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Solar photocatalysis, photodegradation of a commercial detergent in aqueous TiO2 dispersions under sunlight irradiation
Solar Energy 77 (2004) 525–532 www.elsevier.com/locate/solener
Solar photocatalysis, photodegradation of a commercial detergent in aqueous TiO2 dispersions under sunlight irradiation Toshiyuki Oyama a, Akio Aoshima a, Satoshi Horikoshi a, Hisao Hidaka Jincai Zhao b, Nick Serpone c a
a,*
,
Frontier Research Center for the Global Environment Science, Meisei University, 2-1-1 Hodokubo, Hino-shi, Tokyo 191-8506, Japan b Molecular Sciences Center, Institute of Chemistry, Chinese Academy of Science, Beijing 100 080, China c Dipartimento di Chimica Organica, Universita´ di Pavia, Via Taramelli 10, Pavia 27100, Italia Received 17 November 2003; received in revised form 16 April 2004; accepted 16 April 2004 Available online 6 August 2004 Communicated by: Associate Editor Sixto Malato-Rodrı´guez
Abstract A commercial detergent whose major components are an anionic surfactant and a fluorescent whitening agent can be photodegraded in aqueous TiO2 dispersions under irradiation with concentrated sunlight in the presence of air. The degradation process followed apparent first-order kinetics in terms of the total sunlight energy impinging on the photoreactive system. The effects of (a) TiO2 loading, (b) circulation flow rate, and (c) pH of the reactant solution on the kinetics of decomposition of the detergent were examined. Under the prevailing conditions, the optimal operational parameters for this detergent were, respectively: TiO2 loading, 6 g l1; circulation flow rate, 4.9 l min1; and pH, 4.9. The rate of increase of the surface tension was greater than the rate of decrease of the concentration of the detergent. This study adds to our knowledge base in the effective use of sunlight irradiation to detoxify wastewaters containing undesirable detergents. 2004 Elsevier Ltd. All rights reserved. Keywords: Solar photocatalysis; Titanium dioxide photodegradation; Detergent wastewaters; Concentrated sunlight
1. Introduction Organic chemical products such as detergents, dyes, pesticides, herbicides and endocrine disruptors lead to serious environmental contamination. In particular, var-
* Corresponding author. Tel.: +81 42 599 7785; fax: +81 42 599 7785/44 591 6635. E-mail address: [email protected] (H. Hidaka).
ious types of detergents are widely employed in the domestic and industrial fields. They are one of the major causes of pollution of surface and ground waters. To the extent that the main ingredient of detergents containing a fabric-whitening agent is only slowly and inefficiently biodegraded by bacteria, wastewater detergents can cause undue ecological damage to animals and plants. Accordingly, remediation technologies are required to decompose and completely mineralize such organic pollutants. A most attractive technology in this regard is the
0038-092X/$ - see front matter 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.solener.2004.04.020
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TiO2-mediated photocatalytic method through which wastewater pollutants can be degraded oxidatively by the strongly oxidizing OH radical species. With this methodology, an organic pollutant can be degraded in aqueous dispersions containing the TiO2 photocatalyst by UV irradiation from a suitable UV-light source and in the presence of oxygen (Hidaka et al., 1985; Herrmann, 1999; Hoffmann et al., 1995; Kamat, 1993; Kamat and Vinodgopal, 1993; Legrini et al., 1993). UV irradiation at energies greater than or equal to the bandgap energy of the TiO2 photocatalyst (3.2 eV, anatase form) produces electrons in the conduction band and positive holes in the valence band (Eq. (1)). Subsequent to migration to the surface through diffusion and/or drift (Emeline et al., 2003) TiO2 þ hm ! e þ hþ
ð1Þ
followed by trapping by surface defect sites and adsorbates, the electron reacts with adsorbed oxygen to produce reactive oxygen species and holes react with surface bound OH groups (or H2O) to yield OH radicals that ultimately oxidize the polluting substrates (Eq. (2)). e þ O ! O ð2aÞ 2
2
H2 O ! Hþ þ OH
ð2bÞ
þ O 2 þ H ! OOH
ð2cÞ
hþ þ OH ! OH
ð2dÞ
hþ þ H2 O ! OH þ Hþ
ð2eÞ
OH þ RH ! oxidized intermediates ! CO2 þ H2 O
ð2fÞ
Utilization of sunlight as the UV energy source to drive such reactions is beneficial from an ecological point of view and where appropriate from an economic standpoint. Several studies of photocatalytic degradations carried out under solar exposure have been reported by various workers (see e.g. Hidaka et al., 1989; Muradov, 1994; Minero et al., 1996; Nogueira and Jardim, 1996, 1998; Van Well et al., 1997; Dillert et al., 1999a,b; Gimenez et al., 1999; Romero et al., 1996; Alfano et al., 2000; Jimenez et al., 2000; Malato et al., 2000, 2002). Most such reports employed solar collectors to irradiate the photoreactive systems examined. In earlier studies, we reported the TiO2-mediated photocatalytic degradation of several surfactants such as sodium dodecylbenzenesulfonate (NaDBS; Hidaka et al., 1989; Zhang et al., 2003), benzyldodecyldimethyl ammonium chloride (BDAC) and p-nonylphenyl-polyoxyethylene (NPPE; Hidaka et al., 1989), and sodium benzenesulfonate (NaBS; Zhang et al., 2001) together with the methylene blue dye (Zhang et al., 2002) using
artificial UV sources to examine the dynamics and the mechanistic details of the photodegradative process(es). In the present article, we report a recent study carried out to assess the photodegradation of a representative commercial detergent (henceforth referred to simply as detergent) that contains a fluorescent fabric-whitening agent in an aqueous TiO2 dispersion UV-irradiated by concentrated sunlight using a relatively inexpensive circular solar collector. The effects of such operational parameters as (a) titanium dioxide loading, (b) circulation flow rate, and (c) changes in pH of the dispersion on the overall process dynamics were examined to evaluate whether it was also practical to use solar (concentrated) irradiation to degrade a real wastewater polluted with detergents as others have reported albeit for other classes of pollutants (see e.g. Guillard et al., 2003; Kositzi et al., 2004; Malato et al., 2003, 2004).
2. Experimental section 2.1. Materials Nanoparticulate TiO2 (P-25; surface area, 53 m2 g1; mean diameter, 30 nm; composition, 87% anatase and 13% rutile) was kindly supplied by the Aerosil Nippon Co. The commercial detergent was purchased from Lion Co. Ltd. of Japan. The major ingredients in this detergent are an a-sodium fatty acid ester sulfonate, a linear alkylaryl sulfonate, a sodium salt of a fatty acid, aluminosilicate, sodium carbonate, enzymes, and a fabric-whitening agent (namely, bistriazinyl derivatives of 4,4 0 -diaminostilbene-2,2 0 -disulfonic acid). Ion-exchanged water was used throughout to prepare the desired experimental solutions and dispersions. 2.2. Solar photodegradation set-up and analysis A 3-l volume of an aqueous detergent/TiO2 mixture (0.1 g of detergent in 1 l of water; various loadings of TiO2) was prepared and placed in a 10-l feed tank. The tank containing the reaction mixture was placed in a water bath. Subsequently, the contents were dispersed by ultrasonication for 30 min to obtain an optimally dispersed system and complete adsorption/ desorption equilibration. The pH of the TiO2 dispersions was adjusted with either dilute NaOH or HClO4 solutions, as needed. The detergent was photocatalytically degraded in a batch mode reactor with a circulation system (PR-KV2, Furue Sci. Co. Ltd.) exposed to sunlight using a mirror concentrator (Mitaka Kohki Co. Ltd.) designed to face the sun constantly so as to utilize maximal sunlight radiation during daylight. The features of the reactor setup (Fig. 1) consisted essentially of a round concave mirror (aperture diameter, 1.0 m; mirror area, 0.785 m2) for geometric concentration of
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Fig. 1. (a) Schematic diagram of the experimental setup; (b) photograph showing the actual solar concentrator and reactor setup.
sunlight equivalent to 70 suns, and a flask-type photoreactor (Pyrex walls; volume, 1.3 l, bottom diameter, 10 cm). Most of the solar experiments were carried out at a flow rate of 4.9 l min1 (see below) to maintain the TiO2 dispersion sufficiently stirred in the plastic tubes, the feed tank, and the flask reactor, as well as to maintain air saturation of the reactant solution with air oxygen. The temperature of the dispersion in the flask reactor during solar irradiation ranged between 30 and 58 C. Appropriate samples were collected at various time intervals. After centrifugation, the samples were filtered
through a Millipore membrane (0.2 lm) to remove the titania particulates. Filtrates were used for various analytical assays. The temporal changes in the concentration of the detergent were monitored with a JASCO V-570 UV-visible spectrophotometer (kmax 224 nm; UV absorption of the aromatic group). The surface tension of the bulk solution was determined with an automatic surface tensiometer (CBVP-Z, Kyowa Interface Science Co. Ltd.). The concentration of the fluorescent whitening reagent was measured using a JASCO FP6500 spectrofluorimeter (emission wavelength, 345 nm). Chemical oxygen demand (COD) was assayed with
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a digital COD meter (HC-507, Central Kagaku) without previous separation of the TiO2 particulates. The sunlight intensity impinging on the reactor setup was measured with a UV radiometer (UVR-36, Topcon Co.; maximal measuring wavelength, 362 nm; light irradiance, 1.0–3.5 mW cm2). The total energy of the sunlight incident on the photoreactive system was calculated from the light irradiance {see Serpone and Emeline, 2002 for a description of suggested terms and definitions in photocatalysis and radiocatalysis}, the surface area of the aluminum mirror (concave side; covered with a resin), the reflectivity of the mirror (85%), the transmittance of the Pyrex glass (90%), and the irradiation time using an on-site computer integrated to the reactor setup (Fig. 1).
0 (a)
ln(Ct/C0)
528
-1 (b)
-2
0
50 Light energy (kJ)
100
Fig. 3. First-order kinetic plots derived from the data of Fig. 2 in terms of total solar light energy impinging on the photoreactive system: (a) sunlight irradiation in the absence of TiO2; (b) sunlight irradiation in the presence of TiO2.
3. Results and discussion 3.1. Photocatalyzed decomposition of the detergent
Ct/C0
1.0
(a) (b)
(c) 0
50 Irradiation time (min)
0.02
0.01
0 0
2
4 6 8 TiO2 loading (g litre-1)
10
12
Fig. 4. Dependence of the apparent rate constant on the TiO2 loading in the photodegradation of the detergent. Initial concentration of detergent, 0.1 g l1; volume of the reactant solution, 3 l; circulation flow rate, 4.9 l min1; initial pH, 8.9.
0.5
0
0.03 Apparent rate constant (kJ-1)
The temporal concentration changes of the aqueous detergent solutions (measured pH, 8.9) under sunlight irradiation are illustrated in Fig. 2 (error bars shown in Figs. 2–7 represent the standard deviation). Because of the compositional complexity of the detergent, the data are reported as Ct/C0 ratios from the absorbance data at 224 nm since the ratio At/A0 scales with Ct/C0. The detergent decomposed neither in the dark nor under concentrated sunlight irradiation in the absence of TiO2, the principal sunlight absorber. By contrast, the detergent degraded significantly in TiO2 dispersions under sunlight irradiation through first-order kinetics
100
Fig. 2. Photocatalytic degradation of the commercial detergent in aqueous TiO2 dispersions under sunlight irradiation: (a) dark reaction; (b) sunlight irradiation in the absence of TiO2; (c) sunlight irradiation in the presence of TiO2. Experimental conditions: initial concentration of the detergent, 0.1 g l1; volume of the reactant solution, 3 l; TiO2 loading, 6.0 g l1; circulation flow rate, 4.9 l min1; initial pH, 8.9.
(k = 0.025 ± 0.004 min1). These results show that the decrease in the concentration of detergent in aqueous TiO2 dispersions upon sunlight irradiation is due mostly to the photocatalytic process. The photocatalytic decomposition of organic compounds in aqueous TiO2 dispersions under solar UV light at constant intensity typically follows pseudo first-order kinetics at relatively low pollutant load. However, sunlight intensity changes daily and throughout the day. Accordingly, to compare the results of different experimental runs obtained under different solar irradiation conditions or during different days, a kinetic analysis involving irradiation time as the variable is not appropriate. The kinetic expression embodied by Eqs. (3) and (4) is useful in
T. Oyama et al. / Solar Energy 77 (2004) 525–532
1.0
80
0.8
0.01
60
0.6 40 0.4 20
0.2 0 0 0
2
4 6 8 10 Flow rate (litre min-1)
12
14
Fig. 5. Dependence of the apparent rate constant on the circulation flow rate in the photodegradation of the detergent. Initial concentration of detergent, 0.1 g l1; volume of the reactant solution, 3 l; TiO2 loading, 6.0 g l1; initial pH, 2.8.
lnðC t =C 0 Þ ¼ k app Etot Z ¼ k app S E dt
ð3Þ ð4Þ
analyzing solar experimental results (see e.g. Dillert et al., 1999a,b); where Ct is the time-dependent concentration of non-degraded reactant(s), C0 reflects the initial concentration of reactant(s), S is the area of the mirror solar concentrator, t is the irradiation time, E is the light irradiance (W cm2), kapp is the apparent rate constant, and Etot = SE dt represents the total light energy radiation (in Joules) impinging on the photoreactive system. The validity of Eqs. (3) and (4) was demonstrated in earlier solar experiments (Zhang et al., 2001, 2002, 2003). Clearly, the degradation kinet-
0
20
40
60
80
100
120
COD (mg litre-1)
Ct/C0
Apparent rate constant (kJ-1)
0.02
529
0
Light energy (kJ) Fig. 7. Decrease of COD (triangles) of the detergent and decrease of the concentration of the detergent (circles) and that of the fluorescent fabric-whitening agent (squares) upon irradiation with sunlight. Initial concentration of the detergent, 0.1 g l1; volume of the reactant solution, 3 l; TiO2 loading, 6.0 g l1; circulation flow rate, 4.9 l min1; initial pH = 4.9.
ics are independent of the solar light irradiance in the range 1.0–3.5 mW cm2 measured at 362 nm. Fig. 3 illustrates the first-order kinetic plots derived from the data of Fig. 2. Light energy was calculated by use of the light irradiance measured between 330 and 380 nm (maximal emission at 362 nm) correcting for mirror reflectance and glass transmittance. The degradation of the detergent under solar radiation exposure in the presence of the TiO2 photocatalyst was 20-fold faster (1.55 ± 0.01 · 102 kJ1) than in the absence of TiO2 (0.090 ± 0.002 · 102 kJ1). Such apparent rate constants as obtained by this procedure can be used to compare the effects that the experimental operational parameters have on the photodegradation process carried out under sunlight (one sun) or concentrated sunlight irradiation.
Apparent rate constant (kJ-1)
0.03
3.2. Dependence of degradation kinetics on TiO2 loading 0.02
0.01
0 2
4
6 pH
8
10
Fig. 6. Dependence of the apparent reaction rate constant of photocatalytic degradation of the detergent on the pH of the solution. Initial concentration of the detergent, 0.1 g l1; volume of the reactant solution, 3 l; TiO2 loading, 6.0 g l1; circulation flow rate, 4.9 l min1.
The effects of TiO2 loading on process kinetics (kapp) are depicted in Fig. 4. The apparent rate constants of the photocatalyzed decomposition increase rather rapidly initially with TiO2 loading reaching a plateau at 6 g l1 (maximal kapp = 0.021 ± 0.001 kJ1). We attribute this behavior to increased absorption of photons at the higher loadings. However, at these higher TiO2 loadings, scattering becomes a significant problem since the particulates can filter the solar radiation from the rest of the dispersion. In this regard, the scattering coefficient is known to be significantly greater than the absorption coefficient for aqueous TiO2 dispersions over a large range of wavelengths (see e.g. Cabrera et al., 1996). This notwithstanding, an interesting advantage of the higher loadings of photocatalyst in the suspensions is the easier separation of the particulates from what occurs at the
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lower loadings (Xi and Geissen, 2001). This is particularly relevant in a practical treatment process when the photocatalyst needs to be recovered from the degraded/treated wastewaters. The TiO2 particulates in the degraded solution were more readily separated than in pure water. After the irradiation experiment, the TiO2 particulates in the dispersion settled in 9 h. Clearly, a compromise must be reached in real situations between practical kinetics and recovery of photocatalyst. 3.3. Effect of circulation flow rate Apparent first-order rate constants for the degradation of the detergent are plotted against flow rate in Fig. 5. The increase of flow rate of the dispersion through the solar irradiated photoreactor enhanced the rate of photodegradation of the detergent owing to the vigorous turbulent flow, which maintained the TiO2 particulates and the detergent solution suitably dispersed in a manner that optimized mass transfer of the substrate onto the TiO2 photocatalyst particles and maximized aeration of the dispersion to introduce the essential reagent oxygen. Oxygen is an essential reagent in a heterogeneous photocatalytic reaction because it minimizes/ suppresses recombination of photogenerated electrons (e) and holes (h+). In all our experiments, the feed tank was constantly left open to the atmosphere as a means of introducing air oxygen into the aqueous TiO2 dispersions. Perusal of the data of Fig. 5 shows that maximal kapp (= 0.011 ± 0.001 kJ1) is reached at a flow rate of 2 l min1. Nonetheless, a flow rate of 4.9 l min1 was chosen for further solar experimental runs to make certain that the flow rate had no further effect on the degradation kinetics. Although higher flow rates are beneficial to disperse TiO2 aggregates, excessive foaming action occurred in the detergent solution. The foam so produced in the tank disappeared with irradiation time. As a test, the initial dispersion and the dispersion after 2 h of irradiation were vigorously shaken for 5 min resulting in considerable foaming action in the former but none in the irradiated dispersions. 3.4. pH dependence of the photocatalyzed degradation Rates of the photocatalyzed degradation of the detergent depend on the pH of the aqueous TiO2 dispersions as illustrated in Fig. 6, which reports apparent rate constants versus pH. The maximal kapp constants are obtained in the pH range 5–5.5. The point of zero charge (pzc) of P-25 TiO2 is known to occur at pH 6.8 (Zhao et al., 1993) at which the surface of the particles is electrically neutral. Below the pzc, the surface of TiO2 is positively charged and above the pzc it is negatively charged. We should point out, however, that adsorbates, impurities and additives have a strong influ-
ence on the position of the pzc of TiO2 so that it would not be unlikely for the maximal pH range of 5–5.5 to reflect closely the range in which the pzc of TiO2 may be under the prevailing conditions. Since the maximal rate was obtained experimentally at pH = 4.9, further experiments were done at this pH. Adsorption of the anionic detergent on the surface of TiO2 particles is strongly affected by the nature of the surface, which in turn affects the degradative process. As well, changes in surface potential as may occur by changes in pH (together with adsorbates, impurities, synthetic methods, surface reconstruction, and others) also impacts on the relative concentration of the charge carriers on the surface and thus on the relative efficiency of the photooxidative process (Emeline et al., 2003). Accordingly, the drop in the rate below and above the pH range 5–5.5 is probably due to a number of factors the nature of which was not explored in the present study. 3.5. Reduction of COD and change of surface tension upon exposing the detergent dispersion to sunlight radiation The time profile of the reduction of COD in the photodegradation of the detergent following exposure to concentrated sunlight is illustrated in Fig. 7. The decay of COD follows a gradual linear decrease by zero-order kinetics (k = 0.65 ± 0.03 mg l1 kJ1) to zero COD after exposure of the dispersion to a total of 130 kJ of sunlight energy. The initial increase in COD and then decrease with increase in total light energy reflects strong competitive adsorption of various initial reactants, degradation intermediates and byproducts on the TiO2 surface, and is also due to differences of oxidation efficiencies of the various organic species by KMnO4. Thus, compared to the decrease of COD, the detergent degraded faster indicating that complete mineralization of the detergent may require longer irradiation times (i.e. continued exposure to sunlight energy). The surface tension of the aqueous TiO2 dispersion was nearly identical to that of water (72 mN m1 at 25 C) after only a short irradiation time of 25 min. This increase in surface tension of the dispersion was relatively faster than the temporal decrease of the concentration of the detergent (see e.g. Fig. 2), which necessitates several hours for complete degradation. 3.6. Photocatalytic degradation of the fabric-whitening agent The photocatalyzed degradation of the fluorescent fabric-whitening agent in the commercial detergent was monitored by measuring the temporal fluorescence intensity using a spectrofluorimeter (excitation wavelength, 345 nm). Again, concentration ratios (Ct/C0)
T. Oyama et al. / Solar Energy 77 (2004) 525–532
have been used to illustrate the degree of degradation of this whitening agent. We ascertained experimentally that the fluorescence intensity scaled linearly with the concentration of the detergent. The fabric-whitening agent degraded fairly rapidly (kapp = 0.21 ± 0.02 kJ1) as evidenced by the results presented in Fig. 7, whereas degradation of the detergent was two times slower (kapp = 0.11 ± 0.01 kJ1).
4. Conclusions A typical commercial detergent whose biodegradation is remarkably slow was photodegraded fairly rapidly under concentrated sunlight irradiation conditions. TiO2 loading and flow rates influenced the degradation process. The present study demonstrates that a TiO2 loading of 6 g l1 and a flow rate of 4.9 l min1 are practical operational parameters for an apparatus of the type used in this study and for the detergent examined. Other feeds will no doubt require modifications of these optimal conditions. Foaming behavior increased with flow rate. The increase in surface tension as the degradation proceeded was faster than the temporal degradation of the detergent. The present results on the TiO2 photocatalytic remediation of a wastewater containing a commercial detergent with (concentrated) sunlight irradiation add to our knowledge base and demonstrate the practicality of this methodology.
Acknowledgments We are grateful to the Frontier Research Foundation and to the Japanese Ministry of Education, Sports, Culture, Science and Technology for a Grant-in-Aid for Science Research (No. 10640569 to H.H.) in support of this work. We also wish to acknowledge the National Natural Science Foundation of China for sponsoring the studies in Beijing (Nos. 29677019 and 29725715 to J.Z.), and the Ministero dellÕIstruzione, dellÕUniversita e della Ricerca (MIUR – Roma; to N.S.) for support of the research carried out in Pavia.
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Solar photocatalysis, photodegradation of a commercial detergent in aqueous TiO2 dispersions under sunlight irradiation Toshiyuki Oyama a, Akio Aoshima a, Satoshi Horikoshi a, Hisao Hidaka Jincai Zhao b, Nick Serpone c a
a,*
,
Frontier Research Center for the Global Environment Science, Meisei University, 2-1-1 Hodokubo, Hino-shi, Tokyo 191-8506, Japan b Molecular Sciences Center, Institute of Chemistry, Chinese Academy of Science, Beijing 100 080, China c Dipartimento di Chimica Organica, Universita´ di Pavia, Via Taramelli 10, Pavia 27100, Italia Received 17 November 2003; received in revised form 16 April 2004; accepted 16 April 2004 Available online 6 August 2004 Communicated by: Associate Editor Sixto Malato-Rodrı´guez
Abstract A commercial detergent whose major components are an anionic surfactant and a fluorescent whitening agent can be photodegraded in aqueous TiO2 dispersions under irradiation with concentrated sunlight in the presence of air. The degradation process followed apparent first-order kinetics in terms of the total sunlight energy impinging on the photoreactive system. The effects of (a) TiO2 loading, (b) circulation flow rate, and (c) pH of the reactant solution on the kinetics of decomposition of the detergent were examined. Under the prevailing conditions, the optimal operational parameters for this detergent were, respectively: TiO2 loading, 6 g l1; circulation flow rate, 4.9 l min1; and pH, 4.9. The rate of increase of the surface tension was greater than the rate of decrease of the concentration of the detergent. This study adds to our knowledge base in the effective use of sunlight irradiation to detoxify wastewaters containing undesirable detergents. 2004 Elsevier Ltd. All rights reserved. Keywords: Solar photocatalysis; Titanium dioxide photodegradation; Detergent wastewaters; Concentrated sunlight
1. Introduction Organic chemical products such as detergents, dyes, pesticides, herbicides and endocrine disruptors lead to serious environmental contamination. In particular, var-
* Corresponding author. Tel.: +81 42 599 7785; fax: +81 42 599 7785/44 591 6635. E-mail address: [email protected] (H. Hidaka).
ious types of detergents are widely employed in the domestic and industrial fields. They are one of the major causes of pollution of surface and ground waters. To the extent that the main ingredient of detergents containing a fabric-whitening agent is only slowly and inefficiently biodegraded by bacteria, wastewater detergents can cause undue ecological damage to animals and plants. Accordingly, remediation technologies are required to decompose and completely mineralize such organic pollutants. A most attractive technology in this regard is the
0038-092X/$ - see front matter 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.solener.2004.04.020
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TiO2-mediated photocatalytic method through which wastewater pollutants can be degraded oxidatively by the strongly oxidizing OH radical species. With this methodology, an organic pollutant can be degraded in aqueous dispersions containing the TiO2 photocatalyst by UV irradiation from a suitable UV-light source and in the presence of oxygen (Hidaka et al., 1985; Herrmann, 1999; Hoffmann et al., 1995; Kamat, 1993; Kamat and Vinodgopal, 1993; Legrini et al., 1993). UV irradiation at energies greater than or equal to the bandgap energy of the TiO2 photocatalyst (3.2 eV, anatase form) produces electrons in the conduction band and positive holes in the valence band (Eq. (1)). Subsequent to migration to the surface through diffusion and/or drift (Emeline et al., 2003) TiO2 þ hm ! e þ hþ
ð1Þ
followed by trapping by surface defect sites and adsorbates, the electron reacts with adsorbed oxygen to produce reactive oxygen species and holes react with surface bound OH groups (or H2O) to yield OH radicals that ultimately oxidize the polluting substrates (Eq. (2)). e þ O ! O ð2aÞ 2
2
H2 O ! Hþ þ OH
ð2bÞ
þ O 2 þ H ! OOH
ð2cÞ
hþ þ OH ! OH
ð2dÞ
hþ þ H2 O ! OH þ Hþ
ð2eÞ
OH þ RH ! oxidized intermediates ! CO2 þ H2 O
ð2fÞ
Utilization of sunlight as the UV energy source to drive such reactions is beneficial from an ecological point of view and where appropriate from an economic standpoint. Several studies of photocatalytic degradations carried out under solar exposure have been reported by various workers (see e.g. Hidaka et al., 1989; Muradov, 1994; Minero et al., 1996; Nogueira and Jardim, 1996, 1998; Van Well et al., 1997; Dillert et al., 1999a,b; Gimenez et al., 1999; Romero et al., 1996; Alfano et al., 2000; Jimenez et al., 2000; Malato et al., 2000, 2002). Most such reports employed solar collectors to irradiate the photoreactive systems examined. In earlier studies, we reported the TiO2-mediated photocatalytic degradation of several surfactants such as sodium dodecylbenzenesulfonate (NaDBS; Hidaka et al., 1989; Zhang et al., 2003), benzyldodecyldimethyl ammonium chloride (BDAC) and p-nonylphenyl-polyoxyethylene (NPPE; Hidaka et al., 1989), and sodium benzenesulfonate (NaBS; Zhang et al., 2001) together with the methylene blue dye (Zhang et al., 2002) using
artificial UV sources to examine the dynamics and the mechanistic details of the photodegradative process(es). In the present article, we report a recent study carried out to assess the photodegradation of a representative commercial detergent (henceforth referred to simply as detergent) that contains a fluorescent fabric-whitening agent in an aqueous TiO2 dispersion UV-irradiated by concentrated sunlight using a relatively inexpensive circular solar collector. The effects of such operational parameters as (a) titanium dioxide loading, (b) circulation flow rate, and (c) changes in pH of the dispersion on the overall process dynamics were examined to evaluate whether it was also practical to use solar (concentrated) irradiation to degrade a real wastewater polluted with detergents as others have reported albeit for other classes of pollutants (see e.g. Guillard et al., 2003; Kositzi et al., 2004; Malato et al., 2003, 2004).
2. Experimental section 2.1. Materials Nanoparticulate TiO2 (P-25; surface area, 53 m2 g1; mean diameter, 30 nm; composition, 87% anatase and 13% rutile) was kindly supplied by the Aerosil Nippon Co. The commercial detergent was purchased from Lion Co. Ltd. of Japan. The major ingredients in this detergent are an a-sodium fatty acid ester sulfonate, a linear alkylaryl sulfonate, a sodium salt of a fatty acid, aluminosilicate, sodium carbonate, enzymes, and a fabric-whitening agent (namely, bistriazinyl derivatives of 4,4 0 -diaminostilbene-2,2 0 -disulfonic acid). Ion-exchanged water was used throughout to prepare the desired experimental solutions and dispersions. 2.2. Solar photodegradation set-up and analysis A 3-l volume of an aqueous detergent/TiO2 mixture (0.1 g of detergent in 1 l of water; various loadings of TiO2) was prepared and placed in a 10-l feed tank. The tank containing the reaction mixture was placed in a water bath. Subsequently, the contents were dispersed by ultrasonication for 30 min to obtain an optimally dispersed system and complete adsorption/ desorption equilibration. The pH of the TiO2 dispersions was adjusted with either dilute NaOH or HClO4 solutions, as needed. The detergent was photocatalytically degraded in a batch mode reactor with a circulation system (PR-KV2, Furue Sci. Co. Ltd.) exposed to sunlight using a mirror concentrator (Mitaka Kohki Co. Ltd.) designed to face the sun constantly so as to utilize maximal sunlight radiation during daylight. The features of the reactor setup (Fig. 1) consisted essentially of a round concave mirror (aperture diameter, 1.0 m; mirror area, 0.785 m2) for geometric concentration of
T. Oyama et al. / Solar Energy 77 (2004) 525–532
527
Fig. 1. (a) Schematic diagram of the experimental setup; (b) photograph showing the actual solar concentrator and reactor setup.
sunlight equivalent to 70 suns, and a flask-type photoreactor (Pyrex walls; volume, 1.3 l, bottom diameter, 10 cm). Most of the solar experiments were carried out at a flow rate of 4.9 l min1 (see below) to maintain the TiO2 dispersion sufficiently stirred in the plastic tubes, the feed tank, and the flask reactor, as well as to maintain air saturation of the reactant solution with air oxygen. The temperature of the dispersion in the flask reactor during solar irradiation ranged between 30 and 58 C. Appropriate samples were collected at various time intervals. After centrifugation, the samples were filtered
through a Millipore membrane (0.2 lm) to remove the titania particulates. Filtrates were used for various analytical assays. The temporal changes in the concentration of the detergent were monitored with a JASCO V-570 UV-visible spectrophotometer (kmax 224 nm; UV absorption of the aromatic group). The surface tension of the bulk solution was determined with an automatic surface tensiometer (CBVP-Z, Kyowa Interface Science Co. Ltd.). The concentration of the fluorescent whitening reagent was measured using a JASCO FP6500 spectrofluorimeter (emission wavelength, 345 nm). Chemical oxygen demand (COD) was assayed with
T. Oyama et al. / Solar Energy 77 (2004) 525–532
a digital COD meter (HC-507, Central Kagaku) without previous separation of the TiO2 particulates. The sunlight intensity impinging on the reactor setup was measured with a UV radiometer (UVR-36, Topcon Co.; maximal measuring wavelength, 362 nm; light irradiance, 1.0–3.5 mW cm2). The total energy of the sunlight incident on the photoreactive system was calculated from the light irradiance {see Serpone and Emeline, 2002 for a description of suggested terms and definitions in photocatalysis and radiocatalysis}, the surface area of the aluminum mirror (concave side; covered with a resin), the reflectivity of the mirror (85%), the transmittance of the Pyrex glass (90%), and the irradiation time using an on-site computer integrated to the reactor setup (Fig. 1).
0 (a)
ln(Ct/C0)
528
-1 (b)
-2
0
50 Light energy (kJ)
100
Fig. 3. First-order kinetic plots derived from the data of Fig. 2 in terms of total solar light energy impinging on the photoreactive system: (a) sunlight irradiation in the absence of TiO2; (b) sunlight irradiation in the presence of TiO2.
3. Results and discussion 3.1. Photocatalyzed decomposition of the detergent
Ct/C0
1.0
(a) (b)
(c) 0
50 Irradiation time (min)
0.02
0.01
0 0
2
4 6 8 TiO2 loading (g litre-1)
10
12
Fig. 4. Dependence of the apparent rate constant on the TiO2 loading in the photodegradation of the detergent. Initial concentration of detergent, 0.1 g l1; volume of the reactant solution, 3 l; circulation flow rate, 4.9 l min1; initial pH, 8.9.
0.5
0
0.03 Apparent rate constant (kJ-1)
The temporal concentration changes of the aqueous detergent solutions (measured pH, 8.9) under sunlight irradiation are illustrated in Fig. 2 (error bars shown in Figs. 2–7 represent the standard deviation). Because of the compositional complexity of the detergent, the data are reported as Ct/C0 ratios from the absorbance data at 224 nm since the ratio At/A0 scales with Ct/C0. The detergent decomposed neither in the dark nor under concentrated sunlight irradiation in the absence of TiO2, the principal sunlight absorber. By contrast, the detergent degraded significantly in TiO2 dispersions under sunlight irradiation through first-order kinetics
100
Fig. 2. Photocatalytic degradation of the commercial detergent in aqueous TiO2 dispersions under sunlight irradiation: (a) dark reaction; (b) sunlight irradiation in the absence of TiO2; (c) sunlight irradiation in the presence of TiO2. Experimental conditions: initial concentration of the detergent, 0.1 g l1; volume of the reactant solution, 3 l; TiO2 loading, 6.0 g l1; circulation flow rate, 4.9 l min1; initial pH, 8.9.
(k = 0.025 ± 0.004 min1). These results show that the decrease in the concentration of detergent in aqueous TiO2 dispersions upon sunlight irradiation is due mostly to the photocatalytic process. The photocatalytic decomposition of organic compounds in aqueous TiO2 dispersions under solar UV light at constant intensity typically follows pseudo first-order kinetics at relatively low pollutant load. However, sunlight intensity changes daily and throughout the day. Accordingly, to compare the results of different experimental runs obtained under different solar irradiation conditions or during different days, a kinetic analysis involving irradiation time as the variable is not appropriate. The kinetic expression embodied by Eqs. (3) and (4) is useful in
T. Oyama et al. / Solar Energy 77 (2004) 525–532
1.0
80
0.8
0.01
60
0.6 40 0.4 20
0.2 0 0 0
2
4 6 8 10 Flow rate (litre min-1)
12
14
Fig. 5. Dependence of the apparent rate constant on the circulation flow rate in the photodegradation of the detergent. Initial concentration of detergent, 0.1 g l1; volume of the reactant solution, 3 l; TiO2 loading, 6.0 g l1; initial pH, 2.8.
lnðC t =C 0 Þ ¼ k app Etot Z ¼ k app S E dt
ð3Þ ð4Þ
analyzing solar experimental results (see e.g. Dillert et al., 1999a,b); where Ct is the time-dependent concentration of non-degraded reactant(s), C0 reflects the initial concentration of reactant(s), S is the area of the mirror solar concentrator, t is the irradiation time, E is the light irradiance (W cm2), kapp is the apparent rate constant, and Etot = SE dt represents the total light energy radiation (in Joules) impinging on the photoreactive system. The validity of Eqs. (3) and (4) was demonstrated in earlier solar experiments (Zhang et al., 2001, 2002, 2003). Clearly, the degradation kinet-
0
20
40
60
80
100
120
COD (mg litre-1)
Ct/C0
Apparent rate constant (kJ-1)
0.02
529
0
Light energy (kJ) Fig. 7. Decrease of COD (triangles) of the detergent and decrease of the concentration of the detergent (circles) and that of the fluorescent fabric-whitening agent (squares) upon irradiation with sunlight. Initial concentration of the detergent, 0.1 g l1; volume of the reactant solution, 3 l; TiO2 loading, 6.0 g l1; circulation flow rate, 4.9 l min1; initial pH = 4.9.
ics are independent of the solar light irradiance in the range 1.0–3.5 mW cm2 measured at 362 nm. Fig. 3 illustrates the first-order kinetic plots derived from the data of Fig. 2. Light energy was calculated by use of the light irradiance measured between 330 and 380 nm (maximal emission at 362 nm) correcting for mirror reflectance and glass transmittance. The degradation of the detergent under solar radiation exposure in the presence of the TiO2 photocatalyst was 20-fold faster (1.55 ± 0.01 · 102 kJ1) than in the absence of TiO2 (0.090 ± 0.002 · 102 kJ1). Such apparent rate constants as obtained by this procedure can be used to compare the effects that the experimental operational parameters have on the photodegradation process carried out under sunlight (one sun) or concentrated sunlight irradiation.
Apparent rate constant (kJ-1)
0.03
3.2. Dependence of degradation kinetics on TiO2 loading 0.02
0.01
0 2
4
6 pH
8
10
Fig. 6. Dependence of the apparent reaction rate constant of photocatalytic degradation of the detergent on the pH of the solution. Initial concentration of the detergent, 0.1 g l1; volume of the reactant solution, 3 l; TiO2 loading, 6.0 g l1; circulation flow rate, 4.9 l min1.
The effects of TiO2 loading on process kinetics (kapp) are depicted in Fig. 4. The apparent rate constants of the photocatalyzed decomposition increase rather rapidly initially with TiO2 loading reaching a plateau at 6 g l1 (maximal kapp = 0.021 ± 0.001 kJ1). We attribute this behavior to increased absorption of photons at the higher loadings. However, at these higher TiO2 loadings, scattering becomes a significant problem since the particulates can filter the solar radiation from the rest of the dispersion. In this regard, the scattering coefficient is known to be significantly greater than the absorption coefficient for aqueous TiO2 dispersions over a large range of wavelengths (see e.g. Cabrera et al., 1996). This notwithstanding, an interesting advantage of the higher loadings of photocatalyst in the suspensions is the easier separation of the particulates from what occurs at the
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lower loadings (Xi and Geissen, 2001). This is particularly relevant in a practical treatment process when the photocatalyst needs to be recovered from the degraded/treated wastewaters. The TiO2 particulates in the degraded solution were more readily separated than in pure water. After the irradiation experiment, the TiO2 particulates in the dispersion settled in 9 h. Clearly, a compromise must be reached in real situations between practical kinetics and recovery of photocatalyst. 3.3. Effect of circulation flow rate Apparent first-order rate constants for the degradation of the detergent are plotted against flow rate in Fig. 5. The increase of flow rate of the dispersion through the solar irradiated photoreactor enhanced the rate of photodegradation of the detergent owing to the vigorous turbulent flow, which maintained the TiO2 particulates and the detergent solution suitably dispersed in a manner that optimized mass transfer of the substrate onto the TiO2 photocatalyst particles and maximized aeration of the dispersion to introduce the essential reagent oxygen. Oxygen is an essential reagent in a heterogeneous photocatalytic reaction because it minimizes/ suppresses recombination of photogenerated electrons (e) and holes (h+). In all our experiments, the feed tank was constantly left open to the atmosphere as a means of introducing air oxygen into the aqueous TiO2 dispersions. Perusal of the data of Fig. 5 shows that maximal kapp (= 0.011 ± 0.001 kJ1) is reached at a flow rate of 2 l min1. Nonetheless, a flow rate of 4.9 l min1 was chosen for further solar experimental runs to make certain that the flow rate had no further effect on the degradation kinetics. Although higher flow rates are beneficial to disperse TiO2 aggregates, excessive foaming action occurred in the detergent solution. The foam so produced in the tank disappeared with irradiation time. As a test, the initial dispersion and the dispersion after 2 h of irradiation were vigorously shaken for 5 min resulting in considerable foaming action in the former but none in the irradiated dispersions. 3.4. pH dependence of the photocatalyzed degradation Rates of the photocatalyzed degradation of the detergent depend on the pH of the aqueous TiO2 dispersions as illustrated in Fig. 6, which reports apparent rate constants versus pH. The maximal kapp constants are obtained in the pH range 5–5.5. The point of zero charge (pzc) of P-25 TiO2 is known to occur at pH 6.8 (Zhao et al., 1993) at which the surface of the particles is electrically neutral. Below the pzc, the surface of TiO2 is positively charged and above the pzc it is negatively charged. We should point out, however, that adsorbates, impurities and additives have a strong influ-
ence on the position of the pzc of TiO2 so that it would not be unlikely for the maximal pH range of 5–5.5 to reflect closely the range in which the pzc of TiO2 may be under the prevailing conditions. Since the maximal rate was obtained experimentally at pH = 4.9, further experiments were done at this pH. Adsorption of the anionic detergent on the surface of TiO2 particles is strongly affected by the nature of the surface, which in turn affects the degradative process. As well, changes in surface potential as may occur by changes in pH (together with adsorbates, impurities, synthetic methods, surface reconstruction, and others) also impacts on the relative concentration of the charge carriers on the surface and thus on the relative efficiency of the photooxidative process (Emeline et al., 2003). Accordingly, the drop in the rate below and above the pH range 5–5.5 is probably due to a number of factors the nature of which was not explored in the present study. 3.5. Reduction of COD and change of surface tension upon exposing the detergent dispersion to sunlight radiation The time profile of the reduction of COD in the photodegradation of the detergent following exposure to concentrated sunlight is illustrated in Fig. 7. The decay of COD follows a gradual linear decrease by zero-order kinetics (k = 0.65 ± 0.03 mg l1 kJ1) to zero COD after exposure of the dispersion to a total of 130 kJ of sunlight energy. The initial increase in COD and then decrease with increase in total light energy reflects strong competitive adsorption of various initial reactants, degradation intermediates and byproducts on the TiO2 surface, and is also due to differences of oxidation efficiencies of the various organic species by KMnO4. Thus, compared to the decrease of COD, the detergent degraded faster indicating that complete mineralization of the detergent may require longer irradiation times (i.e. continued exposure to sunlight energy). The surface tension of the aqueous TiO2 dispersion was nearly identical to that of water (72 mN m1 at 25 C) after only a short irradiation time of 25 min. This increase in surface tension of the dispersion was relatively faster than the temporal decrease of the concentration of the detergent (see e.g. Fig. 2), which necessitates several hours for complete degradation. 3.6. Photocatalytic degradation of the fabric-whitening agent The photocatalyzed degradation of the fluorescent fabric-whitening agent in the commercial detergent was monitored by measuring the temporal fluorescence intensity using a spectrofluorimeter (excitation wavelength, 345 nm). Again, concentration ratios (Ct/C0)
T. Oyama et al. / Solar Energy 77 (2004) 525–532
have been used to illustrate the degree of degradation of this whitening agent. We ascertained experimentally that the fluorescence intensity scaled linearly with the concentration of the detergent. The fabric-whitening agent degraded fairly rapidly (kapp = 0.21 ± 0.02 kJ1) as evidenced by the results presented in Fig. 7, whereas degradation of the detergent was two times slower (kapp = 0.11 ± 0.01 kJ1).
4. Conclusions A typical commercial detergent whose biodegradation is remarkably slow was photodegraded fairly rapidly under concentrated sunlight irradiation conditions. TiO2 loading and flow rates influenced the degradation process. The present study demonstrates that a TiO2 loading of 6 g l1 and a flow rate of 4.9 l min1 are practical operational parameters for an apparatus of the type used in this study and for the detergent examined. Other feeds will no doubt require modifications of these optimal conditions. Foaming behavior increased with flow rate. The increase in surface tension as the degradation proceeded was faster than the temporal degradation of the detergent. The present results on the TiO2 photocatalytic remediation of a wastewater containing a commercial detergent with (concentrated) sunlight irradiation add to our knowledge base and demonstrate the practicality of this methodology.
Acknowledgments We are grateful to the Frontier Research Foundation and to the Japanese Ministry of Education, Sports, Culture, Science and Technology for a Grant-in-Aid for Science Research (No. 10640569 to H.H.) in support of this work. We also wish to acknowledge the National Natural Science Foundation of China for sponsoring the studies in Beijing (Nos. 29677019 and 29725715 to J.Z.), and the Ministero dellÕIstruzione, dellÕUniversita e della Ricerca (MIUR – Roma; to N.S.) for support of the research carried out in Pavia.
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