling removal and catalyst activation. The catalyst activation by means of UV radiation should be an effective alternative for materials where calcination cannot be applied (plastic based reactors). However, further experiments need to compare photocatalytic efficiency according to TiO2 activation by thermal (low temperature) and UV methods. Finally, the efficiency of the photoreactor should be improved using higher intensity lamp;
60 no UV activation_1 no UV activation_2 after UV activation
UV664 removal (%)
50 40 30 20 10 0 0
50
100
150
200
irradiation time (min)
Fig. 7. Reduction in adsorption contribution: comparison among three different experiments at pH 7.
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Ling and co-workers [14] found that 50% photodegradation of MB occurred after 15.2 min using 1.5 mW/cm2, whereas 11 min was needed using 5 mW/cm2 of light intensity. In order to evaluate the possibility of using the photocatalytic process for wastewater treatment, the followings point should be evaluated: (i) the flowrate to be treated, (ii) the interference of organic matter and dissolved ionic species in the stream, (iii) the possibility of pH adjustment in order to optimize the removal of specific pollutants, (iv) the need for periodical cleaning because of fouling of catalyst surface, (v) the need for reactor reactivation (particularly if the support material cannot be calcined), (vi) the costs of energy, chemical (for pH adjustment), reactivation and/or cleaning process. 5. Conclusions This study focused on the evaluation of methylene blue (MB) removal by means of a novel photocatalytic reactor made of stacked polymethylmethacrylate (PMMA) rings coated with a thin-film of TiO2 prepared using a sol–gel method. The system operated in a recirculation mode under various conditions (with/without catalyst, with/without UV radiation, different pH conditions, after reactor reactivation by UV radiation). Changes in MB concentration (UV664 peak area) at pH 7 amounted to 24 and 48% after 1 and 3 h, respectively. At pH 9, removal efficiency reached 80% after 10 min. The increased removal at pH 9 is due to the photocatalyst surface becoming negatively charged at pH values higher than the point of zero charge, coupled with the cationic charge of MB at these pH values. These results show that TiO2 photocatalysis provides a promising technology to improve the quality of effluent from textile wastewater treatment plants. However, fouling and reactivation of the photocatalyst are issues to be considered in order to evaluate the possibility of using photocatalysis for wastewater treatment.
Acknowlegments The authors are grateful to Jeffrey Brownson for his valuable advices regarding photocatalyst UV reactivation, and Timothy Lee, Joanna Skluzacek, and Kenneth Walz, for technical assistance. References [1] D. Georgiou, A. Aivazidis, J. Hatiras and K. Gimouhopoulos, Treatment of cotton textile wastewater using lime and ferrous sulfate, Wat. Res., 37(9) (2003) 2248–2250. [2] S.P. Petrova and P.A. Stoychev, Ultrafiltration purification of waters contaminated with bifunctional reactive dyes, Desalination, 154 (2003) 247–252. [3] P.C.C. Faria, J.J.M. Órfão and M.F.R. Pereira, Adsorption of anionic and cationic dyes on activated carbons with different surface chemistries, Wat. Res., 38(8) (2004) 2043–2052. [4] P.A. Carneiro, M.E. Osugi, J.J. Sene, M.A. Anderson and M.V.B. Zanoni, Evaluation of color removal and degradation of a reactive textile azo dye on nanoporous TiO2 thin-film electrodes, Electrochim. Acta, 49 (2004) 3807–3820. [5] M.A. Aguado, M.A. Anderson and C.G. Hill, Jr., Influence of light intensity and membrane properties on the photocatalytic degradation of formic acid over TiO2 ceramic membranes, J. Molec. Cataly., 89 (1–2) (1994) 165–178. [6] A.G. Rincón and C. Pulgarin, Effect of pH, inorganic ions, organic matter and H2O2 on E. coli K12 photocatalytic inactivation by TiO2: Implications in solar water disinfection, Appl. Catal. B: Environ, 51(4) (2004) 283–302. [7] M. Bekbolet, C.S. Uyguner, H. Selcuk, L. Rizzo, A.D. Nikolaou, S. Meric and V. Belgiorno, Application of oxidative removal of NOM to drinking water and formation of disinfection by-products, Desalination, 176 (2005) 155–166. [8] A. Houas, H. Lachheb, M. Ksibi, E. Elaloui, C. Guillard and J.M. Hermann, Photocatalytic degradation pathway of methylene blue in water, Appl. Catal. B: Environ., 31 (2001) 145–157. [9] A. Özkan, M.H. Özkan, R. Gürkan, M. Akçay and M. Sökmen, Photocatalytic degradation of a textile azo dye, Sirius Gelb GC on TiO2 or Ag-TiO2 particles in the absence and presence of UV irradiation: the effects of some inorganic anions on the
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[10]
[11]
[12]
[13]
photocatalysis, J. Photochem. Photob. A: Chem., 163 (2004) 29–35. G. Chester, M. Anderson, H. Read and S. Esplugas, A jacketed annular membrane photocatalytic reactor for wastewater treatment: degradation of formic acid and atrazine. J. Photochem. Photobiol. A: Chem., 71 (1993) 291–297. R. Franke and C. Franke, Model reactor for photocatalytic degradation of persistent chemicals in ponds and waste water, Chemosphere, 39(15) (1999) 2651–2659. C.M. Ling, A.R. Mohamed and S. Bhatia, Performance of photocatalytic reactors using immobilized TiO2 film for the degradation of phenol and methylene blue dye present in water stream, Chemosphere, 57 (2004) 547–554. M.A. Anderson, M.J. Gieselmann and X. Qunyin, Titania and alumina ceramic membranes, J. Membr. Sci., 39 (1988) 243–258.
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[14] S. Matsuo, N. Sakaguchi, K. Yamada, T. Matsuo and H. Wakita, Role in photocatalysis and coordination structure of metal ions adsorbed on titanium dioxide particles: a comparison between lanthanide and iron ions, Appl. Surf. Sci., 228 (2004) 233–244. [15] C. Guillard, H. Lachheb, A. Houas, M. Ksibi, E. Elaloui and J.M. Hermann, Influence of chemical structure of dyes, of pH and of inorganic salts on their photocatalytic degradation by TiO2 comparison of the efficiency of powder and supported TiO2, J. Photochem. Photob. A: Chem., 158 (2003) 27– 36. [16] H. Lachheb, E. Puzenat, A. Houas, M. Ksibi, E. Elaloui, C. Guillard and J.M. Hermann, Photocatalytic degradation of various types of dyes (alizarin S, crocein orange G, methyl red, congo red, methylene blue) in water by UV-irradiated titania, Appl. Catal. B: Environ., 39 (2002) 75–90.
Removal of methylene blue in a photocatalytic reactor using polymethylmethacrylate supported TiO2 nanofilm L. Rizzoa*, J. Kochb, V. Belgiornoa, M.A. Andersonb a
University of Salerno, Department of Civil Engineering, Fisciano (SA), Italy Tel. +39 (089) 96-4076; Fax +39 (089) 96-4100; email: [email protected] b University of Wisconsin-Madison, Environ. Chem. & Tech. Prog. Madison, WI, USA
Received 11 November 2005; revised 28 February 2006; accepted 28 February 2006
Abstract In this study, methylene blue (MB) was chosen as a model dye to test a novel photocatalytic reactor using titanium dioxide (TiO2) nanofilm. TiO2 photocatalysis is a light-activated process that has been successfully applied to remove organic and inorganic pollutants from water and wastewater. The reactor, made of stacked polymethylmethacrylate (PMMA) rings coated with a thin-film of TiO2 produced using a sol–gel method, operated in cycles from 2 to 77 h in a recirculation mode (by means of a pump and 2 L reservoir) under various conditions (with/without catalyst, with/without UV radiation, different pH conditions, after reactor reactivation by UV radiation). The MB removal efficiency was evaluated using UV absorbance at 664 nm. MB removal efficiency results (80% after 10 min at pH 9) show that TiO2 photocatalysis provides a promising technology to improve the quality of effluent from textile wastewater treatment plants. However, fouling and reactivation of photocatalyst are issues to be considered in order to evaluate the possibility in using the photocatalytic process for wastewater treatment. Keywords: Photocatalysis; Immobilized TiO2 nanofilm; Methylene blue; Polymethylmethacrylate reactor; UV activation of TiO2
1. Introduction The presence of organic dyes in textile wastewaters may result in poor water quality if proper *Corresponding author.
treatment methods are not applied. Due to the synthetic nature of organic dyes, biological treatment of wastewater alone is usually not effective for the removal of these chemical species. To meet increasingly stringent regulations, wastewater
Presented at the 9th Environmental Science and Technology Symposium, September 1–3, 2005, Rhodes, Greece. Organized by the Global NEST organization and prepared with the editorial help of the University of Aegean, Mytilene, Greece and the University of Salerno, Fisciano (SA), Italy. 0011-9164/07/$– See front matter © 2007 Published by Elsevier B.V. doi:10.1016/j.desal.2006.02.081
2
L. Rizzo et al. / Desalination 211 (2007) 1–9
treatment plants have applied additional processes, like coagulation, membranes, or adsorption for the removal of these contaminants [1–3]. However, these processes simply transfer pollutants from their water matrix into biosolids, rather than completely eliminating these chemicals. For this reason, photocatalysis using titanium dioxide (TiO2) as the semiconducting photocatalyst has been studied as an alternative to conventional processes. Heterogeneous photocatalysis is a process by which the irradiation of a metal oxide semiconductor produces photo-excited electrons (e) and positively charged holes (h+). The photo-excitation of semiconductor particles, by means of light with a higher energy than the electronic band gap energy of the semiconductor, generates excess electrons in the conduction band (ecb) and an electron vacancy in the valence band (h+vb). Although several semiconductors exist, TiO2 is the most widely used catalyst, mainly because of its photostability, non-toxicity, low cost and water insolubility under most environmental conditions [4]. In recent years, TiO2 photocatalysis has been successfully applied to remove organic and inorganic pollutants [5], to inactivate microorganisms [6], and to control disinfection by-product formation [7]. The most commonly proposed mechanism for the mineralization of most organic pollutants is the following [8,9]: 1. Absorption of efficient photons TiO2 + hν → ecb + h+vb; 2. Oxygen adsorption leading to O2•– free radicals (O2)ads + ecb → O2•–; 3. Formation of OH• radical by photo-holes (H2O ↔ H+ + OH–)ads + h+vb → H+ + OH•; 4. Oxidation of organic (R) by OH• radical or holes R + OH• → R’• + H2O; R + h+ → R•+ → degradation products Although TiO2 photocatalysis was found to be effective for the destruction of a wide variety of
environmental contaminants present in water and wastewater, this technology has not yet been successfully commercialized in part because of the costs and problems connected to the separation of TiO2 particles from the suspension after treatment. In order to solve this problem, supported photocatalysts have been developed [10–12]; in particular, titania powder has been immobilized on supports transparent to UV radiation. Unfortunately, the surface area of active catalyst exposed to solution is lower in supported systems than in suspended systems, reducing the catalytic activity. In this study, methylene blue (MB) was chosen as a model dye to test a novel photocatalytic reactor. The reactor, containing polymethylmethacrylate waveguides coated with TiO2 nanofilm produced using a sol–gel method, operated in cycles from 2 to 77 hours in a recirculation mode, under various conditions (with/without catalyst, with/without UV radiation, different pH conditions, after reactor reactivation by UV radiation). MB degradation was tracked by measuring UV absorbance. We also measured pH, dissolved oxygen, and temperature. 2. Materials and methods 2.1. Materials The methylene blue (MB) from Alfa Aesar was used as a model dye. We added 20 mg of MB to 3 L of deionized water to reach a concentration of 18 µM/L. 2.2. TiO2 sol preparation The active TiO2 nanofilm (5 nm average diameter of TiO2 particles) was prepared by a sol– gel method [13]. Briefly, Ti(IV)-tetraisopropoxide (33 mL) was added to 200 mL of ultrapure water with 1.43 mL of concentrated HNO3 as a catalyst. The mixture was peptized for five days and stored under 4°C, dark conditions. Upon use, the sol was dialyzed using 3500 molecular weight cut off di-
L. Rizzo et al. / Desalination 211 (2007) 1–9
alysis tubing in ultrapure water overnight to pH ~2.5. For coating purposes, 25 mL TiO2 sol was diluted in 75 mL methanol (5.8 g TiO2/L). 2.3. The photocatalytic system An annular photocatalytic reactor (PCR) under recycle mode was used to study the degradation of MB. The system consisted of the PCR, a 1 L open reservoir equipped with a stirrer to ensure complete mixing, and a peristaltic pump to force the solution from the reservoir to the reactor (Fig. 1). The PCR was made of 35 stacked polymethylmethacrylate (PMMA) rings to form a reactor height of 24.5 cm. PMMA was choose because this kind of plastic is UV transparent and easy moldable. Each PMMA ring had an inner diameter of 33 mm, an outer diameter of 70 mm, and a height of 7 mm. Water was divided among the 35 rings and forced to follow a fixed route inside each ring before leaving the reactor, resulting in increased irradiation time. The UV lamp (335 nm), housed vertically in the inner reactor center, had an intensity of 3.0 mW/cm2 at the inner ring diameter and 1.1 mW/cm2 at the outer ring diameter. All experiments were performed at room temperature.
3
2.4. Reactor coating PMMA rings were ultrasonically spray-coated with TiO2 sol; 25 ml TiO2 sol was diluted in 75 ml ethanol and sprayed at a constant rate of 76.2 cm/min to achieve a spray coating of 0.011 ml/cm2. Then the rings were air-dried for 2 h. Finally they were put in the oven at around 70°C for 45 min. 2.5. Reactivation of the reactor by UV radiation PMMA rings cannot be heated past 80°C, but the TiO2 photocatalyst must be reactivated over time. In this study, the TiO2 coated PMMA rings were reactivated with UV light 6 months later the PMMA rings were spray-coated with TiO2 sol. In particular, the rings were put in a black box and left under radiation of full-spectrum UV lamp for 45 min. 2.6. Analysis Absorbance measurements were performed using a UV-Vis spectrophotometer [Hewlett Packard 8452A and Shimadzu Corporation UV2401 (PC)]. Dissolved oxygen and temperature (YSI 550A), as well as pH (Fisher Scientific Accumet 50) were monitored during the experiments. Irradiation intensity was measured using a photometer (IL 1400A). 3. Results 3.1. MB photocatalytic degradation and absorbance behavior
Fig. 1. Schematic diagram of photocatalytic system.
Photocatalytic destruction of the MB solution was measured using UV absorbance at 664 nm (UV664). The 8-h photocatalytic experiment at pH 7 without adjustment resulted in reduction of UV664 as shown in Fig. 2. The UV664 decreased by 24, 40, 48 and 70% after 1, 2, 3, and 8 h, respectively. During this experiment, the absorbance peak shifted from 664 nm to 654 nm after 480 min as shown in Figs. 2 and 3. The reduction in absorbance is likely due to the degradation of the MB
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L. Rizzo et al. / Desalination 211 (2007) 1–9 1.4 0h
absorbance (1/cm)
1.2 1 0.8 0.6 0.4
8h 0.2 0 200
300
400
500
600
700
wavelength (nm)
Fig. 2. MB absorbance spectra resulting from 8-h photocatalytic experiment.
1.4
664
662 1.2
abs at 664 nm max abs
660
1 656 0.8
654
0.6 0.4 0.2 0 0
100
200
300
400
500
time (min.)
Fig. 3. Absorbance peak shift during 8-h photocatalytic experiment.
chromophore, and the peak shift is due to de-methylation occurring at the catalyst surface [14]. 3.2. Influence of pH during MB photocatalytic degradation Since pH affects both the surface charge of the TiO2 particles comprising the thin-film sup-
ported on the PMMA waveguides, and the ionization state of ionizable organic molecules, the pH of the solution is an important variable in this process. By adjusting the pH of the solution, one can significantly improve upon the photocatalytic removal efficiency due to increased adsorption of the MB. The photocatalyst surface becomes negatively charged for pH values higher than the point
L. Rizzo et al. / Desalination 211 (2007) 1–9
of zero charge (pzc) and positively charged for pH values lower than the pzc, according to the following equilibria [15]:
pH < pzc: Ti-OH + H + ⇔ TiOH +2
(1)
pH > pzc: Ti-OH + OH − ⇔ TiO− + H 2 O
(2)
Since MB is a cationic dye, increased MB removal for higher pH values was expected. In a second experiment, the system operated continuously for 77 h under different conditions. The temperature was almost constant during the experiments (23–24°C), while the dissolved oxygen concentration ranged from 6.0 to 6.9 mg/L (71–82% saturation). The lowest value was recorded at pH 9 (10 min after pH adjustment), which corresponded to the highest MB removal rate. Before the 77-h experiment, the reactor operated with uncoated waveguides and the UV light on. During this test, no variation in UV absorbance was detected. Next, the TiO2 thin-film was immobilized on the waveguides and the reactor operated with the UV light off for 24 h. During this time, no significant variation in UV664 was observed (Fig. 4). Thus, the removal of MB detected after the lamp was turned on (pH 7) can
5
only be a result of photocatalysis and not a result of adsorption on the TiO2 surface. After 38 and 72 h, the pH was adjusted to 4 and 9, respectively. The MB removal was completely inhibited at pH 4. Conversely, at pH 9, an almost instantaneous decrease in UV664 peak indicated removal up to 80% in the first 10 min. This represents a higher removal efficiency than reported in previous studies with MB utilizing both UV irradiated aqueous suspensions of TiO2 [16] and in a photocatalytic reactor using immobilized TiO2 films [14]. According to the stoichiometry of the overall oxidation reaction [Eq. (3)] the pH of the solution is expected to decrease, although pH value depends on several conditions: (i) water dissociation equilibrium, (ii) the surface charge of titania with respect to its pzc [Eqs. (1) and (2)] and (iii) the ionization state of the organic reactants and of their metabolites [16]. C16 H18 N 3S+ + 51/ 2O 2 → 16CO 2 + 3NO3− + 6H + + 6H 2 O
Compared to the pH 9 solution, the pH 7 solution showed lower removal efficiency, likely because this pH value is closer to the pH of the pzc.
1.6 UV on (pH 7)
1.4
UV664 (1/cm)
1.2 1 0.8 pH 4
0.6
pH 9
0.4 0.2 0 10
(3)
100
1000 time (min.)
Fig. 4. Behavior of UV664 in the different experimental conditions.
10000
6
L. Rizzo et al. / Desalination 211 (2007) 1–9
No significant pH variation was detected in pH 4 experiment, as compared to the pH 9 experiment where the final pH value (after 180 min) was 6.9 (Fig. 5). In order to evaluate the contribution of the adsorption mechanism in UV664 reduction at pH 9, a test was performed using an initial concentration of 1.4 mg MB/L. In the first 60 min, the
reactor was operated without UV light and 15% UV664 removal was detected; then, with the UV light turned on, after 60 min UV664 decreased by 40% (Fig. 6). However, 35% was removed in the first 10 min showing that the photocatalytic oxidation of MB mainly occurs early, while adsorption develops more gradually.
UV664 removal (%)
100 90 80
8.8
70 60
7.4
7.8
8.6
6.9
50 40
6.9
30 20 10
6.9
6.9 4.1
0
4.4
4.4
0
50
100
150
200
time (min.) initial pH 7
initial pH 4
initial pH 9
Fig. 5. Initial pH influence on UV664 removal.
45 UV on
40 UV664 removal (%)
35 30 25 20 15
UV off
10 5 0 0
10
20
30 time (min)
Fig. 6. Evaluation of adsorption contribution in UV664 removal at pH 9.
40
50
60
L. Rizzo et al. / Desalination 211 (2007) 1–9
3.3. Effect of UV reactivation Since 6 months after the first set of experiments the photocatlytic system was not able to remove MB, the same reactor was reactivated according to the procedure described in the “Materials and methods” section. The UV reactivated reactor was less effective in removing MB compared to the performances obtained by the same reactor immediately after the first activation at low temperature (Fig. 7). At pH 7, only 8% was removed after 60 min, and 18% after 180 min in the reactivated test. In the first two experiments without UV reactivation 24%/16%, and 48%/34% were removed after 60 min and 180 min respectively. The decrease in MB removal is likely due to the progressive fouling of the catalyst rather than the activation process (either thermal or UV radiation). Indeed, after the first test (no UV activation_1, Fig. 7) the reactor appeared bluecoloured due to MB adsorption; the blue colour of the reactor was stronger at the end of the second test (no UV activation_2, Fig. 7), thus less sites were available for MB adsorption in the third test (after UV activation, Fig. 7). 3.4. Discussion In this study we showed that positive results
7
were achieved in MB removal using an immobilized TiO2 nanofilm photoreactor. The MB removal amounted to 24 and 48% after 1 and 3 h, respectively at pH 7, and 80% after 10 min at pH 9. The MB removal was due to both photocatalytic degradation and adsorption. In particular, due to the cationic nature of MB, the adsorption mechanism provided significant contribution at pH 9 (higher than the pzc of TiO2), when the titania surface is negatively charged. However, experimental tests showed that the efficiency may significantly decrease in long term use due to both fouling of catalyst and reducing of catalytic activity. The exposure of the reactor to UV radiation may be useful to both mineralize organic compounds adsorbed on the catalyst and to activate the catalyst. The results obtained using the photoreactor after UV radiation exposure suggest that longer exposure time (>45 min) may improve both fouling removal and catalyst activation. The catalyst activation by means of UV radiation should be an effective alternative for materials where calcination cannot be applied (plastic based reactors). However, further experiments need to compare photocatalytic efficiency according to TiO2 activation by thermal (low temperature) and UV methods. Finally, the efficiency of the photoreactor should be improved using higher intensity lamp;
60 no UV activation_1 no UV activation_2 after UV activation
UV664 removal (%)
50 40 30 20 10 0 0
50
100
150
200
irradiation time (min)
Fig. 7. Reduction in adsorption contribution: comparison among three different experiments at pH 7.
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Ling and co-workers [14] found that 50% photodegradation of MB occurred after 15.2 min using 1.5 mW/cm2, whereas 11 min was needed using 5 mW/cm2 of light intensity. In order to evaluate the possibility of using the photocatalytic process for wastewater treatment, the followings point should be evaluated: (i) the flowrate to be treated, (ii) the interference of organic matter and dissolved ionic species in the stream, (iii) the possibility of pH adjustment in order to optimize the removal of specific pollutants, (iv) the need for periodical cleaning because of fouling of catalyst surface, (v) the need for reactor reactivation (particularly if the support material cannot be calcined), (vi) the costs of energy, chemical (for pH adjustment), reactivation and/or cleaning process. 5. Conclusions This study focused on the evaluation of methylene blue (MB) removal by means of a novel photocatalytic reactor made of stacked polymethylmethacrylate (PMMA) rings coated with a thin-film of TiO2 prepared using a sol–gel method. The system operated in a recirculation mode under various conditions (with/without catalyst, with/without UV radiation, different pH conditions, after reactor reactivation by UV radiation). Changes in MB concentration (UV664 peak area) at pH 7 amounted to 24 and 48% after 1 and 3 h, respectively. At pH 9, removal efficiency reached 80% after 10 min. The increased removal at pH 9 is due to the photocatalyst surface becoming negatively charged at pH values higher than the point of zero charge, coupled with the cationic charge of MB at these pH values. These results show that TiO2 photocatalysis provides a promising technology to improve the quality of effluent from textile wastewater treatment plants. However, fouling and reactivation of the photocatalyst are issues to be considered in order to evaluate the possibility of using photocatalysis for wastewater treatment.
Acknowlegments The authors are grateful to Jeffrey Brownson for his valuable advices regarding photocatalyst UV reactivation, and Timothy Lee, Joanna Skluzacek, and Kenneth Walz, for technical assistance. References [1] D. Georgiou, A. Aivazidis, J. Hatiras and K. Gimouhopoulos, Treatment of cotton textile wastewater using lime and ferrous sulfate, Wat. Res., 37(9) (2003) 2248–2250. [2] S.P. Petrova and P.A. Stoychev, Ultrafiltration purification of waters contaminated with bifunctional reactive dyes, Desalination, 154 (2003) 247–252. [3] P.C.C. Faria, J.J.M. Órfão and M.F.R. Pereira, Adsorption of anionic and cationic dyes on activated carbons with different surface chemistries, Wat. Res., 38(8) (2004) 2043–2052. [4] P.A. Carneiro, M.E. Osugi, J.J. Sene, M.A. Anderson and M.V.B. Zanoni, Evaluation of color removal and degradation of a reactive textile azo dye on nanoporous TiO2 thin-film electrodes, Electrochim. Acta, 49 (2004) 3807–3820. [5] M.A. Aguado, M.A. Anderson and C.G. Hill, Jr., Influence of light intensity and membrane properties on the photocatalytic degradation of formic acid over TiO2 ceramic membranes, J. Molec. Cataly., 89 (1–2) (1994) 165–178. [6] A.G. Rincón and C. Pulgarin, Effect of pH, inorganic ions, organic matter and H2O2 on E. coli K12 photocatalytic inactivation by TiO2: Implications in solar water disinfection, Appl. Catal. B: Environ, 51(4) (2004) 283–302. [7] M. Bekbolet, C.S. Uyguner, H. Selcuk, L. Rizzo, A.D. Nikolaou, S. Meric and V. Belgiorno, Application of oxidative removal of NOM to drinking water and formation of disinfection by-products, Desalination, 176 (2005) 155–166. [8] A. Houas, H. Lachheb, M. Ksibi, E. Elaloui, C. Guillard and J.M. Hermann, Photocatalytic degradation pathway of methylene blue in water, Appl. Catal. B: Environ., 31 (2001) 145–157. [9] A. Özkan, M.H. Özkan, R. Gürkan, M. Akçay and M. Sökmen, Photocatalytic degradation of a textile azo dye, Sirius Gelb GC on TiO2 or Ag-TiO2 particles in the absence and presence of UV irradiation: the effects of some inorganic anions on the
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[14] S. Matsuo, N. Sakaguchi, K. Yamada, T. Matsuo and H. Wakita, Role in photocatalysis and coordination structure of metal ions adsorbed on titanium dioxide particles: a comparison between lanthanide and iron ions, Appl. Surf. Sci., 228 (2004) 233–244. [15] C. Guillard, H. Lachheb, A. Houas, M. Ksibi, E. Elaloui and J.M. Hermann, Influence of chemical structure of dyes, of pH and of inorganic salts on their photocatalytic degradation by TiO2 comparison of the efficiency of powder and supported TiO2, J. Photochem. Photob. A: Chem., 158 (2003) 27– 36. [16] H. Lachheb, E. Puzenat, A. Houas, M. Ksibi, E. Elaloui, C. Guillard and J.M. Hermann, Photocatalytic degradation of various types of dyes (alizarin S, crocein orange G, methyl red, congo red, methylene blue) in water by UV-irradiated titania, Appl. Catal. B: Environ., 39 (2002) 75–90.