Synergetic Effects of Ultraviolet and Microwave Radiation for

Jun 4, 2012 - The properties of TiO2 nanoparticles did not change before and after ... Wei Zhong , Ran Miao , Abdelhamid M. El-Sawy , Curtis J. Guild ...
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Synergetic Effects of Ultraviolet and Microwave Radiation for Enhanced Activity of TiO2 Nanoparticles in Degrading Organic Dyes Using a Continuous-Flow Reactor Homer C. Genuino, Dambar B. Hamal, You-Jun Fu, and Steven L. Suib* Department of Chemistry, University of Connecticut, 55 North Eagleville Road, Storrs, Connecticut 06269-3060, United States S Supporting Information *

ABSTRACT: A novel continuous-flow reactor was developed to investigate the synergetic effects of ultraviolet (UV) and microwave (MW) radiation on TiO2 nanoparticles for the enhancement of photodegradation of Direct Red-81 (DR-81) and Bromothymol Blue (BTB) dyes. The efficiency of the combined UV and MW radiation was higher than the sum of the isolated and corresponding thermal effects and directly proportional to the MW power. The % photodegradation of DR-81 after 105 min irradiation at ambient conditions was 40%, 68%, 72%, and 100% using UV/MW100W, UV/MW300W, UV/MW500W, and UV/MW700W methods, respectively. The % photodegradation of BTB under the same conditions was 58%, 78%, 82%, and 88%, respectively. High dissolved oxygen concentration increased DR-81 photodegradation, whereas ambient air conditions were optimum for BTB. The extent of photomineralization of both dyes was dependent on MW power. Degradation products showed that both dyes were successfully oxidized through different intermediate species. The properties of TiO2 nanoparticles did not change before and after reaction; however, the positive surface charge was reduced by as much as 14 mV. Accelerated rates of dye degradation on incorporation of MW to UV were attributed to the generation of more hydroxyl and superoxide anion radicals and an increase in hydrophobicity of TiO2. many organic contaminants including dyes.6−9 The fundamental principles of photocatalysis over UV-illuminated TiO2 are well established.10−12 In the photooxidation reaction with TiO2, photoexcitation with light of energy greater than the bandgap of TiO2 promotes an electron (e−) from the valence band (VB) to the conduction band (CB), and leaves an electronic vacancy or hole (h+) in the former. The highly oxidative h+ (E° = +2.7 V) may directly react with the surface-sorbed organic molecules13 or indirectly oxidize the organic species by •OH and superoxide anion radicals (•O2−), which are formed when O2 and photogenerated e−s react. An aqueous TiO2 dispersion absorbs nearly 99% of the MW radiation,1 which makes TiO2 an ideal candidate for utilization in the UV/MW systems. Specific effects of the MW radiation field on the TiO2-assisted photolysis are demonstrated by an increase in the number of surface-active species and in the dynamics of surface reactions.14 For instance, changes occur in the electronic charge of TiO2 particle surface and its affinity to the substrate under MW radiation.14 Integrated UV/MW reactors in batch configuration have been previously used as a laboratory-scale technology for AOPs

1. INTRODUCTION Photocatalytic reactions involving both ultraviolet (UV) and microwave (MW) radiation produce highly reactive electronically excited molecules caused by a different kind of reactivityenhancing stimulation.1−3 For this advanced oxidation process (AOP), the basic mechanism is the generation of reactive oxygen free radicals and the subsequent attack by these on the pollutant species (e.g., organic compounds). Hydroxyl radicals (•OH), for example, react rapidly and nonselectively with most organic compounds either by addition to a CC bond or by abstraction of an H atom.3,4 These processes usually result in a series of oxidative degradation reactions (partial oxidation) ultimately yielding less toxic mineralization products (CO2 and H2O).4 The effects of MW radiation are apparent only when coupled with the photocatalyst-assisted process occurring under UV light.1 These synergetic effects are mainly due to (1) enhanced formation of reactive oxygen species on the surface of the photocatalyst, (2) possible changes in the activity of bulk solvent (e.g., water), and (3) changes that take place at the surface of the photocatalyst originating from the absorption of MW radiation.1,2 The use of TiO 2 photocatalysts for environmental remediation has been of great interest due to its high efficiency, photochemical stability, nontoxic nature, and inexpensive cost.5 TiO2 has been successfully applied in the decomposition of © 2012 American Chemical Society

Received: April 25, 2012 Revised: June 4, 2012 Published: June 4, 2012 14040

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Figure 1. Schematic diagram of the continuous-flow UV/MW reactor design for dye degradation.

aquatic environment.20−22 Azo dyes are of environmental interest because ∼7 × 105 tons of them are produced in the world annually22 and ∼15% of the total world production of dyes is lost during textile dyeing, most of which are discharged in textile wastewater effluents unregulated.23 The use of DR-81 dye is therefore relevant for this study. BTB is a textile dye derivative often deployed as a pH indicator (Figure S1). BTB dye has been used as a model compound to study TiO2-assisted photodegradation treatment of wastewater contaminated from color dyeing of paper, plastic, and natural and artificial fibers.24,25 Thus, BTB is a useful probe molecule that is most likely not degraded by direct oxidation and thermal treatment, and can only be degraded via free-radical pathways. 26 Moreover, BTB does not show strong absorption of UV-A light (315−400 nm). Using the developed UV/MW method, we thoroughly investigated two important parameters that affect the rate of photocatalytic degradation of DR-81 and BTB dyesmicrowave power and dissolved oxygen concentration. UV−visible spectrophotometry, electrospray ionization/mass spectrometry, and total organic carbon assay techniques were utilized to study the nature and extent of dye degradation. The structure, morphology, and particle size of TiO2 nanoparticles before and after reaction were also examined. By monitoring the formation of •OH and •O2− under different conditions and the alteration of the surface charge on TiO2, we demonstrated new evidence of the synergetic effects of using UV and MW (nonthermal) radiation for enhanced dye-degrading activity of nanosized TiO2 in a continuous-flow reaction.

to enhance the photodegradation of organic pollutants in aqueous TiO2 dispersions.1−3 In a typical design, the reaction vessel containing the organic pollutant and TiO2 is placed inside the reactor and simultaneously illuminated with UV (and/or visible light) on one side and MW radiation on the other side. This arrangement has allowed investigation of the photodegradation rates and mechanisms of a variety of compounds including dyes,9 aldehydes,15 phenols,15−17 and carboxylic acids15,18 However, coupling MW radiation with heterogeneous photocatalysis in a simple continuous-flow design serves as an alternative option to prevent technical difficulties, including safety hazards, that can potentially be encountered in batch reactor designs.3,19 Other notable advantages of using a continuous-flow reactor include the following: (1) the temperature of the dispersion can easily be controlled since the reservoir is outside the reactor, thus, preventing solvent loss, and that reactions can be performed at temperatures below the boiling point of the solvent or under low-temperature conditions; (2) UV and MW radiation can only interact with the sample with greater absorption of incident radiation, and thus cannot influence the performance of analytical instruments attached to the vessel; (3) control of dissolved oxygen concentration, addition of oxidant/s, sampling, and catalyst recovery can conveniently be done; and last, (4) a larger volume of contaminated water can potentially be remediated continuously. Therefore, in the present work, a UV/MW continuous-flow configuration was developed for the degradation of Direct Red-81 (DR-81) and Bromothymol Blue (BTB) dyes as model pollutants. DR-81 is an azo dye which contains two azo bonds (−N N−) (Figure S1 in the Supporting Information). Azo dyes are among the most widely used synthetic dyes and usually become the major pollutants in textile wastewaters.20,21 Azo dyes are generally toxic and even mutagenic to living organisms in the

2. EXPERIMENTAL SECTION 2.1. Dyes and Photocatalyst. DR-81 (C29H19N5O8S2Na2, FW 675.6 g mol−1) and BTB (C27H28Br2O5S, FW 624.4 g mol−1) dyes were purchased from Sigma Chemical Co. (MO) 14041

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and Fisher Scientific Co. (NJ), respectively, and used as received. The initial concentrations of DR-81 and BTB dyes prepared from their stock solutions using distilled deionized water (DDW) were 1.5 × 10−5 and 3.2 × 10−5 M, respectively. Titanium dioxide (P-25, Degussa Aeroxide; anatase 84%, rutile 16%; specific surface area 50 (±15) m2 g−1; average primary particle size 21 nm)27 was used as the photocatalyst in all experiments. Previous study has shown that the number of • OH produced by using this commercial TiO2 in aqueous samples during UV/MW irradiation was 2-fold greater than for pure anatase and rutile TiO2 forms1 and, thus, generally the most photoactive. Spent TiO2 photocatalysts were recovered by centrifugation and dried in the oven at 80 °C overnight. Structural characterization of TiO2 was carried out by powder X-ray diffraction (XRD). The XRD patterns of TiO2 before and after reaction with dye by UV/MW method were recorded on a Scintag 2000 PDS diffractometer using Cu Kα radiation (1.5418 Å wavelength), a beam voltage of 45 kV, and a current of 40 mA. Transmission electron microscopy (TEM) studies of the same TiO2 samples were conducted on a JEOL JEM-2010 FasTEM operating at 200 kV. Each TiO2 sample was suspended in 2-propanol. A drop of the suspension was loaded onto a carbon-coated copper grid and allowed to dry. The zeta potentials (average of five runs, error bars at 95% confidence intervals) of aqueous TiO2 dispersions treated under MW and UV/MW radiation with and without dyes were also measured at 25 °C using a ZetaPlus zeta potential analyzer (Brookhaven Instruments Corp., NY). 2.2. Reactor Design and Catalytic Experiments. The experimental setup for the continuous-flow UV/MW method is depicted in Figure 1. In a typical experiment, 100 mg of TiO2 photocatalyst was dispersed in a 400 mL sample dye solution in a 600 mL glass beaker/reservoir by vigorous stirring for 15 min. The beaker was covered with an aluminum foil to minimize possible interferences by ambient light and CO2 from the atmosphere. Prior to irradiation, the resulting dispersion was circulated through the UV/MW reactor system for 30 min using a Minipuls3 Gilson M312 (Middleton, WI) peristaltic pump. The dispersion was stirred vigorously and continuously while being circulated at an optimum flow rate of 88 mL min−1 ensuring a steady-state condition. The dispersion was first introduced to the UV reactor through a quartz tube (37 cm (length) × 0.8 cm (internal diameter), volume 19 cm3), then to the MW reactor through another quartz tube (53 cm (length) × 0.8 cm (internal diameter), volume 27 cm3), and finally back to the same beaker. Because the MW energy is substantially lower than UV radiation and cannot disrupt the bonds of common organic molecules, we hypothesized that, in this new design, the UV radiation causes the photoinitiation for a chemical change to occur first, and the MW radiation then affects the course of the subsequent reactions. The reverse arrangement has not been evaluated, however. Low-pressure black-light (λ = 315−400 nm, ∼90% in the 350 nm range (Figure S2 in the Supporting Information), ∼24 W, photons of light ∼(1.5−5) × 1016 s−1 cm−3) fluorescent lamps were used as a constant source of UV radiation. Sixteen UV lamps were installed equally spaced on a Rayonet RPR-200 (Southern New England Ultraviolet Co., CT) photochemical chamber reactor. These types of lamps and photoreactors have been recently utilized to mimic the UV component of sunlight for pollutant degradation.28,29 Here, we assumed that there was no loss of irradiance as no water cooling jacket was installed around the quartz tubes. The continuous source of MW

Figure 2. Powder X-ray diffraction patterns and pictures of TiO2 photocatalysts (A) before and after UV/MW reaction with (B) Direct Red-81 dye and (C) Bromothymol Blue dye. The colors of TiO2 powders collected during the course of the reaction are due to the adsorbed dyes, which slightly lower the peak intensities.

radiation was an internally tunable, cylindrical single-mode cavity CMPR 250 microwave reactor operating at 2.45 GHz (Wavemat Processing System, MI). This MW reactor is capable of producing a continuous power up to 1250 W and designed for both solvent and dry-media reactions. The actual power settings used were ca. 100 W (e.g., UV/MW100W), 300, 500, and 700 W. A 5.0 mL sample dispersion was drawn from the beaker from 0 to 105 min in 15 min intervals. To eliminate thermal effects, the temperature of the bulk dispersion in the beaker was maintained at 25 (±2) °C using an ice bath. Conventional heating at a constant temperature of 60 °C was likewise performed using a hot plate. In a separate experiment, a UV/MW700W-treated TiO2 sample was used for the degradation of DR-81. To investigate the effects of dissolved oxygen (DO) concentration on dye degradation, ultrahigh pure O2 or N2 gas (Airgas Co., NH) was bubbled continuously into the beaker. The DO concentration and pH of dispersions were measured in situ using a YSI 550A (YSI Environmental Inc., OH) DO meter and an Oakton pH 510 Series (Oakton Instruments, OH) pH meter, respectively. The reactors were cleaned after each complete run by flowing dilute bleach solution through the tubings for 15 min followed by a 5 L DDW rinse. 2.3. Analysis of Reactants and Products. The dye solutions were separated from TiO2 by centrifugation of the dispersion at 8500 revolutions min−1 for at least 30 min using the AccuSpin 400 centrifuge (Fisher Scientific Co., NJ). The concentration of dyes was quantified from their absorbance values measured by a Shimadzu UV-2450 UV−visible spectrophotometer (Shimadzu Scientific Instruments, Tokyo, Japan). The decrease in absorption intensity at 510 nm for DR81 dye and 431 nm for BTB dye (Figure S3 in the Supporting Information) was monitored to calculate the concentration and determine the % photodegradation (C/Co × 100%, where C and Co are the final and initial (0 min) concentrations, respectively) (average of three trials, error bars at 95% confidence intervals). A series of external standard dye solutions were used for calibration (Figures S4 and S5). To measure the extent of dye photomineralization after 105 min of UV/MW irradiation, the total organic carbon (TOC) concentration was measured on the Apollo 9000 combustion 14042

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Figure 3. Representative low-resolution and high-resolution transmission electron microscopy images of TiO2 photocatalysts (A−C) before and after 105 min UV/MW reaction with (D−F) Direct Red-81 dye and (G−I) Bromothymol Blue dye.

of the TiO2 powders are also shown in Figure 2. P25 TiO2 was a mixture of anatase and rutile phases (Figure 2A). No significant change in the XRD patterns of TiO2 was observed after reaction with DR-81 (Figure 2B) and BTB (Figure 2C) dyes under UV/MW700W irradiation, except for slight decreases in peak intensities due to adsorbed dyes. The ratios of anatase to rutile intensities for fresh and spent catalysts were in good agreement, suggesting that no transformation of phase occurred after irradiation. This further indicates that the catalyst is stable toward irradiation because the bulk structure and phase composition were conserved. Furthermore, the TiO2 photocatalyst retained its uniform morphology and primary particle size (15−25 nm), as shown in the low-resolution and highresolution TEM images before (Figure 3A−C) and after reaction with DR-81 (Figure 3D−F) and BTB (Figure 3G−I) dyes under UV/MW700W irradiation. 3.2. Degradation Profiles for Methods Other than UV/ MW. Pure DR-81 and BTB aqueous solutions showed weak and strong absorptions in the UV and visible spectral regions, respectively (Figure S3 in the Supporting Information). For DR-81 dye, absorptions occurred at 275, 397, and 510 nm, whereas absorptions occurred at 279, 431, and 616 nm for BTB dye. The degradation profiles for DR-81 and BTB dyes are shown in Figure 4, A and B, respectively. Dark experiments under ambient air conditions showed that both dyes did not degrade with or without the photocatalyst. Microwave radiation alone (MW300W) was also ineffective in degrading dyes (Figure S7), suggesting that excitement of dye molecules was negligible.

TOC analyzer (Teledyne Tekmar, OH). The calibration and calibration control standard solutions were prepared from a 1000 mg L−1 organic carbon stock solution. A typical TOC calibration curve prepared is presented in Figure S6. An API 2000 mass spectrometer (Applied Biosystem/MDS SCIEX, CA) using electrospray ionization (ESI) source in negative mode was used to analyze the dye degradation products under different conditions. A 1:1 ratio of HPLC-grade acetonitrile to dye sample solution was used. The operating parameters used for this technique are summarized in Table S1 in the Supporting Information. Hydroxyl radicals were indirectly detected using a 1 mM terephthalic acid (TA) solution (adjusted to pH 7 using a saturated NaOH solution) as an indicator. Fluorescence spectra were collected on an Agilent Cary 50 Eclipse fluorescence spectrophotometer using a 1.0 cm quartz cuvette. The excitation wavelength was set at 320 nm, whereas the emission wavelength was in the range of 340−600 nm. Both the excitation and emission slit widths were set at 2.5 nm. The formation of superoxide anion radicals was also indirectly monitored using a 3 mM tetranitromethane (TNM) solution as an indicator. The TNM solutions were analyzed using a UV−visible spectrophotometer on a scanning mode (280−400 nm).

3. RESULTS 3.1. Characterization of TiO2 Nanoparticles. The XRD patterns and TEM images of fresh and spent TiO2 photocatalysts are presented in Figures 2 and 3, respectively. Pictures 14043

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coupled with TiO2-assisted UV irradiation for degrading DR-81 and BTB dyes. The same plots (Figure 4A, B) show how an increase in MW power enhanced the rate and % photodegradation of DR-81 and BTB dyes at 25 (±2) °C and ambient air conditions. For example, after 105 min irradiation using UV/MW100W, UV/MW300W, UV/MW500W, and UV/ MW700W methods, the average % degradations of DR-81 dye were 40%, 68%, 72%, and 100%, respectively (Figure 4A). Typical UV−visible spectra for DR-81 dye degradation are shown in Figure S8 in the Supporting Information. The corresponding k values obtained for DR-81 degradation also increased with increasing MW power (Table 1). For BTB dye, after 105 min irradiation using UV/MW100W, UV/MW300W, UV/MW500W, and UV/MW700W methods, the average % degradations were 58%, 78%, 82%, and 88%, respectively (Figure 4B). Typical UV−visible spectra for BTB dye degradation are shown in Figure S9. Similar to DR-81, the corresponding k values obtained for BTB degradation increased with increasing MW power (Table 1). Any variations in the % degradation observed between UV/heat and UV/MW methods were consequently due to some microwave nonthermal effects. 3.4. UV/MW Method: Effects of Dissolved Oxygen Concentration. The minimum MW power which showed a significant effect on the degradation of dyes was 300 W. Figure 5 presents the effects of O2 and N2 saturation on the degradation of dyes using the UV/MW300W method. The DO concentrations of dye dispersions saturated with O2 and N2, and at ambient air conditions, ranged from 38.4 to 39.2, 0.14 to 0.27, and 7.6 to 8.6 mg L−1, respectively, measured at 25 (±2) °C. The photodegradation of DR-81 dye was enhanced at a high DO concentrationfor example, the average % degradation after 105 min UV/MW300W irradiation under O2 saturation was 88%, whereas 68% and 41% were obtained under ambient air and N2 saturation, respectively (Figure 5A). The decreasing order of k values for these reactions was O2 saturation > ambient air > N2 saturation (Table 1). Relatively lower DO concentration was optimal for BTB dye degradation at longer irradiation times (≥60 min) (Figure 5B). The average % degradations after 105 min UV/MW300W irradiation under ambient air, O2 saturation, and N2 saturation were 88%, 66%, and 64%, respectively. The decreasing order of k values for these reactions was ambient air > N2 saturation > O2 saturation (Table 1). The same trends were expected to be achieved at higher MW power settings (500 and 700 W). 3.5. Total Organic Carbon. MW power influenced the extent of photomineralization of DR-81 and BTB dyes. Figure 6 shows that incorporation of MW100W to UV radiation accounted for ∼7% and ∼2% enhancement in the average (n = 3) % decrease in TOC concentration of DR-81 and BTB dyes, respectively. Moreover, the rate of photomineralization of DR-81 dye was always faster than the rate of photo-

Figure 4. Degradation profiles: effects of incident microwave power on the photodegradation of (A) Direct Red-81 dye and (B) Bromothymol Blue dye.

The temperature of the dispersions was expected to generally increase with continuous irradiation. Heat treatment (60 °C) for 105 min removed ∼10% of DR-81, whereas the BTB dye did not exhibit any decolorization at all (not shown). Further evidence of the insensitivity of BTB to heat treatment was observed from the larger difference in the average % degradation between UV/heat and UV/heat/TiO2 treatments in BTB (21% difference) than in DR-81 (8% difference) (Figure 4). A pseudo-first-order rate constant (k) value of 0.0039 (±0.0007) min−1 for DR-81 degradation using UV/heat treatment method was also higher than the k value obtained for BTB (0.0032 (±0.0008) min−1). As expected, faster photodegradation rates were observed by adding TiO2 photocatalysts in dye solutions. The k values for DR-81 and BTB dye degradation using UV/heat/TiO2 method were 0.0049 (±0.0007) and 0.0068 (±0.0008) min−1, respectively. 3.3. UV/MW Method: Effects of Microwave Power on Dye Degradation. As further illustrated in Figure 4, MW radiation was more effective than thermal treatment when

Table 1. Average Pseudo-First-Order Rate Constants for the Photodegradation of Direct Red-81 (DR-81) and Bromothymol Blue (BTB) Dyes: Effects of Microwave Power and Dissolved Oxygen Concentration k /min−1 (standard deviation) dye + TiO2 (ambient air) DR-81 BTB dye + TiO2 DR-81 BTB

UV/heat 0.0039 (±0.0007) 0.0032 (±0.0008)

MW

UV/MW100W

0.000016 (±1 × 10−6) 0.0054 (±0.0007) 0.000099 (±1 × 10−6) 0.0089 (±0.0008) O2 saturation (UV/MW300W) 0.0183 (±0.001) 0.0098 (±0.0003) 14044

UV/MW300W 0.0102 (±0.0003) 0.0148 (±0.0007)

UV/MW500W

UV/MW700W

0.0112 (±0.002) 0.0313 (±0.007) 0.0180 (±0.0006) 0.0216 (±0.0001) N2 saturation (UV/MW300W) 0.0057 (±0.0001) 0.0102 (±0.0005)

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Figure 5. Degradation profiles: effects of dissolved oxygen concentration on the photodegradation of (A) Direct Red-81 dye and (B) Bromothymol Blue dye using the UV/MW300W method.

S10 in the Supporting Information). MW300W alone did not produce any detectable •OH radicals under most experimental conditions (Figure S11). As expected, there were also no •OH radicals generated under dark conditions. (Figure S12). 3.7. Superoxide Anion Radicals. In addition to •OH, • O2− radicals were generated in TiO2 dispersions using the UV/MW method. In the presence of •O2−, tetranitromethane (TNM) or C(NO2)4 is easily reduced to form nitroform anion (C(NO2)3−), which has a strong absorption band centered at 350 nm.28 Figure 8 shows the variations in broad and strong absorption bands of C(NO2)3− formed when TNM solutions were treated with TiO2 and UV/MW300W radiation under ambient air (Figure 8A), O2 saturation (Figure 8B), and N2 saturation (Figure 8C). Interestingly, more •O2− radicals were initially generated under ambient air conditions than at high DO concentrations. For example, the absorption intensity when TNM was irradiated for 15 min under ambient air was about 3× larger than that observed under O2 saturation. Low levels of DO furnished minimal quantities of •O2−. Continuous UV/ MW 300W irradiation (>60 min) significantly decreased absorption intensity accompanied by a peak shift to a shorter wavelength with a maxima at ∼330 nm. 3.8. Surface Charge of TiO2 Dispersions. To further examine the effects of UV and MW radiation on TiO2, zeta potential (surface electrical charge) measurements were carried out to obtain information regarding the surface properties of TiO2 dispersion. TiO2 in pure water formed a stable colloidal dispersion with an average zeta potential of +27.5 mV (Figure 9). Zeta potential became less positive with increasing MW700W and UV/MW700W irradiation time under ambient air conditions. MW700W alone caused a 42% reduction in the zeta potential of TiO2 dispersion after 105 min irradiation. Using the UV/ MW700W method, the reduction of the zeta potential by MW700W was enhanced by ∼15%. The pH of the TiO2 dispersions changed slightly after irradiation (Table S2 in the Supporting Information). The pH of the dispersions ranged from 5.70 to 4.70, 5.85 to 4.45, and 5.25 to 4.84 with DR-81 dye, BTB dye, and TiO2 alone. In the presence of DR-81 dye, the zeta potential became negative then became positive again, which stabilized from 45 min (+12.15 mV) to 105 min (+11.22 mV) irradiation. In the presence of BTB dye, the zeta potential increased gradually from −13.13 to +12.53 mV after 105 min irradiation. 3.9. Product Analysis. The oxidized species formed during irradiation of DR-81 and BTB dyes using the UV/MW method

Figure 6. Effects of incident microwave power on the extent of photomineralization of Direct Red-81 and Bromothymol Blue dyes after 105 min irradiation using UV and different UV/MW methods.

mineralization of BTB dye. For example, the average % decrease in the TOC concentration of DR-81 dye after 105 min irradiation using UV/MW700W method was 43%, about 2× higher than what was observed for BTB dye under the same experimental conditions. Thus, the halogenated aromatic ringcontaining dye (BTB) is more resistant to photomineralization than the azo dye (DR-81). 3.6. Hydroxyl Radicals. More •OH radicals were generated in bulk TiO2 dispersions when MW radiation was coupled with UV photolysis. The •OH radicals were successfully detected indirectlythe emissions in the visible region with maxima at 427 nm were attributed to the intensely fluorescent product, monohydroxyterephthalate, formed by the reaction between terephthalic acid (TA) and photogenerated •OH (Figure 7).28,29 Emission intensities increased as more •OH radicals were generated with continued irradiation. The fluorescence spectra of TA in the presence of TiO2 dispersions, which were irradiated with UV alone under ambient air (Figure 7A) and UV/MW300W under ambient air (Figure 7B), O2 saturation (Figure 7C), and N2 saturation (Figure 7D), were different. Higher emission intensities (approximately by a factor of 2) were achieved at a high DO concentration than at ambient air conditions. Considerably low emission intensities (approximately by a factor of 3) were obtained for N2 saturated dispersions. At constant UV light intensity, the amount of •OH radicals produced increased with increasing MW power (Figure 14045

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Figure 7. Fluorescence spectra of 1 mM terephthalic acid solutions with TiO2 irradiated using (A) UV and UV/MW300W methods under (B) ambient air, (C) O2 saturation, and (D) N2 saturation, indicating the formation of hydroxyl (•OH) radicals.

The pure BTB dye solution had characteristic peaks at m/z = 127, 315, 463, 544, and 623 (Figure 10B (bottom)). Addition of TiO2 to pure BTB dye solution (0 min) did not show any significant decrease in concentration due to adsorption, in contrast with that observed for DR-81 dye. After 105 min reaction with TiO2 under UV/MW300W radiation and ambient air, the intensity of the peak at m/z = 623 decreased, forming a degradation product with characteristic peak at m/z = 329. The intensity of peak at m/z = 639 also significantly increased. Recall from Figure 6B that photodegradation of BTB dye was more favorable under ambient air conditions than at high DO concentration at longer reaction times (≥60 min). Selected ESI/MS spectra obtained during shorter irradiation times are shown in Figures S32−S35 in the Supporting Information. The peaks at m/z = 329 and 639 started to increase and the intensity of the peak at m/z = 623 started to decrease at shorter irradiation times at higher MW power settings (500 W (Figures S36−S43) and 700 W (Figures 10B (top) and Figures S44− S49)). Under N2 saturation, the peaks at m/z = 329 and 639 appeared gradually with continuous irradiation (Figures S50− S54), consistent with the spectrophotometric results, which showed that, even at low levels of DO, photodegradation still occurred (Figure 5B).

were qualitatively identified by ESI/MS experiments. The present study focused mostly on the main intermediates. In the spectrum from the pure DR-81 solution, three peaks at m/z = 652 ([M+Na-2H]2−), 630 ([M-H]−), and 314.5 ([M-2H]2−) belong to the parent molecule, whereas the peaks at m/z = 156, 260, 341, and 630 were different fragments from the parent caused by in-source collision-induced ionization (Figure 10A (bottom)) (intensities for pure DR-81 dye solution as shown were reduced by a factor of 10 for better plotting). After addition of TiO2 (0 min), no new peaks were observed, but the intensity of the signals decreased, suggesting that the concentration of the dye decreased due to strong adsorption on TiO2 (Figures 10A and Figures S13 and S14 in the Supporting Information). After 75 min reaction with TiO2 under UV/MW300W irradiation and ambient air, the peak at m/ z = 630 corresponding to the parent compound disappeared forming several peaks of known and unknown intermediates with characteristic peaks at m/z = 127, 171, 201, 215, 329, 347, 361, and 379. These peaks started to appear and the peak at m/ z = 630 started to disappear at shorter irradiation times using the UV/MW500W and UV/MW700W methods (Figure 10A (top) and Figures S15−S18). Peaks at m/z = 329 and 347 were related to a water loss and had similar fragmentation patterns. These results support the initial observation that the photodegradation of DR-81 dye was enhanced at high MW power (Figures 5 and 7). At low DO concentration (N2 saturation), the peak at m/z = 630 was still present and only the peak at m/ z = 329 appeared after 105 min of UV/MW300W irradiation (Figures S19−S24), suggesting slower rate of degradation. Under O2 saturation, the peak at m/z = 630 peak disappeared and an additional peak at m/z = 277 appeared after 105 min of UV/MW300W irradiation (Figures S25−S31), suggesting probable formation of a different oxidized species.

4. DISCUSSION On the basis of experimental results and previous work, we explain here the synergetic effects of UV and MW radiation for the enhanced degradation of DR-81 and BTB dyes involving (1) UV light activation and (2) MW power activation of TiO2 nanoparticles and interaction with dye molecules, as illustrated in Figure 11A. Scheme 1 suggests important steps in the degradation process. 14046

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Figure 8. UV−visible spectra of 3 mM tetranitromethane solutions with TiO2 irradiated using UV/MW300W method under (A) ambient air, (B) O2 saturation, and (C) N2 saturation, indicating the formation of superoxide anion (•O2−) radicals.

TiO2 and participate in the surface-mediated redox reaction. Moreover, the VB h+s have the ability to react with the surfaceadsorbed H2O or OH− to form highly reactive, nonselective • OH radicals that participate in the photooxidation of the organic dyes. On the other hand, the photogenerated e−s have the ability to reduce O2 to form highly reactive •O2− radicals, which in this study, seem to be primarily responsible for the degradation of BTB dye. Depending upon the structure and nature of the substrate molecules and the conditions of the reaction (ambient air and pure O2, and N2 atmospheres), either • OH or •O2− can play a dominant role for the photooxidation reaction. The photoreduction of TNM solution is altered significantly (increase or decrease of molar absorptivity) (Figure 8), which could be due to regeneration of O2 from the metal oxide catalyst with continuous irradiation.28 Absorption of light by the organic dyes produces an excited state, which can then react with the ground-state O2 and H2O. Excited dye molecules (free or adsorbed) are converted to smaller molecules or else mineralized to CO2 and H2O (Figure 6). The MW reactor passes nonionizing radiation at a frequency near 2.45 GHz through the TiO2 dispersion, causing dielectric heating primarily by absorption of MW energy in water. At first glance, the role of MW power activation in enhancing the rate of photodegradation is ascribed to the generation of more •OH

In TiO2-mediated photocatalyzed reactions, UV light (hν > 3.2 eV) excites e−s from the VB to the CB of the TiO2 photocatalyst, leaving behind an equal number of h+s in the VB. These photoinduced e−s and h+s migrate toward the surface of

Figure 9. Average zeta potential of TiO2 nanoparticle dispersions with and without Direct Red-81 and Bromothymol Blue dyes as a function of MW700W and UV/MW700W irradiation time. 14047

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Figure 11. (A) Generation of •OH and •O2− radicals by UV and MW power activation. (B) Variation in the surface charge of TiO2 showing (1) adsorption of anionic dyes on the positively charged TiO2 surface, (2) degradation of adsorbed dyes under UV/MW irradiation, leaving oxidation products and regeneration of positively charged TiO2 surface, and (3) lowering of electric charge of naked TiO2, which weakens the electrostatic interaction with dyes but promotes adsorption due to an increase in hydrophobicity of TiO2.

or near the surface that may increase directly the quantity of OH radicals.1,31 A model proposed by Booske et al. suggests that MW radiation can couple with low-frequency elastic lattice oscillations of the crystalline solid that creates a nonthermal distribution.1,32 Thus, MW power activation may help enhance the diffusion of charge carriers to the TiO2 surface and separation on the active sites. The surface of the TiO2 nanoparticles can be perturbed significantly by MW radiation.14,33 A synergetic effect is again observed because the change in the surface properties of TiO2 using UV/MW is greater than using MW alone. This effect is demonstrated in the reduction of the positive zeta potential of TiO2 dispersions upon irradiation (Figure 9). This effect could be due to the change in the relative quantities of photogenerated e−s and h+s at the surface of TiO2 during irradiation1,34 and the difference in dielectric properties between TiO2 nanoparticles and adsorbed species (Maxwell− Wagner interfacial polarization).35,36 After 105 min UV/MW irradiation, a positively charged TiO2 surface is retained; hence, in the presence of dye molecules, electrostatic interactions between the Ti−OH2+ surface and the negatively charged sulfonic (−SO3−) groups still occur (Figure 11B). A more negative zeta potential was observed for TiO2 dispersions with DR-81 dye than with BTB dye, suggesting that these dyes

Figure 10. Electrospray ionization/mass spectra in a negative ion mode recorded before and after UV/MW300W and UV/MW700W irradiation of (A) Direct Red-81 dye and (B) Bromothymol Blue dye.

•

radicals as compared with the UV light activation alone. At high power, MW radiation can thermally activate the dye molecules from the ground state to the excited state,3 can presumably help renew the catalytic sites on the surface of the TiO2 by ionic conduction, and can consequently increase the migration velocity and lifetime of the photoinduced charge carriers (e−/ h+ pairs). Recombination of photogenerated h+VB with e−CB after UV irradiation of TiO2 is known to compete with the formation of reactive oxygen species such as •OH radicals. Thus, incorporation of MW radiation in UV photolysis increases the lifetime of the photoinduced charge carriers; hence, producing more •OH radicals. The longer the lifetime of the photoinduced charge carriers, the greater the number of • OH radicals available for the degradation of the substrate molecules. Results in Figure 4 suggest that the increase in temperature alone due to UV/MW thermal effects does not account for the increase in •OH radicals, but more importantly, due to the nonthermal interactions between the MW radiation field and the TiO2 nanoparticle surface.1,18,30 Such interactions can give rise to the generation of additional localized defects on 14048

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Scheme 1. Degradation Processes Using (1) UV Light and (2) Microwave Radiation

adsorb on surfaces differently due to the presence of −SO3− substituents in the former. The zeta potential becomes positive (DR-81 > BTB) as irradiation continues, which strongly indicates the disappearance of dye molecules. An increase in the hydrophobic character of TiO2 surface may also promote photodegradation of dyes by the concomitant use of MW radiation.2,3,14,37 Therefore, although a lower positive surface charge on TiO2 can weaken the electrostatic interaction with the dye, a higher degree of hydrophobicity can facilitate the adsorption of dyes on pretreated TiO2 and consequently impact the degradation rate. This hypothesis was proven when we found that DR-81 dye was also completely degraded after 105 min UV/MW700W irradiation using the pretreated TiO2 with relatively less positive zeta potential in water than the fresh TiO2 nanoparticles. Another interesting result showcasing synergetic effect was observed when the use of UV/MW method resulted in a greater loss of TOC than using UV alone (Figure 6). Since the UV light source has a constant emission of photons, any MW effect that arises from using UV/MW method at varying MW power is responsible for the reduction of TOC concentration. The efficiency of the dye degradation reactions using TiO2 and UV/MW method is directly proportional to the incident MW

power. Previous studies on phenol showed that an increase in MW power increases the number of •OH radicals, which also increase the efficiency of photooxidation.17,38 Accelerated degradation rates can also be attributed to the uniform and greater absorption of incident UV and MW radiation by the entire TiO2 dispersion flowing through the quartz tubes illuminated at all possible angles (360°), as opposed to radiation on only a small portion of the dispersion (conventional batch reactor). The present design therefore eliminates the issues of unequal incident radiation and degree of light penetration. The presence of TiO2 and UV/MW radiation under pure O2 or ambient air atmospheres are necessary for the generation of a substantial number of •OH radicals (Figure 7). Formation of • O2− is more favored in the presence of ambient air than excess O2 (Figure 8). Ambient O2 can act as a scavenger of the photogenerated e−s to yield more •O2−.1,39 The % degradation of DR-81 dye is lower in the absence of enough O2 and this can be attributed to the recombination of e−/h+ pairs.39 Previously, the photodegradation of 4-nitrophenol in TiO2 dispersions was found to decrease remarkably at low DO concentration.40 Recall that, for DR-81 dye, O2 saturation enhances photodegradation; whereas for the BTB dye, ambient air conditions 14049

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favor photodegradation (Figure 5). This difference in reactivity between two dyes can be attributed to the following factors: (1) different chemical nature of dyes39 (azo dye versus halogenated dye), (2) characteristic features of the degradation and adsorption of dyes on the TiO2 nanoparticle surface related to structure, and (3) reactivity of oxygen radicals toward dyes formed under certain conditions. In this study, the rates of DR81 and BTB photodegradation are correlated with the rates of • OH and •O2− formation, respectively. N2 saturation suppresses the •OH formation (Figure 7D). The concentrations of •OH and •O2− in aqueous TiO2 dispersions are relatively low at very low DO concentration. Despite these general observations, the MW nonthermal distribution might enhance the rate of oxygen transference and diminishes the need of DO.41 This is not observed in conventional photocatalysis, wherein the rate of oxygen transference is so slow that a high DO concentration in TiO2 dispersion is necessary for sufficient oxygen adsorption on the surface of the photocatalyst. Horikoshi et al. found that under low DO conditions, the photodegradation rate in the presence of MW irradiation is higher than that in the absence of MW radiation.9,38 This inhibition effect is mainly attributed to the scavenging character of anions on the •OH during the photocatalysis.42 Therefore, even under pure N2 atmosphere, • OH radicals are still generated (Figure 7D) due to the surface defects that could prohibit recombination;1,42 hence, photodegradation of DR-81 and BTB dyes still occurs (Figure 6). In the ESI mass spectrometry experiments, the disappearance of the peak at m/z = 630 ([M−H]−), 652 ([M+Na−2H]−), and 314.5 ([M−2H]2‑) for DR-81 dye and the decrease in the intensity of peak at m/z = 623 (or 624) ([M−H]−) for BTB dye strongly suggest photodegradation. For DR-81 dye, the formation of new peaks with characteristic m/z < 630 suggests that smaller fragments of dyes are formed after the reaction (Figure 12A). These intermediates and/or products are presumably the result of the removal of the azo group and the subsequent attack of •OH on the benzene rings. The azo groups of DR-81 dye are susceptible to photodegradation,4,40 which makes DR-81 easy to degrade, resulting in the discoloration of dye solution. For BTB dye, the intensity of the peaks at m/z = 329 and 639 increased, presumably due to cleavage of the C−O bond on the central C resulting in three bromine-substituted benzene groups and further oxidation of three −CH3 on each of these groups by •O2− to −COO− or an initial attack by •O2− on the central C followed by dehydration (Figure 12B). Clearly, further characterizations are needed (for example, by high performance liquid chromatography−mass spectrometry and nuclear magnetic resonance techniques) to be able to confirm these structures, quantify the products, and understand the mechanisms of degradation using the UV/MW method presented here.

Figure 12. Proposed structures of some intermediates and/or degradation products of (A) Direct Red-81 dye and (B) Bromothymol Blue dye after UV/MW700W irradiation based on the ESI/MS experiments.

degradation of DR-81 and BTB dyes, respectively, especially at longer irradiation time. The necessity to optimize these two factors has been discussed. TOC assays showed that photomineralization increases with increasing incident MW power and DR-81 dye mineralized faster than BTB dye. The use of TiO2 and MW in UV photolysis resulted in the generation of more oxygen radicals (•OH and •O2−), thereby increasing the rates of photooxidation. The rates of DR-81 and BTB dye degradation can be correlated to •OH and •O2− formation, respectively. MW activation enhanced the activity of TiO2 when coupled with UV light photoactivation. For example, MW radiation changed the surface electric charge (zeta potential) and the hydrophobicity of TiO2, both of which have consequences on the adsorption or reactivity and degradation of dyes. Results of ESI/MS experiments at different conditions shed light on the nature of dye photocatalytic oxidation. The ensemble of these results suggests that the developed method has potential application for the purification of a large volume of water contaminated with a wide range of pollutants (e.g., wastewater). In the future, particular interest should be given to the removal of dye mixtures to realistically model water contaminants and to evaluate the possible

5. CONCLUSION A continuous-flow UV/MW irradiation method has been developed and utilized for the enhanced photodegradation of DR-81 and BTB dyes in TiO2 nanoparticle dispersions in water. This method proved to be more effective in degrading both dyes than using UV alone, MW alone, and UV/heat methods. Spectrophotometric monitoring of the temporal decrease in the concentration of the dyes showed that photodegradation is directly proportional to incident MW power. High DO concentration and ambient air conditions favored the photo14050

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inhibitory effects in the photodegradation studies that are presented here.

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ASSOCIATED CONTENT

S Supporting Information *

Supporting tables (Table S1−S2) and figures (Figures S1−S54) are provided. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: (+1) 860-486-2797. Fax: (+1) 860-486-2981. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for having received research support from the U.S. Department of Energy (DE-FG02-86ER13622.A000), Office of Basic Energy Sciences, Division of Chemical, Biological and Geosciences, and the Center for Environmental Sciences and Engineering (CESE), University of Connecticut. We also thank Dr. Lichun Zhang for collecting TEM data, and Prof. Joseph Bushey, Prof. Christian Brückner, and Prof. Challa Kumar for helping and allowing us to use their instruments. The guidance, insightful comments, and suggestions of Prof. Joseph Bushey, Dr. Francis Galasso, and Dr. Stephen Hay are likewise appreciated.

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REFERENCES

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