Ind. Eng. Chem. Res. 2005, 44, 3419-3425
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Photocatalytic Enhanced Oxidation of Fluorene in Water with Ozone. Comparison with Other Chemical Oxidation Methods Fernando J. Beltra´ n,* Francisco J. Rivas, Olga Gimeno, and Marı´a Carbajo Departamento de Ingenierı´a Quı´mica y Energe´ tica, Universidad de Extremadura. 06071 Badajoz. Spain
Fluorene, a polynuclear aromatic hydrocarbon, has been chosen as a model compound to study the synergism between ozonation and photocatalytic oxidation in water. Thus, for this purpose, different oxidation methods have been tested: single ozonation (O3), single adsorption (TiO2), ozone photolysis (UVA/O3), TiO2 photocatalysis (TiO2/UVA), TiO2 catalytic ozonation (TiO2/O3), and TiO2 photocatalytic ozonation (TiO2/UVA/O3). These processes have been carried out at two pH levels (2 and 5). In the ozone-involving cases, a positive effect of pH was observed. At a fixed pH, the reactivity order for fluorene oxidation was TiO2/UVA/O3 > UVA/O3 > O3 ≈ TiO2/O3 ≈ TiO2/UVA > TiO2. Since, after a few seconds from the start of oxidation, ozone accumulated in water, reactions of ozone developed in the slow kinetic regime, especially at pH 2. Maleic, formic, and oxalic acids were identified as intermediates or end products of oxidation. The stability and activity of the TiO2 catalyst was also studied for the TiO2/UVA/O3 process. Introduction After a period in which ozone has been studied and used as an alternative oxidant to chlorine for water treatment1 or in combination with hydrogen peroxide and/or radiation (mainly 254 nm UVB radiation),2,3 this oxidizing agent is now being investigated in combination with catalysts (catalytic ozonation) to improve the removal rates of recalcitrant compounds (pollutants) in water.3-5 Additionally, another ozone process which is awakening the interest of different researchers combines the use of catalysts and light (photocatalysis)3 in a system called photocatalytic ozonation. By using the combination between ozone, radiation, and semiconductors, three possible ways of reaction; photolysis, ozonation, and advanced oxidation (including ozone photolysis and photocatalysis), can develop to remove pollutants from water. These three processes separately have been the subject of different works. Among them, one can highlight ozone photolysis (UVB) and photocatalysis with semiconductors using, in this case, visible and A type ultraviolet radiations, VUVA, and, in most cases, TiO2 as semiconductor.6,7 Despite the potential oxidizing effect of photocatalytic ozonation, not a many works, however, have been so far published. Examples on this matter are the works of Sa´nchez et al. (1998),8 Wada et al. (2002),9 Kopf et al. (2000),10 and Wang et al. (2002)11 dealing with the O3/VUV/TiO2 of aniline, cyanide wastewater, chloroacetic acid, and formic acid, respectively. Because of their mutagenic and carcinogenic character, polynuclear aromatic hydrocarbons (PAHs) are wellknown priority pollutants of water.12 Different ways, involving industrial and municipal activities or even natural processes (i.e., runoff due to rainwater), are the sources of PAH’s pollution of surface waters. In fact, PAHs have been found in many water environments13 and maximum contaminant levels for these compounds have been established (i.e., the EU Commission has a MCL of 0.1 µg L-1).14 The presence of unsaturated moieties in PAH molecules (aromatic rings), however, makes ozonation an appropriate technology to remove these * To whom correspondence should be addressed. E-mail:
[email protected]. Fax: 34-924-289385. Tel.: 34-924-289387.
compounds from water.15 Nevertheless, as far as their reactivity with ozone is concerned, PAHs can be catalogued either as slow or fast reacting compounds. Thus, PAHs such as phenanthrene react very fast with ozone (through the so-called direct ozone reactions) and no advanced oxidation or ozone combination with other reagents (catalyst, hydrogen peroxide, etc.) or light is required.16 Other PAHs, such as fluorene, react directly with ozone at much slower rates and ozonation develops in the slow kinetic regime. Then, an advanced oxidation process might be necessary to improve its removal rate.16 As a consequence, fluorene has been chosen in this work as a model compound to study its advanced oxidation with the combination of ozone and ultraviolet (A type) radiation and a semiconductor TiO2 in a process called photocatalytic ozonation (TiO2/UVA/O3). Fluorene is treated here at two pH values (5 and 2) with different oxidation processes, the results are compared and the stability and activity of the catalyst is observed. In a forthcoming part, the mechanism and kinetics of the fluorene TiO2/UVA/O3 oxidation will be presented. Experimental Section Fluorene was obtained from Aldrich and used as received. Ozone was produced from pure oxygen in a laboratory 301.7 Sander ozonator able to supply a maximum of 10 g/h of ozone. A commercial TiO2 Degussa P25 (70% anatase and 30% rutile) was used as the catalyst with an average particle size of 30 nm and BET surface area of 50 m2 g-1, according to the manufacturer. Experiments were carried out in a 1 L capacity tubular borosilicate glass photoreactor (450 mm long, 80 mm diameter). The reactor walls were covered by aluminum foil and an insulating material to avoid release of radiation and heat to the ambient. Degradation of fluorene was carried out at atmospheric pressure in 0.9 L of unbuffered (pH 5) or buffered (pH 2, phosphoric acid) aqueous solutions containing approximately 2 mg L-1 (about 10-5 M) of the organic compound. For the O3 based processes, an ozone-oxygen mixture was continuously bubbled into the solution throughout a diffuser placed at the bottom of the
10.1021/ie048800w CCC: $30.25 © 2005 American Chemical Society Published on Web 04/19/2005
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Figure 1. Scheme of the photoreactor used.
reactor. The gas flow rate was kept constant at 50 L h-1 having an average ozone inlet concentration of 5 mg L-1. In photolytic experiments, the aqueous solution was irradiated with a high-pressure mercury lamp (Heraeus, TQ 718700 W) immersed in a glass well placed at the middle of the reactor. The lamp bandwidth was in the range 238-579 nm with three main wavelengths emitting at 254, 313, and 366 nm. However, UVB radiation was cut off because of the presence of the glass well. This was provided with a jacket where water was circulated to keep the temperature constant. Nonetheless, this was only partially achieved since temperature rose about 3 °C during the reaction time (20 min). Incident radiation at 313 and 366 nm was 0.189 and 0.331 einstein h-1 and the corresponding radiation powers were 20.1 and 30.0 W, respectively. Figure 1 shows a scheme of the photoreactor. When TiO2 was added, the solid was maintained in suspension by magnetic stirring with a concentration of 0.5 or 1.5 g L-1. Prior to the analysis, the solid was removed from samples by a 5415D Eppendorf Centrifuge and further filtration through a Millex-HA filter (Millipore, 0.45 µm). Fluorene was analyzed by high-performance liquid chromatography (Hewlett-Packard Series 1100) with a Kromasil C18 column and a Hewlett-Packard 1046 programmable fluorescence detector (excitation and emission wavelengths were 201 and 311 nm, respectively). A 50:50 v:v aqueous solution of acetonitrile and water was used as the eluting solvent at 1 mL min-1 flow rate. Intermediates were also analyzed through a similar procedure but in a different column (Supelcogel C-610H), UV detector (at 210 nm) and water-phosphoric acid (0.1%) as the mobile phase. The pH of the reaction media was followed by means of a Radiometer Copenhagen pH-meter (HPM82). Ozone in the gas phase was analyzed by means of a GM109 Anseros ozomat analyzer based on the absorption of ozone at 254 nm. The concentration of dissolved ozone was determined by the indigo method.17 Results and Discussion Experiments were carried out at two pH values (2 and 5). In a previous paper,15 fluorene ozonation was observed to mainly develop through hydroxyl free radical re-
Figure 2. (a) Evolution of the dimensionless concentration of fluorene with time corresponding to different oxidation experiments at pH 5: (9) O3; (O) TiO2/UVA; (b) TiO2/UVA/O3; (0) TiO2/ O3; (2) UVA/O3. Other conditions: 20 °C, initial fluorene concentration 1.8 mg L-1 (average value), gas flow rate 50 L h-1, gas ozone concentration 5 mg L-1, TiO2 concentration 1.5 g L-1, and radiation density flux 0.14 einstein h-1. In TiO2 processes, after a first period to reach the adsorption equilibrium of fluorene on the catalyst surface, light, ozone, or both agents were applied. (b) Evolution of the dimensionless concentration of fluorene with time corresponding to different oxidation experiments at pH 2: (4) O3; (O) TiO2/UVA/O3; (b) TiO2/O3; (0) TiO2/UVA; (2) UVA/O3. Other conditions are as above.
action for pH values higher than 7. Then, pH 5 and, in particular, pH 2 were chosen to avoid or, at least, reduce the competition of hydroxyl radicals coming from ozone decomposition. Also, pH 2 was chosen to slow the rapidity of the TiO2/UVA/O3 ozonation that led to nearly total removal of fluorene in less than 1.5 min (see Figure 2a). First, experiments of oxygen oxidation, UVA radiation, and adsorption were carried out to check any possible contribution of photolysis and adsorption processes. Regarding single oxygen oxidation and UVA photolysis, no degradation of fluorene was observed at all regardless of the pH value, an expected behavior since this compound is nonvolatile and does not absorb this type of radiation. On the contrary, adsorption of fluorene on the TiO2 surface presented two stages: a first fast stage of less than 1 min where about 65% of fluorene fed to the reactor was removed from water and a second slower one until saturation was reached. Also, fluorene adsorption at pH 2 was slightly higher than at pH 5 (65% against 58%, respectively, in 1 min). The comparative results from different ozonation systems and TiO2/O2 photocatalysis, expressed as the variation of the remaining concentration of fluorene with time, are shown in Figure 2, parts a and b, for pH 5 and 2, respectively. Notice that pH of water remained practically constant in ozonations at pH 2 (that were buffered) while diminished between 2 and 3 units in experiments at pH 5, due to the unbuffering system and carboxylic
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acid formation. Regarding single ozonation, about 85% of fluorene was removed in about 2 min. In this case, given the low direct reactivity between ozone and fluorene,15 hydroxyl radicals are also expected to participate in the oxidation process, especially at pH 5. In Figures 2, it can also be seen that combined oxidation processes involving two agents (ozone and TiO2 or light or the two latter) do not improve or slightly improve the results observed when single ozonation was used at pH 5. It should be noted, however, that in experiments involving TiO2, fluorene was first allowed to adsorb on the TiO2 surface for 20 min (to reach nearly equilibrium conditions), thereafter the other agent (ozone, light or both) was used. In this way, once ozone or light has been applied, disappearance of fluorene was exclusively due to oxidation effects. In combined processes, after 2 min reaction, photocatalysis, ozone photolysis and catalytic ozonation resulted in 77, 85, and 79% fluorene removal, respectively, at pH 5. However, all these processes led to similar percentage removal of fluorene after 4 min reaction. Finally, photocatalytic ozonation was the best process as far as fluorene removal rate was concerned. Thus, more than 95% removal of fluorene was achieved in 1 min of reaction with this oxidation technology at pH 5. These results are likely due to the improvement of hydroxyl radical concentration from reactions of positive holes and electrons formed in the absorption of UVA light on the semiconductor surface.18 Development of these reactions, yielding the improvement of oxidation, was likely due to the presence of ozone that avoids the recombination of carriers: positive holes and excited electrons, by trapping the second ones and increasing, in this way, the concentration of hydroxyl radicals. Possible reactions in this mechanism are as follows. •Light absorption that promotes one electron from the valence band to the conduction band of the semiconductor. This results, in addition, in a positive oxidant hole in the valence band: hv < 387 nm
TiO298e- + h+
(1)
•Hydroxyl radical generation from the oxidizing action of the positive hole:
Ti(IV) + H2O a Ti(IV)-H2O
(2)
Ti(IV)-H2O + h + f Ti(IV)-HO• + H+
(3)
•Hydroxyl radical generation through the oxidizing action of ozone that prevents or reduces carrier recombination:
Ti(IV) + e- f Ti(III)
(4) -
Ti(III) + O3 f Ti(IV)-O3 -
+
Ti(IV)-O3 + H f Ti(IV)-HO + O2
(5) (6)
•Carrier recombination:
e- + h+ f heat
(7)
Once hydroxyl radicals are formed, they can react with fluorene in a process that can develop on the surface of the semiconductor or in solution after adsorption and/ or desorption steps.
Figure 3. Time evolution of concentrations of fluorene and intermediates from the ozonation of fluorene at pH 2. (2) fluorene; (4) maleic acid; (O) oxalic acid. Other conditions: 20 °C, initial fluorene concentration 1.9 mg L-1, gas flow rate 50 L h-1, and gas ozone concentration 5 mg L-1.
Effect of pH. In Figure 2, parts a and b, it can also be deduced that ozonation rates of fluorene at pH 2 are lower than at pH 5. Ozone photocatalysis leads to the faster removal rate of fluorene, regardless of pH. Total removal of fluorene was only observed with photocatalytic ozonation (TiO2/UVA/O3), ozone photolysis (O3/UVA) and photocatalysis (TiO2/UVA). However, the time required was much higher at pH 2 than at pH 5, especially when two agents were applied. The poorer results at pH 2 when ozone is present are undoubtedly due to the lower generation of hydroxyl radicals coming from ozone decomposition although the simultaneous presence of ozone, TiO2 and light leads to similar results than at pH 5, likely due to steps 1-6. It should be highlighted that while hydroxyl radical generation from ozone decomposition diminishes with pH decrease, adsorption of fluorene increases and, consequently, the possibility of surface reactions. Therefore, both aspects balance the results of photocatalytic ozonation at pHs 2 and 5. Intermediates Detected. In this work, only carboxylic acids were identified in the oxidation processes studied. These results give an idea about the fast oxidation of fluorene during the first seconds of the process since no aromatic compounds were detected. Thus, maleic, formic and oxalic acids were the only compounds identified. In fact, in most of the cases, analytical chromatograms just showed the peaks corresponding to these acids. Thus, given the nature of them, one can assume that they represent (together to the unreacted fluorene), the major fraction, if not all, of the total organic carbon remaining in solution. Figures 3-6 show, as an example, the distribution curves of remaining fluorene and intermediate concentrations with time corresponding to experiments of single ozonation and photocatalytic ozonation, at pH 2 and 5, respectively. For the case of ozonation, only maleic and oxalic acids were detected. In these experiments, maleic acid concentration reached a maximum value between 2.5 and 4 min depending on the pH value while oxalic acid accumulated in water. Accumulation of oxalic acid in water indicates that the applied process did not result in significant generation of hydroxyl radicals. The presence of a high hydroxyl radical concentration allows for the removal of oxalic acid from water.19 In addition to single ozonation, in photocatalytic oxidation and ozone photolysis, oxalic acid also accumulated in water, although in the latter process (O3/ UVA), oxalic acid concentration was decreasing with
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Figure 4. Time evolution of concentrations of fluorene and intermediates from the photocatalytic ozonation of fluorene at pH 2. Key: (2) fluorene; (4) formic acid; (O) oxalic acid. Other conditions: 20 °C, initial fluorene concentration 1.7 mg L-1, gas flow rate 50 L h-1, gas ozone concentration 5 mg L-1; catalyst concentration 1.5 g L-1, and density flux of radiation 0.14 einstein h-1.
Figure 5. Time evolution of concentrations of fluorene and intermediates from the ozonation of fluorene at pH 5. Key: (2) fluorene; (b) maleic acid; (O) oxalic acid. Other conditions: 20 °C, initial fluorene concentration 1.4 mg L-1, gas flow rate 50 L h-1, and gas ozone concentration 5 mg L-1.
Figure 6. Time evolution of concentrations of fluorene and intermediates from the photocatalytic ozonation of fluorene at pH 5. Key: (2) fluorene; (b) maleic acid; (O) oxalic acid. Other conditions: 20 °C, initial fluorene concentration 2 mg L-1, gas flow rate 50 L h-1, gas ozone concentration 5 mg L-1, catalyst concentration 0.5 g L-1, and density flux of radiation 0.14 einstein h-1.
time when the reaction was stopped (after 20 min). The low absorptivity of ozone above 300 nm can justify this result. Photocatalytic ozonation was the only process where the total removal of oxalic acid was achieved. In fact, with this process, not only hydroxyl radicals can be the oxidizing agent but also TiO2 oxalate complexes that also react fast with ozone.20 Finally, also notice that a positive effect of TiO2 concentration on the fluorene removal in TiO2/UVA/O3 processes at pH 5 can be deduced from Figures 2a and 6. Thus, from these figures it is seen that increasing the TiO2 concentration from
Figure 7. Time evolution of estimated dimensionless TOC corresponding to different oxidation systems of fluorene at pH 2. Key: (O) O3; (0) UVA; (b) UVA/O3; (4) UVA/TiO2; (1) O3/TiO2; (9) UVA/O3/TiO2. Other conditions are as in Figure 2.
Figure 8. Time evolution of estimated dimensionless TOC corresponding to different oxidation systems of fluorene at pH 5. Key: (O) O3; (b) UVA; (0) UVA/O3; (4) UVA/TiO2; (1) O3/TiO2; (9) UVA/O3/TiO2. Other conditions are as in Figure 2 except for the TiO2 concentration in UVA/O3/TiO2 process: 0.5 g L-1.
0.5 to 1.5 g L-1 gives rise to 75 and 93% removal of fluorene, respectively, after 1 min of reaction. Mineralization Results. Mineralization is the ultimate goal of the oxidation processes of organic pollutants in water. This is usually determined by the measurement of total organic carbon (TOC). In this work, however, due to the low concentration of carbon in the starting fluorene solution (about 1.6 mg L-1) TOC could not be measured. Nonetheless, an estimation of TOC can be made from intermediates detected. It is well-known that oxidation of PAHs at low concentrations leads to carboxylic acids as final products, especially in the cases where ozone is applied.21 As shown above, some carboxylic acids were identified and their concentrations determined in this work. In addition, during the HPLC determination no other significant peaks appeared, especially when more than 20% conversion of fluorene was achieved. This means that measurements of remaining fluorene and carboxylic acids as concentration of carbon could give an idea about the mineralization reached. This is especially true for the cases where only oxalic acid was present because this compound usually represents the very end product of the oxidation process. Thus, Figures 7 and 8 presents the evolution of TOC determined from the concentrations of remaining fluorene and identified carboxylic acids corresponding to the different oxidation processes applied (radiation alone did not reduce TOC at all). As can be seen from these figures total mineralization was achieved in ozone photolysis and ozone photocatalytic processes while approximately 50% mineralization was reached in photocatalytic oxidation and when ozone
Ind. Eng. Chem. Res., Vol. 44, No. 10, 2005 3423 Table 1. Yields of Unsaturated (YUA) and Saturated (YSA) Carboxylic Acids during Fluorene Oxidation Processes: (A) at pH 2; (B) at pH 5 O3
O3/UVA
YUA
YSA
YUA
YSA
1 2 3 4 5 6 7 8 9 10 15 20
0.04 0.05 0.06 0.07 0.07 0.05 0.04 0.04 0.04 0.04 0.04 0.04
2.74 2.74 2.75 2.76 2.77 2.78 2.78 2.78 2.81 2.84 17.93 11.56
0.05 0.07 0.07 0.05 0.04
2.46 8.45 8.97 8.33 6.21
0.04
6.25
0 0 0
3.98 3.04 2.50
1 2 3 4 5 6 7 8 9 10 15 20
0.11 0.12 0.12 0.11 0.11 0.10 0.10 0.09 0.08 0.08 0.07 0.04
2.25 3.89 3.91 3.98 3.99 4.02 4.09 4.15 4.27 4.34 4.39 4.52
0.08 0.11 0.11 0.13 0.11 0.10 0.07 0.07 0.06 0.05 0.05 0.03
1.97 1.99 3.69 3.98 4.02 4.06 4.10 4.13 4.27 3.92 2.0 1.97
a
O3/TiO2
O3/UVA/TiO2
t, min
YUA (a) at pH 2a 0 0 0 0 0 0 0 0 0 0 0 0 (b) at pH 5a 0.05 0.04 0.04 0.04 0.04 0 0 0 0 0 0 0
UVA/TiO2
YSA
YUA
YSA
YUA
YSA
13.33 7.43 7.43 6.56 5.98
3.08 0 0
0.05 0.05 0.05 0.06 0.05 0.05 0.05 0.05 0.04 0 0 0
0 0 0 0 0 0 0 2.88 3.68 3.73 3.78 3.95
0 0 0 0 0 0 0 0 0 0 0 0
0 3.06 3.11 3.52 3.69 3.91 4.08 4.56 5.13 5.91 6.96 8.02
2.54 3.31 3.30 2.77 2.63 2.42 2.18 0 0 0 0 0
0.09 0.09 0.09 0.09 0.08 0.08 0.07 0 0 0 0 0
5.75 4.76 4.53 0 0 0 0 0 0 0 0 0
0.06 0.06 0.07 0.06 0.05 0.05 0.04 0.04 0.04 0.04 0 0
0 0 0 4.57 4.43 3.97 3.62 3.45 3.09 3.03 0 0
3.12
UA: Unsaturated carboxylic acids. SA: Saturated carboxylic acid.
Table 2. Reaction Factors (E)a during Fluorene Ozone Processes pH 2
pH 5
t, min
O3
O3/TiO2
O3/UVA
O3/UVA/TiO2
O3
O3/TiO2
O3/UVA
O3/UVA/TiO2
2 5 9 20
0.04 0.11 0.09 0.14
0.10 0.15 0.14 0.14
0.21 0.17 0.18 0.19
0.35 0.19 0.24 0.26
0.06 0.19 0.24 0.28
0.14 0.21 0.26 0.33
0.25 0.27 0.30 0.40
0.39 0.37 0.43 0.46
a
Determined from eq 8.
alone or combined with TiO2 was applied. In Figure 7 (for the photocatalytic reaction), it is seen that estimated TOC first decreases and then increases to reach a constant value (due to a steady-state concentration of oxalic acid). In this experiment, however, some unidentified peaks were observed in the HPLC analysis during the first minutes of oxidation. TOC corresponding to these intermediates was not accounted for. This explains the anomalous TOC trend observed in this case. Yields of Intermediates. Yield of a given intermediate or end product in the oxidation of fluorene can be defined as the ratio between the concentration of the intermediate/end product and that of the difference between the concentrations of the starting and remaining fluorene. To discuss the evolution of products of similar nature with time during the oxidation processes of fluorene, in this work, yields of unsaturated carboxylic acids (maleic acid), on one hand, and saturated carboxylic acids (oxalic and formic acids), on the other hand, have been determined (see Table 1 for results). As can be seen from Table 1, as a general rule, yields of unsaturated acids were 1 or 2 orders of magnitude lower than those of saturated acids except in the case of catalytic ozonation (pH 2). This, again, confirms the fast rate of oxidation experienced in these processes, especially in the photocatalytic ozonation. Another point of interest is the high addition of oxygen to the organic molecules during oxidation which makes preoxidation as a suitable methodology to improve the biodegrad-
ability of the water. Thus, yield values higher than unity represent total mole increases of more than 100% if compared to the total moles initially present of fluorene. Ozone Concentration and Kinetic Regimes of Ozonation. Because of the higher oxidation rates and mineralization achieved in ozone processes (specially combined ozonations with light or light and TiO2), measurements of ozone concentration were also made to establish the kinetic regime of the ozone processes or the relative importance of chemical (photochemical) reactions and mass transfer in the oxidation reactor used. In ozonation experiments, ozone rapidly accumulates in water which means that all ozonation processes develop in the slow kinetic regime.22 In fact, ozone saturation was reached in about 1 min with concentrations between 0.6 and 0.8 mg L-1 at pH 5 to 0.8-1.1 mg L-1 at pH 2, depending on the type of ozonation. Reactions of ozone are fast in the following order: single ozonation ≈ catalytic ozonation < ozone photolysis < photocatalytic ozonation. The reaction factors were also calculated from eq 823 (see Table 2):
E)
/ CO - CO3 3 / CO 3
(8)
/ where CO3 and CO correspond to the ozone concentra3 tion in the bulk water (which is experimentally measured) and at the interface in equilibrium with the
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Figure 9. Time evolution of fluorene concentration in consecutive UVA/O3/TiO2 runs carried out with the same catalyst at pH 2. Run number: (O) 1; (b) 2; (0) 3; (9) 4; (4) 5. Other conditions are as in Figure 2.
Figure 10. Time evolution of fluorene concentration in consecutive UVA/O3/TiO2 runs carried out with the same catalyst at pH 5. Run number: (O) 1; (b) 2; (0) 3; (9) 4; (4) 5. Other conditions are as shown in Figure 2.
exiting gas (since perfect mixing is considered for both the water and gas phases), respectively. The equilibrium concentration was determined from Henry’s law.24 The reaction factor represents the ratio between the actual ozonation rate and that from the maximum physical absorption. As can be seen from Table 2, values of E are always lower than unity, which means that ozone reactions develops in the slow kinetic regime, that is, chemical or photochemical reactions control the process rate in all cases. Hence, the effect of mass transfer is not important and a kinetic equation can be derived from reaction mechanisms and confirmed from experimental results. Also, the magnitude of E values confirms the importance of ozone reactivity in the different processes studied with the ozone photocatalysis as the most effective one to remove fluorene and intermediates from water. Activity and Stability of Catalyst. To test the activity and stability of the catalyst, a series of ozone photocatalytic runs were also carried out by reusing the catalyst. Figures 9 and 10 show the results obtained at pH 2 and 5, respectively, in five consecutive photocatalytic ozonation runs where the same catalyst was used. While at pH 2 no activity differences are observed between runs, at pH 5 a slight decrease in activity starts to be noticed after the fourth run. Since no titanium leaching was observed, the decrease of activity at pH 5 can be attributed to some loss of active surface sites. Studies are now in progress to ascertain the reason for such a loss. Conclusions Fluorene, as a representing polynuclear aromatic compound refractory to direct ozonation,15 is rapidly
removed from water through different oxidation technologies involving ozone (likely through hydroxyl radicals), light and a semiconductor photocatalyst such as TiO2. The ozone photocatalytic process shows the highest performance, measured not only through the removal of fluorene but also intermediates. On the opposite site, single light photolysis did not yield any fluorene removal. Thus, estimated total TOC removal was only observed in the ozone photocatalytic process (total mineralization was achieved in less than 10 and 15 min at pH 5 and 2, respectively) and ozone photolysis at pH 5. Carboxylic acids, maleic, formic, and oxalic acids, were the only intermediates detected showing the hight oxidation rate achieved in these processes. In many cases, oxalic acid was the only product still remaining in water after 20 min of reaction. Yields of unsaturated and saturated compounds showed the high accumulation of oxygen in the intermediates which makes ozonation processes, especially photocatalytic ozonation, appropriate technologies to improve biodegradability. Measurements of dissolved and equilibrium ozone concentrations indicated that ozone processes develop in the slow kinetic regime and, hence, a kinetic study can be accomplished neglecting the influence of mass transfer (this will be shown in a forthcoming publication). Finally, the semiconductor used, TiO2, was observed to keep its activity and stability at pH 2 with some decrease of the former one at pH 5. No leaching of titanium was detected in water. Because of the very high rates of fluorene (and estimated TOC) removal in the ozone photocatalysis (TiO2) process and the high activity this catalyst has shown when ozone is applied, photocatalytic ozonation can be considered as a challenging process to be improved and studied. As a result, more work, that will be the subject of future publications, is now in progress to elucidate aspects related to the state of the catalyst active surface, effects on the removal of other priority pollutants and kinetics. Acknowledgment This work has been supported by the CICYT of Spain and The European Region Development Funds of the European Commission (Project PPQ2003/00554). Dr. Olga Gimeno and Mrs. Marı´a Carbajo also thank the Spanish Ministry of Science and Education for providing them a Ramo´n y Cajal contract and a FPU grant, respectively. Literature Cited (1) Langlais, B., Reckhow, D. A., Brink, D. R., Eds. Ozone in water treatment: application and engineering. Lewis Publ.: Chelsea, Michigan, 1991. (2) Beltra´n, F. J. Ozone-UV radiation-Hydrogen peroxide oxidation technologies. In Chemical degradation methods for wastes and pollutants. Environmental and Industrial Applications; Tarr, M., Ed.; Marcel Dekker: New York, 2003; pp 1-74. (3) Beltra´n, F. J. Ozone reaction kinetics for water and wastewater systems; Lewis Publ.: Boca Rato´n, FL, 2003. (4) Legube, B.; Karpel Vel Leitner, N. Catalytic ozonation: a promising advanced oxidation technology for water treatment. Catal. Today 1999, 53, 61-72. (5) Kasprzyk-Hordern, B.; Ziolek, M.; Nawrocki, J. Catalytic ozonation and methods of enhanicing molecular ozone reactions in water treatment. Appl. Catal., B: Environ. 2003, 46, 639-69. (6) Kaneko, M., Okura, I., Eds. Photocatalysis. Science and Technology. Kodansha, Ltd.: Tokyo, Springer-Verlag: Berlin, 2002.
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Received for review December 13, 2004 Revised manuscript received March 2, 2005 Accepted March 28, 2005 IE048800W
