Different Net Effect of TiO2 Sintering Temperature on the ...

Journal of Environmental Science and Health Part A, 41:955–966, 2006 C Taylor & Francis Group, LLC Copyright ! ISSN: 1093-4529 (Print); 1532-4117 (Online) DOI: 10.1080/10934520600689233

Different Net Effect of TiO2 Sintering Temperature on the Photocatalytic Removal Rates of 4-Chlorophenol, 4-Chlorobenzoic Acid and Dichloroacetic Acid in Water Rosario Enr´ıquez1,2 and Pierre Pichat1 1

Laboratoire “Photocatalyse, Catalyse et Environnement,” CNRS UMR “IFoS,” Ecole Centrale de Lyon, France 2 ´ Laboratorios de Ingenier´ıa Ambiental, Universidad del Mar, Puerto Angel, Oaxaca, México Our purpose was to show that the sintering temperature of TiO2 can have a different net effect (thought to arise from a decrease in surface area against a decrease in recombination rate of charge carriers) on the photocatalytic removal rate of various organic pollutants in water. For that, we have chosen four chlorinated pollutants, viz. 4-chlorophenol (4-CP), 2,5-dichlorophenol (2,5-DCP), 4-chlorobenzoic acid (4-CBA) and dichloroacetic acid (DCAA). Their photocatalytic removal was studied over four TiO2 samples (from Millennium Chemicals or affiliate) all obtained identically by TiOSO4 thermohydrolysis with subsequent calcination at various temperatures, TiO2 Degussa P25 was used for comparison. At equal TiO2 mass in the slurry photoreactor, the pseudo-first-order removal rate constant k increased with the calcination temperatures for the three aromatic pollutants, whereas it was the opposite for the aliphatic acid. Results obtained with P25 were consistent with the reasoning based on the combined effects of surface area and charge recombination rate. Similar k values for 4-CP and 2,5-DCP, irrespective of the TiO2 , further illustrate the importance of the molecular structure. For 4-CBA, the possibility of decarboxylation in addition to an attack on the ring, as well as a much higher extent of adsorption, can explain a higher k with respect to the chlorophenols. The implication of these results is that the hole attack mechanism

Received January 23, 2006. Address correspondence to Pierre Pichat, “Photocatalyse et Environnement,” CNRS, Ecole Centrale de Lyon (STMS), 69134, Ecully CEDEX, France; E-mail: pichat@ ec-lyon.fr

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Enr´ıquez and Pichat for carboxylic acids is much more sensitive to surface area variation than would be the (diffusible) OH radical mechanism for aromatics which could react in the near-surface solution-phase. Key Words: Dichloroacetic acid; 4-Chlorophenol; 2,5-Dichlorophenol; 4-Chlorobenzoic acid; Photocatalysis; TiO2 .

INTRODUCTION Photocatalysis using TiO2 for water purification is a well documented area of research[1] even if applications seem limited to niches[2] because of relatively low rates. In particular, this technology is appropriate for isolated locations[3] and can use solar light.[4–7] There have been numerous papers that have tried to study relationships between the properties of TiO2 and the photocatalytic reaction rates in view of improving the efficacy for purifying water. Such relationships are difficult to establish because several interrelated parameters influence the photocatalytic activity, inter alia, competing adsorption between water and pollutants, light absorption, recombination rate of photoproduced charges, back reactions, all of which depend on the structural and textural characteristics of TiO2 . Other variables to be considered are the physicochemical properties of the compounds to be oxidized, not only because of various extents of adsorption on TiO2 , but also because different photocatalytic degradation mechanisms can be involved, depending on the molecular structure. For instance, the importance of the adsorbed amount is not clear for pollutants with different chemical functionalities, and has led one to investigate whether photocatalytic reactions might occur within the solution layers in the vicinity of the TiO2 surface.[8] Most often, the ranking of the efficacy of TiO2 samples is assessed by use of only one compound. However, recent papers have shown that this ranking may vary according to the compound chosen.[9–11] Here, we have employed a series of TiO2 samples which derived from one another by changing the sintering temperature applied to the parent TiO2 . That offers the interest of allowing one to focus on the influence of one parameter, viz. the sintering temperature, whereas several uncontrolled factors affect the TiO2 properties when samples of different sources are tested. In our laboratory, these TiO2 specimens have been compared for several photocatalytic reactions both in the gas and aqueous phases.[12,13] This type of investigation has recently included the photocatalytic removal of phenol, anisole and pyridine in water.[14] Calcination decreases the surface area by both the fusion of elementary crystallites and the decline in the volume of pores. The density of surface irregularities, such as kinks and edges, is also decreased, which results in a lower recombination rate of photoproduced charges as has been illustrated

Net Effect of TiO2 Sintering Temperature on Photocatalytic Activity

by time-resolved microwave conductivity for some of the TiO2 samples used in our study.[15] Calcination can thereby have opposing effects on the rate of photocatalytic reactions. Consequently, the first element of our strategy was to use, as aforementioned, a series of closely related TiO2 specimens which we have also compared with TiO2 Degussa P25, the usual literature reference. A second element of our strategy was to choose probe compounds whose similarities and dissimilarities are susceptible to provide information about the respective importance of surface area and recombination rate of photogenerated charges. These compounds were: (i) dichloroacetic acid (DCAA) which is known to be predominantly decarboxylated since the two Cl atoms render the H atom borne by the same C atom not easily removable, (ii) 4-chlorophenol (4-CP) because it is poorly adsorbed on TiO2 and the hydroxylic H atom can be abstracted, (iii) 2,5-dichlorophenol (2,5-DCP) because its close structural similarity with 4-CP should permit one to check that alike compounds are degraded at about the same rates on the various TiO2 , and (iv) 4-chlorobenzoic acid (4-CBA) because of its common chemical functionalities with both DCAA and 4-CP, and because it adsorbs on TiO2 in greater amount than 4-CP and 2,5-DCP. In addition, these compounds have been extensively studied, hence there is information in the literature about how the photocatalytic degradation proceeds. Finally, we selected chlorinated molecules as chlorinated organic pollutants are of concern in water treatment. The photocatalytic removal of each of these probe molecules over each of the chosen TiO2 sample has been investigated. The results and their implications are presented hereafter.

MATERIALS AND METHODS Materials Four TiO2 prepared by thermohydrolysis of TiOSO4 were utilized in this study. Three were produced by Millennium Chemicals: PC500, PC50 and PC10, the latter two being derived from the first one by sintering at increasing temperature (not revealed by the company) in that order. The fourth TiO2 was a Rhodia sample. It was one of the TiO2 originally synthesized at the laboratory scale as precursors of the TiO2 PCs now manufactured by Millennium Chemicals; therefore, it perfectly fits in the PCs series. TiO2 Degussa P25 was used for comparison. TiO2 characteristics are given in Table 1. 4-CBA, 4-CP, 2,5-DCP and DCAA were obtained from Aldrich. All these chemicals having the highest available purity were used as received. A Milli-Q plus system (Millipore) provided the water used in all cases.

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Enr´ıquez and Pichat Table 1: Powder TiO2 characteristics. Specific Area BET (m2 g−1 )

Porous Average radius (nm)

Crystallite size (nm)

Crystalline structure

PC 10

10

24.1

65–75

PC 50

54

20.1

20–30

Rhodia

150

8.6

n.d.

PC 500

317

6.1

5–10

55

Non-porous

30

Anatase (100%) Anatase (100%) Anatase (100%) Anatase (100%) Anatase/Rutile (80%/20%)

Sample

P25

Photoreactor and Light Source Photocatalytic degradations were performed in a cylindrical batch photoreactor (ca. 80 mL) with a bottom optical window (ca. 11 cm2 ) made of Pyrex. This photoreactor was open to air. It was placed above a Phillips HPK-125 W high pressure mercury lamp whose radiation was filtered trough a circulating water cuvette (thickness = 2.2 cm) equipped with a 340 nm cut-off filter (Corning 0–52). The radiant power, received by the reactor optical window and measured with a UDT 21 A powermeter, was found to be ca 33.3 mW cm−2 . The corresponding number of photons per second potentially absorbable by ◦ 0.02) × 1017 . TiO2 was estimated to be (1.05 +

Procedures 20 mL of a stock solution with the desired concentration of 4-CBA, 4-CP, 2,5-DCP or DCAA was poured into the reactor containing 2.2 g L−1 of TiO2 powder. The suspension was magnetically stirred and maintained in the dark for 1 h prior to irradiation. No buffers were used; the TiO2 content, the pollutant concentration and the equilibrium with atmospheric CO2 fixed the pH of the suspension. Some physical properties of the pollutants and the initial concentration are listed in Table 2.

Analyses Samples were filtered through 0.45 µm Millipore filters before high performance liquid chromatography (HPLC) analyses. For the aromatic compounds analyses, the instrumentation consisted of a LDC Constametric 3200 isocratic pump and a LDC Spectromonitor UV detector fixed at 226, 228, 240 nm for 4-CP, 2,5-DCP and 4-CBA, respectively. An ODS-2 Waters column (25 cm × 4.6 mm) was used and the mobile phase (flow rate: 1 mL min−1 ) was a mixture

Net Effect of TiO2 Sintering Temperature on Photocatalytic Activity Table 2: Physical properties and initial concentration of the pollutants studied.

Pollutant

Molar weight (g)

4-CBA 4-CP 2,5-DCP DCAA

156.57 128.56 163.00 128.94

Initial concentration Solubility (mg/L)

Log Kow

pKa

(ppm)

(µmol L−1 )

2.65 2.39 3.06 0.92

3.98 9.41 7.51 1.26

22.8 21.3 20.6 50.0

145.6 155.1 126.4 165.7

72 24 × 10 3 614 1 × 106

of methanol and water (60/40 vol% : 4-CBA; 40/60 vol% : 4-CP; 55/35 vol%: 2,5-DCP); the pH of the eluent was adjusted to ca. pH 3 by H3 PO4 except for 2,5-DCP analyses. For DCAA analysis, the HPLC-UV apparatus consisted of a solvent delivery system (Waters 600 controller) fitted with an Aminex HPX column and an UV detector (Waters, model 486) fixed at 210 nm. The mobile phase was 5 mmol L−1 of H2 SO4 in water, and the flow rate was 0.2 mL min−1 .

RESULTS AND DISCUSSION Using the standard conditions described in the experimental section, we have studied the kinetics of the photocatalytic disappearance of the four chlorinated compounds. The activity of the different TiO2 samples was determined from the pseudo-first–order rate constant, k, as given by

ln

Co = kt, C

where Co was the initial concentration after the dark adsorption step and C was the concentration measured at irradiation time t. The inset in Figure 1, presented as an example, shows that an apparent first-order kinetics did allow one to compare the photocatalytic removals. k values based on the first 20–30 min of irradiation have been used to minimize effects of competition between the removal of intermediate products and that of the parent compound. In the following paragraphs, we successively compare the values of k for the four pollutants over the various TiO2 specimens taken two by two in the order of increasing calcination temperature and thereby of decreasing surface area. It must be emphasized that these k values correspond to an equal TiO2 mass in the photocatalytic reactor.

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Figure 1: Photocatalytic degradation of 4-chlorophenol over the TiO2 samples indicated. Inset: fitting of pseudo-first-order kinetic constant.

Comparison PC500/Rhodia Pseudo-first-order rate constants for each of the chlorinated compound removal with PC500 and Rhodia are shown in Table 3. No significant difference for the photocatalytic activity between these TiO2 samples was observed for the removal of the chlorinated phenols despite a decrease in surface area by about 55% for Rhodia relative to PC500. It was also the case for the photocatalytic removal of phenol when comparing PC500 to a Millennium sample (PC105) having a surface area of ca. 90 m2 g−1 .[14] By contrast, a decrease in k by a factor of more than 2 for DCAA removal over Rhodia compared with PC500 was observed. This latter observation indicates that, for removal of DCAA, the decrease in surface area and especially the collapsing of the micropores (into which DCAA can penetrate), caused by the calcination used to obtain Rhodia from PC500, plays a greater role Table 3: Comparison of first-order rate constants (in min−1 ) for the removal of the test pollutants. Case of PC500 (317 m2 g−1 ) and Rhodia (150 m2 g−1 ). TiO2 samples PC 500 Rhodia kRhodi a/kPC500

Cl2 HC-COO− 0.2 0.087 0.43

ClC6 H4 COO− 0.034 0.05 1.5

Cl2 C5 H3 OH 0.013 0.014 1.1

ClC5 H4 ·OH 0.014 0.011 0.8


than the expected decrease in the density of defects in TiO2 .[15] From that it can be inferred that the principal initial degradation pathway of DCAA is decarboxylation produced by surface-trapped holes and also that the PC500 micropores appear to enhance the removal of this small-size compound. In the case of 4-CP and 2,5-DCP, the detrimental effect of the decrease in surface area would be compensated by the decrease in the density of defects. To explain the difference with DCAA, that is, the lesser impact of the surface area, we hypothesize that chlorinated phenols, which are poorly adsorbed, can react not only on the surface but also in the multilayers of water in the neighboring of the TiO2 surface as it has been derived by Cunningham and co-workers[8] from experiments, and suggested by Turchi and Ollis[16] from an analysis of the kinetic possibilities. In the same line of reasoning, it has been inferred that ◦ OH radicals can migrate from the surface into the aqueous solution.[17] As 4-CBA can be initially degraded by an attack on its nucleus and by decarboxylation, an intermediate change between the changes observed for chlorophenols and DCAA was expected when comparing Rhodia to PC 500. In fact, an increase by a factor of about 1.5 was found (Table 3). This result can be explained by the higher extent of 4-CBA adsorption, measured in the dark at equal mass of TiO2 , on the Rhodia sample in spite of the lower surface area; this increase in adsorption might have come from a lower amount of constitutional water in TiO2 , which would facilitate the access of 4-CBA to the surface.

Comparison Rhodia/PC50 Table 4 shows a decrease in k for DCAA removal over PC50 compared with Rhodia, whereas for chlorinated phenols k is increased by a factor of about 3. For 4-CBA, k is only slightly increased. In other words, changes in TiO2 related to a higher calcination temperature have a negative effect on DCAA removal and a positive effect on the removal of both chlorinated phenols, whereas negative and positive effects seem to compensate almost exactly in the case of 4-CBA removal. As in the case of the comparison between PC500 and Rhodia, these observations can be taken as an indication that the initial main pathway of DCAA removal is decarboxylation induced by photoproduced holes—which Table 4: Comparison of first-order rate constants (in min−1 ) for the removal of the test pollutants. Case of Rhodia (150 m2 g−1 ) and PC50 (54 m2 g−1 ). TiO2 samples Rhodia PC 50 kPC50 /kRhodia

Cl2 HC-COO− 0.087 0.046 0.53

ClC6 H4 COO− 0.05 0.06 1.2

Cl2 C5 H6 OH 0.014 0.044 3.1

ClC5 H4 OH 0.011 0.028 2.5

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requires a direct contact of DCAA with TiO2 . The other initial pathway, that is the removal of the H atom of the CHCl2 group by a ◦ OH radical, would be less important; note that this atom is indeed poorly labile because of the electron-attracting effect of the Cl atoms. With these assumptions, a negative effect on k of the decrease in surface area greater than the positive effect of the decrease in the recombination rate of the photoproduced charges resulting from a lower density of structural defects in PC50 can be rationalized. These observations are also consistent with an attack of 4-CP and 2,5DCP by ◦ OH radicals in the water multilayers close to the TiO2 surface, so that the number of these radicals would have a greater importance than the surface area per se. As 4-CBA can be initially degraded by both hole-induced decarboxylation and addition of a hydroxyl radical onto the aromatic ring, an intermediate behavior between DCAA and chlorinated phenols can be expected in accord with what was observed.

Comparison PC 50/PC10 Increasing the calcination temperature to obtain PC10 resulted in a small rise in photocatalytic activity for the aromatic pollutants (Table 5). These higher activities would confirm that for these compounds the most important parameter would be the decrease in the density of structural defects, whereas the decrease in surface area (Table 1) would have a lesser influence. This is understandable for 4-CP and 2,5-DCP which are poorly adsorbed and, as was mentioned above, are accordingly supposed to be capable of reacting in the water multilayers close to the TiO2 surface. An increase in k for 4-CBA comprised between those observed for 4-CP and 2,5-DCP is a priori surprising, since the surface area should weight more in the case of 4-CBA. However, dark adsorption measurements showed that the percentage of 4-CBA adsorbed was about the same on PC10 and on PC50, possibly because of a decrease in the amount of TiO2 constitutional water allowing an easier access of 4-CBA to the TiO2 surface. Therefore the possibility for 4-CBA to react in the adsorbed phase would not be decreased over PC10 with respect to PC50, and the reduction in the density of defects would exert the same positive effect as in the case of the chlorinated phenols. Table 5: Comparison of first-order rate constants (in min−1 ) for the removal of the test pollutants. Case of PC50 (54 m2 g−1 ) and PC10 (10 m2 g−1 ). TiO2 samples PC 50 PC 10 kPC10 /kPC50

Cl2 HC-COO− 0.046 0.031 0.7

ClC6 H4 COO− 0.06 0.08 1.3

Cl2 C5 H3 OH 0.044 0.05 1.1

ClC5 H4 OH 0.028 0.045 1.6

Net Effect of TiO2 Sintering Temperature on Photocatalytic Activity Table 6: Comparison of the first-order rate constants (in min−1 ) for the removal of the test pollutants. Cases of Degussa P25 (55 m2 g−1 ) and PC10 (10 m2 g−1 ). TiO2 samples

Cl2 HC-COO−

PC 10 Degussa P25 kPC10 /kP 25

0.031 0.1 0.3

ClC6 H4 COO− 0.08 0.09 0.9

Cl2 C5 H3 OH 0.05 0.06 0.8

ClC5 H4 OH 0.045 0.053 0.8

By contrast, the increase in the TiO2 calcination temperature to obtain PC10 instead of PC50 decreased k for DCAA (Table 5) as expected if DCAA is supposed to react mainly on the surface. Indeed, the DCAA dark adsorption percentage declined from 13 to 2%, approximately; different adsorptive behaviors for DCAA and 4-CBA are understandable considering the miscibility of DCAA in water (1000 g/L), whereas 4-CBA is comparatively very poorly soluble (0.07 g/L) and then has a greater tendency to partition to the TiO2 surface.

Comparison of TiO2 Millennium PCs and TiO2 Rhodia with TiO2 Degussa P25 The greater importance, in the PCs series, of the density of defects with regard to the TiO2 surface area for the aromatic pollutants which we supposed to be able to react in the vicinity of TiO2 and not uniquely on the surface, is corroborated by the k values calculated for P25 (Table 6). This latter sample is directly elaborated at high temperature in a flame reactor; this process is prone to reduce the density of structural defects as is the high calcination temperature employed to synthesize PC10. The higher surface area of P25 compared with PC10 does not matter for these aromatics, again because they presumably do not react principally at the surface. By contrast, for DCAA removal, P25 is more active than PC10 in line with its higher surface area (55 against 10 m2 g−1 ), and more active than PC50 (see Tables 5 and 6)—which has an equal surface area—likely because of a lower density of defects. So, the reasoning used in the PCs series along with the Rhodia sample allows us to qualitatively account for the ranking of the photocatalytic activity of P25 relative to those of the PCs samples for the removal of the pollutants investigated.

CONCLUSIONS On the whole, a difference in k as a function of the degree of calcination appears between the chlorinated aromatics and the aliphatic chlorinated acid studied. For the aromatic compounds, k increased with the TiO2 calcination

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temperature. A contrasting tendency was observed for DCAA in agreement with results regarding the surface area dependency of the removal rate of acetic acid.[9,10] For 2,5-DCP and 4-CP, similar k over the first 30 min of irradiation are in line with the similar molecular structure of these compounds. An increase in k with increasing sintering temperature according to a very similar pattern has also been observed for phenol reacted over the Millennium samples.[14] This observation shows that the TiO2 samples are “viewed” analogously by these phenols, even though the Cl atoms change the electron density on the aromatic ring and the solubility in water. For 4-CBA, the possibility of decarboxylation in addition to an attack on the ring, as well as a much higher extent of adsorption, can explain the higher k with respect to the two other aromatics studied here, regardless of the TiO2 specimen including P25. The mercury emission lines of the Phillips HPK-125 W lamp in the visible spectral range were not filtered as they cannot create electron-hole pairs in TiO2 . However, a removal of 4-CP and 2,4-DCP under visible light irradiation in TiO2 aqueous suspensions has been reported very recently and has been attributed to light absorption by the phenol derivative forming a complex with TiO2 and subsequent transfer of electrons to TiO2 .[18] Accordingly, this phenomenon was shown to be enhanced by an increase in TiO2 surface area. Since the effect of surface area we observed for 2,5-DCP and 4-CP was opposite, it can be concluded that visible light-induced removal of these chlorinated phenols was not significant under the conditions of our study [in particular, the irradiation (a 300 W Xe arc was employed in the other study [18] ), and consequently cannot interfere in the reasoning presented in the preceding section. Both from the comparison of the activities over the powder TiO2 samples and from a previous paper[12] where we have studied the effect of the various decreases in TiO2 accessibility resulting from the immobilization using a silica binder, it can be deduced that chlorophenols can react at some distance from the surface in the well-organized layers of water. This possibility was proposed by Turchi and Ollis[16] on the basis of kinetics considerations and supported by a series of experiments reported and discussed by Cunningham and co-workers;[8] also, from the interpretation of photoelectrochemical measurements, it was deduced that surface formed ◦ OH radicals can escape into solution.[17] By contrast, DCAA can react only when in close contact with TiO2 , as expected if one assumes that hole-induced decarboxylation is the only initial reaction pathway. These conclusions are further supported by comparing the effect of TiO2 fixation using SiO2 on the 18 O2 -Ti16 O2 isotopic exchange and the removal rates in water.[12] Therefore, it is understandable that a low recombination rate of the photoproduced charges–which is favored, as shown


by TRMC results[15] by high calcination temperature of TiO2 –weights more for the removal of the chlorophenols. By contrast, the surface area is a more important factor for the removal of DCAA. A behavior of 4-CBA intermediate between that of chlorophenols and that of DCAA is expected, since this molecule contains both a carboxylic group and a benzene ring. The ranking of P25 relative to three TiO2 aerogels has also been reported[11] to be different for the removal of 4-CBA than for those of 4-CP and 4-nitrophenol: P25 was the most active photocatalyst for these substituted phenols. The present interpretation is consistent with these results. The aerogels (surface area from 73 to 96 m2 g−1 ) were calcined at only 673 K and accordingly should contain more crystalline irregularities than the flame-reactor prepared P25. We are aware that our interpretation may be completed and/or amended by taking into account the potential effects of other TiO2 properties also affected by the degree of calcination, such as the surface charge and the surface coverage by hydroxyl groups. More results, possibly derived from the use of other adequately selected probe molecules or obtained by other investigation means, are needed to arrive at a more comprehensive understanding of the organic pollutant-depending net influence of all these variables. Clearly, these results show that one test pollutant does not suffice to quantitatively compare the photocatalytic activities of TiO2 samples. On the other hand, from a practical perspective, our results show that there is not a universal photocatalyst for water treatment. The photocatalytic activity resulting from a compromise between notably the specific surface area and the recombination rate of photoproduced charges, it may be relevant to use successive photocatalytic reactors, each containing a different grade of TiO2 , to treat water in a continuous mode.

ACKNOWLEDGMENTS R. E. is grateful to the CONACYT (Mexico) for her scholarship. The gift of titania samples by Millennium Chemicals is gratefully acknowledged. The authors warmly thank Ms. M.-N. Mozzanega (CNRS) for her helpful laboratory assistance.

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4. Robert, D.; Malato, S. Solar photocatalysis: a clean process for water detoxification. Sci. Total Environ. 2002, 291, 85–97. 5. Malato, S.; Blanco, J.; Vidal, A.; Alarcon, D.; Maldonado, M.I.; Caceres, J.; Gernjak, W. Applied studies in solar photocatalytic detoxification: an overview. Solar Energy 2003, 75, 329–336. 6. Pichat, P.; Vannier, S.; Dussaud, J.; Rubis, J.-P. Field solar photocatalytic purification of pesticides-containing rinse waters from tractor cisterns used for grapevine treatment. Solar Energy 2004, 77, 533–542. 7. Bousselmi, L.; Geissen, S.U.; Schroeder, H. Textile wastewater treatment and reuse by solar catalysis: results from a pilot plant in Tunisia. Wat. Sci. Technol. 2004, 49, 331–337. 8. Cunningham, J.; Al Sayyed, G.; Srijaranai, S. Adsorption of model pollutants onto TiO2 particles in relation to photoremediation of contaminated water. In Aquatic and surface photochemistry; Helz, G.R.; Zepp, R.G.; Crosby, D.G., Eds.; Lewis Publishers: Boca Raton, FL, 1994; 317–348. 9. Kominami, H.; Murakami, S.; Kato, J.-I.; Kera, Y.; Ohtani, B. Correlation between some physical properties of titanium dioxide particles and their photocatalytic activity for some probe reactions in aqueous systems. J. Phys. Chem. B 2002, 106, 10501–10507. 10. Heinz, O.; Robert, D.; Weber, J.V. Comparison of the degradation of benzamide and acetic acid on different TiO2 . J. Photochem. Photobiol. A: Chem. 2000, 135, 77–80. 11. Malinowska, B.; Walendziewski, J.; Robert, D.; Weber, J.V.; Stolarski, M. The study of photocatalytic activities of titania and titania-silica aerogels. J. Appl. Catal. B 2003, 46, 441–451. 12. Enriquez, R.; Beaugiraud, B.; Pichat, P. Mechanistic implications of the effect of TiO2 accessibility in TiO2 -SiO2 coatings upon chlorinated organics photocatalytic removal in water. Water Sci. Technol. 2004, 49, 147–152. 13. Enr´ıquez, R. Mécanismes photocatalytiques et relations activité-propriétés de TiO2 en phase gazeuse et aqueuse. Ph. D. Thesis, 2002, France, Ecole Centrale de Lyon. 14. Agrios, A.G.; Pichat, P. Recombination rate of photogenerated charges versus surface area: opposing effects of TiO2 sintering temperatura on photocatalytic removal of phenol, anisole and pyridine in water. J. Photochem. Photobiol. A: Chem. In press. 15. Mietton-Ceulemans, E. Corrélations entre l’activité photocatalytique pour la dégradation du méthanol dans l’air, la mobilité des charges photogénérées et les propriétés de surface de TiO2 . Ph. D.Thesis, 2001, France. Université Lyon 1. 16. Turchi, C.S.; Ollis, D.F. Photocatalytic degradation of organic water contaminants: Mechanisms involving hydroxyl radical attack. J. Catal. 1990, 122, 178–192. 17. Peterson, M.W.; Turner, J.A.; Nozik, A.J. Mechanistic studies of the photocatalytic behavior of TiO2 . Particles in a photoelectrochemical slurry cell and the relevance to photodetoxification reactions. J. Phys. Chem. 1991, 95, 221–225. 18. Kim, S.; Choi, W. Visible-light-induced photocatalytic degradation of 4-chlorophenol and phenolic compounds in aqueous suspension of pure titania: demonstrating the existence of a surface-complex-mediated path. J. Phys. Chem. B 2005, 109, 5143–5149.