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Pumice stone supported titanium dioxide for removal of pathogen in drinking water and recalcitrant in wastewater
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Solar Energy 82 (2008) 1099–1106 www.elsevier.com/locate/solener
Pumice stone supported titanium dioxide for removal of pathogen in drinking water and recalcitrant in wastewater M. Subrahmanyam a,*, P. Boule b, V. Durga Kumari a, D. Naveen Kumar a, M. Sancelme b, A. Rachel b b
a Catalysis and Physical Chemistry Division, Indian Institute of Chemical Technology, Hyderabad 500007, India Laboratoire de Photochimie Mole´culaire et Macromole´culaire, Universite´ Blaise Pascal CNRS, UMR 6505 (Clermont-Ferrand), F-63177 Aubie`re cedex, France
Received 28 September 2006; accepted 16 May 2008 Available online 17 June 2008 Communicated by: Associate Editor Gion Calzaferri
Abstract Bactericidal and organic degradation effects of TiO2 on pumice stone are described in this paper. Immobilization of TiO2 on pumice stone is easy and efficient method to obtain photocatalytic reactions without the problem of filtration. Pumice stone is soft and available as pellets that can be used in pellets fixed (with cement or poly carbonate) on a slanting plank/glass by coating the preformed TiO2 over the pellets using simple paint brush for applying the photo catalyst. The treatment of inactivation of bacteria especially E. coli existing in real river waters and also different model organic substrate degradations like acid orange-7, resorcinol, 4, 6-dinitro-o-cresol, 4-nitrotoluene-2-sulfonicacid, isoproturan are studied. Furthermore, TiO2 over pumice stone loaded in a multi tube reactor gave similar results for the disinfection and detoxification studies. Ó 2008 Elsevier Ltd. All rights reserved. Keywords: Photocatalysis; Disinfection; Detoxification; TiO2; Pumice stone
1. Introduction Water disinfection is recognized by the WHO as one of the most important challenges for human health protection. The spread of water borne infection is a problem in both developed and underdeveloped countries. Photocatalysis is an advanced oxidation processes (AOP) that has been shown to possess enhanced disinfection capabilities in recent years (Hu et al., 2006; Coleman et al., 2005; Rincon and Pulgarin, 2006; Duffy et al., 2004; Dunlop et al., 2002). A few studies on the durability of photo catalytic disinfection using real water sources have been reported (Gumy et al., 2006; Ljubas, 2005; Robertson et al., 2005).
*
Corresponding author. Tel.: +91 40 27193165; fax: +91 40 27160921. E-mail address: [email protected] (M. Subrahmanyam).
0038-092X/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.solener.2008.05.007
It has been generally accepted that inactivation of microorganisms is mainly due to the oxidative radicals (mainly OH*) produced by irradiated TiO2 (Villen et al., 2006). The effective bactericidal properties of photocatalysis in static and in dynamic way on a diverse range of coli forms over various photocatalysts and with different reactor systems are of recent interest (McLoughlin et al., 2004; Melian et al., 2000). Disinfection rates or inactivation times are usually not comparable from study to study due to a wide range of operational parameters and reactor configuration used in different laboratories (Sagawe et al., 2004). Reactor configurations range from small volume pyrex beakers or petri dishes, typically 1–10 cm3, illuminated from the side or top of a tubular lamp to the modification of commercial disinfection apparatus. To overcome these problems trials were designed to asses the efficiency of using titanium
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M. Subrahmanyam et al. / Solar Energy 82 (2008) 1099–1106
dioxide over pumice in continuous flow disinfection process, which if successful, can be further developed into a low cost method of fixing the TiO2 over support in order to use thin film fixed bed reactor as well as a multi-tube reactor. Research into continuous flow systems for disinfection purposes has been limited with only few experimental trials having been carried out to date (McLoughlin et al., 2004). Many techniques are proposed for the immobilization of TiO2 on solid supports (Noorjahan et al., 2003; Noorjahan et al., 2004). Porous lavas more precisely pumice stone are promising supports of TiO2 when used as a photo catalyst (Subba Rao et al., 2004; Rachel et al., 2002). The present study has been undertaken to provide a practical reactor assessment of photocatalytic treatment of TiO2 supported over pumice stone by coating with a technique giving a TiO2 thin film over pumice stone. It is fixed to a slanting unit and also loaded in a multi tube (four tubes) glass reactor and they are used for inactivation of bacteria in drinking water and degradation of model organics in wastewater. The results generated using the reactors and its potential application for water disinfection and detoxification are studied within the frame-work program of the project sponsored by the Indo-French Center of Promotion of Advanced Research (IFCPAR), New Delhi under the project No: 2205-2. 2. Experimental Two separate reactors were used during the experiments and both are configured as continuous flow re-circulating systems. The reactors were positioned as non-tracking static systems with each in slanting position tilted and the units are aligned to maximize sunlight capture. Photocatalytic reactions were carried out using a TiO2 P-25 of Degussa supported over pumice as presented in Fig. 1. The supported photocatalyst comprises of impregnation of pumice stone pellets with commercially available TiO2 by simply brushing with TiO2 milk or impregnating TiO2 milk with conventional soaking, drying and heat treatment methods. Pumice stone is a soft material and their fixing over the pellets on a hard slanting plank surface is with cement or polycarbonate One of the resulting unit is a TiO2 thin-film containing fixed bed photocatalytic reactor as is shown in Fig. 2. Pellets of pumice stone 2–3 mm and ffi5 mm are kindly provided by Eyraud S. A. (Lyon, France). Furthermore, the unit also used with impregnated TiO2 over pumice granules and loaded as a photocatalyst in a multi-tube reactor as shown in Fig. 3. The multi tube reactor was used with 30 wt% TiO2 over pumice stone of 2 mm size thickness after optimization of different TiO2 loadings of 10, 30, 50 wt%. Model pollutants were chosen for detoxification experiments. In all experiments a total volume of 8 L of water was circulated at a constant flow rate of 2 L/min through the solar reactor via a circulation pump connected to a reservoir. All the experiments were carried out in June, July, August at
Fig. 1. A photograph of pellets of pumice stone (magnification 250); (a) pumice stone (b) pumice impregnated with TiO2 P25.
Clermont Ferrand in France and Hyderabad in India. At the beginning of each test a fresh volume of distilled water was admitted into the reactor via the storage reservoir. The water was circulated for 30 min and a sample of the water taken from the reservoir as a control. This control was kept in the dark and was enumerated for bacterial content at the beginning and at the end of the experiment. The process water was then inoculated with bacteria to gain the initial concentration of 5 105 CFU/ml. After this inoculation the water was then circulated through the photo reactor in the dark until the complete mix was achieved and uniform concentrations existed through out the photo reactor. E. coli is extensively used as a treatment efficiency indicator and if not detected the treated water is regarded as free from fecal contamination. At the start of each trial a sample is taken from the reservoir at time, t = 0 and the reactor is covered. During the experiment samples were taken at discrete time intervals from the reservoir during the duration of each test. The samples were then taken directly to the laboratory for the bacterial enumeration. Each trial was repeated before the final results were obtained. The disinfection procedures lead to the formation of disinfection products that are mainly from naturally occurring organic compounds. In view of this the reactivity influence of the devices in Figs 2 and 3 against acid orange-7, resorcinol, 4-nitro-2-sulfonic acid, 4,6-dinirtro-o-cresol, isopro-
M. Subrahmanyam et al. / Solar Energy 82 (2008) 1099–1106
1101
Fig. 2. TiO2 coated pumice stone in a fixed bed reactor used for the irradiation of solutions in artificial UV light or sunlight. Slanting unit (20 40 cm). The volume to fill is ffi8 L: upper part: over view; middle: lateral view; bottom: section of the photocatalytic panel. (1) TiO2 impregnated pellets of pumice stone; (2) peristaltic pump (double channel) (3) battery, (4) spacer (8 mm thick); (5) feed; (6) wooden support (slope ffi1%); (7) PMMA sheet (5 mm sheet); (8) glass pane (6 mm thick).
toron, degradation studies are also performed in order to know the overall performance of the units for disinfection and detoxification studies.
CH3
OH
SO3H
OH
4-NT-2-SA
Resorcinol C H3
NO2
OH
N O2
(CH 3 ) 2 C H
N HC ON (C H3 ) 2
N O2 (4,6-DN OC )
ISOPROTURON
The color disappearance of the solution was monitored by UV–Vis spectra photometry on Cary 3 (Varian). For AO-7, 4,6-dinitro-o-cresol, and resorcinol solutions are controlled with spectra photometric monitoring and also cross checked with the data HPLC analysis which gave similar results. The disappearance of 4-NT-2-SA compound was quantified by HPLC using a photodiode array detector and column C18 250 mm 4.6 mm and eluent was methanol/water (35: 65 v/v). The percentage of disappearance isoproturan in water is monitored with TOC analysis using Shimadzu TOC analyzer 5050A. All solutions were exposed to sunlight using the device presented in Fig. 2 and/or Fig. 3. The unit in Fig. 2 consists of a plank 20 40 cm covered with a photocatalyst. The solution is spread at the top with a peristaltic pump (35 ml min1) and collected at the bottom in a volumetric jar that permits to correct the evaporation by making up the volume with purified water. The slanting unit was covered with a polymethylmetacrylate
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Fig. 3. Multi-tube photocatalytic reactor with TiO2 supported pumice stone as photocatalyst; (1) tubes filled with catalyst (L) = 750 mm, diameter 5 mm; (2) reflecting surface; (3) volumetric jar with solution (4) peristaltic pump; (5) battery.
cover to reduce the evaporation. For indoor experiments the light source consists of 4 lamps Philips TLD 15 W emitting between 300 and 450 nm. Water used for the solutions was purified by Milli-Q system from Millipore and controlled by its resistivity (>18 MX cm). The organic compounds were a dye acid orange-7 (AO-7) of high purity grade from ACROS. 4, 6 di nitro-o-cresol and isoproturan, 4-NT-2-SA are from Fluka grade. Also it was controlled that the direct photolysis of acid orange-7 and isoproturon, 4-NT-2-SA disappears very slowly in sunlight in the absence of photocatalyst. The main purpose of this study was to evaluate the photocatalytic inaction of E. coli bacteria. Solar experiments were conducted using the photocatalyst supported on pumice stone as loaded devices in Figs. 2 and 3. The volume to fill is approximately 8 L liquid for treatment with the known amount of E. coli containing distilled water as well as with the live samples of drinking water collected from the river Allier near Clermont Ferrand. 3. Results and discussion The efficiency of E. coli photo inactivation with a solar light was evaluated using distilled water and natural water containing the pathogen. The results obtained are given in Table 1. The survival rating of E. coli was determined by counting the number of viable cells in terms of CFU and the same was performed with the help of biology division. The E. coli bacteria killing tests are made at 2 L/h feed rate. The experiments performed with E. coli in distilled water as
Table 1 Photocatalytic inactivation of E. coli in (A) distilled water and (B) river water Time (min)
(A) distilled water CFU concentration
(B) River water CFU concentration
0 2 90
1.3 105 0 1.0 103 1.0 101
5 105 2 102 –
The presence of clouds in the sky markedly increases the time of killing E. coli.
well with river water collected samples showed the decrease in cell concentration after treating the solution maintained in the dark. For practical application of photocatalytic disinfection, it is important to determine the length of the irradiation period to ensure death of bacteria. Aqueous suspensions of bacteria were passed over the unit during different periods of time, in dark and in light and sampled every hour during the run time. The effect on bacterial inactivation of the unit with intermittent organic degradation studies also conducted in order to know its practical application for the day-to-day utility. Time required for inactivation of E. coli and the deactivation efficiency using pumice stone supported TiO2 was 20–40 min. The reactor geometry determines the light distribution and its availability for fixed TiO2 excitation. The ‘‘blank” was obtained with bare pellets of pumice stone fixed on white cement. It clearly appears that the best efficiency is obtained with impregnated pumice stone. The number of colony forming units for river water
M. Subrahmanyam et al. / Solar Energy 82 (2008) 1099–1106
was reduced by 60% after 20 min and no cells were found at the sample collected at 40 min. Photo catalytic treatment of bacterial cells were allowed up to 30 h to recover from disinfection. Following the recovery period, the number of CFU’s remained below detectable limits showing the photo catalytic disinfection had caused irreversible damage to the bacterial cells. For instance, it has been showed that E. coli (103 cells/min) was totally inactivated in 40 min using TiO2 supported pumice stone in fixed bed form and loaded in multi tube reactor unit. The pH of the solution changed from 6.60 to 5.88 after photo catalytic treatment. The change in pH of the solution with time on irradiation was affected by the bacteria inactivation. On the basis of these experimental observations, the disinfection of E. coli with the usage of the present device is found to be useful under solar light irradiation for the drinking water treatment application. The effect of TiO2 loading over pumice stone supported material for photocatalytic inactivation/degradation were also tested with multi-tube reactor. The optimal TiO2 concentration i.e. at present 30 wt% found to be the optimum one.
1103
Actually the present investigation cannot be recommended for rough wastewater that contains too much organic matter, but for the elimination of bacteria especially E. coli and other compounds that resist to biological treatment. Acid orange-7 was chosen as a substrate since it was experimentally proved that its direct photolysis in sunlight is not efficient. The colour transformation of a solution of AO-7 (5 105 M) was measured with units in Figs. 2 and 3. The catalyst was used for the degradation of 5 L of dye solution of (105 M) during a week. The decolorization progress of AO-7 recorded the photographs over time on stream using the unit Fig. 2 is provided in Fig. 4 and the respective color disappearance spectrum with time on stream is shown in Fig. 5. It was observed that a solution was completely decolourised over a short period as seen from visual photographs of Fig. 4 and from Fig. 5a and b spectral data generated from both the reactors. After washing with pure water the same catalyst was used with a dye solution of 5 105 M and the kinetics were compared with the first one. The transformation was highly reproducible. The experiment was continued for two days and some
Fig. 4. The photographs at different stages of photocatalytic transformation of acid orange-7 on immobilized TiO2 over pumice stone device in Fig. 2. (1) Original; (2) 30 min; (3) 90 min; (4) 150 min; (5) 180 min; (6) after degradation.
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M. Subrahmanyam et al. / Solar Energy 82 (2008) 1099–1106
Fig. 5. (a) The UV spectrum of acid orange-7 solution 5 105 M run over pumice supported photo catalyst device for a duration of (1) 0; (2) 1; (3) 4; (4) 5; (5) 5 1/2 (6) 6 1/2 h treatment time. (b) The experiment performed in Fig. 5a but with a multi-tube reactor shown in Fig. 3.
times for three days. The overall efficiency and the trend in reproducibility has maintained very well. Furthermore, the percent TOC degradation of 4-NT2SA (Table 2), Isoproturon and (Table 3) and the percent disappearance of resorcinol (Fig. 6); 4,6 dinitro-o-cresol
Fig. 7 compounds were monitored over the units in Figs. 2 and 3. All the above data substantiates well with the present capabilities of the units for the photo catalytic disinfection of urban waters and transformations of organics as well. The influence of reusing on the photocatalytic effi-
Table 2 Photocatalytic degradation of 6 104 M, 4- nitro toluene-2-sulfonic acid (4-NTSA) in a multi-tube reactor
Table 3 Photocatalytic degradation of 1.94 104 M, isoproturon in a multi-tube reactor
Time on irradiation (min)
% TOC reduction
Time on irradiation (min)
% TOC reduction
60 160 220 280
29 46 76 83
60 120 220 280
51 55 59 60
M. Subrahmanyam et al. / Solar Energy 82 (2008) 1099–1106
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ciency with TiO2 immobilized on pumice stone was controlled very well even in drastic conditions performed like bacteria inactivation after the use of the unit for treatment with AO-7 solution and also after the use of bacteria and AO-7 solution combinate systems treatment. 4. Conclusions
Fig. 6. Spectra of resorcinol (5 105 M) solution after treatment over slanting unit in Fig. 2 with time (h) of irradiation i.e 0, 1, 4, 5.
The water that has passed through the pumice stone supported reactors demonstrated strong bactericidal ability as well as detoxification affects by showing reduction in TOC, extent of decrease in color over treatment period. The photocatalytic deactivation of total E. coli by solar light has been attempted for real river waters. The technical feasibility and performance of photocatalytic effect of TiO2 coatings over pumice stone in continuous reactor process studied proved to improve the potability of drinking water collected from real waters.
Fig. 7. (a) UV–Vis absorption spectra of 4,6-dinitro-o-cresol (5 105 M) after treatment through 30 wt% TiO2/pumice loaded in multi tube reactor (1) Original solution (2) 60 (3) 120 (4) 180 (5) 240 min duration. (b) Photocatalytic degradation of 4,6-dinitro-o-cresol (5 105 M) TOC reduction after treatment over TiO2 supported pumice stone in a multi-tube reactor containing 30 wt% TiO2/pumice.
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The result provides that an apparatus for photocatalytic degradation of pathogens and recalcitrant compounds in drinking water and wastewater using a special type of reactor wherein the pumice stone is coated with TiO2. Also the device is wherein the TiO2 supported photocatalyst is used in a multi tube reactor. The inactivation of bacteria in drinking water is a useful method for the treatment of drinking water and for degradation of organic recalcitrant pollutants since it avoids the post-treatment filtration. The results suggest that TiO2 supported on pumice stone form is economical and efficient process for the treatment of bacteria containing drinking water and organics containing wastewater effluents at smaller scales that may be adopted for diluted wastewater Acknowledgement The authors acknowledge IFCPAR, New Delhi for financial support (No.: 2205-2). References Coleman, H.M., Marquis, C.P., Scott, J.A., Chin, S.S., Amal, R., 2005. Bactericidal effects of titanium dioxide-based photocatalysts. Chem. Eng. J. 113, 55–63. Duffy, E.F., Touati, F.Al., Kehoe, S.C., McLoughlin, O.A., Gill, L.W., Gernjak, W., Oller, I., Maldonado, M.I., Malato, S., Cassidy, J., Reed, R.H., McGuigan, K.G., 2004. A novel TiO2-assisted solar photocatalytic batch-process disinfection reactor for the treatment of biological and chemical contaminants in domestic drinking water in developing countries. Solar Energy 77, 649–655. Dunlop, P.S.M., Byrne, J.A., Manga, N., Eggins, B.R., 2002. The photocatalytic removal of bacterial pollutants form drinking water. J. Photochem. Photobiol. A 148, 355–363. Gumy, D., Morais, C., Bowen, P., Pulgarin, C., Giraldo, S., Hajdu, R., Kiwi, J., 2006. Catalytic activity of commercial of TiO2 powders for
the abatement of the bacteria (E. coli) under solar simulated light: Influence of the isoelectric point. Appl. Catal. B 63, 76–84. Hu, X., Hu, C., Qu, J., 2006. Photocatalytic decomposition of acetaldehyde and E. coli using NiO/SrBi2O4 under visible light irradiation. Appl. Catal. 69, 18–24. Ljubas, D., 2005. Solar photocatalysis-a possible step in drinking water treatment. Energy 30, 1699–1710. McLoughlin, O.A., Kehoe, S.C., McGuigan, K.G., Duffy, E.F., Touati, F. Al., Gernjak, W., Alberola, I.O., Rodriguez, S.M., Gill, L.W., 2004. Solar disinfection of contaminated water: a comparison of three smallscale reactors. Solar Energy 77, 657–664. Melian, J.A.H., Rodriguez, J.M.D., Suarez, A.V., Rendon, E.T., Campo, C.V., do Arana, J., Pena, J.P., 2000. The photocatalytic disinfection of urban wastewaters. Chemosphere 41, 323–327. Noorjahan, M., Durga Kumari, V., Subrahmanyam, M., Boule, P., 2004. A novel and efficient photocatalyst: TiO2-HZSM-5 combinate thin film. Appl. Catal. B 47, 209–213. Noorjahan, M., Pratap Redy, M., Durga Kumari, V., Lavedrine, B., Boule, P., Subrahmanyam, M., 2003. Photocatalytic degradation of Hacid over a novel TiO2 thin film fixed bed reactor and in aqueous suspensions. J. Photchem. Photobiol. A Chem. 156, 179–187. Rachel, A., Lavedrine, B., Subrahmanyam, M., Boule, P., 2002. Use of porous lavas as supports of photocatalysts. Catal. Commun. 3, 165– 171. Rincon, A.G., Pulgarin, C., 2006. Comparative evaluation of Fe3+ and TiO2 photoassisted processes in solar photocatalytic disinfection of water. Appl. Catal. B 63, 222–231, and references therein. Robertson, J.M.C., Robertson, P.K., Lawton, L.A., 2005. A comparison of the effectiveness of TiO2 photo catalysis and UVA photolysis for the destruction of three pathogenic microorganisms. J. Photochem. Photobiol. A 175, 51–56. Sagawe, G., Brandi, R.J., Bahnemann, D., Cassano, A.E., 2004. Photocatalytic reactors for treating water pollution with solar illumination III: a simplified analysis for re-circulating reactors. Solar Energy 77, 471–489. Subba Rao, K.V., Subrahmanyam, M., Boule, P., 2004. Immobilzed TiO2 photocatalyst during long-term use: decrease of its activity. Appl. Catal. B 49, 239–249, and references therein. Villen, L., Manjon, F., Garcia-Fresnadillo, D., Orellana, G., 2006. Solar water disinfection by photocatalytic singlet oxygen production in heterogeneous medium. Appl. Catal. B 69, 1–9.
Solar Energy 82 (2008) 1099–1106 www.elsevier.com/locate/solener
Pumice stone supported titanium dioxide for removal of pathogen in drinking water and recalcitrant in wastewater M. Subrahmanyam a,*, P. Boule b, V. Durga Kumari a, D. Naveen Kumar a, M. Sancelme b, A. Rachel b b
a Catalysis and Physical Chemistry Division, Indian Institute of Chemical Technology, Hyderabad 500007, India Laboratoire de Photochimie Mole´culaire et Macromole´culaire, Universite´ Blaise Pascal CNRS, UMR 6505 (Clermont-Ferrand), F-63177 Aubie`re cedex, France
Received 28 September 2006; accepted 16 May 2008 Available online 17 June 2008 Communicated by: Associate Editor Gion Calzaferri
Abstract Bactericidal and organic degradation effects of TiO2 on pumice stone are described in this paper. Immobilization of TiO2 on pumice stone is easy and efficient method to obtain photocatalytic reactions without the problem of filtration. Pumice stone is soft and available as pellets that can be used in pellets fixed (with cement or poly carbonate) on a slanting plank/glass by coating the preformed TiO2 over the pellets using simple paint brush for applying the photo catalyst. The treatment of inactivation of bacteria especially E. coli existing in real river waters and also different model organic substrate degradations like acid orange-7, resorcinol, 4, 6-dinitro-o-cresol, 4-nitrotoluene-2-sulfonicacid, isoproturan are studied. Furthermore, TiO2 over pumice stone loaded in a multi tube reactor gave similar results for the disinfection and detoxification studies. Ó 2008 Elsevier Ltd. All rights reserved. Keywords: Photocatalysis; Disinfection; Detoxification; TiO2; Pumice stone
1. Introduction Water disinfection is recognized by the WHO as one of the most important challenges for human health protection. The spread of water borne infection is a problem in both developed and underdeveloped countries. Photocatalysis is an advanced oxidation processes (AOP) that has been shown to possess enhanced disinfection capabilities in recent years (Hu et al., 2006; Coleman et al., 2005; Rincon and Pulgarin, 2006; Duffy et al., 2004; Dunlop et al., 2002). A few studies on the durability of photo catalytic disinfection using real water sources have been reported (Gumy et al., 2006; Ljubas, 2005; Robertson et al., 2005).
*
Corresponding author. Tel.: +91 40 27193165; fax: +91 40 27160921. E-mail address: [email protected] (M. Subrahmanyam).
0038-092X/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.solener.2008.05.007
It has been generally accepted that inactivation of microorganisms is mainly due to the oxidative radicals (mainly OH*) produced by irradiated TiO2 (Villen et al., 2006). The effective bactericidal properties of photocatalysis in static and in dynamic way on a diverse range of coli forms over various photocatalysts and with different reactor systems are of recent interest (McLoughlin et al., 2004; Melian et al., 2000). Disinfection rates or inactivation times are usually not comparable from study to study due to a wide range of operational parameters and reactor configuration used in different laboratories (Sagawe et al., 2004). Reactor configurations range from small volume pyrex beakers or petri dishes, typically 1–10 cm3, illuminated from the side or top of a tubular lamp to the modification of commercial disinfection apparatus. To overcome these problems trials were designed to asses the efficiency of using titanium
1100
M. Subrahmanyam et al. / Solar Energy 82 (2008) 1099–1106
dioxide over pumice in continuous flow disinfection process, which if successful, can be further developed into a low cost method of fixing the TiO2 over support in order to use thin film fixed bed reactor as well as a multi-tube reactor. Research into continuous flow systems for disinfection purposes has been limited with only few experimental trials having been carried out to date (McLoughlin et al., 2004). Many techniques are proposed for the immobilization of TiO2 on solid supports (Noorjahan et al., 2003; Noorjahan et al., 2004). Porous lavas more precisely pumice stone are promising supports of TiO2 when used as a photo catalyst (Subba Rao et al., 2004; Rachel et al., 2002). The present study has been undertaken to provide a practical reactor assessment of photocatalytic treatment of TiO2 supported over pumice stone by coating with a technique giving a TiO2 thin film over pumice stone. It is fixed to a slanting unit and also loaded in a multi tube (four tubes) glass reactor and they are used for inactivation of bacteria in drinking water and degradation of model organics in wastewater. The results generated using the reactors and its potential application for water disinfection and detoxification are studied within the frame-work program of the project sponsored by the Indo-French Center of Promotion of Advanced Research (IFCPAR), New Delhi under the project No: 2205-2. 2. Experimental Two separate reactors were used during the experiments and both are configured as continuous flow re-circulating systems. The reactors were positioned as non-tracking static systems with each in slanting position tilted and the units are aligned to maximize sunlight capture. Photocatalytic reactions were carried out using a TiO2 P-25 of Degussa supported over pumice as presented in Fig. 1. The supported photocatalyst comprises of impregnation of pumice stone pellets with commercially available TiO2 by simply brushing with TiO2 milk or impregnating TiO2 milk with conventional soaking, drying and heat treatment methods. Pumice stone is a soft material and their fixing over the pellets on a hard slanting plank surface is with cement or polycarbonate One of the resulting unit is a TiO2 thin-film containing fixed bed photocatalytic reactor as is shown in Fig. 2. Pellets of pumice stone 2–3 mm and ffi5 mm are kindly provided by Eyraud S. A. (Lyon, France). Furthermore, the unit also used with impregnated TiO2 over pumice granules and loaded as a photocatalyst in a multi-tube reactor as shown in Fig. 3. The multi tube reactor was used with 30 wt% TiO2 over pumice stone of 2 mm size thickness after optimization of different TiO2 loadings of 10, 30, 50 wt%. Model pollutants were chosen for detoxification experiments. In all experiments a total volume of 8 L of water was circulated at a constant flow rate of 2 L/min through the solar reactor via a circulation pump connected to a reservoir. All the experiments were carried out in June, July, August at
Fig. 1. A photograph of pellets of pumice stone (magnification 250); (a) pumice stone (b) pumice impregnated with TiO2 P25.
Clermont Ferrand in France and Hyderabad in India. At the beginning of each test a fresh volume of distilled water was admitted into the reactor via the storage reservoir. The water was circulated for 30 min and a sample of the water taken from the reservoir as a control. This control was kept in the dark and was enumerated for bacterial content at the beginning and at the end of the experiment. The process water was then inoculated with bacteria to gain the initial concentration of 5 105 CFU/ml. After this inoculation the water was then circulated through the photo reactor in the dark until the complete mix was achieved and uniform concentrations existed through out the photo reactor. E. coli is extensively used as a treatment efficiency indicator and if not detected the treated water is regarded as free from fecal contamination. At the start of each trial a sample is taken from the reservoir at time, t = 0 and the reactor is covered. During the experiment samples were taken at discrete time intervals from the reservoir during the duration of each test. The samples were then taken directly to the laboratory for the bacterial enumeration. Each trial was repeated before the final results were obtained. The disinfection procedures lead to the formation of disinfection products that are mainly from naturally occurring organic compounds. In view of this the reactivity influence of the devices in Figs 2 and 3 against acid orange-7, resorcinol, 4-nitro-2-sulfonic acid, 4,6-dinirtro-o-cresol, isopro-
M. Subrahmanyam et al. / Solar Energy 82 (2008) 1099–1106
1101
Fig. 2. TiO2 coated pumice stone in a fixed bed reactor used for the irradiation of solutions in artificial UV light or sunlight. Slanting unit (20 40 cm). The volume to fill is ffi8 L: upper part: over view; middle: lateral view; bottom: section of the photocatalytic panel. (1) TiO2 impregnated pellets of pumice stone; (2) peristaltic pump (double channel) (3) battery, (4) spacer (8 mm thick); (5) feed; (6) wooden support (slope ffi1%); (7) PMMA sheet (5 mm sheet); (8) glass pane (6 mm thick).
toron, degradation studies are also performed in order to know the overall performance of the units for disinfection and detoxification studies.
CH3
OH
SO3H
OH
4-NT-2-SA
Resorcinol C H3
NO2
OH
N O2
(CH 3 ) 2 C H
N HC ON (C H3 ) 2
N O2 (4,6-DN OC )
ISOPROTURON
The color disappearance of the solution was monitored by UV–Vis spectra photometry on Cary 3 (Varian). For AO-7, 4,6-dinitro-o-cresol, and resorcinol solutions are controlled with spectra photometric monitoring and also cross checked with the data HPLC analysis which gave similar results. The disappearance of 4-NT-2-SA compound was quantified by HPLC using a photodiode array detector and column C18 250 mm 4.6 mm and eluent was methanol/water (35: 65 v/v). The percentage of disappearance isoproturan in water is monitored with TOC analysis using Shimadzu TOC analyzer 5050A. All solutions were exposed to sunlight using the device presented in Fig. 2 and/or Fig. 3. The unit in Fig. 2 consists of a plank 20 40 cm covered with a photocatalyst. The solution is spread at the top with a peristaltic pump (35 ml min1) and collected at the bottom in a volumetric jar that permits to correct the evaporation by making up the volume with purified water. The slanting unit was covered with a polymethylmetacrylate
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Fig. 3. Multi-tube photocatalytic reactor with TiO2 supported pumice stone as photocatalyst; (1) tubes filled with catalyst (L) = 750 mm, diameter 5 mm; (2) reflecting surface; (3) volumetric jar with solution (4) peristaltic pump; (5) battery.
cover to reduce the evaporation. For indoor experiments the light source consists of 4 lamps Philips TLD 15 W emitting between 300 and 450 nm. Water used for the solutions was purified by Milli-Q system from Millipore and controlled by its resistivity (>18 MX cm). The organic compounds were a dye acid orange-7 (AO-7) of high purity grade from ACROS. 4, 6 di nitro-o-cresol and isoproturan, 4-NT-2-SA are from Fluka grade. Also it was controlled that the direct photolysis of acid orange-7 and isoproturon, 4-NT-2-SA disappears very slowly in sunlight in the absence of photocatalyst. The main purpose of this study was to evaluate the photocatalytic inaction of E. coli bacteria. Solar experiments were conducted using the photocatalyst supported on pumice stone as loaded devices in Figs. 2 and 3. The volume to fill is approximately 8 L liquid for treatment with the known amount of E. coli containing distilled water as well as with the live samples of drinking water collected from the river Allier near Clermont Ferrand. 3. Results and discussion The efficiency of E. coli photo inactivation with a solar light was evaluated using distilled water and natural water containing the pathogen. The results obtained are given in Table 1. The survival rating of E. coli was determined by counting the number of viable cells in terms of CFU and the same was performed with the help of biology division. The E. coli bacteria killing tests are made at 2 L/h feed rate. The experiments performed with E. coli in distilled water as
Table 1 Photocatalytic inactivation of E. coli in (A) distilled water and (B) river water Time (min)
(A) distilled water CFU concentration
(B) River water CFU concentration
0 2 90
1.3 105 0 1.0 103 1.0 101
5 105 2 102 –
The presence of clouds in the sky markedly increases the time of killing E. coli.
well with river water collected samples showed the decrease in cell concentration after treating the solution maintained in the dark. For practical application of photocatalytic disinfection, it is important to determine the length of the irradiation period to ensure death of bacteria. Aqueous suspensions of bacteria were passed over the unit during different periods of time, in dark and in light and sampled every hour during the run time. The effect on bacterial inactivation of the unit with intermittent organic degradation studies also conducted in order to know its practical application for the day-to-day utility. Time required for inactivation of E. coli and the deactivation efficiency using pumice stone supported TiO2 was 20–40 min. The reactor geometry determines the light distribution and its availability for fixed TiO2 excitation. The ‘‘blank” was obtained with bare pellets of pumice stone fixed on white cement. It clearly appears that the best efficiency is obtained with impregnated pumice stone. The number of colony forming units for river water
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was reduced by 60% after 20 min and no cells were found at the sample collected at 40 min. Photo catalytic treatment of bacterial cells were allowed up to 30 h to recover from disinfection. Following the recovery period, the number of CFU’s remained below detectable limits showing the photo catalytic disinfection had caused irreversible damage to the bacterial cells. For instance, it has been showed that E. coli (103 cells/min) was totally inactivated in 40 min using TiO2 supported pumice stone in fixed bed form and loaded in multi tube reactor unit. The pH of the solution changed from 6.60 to 5.88 after photo catalytic treatment. The change in pH of the solution with time on irradiation was affected by the bacteria inactivation. On the basis of these experimental observations, the disinfection of E. coli with the usage of the present device is found to be useful under solar light irradiation for the drinking water treatment application. The effect of TiO2 loading over pumice stone supported material for photocatalytic inactivation/degradation were also tested with multi-tube reactor. The optimal TiO2 concentration i.e. at present 30 wt% found to be the optimum one.
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Actually the present investigation cannot be recommended for rough wastewater that contains too much organic matter, but for the elimination of bacteria especially E. coli and other compounds that resist to biological treatment. Acid orange-7 was chosen as a substrate since it was experimentally proved that its direct photolysis in sunlight is not efficient. The colour transformation of a solution of AO-7 (5 105 M) was measured with units in Figs. 2 and 3. The catalyst was used for the degradation of 5 L of dye solution of (105 M) during a week. The decolorization progress of AO-7 recorded the photographs over time on stream using the unit Fig. 2 is provided in Fig. 4 and the respective color disappearance spectrum with time on stream is shown in Fig. 5. It was observed that a solution was completely decolourised over a short period as seen from visual photographs of Fig. 4 and from Fig. 5a and b spectral data generated from both the reactors. After washing with pure water the same catalyst was used with a dye solution of 5 105 M and the kinetics were compared with the first one. The transformation was highly reproducible. The experiment was continued for two days and some
Fig. 4. The photographs at different stages of photocatalytic transformation of acid orange-7 on immobilized TiO2 over pumice stone device in Fig. 2. (1) Original; (2) 30 min; (3) 90 min; (4) 150 min; (5) 180 min; (6) after degradation.
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Fig. 5. (a) The UV spectrum of acid orange-7 solution 5 105 M run over pumice supported photo catalyst device for a duration of (1) 0; (2) 1; (3) 4; (4) 5; (5) 5 1/2 (6) 6 1/2 h treatment time. (b) The experiment performed in Fig. 5a but with a multi-tube reactor shown in Fig. 3.
times for three days. The overall efficiency and the trend in reproducibility has maintained very well. Furthermore, the percent TOC degradation of 4-NT2SA (Table 2), Isoproturon and (Table 3) and the percent disappearance of resorcinol (Fig. 6); 4,6 dinitro-o-cresol
Fig. 7 compounds were monitored over the units in Figs. 2 and 3. All the above data substantiates well with the present capabilities of the units for the photo catalytic disinfection of urban waters and transformations of organics as well. The influence of reusing on the photocatalytic effi-
Table 2 Photocatalytic degradation of 6 104 M, 4- nitro toluene-2-sulfonic acid (4-NTSA) in a multi-tube reactor
Table 3 Photocatalytic degradation of 1.94 104 M, isoproturon in a multi-tube reactor
Time on irradiation (min)
% TOC reduction
Time on irradiation (min)
% TOC reduction
60 160 220 280
29 46 76 83
60 120 220 280
51 55 59 60
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ciency with TiO2 immobilized on pumice stone was controlled very well even in drastic conditions performed like bacteria inactivation after the use of the unit for treatment with AO-7 solution and also after the use of bacteria and AO-7 solution combinate systems treatment. 4. Conclusions
Fig. 6. Spectra of resorcinol (5 105 M) solution after treatment over slanting unit in Fig. 2 with time (h) of irradiation i.e 0, 1, 4, 5.
The water that has passed through the pumice stone supported reactors demonstrated strong bactericidal ability as well as detoxification affects by showing reduction in TOC, extent of decrease in color over treatment period. The photocatalytic deactivation of total E. coli by solar light has been attempted for real river waters. The technical feasibility and performance of photocatalytic effect of TiO2 coatings over pumice stone in continuous reactor process studied proved to improve the potability of drinking water collected from real waters.
Fig. 7. (a) UV–Vis absorption spectra of 4,6-dinitro-o-cresol (5 105 M) after treatment through 30 wt% TiO2/pumice loaded in multi tube reactor (1) Original solution (2) 60 (3) 120 (4) 180 (5) 240 min duration. (b) Photocatalytic degradation of 4,6-dinitro-o-cresol (5 105 M) TOC reduction after treatment over TiO2 supported pumice stone in a multi-tube reactor containing 30 wt% TiO2/pumice.
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The result provides that an apparatus for photocatalytic degradation of pathogens and recalcitrant compounds in drinking water and wastewater using a special type of reactor wherein the pumice stone is coated with TiO2. Also the device is wherein the TiO2 supported photocatalyst is used in a multi tube reactor. The inactivation of bacteria in drinking water is a useful method for the treatment of drinking water and for degradation of organic recalcitrant pollutants since it avoids the post-treatment filtration. The results suggest that TiO2 supported on pumice stone form is economical and efficient process for the treatment of bacteria containing drinking water and organics containing wastewater effluents at smaller scales that may be adopted for diluted wastewater Acknowledgement The authors acknowledge IFCPAR, New Delhi for financial support (No.: 2205-2). References Coleman, H.M., Marquis, C.P., Scott, J.A., Chin, S.S., Amal, R., 2005. Bactericidal effects of titanium dioxide-based photocatalysts. Chem. Eng. J. 113, 55–63. Duffy, E.F., Touati, F.Al., Kehoe, S.C., McLoughlin, O.A., Gill, L.W., Gernjak, W., Oller, I., Maldonado, M.I., Malato, S., Cassidy, J., Reed, R.H., McGuigan, K.G., 2004. A novel TiO2-assisted solar photocatalytic batch-process disinfection reactor for the treatment of biological and chemical contaminants in domestic drinking water in developing countries. Solar Energy 77, 649–655. Dunlop, P.S.M., Byrne, J.A., Manga, N., Eggins, B.R., 2002. The photocatalytic removal of bacterial pollutants form drinking water. J. Photochem. Photobiol. A 148, 355–363. Gumy, D., Morais, C., Bowen, P., Pulgarin, C., Giraldo, S., Hajdu, R., Kiwi, J., 2006. Catalytic activity of commercial of TiO2 powders for
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