Environ. Sci. Technol. 2004, 38, 2198-2208
Environmental Remediation by an Integrated Microwave/UV Illumination Technique. 8. Fate of Carboxylic Acids, Aldehydes, Alkoxycarbonyl and Phenolic Substrates in a Microwave Radiation Field in the Presence of TiO2 Particles under UV Irradiation SATOSHI HORIKOSHI, FUKUYO HOJO, AND HISAO HIDAKA* Frontier Research Center for the Global Environment Science, Meisei University, 2-1-1 Hodokubo, Hino, Tokyo 191-8506, Japan NICK SERPONE* Department of Chemistry and Biochemistry, Concordia University, 7141 Sherbrooke Street West, Montre´al, Quebec, Canada H4B-1R6, and Dipartimento di Chimica Organica, Universita di Pavia, Via Taramelli 10, Pavia 27100, Italy
Thermal and nonthermal effects originating when a system is subjected to a microwave radiation field in the TiO2photocatalyzed transformation of model substances containing various functional groups (e.g., benzoic acid, phthalic acid, o-formylbenzoic acid, phthalaldehyde, succinic acid, dimethyl phthalate, diethyl phthalate, and phenol) have been examined under simultaneous irradiation by ultraviolet (UV) and microwave (MW) radiations. Characteristics of the microwave effects and the fate of each substrate during the microwave-assisted photocatalytic process were monitored by UV absorption spectroscopy, HPLC methods, total organic carbon assays, and identification of intermediates using electrospray mass spectral techniques. Microwave thermal and nonthermal effects were delineated by comparing results from MW-generated internal heat versus conventional external heating, and at constant ambient temperature under a microwave field. Factors involved in the nonthermal component of the microwave radiation were inferred for the initial adsorption of the substrate and its subsequent degradation occurring on the surface of TiO2 particles. Microwave effects bear on the mechanism through which a model substrate undergoes oxidative degradation. A characteristic feature of these effects was briefly examined by considering the behavior of polar (dipole moments) substrates in a microwave radiation field.
1. Introduction The oxidative degradation of organic compounds occurring on a UV-irradiated metal oxide photocatalyst, a methodology * Corresponding author phone: +81-42-591-6635; fax: +81-42599-7785; e-mail:
[email protected] (H.H.), serpone@ vax2.concordia.ca (N.S.). 2198
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that is a pillar of advanced oxidation technologies (AOTs), has been examined extensively as evidenced in several excellent reviews (1-5). Practical applications of the photocatalytic process in the remediation of polluted air are far more advanced than is otherwise the case in the treatment of wastewaters. Microwave (MW) radiation is widely used domestically (cooking), and industrially in heating (e.g., rubber), in drying devices, and in communication technologies (LAN), among others. Microwave technology also finds applications in combinatorial chemistry (6, 7). There is growing interest in using microwave radiation to drive or otherwise assist chemical reactions. Various types of organic and inorganic reactions, once performed using classical heating methods (e.g., oil bath, heating mantle), are now routinely performed using microwave radiation (8-11). Giguere and co-workers (12) and Gedye et al. (13) were among the first to report microwave-assisted organic syntheses. Microwave-induced catalyzed reactions with metal and metal oxide catalysts have been described for syntheses and for various redox processes (14-23). Most of these studies used microwave radiation as a simple heat source. The domestic microwave oven (frequency 2.45 GHz) is an inexpensive apparatus relative to more classical heating sources. Recent papers demonstrated improvement of the degradative efficiency by coupling microwave radiation to the photocatalytic degradation of various substrates, e.g., rhodamine-B dye (24a-e), bisphenol-A (24f), and 2,4-dichlorophenoxyacetic acid (24g). Some of the many features of the microwave equipment have been examined, and microwave radiation has been used to power a microwave plasma electrodeless UV lamp (24h,i). Coupling of two different free energy sources (MW and UV radiations) in environmental remediation presents an interesting and attractive approach. Most recently, we reported (24j) on the practicality and usefulness of a domestic microwave oven minimally modified to incorporate a Teflon batch reactor and an electrodeless double glass cylindrical plasma lamp (DGCPL) to photodegrade a variety of pollutants. Thermal and nonthermal effects govern microwaveassisted reactions when a system is subjected to a microwave radiation field (24e-g). In this regard, an additional number of •OH radicals are formed in aqueous TiO2 dispersions when such dispersions are subjected to simultaneous irradiation by UV light and microwave radiation (PD/MW) as evidenced by electron paramagnetic resonance (EPR) spctroscopy (24k). Also, the hydrophobicity of the TiO2 particle surface is increased under irradiation with microwaves and UV/vis light (24e). The increased number of •OH radicals and changes in the hydrophilic/hydrophobic characteristics were attributed to nonthermal effects of the microwave radiation. Features of nonthermal effects in chemical reactions have been investigated both theoretically and experimentally (8, 9, 2531). Our own interests in this area lie in assessing the influence of thermal and nonthermal effects in heterogeneous photocatalyzed reactions, and in determining what effect(s) microwave radiation has on the surface (and/or bulk) of a metal oxide solid. As part of our systematic studies of microwave-assisted heterogeneous photocatalysis, the present study focused on four strategies: (i) Examine the effects of thermal and nonthermal factors in the degradation of benzoic acid (benzene ring and a carboxylic acid function) by the characteristic differences of the mechanism, and suggest a model to describe the initial adsorption when microwave radiation is involved in the photocatalyzed degradation (see 10.1021/es034823a CCC: $27.50
2004 American Chemical Society Published on Web 03/03/2004
FIGURE 1. Experimental setup used for the UV and MW irradiation of the contents in the cylindrical reactor system employing a single-mode resonant cavity (A) and a multiple-mode applicator (B). also ref 24b). (ii) Examine thermal and nonthermal effects as they might influence the adsorption of methoxycarbonyl and ethoxycarbonyl substrates (dimethyl phthalate and diethyl phthalate) on the TiO2 surface. As a classified endocrine disruptor, it was important to investigate the decomposition of diethyl phthalate using the microwave-assisted photocatalytic technique. (iii) Probe further the thermal and nonthermal effects from the behavior of phthalic acid (two carboxylic groups), o-formylbenzoic acid (one carboxylic acid function and an aldehyde moiety), phthalaldehyde (two aldehyde functions), and phenol as model substrates. (iv) Probe and identify the effects of microwave radiation on polar substances with relatively high dipole moments when substrates and the metal oxide photocatalyst (TiO2) are subjected to a microwave radiation field.
2. Experimental Section 2.1. Chemical Reagents and Degradation Procedures. The photocatalyst was Degussa P-25 TiO2 (powder, specific surface area 53 m2 g-1 by the BET method, particle size 2030 nm by TEM microscopy, composition 83% anatase and 17% rutile by X-ray diffraction). High purity grade benzoic acid, phthalic acid, o-formylbenzoic acid, phthalaldehyde, dimethyl phthalate, diethyl phthalate, phenol, and succinic acid were supplied by Tokyo Kasei Co. Ltd. Continuous microwave irradiation was carried out with a Shikoku Keisoku ZMW-003 apparatus containing a microwave generator (2.45 GHz, maximal power 1.5 kW) fabricated by Shibaura Mechatronics Co., Ltd. (Figure 1, microwave cavity A). Other properties and details of the microwave system were reported previously (24b,f). The microwave power radiated from a magnetron was ca. 220 W as monitored by a power monitor. Unless noted otherwise, the UV irradiation source was a Toshiba 75 W mercury lamp (irradiance ca. 0.3-0.4 mW cm-2, wavelength range 310-400 nm, maximal emission λ ) 360 nm). An aqueous sample of the substrate (0.1 mM, 30 mL) was placed in a 250 mL Pyrex cylindrical reactor (L ) 45 × 290 mm, Taiatsu Techno Co., maximum pressure 1 MPa,
temperature 150 °C) together with TiO2 particles (loading 60 mg), after which the stirred dispersion was irradiated by microwave and UV light radiation. The aqueous dispersion was supersonicated for ca. 30 s under air-equilibrated conditions prior to irradiation. The microwave reactor was sealed with two Teflon rings and a stainless steel cap. Microwave irradiation was carried out on the right side of the reactor through a waveguide. The UV source was located on the backside of the reactor device such that UV and microwave radiations crossed at 90° at the center of the dispersion. Under our conditions, the 250 mL Pyrex cylindrical reactor used absorbed ca. 12% of the total microwave energy, as estimated from the difference between the incident energy and the reflected energy measured by the power monitor (Figure 1). Absorption of microwaves by the waveguide and other components was also taken into account. Induction heating is caused by the absorption of the microwaves by the Pyrex glass reactor. However, since dielectric heating of water occurs efficiently, by comparison there was almost no effect of conventional heating by the glass. The rate of increase of temperature for water was greater than that of the Pyrex glass under otherwise similar microwave irradiation conditions. 2.2. Experimental Techniques. Oxidation of the substrates was carried out by four different routes: (i) TiO2 photocatalytic degradation with coupled UV light and microwave radiation (PD/MW), (ii) TiO2 photocatalytic degradation with UV light (PD), (iii) microwave radiation in the absence of the TiO2 catalyst (MW), and (iv) TiO2 photocatalytic degradation with UV light and conventional external heating (PD/TH). For the thermal and the TiO2-photocatalyzed reaction, the cylindrical reactor was coated with a metal thin film (contained two electrodes) on one side at the bottom of the reactor to provide conventional heating (voltage applied PD/TH (40%) > PD (37%) > MW ( HCOOH; for the PD/TH route the order was CH3COOH > HCOOH. 3.3. Ring Opening Influenced by MW Nonthermal Effects. To assess the influence of MW nonthermal effects on cleavage of the benzoic acid ring, the microwave-assisted photocatalyzed degradation was carried out under constant ambient temperature in a microwave oven applicator (Figure 1, microwave cavity B). In this way, nonthermal microwave effects could be discerned from differences between the PD and PD/MW routes since the thermal MW component was suppressed at a constant temperature of 25 ( 1 °C. The temporal decrease of UV absorption by benzoic acid through the PD/MW25, PD, and MW25 routes was monitored spectroscopically, the results from which are summarized in Table 1. Note that the experiment was conducted by cooling the whole reaction system. Microwave irradiation alone was uneventful at 25 ( 1 °C. By contrast, both the PD and PD/MW routes showed significant changes in the concentration of benzoic acid, with the latter route slightly more so, at least during the first 5 min (wavelength 198 nm). Otherwise, the two routes showed similar extents of degradation. Despite the rather small magnitude of the microwave effect, non-
SCHEME 1. Proposed Mechanism for the Degradation of Benzoic Acid by the PD and PD/MW Routes
TABLE 1. Summary of Loss of UV Absorption during Irradiation of Benzoic Acid for 1, 5, 10, and 15 min by the PD and PD/MW25 Routes at Constant Ambient Temperature (25 °C) time (min)
loss yield (%) 198 nm 226 nm
time (min)
loss yield (%) 198 nm 226 nm
MW25 1 5
dimethyl phthalate > diethyl phthalate > benzoic acid > phthalaldehyde ≈ phenol. Unfortunately, the accelerating effect in the decomposition of these substrates does not find a simple correlation with the dipole moments because the reaction occurs in a heterogeneous system of water and TiO2 particles wherein there exist many competing events, not least of which are the several kinds of intermediates generated during the degradation of the substrates. Accordingly, the overall effect on the reactive system is bound to vary frequently and periodically. The principles underlying the characteristic heating of microwave radiation are dipole orientation (dipole rotation) and ionic conduction. For the former, the orientation of the dipoles changes with the alternating change(s) of the directions of the microwave radiation field. In the case of polarization of a highly dielectric substance such as a polar water molecule, the dipole is considered to float in a viscous fluid and is forced to align with the electric field (Scheme 3a). However, when the dipole is subjected to a highfrequency alternating electric field of the microwave radiation, rotation (reversing) of the dipole cannot adequately follow the rate of change of direction of the electric field. This leads to a time delay, causing a substantial quantity of energy to be spent that turns into heat. In dielectric substances, further generation of heat by microwave irradiation is most pronounced for substances having large dielectric losses, which depend on the dissipation factor, r tan δ (where r reflects the dielectric constant and δ is the dielectric loss angle). In the present context and under our experimental conditions, the polar substrate(s) and water that take part in the degradation process undergo dipole rotation (i.e., orientation polarization). However, whether rotation of the substrate occurs through aligning its dipole with the field at the underlying microwave frequency in water (2.45 GHz) and the dissipation factor is unknown. Accordingly, whether the substrate is heated internally also remains unknown (Scheme 3b). For the TiO2 particles, ionic conduction can occur because of the positively charged surface (see Scheme 3c). However, the TiO2 particles cannot undergo the rapid motion necessitated by the high-frequency microwave radiation field. It is not unlikely, however, that ionic conduction on TiO2 particles causes some peculiar diffusion and aggregation of TiO2 particles, which for the PD route changes the size of the effective reaction area on the TiO2 surface. Adsorption and desorption events of the substrates and water on TiO2 particles (total surface area 3.18 m2 taken up
mostly by H2O) are governed by simple Coulombic forces and by nonthermal effects of the microwave radiation field. It is tempting to speculate that the cause for diffusion of the substrates is partially due to formation of micro/nanoscale hot spots on the TiO2 surface (39), although we were unable to measure such spots by thermographic and IR thermometric methods. It is possible that the temperature of the microwaveirradiated heterogeneous medium may not be uniform. Moreover, the plasma in water may depend on the nature of the interface between water and the TiO2 surface (40). In any event, nonthermal effects are expected to play a role in surface reactions occurring on TiO2 particles, the extent of which varies with the nature of the substrate.
Acknowledgments We greatly appreciate financial support from the Frontier Research Promotion Foundation (to H.H.) of the Japanese Ministry of Education, Culture, Sports, Science and Technology, the Natural Sciences and Engineering Research Council of Canada (to N.S.), and the Ministero dell’Universita, Istruzione e Ricerca (MUIRsRoma), via its program “Rientro dei Cervelli” (to N.S.). We are grateful to N. Watanabe, T. Itokawa, A. Tokunaga, and A. Saitou for technical assistance in some of the experiments. We also express our sincere thanks to the personnel of Shikoku Keisoku Instrument Co. Ltd. and Shibaura Mechatronics Co., Ltd. For their technical support with the microwave device.
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Received for review July 24, 2003. Revised manuscript received December 21, 2003. Accepted December 26, 2003. ES034823A
