Environ. Sci. Technol. 2001, 35, 1544-1549
Lanthanide Oxide-Doped Titanium Dioxide Photocatalysts: Novel Photocatalysts for the Enhanced Degradation of p-Chlorophenoxyacetic Acid K . T . R A N J I T , * ,† I . W I L L N E R , † S. H. BOSSMANN,‡ AND A. M. BRAUN‡ Institute of Chemistry and Farkas Center for Light-Induced Processes, The Hebrew University of Jerusalem, Jerusalem, 91904 Israel, and Lehrstuhl fu ¨ r Umweltmesstechnik der Universita¨t Karlsruhe (TH), Engler-Bunte-Institut, Richard-Willsta¨tter-Allee 5, 76128 Karlsruhe, Germany
The photocatalytic degradation of p-chlorophenoxyacetic acid has been investigated in oxygenated aqueous suspensions of lanthanide oxide-doped TiO2 photocatalysts. Complete mineralization was achieved. The enhanced degradation is attributed to the formation of Lewis acidbase complex between the lanthanide ion and the substrate.
Introduction Semiconductor photocatalysis has attracted the attention of several researchers for the past decade for the light-stimulated degradation of aqueous pollutants (1-3) and atmospheric pollutants (4, 5). Semiconductor photocatalysis has been used successfully for the remediation of contaminants for a variety of compounds (6-10) such as phenols, aromatic carboxylic acids, dyes, surfactants, and pesticides as well as for the reductive deposition of heavy metals (e.g., Pt4+, Au3+, Rh3+, Cr3+, etc.) from aqueous solutions to surfaces (11-14). Several semiconductors exhibit band-gap suitable for catalyzing the degradation of organic compounds. Among them, TiO2 has proven to be a benchmark catalyst for detoxification of organic pollutants. Photoexcitation of the semiconductor leads to the formation of an electron-hole pair. The excited-state conduction band electrons can recombine with the holes and dissipate the input energy, get trapped in surface states, or react with electron donors and electron acceptors adsorbed on the semiconductor surface. To effectively compete with electron-hole recombination and trap effectively the conduction band electrons or the valence band holes, the respective electron acceptor or donor should be confined to the semiconductor surface. Several methods have been suggested to control the interfacial electron transfer at the semiconductor-electrolyte interface. These include electrostatic association of electron acceptors at the semiconductor surface (15), encapsulation of electron acceptors on receptor functionalized semiconductors (16, 17), and immobilization of semiconductor photocatalyzed in redox functionalized polymers (18, 19). The mechanism of TiO2-photocatalyzed reactions has been the subject of extensive research (20-22). The detailed * Corresponding author present address: Department of Chemistry, University of Houston, Houston, TX 77204-5641. † The Hebrew University of Jerusalem. ‡ Lehrstuhl fu ¨r Umweltmesstechnik der Universita¨t Karlsruhe (TH). 1544
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mechanism differs from one pollutant to another, but it is widely recognized that superoxide and, in specific hydroxyl radicals, •OH act as active reagents for the mineralization of the organic compounds. These radicals are formed by the scavenging of electron-hole pairs by oxygen and water:
O2 + ec.b- f O2•-
(1)
H2O + h+f •OH + H+
(2)
•
OH + •OH f H2O2
(3)
H2O2 + O2•- f •OH + OH- + O2
(4)
Insertion of •OH radicals into C-H bonds leads ultimately to the complete degradation of the organic substrate. Complete mineralization of a variety of organic compounds such as hydrocarbons, phenols, carboxylic acids, olefinic compounds has been realized (23, 24). The hydroxyl radical can also recombine to yield H2O2. This leads to inefficient degradation of the pollutant. For effective degradation of the pollutant, the organic material should be preconcentrated at the semiconductor surface in order to effectively trap the respective reactive radicals. Concentration of the organic pollutant at the semiconductor surface has been achieved by immobilization of hydrophobic layers (25) and by surface modification of the photocatalyst with chelating agents (26). Other methods to preconcentrate the pollutant at the photocatalyst surface include surface modification of the catalyst with electron acceptors groups (27) and selective doping of the semiconductor (28). Lanthanide ions are known for their ability to form complexes with various Lewis bases (e.g., acids, amines, aldehydes, alcohols, thiols, etc.) in the interaction of these functional groups with the f-orbitals of the lanthanides. This property is used in NMR spectroscopy where the magnetic features of the Eu3+ or Pr3+ yield significant chemical shifts in the protons of the associated organic ligands (29, 30). Thus, incorporation of lanthanide ions in a TiO2 matrix could provide a means to concentrate the organic pollutant at the semiconductor surface. The sol-gel method enables the synthesis of high surface area TiO2 particles (31, 32). Here we wish to report on the enhanced degradation of p-chlorophenoxyacetic acid; a model pollutant by the lanthanidemodified TiO2 as compared to the nonmodified photocatalyst. The photodegradation of p-chlorophenoxyacetic acid has important practical implications. The chlorinated aromatic pesticides in the environment represent a serious hazard because of their persistence, toxicity, and widespread utilization (33, 34). Their degradation, hence, is very important. The degradation of these compounds is possible through chemical, photochemical, and biological processes, but many of these methods require long treatment periods and are difficult to apply practically in disposal systems. In most cases, the degradation methods lead to the formation of products, which are more toxic than the original pollutant itself (35, 36). The degradation of halogenated phenoxyacetic acids and their reaction intermediates have been studied by irradiated titanium dioxide suspensions (37-45), by Fenton reagent (46-52), and by UV irradiation (53-55). Several intermediates were detected by gas chromatographic-mass spectrometric techniques, and a mechanism for the photodegradation was formulated. Photocatalytic degradation provides a relatively convenient and efficient way for the degradation of chlorinated aromatic compounds. Thus, 10.1021/es001613e CCC: $20.00
2001 American Chemical Society Published on Web 03/02/2001
SCHEME 1. Preparation of the Lanthanide Oxide-Doped TiO2 Catalysts
photocatalytic degradation of p-chlorophenoxyacetic acid could provide a model system for the light-induced degradation of chlorinated pesticides.
Experimental Section The nonmodified TiO2, europium (Eu3+), praseodymium (Pr3+), and ytterbium (Yb3+) doped TiO2 were prepared as shown in Scheme 1. The preparation of these catalysts has been reported previously (28). In brief, TiO2 catalysts were prepared by acid-catalyzed sol-gel method employing titanium(IV) isopropoxide as the source of titanium ions. The gelation time was found to vary depending on the nature of the lanthanide ions. The resulting gel formed was first dried in air at 100 °C for 2 h and then calcined in air at 550 °C for 14 h. The X-ray diffractograms of the calcined samples were recorded using a Philips PW 1050 powder diffractometer. The diffraction patterns were recorded at room temperature using a Ni-filtered Cu KR radiation (λ ) 1.5418 Å) for the samples. Surface area measurements were measured by nitrogen adsorption at -196 °C by the dynamic BET method using a micromeritics II 2370 surface area analyzer. Photochemical degradation of p-chlorophenoxyacetic acid was examined. Aqueous solution of the pollutant (3 × 10-4 M) and 2.0-2.4 mg of the respective photocatalyst were placed in a quartz cuvette. The suspension was irradiated with a 200-W Xe(Hg) lamp (Oriel). During irradiation, the suspension was stirred continuously and purged with oxygen. After irradiation, the suspension was filtered to remove essentially all the catalyst, and the solution was analyzed
spectroscopically (Uvikon-860 Kontron spectrophotometer) and also by GC-MS/FTIR. DOC Analyses. The analysis of DOC (dissolved organic carbon) was carried out using a Dohrmann DC-190 TOC (total organic carbon) analyzer (T ) 680 °C) from Rosemount Analytical. The organic components were oxidized in an oxygen atmosphere at a platinum/alumina contact. The amount of carbon dioxide formed was measured using a nondispersive IR detector. The total carbon content was analyzed in this manner, and the TOC was calculated by subtraction of inorganic carbon. The latter is obtained by treating the samples with concentrated phosphoric acid. The calibration was performed using salicylic acid, oxalic acid, and potassium hydrophthalate (KHP). All calibration samples could be fitted with a linear calibration curve. The samples were injected three consecutive times (injection volume, 50 µL), and the average values were reported. GC-MS/FTIR Analysis. For the quantitative and qualitative analysis of the reaction products generated during the photolysis experiments, the samples (2 µL) were injected into a GC (HP 5971A MSD, mass selective detector) coupled with a HP 5965B 1D (infrared detector). A HP-INNOWAX capillary column (cross-linked poly(ethylene glycol)) was employed. All reaction products were identified by a combination of MS and FTIR spectroscopy in comparison with analytical databases. Furthermore, the calibration curves for the compounds investigated were also analyzed. In all cases, linear dependence of both MS and FTIR areas were obtained. VOL. 35, NO. 7, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. (A) Absorption spectra of an aqueous solution of p-chlorophenoxyacetic acid (3 × 10-4 M): (a) before irradiation, (b) after 30 min of irradiation, (c) 45 min of irradiation, (d) 60 min of irradiation in the presence of Eu2O3/TiO2 catalyst (Ti/Eu ) 100) (2.4 mg in 2.5 mL of solution). (B) Absorption spectra of an aqueous solution of p-chlorophenoxyacetic acid (3 × 10-4 M): (a) before irradiation, (b) after 30 min of irradiation, (c) 60 min of irradiation in the presence of TiO2 catalyst (2.5 mg in 2.5 mL of solution).
Results and Discussion The TiO2-based catalysts were prepared as described in Scheme 1. The molar ratio of Ti(IV):Ln(III) prior to the formation of the gel is usually 100:1. In the case of europiumdoped TiO2, a gel with a composition of Ti(IV):Eu(III) corresponding to a ratio of 20:1 was prepared. The calcined samples were analyzed by XRD and XPS. The X-ray diffractograms of the samples reveal that in the case of the bare TiO2, the anatase phase is the major constituent of the photocatalyst (80%) accompanied by a rutile phase (20%). In the case of Eu2O3/TiO2 and Yb2O3/TiO2 the anatase phase is predominant (>95%), while in the case of Pr2O3/TiO2 catalyst the X-ray diffractogram reveals the presence of only the anatase phase. The surface areas of the catalysts are as follows: TiO2, 78.1 m2/g; Eu2O3/TiO2, 102.1 m2/g; Pr2O3/TiO2, 125.0 m2/g; and Yb2O3/TiO2, 55.4 m2/g, respectively. XPS measurements reveal that the lanthanide does exist in the resulting TiO2 photocatalysts as oxides Ln2O3. Lanthanide concentrations were evaluated from their 3d lines and 4d lines. The photodegradation of p-chlorophenoxyacetic acid by Eu2O3/TiO2, Pr2O3/TiO2, and Yb2O3/TiO2 was examined and compared to the photodegradation yields by nonmodified TiO2. Figure 1A shows the absorbance spectra of p-chlorophenoxyacetic acid solution at time intervals of irradiation with the Eu2O3/TiO2 photocatalyst. A significant decrease in the absorbance of the substrate is observed indicating its photodegradation. From the figure it is evident that the 1546
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FIGURE 2. (A) Adsorption isotherm of p-chlorophenoxyacetic acid over Eu2O3/TiO2 catalyst (Ti/Eu ) 100). (B) Langmuir analysis of the adsorption isotherm of p-chlorophenoxyacetic acid over Eu2O3/TiO2 catalyst (Ti/Eu ) 100). substrate is completely degraded in the presence of Eu2O3/ TiO2 catalyst. The absorbance spectra of p-chlorophenoxyacetic acid over Pr2O3/TiO2 and Yb2O3/TiO2 catalysts were similar to the one obtained over Eu2O3/TiO2 catalyst, indicating the mineralization of p-chlorophenoxyacetic acid. Figure 1B shows the absorbance spectra of p-chlorophenoxyacetic acid in the presence of bare TiO2 catalyst. A small decrease in the absorbance of the substrate is observed. In addition, the absorbance spectra reveal the formation of at least a new intermediate with an absorbance maximum at λ ) 250 nm and a tailing absorbance with a shoulder in the region (λ ) 300-400 nm). The broad absorption spectra in the region (λ ) 300-400 nm) are indicative of the formation of aromatic intermediates. In contrast, the photodegradation of p-chlorophenoxyacetic acid by the lanthanide oxide-doped TiO2 catalyst leads to almost complete mineralization. The substrate is degraded at an impressive rate in the presence of Eu2O3/TiO2, Pr2O3/TiO2, and Yb2O3/TiO2 catalysts as compared to the nonmodified TiO2. For example, after 60 min of irradiation, 85%, 90%, and 85% of p-chlorophenoxyacetic acid are degraded by the Eu2O3/TiO2, Pr2O3/TiO2, and Yb2O3/TiO2 catalysts, respectively. In contrast, the photodegradation by nonmodified TiO2 photocatalysts leads to the formation of intermediates. To account for the enhanced photocatalytic activity of Eu2O3/TiO2, Pr2O3/TiO2, and Yb2O3/TiO2 catalyst as compared to the nonmodified TiO2, the adsorption of p-chlorophenoxyacetic acid to the modified and nonmodified TiO2 were examined. Figure 2A shows the adsorption isotherms of p-chlorophenoxyacetic acid on Eu2O3/TiO2, and Figure 3A shows the adsorption isotherms on the bare TiO2. The amount
FIGURE 4. Degradation of p-chlorophenoxyacetic acid (3 × 10-4 M) over (a) Eu2O3/TiO2 (Ti/Eu ) 100), (b) Pr2O3/TiO2, (c) Yb2O3/TiO2, and (d) nonmodified TiO2 catalysts at time intervals of irradiation analyzed spectroscopically.
FIGURE 3. (A) Adsorption isotherm of p-chlorophenoxyacetic acid over nonmodified TiO2 catalyst. (B) Langmuir analysis of the adsorption isotherm of p-chlorophenoxyacetic acid over nonmodified TiO2 catalyst. of p-chlorophenoxyacetic acid adsorbed into the photocatalyst increases as the bulk concentration of the substrate is elevated, and then it reaches a saturation value that represents the maximum loading of p-chlorophenoxyacetic acid on the catalyst. For Eu2O3/TiO2, the saturation value of 2 × 10-6 mol g-1 of p-chlorophenoxyacetic acid on the catalyst is obtained at a bulk concentration of 2 × 10-3 M pchlorophenoxyacetic acid. For Pr2O3/TiO2 and Yb2O3/TiO2, the saturation value for the amount of adsorbed p-chlorophenoxyacetic acid is 1.6 × 10-6 and 7.2 × 10-7 mol g-1, respectively. The adsorption isotherm can be analyzed in terms of Langmuir theory (56) (Figures 2B and 3B). A linear relationship between [C]eq/Cads as a function of [S]eq is observed where [C]eq represents the equilibrium bulk concentration of p-chlorophenoxyacetic acid and Cads is the amount (in moles) of p-chlorophenoxyacetic acid adsorbed onto the photocatalysts. From Figure 2B, the derived adsorption constant on Eu2O3/TiO2 (Ti/Eu ) 100) is Kads ) 1.15 × 103 M-1. For the nonmodified TiO2 catalyst, the saturation value of 6.0 × 10-7 mol g-1 on the catalyst is obtained at a bulk concentration of 3.0 × 10-3 M. The derived adsorption constant from Figure 3B on nonmodified TiO2 is Kads ) 4.5 × 102 M-1. The derived adsorption constants of p-chlorophenoxyacetic acid to Pr2O3/TiO2 and Yb2O3/TiO2 were found to be Kads ) 1.23 × 103 and 9.96 × 102 M-1, respectively. The adsorption constants of p-chlorophenoxyacetic acid to the lanthanide-modified TiO2 are ca. 3-fold higher as compared to the nonmodified TiO2. This is attributed to the formation of Lewis acid-base complexes between the lanthanide ions in the modified TiO2 and the
carboxylic acid residue of the substrate. Concentration of the substrate at the photocatalytic surface could then provide the mechanism for the enhanced mineralization of pchlorophenoxyacetic acid by the modified photocatalyst. The photodegradation of p-chlorophenoxyacetic acid in the presence of Eu2O3/TiO2, Pr2O3/TiO2, and Yb2O3/TiO2 catalysts is enhanced as compared to the nonmodified TiO2. From Figure 4, 85% of substrate is decomposed by Eu2O3/ TiO2 (curve a), 90% is decomposed by Pr2O3/TiO2 (curve b), and 85% is decomposed by Yb2O3/TiO2 (curve c) while intermediates are formed by nonmodified TiO2 after 60 min of irradiation. The concentrations of the solutions after irradiation were also monitored by GC-MS analyses. Table 1 shows the results obtained from the GC-MS study for the degradation of p-chlorophenoxyacetic acid by the lanthanide oxide-doped catalysts. The GC-MS results clearly reveal the superior performance of the lanthanide oxide-doped catalysts. For example, from Table 1 we can see that, after 30 min of irradiation, the concentration of p-chlorophenoxyacetic acid is 23.5 ppm with the Eu2O3/TiO2 catalyst whereas it is 45.2 ppm with the nonmodified catalyst. After 60 min of irradiation, almost complete mineralization of p-chlorophenoxyacetic acid is achieved by the lanthanide oxide-doped catalysts while the concentration of p-chlorophenoxyacetic acid is still 40.0 ppm with the nonmodified catalyst. The spectroscopic assay of the degradation rates of p-chlorophenoxyacetic acid coincides with the results obtained from the GC-MS analysis of the irradiated samples. Lanthanide ions themselves exhibit absorption and emission characteristics (57). To verify if the lanthanide oxides themselves are involved in light absorption, energy transfer to the TiO2 semiconductor, and photodegradation process, it is worthwhile to compare the rate of photodegradation of p-chlorophenoxyacetic acid by Eu2O3/TiO2 catalysts that include different amount of europium oxide. Figure 5 shows the degradation of p-chlorophenoxyacetic acid by the two Eu2O3/TiO2 catalysts. The rate of degradation is faster in the presence of Eu2O3/TiO2 catalyst (Ti/Eu ) 100) as compared to the photocatalyst containing Ti/Eu ) 20. The fact that the Eu2O3/TiO2 catalyst (Ti/Eu ) 20) is less efficient than the VOL. 35, NO. 7, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. GC-MS Results Obtained for Degradation of p-Chlorophenoxyacetic Acid over Lanthanide Oxide-Doped TiO2 Catalysts and Nonmodified Catalysts TiO2
Eu2O3/ TiO2
Pr2O3/ TiO2
Yb2O3/ TiO2
time (min)
GC-FID/MS (ppm)
DOC (ppm)
GC-FID/MS (ppm)
DOC (ppm)
GC-FID/MS (ppm)
DOC (ppm)
GC-FID/MS (ppm)
DOC (ppm)
0 30 45 60
55.4 45.2 nda 40.0
55.5 52.5 nd 45.0
55.4 23.5 14.7 0
55.5 41.5 30.4 5.0
55.4 17.2 11.8 0
55.5 35.1 25.7 12.4
55.4 21.4 13.0 8.7
55.5 37.8 30.4 16.8
a
nd, not determined.
FIGURE 5. Rate of photodegradation of p-chlorophenoxyacetic acid (3 × 10-4 M) over (a) Eu2O3/TiO2 catalyst (Ti/Eu ) 20) and (b) Eu2O3/ TiO2 catalyst (Ti/Eu ) 100). In all experiments, 2.3-2.5 mg of the catalyst in 2.5 mL of the solution was used. catalyst containing Ti/Eu ) 100 implies that the improved adsorption of the pollutants to the europium oxide-doped photocatalysts is not the only operative mechanism that controls the activity of the photocatalysts since the adsorption of p-chlorophenoxyacetic acid is slightly higher on the Ti/Eu catalyst. The Eu2O3/TiO2 catalyst (Ti/Eu ) 20) absorbs ca. 20% more incident light as compared to the Ti/Eu ) 100 catalyst. However, the initial rates for the two catalysts seem to be similar, and the conversion differs on longer irradiation. Thus, we are at the moment unable to clearly explain the differences obtained between the two catalysts. The discussion henceforth pertains to the mechanism leading to the enhanced activity of the lanthanide oxidedoped titania catalysts. The crystalline phase of the titania catalyst is an important factor that determines its activity; previous reports have indicated that the rutile phase (58, 59) is less active in the photodegradation of organic compounds. The lanthanide oxide-containing catalysts exist predominantly in the anatase phase whereas the nonmodified TiO2 catalyst contains rutile phase (ca. 20%). However the significant differences obtained in the photocatalytic activities cannot be attributed to the differences in the composition. Another factor that could influence significantly the photocatalytic activity is the surface area of the catalysts. The surface areas of Eu2O3/TiO2 and PrO3/TiO2 are similar (102.1 and 125.0 m2 g-1, respectively) while that of Yb2O3/TiO2 is about 25% lower than that of Eu2O3/TiO2. The surface area 1548
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of the nonmodified TiO2 is 78.1 m2 g-1. The photocatalytic activity of Yb2O3/TiO2 is similar to the other lanthanide oxidedoped catalysts, which suggest clearly that the differences in the surface area cannot be a controlling factor in influencing the photocatalytic activity. The electronic structure and surface properties such as space charge layer, surface states, and their concentration are important parameters that are affected by doping of a semiconductor with altervalent ions. Thus doping of TiO2 semiconductor with a lower valent ions such as Ln3+ (Ln ) Eu, Pr, or Yb) increases the work function; hence, the Fermi energy level is shifted to lower values. Consequently, on contact with an electrolyte, the depletion layer of the p-doped semiconductor becomes thicker as compared to the nonmodified semiconductor. As a result, the capacity of the space charge region to separate the electron-hole pair is reduced. Thus when TiO2 semiconductor is doped with a p-type donor the activity is expected to decrease. However in the present investigation we have found that the activity of the lanthanide oxide-doped TiO2 semiconductor is significantly higher than the nonmodified TiO2 semiconductor. This would suggest that the equilibrium dark adsorption of the pollutant at the photocatalyst surface have an important role in determining the photocatalytic activity. The adsorption of p-chlorophenoxyacetic acid was found to be higher on the lanthanide oxide-modified catalysts as compared to the nonmodified catalysts. This can be attributed to the formation of Lewis acid-base complexes between the lanthanide ions and p-chlorophenoxyacetic acid. Concentration of the pollutant at the photocatalyst could provide the mechanism for the enhanced mineralization of pchlorophenoxyacetic acid by the modified catalysts. The photocatalytic oxidation involves complicated multistage processes. The identification of different intermediates is dependent on the stability of the intermediate itself as some of the species undergo further fast oxidation. As a result, observation and identification of trace quantities of intermediates becomes a challenge. In the present work in the case of lanthanide oxide-doped TiO2 catalysts, p-chlorophenol was detected as the only intermediate. This is in contrast to previous studies (60, 61) where other intermediate aromatic products such as hydroquinone and chloroquinone have been detected. The GC-MS results indicate that the plausible mechanism for the degradation involves the oxidative decarboxylation of the aliphatic chain. The mechanism of photooxidative decarboxylation was originally suggested for aliphatic acids by Yoneyama et al. (62) as described below:
R-OCH2COO- + h+ f R-OCH2• + CO2
(5)
R-OCH2• + O2 f R-OCH2OO• f R-OC(O)H + •OH (6) R-OC(O)H + H2O f R-OH + HCOO- + H+
(7)
In case of the nonmodified TiO2 catalyst, several intermediates (hydroquinone, chlorophenol, phenol, etc.) were observed but their concentrations were low to be quantitatively
estimated. The results clearly indicate the incomplete mineralization of p-chlorophenoxyacetic acid by the nonmodified TiO2 catalyst. This indicates that, in the case of the nonmodified TiO2 catalyst, several reaction pathways may be operative such as oxidative hydroxylation, oxidative decarboxylation, dehalogenation, etc. The details of the mechanism are however not clear at the moment. To conclude, we have designed a novel class of photocatalysts for the degradation of p-chlorophenoxyacetic acid. The photocatalysts consist of lanthanide oxide/TiO2 composites. The present study has demonstrated that europium, praseodymium, and ytterbium oxide-doped TiO2 exhibit significantly higher activity as compared to the nonmodified TiO2 catalyst. The enhanced degradation by the lanthanide oxide-doped TiO2 is general, and other organic compounds containing carboxylic acid groups such as salicylic acid, transcinnamic acid, p-nitrobenzoic acid, and amine-containing compounds such as aniline are completely mineralized (28). Since the photodegradation involves oxidative mineralization, the intermediates are bound to the lanthanide ions, and their surface degradation prevents their appearance in solution. The loading of the lanthanide ions is also an important parameter. High loading of the TiO2 photocatalyst decreases the photocatalytic activity. The present study has shown the usefulness of doping TiO2 catalyst by lanthanide oxides to achieve enhanced degradation of organic substrates. Photocatalytic detoxification has been demonstrated as an alternative method for the cleanup of organic substrates.
Acknowledgments This research is supported by the Israel Ministry of Science (MOS), Israel, and the Bundesministerium fu ¨ r Bilding, Wissenschaft, Forschung und Technologie (BMBF), Germany.
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Received for review August 22, 2000. Revised manuscript received January 9, 2001. Accepted January 24, 2001. ES001613E
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