Quantum Efficiencies of 4-Chlorophenol Photocatalytic Degradation

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Ind. Eng. Chem. Res. 2007, 46, 43-51

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Quantum Efficiencies of 4-Chlorophenol Photocatalytic Degradation and Mineralization in a Well-Mixed Slurry Reactor Marı´a L. Satuf, Rodolfo J. Brandi, Alberto E. Cassano, and Orlando M. Alfano* INTEC (UniVersidad Nacional del Litoral and CONICET), Gu¨emes 3450, S3000GLM Santa Fe, Argentina

A study of the quantum efficiency of the photocatalytic degradation of 4-chlorophenol (4-CP) in a TiO2 slurry reactor is presented. Also, in order to assess the final conversion of the pollutant into CO2, the efficiency of the mineralization process is defined and evaluated. To calculate these efficiencies, the radiation absorbed by the heterogeneous reacting system needs to be determined. A one-dimensional, one-directional radiation field model is proposed to compute the photon absorption inside the slurry photocatalytic reactor. Degradation rates and quantum efficiencies are analyzed considering the influence of three operating variables: pH, catalyst loading, and irradiation intensity. Important dependencies on these variables were found. The best efficiencies were obtained in acidic conditions, at the highest employed catalyst concentration (1.0 × 10-3 g cm-3) and at the lowest level of irradiation. ηi )

I. Introduction In recent years, research on water-purification methods has become an active area as a result of the increasing worldwide concern on environmental problems. In this field, heterogeneous photocatalysis appears to be a promising technique, with significant advantages over conventional water treatment methods: it can provide the complete mineralization of pollutants at ambient temperature and pressure, it is effective over a wide range of organic compounds, and it offers the possibility to employ solar radiation as the source of radiant energy. Titanium dioxide (TiO2) is the most extensively used photocatalyst, as it is considered very efficient, nontoxic, stable to photocorrosion, and inexpensive. The initial step in the photocatalytic process involves the absorption of photons of suitable energy by the TiO2 particles and the consequent generation of electrons and holes. These species can migrate to the particle surface and take part in oxidation-reduction reactions that finally lead to the mineralization of the pollutants.1 Several parameters have been defined to evaluate the efficiency of photocatalytic processes and to compare experimental results obtained from different photocatalytic systems.2 Among them, the quantum yield (Φλ) and the quantum efficiency (η) are two of the most frequently used. These parameters may present variations according to the type of catalyst employed, the operating conditions of the experiments (radiation wavelength, catalyst loading, initial reactant concentration, range of incident intensities, effects of reactions products, pH, etc.), and, in particular, the nature of the reaction considered.3 For monochromatic radiation, Φλ is applied, whereas η is used when the energy source is polychromatic. The quantum efficiency η can be defined as the ratio of the number of reactant molecules degraded (or product molecules formed) during a given time t to the total number of photons absorbed by the species to be activated, over the employed spectral range, during the same period of time.4 In terms of rates, η can be evaluated as * To whom correspondence should be addressed. E-mail: alfano@ intec.unl.edu.ar. Phone: +54-342-4511546. Fax: +54-342-4511087.

[reaction rate of species i] (1) [rate of photon absorption by the species to be activated] Because of the simultaneous existence of absorption and scattering in heterogeneous systems, the determination of the rate of absorbed photons in slurry reactors becomes complex. Several approaches have been reported to assess the photon absorption in aqueous TiO2 suspensions. Palmisano et al.5 employed actinometric measurements to evaluate the incident radiation and the radiation transmitted through a reactor medium. By applying a macroscopic photon balance, they obtained the rate of reflected photons and the rate of photon absorption. From these results, the quantum yield for the photodegradation of phenol was computed. Sun and Bolton6 determined the quantum yield of the generation of hydroxyl radicals in a heterogeneous system, employing a modified integrating sphere method to measure the true fraction of light absorbed by TiO2 suspensions. Then, Serpone7 proposed a protocol to assess the quantum yields for the photooxidation of organic substrates by calculating a relative photonic efficiency and considering the quantum yield of phenol as reference. The integrating sphere method of Sun and Bolton was employed to compute the fraction of light absorbed by the catalyst in the evaluation of the phenol quantum yield. Stafford et al.8 evaluated the quantum yield of the photocatalytic degradation of 4-chlorophenol in an annular slurry reactor, at different radiation wavelengths and TiO2 concentrations. The rates of photon absorption in the reactor were calculated with the values of the incident radiation and a correction that considers the radiation transmitted through the slurry. Thereafter, Cabrera et al.9 measured the quantum efficiencies of the photocatalytic decomposition of trichlorethylene in water, comparing the activities of three TiO2 catalyst brands: Aldrich, Degussa P25, and Hombikat UV 100. The polychromatic radiation absorption inside the reactor was computed by solving the radiative transfer equation (RTE) in the heterogeneous system. Subsequently, Salaices et al.10 presented an experimental method to evaluate the rate of absorbed photons in aqueous TiO2 suspensions in an annular photocatalytic reactor. They applied a macroscopic radiation balance to determine the extinction coefficient for several TiO2 catalyst samples and calculated the quantum efficiency for the photodegradation of phenol. Curco´ et al.11 estimated the

10.1021/ie0604019 CCC: $37.00 © 2007 American Chemical Society Published on Web 12/06/2006

44 Ind. Eng. Chem. Res., Vol. 46, No. 1, 2007

absorbed radiation in a photocatalytic reactor employing exponential and probabilistic models for light propagation. The optical parameters needed to solve the models were determined experimentally from transmittance measurements. Introducing these results into kinetic expressions, they related the effects of the catalyst concentration and the radiation absorption on the degradation rates of Cr(IV) and phenol. Then, Brandi et al.12 developed a method to measure the quantum yields for photocatalytic reactions in slurry reactors. The value of the photonic absorption rate was obtained by solving the RTE inside the heterogeneous reactor. The quantum yields for phenol and 1,4-dioxane at two different wavelengths were reported. Statistical methods have also been applied to estimate the radiation distribution inside heterogeneous photoreactors, with the Monte Carlo approach being the most extensively used.13,14 Chlorophenols are compounds widely used in the chemical industry, representing an important group of water pollutants. In the present work, we evaluate the quantum efficiencies of the 4-chlorophenol (4-CP) photocatalytic degradation and mineralization under different experimental conditions. The study was carried out in a perfectly stirred slurry reactor irradiated by UV lamps. Considering the importance of the correct evaluation of radiation absorption, a one-dimensional, one-directional radiation field model is proposed and solved to obtain the rate of photon absorption in the heterogeneous reactor. The optical properties of TiO2 suspensions and the phase function for scattering, required to solve the radiation model, were obtained following the procedure reported in a previous work.15 Rigorously speaking, the quantum efficiency and, to a lesser extent, the quantum yield are imperfectly defined properties to give an idea of the efficiency of the photons employed to promote a particular reaction, because they depend on too many additional reaction variables. Thus, both of them are meaningless figures if, at least, the following experimental conditions are not clearly stated when they are reported: (i) the temperature; (ii) the pH in aqueous media; (iii) the employed catalyst; (iv) the physical, chemical, and optical properties of the catalyst; (v) the purity of the reactant and/or the existence of impurities; (vi) the catalyst concentration; (vii) the initial reactant concentration; (viii) the irradiation rate; and finally, for quantum yields, (ix) the wavelength of monochromatic radiation. This work is intended to show some of these variations, with a detailed definition of the corresponding operating conditions. II. Experimental II.1. Materials. 4-CP (Aldrich, >99%) was employed as the model pollutant. Reagent-grade perchloric acid and sodium hydroxide were used to adjust the pH of the reacting suspensions. TiO2 powder (>99% Anatase, 9.6 m2 g-1 of specific surface area) was obtained from Aldrich Chemicals. Deionized and double-distilled water was used to prepare all solutions. II.2. Experimental Setup and Procedure. Figure 1 shows a schematic representation of the experimental setup. The photocatalytic degradation of 4-CP was carried out in a cylindrical reactor made of stainless steel, with an inner wall of Teflon. It operates as a slurry reactor inside the loop of a batch recycling system. The reactor has two circular, flat windows made of borosilicate glass. Radiant energy was supplied by two sets of four UV lamps (TL 4W/08 black light UVA lamps from Phillips), placed in front of each window. The lamps emit in the 300-400 nm range with an emission peak at ca. 350 nm. Ground glass plates, situated between the lamps and the reactor windows, were used to produce diffuse

Figure 1. Schematic representation of the experimental setup: (1) reactor; (2) windows; (3) lamps; (4) tank; (5) sampling valve; (6) thermostatic bath; (7) oxygen supply; and (8) pump. Table 1. Experimental Setup: Dimensions and Main Characteristics component reactor

lamps (four from each side)

pump thermostatic bath

parameter

value

inner diameter length reactor volume total system volume nominal power

8.6 cm 5 cm 290 cm3 1000 cm3 4W

emission range radiation flux (100%)

300-400 nm 7.55 × 10-9 Einstein cm-2 s-1 1.6 cm 13.6 cm 100 cm3 s-1 293 K

diameter arc length flow rate temperature

inlet radiation. The radiation fluxes that reached the reacting space from each side of the reactor were measured by ferrioxalate actinometry.16 Specially designed optical filters17 were interposed between the lamps and the reactor to modify the level of incident radiation. The storing tank was equipped with a sampling valve, a gas inlet for oxygen supply, and a watercirculating jacket to ensure isothermal conditions during the reaction time. The system was maintained under overpressure of oxygen to guarantee the renewal of the oxygen consumed by the photochemical reaction. The pump flow rate was adjusted in order to provide good mixing conditions, low conversion per pass in the reactor, and uniform concentration of the catalyst throughout the system. Each run lasted 8 h, and samples were taken from the tank every hour. The dimensions and main characteristics of the experimental setup are summarized in Table 1. A control experiment was carried out to evaluate direct photolysis. It was conducted following the same procedure of the rest of the experiments but without the addition of TiO2. No detectable changes occurred in the concentration of 4-CP after 10 h of irradiation. Therefore, the effect of direct photolysis can be neglected. II.3. Analysis. The concentration of 4-CP in the samples was measured by high-performance liquid chromatography (HPLC) using a Waters chromatograph equipped with a LC-18 Supelcosil reversed-phase column (Supelco). The eluent was a ternary mixture of water (containing 1% v/v acetic acid), methanol, and acetonitrile (60:30:10), pumped at a rate of 1 mL min-1. UV detection of 4-CP was performed at 280 nm. Total organic carbon (TOC) was determined with a Shimadzu TOC-5000 A analyzer. III. Theoretical In our reacting system, η4-CP can be expressed as the ratio of the initial volumetric rate of 4-CP degradation to the local

Ind. Eng. Chem. Res., Vol. 46, No. 1, 2007 45

volumetric rate of photon absorption (LVRPA), both averaged over the reactor volume (VR):

η4-CP )

〈r4-CP(x,t0)〉VR

(2)

〈ea(x)〉VR

The 4-CP initial reaction rate was calculated from experimental runs, whereas the photon absorption in the reactor was obtained by solving the radiative transfer equation (RTE). In the degradation process of organic compounds, it is important to ensure the disappearance of the pollutant as well as the absence of toxic intermediate products. Ideally, unless photocatalysis is used as a pretreatment process, the complete conversion of the total organic carbon (TOC) into carbon dioxide should be obtained. Therefore, a useful tool to evaluate decontamination technologies is the TOC conversion at the end of the experimental procedure. In this sense, we define the quantum efficiency of the mineralization process, ηTOC, as the ratio of the amount of organic carbon degraded in a given time to the amount of photons absorbed during the same time,

Figure 2. Coordinate system for the one-dimensional, one-directional radiation model.

Radiation intensity in the medium is independent of the azimuthal angle. The arrangement of the lamps and the ground glass plates ensure the arrival of diffuse radiation with azimuthal symmetry at the reactor windows. If the boundary conditions for the RTE are azimuthally symmetric, then the radiation can be modeled with one angular variable.19 Under these assumptions, a one-dimensional (x in space), one-directional (θ in the direction of radiation propagation) radiation transport model was applied to solve the RTE in the reacting system,20

ηTOC ) [amount of TOC converted] ) [amount of photons absorbed by the species to be activated] (TOC0 - TOCf)VT (3) 〈ea(x)〉VRVR(tf - t0) where VT is the total system volume and (tf - t0) is the time length of the experimental run. III.1. Reaction Rate of 4-CP. Considering that (i) there is a differential conversion per pass in the reactor, (ii) the system is perfectly mixed, (iii) there are no mass transport limitations, and (iv) the chemical reaction occurs only at the solid-liquid interface (there is no parallel homogeneous reaction), the mass balance for 4-CP in the system takes the following form,18

L

VR dC4-CP(t) |Tk ) - 〈r4-CP(x,t)〉VR dt VT

(4)

where L is the liquid hold-up (L ≈ 1), C4-CP is the molar concentration of 4-CP, t denotes the reaction time, Tk refers to the tank, and 〈r4-CP(x,t)〉VR is the 4-CP reaction rate averaged over the reactor volume. The expression to calculate the initial reaction rate from experimental data can be derived from eq 4:

〈r4-CP(x,t0)〉VR ) -L

(

)

VT C4-CP(t) - C4-CP(t0) lim VR tft0 t - t0

(5)

Tk

For every experimental run, limtft0 [(C4-CP(t) - C4-CP(t0))/(t - t0)]Tk was obtained from the initial slope of the plot of C4-CP vs time. III.2. TOC Degradation. To evaluate the mineralization quantum efficiency, we computed the difference between the initial value of TOC and the amount of TOC at the end of the run for the total reaction time. III.3. Radiation Model. The RTE was solved to calculate the LVRPA. The radiation model (Figure 2) considers the following: (i) The main changes in the radiation spatial distribution occur along the x coordinate axis, due to the significant extinction produced by the catalyst particles. This effect determines a short mean free path. Consequently, radiation propagation is modeled with only one spatial variable. (ii)

µ

∂Iλ(x,µ) σλ + βλIλ(x,µ) ) ∂x 2

∫

1 I (x,µ′)p(µ,µ′) µ′)-1 λ

dµ′

(6)

where Iλ is the spectral radiation intensity; x is the axial coordinate; βλ is the spectral volumetric extinction coefficient; σλ is the spectral volumetric scattering coefficient; µ is the direction cosine of the ray with respect to the propagating direction for which the RTE is written (µ ) cos θ); µ' is the cosine of an arbitrary ray before scattering; and p represents the phase function for scattering. The extinction coefficient, βλ, is defined as the sum of the absorption and the scattering coefficients (βλ ) κλ + σλ). The phase function p(µ,µ′) represents the probability that the incident radiation from direction µ′ will be scattered and incorporated into the direction µ. To model the scattering effects of TiO2 particles, we adopted the Henyey and Greenstein (HG) phase function (pHG,λ):21

pHG,λ(µ0) )

1 - gλ2 (1 + gλ2 - 2gλµ0)3/2

(7)

pHG,λ is a simple and suitable function to solve problems with multiple scattering. It is determined by a single free parameter: the asymmetry factor gλ, and, according to the value of gλ, pHG,λ varies smoothly from isotropic (gλ ) 0) to a narrow forwardly directed peak (gλ ) 1) or to a narrow backwardly directed peak (gλ ) -1).22 The values of gλ and the specific coefficients (per catalyst mass concentration Cm) β/λ, σλ/, and κ/λ at pH 6.5 were obtained from a previous work.15 The method employed for determining these optical properties involved diffuse reflectance and transmittance spectrophotometric measurements of TiO2 suspensions, the evaluation of the radiation field in the spectrophotometer sample cell, and the application of a nonlinear optimization program to adjust the model predictions to the experimental data. Figure 3a depicts the spectral distribution of σλ/, κ/λ, gλ at the natural pH of the TiO2 suspensions (pH 6.5) and the relative emission of the lamps (Er,λ). The influence of the pH on the optical properties of the catalyst suspensions was also analyzed in the present work. It should be stressed that important changes were found in acidic conditions. The spectral values of the extinction coefficient at

46 Ind. Eng. Chem. Res., Vol. 46, No. 1, 2007

Iλ(LR,-µ) ) ILR,λ + ΓW,λ(µ)Iλ(LR,µ)

1 > µ > µc

Iλ(LR,-µ) ) ΓW,λ(µ)Iλ(LR,µ)

µc > µ > 0 (8d)

(8c)

where µc ) cos θc. The radiation fluxes coming from the lamps and entering the -9 Einstein reacting space at x ) 0 (∫λ q+ λ (0) dλ ) 7.65 × 10 -2 -1 cm s ) and at x ) LR (∫λ qλ (LR) dλ ) 7.45 × 10-9 Einstein cm-2 s-1) were measured experimentally by using potassium ferrioxalate actinometry.16 For the one-dimensional, onedirectional model with azimuthal symmetry and polychromatic radiation, these fluxes can be expressed as

∫λ qλ+(0) dλ ) ∫λ 2π∫µ1 I0,λµ dµ dλ ) ∫λ πI0,λ(1 - µc2) dλ ) π(1 - µc2)∫λ I0,λ dλ

(9)

∫λ qλ-(LR) dλ ) ∫λ 2π∫-µ-1 IL ,λµ dµ dλ ) ∫λ πIL ,λ[1 - (-µc2)]dλ ) π[1 - (-µc2)]∫λ IL ,λ dλ

(10)

c

c

R

R

R

From eqs 9 and 10, and the spectral distribution of Er,λ, the values of I0,λ and ILR,λ can be obtained. The net radiation method21 was adapted to compute the global window reflection coefficient as a function of µ, Figure 3. Spectral distribution of the catalyst optical properties and the relative emission of the lamps: broken and dotted line, σ/λ ; solid line, κ/λ; broken line, gλ; dotted line, Er,λ at (a) natural pH (∼6.5); (b) pH 2.5.

pH 2.5 are ∼3× lower than the ones obtained at natural pH, with the consequent decrease in the absorption and scattering coefficients. On the other hand, the optical properties between pH 6.5 and 11 do not vary appreciably. The values of σλ/, κ/λ, and gλ at pH 2.5, calculated employing the method described in Satuf et al.,15 are shown in Figure 3b. Accordingly, when working under acidic conditions (pHs < 6.5), these optical properties were used to compute the LVRPA and the corresponding quantum efficiencies. On the contrary, in the pH range of 6.5-11, the optical properties depicted in Figure 3a were employed. The optical effects of the reactor windows, i.e., absorption, refraction, and reflection, are taken into account to obtain the boundary conditions for eq 6. At x ) 0 (Figure 2), radiation intensities in the forward direction (µ > 0) are the result of two contributions: (i) the transmitted portion of the radiation arriving from outside of the reactor and (ii) the reflected portion of the radiation coming from the suspension. (i) The radiation coming from the lamps arrives in a diffuse way at the external side of the glass window. To reach the reacting space, radiation crosses two interfaces: air-glass and glass-suspension. Because of refraction, the angular directions of the intensities entering the suspension (I0,λ) are comprised between 0 and the critical angle θc, given by θc ) sin-1 na/ns. (ii) The intensities coming from the reacting medium, Iλ(0,µ), undergo multiple specular reflections at the reactor window. The global window reflection coefficient, ΓW,λ, represents the reflected fraction of the radiation that returns to the suspension. By applying a similar analysis at x ) LR, the boundary conditions take the following form,

Iλ(0,µ) ) I0,λ + ΓW,λ(-µ)Iλ(0,-µ)

1 > µ > µc

(8a)

Iλ(0,µ) ) ΓW,λ(-µ)Iλ(0,-µ)

µc > µ > 0

(8b)

ΓW,λ(µ) ) Fs-g(µ) + [1 - Fs-g(µ)][1 - Fg-s(µ*)] Fg-a(µ*)τλ2(µ*) 1 - Fg-s(µ*)Fg-a(µ*)τλ2(µ*)

(11)

where Fs-g is the suspension-glass reflectivity; Fg-s is the glass-suspension reflectivity; Fg-a is the glass-air reflectivity; τλ represents the internal glass window transmittance; and µ* is the cosine of the refracted angle in the glass. The interface reflectivities were calculated by using Snell’s law and Fresnel’s equations.21 The discrete ordinate method23 was then applied to solve the radiation model (eqs 6, 8a-8d). This method transforms the RTE into a set of algebraic equations that can be solved numerically. The solution of the RTE provides the values of Iλ(x,µ) at each point and each direction in the heterogeneous reacting medium. In polar coordinates, parts a-c of Figure 4 show the angular distribution of radiation intensities at different spatial positions inside the reactor (along the x-axis) for Cm ) 0.1 × 10-3 g cm-3 at natural pH. At x ) 0 in the forward direction (µ > 0), Figure 4a shows the incoming radiation from the lamps as well as the reflected radiation at the reactor window. As stated in the boundary conditions, between µ ) 1 and µ ) µc, both contributions are present, whereas between µ ) µc and µ ) 0, only the reflected radiation is observed. At the same x-position but for µ < 0, the backscattering produced by the TiO2 particles is depicted in the figure. It should be mentioned that, due to the extinction produced by TiO2 particles, no radiation arrives at x ) 0 directly from the lamps of the opposite window, even at this low catalyst concentration (Cm ) 0.1 × 10-3 g cm-3). Therefore, all the radiation in the backward direction (µ < 0) is a consequence of backscattering. The loss due to this effect represents ∼24% of the incident radiation. This figure highlights the importance of considering the scattering phenomenon in heterogeneous systems. At x ) LR/2 (Figure 4b), the result of the radiation arriving from both windows is

Ind. Eng. Chem. Res., Vol. 46, No. 1, 2007 47

Figure 5. LVRPA profiles for different catalyst concentrations (at natural pH): (a) solid line, 1.0 × 10-3 g cm-3; broken line, 0.5 × 10-3 g cm-3; and dotted line, 0.25 × 10-3 g cm-3; (b) solid line, 0.1 × 10-3 g cm-3; and broken line, 0.05 × 10-3 g cm-3.

For the one-dimensional model, the average value of ea(x) in the reactor volume, necessary to calculate the quantum efficiencies, can be computed as

〈ea(x)〉VR )

1 VR

∫V

ea(x) dV

(13)

R

By introducing the result of eq 13 into eqs 2 and 3, η4-CP and ηTOC can be readily calculated. Figure 4. Radiation intensity vs θ in polar coordinates, at different spatial positions inside the reactor (µ ) cos θ): (a) x ) 0; (b) x ) LR/2; (c) x ) LR; ordinate axis ) I × 109 Einstein cm-2 sr-1 s-1.

depicted. As can be expected, an angular symmetry of radiation intensities is observed at this spatial position. Notice that radiation has been significantly attenuated due to the extinction by the TiO2 particles (the scale of Figure 4b is 1 order of magnitude smaller than the scales of parts a and c of Figure 4). At x ) LR (Figure 4c), the directional distribution of radiation is a mirror replica of the one at the opposite window (x ) 0). The LVRPA for polychromatic radiation is calculated from the values of Iλ(x,µ) as

ea(x) ) 2π

1 Iλ(x,µ) dµ dλ ∫λ κλ∫µ)-1

(12)

In Figure 5, the LVRPA profiles for different TiO2 concentrations at natural pH are shown. Photon absorption presents strong variations along the x-axis. For Cm g 0.25 × 10-3 g cm-3 (Figure 5a), >90% of the radiation is absorbed in the first 0.5 cm of the reactor. Smoother profiles are obtained as long as Cm decreases, as shown in Figure 5b.

IV. Results and Discussion The reaction rate and the quantum efficiency of the photocatalytic degradation of 4-CP, as well as the TOC degradation and the quantum efficiency of the mineralization process, were analyzed considering the effect of three operating parameters: initial pH of the reacting suspension, catalyst loading, and level of irradiation. The effect of the 4-CP concentration on the degradation rates and quantum efficiencies was not studied in the present work. Hence, in all experiments, the initial concentration of 4-CP was 1.4 × 10-7 mol cm-3. IV.1. Effect of pH. The influence of the initial pH on the 4-CP degradation was studied in a first set of experiments. The runs were performed at the highest level of irradiation (100%) with a TiO2 concentration of 0.5 × 10-3 g cm-3. Figure 6a shows the experimental values of the 4-CP dimensionless concentration vs time at five different initial pH values: 2.5, 4.0, 6.5, 7.5, and 8.0. Significant differences in the reaction rate are observed, and these results consequently affect the quantum efficiency. As described in Section III.3, important changes in the optical properties of the TiO2 suspensions were also found when varying the pH, thus modifying the LVRPA and the corresponding quantum efficiencies. As a result, the pH affects the quantum

48 Ind. Eng. Chem. Res., Vol. 46, No. 1, 2007

Figure 6. (a) Degradation of 4-CP at different initial pHs; keys: (]), pH 2.5; (3), pH 4.0; (4), pH 6.5; (O), pH 7.5; (0), pH 8.0. (b) Quantum efficiency of the 4-CP degradation (%) vs pH. Bars represent a 95% confidence interval.

Figure 7. (a) TOC degradation at different initial pHs; keys: (]), pH 2.5; (3), pH 4.0; (4), pH 6.5; (O), pH 7.5; (0), pH 9.3. (b) Quantum efficiency of the mineralization (%) vs pH. Bars represent a 95% confidence interval.

efficiency through both the reaction rate and the photon absorption rate. In Figure 6b, the values of η4-CP, calculated from eqs 2, 5, and 13, are represented over a pH range between 2.5 and 10. The minimum efficiency is found at the natural pH of the reacting suspension (pH ≈ 6.5). As pH decreases, a strong increase in η4-CP is observed. On the other hand, a slight

Figure 8. (a) Degradation of 4-CP employing different catalyst concentrations; keys: (0), 0.05 × 10-3 g cm-3; (O), 0.1 × 10-3 g cm-3; (4), 0.5 × 10-3 g cm-3; (3), 1.0 × 10-3 g cm-3. (b) Quantum efficiency of the 4-CP degradation (%) vs catalyst concentration. Bars represent a 95% confidence interval.

Figure 9. (a) TOC degradation employing different catalyst concentrations; keys: (0), 0.05 × 10-3 g cm-3; (O), 0.1 × 10-3 g cm-3; (4), 0.5 × 10-3 g cm-3; (3), 1.0 × 10-3 g cm-3. (b) Quantum efficiency of the mineralization (%) vs catalyst concentration. Bars represent a 95% confidence interval.

enhancement of the quantum efficiency is found for alkaline conditions at pH > 9. A possible explanation for this behavior can be attributed to the pH dependence of the superficial charge of the catalyst particles. At acid pH values, the TiO2 surface

Ind. Eng. Chem. Res., Vol. 46, No. 1, 2007 49 Table 2. Effect of Different Irradiation Conditions on the 4-CP Degradation: Degradation Quantum Efficiencies Cm ) 0.1 × 10-3 g cm-3 irradiation level (%) 30 67 100

〈r4-CP(x,t0)〉VR × (mol cm-3 s-1)

1010

0.28 0.53 0.62

〈ea(x)〉VR ×

Cm ) 1.0 × 10-3 g cm-3

(Einstein cm-3 s-1)

η4-CP (%)

〈r4-CP(x,t0)〉VR × 1010 (mol cm-3 s-1)

〈ea(x)〉VR × 1010 (Einstein cm-3 s-1)

η4-CP (%)

7.46 17.81 25.81

3.75 2.98 2.40

0.68 0.90 1.06

7.67 18.32 26.56

8.87 4.91 3.99

1010

Table 3. Effect of Different Irradiation Conditions on the TOC Degradation: Mineralization Quantum Efficiencies Cm ) 0.1 × 10-3 g cm-3 irradiation level (%) 30 67 100

TOC converted × (mol) 0.08 0.18 0.25

103

photons absorbed × (Einstein)

Cm ) 1.0 × 10-3 g cm-3 103

6.24 14.90 21.59

carries a net positive charge, whereas chlorophenols and their intermediates products remain neutral or negatively charged. Therefore, acidic conditions could facilitate the adsorption of the organic molecules and enhance degradation.24 Several additional effects of the pH on the photocatalytic process have been reported elsewhere. Among them, we can mention the influence of the pH on the shift of the valence and the conduction band of the semiconductor,25 the modification of the rheological properties of TiO2 suspensions,26 a change in the electron-hole recombination rate,27 and different reaction mechanisms under acidic or alkaline conditions.25 The effect of pH on the TOC overall degradation was also analyzed. Figure 7a shows the depletion of TOC during the experimental runs for different initial pH values, whereas Figure 7b depicts ηTOC as a function of pH (calculated from eqs 3 and 13). Under acidic conditions, as can be noted in Figure 7b, the efficiency of the mineralization process decreases as pH increases, following a behavior similar to that for η4-CP, and reaches a minimum at neutral pH. For pH values > 8, although the efficiency of the 4-CP degradation increases slightly, the TOC degradation efficiency declines. This effect on the mineralization of 4-CP can be attributed to the higher number and concentration of intermediate species and the formation of more stable byproducts (bicyclic compounds) at alkaline pH values.25 IV.2. Effect of the Catalyst Loading. The effect of the catalyst loading on the 4-CP degradation is represented in Figure 8a, where the plots of the experimental 4-CP dimensionless concentration vs time are depicted for different catalyst concentrations, between 0.05 × 10-3 and 1.0 × 10-3 g cm-3. In all runs, the level of irradiation was 100% and the initial pH of the reacting suspension was 2.5. As can be observed in the figure, the reaction rate increases with the catalyst loading, but this effect becomes less significant when the TiO2 concentration is >0.5 × 10-3 g cm-3. This behavior can be explained by taking into account that the reaction is activated by the absorption of photons by the catalyst and, in our reacting system, the maximum radiation absorption is already achieved at approximately Cm ) 0.5 × 10-3 g cm-3. The variation of η4-CP with the TiO2 concentration is presented in Figure 8b. Notice that η4-CP always increases with the catalyst loading but reaches a sort of plateau around Cm ) 0.5 × 10-3 g cm-3. Therefore, higher amounts of catalyst will not increase significantly the efficiency of the 4-CP degradation. Figure 9a shows the experimental values of the TOC dimensionless concentration vs time for different TiO2 concentrations. It can be noticed that the TOC conversion after 8 h of irradiation is enhanced by the catalyst loading, but for TiO2 concentrations > 0.5 × 10-3 g cm-3, no considerable changes

ηTOC (%)

TOC converted × 103 (mol)

photons absorbed × 103 (Einstein)

ηTOC (%)

1.28 1.21 1.16

0.21 0.34 0.42

6.42 15.32 22.22

3.27 2.22 1.89

in the mineralization of the pollutant are obtained. ηTOC as a function of Cm is presented in Figure 9b. The behavior of ηTOC with the amount of catalyst is similar to that for η4-CP: the efficiency of the mineralization increases with the TiO2 concentration and reaches a maximum value about Cm ) 0.5 × 10-3 g cm-3. For concentrations of titania >0.5 × 10-3 g cm-3, ηTOC remains almost unaffected. IV.3. Effect of the Irradiation Level. Table 2 presents the values of the initial reaction rate, the rate of photon absorption, and η4-CP at three irradiation levels, 30, 67, and 100%, and for two catalyst loadings, 0.1 × 10-3 and 1.0 × 10-3 g cm-3. The initial pH in all runs was 2.5. Although the reaction rate is always enhanced with the irradiation level, as can be observed in the table, the efficiency of the process declines considerably. The decrease in the efficiency is observed for both TiO2 loadings, although the changes are more pronounced at the highest Cm. The obtained results can be attributed to the undesired increase in the recombination rate of the photogenerated electrons and holes in the catalyst particle when the radiation level rises. This effect, typical for high levels of irradiation, originates a nonlinear relation (square root dependence) between the reaction rate and the radiation absorption.28 As a result, the η4-CP declines. The best 4-CP degradation efficiency was obtained at Cm ) 1.0 × 10-3 g cm-3 and at 30% irradiation level: 8.87%. The effects of the irradiation levels on the TOC degradation are summarized in Table 3. The moles of TOC converted and the moles of photons (Einsteins) absorbed at the end of the experimental runs, as well as ηTOC, are reported under the same conditions used to build Table 2. For the two catalyst loadings assayed, it can be observed that the amount of TOC converted improves with the level of irradiation but the mineralization efficiency decreases. The higher rate of recombination of electrons and holes at higher levels of irradiation can also account for this effect on ηTOC. The best mineralization efficiency, 3.27%, was found at the highest TiO2 concentration (1.0 × 10-3 g cm-3) and at the lowest irradiation level (30%). V. Conclusions The quantum efficiencies of the 4-chlorophenol (4-CP) photocatalytic degradation and of the mineralization process in a slurry reactor have been determined under different operating conditions. The influence of the initial pH, catalyst loading, and irradiation level on both reaction rate and TOC degradation has also been assessed. A one-dimensional, one-directional radiation model, derived from the radiative transfer theory, was solved to obtain the true value of absorbed photons in the photocatalytic reactor. On the basis of the reported results, the following conclusion can be asserted:

50 Ind. Eng. Chem. Res., Vol. 46, No. 1, 2007

(i) When computing the photon absorption, it is very significant to consider the effect of the scattering produced by the heterogeneous medium. The loss of radiation due to the backscattering can represent ∼24% of the incoming radiation, even at low catalyst concentrations. (ii) The effect of pH on the quantum efficiency of a photocatalytic reaction depends strongly on the nature of the reactant. For 4-CP, the process is enhanced at acidic pH values. (iii) When the initial pH is changed, the efficiency of the mineralization process (ηTOC) does not always follow the same behavior as the efficiency of the reactant degradation (η4-CP). In acidic conditions, both η4-CP and ηTOC are improved, but at pH values > 8, η4-CP is slightly enhanced whereas ηTOC declines, probably due to the presence of more stable intermediate products. (iv) η4-CP and ηTOC always increase with the catalyst loading (Cm) and reach a plateau when Cm ≈ 0.5 × 10-3 g cm-3. Higher amounts of catalyst do not improve the efficiencies significantly. (v) Regarding the irradiation conditions, the best efficiencies were found at the lowest irradiation rates. The maximum values of η4-CP, 8.87%, and ηTOC, 3.27%, were obtained at pH 2.5, for Cm ) 1.0 × 10-3 g cm-3, and at the lowest level of irradiation (30%). (vi) We have found many different values of quantum efficiencies for the same reaction, confirming the idea originally exposed that, without the exact definition of all properties of the catalyst and the reaction operating conditions, it is impossible to obtain a unique value to be used for comparison purposes. Acknowledgment The authors are grateful to Universidad Nacional del Litoral (UNL), Consejo Nacional de Investigaciones Cientı´ficas y Te´cnicas (CONICET), and Agencia Nacional de Promocio´n Cientı´fica y Tecnolo´gica (ANPCyT) for the financial support. They also thank Antonio C. Negro for his valuable help during the experimental work. Nomenclature C ) molar concentration (mol cm-3) Cm ) catalyst mass concentration (g cm-3) ea ) local volumetric rate of photon absorption (Einstein cm-3 s-1) Er ) relative emission of the lamps (dimensionless) g ) asymmetry factor (dimensionless) I ) radiation intensity (Einstein cm-2 sr-1 s-1) L ) length (cm) n ) refractive index (dimensionless) p ) phase function (dimensionless) q ) net radiative flux (Einstein cm-2 s-1) r ) reaction rate (mol cm-3 s-1) t ) time (s) V ) volume (cm3) x ) axial coordinate (cm) x ) position vector (cm) Greek Letters  ) hold-up (dimensionless) β ) volumetric extinction coefficient (cm-1) β* ) specific extinction coefficient (cm2 g-1) Γ ) global reflection coefficient (dimensionless) θ ) spherical coordinate (rad) θc ) critical angle (rad) κ ) volumetric absorption coefficient (cm-1) κ* ) specific absorption coefficient (cm2 g-1)

µ ) direction cosine of the ray for which the RTE is written µ' ) direction cosine of an arbitrary ray before scattering µ0 ) cosine of the angle between the direction of the incident and the scattered rays µ* ) cosine of the refracted angle µc ) cosine of the critical angle θc F ) interface reflectivity (dimensionless) σ ) volumetric scattering coefficient (cm-1) σ* ) specific scattering coefficient (cm2 g-1) Φ ) quantum yield (mol Einstein-1) η ) quantum efficiency (mol Einstein-1) τ ) glass window internal transmittance (dimensionless) Subscripts a ) air 4-CP ) 4-chlorophenol f ) final condition g ) glass HG ) Henyey and Greenstein L ) liquid phase LR ) relative to the reactor window at x ) LR R ) reactor s ) suspension T ) total system Tk ) tank TOC ) total organic carbon W ) reactor window 0 ) initial condition; also, relative to the reactor window at x )0 λ ) dependence on wavelength Special Symbols 〈〉 ) denotes average value over a given space Literature Cited (1) Bahnemann, D. Photocatalytic Water Treatment: Solar Energy Applications. Solar Energy 2004, 77, 445. (2) Salaices, M.; Serrano, B.; de Lasa, H. I. Experimental Evaluation of Photon Absorption in an Aqueous TiO2 Slurry Reactor. Chem. Eng. J. 2002, 90, 219. (3) Herrmann, J. M. Heterogeneous Photocatalysis: Fundamentals and Applications to the Removal of Various Types of Aqueous Pollutants. Catal. Today 1999, 53, 115. (4) Braun, A. M.; Maurette, M. T.; Oliveros, E. Photochemical Technology; Wiley: New York, 1991. (5) Palmisano, L.; Augugliaro, V.; Campostrini, R.; Schiavello, M. A. Proposal for the Quantitative Assessment of Heterogeneous Photocatalytic Processes. J. Catal. 1993, 143, 149. (6) Sun, L.; Bolton, J. R. Determination of the Quantum Yield for the Photochemical Generation of Hydroxyl Radicals in TiO2 Suspensions. J. Phys. Chem. 1996, 100, 4127. (7) Serpone, N. Relative Photonic Efficiencies and Quantum Yields in Heterogeneous Photocatalysis. J. Photochem. Photobiol., A 1997, 104, 1. (8) Stafford, U.; Gray, K. A.; Kamat, P. V. Photocatalytic Degradation of 4-Chlorophenol: The Effects of Varying TiO2 Concentration and Light Wavelength. J. Catal. 1997, 167, 25. (9) Cabrera, M. I.; Alfano, O. M.; Cassano, A. E. Quantum Efficiencies of the Photocatalytic Decomposition of Trichloroethylene in Water. A Comparative Study for Different Varieties of Titanium Dioxide Catalysts. J. AdV. Oxid. Technol. 1998, 3, 220. (10) Salaices, M.; Serrano, B.; de Lasa, H. I. Photocatalytic Conversion of Organic Pollutants. Extinction Coefficients and Quantum Efficiencies. Ind. Eng. Chem. Res. 2001, 40, 5455. (11) Curco´, D.; Gime´nez, J.; Addardak, A.; Cervera-March, S.; Esplugas, S. Effects of radiation absorption and catalyst concentration on the photocatalytic degradation of pollutants. Catal. Today 2002, 76, 177. (12) Brandi, R. J.; Citroni, M. A.; Alfano, O. M.; Cassano, A. E. Absolute Quantum Yields in Photocatalytic Slurry Reactors. Chem. Eng. Sci. 2003, 58, 979.

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ReceiVed for reView March 30, 2006 ReVised manuscript receiVed September 25, 2006 Accepted October 9, 2006 IE0604019