Magnetically Agitated Photocatalytic Reactor for Photocatalytic

Sep 21, 2005 - (4) Mazyck, D. W.; Drwiega, J.; Lee, S.-W.; Wu, C.-Y.; Sigmund, W.;. Chadik, P.; Park, J.-H.; Meisel, M. W. Development and characteriz...
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Environ. Sci. Technol. 2005, 39, 8052-8056

Magnetically Agitated Photocatalytic Reactor for Photocatalytic Oxidation of Aqueous Phase Organic Pollutants W I L L I A M L . K O S T E D T I V , * ,† JACK DRWIEGA,‡ DAVID W. MAZYCK,† SEUNG-WOO LEE,§ WOLFGANG SIGMUND,§ CHANG-YU WU,† AND PAUL CHADIK† Department of Environmental Engineering Sciences, University of Florida, 321 A. P. Black Hall, P.O. Box 116450, Gainesville, Florida 32611-6450, Jones Edmunds and Associates, 324 South Hyde Park Avenue, Suite 250, Tampa, Florida 33606-4110, and Department of Materials Science and Engineering, University of Florida, 225 Rhines Hall, P.O. Box 116400, Gainesville, Florida 32611-6400

A magnetically agitated photocatalytic reactor (MAPR) has been developed and assessed for oxidation of phenol. The MAPR uses a titanium dioxide composite photocatalyst with a ferromagnetic barium ferrite core. The catalyst motion was controlled with a dual-component magnetic field. First, a permanent magnet above the reactor provided a static magnetic field to counteract the force of gravity, hence increasing catalyst exposure to UV. Second, an alternating magnetic field generated by a solenoid was used to agitate the catalyst, thus increasing mass transfer between pollutants and byproducts to the catalyst. Optimal performance of the MAPR was achieved with the permanent magnet present and 1 A of alternating current to the solenoid between 20 and 80 Hz. Operating with a 60Hz signal at 1 A with the permanent magnet present and 100 mg of catalyst, the system reduced an 11 mg/L phenol concentration by 97% and decreased nonpurgeable dissolved organic carbon by 93% in 7 h using three 8-W 365-nm peak UV lamps.

Introduction Photocatalysis offers an advanced technology for the elimination of toxic organic compounds from water. Many of the current technologies simply transfer the pollutant out of the water or from one phase to another, requiring additional treatment and/or disposal. In photocatalysis, the organic contaminants are oxidized, ultimately to carbon dioxide and water, leaving no waste to dispose of. Water and hydroxide ions react with the electron holes to form hydroxyl radicals, proven the primary oxidant in the photocatalytic oxidation of organics (1). These hydroxyl radicals have an oxidation potential higher than that of ozone or hydrogen peroxide, second only to fluorine (2). Repeated hydroxyl radical attack * Corresponding author phone: (352)294-0083; fax: (352)392-3076; e-mail: [email protected]. † Department of Environmental Engineering Sciences, University of Florida. ‡ Jones Edmunds and Associates. § Department of Materials Science and Engineering, University of Florida. 8052

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can eventually lead to complete oxidation of the contaminants. A more detailed explanation of the mechanism can be found in a review by Hoffman et al. (3). The past few decades of research have focused on strategies to advance photocatalysis, especially with regards to the full-scale implementation of this technology. Early in the development of photocatalysis, an effective method to separate the nanosized photocatalyst from water was unknown, so researchers explored coating titania on reactor walls and wire mesh, supporting the titania on or within sorbents, and other engineering solutions (4-10). Concurrent to these efforts, a Canadian firm, Purifics, has solved, patented, and installed several photocatalytic reactors (Photo-Cat) with slurry separation and reuse using a backpulsed membrane (11). However, even this very novel and aesthetic approach may have limitations, particularly for laminar flow conditions and air application. To achieve high mass transfer rates between contaminant and photocatalyst (Figure 1S, Supporting Information), a magnetically agitated photocatalytic reactor (MAPR) was designed. Indeed, others have also developed and used magnetic photocatalysts (12-13), but their intended use of these composites was for separation purposes whereas in this study a magnetic field was used to control the movement/ agitation of these particles. The present work examines the use of a permanent magnet to generate a static magnetic field and an alternating magnetic field generated by a solenoid to counteract gravity and agitate the catalyst, respectively. This work is the first known to take advantage of the magnetic property of the synthesized photocatalyst for both agitation and control using a magnetic field gradient generated by an alternating current.

Materials and Methods Silica Coating. Barium ferrite powder obtained from Alfa Aesar was coated with silica using a base-catalyzed sol-gel procedure (12, 14, 15) to provide insulation between the magnetic core and the photocatalyst layer for the purpose of preventing photodissolution and corrosion of the core. The 1-2-µm barium ferrite was chosen due to previous results indicating inadequate coating adhesion for larger barium ferrite (300-600 µm) (15). The coated and rinsed particles were allowed to dry for 24 h at room temperature before titanium dioxide coating. Presence of the silica layer was verified via energy-dispersive X-ray spectroscopy (EDS), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) (15). There was morphological evidence that the silica coating was of a core-shell type. Titanium Dioxide Deposition. An amorphous layer of titanium dioxide was deposited using a hydrolysis and precipitation technique (13, 16). Once dry, the catalyst was heated in a muffle furnace with a 5 °C/min ramp and held at 500 °C for 60 min to develop the anatase crystalline phase of TiO2, which was verified by X-ray diffraction (XRD) (15). Finally, the catalyst was removed from the furnace and allowed to cool to room temperature before experimentation. TiO2 mass loading was determined to be 28.0% by digesting and comparing dried sols prepared with and without silicacoated barium ferrite particles. The digestion method was adapted from Korn et al. (17), and the samples were analyzed by inductively coupled plasma optical emission spectrometry (ICP-OES) with a Perkin-Elmer 3200 spectrometer. A Brunauer, Emmett, and Teller (BET) surface area analysis using a Quantachrome Nova 2200e revealed a specific surface area of 31.3 m2/g. The composite catalyst and uncoated barium 10.1021/es0508121 CCC: $30.25

 2005 American Chemical Society Published on Web 09/21/2005

FIGURE 1. Drawing of quartz reactor.

FIGURE 2. Schematic of the alternating magnetic field generator. ferrite were imaged with a JEOL JSM 6400 SEM using a 15-kV accelerating voltage at 6000× magnification. Photocatalytic Test Stand. The experimental setup consisted of a magnetic field generation device, UV lamps, and a batch reactor. The 10-mL curved reactor was constructed of three fused quartz tubes as can be seen in Figure 1. The straight upright segments have an inner diameter of 1.27 cm, and the curved segment has an inner diameter of 1.05 cm. The 8-W UV lamps had a 365-nm wavelength peak output and were manufactured by Spectronics Corp. (Westbury, NY). During experimentation, the reactor was sealed with Teflon caps that were not in contact with the solution. The magnetic field was generated with an alternating current signal fed to a series of two solenoids (Figure 2). Each solenoid consisted of 152 m of 18 American wire gauge (AWG) wire wound around an 8.9 cm in diameter wire spool. The wire was wound in 20 layers with 23 turns per layer as a result of modeling to maximize the magnetic field gradient (4). The signal was generated by a BK Precision 4011A function generator. The output from the function generator was delivered to a Guitammer BKA 1000-4 power amplifier with outputs attached to the leads of solenoids in series (Figure 2). Solenoid A was used to increase the impedance of the amplifier load to prevent overloading while solenoid B was

used for the magnetic field generation beneath the reactor. Finally, a 2.54-cm3 permanent magnet was suspended on a sheet of plexiglass above the reactor to provide a magnetic field that counteracted gravity. With the permanent magnet in place, the catalyst within the reactor was both horizontally and vertically dispersed. During all experiments, the test stand was covered with a black box to prevent UV light from entering or leaving. A fan assisted in transferring the heat generated by the solenoid and UV lamps to maintain a final sample temperature of 29 ( 5 °C. Gas Chromatography/Nonpurgeable Dissolved Organic Carbon Analysis of Phenol. Liquid phenol from Fisher Chemicals (90.9%) was used to simulate aqueous organic pollutants. All experiments treated 10 mL of a 11-mg/L phenol solution prepared for each experiment by diluting the concentrated liquid phenol with Barnstead E-Pure 18.2 MΩ/ cm water. The initial dissolved oxygen was found to be 7.5 ( 0.2 mg/L using a YSI 52 dissolved oxygen meter and was not augmented with an external oxygen supply during experiments. The reactor was operated in batch such that data for each time represents a discrete experiment. Samples for gas chromatography (GC) or nonpurgeable dissolved organic carbon analysis were obtained by separating the catalyst from solution with a magnet and storing it in a volatile organic analysis (VOA) vial with Teflon septa at 4 °C until testing. Nonpurgeable dissolved organic carbon (nDOC) was measured on a Tekmar Dohrmann Apollo HS 9000 with an autosampler. Purging and mixing were accomplished by sparging the sample, which had been pH-adjusted with phosphoric acid, with zero-grade air for 3 min. Analysis of phenol concentrations was performed with a gas chromatograph/mass spectrometer (GC/MS; Varian 3900/2100T) using a solid-phase microextraction (SPME) sampling technique with an 85-µm polyacrylate-coated fiber assisted by the addition of reagent-grade sodium chloride. The relative standard deviation for (RSD) for replicate analysis of samples was less than 5% for both reported phenol concentrations and nDOC values. Photodetector. The photomeasurement for light transmittance utilized 200 mg of catalyst and a 1-cm quartz cuvette. A silicon photodetector (UDT Sensors, Inc.) was positioned and clamped on one side of the cuvette. A blue light-emitting diode (LED) was placed on the opposite side of the detector and immobilized. The frequency range tested was from 0 to 70 Hz at 5-Hz intervals. The photodetector setup was isolated from outside light by covering with a black box.

Results and Discussion Catalyst Synthesis. The surface morphology of the barium ferrite core (Figure 2S, Supporting Information) and completed catalyst (Figure 3S, Supporting Information) can be seen in the SEM images. The raw barium ferrite morphology indicated sharp edges and a dominant flat hexagonal shape. The magnetic property caused the particles to agglomerate into stacks. After the catalyst was coated with double layers of silica and titania, the sharp edges were less evident. In addition, the agglomerates were more irregularly shaped possibly due to silica necking between the particles of barium ferrite. A superconducting quantum interference device (SQUID) analysis of the barium ferrite core and the composite photocatalyst demonstrated several important features of the catalyst (18). First, the catalyst and barium ferrite had a clear hysteresis loop indicated by the lag in magnetism of the sample with changes in applied magnetic field intensity. This hysteresis loop is indicative of a ferromagnetic material possessing a remanent magnetization as was expected for barium ferrite. This remanent magnetization was approximately 23 (emu G)/g for the composite photocatalyst and 24 (emu G)/g for the barium ferrite core. In addition, the SQUID VOL. 39, NO. 20, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 3. Comparison of the number of UV lamps and presence of permanent magnet for degradation of phenol in the MAPR (WPM ) with permanent magnet, NPM ) no permanent magnet, 1L ) one 365-nm lamp, 3L ) three 365-nm lamps).

FIGURE 4. nDOC concentration as a function of the number of lamps and the presence of permanent magnet in the MAPR (WPM ) with permanent magnet, NPM ) no permanent magnet, 1L ) one 365-nm lamp, 3L ) three 365-nm lamps). analysis confirmed that coating the barium ferrite caused only a slight decrease in remanent magnetization, and the ferromagnetic property was preserved. Phenol Photolysis. Previous studies have indicated that phenol undergoes photolysis when exposed to 185- or 254nm irradiation (19). Similarly, 254-nm irradiation has been found to be more effective for mineralizing phenol (20). In our experiments, phenol showed no photolytic mineralization during 7 h of exposure from three 365-nm lamps using a quartz reactor. Therefore, data shown herein are a result of photocatalysis and not photolysis. Indeed, a full-scale MAPR system may take advantage of photolytic degradation, yet it is important here to distinguish between photolytic and photocatalytic effects because the two reaction mechanisms may produce different byproducts. Phenol Adsorption. It is important to quantify the extent of adsorption to the catalyst in the absence of UV to isolate what phenol and nDOC concentration changes may be due to oxidation. For this experiment, 100 mg of catalyst was agitated with 1 A of current at 60-Hz signal frequency. The data indicate that phenol concentration and nDOC did not change over a period of 7 h (Figure 4S, Supporting Information). Phenol and nDOC: The Effect of Lamp Number and a Permanent Magnet. Initial experiments employed 100 mg of the composite photocatalyst, 1 A of current, and 60-Hz solenoid frequency. When the permanent magnet was present, the phenol concentration after 5 h was not detected via GC analysis (Figure 3), while close to 70% of the nDOC remained (Figure 4). Without the permanent magnet (NPM) and with one 365-nm lamp (1L) the nDOC concentration decrease was less than the variation in replicate analysis of samples (Figure 4), but the phenol concentration at 7 h decreased to 29% of its original value (Figure 3). Therefore, a large fraction of the phenol was simply being converted to byproducts. To increase the rate and efficiency of photo8054

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FIGURE 5. Catalyst mass effect on phenol photocatalysis in the MAPR (normalized concentration decrease ) C0 - C/C0). catalysis, two 365-nm lamps were added (i.e., for a total of three lamps or 24 W). Although the decrease in phenol concentration after 7 h was approximately the same with and without the permanent magnet (Figure 3), the initial pseudo-first-order rate constant for phenol oxidation was 3 times greater (0.80 vs 0.27 min-1) with the permanent magnet. Data for nDOC (Figure 4) indicated that after 7 h 0.6 mg/L remained with a permanent magnet, whereas when it was not present during the experiment 3.1 mg/L was still remaining. The improvement in photocatalytic degradation from the fixed magnet is believed to be due to its ability to counteract gravity with a magnetic field gradient. Under the influence of this magnetic field gradient, the catalyst was more effectively spread throughout the reactor, thus increasing its exposure to UV. Catalyst Mass Optimization. The quantity of catalyst in the reactor was optimized for phenol and nDOC reduction using 2 h of exposure with one 365-nm lamp. With a 365-nm lamp, which was shown earlier not to cause photolytic mineralization, the phenol removal was greater than nDOC removal for the entire range of catalyst mass (Figure 5). This was most likely an indication that several oxidation byproducts remain in solution after 2 h of irradiation. The trend in phenol removal showed a slight increase as mass increased up to 400 mg and remained constant at higher mass loadings. The nDOC trend demonstrated an increase in removal with mass until 400 mg where there was a peak and then a descent as the mass increased further. This optimum is believed to be the result of two phenomena. An increase in phenol and nDOC removal is expected with an increase in TiO2 concentration (as a function of catalyst mass increasing) due to an increase in the number of reaction sites and thus the quantity of hydroxyl radicals produced. A competing phenomenon is expected as the catalyst concentration reaches a level where the UV light is blocked, reducing the proportion of catalyst receiving photons of sufficient energy to generate hydroxyl radicals (21). Solenoid Frequency Optimization. The sinusoidal signal frequency fed to the solenoid affected the rate at which the magnetic field gradient, and thus force on the catalyst, alternated direction (4). This oscillating force caused the catalyst to move within the reactor, thus increasing the mass transfer of phenol and photodegradation byproducts to the TiO2 on the surface of the composite catalyst. For this experiment, each data point represents a variation in frequency where the current was fixed at 1.25 A and 100 mg of catalyst was irradiated for 2 h with three 365-nm UV lamps. The data demonstrate that nDOC is eliminated most effectively between 20 and 80 Hz where the variation in removal over the frequency range is less than the sample RSD from the Materials and Methods section (Figure 6). The elimination of oxidation byproducts, which is indicated by nDOC, decreases as frequency increases beyond 80 Hz. This effect

FIGURE 6. Solenoid signal frequency effect on phenol photocatalysis in the MAPR (normalized concentration decrease ) C0 - C/C0).

FIGURE 8. Solenoid signal current effect on photocatalysis (normalized concentration decrease ) C0 - C/C0).

FIGURE 7. Solenoid frequency effect on photodetector output.

correlate with an increase in catalyst motion and thus mass transfer of phenol to catalyst. The experimental data agreed with this expected increase from 0 to 1 A; there was a corresponding increase in both reduction of nDOC and phenol concentration (Figure 8). As seen in previous experiments, the normalized concentration decrease for phenol was higher than nDOC for all conditions except 0 A. This is believed to be the result of organic oxidation byproducts. As the current increased above 1 A, the photocatalytic degradation remained constant. This change in the trend indicates that the system performs optimally with 1-A current because further increases in current only increase energy requirements and heat generation.

may be due to a decrease in the ability of the catalyst to effectively respond to changes in the magnetic field at higher frequencies. As the catalyst motion response is diminished, there will be a corresponding decrease in mass transfer and thus photocatalytic degradation. In contrast to the nDOC result, phenol removal showed a less dramatic decrease at higher frequencies. This discrepancy in oxidation byproduct removal and phenol removal may be due to a higher mass transfer dependence of byproduct oxidation over phenol oxidation. This would mean that the first few steps of phenol oxidation to hydroxylated compounds (22) occur at the same rate for higher frequencies and lower mass transfer, but subsequent reaction steps such as loss of aromaticity and mineralization occur at a slower rate because they may be more dependent upon mass transfer. Since the catalyst is heat-treated to 500 °C during synthesis, an increase in hydrophobicity is likely to occur due to dehydroxylation of the silanol groups (23), thus decreasing the adsorption of polar compounds (compounds with more hydroxyl groups such as the oxidation byproducts). Finally, at 0 Hz (a control with the alternating magnetic field absent), the least photocatalysis was observed, confirming that the presence of an alternating magnetic field enhanced photocatalysis. Photodetector Frequency Analysis. This experiment was performed to establish the relationship between solenoid frequency and light transmittance. The photodetector output voltage indicated the flux of photons reaching the solid-state detector, which decreased as the agitation increased (Figure 7). The data showed that as the frequency increases between 25 and 50 Hz the photodetector output voltage decreased, indicating a decrease in light transmittance. Correspondingly, there was an increase in output voltage above 50 Hz, indicating a decrease in agitation. This region is within the maximum phenol oxidation and nDOC decrease in the frequency optimization (Figure 6). Solenoid Current Optimization. The magnetic flux of the alternating magnetic field generated by the solenoid is directly proportional to current according to the Biot-Savart law (4). As such, the increase in magnetic flux is expected to

Acknowledgments This work was supported by the NASA-UF ES-CSTC and the U.S. Environmental Protection Agency Science to Achieve Results program (Grant No. R829602). In addition, the authors thank Brad Willenberg of the Major Analytical Instrumentation Center, Department of Materials Science and Engineering, University of Florida for assistance with the SEM imaging. The quartz reactor and drawings are from Analytical Research Systems, Inc. of Gainesville, FL.

Supporting Information Available Magnetic photocatalyst concept illustration, SEM images of the barium ferrite core and completed photocatalyst, and adsorption of phenol. This material is available free of charge via the Internet at http://pubs.acs.org.

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Received for review April 27, 2005. Revised manuscript received August 11, 2005. Accepted August 12, 2005. ES0508121