Photocatalytic Decolorization and Mineralization of Dyes with

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Photocatalytic Decolorization and Mineralization of Dyes with Nanocrystalline TiO2/SiO2 Materials Javier Maruga´ n,* Marı´a-Jose´ Lo´ pez-Mun˜ oz, Rafael van Grieken, and Jose´ Aguado Department of Chemical and EnVironmental Technology, ESCET, UniVersidad Rey Juan Carlos, C/Tulipa´ n s/n, 28933 Mo´ stoles, Madrid, Spain

The application of photocatalytic technologies for the treatment of dyeing effluents from the textile industry has been shown to be economically competitive, especially when solar light drives the process. However, improvements in the catalysts are required to facilitate the separation stage. This work addresses the synthesis and characterization of nanocrystalline TiO2 (n-TiO2) incorporated into commercial silica particles and the activity of these materials in the photocatalytic treatment of three different azo dyes. The characterization of the n-TiO2/SiO2 photocatalysts shows differences in the average crystallite size that lead to significant differences in the values of the band gap energy of the semiconductor and also in activity. The molecular structure of the dyes also influences the kinetics of degradation, being almost independent of the TiO2 loading for Methyl Orange, which is opposite to the cases of Congo Red and Trypan Blue. This different behavior could arise from the existence of mass-transport restrictions hindering the complex dye molecules from reaching the titania nanocrystals located inside the porous structure of the silica particles. and mineralization of three different azo dyes with increasing molecular complexity.

1. Introduction In the past several years, many works have reported the application of heterogeneous photocatalytic technologies for the treatment of hazardous organic and inorganic compounds present in aqueous media.1 The application of these processes has been shown to be especially interesting for the treatment of dyeing compoundsusuallypresentinwastewatersfromtextileindustries.2-6 A growing interest has been focused on this technology because of the possibility of using solar radiation as the energy source for the decontamination of these effluents.7-9 The use of solar energy to drive the destruction of pollutants presents an undoubted advantage from the environmental viewpoint,10 but also improves the economic feasibility of the process, making it competitive with other technologies for wastewater treatment, such as ozone or H2O2/UV-C.11 On the other hand, dyes have been extensively used as model compounds for the evaluation of newly developed photocatalysts, probably because of the simplicity of the analytical tools required to follow their reactions. Research in the preparation of innovative photocatalysts has been motivated by two main issues: (i) the improvement of the efficiency in the use of solar light by shifting the absorption of TiO2, located in the near UV spectrum, to the visible wavelength range12,13 and (ii) the immobilization of the active semiconductor particles to facilitate the separation of the catalyst once the reaction has finished.1,14,15 Even though nanocrystalline TiO2 has been shown to be a very active photocatalyst,16 many research efforts have been focused on the immobilization of titania nanoparticles onto silica materials using different reactor configurations, such as silica gel particles slurries,17 glass electrodes,18 and fluidized beds.19 The goal of this work was the study of the synthesis and characterization of nanocrystalline TiO2 photocatalysts incorporated into commercial silica particles with different loadings and the correlation of their structural and physicochemical properties with the activity for the photocatalytic degradation * To whom correspondence should be addressed. Tel.: +34 91 664 74 66. Fax: +34 91 488 70 68. E-mail: [email protected].

2. Experimental Section 2.1. Synthesis and Characterization of the Photocatalysts. Preparation of the nanocrystalline TiO2/SiO2 materials (n-TiO2/ SiO2) was carried through a sol-gel method. Hydrolysis of titanium tetraisopropoxide (Aldrich, 97%) and condensation in the presence of a commercial silica material (Tixosil, Manuel Riesgo, specific surface area ) 154 m2‚g-1, pore volume ) 0.30 cm3‚g-1) was followed by a hydrothermal treatment for the crystallization of TiO2 and calcination at 550 °C. More details about the synthesis procedure can be found in previous works using different silica materials.20,21 The synthesized materials are denoted as n-TiO2(x%)/SiO2, where x represents the nominal weight percentage of titania. Chemical analyses for determining the exact titanium contents of the photocatalysts were performed using a Varian VISTA AX inductively coupled plasma atomic emission spectrophotometer (ICP-AES). Samples were dissolved in fluorhydric acid (Scharlab, 40 wt % aqueous solution), and quantification was carried out using the emission line at 336.112 nm after calibration with certified standards. For all of the materials, the deviations of the measured TiO2 contents from the nominal contents of titania incorporated to the synthesis procedure were lower than 5%. The Brunauer-Emmett-Teller (BET) specific surface areas and pore volumes of the samples were calculated from nitrogen adsorption-desorption isotherms at 77 K obtained on a Micromeritics Tristar 3000 apparatus. Pore size distributions were determined using the Barrett-Joyner-Halenda (BJH) model assuming a cylindrical geometry. Powder X-ray diffraction (XRD) patterns were collected on a Philips X’PERT MPD diffractometer using monochromatic Cu KR radiation and scanning 2θ from 20° to 70° with a step size of 0.02°. The step time was 2 s, which was adequate to obtain a good signal-to-noise ratio in the main diffractions of the two typical TiO2 crystalline phases, (101) anatase (2θ ≈ 25.3°) and (110) rutile (2θ ≈ 27.4°). The average crystallite

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Table 1. Characteristics of the Tested Azo Dyes

size was estimated according to the Scherrer equation using the calcite (104) diffraction signal (2θ ≈ 29.4°) to calculate the instrumental width. Diffuse reflectance spectra in the 200-500 nm range were recorded with a Varian Cary 500 Scan UV/vis/NIR spectrophotometer equipped with an integrating sphere diffuse reflectance accessory, using polytetrafluoroethylene as the reference scatterer. The reflectance data were obtained as the KubelkaMunk function, F(R), whose value could be assumed to be proportional to the absorption.22 From these spectra, the band gap values of the different materials were estimated according to the procedure reported by Sakthivel et al.23 2.2. Photocatalytic Reactions. Three different azo dyes purchased from Sigma-Aldrich were tested for degradation: Methyl Orange, Congo Red (Direct Red 28), and Trypan Blue (Direct Blue 14). Table 1 lists their chemical structures, molecular weights, and spectroscopic data on their respective absorption bands in the visible spectrum. As can be seen, the three model dyes were selected to present increasing molecular complexity. The photoreactor consisted of a 1-L vessel with internal annular irradiation by a 150 W medium-pressure mercury lamp (Heraeus TQ-150) placed inside a cooling jacket to maintain the temperature of the solution at 25 ( 0.1 °C. The lamp was switched on 30 min before the beginning of the reaction to stabilize the power of its emission spectrum line at 365 nm. The UV-A irradiance, determined by ferrioxalate actinometry, was 1.10 × 105 einstein‚L-l‚s-1. Reactions were carried out by dissolving 20 mg of the commercial dye in 1 L of deionized water (Milli-Q, 18.2 MΩ· cm) and suspending the amount of catalyst required to obtain a titania concentration of 0.5 gTiO2‚L-1. It was experimentally verified that this catalyst concentration led to a negligible UV-A radiation flux through the outer wall of the reactor with all tested photocatalysts. Afterwards, the suspensions were stirred in the dark for 30 min to reach adsorption equilibrium prior to the irradiation. All experiments were carried out at natural pH without further adjustment. Once the irradiation had been started, samples were withdrawn at regular intervals from the upper part of the reactor, with the catalyst being removed from the liquid phase by filtration through 0.22 µm nylon syringe filters. Color removal was followed by absorbance measurements using a Merck Spectroquant Vega 400 spectrophotometer. Mineralization of the organic content was determined through total organic carbon (TOC) measurements in a Shimadzu 5000 TOC analyzer calibrated with hydrogen potassium phthalate standard solutions.

3. Results and Discussion 3.1. Catalyst Characterization. Figure 1 shows the values of the BET specific surface areas of the tested materials as estimated from the adsorption-desorption isotherms of nitrogen at 77 K. As the TiO2 loading increases, a progressive increase of the specific surface area and pore volume (not shown) is observed. These results are quite anomalous, as an opposite behavior was previously reported for other silica supports.20,21 The reason for the difference is that the value of the surface area of the silica material used in this work is relatively low, as shown in Figure 1 for the 0 wt % TiO2 loading data, corresponding to the silica material after it had been subjected to the synthesis procedure in the absence of a titania source. Consequently, the contribution of the titania nanocrystals to the total surface area of the material is quite significant, even higher than that of the silica particles. The X-ray diffraction patterns of the n-TiO2/SiO2 photocatalysts (not shown) indicated that, in all of the samples, only the anatase crystalline phase was present. Despite the large collecting time, the wide peaks and the low signal-to-noise ratios confirmed that the average crystals sizes of the TiO2 clusters were very low. Figure 2 displays the average crystallite sizes estimated by the Scherrer equation, where an increase in crystal size is observed with increasing titania loading, especially in the range below 20 wt % of TiO2. In any case, all materials show average crystallite sizes below 9.0 nm, which is in agreement with their high specific surface areas. On the other hand, the analysis of the band gap energy (Eg) values estimated from the UV/vis diffuse reflectance spectra (Figure 3) indicates that the lower the TiO2 loading the higher the Eg value, especially for materials with TiO2 loadings below 20 wt %. Similar results were observed previously for TiO2 supported on silica materials and were explained according to the existence of quantum size effects that increase the distance between the lowest unoccupied molecular orbital (LUMO) of the conduction band and the highest occupied molecular orbital (HOMO) of the valence band of semiconductor particles with sizes of few nanometers.24 In summary, from the characterization data, it seems that, for low titania contents, n-TiO2/SiO2 materials present very small titania crystals with relatively high band gap energies, which means a worse use of the radiation spectrum of a given light source. Additionally, these materials show lower specific surface area values. Consequently, higher photoactivities should be expected for materials with high loadings, not only from photochemical considerations, i.e., photon absorption, but also

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Figure 1. BET specific surface area values estimated from the nitrogen adsorption isotherms of the catalysts.

Figure 2. Average crystallite sizes estimated from the peak broadening of the main anatase signal of the powder X-ray diffraction patterns of the catalysts.

Figure 4. Pseudo-first-order kinetic constants for the degradations of (a) Methyl Orange, (b) Congo Red, and (c) Trypan Blue solutions with the different catalysts.

Figure 3. Band gap values estimated from the UV/vis diffuse reflectance spectra of the catalysts.

from the heterogeneous catalysis viewpoint, considering the higher specific surface areas of the catalysts. 3.2. Photocatalytic Degradation of the Dyes. All of the n-TiO2/SiO2 materials presented above were tested in the

photocatalytic degradation of three azo dyes with increasing molecular complexity: Methyl Orange (MO), Congo Red (CR), and Trypan Blue (TB). The lack of photolytic degradation in the absence of TiO2 was first verified for the three dye solutions, confirming the photocatalytic nature of the decolorization process. In all cases, the profiles of dye concentration versus time of irradiation can be successfully reproduced by a pseudofirst-order macroscopic kinetic model, in agreement with the results of many other research groups.5 Figure 4 displays the

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Figure 5. Pseudo-first-order kinetic constants for the mineralizations of a) Methyl Orange, (b) Congo Red, and (c) Trypan Blue solutions with the different catalysts.

values of the kinetic constants calculated by fitting the experimental data to an exponential decay function using a leastsquares linear regression algorithm. The absolute error between two different replicates is below 0.05 h-1. First, it seems that the titania loading of the material produces only a slight increase in the rate of MO photodegradation, whereas the kinetic constants for the degradations of CR and TB increase several times. Moreover, for both CR and TB dye solutions, different slopes are found for low and high loadings of titania, with

the increase in the reaction rate being more pronounced for TiO2 loadings above 30 wt %. These results can be explained by the higher specific surface areas and crystallite sizes of the catalysts with higher TiO2 loadings, together with their lower band gap energy values. Taking into account that photon absorption is a threshold process and that the emission spectrum of the lamp is centered around 365 nm (equivalent to 3.4 eV), the influence on the activity of the material is higher when values of Eg corresponding to a better overlap between the absorption spectrum of the catalysts and the emission spectrum of the lamp are reached. Consequently, even though the variation of Eg is more pronounced at lower TiO2 loadings, the influence on the activity is more significant at high TiO2 loadings. Taking into account the fact that a concentration of 0.5 gTiO2‚L-1 was used in all degradation experiments, differences in activity must be explained in terms of the structural and physicochemical properties of the materials and the molecular structures of the dyes. Consequently, it is very significant that, despite the lower molecular complexity of MO compared to CR and TB, the decolorization of MO solutions is slower for most of the catalysts. It seems that the activity for MO photodegradation is affected by neither the average crystal size of titania nor its band gap energy value. The slight increase in kMO observed with increasing loading of TiO2 could be supported by the slightly higher values of the specific surface area. A possible explanation for the different behavior shown by both CR and TB solutions in comparison to MO solutions could be the existence of differences in the adsorption of the dyes over the TiO2 and SiO2 surfaces. However, preliminary adsorption experiments did not show significant differences in the amount of dye adsorbed on each catalyst. Moreover, the influence of the surface on the activity should produce a monotonic increase in the kinetic constant, which does not explain the existence of two well-defined tendencies depending on the titania loading. In contrast, the results for the photodegradation rates of CR and TB seem to show a clear relation with the average crystal size of the catalysts and with the increase in the band gap energy values produced by the existence of quantum effects. As the value of Eg decreases, an increase in kCR and kTB is observed. However, this explanation is not sufficient to explain why the kinetic constants for catalysts with titania loadings above 30 wt % increase significantly for small variations of the band gap energy. In this case, the more plausible explanation is that the higher molecular complexity of the two diazo dyes CR and TB in comparison to MO (six aromatic rings for CR and TB versus two for MO) reduces the accessibility of the dye molecules to the TiO2 nanocrystals. As the loading of TiO2 increases, the amount of TiO2 on the external surface of the SiO2 particles also increases. Consequently, the lower values of kCR and kTB for catalysts with low TiO2 loadings could also be due to the existence of mass-transport problems with reaching the titania nanocrystals located inside the pores of the silica particles. For MO molecules, this effect should be not as significant, leading to the minor dependence of kMO on the TiO2 loading. This assumption is in agreement with previous results reported for cyanide complexes21 and alcohols25 with different molecular sizes. 3.3. Mineralization of the Dyes. The total mineralization of complex molecules, such as those shown in Table 1, requires many oxidation steps and involves a large number of intermediate species in the degradation pathway. A detailed analysis of the degradation mechanism of the studied dyes is beyond the scope of this work.

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On the other hand, the goal of the photocatalytic treatment of dyestuff is usually bleaching the solutions, together with increasing the biodegradability and reducing the toxicity, making it more suitable for treatment by other conventional means to achieve the total oxidation of the organic content. However, the study of the mineralization results of the photodegradation of dyes is very important, as such results provide useful information about the possible competition between the dye molecules and their degradation products for the oxidant species generated on the photocatalyst surface. In this work, the evolution of the total organic carbon in solution was used to determine the degree of mineralization of the degraded dyes. Once again, the profiles of TOC versus irradiation time can be satisfactorily reproduced by a pseudofirst-order macroscopic kinetic model. Figure 5 displays the values of the kinetic constants calculated by fitting the experimental data to an exponential decay function. For the three studied dyes, the dependence of the mineralization kinetic constant on the titania loading of the catalyst is quite similar to that of decolorization. The dispersion in the k′MO results is very high, and the values do not show a clear dependence. On the other hand, k′CR and k′TB again show two well-defined ranges of variation, with a much higher increase for materials with TiO2 loadings above 30%, similarly to the results for photodegradation. Dye mineralization can be represented by the following simplified mechanism k

k′

dye 98 uncolored intermediates 98 mineralization products where k represents the kinetic constant for dye photodegradation and k′ is the kinetic constant for dye mineralization. This mechanism assumes that the first attack on the dye molecules causes the destruction of the chromogenic group (the group responsible for visible light absorption), usually the -C-Nd N-C- azo group.5 At the beginning of the reaction, only dye molecules are present, but as the reaction proceeds, the intermediates accumulated in the solution begin to compete with the dye molecules for the oxidant species photogenerated in the semiconductor. This competition can be represented by the ratio between the mineralization kinetic constant and the photodegradation kinetic constant, and it allows for a comparison among the different degrees of mineralization of each dye. The degree of mineralization of MO (average k′/k ) 0.74) is much higher than those of CR (average k′/k ) 0.09) and TB (average k′/k ) 0.31). Consequently, although the degradation of CR is faster than that of MO, its degree of mineralization is much lower. On the other hand, TB solutions show a fast degradation kinetics together with a relatively high degree of mineralization. These results are obviously explained by the higher molecular complexity of the CR and TB intermediate species in comparison to those of MO. 4. Conclusions Nanocrystalline TiO2/SiO2 photocatalysts show significant differences in average crystallite size that lead to different values of the band gap energy of the semiconductor and, more importantly, to significant differences in activity in the photodegradation and mineralization of azo dyes such as Congo Red and Trypan Blue. However, the molecular structure of the dye also affects the kinetics of degradation, being independent of the TiO2 loading for other dyes such as Methyl Orange. This different behavior could be also produced by the existence of mass-transport problems in the access of complex dye molecules

to the titania nanocrystals located inside the porous structure of the silica particles. On the other hand, although the degree of mineralization is also very dependent on the molecular structure of the dye, in all cases, better results for degradation and mineralization at constant TiO2 concentration are obtained at increased titania loadings in the n-TiO2/SiO2 catalysts. Acknowledgment Financial support for this work was provided by Universidad Rey Juan Carlos through Project PPR-2004-10 and Comunidad de Madrid through Program S-0505/AMB/0395 “Red Madrilen˜a de Tratamientos Avanzados para Aguas Residuales con Contaminantes No Biodegradables (REMTAVARES)”. Literature Cited (1) Kabra, K.; Chaudhary, R.; Sawhney, R. L. Treatment of hazardous organic and inorganic compounds through aqueous-phase photocatalysis: A review. Ind. Eng. Chem. Res. 2004, 43, 7683-7696. (2) Davis, R. J.; Gainer, J. L.; O’Neal, G.; Wu, I. W. Photocatalytic decolorization of wastewater dyes. Water EnViron. Res. 1994, 66, 50-53. (3) Lizama, C.; Yeber, M. C.; Freer, J.; Baeza, J.; Mansilla, H. D. Reactive dyes decolouration by TiO2 photo-assisted catalysis. Water Sci. Technol. 2001, 44, 197-203. (4) Lachheb, H.; Puzenat, E.; Houas, A.; Ksibi, M.; Elaloui, E.; Guillard, C.; Herrmann, J.-M. Photocatalytic degradation of various types of dyes (Alizarin S, Crocein Orange G., Methyl Red, Congo Red, Methylene Blue) in water by UV-irradiated titania. Appl. Catal. B: EnViron. 2002, 39, 7590. (5) Konstantinou, I. K.; Albanis, T. A. TiO2-assisted photocatalytic degradation of azo dyes in aqueous solution: Kinetic and mechanistic investigations: A review. Appl. Catal. B: EnViron. 2004, 49, 1-14. (6) Bizani, E.; Fytianos, K.; Poulios, I.; Tsiridis, V. Photocatalytic decolorization and degradation of dye solutions and wastewaters in the presence of titanium dioxide. J. Hazard. Mater. 2006, 136, 85-94. (7) Augugliaro, V.; Baiocchi, C.; Bianco Prevot, A.; Garcı´a-Lo´pez, E.; Loddo, V.; Malato, S.; Marcı´, G.; Palmisano, L.; Pazzi, M.; Pramauro, E. Azo-dyes photocatalytic degradation in aqueous suspension of TiO2 under solar irradiation. Chemosphere 2002, 49, 1223-1230. (8) Neppolian, B.; Choi, H. C.; Sakthivel, S.; Arabindoo, B.; Murugesan, V. Solar/UV-induced photocatalytic degradation of three commercial textile dyes. J. Hazard. Mater. 2002, B89, 303-317. (9) Stylidi, M.; Kondarides, D. I.; Verykios, X. E. Pathways of solar light-induced photocatalytic degradation of azo dyes in aqueous TiO2 suspensions. Appl. Catal. B: EnViron. 2003, 40, 271-286. (10) Mun˜oz, I.; Rieradevall, J.; Torrades, F.; Peral, J.; Dome´nech, X. Environmental assessment of different solar driven advanced oxidation processes. Solar Energy 2005, 79, 369-375. (11) Alaton, I. A.; Balcioglu, I. A.; Bahnemann, D. W. Advanced oxidation of a reactive dyebath effluent: Comparison of O3, H2O2/UV-C and TiO2/UV-A processes. Water Res. 2002, 36, 1143-1154. (12) Vinodgopal, K.; Wynkoop, D. E.; Kamat, P. V. Environmental photochemistry on semiconductor surfaces: Photosensitized degradation of a textile azo dye, Acid Orange 7, on TiO2 particles using visible light. EnViron. Sci. Technol. 1996, 30, 1660-1666. (13) Subramanian, V.; Roeder, R. K.; Wolf, E. E. Synthesis and UVvisible light photoactivity of noble-metal-SrTiO3 composites. Ind. Eng. Chem. Res. 2006, 45, 2187-2193. (14) Guillard, C.; Lachheb, H.; Houas, A.; Ksibi, M.; Elaloui, E.; Herrmann, J.-M. Influence of chemical structure of dyes, of pH and of inorganic salts on their photocatalytic degradation by TiO2 comparison of the efficiency of powder and supported TiO2. J. Photochem. Photobiol. A: Chem. 2003, 158, 27-36. (15) Maruga´n, J.; Lo´pez-Mun˜oz, M.-J.; Gernjak, W.; Malato, S. Fe/ TiO2/pH interactions in solar degradation of imidacloprid with TiO2/SiO2 photocatalysts at pilot-plant scale. Ind. Eng. Chem. Res. 2006, 45, 89008908. (16) Zhang, Z.; Wang, C.-C.; Zakaria, R.; Ying, J. Y. Role of particle size in nanocrystalline TiO2-based photocatalysts. J. Phys. Chem. B 1998, 102, 10871-10878. (17) Chun, H.; Yizhong, W.; Hongxiao, T. Preparation and characterization of surface bond-conjugated TiO2/SiO2 and photocatalysis for azo dyes. Appl. Catal. B: EnViron. 2001, 30, 277-285.

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(18) Subramanian, V.; Kamat, P. V.; Wolf, E. E. Mass-transfer and kinetic studies during the photocatalytic degradation of an azo dye on optically transparent electrode thin film. Ind. Eng. Chem. Res. 2003, 42, 2131-2138. (19) Kwon, J. M.; Kim, Y. H.; Song. B. K.; Yeom, S. H.; Kim, Im, B. S., J. B. Novel immobilization of titanium dioxide (TiO2) on the fluidizing carrier and its application to the degradation of azo-dye. J. Hazard. Mater. 2006, B134, 230-236. (20) van Grieken, R.; Aguado, J.; Lo´pez-Mun˜oz, M. J.; Maruga´n, J. Synthesis of size-controlled silica-supported TiO2 photocatalysts. J. Photochem. Photobiol. A: Chem. 2002, 148, 315-322. (21) Lo´pez-Mun˜oz, M.-J.; van Grieken, R.; Aguado, J.; Maruga´n, J. Role of the support on the activity of silica-supported TiO2 photocatalysts: Structure of the TiO2/SBA-15 photocatalysts. Catal. Today 2005, 101, 307314. (22) Anderson, C.; Bard, A. J. Improved photocatalytic activity and characterization of mixed TiO2/SiO2 and TiO2/Al2O3. J. Phys. Chem. B 1998, 101, 2611-2616.

(23) Sakthivel, S.; Janczarek, M.; Kisch, H. Visible Light Activity and Photoelectrochemical Properties of Nitrogen-Doped TiO2. J. Phys. Chem. B 2004, 108, 19384-19387. (24) Lassaleta, G.; Ferna´ndez, A.; Espinos, J. P.; Gonza´lez-Elipe, A. R. Spectroscopic characterization of quantum-sized TiO2 supported on silica: Influence of size and TiO2-SiO2 interface composition. J. Phys. Chem. 1995, 99, 1484-1490. (25) Maruga´n, J.; Hufschmidt, D.; Lo´pez-Mun˜oz, M.-J.; Selzer, V.; Bahnemann, D. Photonic efficiency for methanol photooxidation and hydroxyl radical generation on silica-supported TiO2 photocatalysts. Appl. Catal. B: EnViron. 2006, 62, 201-207.

ReceiVed for reView January 15, 2007 ReVised manuscript receiVed May 22, 2007 Accepted May 24, 2007 IE070093U