Photocatalytic Degradation of Volatile and Nonvolatile Organic

Ind. Eng. Chem. Res. 2003, 42, 3237-3244

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Photocatalytic Degradation of Volatile and Nonvolatile Organic Compounds on Titanium Dioxide Particles Using Fluidized Beds Hidehiro Kumazawa,* Makoto Inoue, and Tomomichi Kasuya Department of Chemical Process Engineering, Toyama University, 3190 Gofuku, Toyama 930-8555, Japan

The photocatalytic degradations of an acid dye, C.I. Acid Blue 40, on titanium dioxide (TiO2) particles suspended in a gas-liquid dispersion, and of methylethyl ketone (MEK) on TiO2 particles suspended in the liquid were carried out using coaxial double-cylinder-type reactors. The former degradation was executed in a three-phase fluidized bed, whereas the latter one was done in a liquid-solid fluidized bed. A black-light-type fluorescent tube was installed in the internal cylinder of the reactor. The photocatalytic degradation of both acid dye and MEK here could be described as first-order with respect to the reactant species (i.e., acid dye or MEK). The apparent first-order rate constants in both systems, however, decreased with increasing initial concentration of reactant species. Such an observation was examined on the basis of the Langmuir-Hinshelwood-type catalytic reaction mechanism and the competition of light absorption due to the reactant species. In both degradation systems, the observed relationship between the apparent first-order rate constant and TiO2 catalyst loading was satisfactorily compared with the prediction on the basis of the premise that the decay of light illumination obeys Lambert-Beer law. Introduction When titanium dioxide (TiO2) absorbs photons with energies greater than its band gap of 3.2 eV, it generates electron-positive hole pairs. Such photogenerated positive holes possess strong oxidizing power of ca. 3.0 V versus normal hydrogen electrode (NHE). Positive holes are then transferred to the surface to react with adsorbed molecules. Thus, TiO2 is a typical photocatalyst and its photocatalytic reaction is classified into a surface reaction. The degradation of organics on illuminated TiO2 catalysts has been extensively studied from the various points of view because it has possible application in the treatment of both wastewater and drinking water.1 The photocatalytic degradation of organic compounds on TiO2 has been analyzed in terms of the Langmuir-Hinshelwood kinetics for the compound.1-7 In the degradation of chlorocarbons on illuminated TiO2 catalysts, the Langmuir-Hinshelwood kinetic mechanism incorporating the inhibitory influence of complete mineralized product HCl was proposed.2-4 For the purpose of practical applications of TiO2 photocatalyst particles to the purification of aquatic environments, uniform solid (TiO2 particle)-liquid contact must be fulfilled under UV illumination. To date, the degradation of organic compounds in aqueous solutions have mainly been carried out using suspensions of TiO2 fine powders, so it is difficult to recover the suspended TiO2 fine powders from the reaction mixture for subsequent reuse. To circumvent the need for filtration to recover the suspended fine powders, TiO2 catalysts supported on glass beads and glass surfaces or TiO2 fine powdercoated ceramic balls were proposed.1,5,6,8 In our preceding work,9 simultaneous upward gas flow was incorporated in a coaxial double-cylinder-type solid-liquid contactor to disperse solid particles uniformly with low * To whom correspondence should be addressed. Telephone and Fax: +81-76-445-6859. E-mail: [email protected].

solid loadings. TiO2 ultrafine powder-coated ceramic ball catalyst was used as suspended catalyst particles. As to such catalyst particles, coated ultrafine powders do not have to be detached from the support particle due to shear stress induced by violently fluidizing liquid motion. Incorporation of upward gas flow was effective in reducing the shear stress induced by liquid motion. In the present work, first, following the previous work,9 the oxidative degradation of an acid dye, C.I. Acid Blue 40, was carried out by a circulation reaction method using a coaxial double-cylinder-type reactor. The acid dye was used as a nonvolatile organic compound. The gas-liquid-solid three-phase fluidized bed was used as the reactor because the simultaneous upward gas flow was imposed in the liquid-solid dispersion to uniformly distribute solid particles. However, the incorporation of upward gas flow has a drawback that it is not suitable for the degradation of volatile reactants. Thereby, the reactor including the liquid distributor was modified to achieve the uniform axial distribution of suspended particles without imposed upward gas flow, that is, to form a solid-liquid two-phase fluidized bed. Then, methylethyl ketone was used as the volatile reactant in this photocatalytic degradation. Next, the photocatalytic degradation of methylethyl ketone (MEK) was carried out by a circulation reaction method using a modified coaxial double-cylinder-type reactor, where the black light is installed in the internal cylinder. The kinetic law for the photocatalytic degradation was determined to apparently be first-order with respect to acid dye and MEK. The dependencies of apparent firstorder rate constant on the initial concentration of reactant species and the catalyst loading were analyzed in terms of a Langmuir-Hinshelwood-type catalytic reaction mechanism and the concept of competing light absorption by catalyst particles and reactant species.

10.1021/ie020723m CCC: $25.00 © 2003 American Chemical Society Published on Web 06/13/2003

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Ind. Eng. Chem. Res., Vol. 42, No. 14, 2003

Experimental Section Photocatalyic Degradation of Acid Dye on TiO2 Particles Suspended in a Gas-Liquid Dispersion. A coaxial double-cylinder-type gas-liquid-solid contactor9 was used as a photocatalytic reactor. A blacklight-type fluorescent tube (20 W, Toshiba, Japan) as the UV light source is installed in the internal cylinder of 42 mm in outer diameter, made of quartz glass. The outer cylinder whose internal diameter is 80 mm is made of acrylic resin. In the annular part between both cylinders, the gas-liquid-solid dispersion is formed. The reactor is operated continuously with respect to the gas phase and batchwise with respect to the solid phase. The liquid phase is overflowed and recirculated. Thirtysix holes of 6 mm in diameter are bored on the lower end plate of the annular part for the liquid to lift up the particles as uniformly as possible, whereas 6 holes of 0.5 mm in diameter are bored on the same plate as the gas sparger.9 The total volume of the apparatus (VT) is 4380 cm3 and the real volume of the reactor (VR) is 1930 cm3. Tap water was used as a fluidization liquid medium and kept constant at 28 °C. Fluidized solid particles were TiO2 spherical particles (ST-B21, Ishihara Sangyo Co. Ltd., Japan) whose mean size and density are 0.56 mm and 1.24 g/cm3, respectively. The TiO2 spherical particle is actually composed of TiO2 ultrafine powders coated on the outer surface of the spherical ceramic particle with the aid of sintering. The photocatalytic activity of TiO2 particles suspended in the gas-liquid-solid contactor was evaluated by photocatalytic degradation of an acid dye, C.I. Acid Blue 40 (molecular weight ) 473 g/mol) in aqueous solutions using a circulation method. The concentration of acid dye was determined by UV spectrophotometry at 606 nm. The liquid sample was taken out of the recirculation pipe. The time courses of the concentration of acid dye were measured for the various combinations of solid (TiO2 catalyst) loading (W) and initial concentration of dye (CD0). The liquid and gas velocities were maintained mainly at 1.83 and 0.46 cm/s, respectively. Photocatalytic Degradation of MEK on TiO2 Particles Suspended in Liquid. A similar coaxial double-cylinder-type liquid-solid contactor was used as a photocatalytic reactor. A detailed description of the liquid-solid contactor is presented in Figure 1. The same black-light-type fluorescent tube as the UV light source is installed in the internal cylinder of 42 mm in outer diameter, made of quartz glass. The outer cylinder whose internal diameter is 60 mm is made of acrylic resin. The reactor is operated batchwise with respect to the solid phase, whereas the liquid phase is overflowed and recirculated. The wire mesh with the opening of 0.4 mm was used as the distributor for the liquid to lift up the particles as uniformly as possible. The coaxial double-cylinder-type photoreactor was scaled down; the internal diameter of the outer cylinder was decreased to 60 mm from 80 mm, and the reactor volume was decreased to 720 cm3 from 1930 cm3. The liquid distributor was replaced with the wire mesh of 0.4-mm opening. At the upper part of the reactor, its diameter is enlarged to 80 mm to reduce the upward liquid flow rate. The wire mesh with the same opening (0.4 mm) is attached to the upper end of the reactor to avoid solid particles flowing out of it. In the annular part between both cylinders, the uniform liquid-solid dispersion could be attained. The total volume of the

Figure 1. Photocatalytic reactor using a liquid-solid fluidized bed. 1, Liquid inlet; 2, liquid outlet; 3, black-light-type fluorescent tube installed in a quartz glass cylinder; 4, wire mesh.

apparatus (VT) is 1840 cm3 and the real reactor volume (VR) is 720 cm3. Tap water dissolving methylethyl ketone (MEK) was used as the fluidization liquid medium and kept constant at 28 °C. Fluidized solid particles were the same TiO2 spherical particles. The photocatalytic activity of TiO2 particles suspended in the present liquid-solid contactor was evaluated by the photocatalytic degradation of MEK in the aqueous solution using a circulation method. The concentration of MEK was determined by UV spectrophotometry at 270 nm. Also the liquid sample was taken out of the recirculation line. The time courses of MEK concentration were measured for the various combinations of solid loading (W) and initial concentration of MEK (CA0). The range of liquid velocity achieving the uniform axial distribution of TiO2 particles was searched for. As a result, the liquid velocity achieving the uniform solid distribution has been found to depend on the solid loading; that is, the liquid velocity was changed between 1.8 and 3.9 cm/s, depending on the solid loading. Experimental Results and Discussion 1. Effect of Gas Velocity in a Three-Phase Fluidized Bed on the Time Dependence of Unconverted

Ind. Eng. Chem. Res., Vol. 42, No. 14, 2003 3239

Figure 2. Effect of gas velocity on the time dependence of an unconverted fraction of dye.

Fraction of Acid Dye. First, the ranges of gas and liquid velocities achieving the uniform longitudinal distribution of TiO2 catalyst particles were searched for. As to ST-B21 particles, the longitudinal solid particle concentration profile was also affected by the liquid velocity rather than the gas velocity like in the ST-B11 particle case.9 Thus, the liquid and gas velocities were maintained mainly at 1.83 and 0.46 cm/s, respectively. The effect of gas velocity on the time dependence of an unconverted fraction of acid dye was investigated at two levels of catalyst loading and shown in Figure 2. The time dependence of the unconverted fractions was shown to be almost independent of the gas velocity ranging from 0.23 to 0.69 cm/s. 2. Dispersion of TiO2 Particles in a Liquid-Solid Fluidized Bed. The range of liquid velocity realizing the uniform longitudinal distribution of TiO2 particles in the two-phase fluidized bed was searched for. The liquid velocity achieving the uniform solid distribution depended on the liquid velocity. Such a liquid velocity increased from 1.8 to 3.9 cm/s as the loading was increased up to 14 wt %. 3. Dependence of Initial Concentration of Reactant Species on the Reaction Kinetics for the Photocatalytic Degradation of Acid Dye in the Three-Phase Fluidized Bed. The time dependencies of acid dye during the photocatalytic degradation in the three-phase fluidized bed were investigated at the various initial concentrations of dye and catalyst loadings. Figure 3 shows the semilogarithmic plots of time dependence of the unconverted fraction of acid dye (CD/ CD0) for the various initial concentrations at two levels of catalyst loading. It is shown to be linear to time at each initial concentration of dye on a semilogarithmic paper. This means that the first-order kinetics relative to dye is operative. The apparent first-order rate constant, however, depends on the initial concentration of dye. When the degradation rate can apparently be expressed as first-order with respect to the concentration of dye,

-VTdCD/dt ) VRkaCD

(1)

an integral form of reaction rate can be described as

CD/CD0 ) exp(-VRkat/VT)

(2)

where ka refers to the apparent first-order reaction rate

Figure 3. Effect of initial concentration of dye on the time dependence of an unconverted fraction of dye at two levels of catalyst loadings.

Figure 4. Dependence of initial dye concentration on the apparent first-order reaction rate constant at catalyst loadings ranging from 2 to 10 wt %.

constant and VR and VT denote, respectively, the volumes of reactor and total equipment, that is, reactor and circulation piping sections. The apparent degradation rate constant (ka) can be determined from the slope of the straight line as drawn in Figure 3. Figure 4 shows the dependence of the apparent firstorder reaction rate constant on the initial concentration of dye at different catalyst loadings plotted on a semilogarithmic paper. The logarithm of the apparent firstorder rate constant is found to be linear to the initial concentration of dye at every catalyst loading covered here. Besides, the first-order reaction rate constant decreases with increasing initial dye concentration. To discuss more about the experimental evidence that the apparent first-order rate constant decreased with increasing initial dye concentration, the effects on the reaction kinetics of both the adsorption of dye and the competing light absorption on a TiO2 catalyst surface were investigated. The photocatalytic degradation rate (-rD) of acid dye will be postulated to be proportional to the concentration

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Figure 5. Dependence of the reciprocal apparent first-order rate constant on the initial concentration of dye.

of the adsorbed species in terms of Langmuir-type adsorption,

-rD ) krKDCD/(1 + KDCD)

(3)

where KD refers to the adsorption equilibrium constant of acid dye. Besides, the adsorption equilibrium constants of intermediate degradation products are assumed to be the same as that of the original dye, KD. The same assumption was employed for studying the kinetics of photocatalytic oxidation of organic solutes on TiO2 thin films by Matthews.1,5 Then, eq 3 can be simplified into

-rD ) krKDCD/(1 + KDCD0)

(4)

The above rate expression means the first-order kinetics with the apparent first-order rate constant ka:

ka ) krKD/(1 + KDCD0)

(5)

If such an assumption is met, the following relation holds:

1/ka ) 1/(krKD) + CD0/kr

(6)

Figure 5 shows the dependence of 1/ka on CD0. If eq 5 or eq 6 is operative, the term 1/ka should be linear to CD0. But linear relations could not be obtained at all of the catalyst loadings. Then, the observed dependence of ka on the initial concentration of dye was expected to be reflected from some competition of light absorption due to the dye in the black light region.

Figure 6. Absorption spectrum of C.I. Acid Blue 40.

The absorption spectrum of C.I. Acid Blue 40 is depicted in Figure 6. The dye has some absorbance in the wavelength region of black light, between 300 and 400 nm, where the absorption on TiO2 is competed. Thereby, the reaction rate may be diminished as the initial concentration of reactant species increases. Actually, the linear relation between the logarithm of the first-order rate constant and the initial concentration of reactant species has been recognized in the case of photo-oxidation of organic impurities in water using TiO2 thin films.5 Then, the apparent first-order rate constants here were plotted against the initial concentration of acid dye on a semilogarithmic paper, and as a result, the linear relationships were recognized between these two factors, that is, the logarithm of the first-order rate constant and the initial dye concentration. The dependence of the first-order rate constant extraporated to zero initial dye concentration on the catalyst loading can be judged to be influenced only by the catalyst loading because the solute (dye) has negligible light absorbance. 4. Dependence of the Apparent First-Order Rate Constant on the Catalyst Loading for the Photocatalytic Degradation of Acid Dye in a ThreePhase Fluidized Bed and of MEK in a LiquidSolid Fluidized Bed. Figure 7 indicated the values of the apparent first-order rate constant extraporated to zero initial dye concentration and those at minimum and maximum initial concentrations against the catalyst loading. The rate constants at three revels of initial dye concentration increased with increasing catalyst loading and tended to approach constant values. Such a trend can be interpreted as follows.9 The photocatalytic activity of TiO2 particles suspended in the gas-liquid dispersion is controlled by UV illumination intensity. The decay of UV illumination intensity (I) is assumed to obey Lambert-Beer law,

I ) I0 exp(-Rx)

(7)

where I0 is the incident illumination intensity at x ) 0, x the radial distance from the outer surface of the internal cylinder, and R the absorption coefficient. It should be further noted that eq 7 is based on a premise that the curvature of the cross section of the reactor can be neglected. The net number of positive holes (Nh) generated on TiO2 particles suspended in the gas-liquid dispersion is believed to be proportional to the total intensity of UV illumination, viz.,


Figure 8. Relationship between the term 1 - ka/ka* and the catalyst loading at the limit extraporated to zero initial dye concentration.

following relation may also hold:

ka* ) ka′(IO/R′) {1 - exp(-R′W*d)} Figure 7. Effect of catalyst loading on the apparent first-order reaction rate constant at two levels of initial dye concentration and at the limit of zero initial dye concentration.

Np ∝

∑

I ) IO

(IO/R){1 - exp(-Rd)} (8)

where d denotes the width of the annulus, i.e., x ) d means the internal surface of the outer cylinder. The “net” number means the number of positive holes after consumption by simultaneously generated electrons. The cross section of suspended particles tends to intercept the illumination path, and thereby the intensity decreases with increasing particle loading. Then, the absorption coefficient (R) was presumed to be proportional to the concentration of suspending particles:

R ) R′W

(9)

Substitution of the above equation into eq 4 yields

Np ∝ (IO/R′W) {1 - exp(-R′Wd)}

(10)

The apparent reaction rate constant (ka) is proportional to the product of the total number of positive holes (Nh) and the total cross-sectional area of catalyst particles under illumination, which is proportional to the catalyst loading (W), so

ka ) ka′NpW ) ka′(IO/R′) {1 - exp(-R′Wd)}

Division of eq 11 by eq 12 yields

ka/ka* )

∫0d exp(-Rx) dx )

(11)

In eq 11, ka is found to be equal to zero at the limit of W f 0. In Figure 7, the apparent reaction rate constant extraporated to zero initial dye concentration (ka) tends to approach a constant value, ka*, asymptotically, which is realized at a particle concentration, W*, so the

(12)

{1 - exp(-R′Wd)}/{1 - exp(-R′W*d)} (13)

The term exp(-R′W*d) can be set nearly equal to zero, and hence the above equation reduces to

ka/ka* ) 1 - exp(-R′Wd)

(14)

ln(1 - ka/ka*) ) -R′dW

(15)

or

Equation 15 implies that the logarithm of term 1 ka/ka* should be proportional to the catalyst loading. In view of eq 15, the term 1 - ka/ka*, which had been calculated from the experimental points at the limit extraporated to zero initial dye concentration plotted in Figure 7, was plotted against the catalyst loading on a semilogarithmic paper in Figure 8. The relationship between the logarithm of 1 - ka/ka* and the catalyst loading can be described approximately by a straight line passing through an intercept, 1.0. Next, the time dependencies of MEK concentration during the photocatalytic degradation in a liquid-solid fluidized bed were investigated at the various initial concentrations of MEK and catalyst loadings. Figures 9 and 10 show the semilogarithmic plots of time dependencies of the unconverted fraction of MEK (CA/CA0) at three levels of initial MEK concentration and catalyst loading, respectively. The apparent first-order rate constants calculated from the straight lines passing through (0, 1) decreased with increasing initial concentration of MEK at every catalyst loading as shown in Figure 11.

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Figure 9. Effect of initial concentration of MEK on time dependencies of an unconverted fraction of MEK at catalyst loadings of 1 and 5 wt %.

Figure 11. Dependence of initial MEK concentration on the apparent first-order rate constant at catalyst loadings ranging from 1 to 14 wt %.

Figure 12. Dependence of the reciprocal apparent first-order rate constant on the initial concentration of MEK. Figure 10. Effect of initial concentration of MEK on the time dependence of an unconverted fraction of MEK at a catalyst loading of 14 wt %.

Similarly, on the basis of eq 6 for the photocatalytic degradation of MEK, that is,

1/ka ) 1/(krKA) + CA0/kr

(16)

the dependence of the reciprocal of the apparent firstorder rate constant (1/ka) on the initial concentration of MEK (CA0) was examined. The values of the term 1/ka were plotted against CA0 at different catalyst loadings in Figure 12, where the linear relation could be approximately obtained at every catalyst loading. The values of kr and KA calculated from the slope and intercept of the straight line at every catalyst loading were plotted against the catalyst loading in Figures 13 and 14, respectively. It is apparent from Figure 13 that the reaction rate constant (kr) increases with increasing catalyst loading and approaches a constant value (kr*). In view of eq 15, the term 1 - kr/kr* was plotted against the catalyst loading on a semilogarithmic paper in Figure 15. The relationship between the logarithm of the term 1 - kr/kr* and the catalyst loading can be described approximately by a straight line passing

Figure 13. Dependence of kr on the catalyst loading for the photocatalytic degradation of MEK.

through an intercept, 1.0. The full curve in Figure 13 was drawn from the calculated values of ka from a solid line drawn in Figure 15 and naturally agrees well with the extrapolated values based on experimental ones. As plotted against in Figure 14, however, the adsorption equilibrium constants were not shown to be constant irrespective of the catalyst loading. Especially, the


Conclusion

Figure 14. Dependence of KA on the catalyst loading for the photocatalytic degradation of MEK.

The photocatalytic degradations of acid dye (C.I. Acid Blue 40) and MEK on TiO2 particles suspending in the gas-liquid dispersion and in the liquid, that is, in threephase and liquid-solid fluidized beds, respectively, could be described as first-order with respect to the reactant species (acid dye or MEK). The first-order rate constant, however, decreased with increasing initial concentration of reactant species, acid dye and MEK. The dependence of the apparent first-order rate constant on the initial concentration of acid dye or MEK was examined by taking account of the Langmuir-Hinshelwood-type catalytic reaction mechanism and competition of light absorption due to acid dye or MEK in the wavelength region of black light. The observed concentration dependencies of the apparent first-order rate constants could be interpreted by the LangmuirHinshelwood-type catalytic reaction mechanism rather than the competition of light absorption for the photocatalytic degradation of MEK and by the competition of light absorption rather than the Langmuir-Hinshelwood-type catalytic reaction mechanism for the photocatalytic degradation of acid dye. The variation of the observed reaction rate constant with the catalyst loading for the said two photocatalytic reaction systems were interpreted by the reactor model where the decay of black light illumination obeyed Lambert-Beer law. Nomenclature

Figure 15. Relationship between the term 1 - kr/kr* and the catalyst loading for the photocatalytic degradation of MEK.

CA ) concentration of MEK, mol/m3 CD ) concentration of acid dye, mol/m3 d ) width of the annulus of the coaxial double cylinder, cm I ) illumination intensity I0 ) illumination intensity at the outer surface of the internal cylinder (x ) 0) K ) Langmuir adsorption constant of MEK, m3/mol ka ) apparent first-order reaction rate constant, s-1 or h-1 ka′ ) proportionality constant appearing in eq 11 kr ) reaction rate constant appearing in eq 3, mol/(m3 s) or mol/(m3 h) Nh ) number of positive holes t ) process time, s VR ) reactor volume, m3 VT ) volume of total equipment, i.e., reactor and circulation piping sections, m3 W ) catalyst loading, wt % x ) radial distance from the outer surface of the internal cylinder, cm Greek Letters R ) absorption coefficient appearing in eq 7 R′ ) proportionality constant appearing in eq 9 Subscript 0 ) initial value Superscript

Figure 16. Absorption spectrum of MEK in the aqueous solution.

* ) asymptotic value

Literature Cited adsorption equilibrium constants at the low catalyst loadings up to ca. 5 wt % were lower than the constant value attained at the catalyst loadings above 7 wt %. There might be little competition of light absorption induced by the aqueous MEK solution as revealed in Figure 16.

(1) Matthews, R. W. Kinetics of Photocatalytic Oxidation of Organic Solutes over Titanium Dioxide. J. Catal. 1988, 111, 264. (2) Pruden, A. L.; Ollis, D. F. Photoassisted Heterogeneous Catalysis: The Degradation of Trichloroethylene in Water. J. Catal. 1983, 82, 404. (3) Hsiao, C.-Y.; Lee, C.-L.: Ollis, D. F. Heterogeneous Photocatalysis: Degradation of Dilute Solutions of Dichloromethane

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(CH2Cl2), Chloroform (CHCl3), and Carbon Tetrachloride (CCl4) with illuminated TiO2 Photocatalyst. J. Catal. 1983, 82, 418. (4) Ollis, D. F.; Hsiao, C.-Y.; Budiman, L.; Lee, C.-L. Heterogeneous Photoassisted Catalysis: Conversions of Perchloroethylene, Dichloroethylene, Chloroacetic Acids, and Chlorobenzenes. J. Catal. 1984, 88, 89. (5) Matthews, R. W. Photooxidation of Organic Impurities in Water Using Thin Films of Titanium Dioxide. J. Phys. Chem. 1987, 91, 3328. (6) Sabate, J.; Anderson, M. A.; Kikkawa, H.; Edwards, M.; Hill, C. G. A Kinetic Study of the Photocatalytic Degradation of 3-Chlorosalicylic Acid over TiO2 Membranes Supported on Glass. J. Catal. 1991, 127, 167. (7) Hidaka, H.; Zhao, J.; Pelizzetti, E.; Serpone, N. Photodegradation of Surfactants. 8. Comparison of Photocatalytic Processes

between Anionic Sodium Dodecylbenzenesulfonate and Cationic Benzyldodecyldimethylammonium Chloride on the TiO2 Surface. J. Phys. Chem. 1992, 96, 2226. (8) Aguado, M. A.; Anderson, M. A.; Hill, C. G., Jr. Influence of Light Intensity and Membrane Properties on the Photocatalytic Degradation of Formic Acid over TiO2 Ceramic Membranes. J. Mol. Catal. 1994, 89, 165. (9) Kumazawa, H.; Kawasaki, H.; Inoue, M. Liquid-Phase Photocatalytic Degradation over TiO2 Particles Suspending in Gas-Liquid Dispersion. Chem. Eng. Commun. 2002, 189, 298.

Resubmitted for review January 23, 2003 Accepted April 10, 2003 IE020723M

Photocatalytic Degradation of Volatile and Nonvolatile Organic

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