Environ. Sci. Technol. 2002, 36, 1357-1366
Environmental Remediation by an Integrated Microwave/ UV-Illumination Method. 1. Microwave-Assisted Degradation of Rhodamine-B Dye in Aqueous TiO2 Dispersions SATOSHI HORIKOSHI AND HISAO HIDAKA* Frontier Research Center for the Global Environment Protection, Meisei University, 2-1-1 Hodokubo, Hino, Tokyo 191-8506, Japan NICK SERPONE* Department of Chemistry and Biochemistry, Concordia University, 1455 de Maisonneuve Boulevard West, Montre´al (Quebec), Canada H3G 1M8
particularly attractive in environmental remediation. Accordingly, they necessitate further examination. Earlier, we examined the photocatalyzed degradation of hydrophobic organic pollutants (e.g., polymers), endocrine disruptors, and pesticides in aqueous TiO2 dispersions (2124). Thermal degradation of poly(vinyl chloride) (PVC) yielded toxic materials. By contrast, no toxic intermediates were produced during the photooxidation of PVC by the TiO2 photocatalytic method, at least as verified by AMES test (25, 26). Complete mineralization of PVC to Cl- ions and CO2 occurred, although longer UV-irradiation times were necessary for the solid/solid process. Enhanced photodegradation occurred on closer contact between the PVC polymer and the TiO2 particles in TiO2-embedded PVC films (26). The major focus of the current study is the degradation of organic pollutants, as exemplified by the rhodamine-B (RhB) dye, catalyzed by TiO2 semiconductor particles under both UV and microwave irradiation. The efficacy of degradation by these two irradiation methods was assessed by examining the relevant kinetics of degradation and identification of intermediates. Experimental optimization of the photodegradation by the integrated UV/microwave technique was also examined.
Experimental Section The photocatalytic decomposition of the cationic rhodamine-B (RhB) dye was examined in aqueous TiO2 dispersions using an integrated microwave/UV-illumination (PD/MW) method. This procedure proved to be superior in the degradation of the dye than the TiO2 photocatalytic degradative method alone. With few exceptions, the integrated PD/MW method also proved superior for other chemical systems. The greater efficacy of the PD/MW technique appears to be the result of the following two considerations: (i) there is enhanced formation of reactive oxygen species (•OH radicals), as attested to by DMPO spintrap ESR methods and their attack on the dye; and although speculative at this time, (ii) the activity of bulk water or the TiO2 particle surface is somehow affected by microwave radiation. The greater efficacy of the PD/MW degradation method was also observed at low concentrations of molecular oxygen and at low radiant excitance of the light source. A brief mechanistic description is given on the basis of results obtained on the two model compounds, (i) benzoic acid and (ii) pyronin-B dye.
Introduction Microwave radiation has found its place in several domestic, industrial, and medical applications. Interesting reports have appeared on the application of microwave radiation in such technologies as (1) organic syntheses, polymerization (1, 2) and dehydration (3-11) processes; (2) inorganic syntheses by ceramic calcination and solidification (12, 13); (3) environmental waste treatment (14); (4) safety and biological aspects (15-17); (5) analyses and extraction (18-20); and (6) food sterilization (19). The microwave technique may also be useful in decomposing nondegradable materials from wastes. Such heterogeneous processes as may occur in photocatalytic and microwave-assisted reactions at solid/ solid, solid/liquid, or solid/gas interfaces might indeed be * Corresponding authors e-mail:
[email protected] and
[email protected]. 10.1021/es010941r CCC: $22.00 Published on Web 02/16/2002
2002 American Chemical Society
Materials and Degradation Procedures. The TiO2 was Degussa P-25 (specific surface area, 53 m2/g by the BET method; particle size, 20-30 nm by TEM; composition, 83% anatase and 17% rutile by X-ray diffraction). Rhodamine-B and benzoic acid (high-purity grade) were supplied by Tokyo Kasei Co. Ltd. The pyronin-B (PyB) dye was from Aldrich (high-purity grade).
The source of microwave radiation was a Shikoku Keisoku ZMW-003 apparatus equipped with a Toshiba microwave generator (2.45 GHz, 1.5 kW). Unless noted otherwise, the UV-irradiation source was a Toshiba 75-Watt Hg lamp with a radiant excitance (27) of 0.3 mW cm-2 in the wavelength range 310-400 nm (maximal emission at λ ) 360 nm). A 30-mL aqueous RhB solution (0.05 mM, pH ∼ 5.5) in the presence of TiO2 particles (loading, 30 mg) contained in a 250-mL Pyrex cylindrical reactor (φ ) 45 × 290 mm; Taiatsu Techno Co.; maximal pressure, 1 MPa; temperature, 150 °C) was irradiated using both continuous microwave radiation and UV light under magnetic agitation. Unless otherwise noted, solutions and dispersions were air-equilibrated. The reactor was sealed by appropriate means, as illustrated in Figure 1. The reactor contents were microwave irradiated from the right side of the light source/microwave-generator setup (Figure 1a, front view). The UV source was located on the backside of the device (Figure 1b, side view). Both radiation sources were orthogonal to each other so that their radiation would cross at 90° at the center of the solution. When required, the RhB dispersion or solution was irradiated simultaneously by both radiation sources, as displayed in Figure 1 (lower part). Experimental Technique. RhB was degraded by employing the following four techniques: (1) an integrated microwave/photocatalytic method in the presence of TiO2 parVOL. 36, NO. 6, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Experimental setup for UV and microwave irradiation.
TiO2 particles alone under aerobic conditions was ca. 4.8% after 3 min of irradiation; initially at 25 °C, the TiO2 particles reached 60 °C after they were microwave-irradiated for 15 min. Note that TiO2 particles had been previously “dried” at 120 °C in an electric furnace for about 30 min. Nonetheless, this latter heat treatment does not remove the OH- terminal groups ubiquitously present on the surface. Analytical Procedures. Temporal concentration changes in RhB occurring during the degradative process were monitored with a JASCO UV/Vis/NIR model V-560V spectrophotometer. The decrease in total organic carbon (TOC) was determined using a Shimadzu TOC-5000A analyzer. The concentrations of NH4+ and NO3- ions were assayed with a JASCO liquid chromatograph (HPLC) equipped with a CD-5 conductivity detector and either a Y-521 cation column or an I-524 anion column. Formation of carboxylic acid intermediates was ascertained with a JASCO HPLC chromatograph using the analytical method involving the BTB coloring agent. The relative concentrations of •OH radicals formed were established relative to an external standard Mn2+ marker and analyzed using the DMPO spin-trap technique and a JEOL JES-TE200 ESR spectrometer. Reproducibility of the experimental results was (10% throughout, under the conditions used unless noted otherwise. Simulation of the absorption spectrum of RhB was accomplished with the ZINDO, version 6.3, software available in the CAChe system, version 4.2 (26).
Results and Discussions
FIGURE 2. Details of the experimental setup with both UV and microwave irradiation capabilities in the oven. ticulates (PD/MW); (2) the TiO2 photocatalytic method alone (PD); (3) microwave irradiation in the absence of TiO2 particulates (MW); and (4) a thermally assisted TiO2 photocatalytic method (PD/TH). The cylindrical reactor used for the thermal and TiO2 photocatalytic reaction was coated with a heating wire on one side at the bottom of the reactor; heating was established by an applied voltage of less than 100 V. The other side of the reactor was not coated so that UV irradiation could be used to photodegrade RhB. In all cases, temperature and pressure were adjusted at the same values as those used for the PD/MW method. The PD/TH technique, in which a heater was used to supply external heat, was compared to the PD/MW method, wherein the thermal energy was delivered by the microwave radiation (note that, unless specified otherwise, the microwave power was 300 W). Experiments were also carried out at constant temperature (25 °C) using a dry ice/hexane slush bath to avoid heating in the PD/MW method and in the TiO2 photocatalytic degradation (see Figure 2). From the difference between 45 mV of MW(incidence) and 0.5 mV of MW(reflection), we estimate that, in the PD/MW process, the aqueous RhB/TiO2 dispersion absorbed ca. 98.8% of the microwave radiation. The temperature reached 110 °C after 10 min, whereas the pressure was ca. 0.2 MPa after 90 min of irradiation. Absorption of microwave radiation by 1358
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Degradation of RhB by the PD/MW Method. The color of the RhB dye solution subjected to the PD/MW degradative method faded considerably more than witnessed with the other degradative methods (MW and UV) for 3 h of irradiation. No degradation occurred by microwave irradiation alone. Degradation of RhB by the PD/MW procedure was nearly complete in contrast to the photocatalytic (PD) method. The PD/TH method was also less effective. Direct UV-light irradiation and direct microwave irradiation of the RhB dye solution in the absence of TiO2 were inconsequential toward degrading the dye. The temporal changes in the spectral features of RhB solutions monitored at 197, 256, 354, and 550 nm (Figure 3a) indicate that microwave irradiation alone had no effect on discoloration of the RhB solution, even after 180 min. However, illumination of RhB/TiO2 dispersions, under otherwise identical conditions, led to about 68% of the RhB transformed after this time. With the integrated PD/MW method (Figure 3c), about 97% of the RhB solution was discolored under our conditions, with the solution displaying no spectral features after 180 min. As depicted in Figure 3d, the thermally assisted photodegradative method (PD/TH) exhibited only slight discoloration of the RhB solutions. Relevant first-order kinetics of the transformation of rhodamine-B at the four monitored wavelengths are listed in Table 1. These data show that discoloration by the PD/ MW method is about 3-fold faster than the PD technique alone and approximately 10-fold faster relative to the coupled PD/TH method. What is intriguing about the data of Table 1, however, is that the thermally assisted photocatalyzed degradation pathway for RhB was nearly 5-fold slower than the PD method alone. No doubt, carrying out the photocatalytic degradation at temperatures higher than ambient influences the complex redox events occurring at the particle/solution interface. The UV absorption spectrum of RhB was simulated using ZINDO calculations of spectral bands. The simulated spectrum reveals bands at 200, 277, 333, and 464 nm in reasonable accord with the experimental spectrum. The band maximum at 197 nm correlates with absorption by the 2-carboxylphenyl substituent, whereas the other absorption features correlate
FIGURE 4. Decrease of total organic carbon (TOC) in the decomposiiton of RhB solution (initial TOC concentration, 18.6 ppm) (a) by microwave irradiation (MWe photocatalyzed oxidation (PD) at TiO2 loadings of 30 and 60 mg (volume of dispersion, 30 mL); (b) thermally assisted photocatalyzed oxidation (PD/TH) with TiO2 and integrated microwave-assisted photocatalyzed (PD/MW) degradation at TiO2 loadings of 30 and 60 mg.
FIGURE 3. Temporal changes in the absorption spectral patterns in the degradation of RhB (0.05 mM in aqueous media) for irradiation times of 30, 60, 120, and 180 min. The radiant exitance of the light source was 0.3 mW cm-2. The temperature for the systems was 120 °C, except for the PD system for which the temperature was 25 °C.
TABLE 1. Summary of Rates of Discoloration of RhB Solutions during 180 min of Irradiation by the Four Methods Used degradation k197 nm k256 nm k354 nm k550 nm kaverage method (10-2 min-1) (10-2 min-1) (10-2 min-1) (10-2 min-1) (10-2 min-1) MW PD PD/MW PD/TH
0.54 1.20 0.11
0.70 1.83 0.16
0.57 1.41 0.15
0.72 2.44 0.14
0.63 ( 0.09 1.7 ( 0.6 0.14 ( 0.02
with the structural features of RhB in the vicinity of the nitrogen atoms and with the hetero-ring bonded to the 2-carboxylphenyl substituent. The ratios of the disappearance kinetics of RhB at the four spectral wavelengths following the PD/MW degradation method are k197 nm/k256 nm/k354 nm/ k550 nm ) 1/1.5/1.2/2.0. Clearly, the disappearance of the spectral feature at 197 nm is relatively less efficient than for the other three spectral bands. These ratios also infer that the events in the PD/MW degradative method are different from those in the photocatalyzed degradation (PD route) for RhB, inasmuch as the relevant ratios for the latter method are k197 nm/k256 nm/k354 nm/k550 nm ) 1/1.3/1.1/1.3. The data of Table 1 and the spectral patterns of Figure 3 inferred that the PD and the PD/MW methods were most promising. The enhanced decrease of the absorption features of RhB and the visual discoloration of RhB solutions (not shown) by the integrated PD/MW method show that this might be the more attractive degradation method among the others. Note that this integrated method is not compound specific. Rather,
the method appears to be generally applicable and useful, with a few exceptions as illustrated in Table 2, which summarizes the relative rates between the PD/MW method (referred in this table as “hybrid”) and the TiO2/UV method (or PD). Figure 4 illustrates the extent of RhB mineralization, as witnessed by the temporal decay of total organic carbon (TOC) by the four degradative methods examined. Also illustrated in Figure 4 are the effects of TiO2 loading (30 mg and 60 mg; volume, 30 mL) on the efficacy of the PD and the PD/MW methods. TOC was not influenced by the MW and the thermally assisted method (PD/TH), even after 3 h of irradiation. By contrast, the PD method alone caused decay of the TOC with the greater loading in TiO2 appearing more effective at longer times. Doubling the quantity of TiO2 particulates does not correlate with twice the rate of photodegradation by either the PD/MW method or the PD method. Nonetheless, the disappearance of TOC by the PD/MW method appears more significant than that by TiO2 photocatalysis alone. Any difference between the two TiO2 loadings for the PD/MW method was greatest after 2 h of irradiation. It is likely that, after this time, there was a greater contact and, thus, greater adsorption of RhB onto the metal-oxide particle surface, thereby enhancing the degradation. Note that the lower TiO2 loading at longer times was just as effective in degrading RhB. Formation of NH4+ and NO3- ions in the decomposition of RhB is illustrated in Figure 5. Again, the PD/MW method leads to more significant changes than either the PD or MW methods in converting the two nitrogen atoms in the RhB structure. The mineralization yields of the two nitrogen atoms, given as the sum of the yields of NH4+ and NO3- ions, respectively, after 3 h of irradiation are 77% (70% + 7%) for the PD/MW procedure, 12.8% (10% + 2.8%) for PD, and 8% (8% + 0%) for the MW method. The ratios of the rates of formation of these two ions (NH4+/NO3-) for the PD and PD/MW methods are 3.6 and 10, respectively, whereas the VOL. 36, NO. 6, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 2. Systems for Which MW Irradiation and UV/TiO2 (PD) Were Combined To Form the Hybrid Method PD/MWa
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TABLE 2 (Continued)
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TABLE 2 (Continued)
a
The PD/MW method proved generally useful with a few exceptions.
FIGURE 5. Temporal evolution of the formation of NH4+ and NO3ions in the decomposition of RhB (0.05 mM) using the PD, MW, and PD/MW methods; radiant exitance was 0.3 mW cm-2. ratios {(NH4+PD/MW/NH4+PD) and (NO3-PD/MW/NO3-PD)} that compare the two degradation methods are 7 for NH4+ and 2.5 for NO3- ions. The increased formation yield of NH4+ ions by the PD/MW method is significant by comparison with the photocatalytic degradation or MW irradiation alone (Figure 5a). Either or both of two possible causes may lead to this observation: (i) in combination, the PD/MW method likely generates additional defect sites on the TiO2 particles 1362
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that leads to a somewhat diminished electron/hole recombination and, thus, to a more efficient photocatalytic process; (ii) the combined method impinges on a mechanistic step that seems to favor a greater reduction of the nitrogen than do either the PD or MW methods. The extent of mineralization of RhB seems to depend on what event is used to monitor the degradation/discoloration of the dye solution, as evidenced from the decrease in absorption and loss of TOC. The yield ratios that compare the PD/MW and PD methods are, for nitrogen atoms, 6/1 (sum of yields of NH4+ and NO3- ions); from TOC decay, 2.3/1; and from UV absorption, 3.4/1. In other words, the conversion of the two nitrogen atoms using the PD/MW versus the PD method was more effective than either ring opening or mineralization of the carbon atoms. Typically, the degradation of quaternary ammonium compounds by the photocatalytic method (PD) in aqueous TiO2 dispersions tends to be slower than for compounds containing no nitrogens. However, as noted here, the PD/MW method seems to resolve this problem. Degradation of Benzoic Acid and Pyronin-B. A better understanding of the process and the mechanistic details in the degradation/discoloration of RhB solutions by the four methods used may be obtained by examining the degradation of two other compounds, namely, benzoic acid, a model of the 2-carboxylphenyl substituent in the RhB structure, and pyronin-B, which models the remaining structure of the RhB dye. Figure 6a illustrates the loss of TOC, and Figure 6b shows the temporal degradation of benzoic acaid monitored by the absorption spectral feature at 226 nm. The initial drop in TOC in the benzoic acid solution (0.1 mM) after 40 min of irradiation followed the order in efficacy PD/MW > PD ∼ PD/TH > MW. However, after a 2-h period, the relative order was PD/MW ∼ PD ∼ PD/TH > MW. The former three degradative methods are equally effective in degrading TOC. In accord with these TOC results, loss of benzoic acid as
FIGURE 6. (a) Temporal evolution of the decrease of TOC during the degradation of an aqueous solution of benzoic acid (0.1 mM; initial TOC concentration 9.5 ppm; sensitivity (0.1 ppm). (b) Temporal decrease of the concentration of benzoic acid (0.1 mM) monitored by the UV spectral band intensity at 226 nm. evidenced at 226 nm occurred with equal efficacy for the microwave-assisted (PD/MW) and thermally assisted (PD/ TH) photodegradation methods after 60 min of irradiation (Figure 6b). Ring opening in benzoic acid by the photocatalytic process was enhanced either by the external thermal source or by the internal microwave-generated heat. Microwave-generated heat also caused TOC to decay for benzoic acid, albeit to a very slight extent, after 2 h of irradiation, contrasting the observations on RhB which was thermally stable under microwave irradiation (compare Figure 6a with Figure 4a). The microwave thermal effect is compounded in the PD/MW method. Note that the two UV absorption spectral features of benzoic acid at 198 and 226 nm decreased concomitantly in intensity with time. Differences in the degradation and formation of different intermediates contribute to establishing the decomposition mechanism. The formation of various carboxylic acid intermediates is illustrated in Figure 7. Regardless of the degradation procedure, the quantity of formic acid produced was the same in the first 60 min of UV irradiation. At longer times, formation of formic acid followed the order PD > PD/MW > PD/TH. Acetic acid also formed in the degradation of RhB for which the order of formation was PD/MW > PD/ TH > PD after 90 min of irradiation. Note that the microwave method alone had no effect on the RhB solution. Succinic acid was also produced by the PD/MW and by the PD method alone, producing equal amounts in the first 60 min of irradiation. The MW method also produced a small quantity of this acid. By contrast, the PD/TH method was inconsequential with respect to formation of succinic acid (see Scheme 1). Acetic acid and formic acid are the precursors to the mineralization of benzoic acid to CO2 and may reflect the initial drop in TOC at early irradiation times with the PD/MW method. The temporal behavior of the UV-vis absorption spectrum of a pyronin-B solution (PyB, 0.1 mM) also subjected to the various degradation methods is shown in Figure 8. Under our conditons, we saw no effect on the degradation of PyB by the MW technique. However, decomposition of PyB by the PD method was more effective (Figure 8b) than by the PD/MW method (Figure 8c) for an irradiation period of 4 h. Influence of Experimental Factors in the Degradation of RhB. The discoloration/degradation of RhB was examined at two different radiant excitance (0.3 and 2 mW cm-2) to
FIGURE 7. Temporal evolution of the formation of carboxylic acid intermediates during the degradation of benzoic acid (0.1 mM).
SCHEME 1. Proposed Simplified Mechanism for Benzoic Acid Degradation by the Integrated PD/MW Degradation Method
assess the effect on the PD/MW and PD methods for equal TiO2 loadings (30 mg; 30 mL). Changes in the decay of TOC are depicted in Figure 9. Clearly, the degradation of RhB by the PD/MW, even at the smaller radiant excitance of 0.3 mW cm-2, was faster than occurred by the PD method at the higher radiant excitance of 2 mW cm-2. At this higher excitance, the extent of degradation was about 6- to 7-fold greater. Evidently, microwave radiation enhances the degradation and makes up for the poor transparency of the dispersion and the lower radiant excitance used. These results show promise of the PD/MW method toward possible use in wastewater treatment. The effect of purging the solution or dispersion with a reactive (oxygen, air) or an inert gas (nitrogen, helium) was also examined. For this purpose, the light source was a highpressure Hg lamp (250 W) with a radiant excitance of 20 mW cm-2. The temporal discoloration of RhB solutions was monitored by UV-vis spectroscopy and by the decay of TOC employing both the PD and the PD/MW methods. The pressure in the solution was reduced by about 0.7 MPa with a pump, after which the appropriate gas was introduced (0.7 MPa) to bring the system back to ambient pressure. This operation was repeated four times, following which the dispersion was subjected to microwave and UV irradiation. Disappearance of the spectral features in the UV-visible absorption spectrum of RhB after a 10-min irradiation period is illustrated in Figure 10. Recombination of photogenerated valence band holes with conduction band electrons after UV irradiation of TiO2 particles is known to compete with formation of reactive oxygen species (•OH and •OOH radicals); their formation is controlled by the quantity of VOL. 36, NO. 6, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 10. Absorption spectra taken after 10 min of irradiation, illustrating the influence of different added gases on the degradation of RhB (0.05 mM) (see text for experimental details).
FIGURE 8. Absorption spectral patterns at various irradiation times relevant to the decomposition of PyB (0.1 mM). Radiant exitance was 0.3 mW cm-2.
FIGURE 9. Temporal evolution of the decrease of TOC during the degradation of RhB solution (0.05 mM, 30 mL) at a radiant exitance of 0.3 and 2 mW cm-2. oxygen in solution. The spectral results demonstrate that color fading of the air-equilibrated RhB solution by the PD/ MW method is enhanced relative to the degradation of O2saturated RhB solutions by the PD method. In fact, the kinetics of decomposition of RhB by the PD method with O2 gas are nearly identical to those when the PD/MW was used but for which ambient air was replaced either by nitrogen or helium gas. The ratios of the loss of absorption at 554 nm for each of the methods used and at the conditions indicated are (PDair, 18.7%)/(PDO2, 25.0%)/(PD/MWN2, 23.6%)/(PD/MWHe, 27.6%)/(PD/MWair, 58.6%)/(PD/MWO2, 92.2%) ) 1/1.3/1.3/ 1.5/3.1/4.9. The discoloration was enhanced by a factor of 3.7 when microwave irradiation was integrated to the photocatalytic method under an oxygen atmosphere (ratio of PD/MW to PD). 1364
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FIGURE 11. Absorption spectra of RhB solutions (0.05 mM, 30 mL) after being subjected to degradation by the PD/MW method, illustrating the effect of microwave power output; TiO2 loading was 30 mg; radiant exitance was 2 mW cm-2; microwave power output was 300 and 225 W. The quantity of RhB dye left in solution after only 10 min of irradiation was 14.4 ppm for PDair, 13.9 ppm for PDO2, 13.5 ppm for PD/MWHe, 13.5 ppm for PD/MWN2, 11.7 ppm for PD/MWair, and 7.1 ppm for PD/MWO2. Thus, the decomposition ratio in terms of TOC left in solution relative to the PDair method was 1/1.1/1.2/1.2/1.6/2.7. Integrating microwave radiation to the PD method for an oxygen-saturated solution led to a 2.5-fold enhancement. The order of degradation between ring opening and TOC under the different gaseous conditions was identical, inferring that ring opening may be the dominant pathway for RhB to degrade to complete mineralization. The effect of microwave power on the degradation of RhB is illustrated in Figure 11. The temperature rise from ambient (25 °C) at power levels of 300 and 225 W was 138.2 and 74.9 °C, respectively, in the first 20 min of irradiation. At longer times, the maximal temperature was 148.6 and 80.9 °C,
respectively. The pressure in the closed reactor at 300 and 225 W was 0.32 and 0.05 MPa, respectively, after 25 min of irradiation. The increase in microwave power enhanced discoloration of RhB solutions, as witnessed by the relevant spectral changes in Figure 11. For example, the decrease in absorption band intensity at 554 nm was 90% at 300 W and 67% at 225 W. Interestingly, when the microwave power output was increased 1.3 times, the enhancement in color fading increased by the same factor of 1.3 {(300 W/225 W ) 1.3) ) (90%(554 nm) /67%(554 nm) ) 1.3)}. However, temperature and pressure gradients for the 300 and 225 W power do not correlate with the extent of color fading at 554 nm {(90%(554 nm)/67%(554 nm) ) 1.3) < (123 °C(148-25)/55 °C(148-25) ) 2.2) < (0.32 MPa/0.05 MPa ) 6.4)}. The extent of color fading by the PD method was 38%, significantly lower than by the PD/MW method. The decrease in TOC for each of the degradation methods was 26% for PD (300 W), 79% for PD/ MW (225 W), and 90% for PD/MW (300 W) after 2 h of irradiation. The decrease of TOC versus the increase in microwave output was relatively small {(300 W/225 W ) 1.3) > (90%TOC/70%TOC ) 1.1)}. The difference in correlation between the microwave power output and TOC or extent of color fading appears to be influenced by the degradation of intermediates. Microwave irradiation is relatively more effective in decomposing RhB than degrading the intermediates formed in the process. Titania particles absorb UV light of energy greater than the band gap of 3.2 eV to generate electron/hole pairs. The holes (h+) are subsequently trapped by surface HO- ions or H2O to yield •OH radicals (and H+). Molecular oxygen interacts with electrons (e-) in the conduction band to yield superoxide radical anions (O2•-), which combine with protons to yield •OOH radicals. Consequently, the photodegradation of RhB was expected to be governed by the concentration of these reactive oxygen species formed at the TiO2 particle surface and by the concentration of preadsorbed organic pollutant. Formation of •OH radicals in each of the degradation methods in aqueous media and in the absence of RhB was analyzed by the DMPO spin-trap ESR technique. The number of •OH radicals formed was determined in water (250 mL) (alone in the dark and under microwave irradiation (350 W)) and in a 250-mL aqueous TiO2 (250 mg) dispersion by the PD method and by the integrated PD/MW method using a Hg lamp UV illumination for 40 s (an external standard Mn2+ marker was used to normalize the intensities). The PD/MW method was more effective than either the PD or the MW methods. Indeed, about 20% more •OH radicals were generated by the PD/MW method than the PD method alone. Note that the number of •OH radicals formed in water by microwave iradiation was negligibly small. The rate of degradation of RhB by the MW method and by the external heat source (metal-wire-coated cylinder) was distinct, as shown in Figures 4 and 5. However, it was difficult to assess whether the microwave radiation or the microwavegenerated heat influenced the decomposition. Accordingly, experiments were designed to evaluate the microwaveirradiation method at a constant temperature. A highpressure Hg lamp was used that had a radiant excitance of 45 mW cm-2 when no cutoff filter (wavelength < 320 nm) was used; the oven-type microwave-irradiation device shown in the diagram of Figure 2 was employed. The temperature of the TiO2/RhB dispersion was maintained at 25 °C using a slush bath of dry ice and hexane; this bath did not absorb microwave radiation. The PD/MW and the MW methods were examined in these experiments. The PD method was also explored for comparison. The relevant decrease in absorption in the degradation of RhB by the PD/MW, the PD, and the MW methods at a constant 25 °C is summarized
FIGURE 12. Absorption spectral patterns following 15 min (top) and 35 min (bottom) of irradiation of RhB solution (0.05 mM) by the various methods indicated while maintaining the temperature constant at 25 °C using a slush bath of dry ice and hexane. in Figure 12. Microwave irradiation alone for 15 min had no effect on the spectrum of the RhB solution. Degradation of RhB by the PD method was somewhat more significant than by the PD/MW method. However, the order reversed when irradiation was extended to 35 min. Consequently, we infer that the heat released by microwave radiation had little effect, if any, on the degradation of RhB.
Acknowledgments We are grateful to Dr. S. Kato of the Research Institute for Solvothermal Technology at Takamatsu, Japan, and Shikoku Keisoku Co. Ltd. for several helpful discussions about the microwave technique. The research in Tokyo was supported by a grant from the Frontier Research Promotion Foundation of Japan (to H.H.), whereas the work in Montreal was sponsored by the Natural Sciences and Engineering Research Council of Canada (to N.S.). The authors also wish to thank T. Itokawa and F. Hojou for their technical assistance in some of the experiments.
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Received for review May 4, 2001. Revised manuscript received October 9, 2001. Accepted December 13, 2001. ES010941R
