Photocatalytic degradation of model organic pollutants on an immobilized particulate TiO2 layer

Applied Catalysis B: Environmental 64 (2006) 290–301 www.elsevier.com/locate/apcatb

Photocatalytic degradation of model organic pollutants on an immobilized particulate TiO2 layer Roles of adsorption processes and mechanistic complexity Josef Kry´sa a,*, Georg Waldner b, Hana Meˇsˇt’a´nkova´ c, Jaromı´r Jirkovsky´ c, Gottfried Grabner d a

Institute of Chemical Technology, Department of Inorganic Technology, Technicka´ 5, CZ-166 28 Prague 6, Czech Republic b Institut fu¨r Materialchemie, Technische Universita¨t Wien, Veterina¨rplatz 1, A-1210 Wien, Austria c J. Heyrovsky´ Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolejsˇkova 3, CZ-182 23 Prague 8, Czech Republic d Max F. Perutz Laboratories, Department of Chemistry, University of Vienna, Campus-Vienna-Biocenter 5, A-1030 Wien, Austria Received 7 July 2005; received in revised form 14 November 2005; accepted 19 November 2005 Available online 26 January 2006

Abstract The kinetics of photocatalytic degradation of four different model organic compounds, formic acid (FA), oxalic acid (OA), 4-chlorophenol (4CP) and the herbicide monuron (3-(4-chlorophenyl)-1,1-dimethylurea) in a self-constructed batch-mode plate photoreactor with a thin flow of contaminated aqueous solution circulating over an illuminated particulate layer of TiO2 P25 (Degussa) was compared. Both OA and FA were adsorbed on TiO2 surface; their mineralization, induced by direct transfer of photogenerated holes, proceeded in a single step, without observable intermediates, following approximately zero order kinetics. Numerical simulations were performed using a newly proposed kinetic model based on the photostationary state assumption. The model allowed an explanation of the observed reaction order as well as the comparison of independent with competitive adsorption of organic compound and oxygen on the photocatalyst surface, yielding a better fit for the case of competition. 4-CP and monuron, which were not adsorbed under the conditions used, were degraded through the action of photogenerated hydroxyl radicals. Their degradation proceeded with lower photoefficiency than for the adsorbed compounds (FA and OA). While the mineralization of both 4-CP and monuron followed zero order kinetics, their degradation was close to first order. The different reaction orders were consistently explained using the photostationary state approach. # 2005 Elsevier B.V. All rights reserved. Keywords: Photocatalysis; Degradation; TiO2; Immobilization; Oxalic acid; Formic acid; 4-Chlorophenol; Monuron

1. Introduction Photocatalysis on TiO2 represents a promising alternative technology for degradation of organic pollutants and inactivation of microorganisms in water. It is based on the photogeneration of separated electrons and positive holes in semiconductor particles. These charge carriers either recombine inside the particle or move to its surface where they can react with adsorbed molecules. Positive holes typically oxidize organic compounds,

* Corresponding author. Tel.: +420 2 2044 4112; fax: +420 2 2044 4410. E-mail address: [email protected] (J. Kry´sa). 0926-3373/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2005.11.007

inducing their oxidative degradation, while electrons mainly reduce molecular oxygen to superoxide radical anions. Recombination of photogenerated positive holes and electrons inside the semiconductor particles is responsible for the relatively low quantum yield of photocatalytic degradation. Two different types of photoreactor are generally employed in photodegradation studies, containing either mixed aqueous slurries or immobilized layers of the TiO2 photocatalyst. Good mass transport characteristics are the great advantage of the slurries. However, the catalyst requires long settlement times to be separated from the purified water or alternatively, fine filters have to be employed. Therefore, attention turned recently to immobilized photocatalysts [1]. Concerning the

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model compounds in photodegradation experiments, oxalic acid [2–4], formic acid [5–7] and 4-chlorophenol [1,8–13] have often been used. The topic of semiconductor photocatalysis studies is frequently the comparison of the effect of various photocatalysts on the degradation of a selected model pollutant with the aim of optimizing process efficiency [14]. There are only few papers comparing the degradation behavior of different organic pollutants using the same type of catalyst. The influence of the chemical structure and adsorption of dyes [15] and sulfonylurea herbicides [16] on photocatalytic degradation on TiO2 was studied recently. Excepting a study of chemisorption of phenols and acids [17], the photodegradation mechanisms of aliphatic carboxylic acids and aromatic substances, such as 4-chlorophenol, have not been systematically compared. A number of methods for immobilization of TiO2 particles on various supports has been described in the literature [18]. In this work, we followed the approach of Vinodgopal et al. [8,9] in preparing particulate layers on glass sheet supports from TiO2 P25 (Degussa) slurries by sedimentation and drying at 300 8C. This paper represents a continuation of previous studies devoted to the investigation of the role of mass transfer and photon flux in controlling the kinetics of the photocatalytic degradation of oxalic acid in aqueous films flowing over illuminated particulate layers of TiO2 P25 (Degussa) in batchmode plate photoreactors [19–21]. The aim of the present work was to compare the kinetics of photocatalytic degradation and mineralization of four different model organic compounds, formic acid, oxalic acid, 4-chlorophenol and the herbicide monuron, in the same reactor. Studies of the same model pollutants have previously been carried out in photoelectrochemical reactors [22,23]. The two groups of model compounds chosen – aliphatic versus aromatic – differ in two important respects, namely in their adsorption properties and in the complexity of their degradation mechanism. Accordingly, comparing their degradation behavior allows us to address the role of these two mechanistic aspects on the degradation kinetics. The experimental results will be discussed with reference to current kinetic models of photocatalysis; a new model will be put forward to allow a more detailed description of the photostationary state approach and to compare two adsorption situations, i.e. independent versus competitive adsorption of the organic model compound and of molecular oxygen on the surface of the photocatalyst.

291

Fig. 1. Morphology of immobilized particulate TiO2 layer (1.0 mg TiO2 cm2) prepared by sedimentation on glass substrate and thermal treatment at 300 8C.

additional drying and annealing at 300 8C for 1 h. The TiO2 amount was estimated gravimetrically to be about 1 mg cm2. The surface morphology of particulate TiO2 layer is shown in Fig. 1. Using X-ray diffraction, the mean size of anatase particles was estimated to be 15 nm. It means that the so-called particles observed by SEM are in fact aggregates of sizes about 1–5 mm. Monuron was purchased from Sigma–Aldrich, oxalic acid from Fluka. 4-Chlorophenol, formic acid, methyl alcohol and acetonitrile for HPLC were Merck products. All chemicals were used without further purification. A batch-mode plate photoreactor with a thin flow (thickness about 1 mm) of contaminated aqueous solution circulating over the immobilized particulate TiO2 layer, irradiated by three ultraviolet sun bed tubes (Lynx 11W BL350, Sylvania, broad maximum at 350
20 nm), was employed (Fig. 2). The mean incident photon flux on the photoreactor plate at wavelengths below 400 nm was determined, using a Si photodiode Hamamatsu S1337-BQ, as 1.0  104 Einstein m2 s1. The photoreactor was constructed from rectangular polymethylmethacrylate trays with troughs at both ends. The

2. Experimental TiO2 P25 (Degussa) was used as the photocatalyst in this study. It consists of a non-porous 70:30% anatase-to-rutile mixture with a BET surface area of 55
12 m2 g1 and crystalline sizes of 30 nm in 0.1 mm diameter aggregates. The immobilized particulate TiO2 layers were prepared on glass sheets (length 15 cm, width 10 cm) by sedimentation from an aqueous suspension of TiO2 P25 (10 g dm3) for 1 h, with

Fig. 2. Scheme of a self-constructed batch-mode plate photoreactor with a thin liquid film of contaminated aqueous solution circulating over an immobilized particulate layer of TiO2 photocatalyst irradiated by UV tubes. Distance of UV tubes from glass plate 12.5 cm, inclination 108.

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trays were dimensioned to accommodate a glass plate 15 cm long and 10 cm wide. The contaminated aqueous solution was pumped from the holding tank employing a centrifugal pump (Nova, Sicce, Italy) to the higher trough. The flow rate of 1.25 dm3 min1 and the corresponding flow velocity of 0.208 m s1 were controlled by a needle valve and rotameter. The overflow produced a liquid film flowing over the TiO2 layer immobilized on a glass plate fixed in the trays. The solution was collected in the lower trough and returned into the holding tank. The dissolved oxygen concentration was 0.25 mM. To follow the degradation kinetics, samples of the reaction mixture were taken at various irradiation times. The concentration of OA was determined by titration with potassium permanganate. The concentration of FA was estimated through its carbon content determined employing a TOC analyzer Shimadzu 5000. The degradation course of both 4-CP and monuron was followed in parallel by means of UV–vis absorption spectroscopy, HPLC and TOC analyses. HPLC analysis was carried out employing Shimadzu chromatographic set with LC-10ADvp pump and SPD-M10Avp diode array detector. A mobile phase acetonitrile/water (35:65, v/v) was applied with a flow rate 1 ml min1. LiChrospher 100 RP-18 column (type LiChroCART 125-4, Merck, Germany) was used. The adsorption equilibrium of OA, monuron and 4-CP on the TiO2 was examined using a suspension (5 g dm3).

OA. Moreover, the difference between both kinetic curves, which should correspond to the OC of eventual degradation intermediates, is very small. It is worth to note that FA, a possible degradation intermediate of OA, did not react with permanganate under the conditions used for the OA titration. This means that the photostationary concentration of FA was very low and that OA represented the only organic compound present in the reaction mixture during the whole mineralization process. A reaction mechanism of the photocatalytic degradation of OA was proposed previously [20], assuming the initial attack of hydroxyl radicals (OH) and the intermediate formation of FA. This mechanism has to be modified in view of the very small difference between OA disappearance and TOC evolution (Fig. 3), suggesting negligible intermediate formation of FA. The initial reaction of OH radical with the hydrogen oxalate anion, which is the most abundant form of OA at natural pH, leads to the hydrogen oxalate radical: HOOCCOO þ  OH ! HOOCCOO þ OH

(1)

This reaction proceeds relatively slowly in homogeneous aqueous solution (k = 4.7  107 mol1 dm3 s1) [24]. The alternative possibility of the direct transfer of a photogenerated positive hole (h+) to adsorbed OA should therefore be taken into account:

3. Results and discussion HOOCCOO þ hþ ! HOOCCOO 3.1. Photocatalytic degradation of simple carboxylic acids Kinetic dependences of both total organic carbon (TOC), measured by TOC analysis, and organic carbon (OC) corresponding to OA, calculated from the OA concentrations determined by permanganometry, are shown in Fig. 3. It can be seen that both dependences on irradiation time are approximately linear, corresponding to zero order kinetics of the photocatalytic degradation as well as of the mineralization of

(2)

The fate of the hydrogen oxalate radical, formed in both reactions (1) and (2), is not known with certainty; fast decarboxylation to form a carbon dioxide radical anion has been suggested [4]: HOOCCOO ! Hþ þ  CO2  þ CO2

(3)

These radical anions are known to undergo fast bimolecular reactions, forming either hydrogen oxalate by recombination or formate and carbon dioxide by disproportionation: 2  CO2  þ Hþ ! HOOCCOO

(4)

2  CO2  þ Hþ ! HCO2  þ CO2

(5)

However, because of the low photostationary concentration of carbon dioxide radical anions produced by continuous irradiation, the bimolecular reactions (4) and (5) of the carbon dioxide radical anions (k = 6.5  108 mol1 dm3 s1) [25] will be negligible compared to their reaction with dissolved oxygen (k = 4.2  109 mol1 dm3 s1) [26]: 

Fig. 3. Kinetics of total organic carbon (TOC) in the course of photocatalytic degradations of oxalic acid and formic acid compared to time profile of substrate carbon content (OC) (for oxalic acid only).

CO2  þ O2 ! CO2 þ  O2 

(6)

An analogous kinetic TOC dependence for the photocatalytic degradation of FA carried out in the same photoreactor is also shown in Fig. 3. Similar to OA, the TOC values of FA decreased approximately linearly with irradiation time.

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The initial reaction of OH radicals with the formate anion, the most abundant form under the used conditions, leads to carbon dioxide radical anions: HCOO þ  OH !  CO2  þ H2 O

(7)

In homogeneous aqueous solutions, this reaction proceeds much faster (k = 5.1  109 mol1 dm3 s1) [27] than the corresponding reaction (1) of OA. Alternatively, direct hole transfer from TiO2 particle to adsorbed formate may be assumed, leading to the same radical: HCOO þ hþ !  CO2  þ Hþ

(8)

This step will be followed by reaction (6), as in the case of OA, to complete the mineralization. Under the conditions used, the O2 radicals produced in reaction (6) will protonate to give HO2 radicals. These radicals are not expected to be reactive toward OA or FA. Thus, their disproportionation leading to hydrogen peroxide and oxygen should be supposed. Hydrogen peroxide can subsequently oxidize OA and FA to CO2. In parallel to the oxidation of the organic substrates, reductive reactions are necessary to close the photocatalytic cycle of TiO2; these mainly consist in electron transfer to adsorbed molecular oxygen that also leads to the formation of O2 radicals. As a result, the following overall reaction equations for the photocatalytic degradation of OA and FA can be derived: 2HOOCCOOH þ O2 þ hþ þ e ! 4CO2 þ 2H2 O 2HOOCH þ O2 þ hþ þ e ! 2CO2 þ 2H2 O

(9) (10)

It can be concluded that both OA and FA can be mineralized in a single step following the initial oxidation attack by hole transfer. Under conditions of continuous irradiation involving low photostationary concentrations of photogenerated charge carriers, their primary transfer reactions will control the overall kinetics of the photocatalytic degradation of OA and FA because the subsequent reactions (3) and (6) are faster. It can be seen in Table 1 that the reaction rates of the disappearance of both OA and FA were very similar even though the rate constants of reactions (1) and (7) differ by two orders of magnitude (it should be noted that the different slopes in Fig. 3 result from the different numbers of carbon atoms contained in OA and FA). This leads to the conclusion that direct hole transfer to adsorbed OA and FA molecules is more efficient than OH attack as the induction step of the oxidative degradation of both OA and FA.

Fig. 4. Time evolution of UV–vis absorption spectra in the course of photocatalytic degradation of monuron.

3.2. Photodegradation of the herbicide monuron Similarly to diuron [28], the adsorption of monuron on titanium dioxide was found to be negligible (see Section 2). UV–vis spectra of the reaction mixture during the photocatalytic degradation of monuron (initial concentration 1.0  104 M) are shown in Fig. 4. The initial spectrum scanned before irradiation corresponds to pure monuron; it consists of two absorption bands with maxima at 205 and 244 nm (e244 = 1.73  104 mol1 dm3 cm1) and a shoulder at about 280 nm not exceeding 320 nm. It can be seen that the absorbance decreased during the photodegradation process without any characteristic spectral changes. It can be expected that both absorbing and non-absorbing intermediates are formed during the photocatalytic degradation of monuron [28]. The former represent aromatic compounds (mainly monuron derivatives with structurally altered urea side chain), while the latter are ring-opening degradation products not absorbing above 230 nm. Using the same extinction coefficient value (i.e. e244 of monuron) for all absorbing components of the reaction mixture, a total concentration could be estimated from the measured absorbance. The results are shown, as open triangle symbols, in Fig. 5 together with the concentrations of monuron determined by HPLC (open squares); it can be seen that the total concentration estimates are higher than the monuron concentrations. The difference, shown in Fig. 5 as solid triangles, could be attributed to the overall contribution of absorbing degradation intermediates of monuron. The corresponding time profile first increases to reach a maximum at about 3 h of irradiation and then gradually decreases. After

Table 1 Initial degradation (dcorg,i/dt) and mineralization (dcTOC/dt) rates of the studied compounds during photocatalytic degradation in a batch-mode plate photoreactor Compound

Initial concentration (103 mol dm3)

dcorg,i/dt (108 mol dm3 s1)

Initial TOC (ppm)

dcTOC/dt (103 ppm s1)

Oxalic acid Formic acid 4-Chlorophenol Monuron

5.0 5.0 0.2 0.1

14.1 14.2 1.2 1.3

120.0 69.0 13.5 13.0

3.55 1.70 0.27 0.17

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Fig. 5. Photocatalytic degradation kinetics of monuron determined by HPLC compared to time profile of all absorbing components estimated from UV–vis absorption spectra of reaction mixture and difference of both curves.

3 h of irradiation, 68% of monuron was transformed while the sum of absorbing intermediates corresponds to 16% of the initial concentration of monuron. This result is in good agreement with the results of a kinetic study of photocatalytic degradation of diuron in aqueous colloidal solutions of Q-TiO2 particles, showing that 25% of the diuron transformation leads to absorbing side chain derivatives while 75% proceeds via ring-opening to give non-absorbing products [28]. The measured TOC values and the corresponding OC values for monuron (calculated from its concentrations determined by HPLC) are shown in Fig. 6. Their difference represents the sum of the OC values of all organic degradation intermediates present in the reaction mixture. For comparison, the estimate of the sum of absorbing intermediates adapted from Fig. 5 is also shown in Fig. 6. Different calibration procedures for TOC, UV–vis spectroscopy and HPLC caused that the calculated initial concentrations of intermediates in Figs. 4 and 5 are not exactly zero. The TOC values decreased

Fig. 6. Kinetics of TOC in the course of photocatalytic degradation of monuron compared to time profile of OC content of monuron (determined by HPLC), difference of both curves corresponding to all degradation intermediates and time profile of absorbing components only (adapted from Fig. 4).

Fig. 7. Time evolution of UV–vis absorption spectra in the course of photocatalytic degradation of 4-chlorophenol.

approximately linearly with irradiation time and reached 33% of their initial value after 15 h of irradiation. The OC values corresponding to all organic degradation intermediates first increased to reach a maximum at 5–6 h and then started to decrease. After 6 h of irradiation, the OC value of monuron corresponded to only 16% of the remaining TOC value while the other 84% represented the sum of all organic degradation intermediates. After 9 h of irradiation, the mineralization reached 50%, the OC of monuron was very low (only about 3% of the initial value) and the reaction mixture contained about 6% of monuron and 94% of organic degradation intermediates. It should be stressed that the time profile adapted from Fig. 5 represents only the absorbing intermediates, while the differential kinetic curve (TOC minus OC of monuron) corresponds to the sum of all organic degradation products, i.e. absorbing plus non-absorbing. 3.3. Photodegradation of 4-chlorophenol Unlike monuron, 4-chlorophenol was measurably adsorbed on a TiO2 suspension (around 5  106 mol g1 for a bulk concentration of 5  104 M). Still, the adsorption ability of 4chlorophenol was by about two orders of magnitude lower than that of oxalic acid. UV–vis absorption spectra of the reaction mixture measured during the photocatalytic degradation of 4-CP (initial concentration 2.0  104 M) are shown in Fig. 7. The initial spectrum corresponds to pure 4-CP and exhibits two characteristic bands with maxima at 226 nm (e266 = 8.14  103 mol1 dm3 cm1) and 280 nm (e280 = 1.43  103 mol1 dm3 cm1). As in the case of monuron, the absorbance also decreased with irradiation time in the whole wavelength range. TOC results and OC values corresponding to 4-CP, calculated from the concentrations determined by HPLC, are shown in Fig. 8 as a function of irradiation time. The difference of these two dependences, corresponding to organic degradation intermediates, can also be seen in Fig. 8. The TOC values decreased with irradiation time following kinetics close to

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zero order, but the time delay between substrate degradation and mineralization was less pronounced than for monuron (Fig. 6). After 14 h of irradiation, the mineralization reached about 94%. The OC values corresponding to the sum of all organic degradation intermediates increased during the first 6 h to reach a flat maximum and then started gradually to decrease. After 6 h of irradiation, the mineralization was completed to about 45%; the OC value of 4-CP was about 55% of the remaining TOC value while the other 45% corresponded to the sum of all organic degradation intermediates. Hydroquinone (HQ) and benzoquinone (BQ) were identified in the reaction mixture by HPLC as degradation intermediates of 4-CP. The dependences of their concentrations on irradiation time are shown in Fig. 9. It can be seen

that the concentrations of both BQ and HQ first increased with irradiation time to reach their maxima at 7 h for BQ and at 5 h for HQ. The observed delay for HQ and different shapes of both time profiles (sharp for HQ and flat for BQ) are in good agreement with the finding that the photocatalytic degradation of BQ proceeded through HQ as an intermediate and vice versa [29]. The degradation kinetics of 4-CP was close to first order. Analyzing the TOC value after 6 h of irradiation (Figs. 8 and 9), the mineralization reached about 45% and the remaining organic substrate contained 55% of 4-CP, 20% of BQ, 8% of HQ and 17% of other organic degradation intermediates. As in the case of monuron described above, an estimate of the overall concentration of all absorbing components was derived using e226 of 4-CP, and compared to the actual concentration of 4-CP, determined by HPLC (Fig. 10). The latter is lower than the overall concentration at all irradiation times. This again shows that other absorbing components are present in the reaction mixture. As mentioned above, two of them were identified as HQ and BQ. Using the extinction coefficients of BQ (e226 = 5.59  103 mol1 dm3 cm1) and HQ (e226 = 4.41  103 mol1 dm3 cm1) as well as their concentrations determined by HPLC, their absorbances at l = 226 nm were calculated and subtracted from the measured absorbances of the reaction mixture. These differential values correspond to the sum of absorbances of 4-CP plus its degradation intermediates except HQ and BQ. Using again e226 of 4-CP for the remaining components, corrected concentrations were estimated, shown in Fig. 10 as solid triangles; the resulting values correspond nicely to the actual concentrations of 4-CP. It can be concluded that HQ and BQ were the dominant absorbing intermediates in the reaction mixture. This is in agreement with the finding that the only peaks observed in the HPLC chromatograms at l = 226 nm were those of 4-CP, HQ and BQ. 4-Chlorocatechol, another

Fig. 9. Time evolution of the concentrations of 4-chlorophenol, hydroquinone and benzoquinone (determined by HPLC) in the course of photocatalytic degradation of 4-chlorophenol.

Fig. 10. Time profiles of the concentration of 4-chlorophenol determined by HPLC, of the sum of all absorbing components estimated from UV–vis absorption spectra based on the extinction coefficient of 4-chlorophenol (e226 = 8.14  103 dm3 mol1 cm1), and of the absorbing components after subtraction of the contributions of hydroquinone and benzoquinone.

Fig. 8. Kinetics of TOC in the course of photocatalytic degradation of 4chlorophenol compared to time profile of OC content of 4-chlorophenol (determined by HPLC) and difference of both curves corresponding to all degradation intermediates.

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expected primary degradation product of 4-CP [30], remained possibly adsorbed on the TiO2 layer. This was proved in an analogous study with TiO2 suspension. 3.4. Comparison of degradation kinetics The initial degradation rates (dcorg,i/dt), defined as the initial slopes of the dependences of the concentration of each model compound on irradiation time, and analogously, the mineralization rates (dcTOC/dt) defined as the slopes of the dependences of TOC values on irradiation time, are summarized in Table 1 for OA, FA, 4-CP and monuron. It is apparent that the initial degradation rates of FA and OA on one hand and of 4-CP and monuron on the other hand were similar. However, the former values were by more than one order of magnitude higher than the latter ones. This indicates major differences in the reaction mechanisms for both couples. While the carboxylic acids OA and FA most probably formed surface complexes on TiO2, which were oxidized by direct charge transfer of photogenerated holes, 4CP and monuron had to undergo indirect oxidation mediated through photogenerated OH radicals. The direct reactions (2) or (8) might proceed more efficiently than hole trapping on surface hydroxyl groups to produce surface-bound OH radicals which subsequently react with dissolved 4-CP or monuron molecules. The mineralization rate of OA was twice higher than the corresponding value of FA, because both compounds were mineralized in a single step following direct hole transfer, producing two CO2 molecules from OA but just one CO2 molecule from FA. Comparing 4-CP with monuron, the corresponding values of initial degradation rates were almost the same (Table 1). However, the mineralization rate was by about 50% higher for the former. This could be due to the structural differences of both molecules. Monuron, with a more complex structure and higher number of carbon atoms, needed probably a higher number of subsequent steps to be mineralized than the simpler molecule 4-CP. As noted above, a substantial fraction of OH radical reactions with monuron will lead to oxidation at the side chain, while 4-CP must be attacked on the ring. The inherently more complex degradation mechanism of monuron is clearly demonstrated by the much greater time delay of TOC formation with respect to substrate degradation (see Figs. 6 and 8). Moreover, the differential kinetic curves corresponding to the total amount of organic intermediates in the reaction mixture were more pronounced in the case of monuron than for 4-CP. 3.5. Reaction order of degradation and mineralization kinetics It was experimentally proved that the effect of flow rate on the degradation kinetics of model compounds was almost negligible. It means that the flow regime became turbulent and the mass transfer phenomena did not play any important role.

3.5.1. Langmuir–Hinshelwood model approach The mechanism of the photocatalytic degradation of organics has frequently been discussed in connection with the adsorption of the compound to be degraded on the photocatalyst surface. The reaction rate in TiO2 slurries is then usually described by the Langmuir–Hinshelwood expression: r¼

dcRH KRH cRH ¼ kRH dt 1 þ KRH cRH

(11)

where cRH is molar concentration of the degraded organic substrate, KRH corresponds to adsorption coefficient of the organic substrate on TiO2 surface and kRH represents degradation rate constant. Several authors [31–35] have treated the kinetics of photocatalytic degradation in photoreactors with immobilized TiO2 using the Langmuir–Hinshelwood relation (11). There is a general agreement that at high concentrations of the organic substrate (KRHcRH  1), the reaction rate (r  kRH) should remain constant and thus zero order kinetics results. On the other hand, at low concentrations (KRHcRH  1), the reaction rate (r  kRHKRHcRH) should be proportional to cRH and therefore the kinetics is of first order with apparent rate constant kRHKRH. A kinetic behavior of this type was observed in detail for the photocatalytic degradation of malonic acid [36] and thymine [37] in TiO2 slurries. However, a generally applicable kinetic treatment of photocatalytic degradation cannot be obtained in this way, because neither the formation of intermediates with distinct adsorption properties and reactivities, nor the presence of coadsorbants, in particular molecular oxygen, are taken into account. The simple Langmuir–Hinshelwood approach may therefore fail in the case of complex degradation mechanisms, but, as we will show below, may also be unable to account for the experimentally observed kinetic order in the ‘‘simple’’ carboxylic acid systems. It has been pointed out earlier that considerable discrepancies may exist between adsorption coefficients KRH resulting from the Langmuir–Hinshelwood treatment of kinetic data and those obtained by classical dark adsorption isotherm measurements [38]. 3.5.2. Photostationary state approach A realistic model for the description of photocatalytic degradation kinetics should take into account the photogeneration of electron–hole pairs, their recombination and charge transfer reactions, as well as the adsorption equilibria of the substrate to be degraded, its intermediate photoproducts, and of molecular oxygen. This will not be possible in the general case, since the chemical nature, the adsorption properties and the reactivity of the intermediates are frequently unknown. However, the special case of the carboxylic acids OA and FA lends itself well for a model treatment since, as shown above, no organic intermediates complicate the degradation mechanism. We have therefore proposed a novel kinetic approach, described in detail in Appendix A, based on the concept of a photostationary equilibrium of the formation and reactions of charge carriers in the semiconductor, combined with the adsorption equilibria of organic substrate and molecular oxygen.

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Fig. 11. Reaction scheme of the photocatalytic oxidation of an electron donor (D) in the presence of an electron acceptor (A) on the illuminated surface of semiconductor photocatalyst.

This model, formally described by the flow scheme shown in Fig. 11, is mathematically based on a set of differential equations defining the individual reaction steps (Tables 2 and 3), which must be numerically integrated. In Appendix A, we show an application aiming at the discrimination between two specific cases of adsorption equilibria, i.e. competitive versus non-competitive adsorption of OA and oxygen on Table 2 Definition of initial values and derivatives of the variables in the kinetic model of photocatalytic degradation assuming either independenta or competitiveb adsorption of electron donor (D) and electron acceptor (A) on the photocatalyst surface Variable Agas Abulk Aads A Light Photon e h+ Heat Dbulk Dads D+ *

Initial value *

Arbitrary [A]bulk Eq. (26)a or (30)b 0 Arbitrary* Iabs Eq. (22) Eq. (21) 0 cD  [D]ads,0 Eq. (25)a or (29)b 0

Derivative d½Agas =dt ¼ v1 þ v2 d½Abulk =dt ¼ v1  v2  v4 þ v5 ¼ 0 d½Aads =dt ¼ v4  v5  v10 d½A =dt ¼ v10 d½Light=dt ¼ v3 d½Photon=dt ¼ v3  v6  v7 ¼ 0 d½e =dt ¼ v4  v10  v11 d½hþ =dt ¼ v7  v12  v13 d½Heat=dt ¼ v11 þ v12 d½Dbulk =dt ¼ v8 þ v9 d½Dads =dt ¼ v8  v9  v13 d½Dþ =dt ¼ v13

These variables are formally needed to keep [A]bulk and Iabs constant.

the TiO2 surface. Comparing the model calculations with experimental data yields a better agreement for the case of competitive adsorption (see below). Applying the numerical photostationary model to all systems studied in this work is beyond the scope of the present paper. However, qualitative pictures can be obtained which are discussed in the following. We have shown above (Fig. 3) that the photocatalytic degradation of both OA and FA followed approximately zero order kinetics. Assuming constant irradiation intensity, the photogeneration rate of electron–hole pairs remains constant. The separated holes and electrons either recombine inside the TiO2 particles or are transferred through the interface to adsorbed molecules, positive holes to OA or FA and electrons to oxygen. These three processes are competitive and determine the quantum yield of the photocatalytic degradation. The recombination rate is proportional to the concentrations of both holes and electrons, whereas the rate of hole transfer is proportional to the hole concentration and to the surface concentration of the carboxylic acid, and the rate of the electron transfer is proportional to the electron concentration and to the surface concentration of oxygen. It can be assumed that the bulk concentration of oxygen remains constant during the whole photodegradation process due to equilibration with air. However, the total bulk plus surface concentration of the carboxylic acid gradually decreases because of its degradation. As long as this

Table 3 Definition of reaction rates for the kinetic model of photocatalytic degradation, shown in Fig. 12, assuming either independent or competitive adsorption of electron donor (D) and electron acceptor (A) on the photocatalyst surface Rate

Definition (independent adsorption)

Definition (competitive adsorption)

v1 ¼ v4 v2 ¼ v5 v3 ¼ v6 ¼ v7 v8 v9 v10 v11 ¼ v12 v13

kA,ads  [A]bulk  (csurf  [A]ads) kA,ads/KA  [A]ads Iabs kD,ads  [D]bulk  (csurf  [D]ads) kD,ads/KD  [D]ads 2ke  [e]  [A]ads kr  [e]  [h+] 2kh  [h+]  [D]ads

kA,ads  [A]bulk  (csurf  [A]ads  [D]ads) kA,ads/KA  [A]ads Iabs kD,ads  [D]bulk  (csurf  [D]ads  [A]ads) kD,ads/KD  [D]ads 2ke  [e]  [A]ads kr  [e]  [h+] 2kh  [h+]  [D]ads

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Fig. 12. Experimental dependence of bulk concentration of oxalic acid on irradiation time and two theoretical fits based on the kinetic model of photocatalytic degradation assuming either independent or competitive adsorption of oxalic acid and oxygen.

concentration is sufficient to saturate the photocatalyst surface, the surface concentration and thus the degradation rate remains constant. As a result, zero order photodegradation kinetics is observed; this interpretation is valid for the initial part of the degradation process as shown in Fig. 3. As the concentration of OA decreases in the course of the degradation process, a gradual transition from zero order to first order kinetics should be observed according to the Langmuir– Hinshelwood model, i.e. the measured rate should slow down. Actually, the experimental results exhibit the opposite tendency (as shown in Fig. 12) to a small, but significant extent, the measured rate gets faster as the degradation proceeds. As shown in Appendix A, this observation can be quantitatively modeled using the photostationary approach with the assumption that the adsorption processes of OA and oxygen are competitive. Qualitatively, this apparently counterintuitive result can be explained by the fact that the decrease of the surface concentration leads to an increase of the concentration of adsorbed oxygen in the case of competitive adsorption, thereby reducing the concentration of electrons. This will result in a reduced rate of electron–hole recombination. The overall result is that the rate of hole transfer does not change much, and even tends to increase, during the whole degradation process. As stated above, the application of the photostationary model to the degradation of the aromatics monuron and 4-CP is hampered by the complexity of the reaction mechanism. Nonetheless, some interpretation is possible here as well. The most conspicuous result as far as reaction order is concerned is the observation, valid for both monuron and 4-CP, of first order substrate degradation and of zero order mineralization kinetics (see Figs. 6 and 8). The starting point of the photostationary model is the assumption that the primary reactive species (photogenerated holes or OH radicals) reach an equilibrium determined by their rates of formation (by irradiation) and disappearance (by reaction with molecules in the system), and that these reactions determine the rate of degradation. The sum of the individual reaction rates of all molecules must therefore be constant. The experimental

observation of a first order substrate degradation means that the rate decreases with time; the photostationary condition requires that this decrease is exactly compensated by increasing rates of reaction of intermediates formed in secondary reactions. Therefore, first order kinetics necessarily presupposes that the concentration of organic molecules in the solution is constant during the degradation of the starting compound, and that the degradation rate constants of the individual molecular components are similar. Monuron and 4-CP were only negligibly adsorbed in the conditions used and thus their degradation was mediated by photogenerated OH radicals. It is known that a large number of reaction products may be produced by reaction of OH radicals with benzene derivatives, indicating complex reaction sequences; in the case of stationary irradiation, these intermediate products in turn can undergo further reactions with OH [39,40]. In most cases, the rate constants of these reactions will be high or even close to diffusion control. For this reason, the total molar concentration of organic molecules in the reaction mixture does not change much for a considerable time as the reaction proceeds, even though degradation of the substrate may already be quite advanced [40]. Under these conditions, a first order degradation kinetics is expected and experimentally observed, as illustrated in Figs. 6 and 8. The experimental first order rate constant (kexp = 1  103 s1 for monuron) should then correspond to the product of an intrinsic second order rate constant of reaction OH radicals with the substrate multiplied by the photostationary concentration of OH radicals, i.e. kexp = kOH [OH]. Provided that the rate constant of OH attack on monuron is close to the diffusion limit, kOH  1010 mol1 dm3 s1, similarly to 4-chloroaniline (7.3  109 mol1 dm3 s1) [41], the corresponding photostationary concentration of OH radicals would be [OH]  1014 mol dm3. CO2 molecules may be formed at various stages during the photodegradation of the aromatics. Because of the large number of intermediates involved, the amount of released CO2 molecules in each generation of intermediates is statistically averaged and remains approximately constant. Since the formation of CO2 molecules is a measure of mineralization, it follows that zero order kinetics is observed in this case. This type of behavior is clearly observed for monuron (Fig. 6). In the case of the structurally simpler molecule 4-CP, the number of reaction intermediates is probably lower, but the mineralization kinetics is still satisfactorily approximated by a zero order function (Fig. 8). 4. Conclusions The comparative kinetic study of the photocatalytic degradation and mineralization of simple organic compounds (oxalic and formic acid) and selected pollutants (4-chlorophenol and the herbicide monuron) of more complex molecular structure showed particularities as well as common features of heterogeneous photocatalysis. The proposed general kinetic concept based on the photostationary state approximation provides a consistent explanation for the different reaction systems, whereas the classical Langmuir–Hinshelwood formalism is unable to explain all kinetic features.

J. Kry´sa et al. / Applied Catalysis B: Environmental 64 (2006) 290–301

In the case of oxalic and formic acid, which were adsorbed on TiO2 surface only negligibly, the photocatalytic mineralization was induced by the direct transfer of photogenerated holes. It proceeded in only one step, without intermediates, following zero order kinetics. For monuron and 4-chlorophenol, due to their negligible adsorption, the degradation was mediated through photogenerated hydroxyl radicals. The photodegradation followed first order kinetics while mineralization proceeded according to zero order. The different kinetic orders were consistently explained on the basis of general mechanistic assumptions using the photostationary state approach. The numerical application of the photostationary model was successful in explaining the particular kinetic behavior of the OA/TiO2 system based on the assumption of competitive adsorption of oxygen and organic substrate on the semiconductor surface. Acknowledgments The authors thank for financial supports by grants KONTAKT 2002-11, KONTAKT 2005-22, to the European Commission, project ABWASZ (contract KA3-CT-199900016) and to the Ministry of Education, Youth and Sport of the Czech Republic (project 1M0577).

The heterogeneous photocatalytic system is based on light absorption by semiconductor particles, typically of TiO2. Pairs of separated positive holes (hVB+) in the valence band and electrons (eCB) in the conduction band are generated:  TiO2 þ hn ! hþ VB þ eCB

(12)

These charge carriers may recombine inside the semiconductor particles, or migrate to the surface where they can be directly transferred through the interface to adsorbed molecules, holes to an electron donor (D) and electrons to an electron acceptor (A): hþ VB

þ

e CB

! heat

(13)

þ hþ VB þ D ! D

(14)

 e CB þ A ! A

(15)

Under continuous irradiation, the photocatalyst absorbs a constant light intensity (Iabs). The rate of absorption is equal to the rate of photogeneration of separated charges in the process (12). Electrons and positive holes are consumed in the processes (13)–(15). The reaction rates of recombination (vr ), hole transfer (vh ) and electron transfer (ve ) can be formally treated as in the case of kinetics of chemical reactions: vr ¼ kr ½hþ ½e 

vh ¼ kh ½hþ ½Dads

(17)

ve ¼ ke ½e ½Aads

(18)

where kr, kh and ke are formally second order rate constants, [h+] and [e] represent the photostationary concentrations of positive holes and electrons and [D]ads and [A]ads are the surface concentrations of adsorbed electron donor and electron acceptor. Under constant irradiation, the rates of photogeneration and consumption of charge carriers will be dynamically equilibrated: d½hþ  ¼ Iabs  kr ½hþ ½e   kh ½hþ ½Dads ¼ 0 dt

(19)

d½e  ¼ Iabs  kr ½hþ ½e   ke ½e ½Aads ¼ 0 dt

(20)

The corresponding photostationary concentrations of charge carriers can be obtained, solving the equation set (19) and (20), as functions of absorbed light intensity (Iabs) and surface concentrations of electron donor ([D]ads) and electron acceptor ([A]ads): ke ½A þ ½h  ¼  2kr ads

sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ke2 ½A2ads ke ½Aads þ Iabs 4kr2 kr kh ½Dads

kh ½D þ ½e  ¼  2kr ads

sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi kh2 ½D2ads kh ½Dads þ Iabs 4kr2 kr ke ½Aads

þ

Appendix A

(16)

299



(21)

(22)

The surface concentrations of electron donor ([D]ads) and electron acceptor ([A]ads) can be estimated assuming a Langmuir adsorption isotherm for both components. At this point, two different model situations may be taken into account: (i) Independent adsorption of both D and A (two types of adsorption sites): KD ¼

½Dads ½Dads ¼ ½SD ½Dbulk ð½Dmax  ½D ads ads ÞðcD  ½Dads Þ

(23)

KA ¼

½Aads ½Aads ¼ max ½SA ½Abulk ð½Aads  ½Aads ÞðcA  ½Aads Þ

(24)

Here, KD and KA represent the adsorption constants of D and A, [D]bulk and [A]bulk are their concentrations in the aqueous phase, [S]D and [S]A symbolize the free adsorption sites for donor and acceptor, while cD and cA are the total concentrations of electron donor and electron acceptor. In the case of the photocatalytic degradation of OA, the mineralization proceeds in a single step; therefore, OA is the unique donor. Its adsorbed concentration

J. Kry´sa et al. / Applied Catalysis B: Environmental 64 (2006) 290–301

300

can be obtained by solving the equation set (23) and (24) as: ½OAads ¼

½O2 ads ¼

1 ½OAmax ads þ cOA þ KOA

1 KO2 ½O2 bulk 2KOA 1 þ KO2 ½O2 bulk ðKOA ðcsurf  cOA Þ  1  KO2 ½O2 bulk sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi þ ðKOA ðcsurf þ cOA Þ þ 1 2 c þKO2 ½O2 bulk Þ2  4KOA OA csurf Þ

2 sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi  1 2 ½OAmax ads þ cOA þ KOA  ½OAmax  ads cOA 2 (25)

The bulk concentration of oxygen, which acts as the electron acceptor, is constant during the photodegradation process due to its continuous equilibration with the gas phase. That is why a classical Langmuir isotherm formula can be used for the adsorbed concentration of oxygen from the equation set (23) and (24), introducing O2 instead of A: ½O2 ads ¼ ½O2 max ads

KO2 ½O2 bulk 1 þ KO2 ½O2 bulk

(26)

(ii) Competitive adsorption between OA and oxygen. The Eqs. (23) and (24) have to be correspondingly adapted: KOA ¼

½OAads ðcsurf  ½OAads  ½O2 ads ÞðcOA  ½OAads Þ

(27)

KO2 ¼

½O2 ads ðcsurf  ½OAads  ½O2 ads Þ½O2 bulk

(28)

where csurf represents the total concentration of the surface sites able to bind either oxalic acid or oxygen. Solving the equation set (27) and (28), the following expressions for the adsorbed concentrations of OA and oxygen result: ½OAads ¼

1 ðKOA ðcsurf þ cOA Þ þ 1 þ KO2 ½O2 bulk 2KOA sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi  ðKOA ðcsurf þ cOA Þ þ 1 2 c þ KO2 ½O2 bulk Þ2  4KOA OA csurf Þ (29)

Both sets of expressions for the adsorbed concentrations of OA and oxygen, i.e. (25) and (26), and (29) and (30), can now be introduced into the relations (21) and (22) to obtain formulas for photostationary concentrations of positive hole and electron. These were used to calculate starting values of the variables participating in the photocatalytic degradation of OA. The general reaction scheme shown in Fig. 11 is common to both adsorption models while the particular sets of differential equations differ according to whether the independent or competitive adsorption of oxalic acid and oxygen are assumed (as summarized in Tables 2 and 3). Numerical simulations were performed employing the software ModelMaker 4.0 (Cherwell Scientific Ltd.) to fit experimental dependences of bulk concentration of OA on irradiation time. Both fits are shown in Fig. 12 for a selected measurement; the corresponding parameters and simulation results are listed in Table 4. Comparing the fits for independent and competitive adsorption, the latter one seems to allow a better description. Analogous results were obtained for the majority of our measurements of the photocatalytic degradation of OA (5 mM) under various bulk concentrations of oxygen (from 0.2 to 1 mM) and using two different incident photon fluxes (1.0  104 and 2.4  104 Einstein m2 s1). Moreover, the optimized values of the rate constants of hole and electron transfer (kh and ke) remained almost constant for various bulk concentrations of oxygen if the competitive adsorption model was applied. Using the independent adsorption model, these values showed larger deviations, as shown in Fig. 13 for the ratio kh/ke; this may be taken as a further indication of the validity of the competitive adsorption model.

Table 4 Parameters and results of a simulation of the photocatalytic degradation of OA assuming either independent or competitive adsorption of electron donor (D) and electron acceptor (A) on photocatalyst surface Parameter

Unit

Independent adsorption Value

[A]bulk [D]ads,0 Iabs KA a kA,ads KD b kD,ads ke kh kr csurf a b

3

mol dm mol dm3 Einstein s1 mol1 dm3 mol1 dm3 s1 mol1 dm3 mol1 dm3 s1 mol1 dm3 s1 mol1 dm3 s1 mol1 dm3 s1 mol dm3

Optimized 4

7.63  10 5.32  103 4.4  105 2.55  10 1 1.0  10 3 6.4  10 3 1.0  10 3 3.16  10 3 1.96  10 7 3.0  10 10 1.73  104

The value of KA was estimated employing literature data [45]. The value of KD was determined as described in Section 2.

Competitive adsorption

No Yes No No No No No Yes Yes No No

Value

Optimized 4

7.63  10 5.17  103 4.4  105 2.55  10 1 1.0  103 6.4  103 1.0  103 4.53  10 4 2.80  10 7 3.0  1010 1.73  104

No Yes No No No No No Yes Yes No No

J. Kry´sa et al. / Applied Catalysis B: Environmental 64 (2006) 290–301

Fig. 13. Optimized rate constants of hole and electron transfer (kh and ke) in the course of photocatalytic degradation of oxalic acid for various bulk concentrations of oxygen.

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