TiO2-fixed-bed reactor for water decontamination using solar light

Pergamon

PIh S0038-092X (96) 00036-9

Solar Energy Vol. 56, No. 5, pp. 471-477, 1996 Copyright © 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0038-092X/96 $15.00 + 0.00

TiO2-FIXED-BED REACTOR FOR WATER DECONTAMINATION USING SOLAR LIGHT RAQUEL F. P. NOGUEIRA and WILSON F. JARDIM t Universidade Estadual de Campinas, Instituto de Quimica, CP 6154, C E P 13081-970, Campinas, SP, Brazil Abstract A photocatalytic reactor using immobilized TiO 2 (Degussa P25) on a glass plate was studied on a bench scale using solar light as the source of radiation. The influence of parameters such as the slope of the plate, solar light intensity, flow rate and molar flow rate, as well as the geometry of the reactor, was studied using dichloroacetic acid (DCA) as a model compound. A linear dependence of degradation with solar light intensity, measured at 365 nm, was observed. Experiments with recirculation as well as a single pass of solution suggested no mass transfer limitations in this system. The mineralization of DCA resulted in the production of quantitative a m o u n t s of chloride ions. An initial concentration of 5 m m o l / L of D C A decayed to 2 m m o l / L in about 2 min of irradiation. An exponential decay of degradation was observed with an increase of the molar flow rate, achieving saturation around 1.5 mmol DCA/min. Copyright © 1996 Elsevier Science Ltd.

1. INTRODUCTION The contamination of our environment has been pointed out as one of the major problems of modern society. As a result of this concern and of the risks posed to human health, new regulations have been adopted to prevent and minimize this risk. New wastewater treatment technologies aiming for the complete mineralization of organic contaminants are presently believed to be the most suitable solution to comply with new contaminant levels in accordance with stricter regulations. Chemical water treatment has developed improvements in oxidative degradation procedures for organic contaminants in aqueous media using catalytic and photochemical methods. The so called Advanced Oxidation Processes (AOP) based on hydroxyl radical formation ('OH), a powerful oxidizing agent, has attracted great interest in recent years (Legrini et al., 1993; Huang et al., 1993). Among AOP, photocatalysis may play an important role in water treatment technologies. Recent literature on photocatalysis applied to decontamination of water and wastewater is now available, showing the mineralization of a variety of contaminants, many resistant to biological degradation. Its potential for application is also discussed (Serpone and Pelizzetti, 1989; Matthews, 1991b; Ollis et al., 1991; Fox, 1992; Mills et al., 1993; Fox and Dulay, 1993; Hermann et al., 1993; Pichat, 1994; Hoffmann et al., 1995). t Author to whom correspondence should be addressed. 471

Some semiconductors, such as CdS, ZnO and WO3, are capable of promoting oxidation of organic contaminants. However, TiO2 is the most widely used, mainly because of its high photoactivity and stability over a wide range of pH and non-toxicity. Upon near-UV irradiation, TiO/generates electron/hole (e-/h +) pairs, eqn (1), in which the holes show a very positive potential (from 2.0 to 3.5 V vs SCE) capable of promoting the oxidation of many organic contaminants. TiO2+hv--,TiO:(e-

+ h +)

(1)

Hydroxyl radicals ('OH) are supposed to be the primary oxidant, as with other AOP, generated by the reaction of the hole with OH-, leading to the destruction of organic contaminants, eqn (2), although direct oxidation by photogenerated holes has also been suggested (Fox and Dulay, 1993). TiOz(h +) +

O H a d s --~ " O H a d s +

TiO2

(2)

TiO/ (anatase) has an energy bandgap of 3.2 eV and is capable of activation by near UV-light with wavelengths up to 388 nm. This absorption corresponds to between 3 and 4% of the solar spectrum. Although this is a small fraction of the spectrum, many studies have been carried out to develop an efficient method for using solar radiation to destroy toxic organic compounds (Alpert et al., 1991; Ollis, 1991; Zhang et al., 1994; Muradov, 1994; Matthews, 1991a; Matthews and McEvoy, 1992; Blanco et al., 1991; Fox, 1992; Hidaka et al., 1989). The use of solar energy has been stimulated in recent years as a result of the potential use

472

R . F . P . Nogueira and W. F. Jardim

of concentrated solar photons for photochemical, photoelectrochemical and thermal processes (Glatzmaier, 1991; Gupta and Anderson 1991; Esser et al., 1994). Nevertheless, the use of this source of energy is still very incipient considering the amount of energy available, especially in Brazil, where, because of its geographical localization (between 5°N and 32°S), there is an average solar incidence of 2000h/year (Moreira and Rosa, 1986; Luiz, 1985). Most of the waste produced in Brazil is concentrated in the industrialized part of the country, mainly at the state of S~o Paulo, where 820,000 tons of hazardous waste are generated per year (Passos et al., 1994). In this scenario, it would be very interesting to develop this photocatalytic process for the decontamination of wastewater using the available solar light. In this work, a fixed-bed solar reactor using TiO2 as a photocatalyst was studied in the photodegradation of a chlorinated organic compound. TiO2 was used in the immobilized form using a fiat glass plate as a support. The first prototype of this reactor was developed in Unicamp by Kondo (1990), later studied by Hilgendorf et al. (1992) and examined in detail by Nogueira (1995). 2. E X P E R I M E N T A L

2.1. Chemicals Titanium dioxide (P25, Degussa), 70:30 anatase form, BET surface area 50 me/g, 30 nm mean particle size, was used as a photocatalyst. Dichloroacetic acid (DCA) from Merck was used in most of the experiments as a model compound. Its high solubility in water, facility of analysis and quantification of the degradation product (C1-), with no losses due to evaporation, were some of the reasons for the use of this compound as a model. The pH of the DCA solutions of different initial concentrations was adjusted to 3 before all experiments. Methylene blue (Merck) was used in preliminary experiments at a concentration of 0.1 mmol/L. 2.2. Ti02 immobilization Immobilization of the catalyst was performed by applying a 10% (wt) TiO2 aqueous suspension of TiO: to flat glass plates. Two different geometries of reactor were tested. The dimensions of plate A were 60 × 90 cm, whereas plate B measured 50 × i10 cm, resulting in approximately the same exposed area of 0.48 m 2. The TiO2 suspension was passed by gravity flow

over the plate and dried with hot air for the immobilization of TiO 2. This procedure was repeated several times until an homogeneous TiO 2 film, with about 10g TiO2/m 2 was attached to the glass. This photocatalyst film absorbed 80% of the solar irradiation (365 nm) and allowed an ideal flow of aqueous solutions over the TiO2 surface. The titanium dioxide film remained stable during the flow of aqueous solutions and no loss of photocatalytic activity was observed during 40 h of use.

2.3. Photodegradation procedure The TiO 2 immobilized glass plate was placed on a wooden support facing towards the equator with an inclination angle of 22 ° as shown in Fig. 1. The reactor worked in a way that the solution containing the substrate to be destroyed was pumped at a fixed flow rate, with the help of a peristaltic pump (Ismatec), to the top of the plate and flowed to the bottom by gravity while illuminated. The reactor was operated at flow rates ranging from 2 to 6 L/h. Two different experiments were performed: (1) single pass mode, when 500mL of solution were passed over the plate and collected at the bottom; and (2) recirculation mode, where 1000 mL of solution were recirculated for 1 h. Samples were withdrawn before and after illumination in the single pass mode, and at 20 min intervals in the recirculation experiments. A correction for the losses caused by water evaporation was made before each determination. 2.4. Analysis Total Organic Carbon (TOC) was determined using a TOC analyzer (TOC 5000, SHIMADZU). Optical absorptions of methylene blue were determined at 660nm ( e = 66700_ 350 L/cm/mol) using a Micronal (model 382) spectrophotometer. Production of chloride ions during the photo-

solar light

wastewater

~~,~2fsupp

°rtcdTiO2

Fig. 1. Schematicview of TiO2-fixed-bedsolar reactor.

Photocatalytic reactor for water decontamination

degradation of DCA was measured using an Orion ion-selective electrode and a double junction reference electrode connected to a Procyon potentiometer at constant ionic strength (NaNO3 0.1 mol/L) using a calibration curve constructed with NaC1 solutions.

473

degraded, while 89% was degraded at a slope of 25 °. This experiment was carried out in duplicate, around 12 a.m. on a summer's day. All the subsequent experiments were carried out using a slope of 22 °.

3.2. Influence of solar light intensity 2.5. Solar light intensity measurements Solar light intensity was measured using a Cole Parmer Radiometer (model 9811-50) at 365 nm at the same inclination angle as the plate. This wavelength was chosen due to absorption of TiO2 in anatase form. The average light intensity was calculated for each experiment. All the experiments, unless specified, were performed under clear sky conditions between 10 a.m. and 2 p.m. at the University campus located in Campinas, Brazil (23 ° south latitude). 3. RESULTS AND DISCUSSION

3.1. Influence of the slope of the plate The slope of the plate impregnated with the photocatalyst influences the extension of photodegradation because of two factors: (1) the thickness of the fluid film which flows over the plate and (2) the light intensity that reaches the system. At lesser slopes, the thickness of the fluid film is higher as it flows by gravity, resulting in a longer retention time. A thick film can result in mass and light transfer limitations. The problem of mass transfer limitations has been discussed for systems which use supported catalyst and is discussed in more detail later in this paper for the present reactor. Light transfer limitations can also happen, according to the Lambert-Beer law, which can have considerable influence depending on the absorption of the sample. The mineralization of methylene blue by O2 photocatalysed by TiO2 has been established previously (Matthews, 1989). Its intense blue color and solubility in water provides a simple visual demonstration of photodegradation as reported by Nogueira and Jardim (1993). The solar radiation that reaches the catalyst on the plate changes according to the time of the day and of the year for a specific slope and vice-versa. So the most suitable angle of inclination has to be determined to get the highest quantum efficiency during the time of the experiments. In a small range, between 22 ° and 25 °, using a methylene blue solution as a model substance (0.1 mmol/L), the photodegradation was favored at 22 ° slope, where 95.8% was

Some authors have shown that the increase of light intensity results in an increase of photodegradation rate of organic compounds with artificial light. At low light intensities, a linear dependence is observed, while at higher intensities this linearity no longer holds and a square root dependence is observed (Okamoto et al., 1985; Blake et al., 1991; Ollis, 1991). This behavior was also reported recently by Vincze and Kemp (1995), using a xenon lamp as a source of radiation. It has been suggested that the square root dependence indicates a limitation caused by the electron/hole recombination. This behavior can be a limitation for the use of concentrated solar systems, when the increase of degradation rate is not followed by the same factor of light concentration. For the present fixed-bed solar reactor, the dependence of photodegradation with solar irradiation is depicted in Fig. 2(a). The variation of light intensity was obtained with diurnal variation of sunlight, between 9 and 12a.m. during the autumn (April-May). The values of light intensity shown are an average of solar global radiation measured at 365 nm during the single pass of a 5 mmol/L DCA solution over the reactor. It can be observed that the degradation increases with increasing solar irradiation, as expected, showing a linear dependence with light intensity in the range of 20 to 30 W/m 2. This indicates that, at this concentration, an increase of

=.~,25

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solar light intensity 365 nm (W/m2) Fig. 2. (a) Influence of average light intensity on the photodegradation of DCA, (b) Apparent quantum yield as a function of light intensity. Co = 5 mmol/L. Q = 5 L/h. Plate A.

474

R.F.P. Nogueira and W. F. Jardim

10 W/m 2 results in about 11% more degradation of DCA at a flow rate of 5 L/h. Below this range, a smaller variation is observed. Figure2(b) shows the apparent quantum yield of photodegradation of DCA as a function of light intensity under the experimental conditions already described, calculated by eqn (3). The average value of 3.4% obtained is higher than the values obtained by Zhang (1994) using solar light and supported photocatalyst in the destruction of trichloroethylene under sunny conditions in the range of 300 to 387.5 nm, which varied between 0.5 and 0.2%. A[DCA]Q (I)365 -I

TiO2 immobilized on sand. In both cases, the thickness of the fluid film interfered with the diffusion of the components, resulting in mass transfer limitations. In the present system, the thickness of the fluid film and the residence time, as well as the volume of the reactor, were calculated according to the following eqns (4) and (5). These values are shown in Table 1: V

Q V A

(3)

A [ D C A ] Q (#mol/s m E) is the mass balance of the reactor and I (pEinstein/s m 2) is the average light intensity. However, these values are still low, probably because of electron/hole recombination. The value 14 W/m 2 corresponds to an experiment performed under a cloudy sky condition. In this case, the intensity was measured at 15 s intervals due to the continuous changing of irradiation with time. It is observed that even under a cloudy sky, photodegradation reached 10%, while 28% was achieved with clear sky conditions (27 W/m2).

3.3. Influence of flow rate Most studies centered on the photodegradation of organic compounds in aqueous medium using TiO2 as photocatalyst have been carried out using the catalyst in suspension. The difficulties inherent in the separation of fine powders like TiOz have stimulated its use in the immobilized form, avoiding the filtration step. Titanium dioxide adheres strongly to a glass surface and this property has been explored in the degradation of contaminants (Serpone et al., 1986; Matthews, 1987a,b; Kondo, 1990). Nevertheless, many authors have pointed out that mass transfer limitations can arise when a supported catalyst is used, since it is a surface reaction, while in slurries of fine catalyst particles (0.1%), the diffusion distance is about 0.5/~m, presenting no such limitation (Turchi and Ollis, 1988). An indication that mass transfer limitations occur is the increase of reaction rate as the flow rate is increased. This behavior was observed by Matthews (1991a) and Matthews and McEvoy (1992) in a system where TiOz was immobilized on the inner wall of a coiled tube, as well as in an open system, using a horizontal plate with

-

0

(4)

-,~

(5)

where V is the volume of the reactor (volume of fluid over the plate), Q is the flow rate, A is the area of the plate, 0 is the retention time, and 6 is the thickness of the fluid film. It has been observed that a very thin film, from 50 to 100/tm, was obtained in such a system. Recirculation experiments at two extreme values of flow rate (2 and 6 L/h) were performed to evaluate the influence of flow rate and possible mass transfer limitations. The volume of 1 L of 5 mmol/L DCA solution was recirculated and exposed to solar light for 1 h. It is necessary to remark that, in this case, the time of irradiation by solar light is different from the total time of experiment, since the reaction takes place during the time the solution flows over the plate that supports the catalyst. Irradiation time is given by the following eqn (6): tirradiatio n = ttotaI " Vreactor/Vtotal

(6)

Figure 3 shows the decrease of DCA concentration at both flow rate values with the concomitant production of chloride ions. It can be observed that the initial degradation rate is the same in both cases, indicating that no mass transfer limitations occur. This is related to the small thickness of fluid film obtained under these conditions, which does not interfere in the diffusion of the substrate present in the sample. Table 1. Variation of retention time, reactor volume and thickness of fluid film with the flow rate for plates A and B Plate A

Plate B

Q (L/h)

0 (min)

Vreaetor (L)

6 (#m)

0 (min)

Vreactor (L)

(#m)

2 3 4 5 6

0.78 0.58 0.47 0.40 0.35

0.026 0.029 0.031 0.033 0.035

55 61 65 69 73

0.98 0.80 0.66 0.56 0.48

0.033 0.040 0.044 0.047 0.048

67 82 90 96 98

Photocatalytic reactor for water decontamination

3.4. Influence of molar flow rate and geometry of reactor

120

100, ~"

475

80-

4~

40-

2

Uw

20-

o~

o time of in-adiation (rain)

Fig. 3. Influence of flow rate on the degradation of DCA with recirculation of the sample. Co = 5 mmol/L. Plate A.

After 1.5 min of irradiation, the degradation differs slightly, showing higher values for 2 L/h. This difference is probably caused by a lower solar light intensity after 1.5 rain of irradiation when working with the flow rate of 6 L/h. It can be also seen in Fig. 3 that the production of chloride ions corresponds to the total oxidation of DCA according to eqn (7): ChC12COO- + O 2 ~ 2 C 1 - + 2CO2 + H +

(7) After 2.7 min of irradiation and at a flow rate of 6 L/h, 6.8 mmol/L of chloride were produced in the photodegradation of 3.3 mmol/L of DCA, while at the flow rate of 2 L/h, 6.0 mmol/L of C1- were generated after 1.9 min of irradiation of a 3.0 mmol/L solution of DCA. These figures indicate the total mineralization of DCA.

plate 1 : f ( x ) = 12.81 + 98.23e 38.46x + 34.21 e -2"16x

plate

+ 17.46e - °'96x

80,

60,

g~ 60

(9)

plateB

A

80,

~ 40.

II

°J

"~ 40 "0

20

20

0 j 0

(8)

plate 2 : f(x) = 15.44 + 76.32 6.41~

100.

100

~

Single pass experiments varying the molar flow rate of DCA (a product of initial concentration by the flow rate) were carried out to evaluate the reactor using two different geometries of solar exposed areas, as described in Section 2.2. Figure 4 shows the percentage of DCA degradation as a function of molar flow rate. It is observed that the degradation decays exponentially with an increase of the molar flow rate of the system, reaching a plateau above 1.5 mmol/min. This plateau indicates a saturation of the TiO2 surface when the maximum degradation rate is achieved, corresponding to 12.8% and 15.4% of degradation for plates A and B, respectively. It is interesting to mention that, despite the fact that five different flow rates were used, the photodestruction of DCA seems to be independent of this parameter, but dependent on molar flow rate, indicating that no mass transfer limitations occur in this system. Mathematical modeling, obtained by a fit of experimental data, describing DCA destruction as a function of molar flow rate for plates A and B is expressed in eqns (8) and (9), where f(x) is the percentage of degradation and x is the molar flow rate•

i

,

i

•

i

•

i

•

0.0 0.5 1.0 1.5 2.0 molar flow rate (mmol/min)

=

•

0.0

i 0.5

•

i 1.0

.

J 1.5

• 2.0

molar flow rate (mmol/min)

Fig. 4. Influence of molar flow rate on the degradation of DCA for different flow rates. Plate (A): 60 × 90cm; plate (B): 50 x 110 cm.

476

R . F . P . Nogueira and W. F. Jardim

T h e g e o m e t r y o f the plate influences in such a w a y t h a t the l o n g e r the plate, h e n c e a h i g h e r r e t e n t i o n time, t h e m o r e efficient is the d e g r a d a tion. As the m o l a r flow rate increases, the d e g r a d a t i o n t e n d s to s a t u r a t i o n a n d the g e o m e t r y n o l o n g e r influences the process.

4. C O N C L U S I O N S A s o l a r f i x e d - b e d r e a c t o r was d e v e l o p e d u s i n g a v e r y stable a n d a c t i v e T i O 2 film s u p p o r t e d o n a glass plate. T h e m i n e r a l i z a t i o n of D C A s h o w e d a l i n e a r d e p e n d e n c e of d e g r a d a t i o n w i t h s o l a r light i n t e n s i t y for clear sky c o n d i t i o n s (20 to 30 W/m2). U n d e r c l o u d y skies, d e g r a d a t i o n also o c c u r r e d , b u t to a s m a l l e r extent. R e a c t o r p e r f o r m a n c e w i t h different flow rates a n d c o n c e n t r a t i o n s of D C A s u g g e s t e d t h a t n o m a s s t r a n s f e r l i m i t a t i o n s o c c u r r e d since a thin fluid film was f o r m e d o v e r the c a t a l y s t surface. A n e x p o n e n t i a l d e c a y of d e g r a d a t i o n w i t h t h e i n c r e a s e of m o l a r flow r a t e was o b s e r v e d . As a c o n s e q u e n c e , h i g h e r d e g r a d a t i o n was o b s e r v e d for l o w e r m o l a r flow rates. D e s p i t e the g o o d p e r f o r m a n c e of the r e a c t o r , f u r t h e r d e v e l o p m e n t s to a c h i e v e b e t t e r d e s t r u c t i o n yields are still necessary. T h e system, a l t h o u g h v e r y efficient in the p h o t o d e s t r u c t i o n o f D C A , is curr e n t l y r e s t r i c t e d to l o w flows. L a r g e v o l u m e s of contaminated wastewaters may pose some probl e m s to be t r e a t e d u s i n g the p r o p o s e d t e c h n o l ogy, since this will d e m a n d r e a c t o r s w i t h v e r y large w o r k i n g a r e a s to p r o d u c e thin films o v e r the i l l u m i n a t e d catalyst. Acknowledgements We thank Prof. Carol Collins for revising the manuscript. This research was partially funded by FAPESP and RHAE.

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