Article pubs.acs.org/IECR
Polyacrylonitrile-TiO2 Fibers for Control of Gaseous Aromatic Compounds Wan-Kuen Jo* and Hyun-Jung Kang Department of Environmental Engineering, Kyungpook National University, Daegu 702-701, Korea ABSTRACT: Polyacrylonitrile (PAN)-supported titania (PAN-TiO2) fibers with different TiO2 to PAN ratios were prepared, and their feasibility for indoor air cleaning applications was examined. For all target aromatic compounds, the decomposition efficiencies of PAN-TiO2 fibers increased as the ratio of PAN:TiO2:N,N-dimethyl formamide increased. For the highest TiO2 ratio (1:1:9), the decomposition efficiencies of the PAN-TiO2 fibers were close to 100% for the target compounds over the 3-h photocatalytic process, whereas for the lowest TiO2 ratio (1:0.05:9), they were initially less than 30% and then gradually decreased to close to zero at the end of the 3-h photocatalytic process. The decomposition efficiencies revealed a descending trend, but photocatalytic reaction rates revealed an ascending trend with increasing initial concentration and stream flow rate. Overall, these findings suggested that the PAN-TiO2 fibers would be effectively used for indoor air aromatic compound cleaning, when operational conditions are optimized.
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environments.16,17 PAN-based TiO2 (PAN-TiO2) composites have been prepared and applied to the photocatalytic decomposition of water pollutants in a few studies.17,18 Im et al.17 found that up to 80% of the dye rhodamine B in the aqueous phase could be decomposed within 38 h when PAANTiO2 fiber webs in the aqueous phase were applied. Similarly, Prahsarn et al.18 showed that PAN-TiO2 nanofiber could decompose 80% of 10 ppm aqueous-phase methylene blue in 24 h. Moreover, they demonstrated that the ratios of TiO2 to PAN in TiO2-PAN composites could influence the surface characteristics of composites and, as a result, the efficiency of their photocatalytic decomposition of aqueous methylene blue. It is be noted that the effects of TiO2 to PAN ratios on photocatalytic decomposition of aqueous methylene blue may differ from those of gaseous pollutants, because mechanisms of photon absorption and reaction kinetics of chemical species differ at the solid−liquid and solid−gas interfaces.19 Accordingly, in the present study, the PAN-supported TiO2 (PANTiO2) fibers with different TiO2 to PAN ratios were prepared, and the feasibility of their application to decomposition of four hazardous aromatic VOCs (benzene, toluene, ethyl benzene, and xylene (BTEX)) at indoor air levels was examined. These applications were conducted under different operational conditions because the effectiveness of such treatments can vary with environmental conditions such as the types and concentrations of gaseous pollutants and stream flow rate in air media.14,20 The target compounds were usually detected at high concentrations in indoor environments.5 The published indoor air quality guideline values for BTEX are 0.003, 0.07, 0.88, and 0.2 ppm, respectively.21,22
INTRODUCTION Exposure to volatile organic compounds (VOCs) has received a great deal of concern because it is closely associated with adverse health effects, and these compounds are prevalent in indoor and outdoor environments. The effects of VOCs on human health have been well documented: specifically, benzene is a human carcinogen associated with leukemia and other blood-related disorders, while other aromatic VOCs are associated with damage to human organs and nervous system.1,2 Other health effects of VOCs include irritation, allergy, asthma exacerbation, and respiratory effects.3,4 Most VOCs are typically found at higher concentrations in indoor than ambient air.5 These higher indoor VOC concentrations have been attributed in part to penetration of ambient VOCs that originate from a variety of urban sources including industrial discharges and the burning of fossil fuels.6,7 Indoor VOC levels have been shown to be further elevated by a range of residential indoor sources such as building finishing materials, household products, and/or cigarette smoke.8 These findings indicate that residences are an important indoor microenvironment for potential exposure to VOCs. Consequently, efforts to exploit control tools for these compounds are necessary to lessen the health risks that indoor air exposure poses to building occupants. Heterogeneous photocatalysis using titanium dioxide (TiO2) has been shown to be a promising advanced oxidation technique for treatment of air and water pollution because it can oxidize a variety of environmental pollutants with high decomposition efficiency instead of accumulating them.9,10 For air treatment applications, an immobilization material is necessary to prevent fine TiO2 powders from blowing away with treated air. A range of substrates have been used as catalyst supports for the photocatalytic degradation of air pollutants. Popular supporting substrates include granular and fibrous activated carbons,11,12 glass beads,13 glass tubes,14 and polymer materials.15 Polyacrylonitrile (PAN) is especially interesting as a potential support material for TiO2 because of its hydrophobic nature, which is preferable for applications in humid © 2013 American Chemical Society
Received: Revised: Accepted: Published: 4475
November 19, 2012 January 27, 2013 March 5, 2013 March 5, 2013 dx.doi.org/10.1021/ie303178u | Ind. Eng. Chem. Res. 2013, 52, 4475−4483
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fluorescent black light lamp (F8T5/BLB-352 nm, Sankyo Denki Co.) with a maximum spectral intensity at 352 nm and two types of 3.1-W UV-LEDs (MS-L510UV365 and MSL510UV380, Moksan Electric Co.) with maximum spectral intensities at 365 and 380 nm, respectively. For each test, other parameters were fixed to their representative values: RPTD, 1:0.5:9; IC, 0.1 ppm; SFR, 1.0 L min−1; RH, 45%; and LST, 8W fluorescent black light lamp. The experimental procedure for the indoor air application tests included the following control program. The SFRs and RHs were measured at the inlet and outlet of the annular-type photocatalytic reactor to confirm the stability of the experimental system, while no standard species were injected into the reactor. After the SFR and RH reached equilibrium, the annular-type photocatalytic reactor was pretreated for 3 h by allowing continuously humidified clean air to flow through the UV-illuminated reactor. When no contamination with VOCs was observed in the system, the UV light source was turned off and target compounds were injected into the reactor to observe the adsorption equilibrium on the surface of PAN-TiO2 fibers in the reactor. After the adsorption process between the titania catalyst and the target compounds reached equilibrium, which was indicated by equality between the inlet and outlet concentrations, the light source was again activated to determine the photocatalytic decomposition efficiency of the PAN-TiO2 fiber. Gas-phase species in the air stream were measured at the upstream and downstream sides of the photocatalytic reactor. Air samples were collected by filling an evacuated 5 L Tedlar bag. Subsequently, air from this bag was drawn through a 0.64cm-o.d. and 10-cm-length stainless steel tube containing 0.2 g of Tenax. Sampling times varied from 1 to 5 min depending on the sampling flow rate. The gaseous compounds collected on the trap were analyzed by coupling an automatic thermal desorber (ATD 400, Perkin-Elmer Co.) to a gas chromatograph (GC, 7890, Agilent Inc.) equipped with a flame ionization detector (FID) and a capillary column (DB-1, Agilent Co.). The trap was thermally desorbed at 250 °C for 10 min, and the VOCs were cryofocused at −30 °C on a cryo trap. The cold trap was then rapidly heated to 250 °C and subsequently flushed to transfer the target compounds to the GC. The initial oven temperature was set to 35 °C for 5 min, then ramped to 200 °C at 4 °C min−1, where it was held for 5 min. In the present study, the target VOCs were identified by their retention times using GC/FID analysis. Quantification of VOCs was performed using calibration curves based on at least five concentrations. The quality control program for VOC measurements included laboratory blank traps and spiked samples. On each day, a laboratory blank sample was analyzed to check for any contamination, and no contamination was found. An external standard was analyzed daily to check the quantitative response of the analytical instrument. The method detection limits for BTEX ranged from 0.003−0.007 ppm depending on the compound. Calculations of Decomposition Efficiencies and Photocatalytic Reaction Rates. The decomposition efficiencies and photocatalytic reaction rates were calculated by using the following equations:
METHODOLOGY Preparation and Characterization of PAN-TiO2 Fibers. The PAN-TiO2 fibers with different PAN to TiO2 ratios were prepared using PAN (Aldrich Inc.) as a carbon source, N,Ndimethyl formamide (DMF, Aldrich Inc.) as a solvent, and TiO2 (titanium(IV) oxide, anatase powder, Aldrich Inc.) as a photocatalyst. Polymer solution was prepared by mixing PAN, TiO2, and DMF at a specified weight ratio (1:0.05:9, 1:0.1:9, 1:0.5:9, or 1:1.0:9). This mixture was then heated at 110 °C for 1 h while stirring, after which it was transferred into a 10 cm−3 syringe equipped with a capillary tip composed of an 18 gauge needle. This solution was electrospun into a fiber web using an electrospinning system equipped with a power supply (CPS60K02VIT, Chungpa EMT Co.). The syringe needle was electrified using 15 kV DC, and the solution was ejected via a syringe pump at a spinning rate of 1.5 cm3 h−1 and a rotation speed of 300 rpm. The distance from the tip to the collector was set to 10 cm. The prepared PAN-TiO2 fibers were characterized using scanning electron microscopy (SEM) and X-ray diffraction (XRD). XRD patterns were determined using a Rigaku D/max2500 diffractometer with Cu Kα radiation operated at 40 kV and 100 mA in the range of 20−80° (2θ) at a scanning rate of 10° min−1. The particle morphology was observed by FE-SEM S-4300 and EDX-350 FE-SEM (Hitachi Co.) at an acceleration voltage of 15 kV. Feasibility of Indoor Air Cleaning Applications. The feasibility of indoor air cleaning applications of the as-prepared PAN-TiO2 fiber was evaluated using a continuous annular-type Pyrex reactor (4.0 cm i.d. and 26.5 cm length) with an inner wall that was covered with a thin PAN-TiO2 fiber. A cylindrical conventional UV lamp or UV LEDs supported by a hexahedral tube were inserted inside the Pyrex reactor and acted as the inside surface boundary layer of the annular reactor. The inlet gas stream was composed of four flow lines. A pure dried air stream was supplied at 1.0−4.0 L min−1 from a compressed cylinder and split into two flow lines. The air stream in one flow line was directed to a humidification device in a water bath. The second line was split into two flow lines: one directed to an empty buffering glass chamber (500 mL) and another that was again split into lines for the mixing glass chamber and the exhaust tube at a specified ratio (10:1) for dilutions. A standard gas stream was prepared by injecting the target compounds into a mixing chamber via an autoprogrammed syringe pump (Model Legato 100, KdScientific Inc.). Finally, the mixed gas stream was transferred into an empty buffering chamber to minimize the inlet concentration fluctuation and fed into the reactor. The stream flow rate (SFR) and the relative humidity (RH) were measured using mass flow controllers (Defender 510, Bios International Co.) and a humidity meter (TR-72S, T & D Co.), respectively. The photocatalytic degradation efficiencies of the BTEX were tested under a variety of experimental conditions by varying the ratio of PAN:TiO 2 :DMF (RPTD), initial concentration (IC), SFR, RT, and light source type (LST). Four RPTDs (1:0.05:9, 1:0.1:9, 1:0.5:9, and 1:1.0:9) were tested. The ICs of each target compound ranged from 0.1 to 2.0 ppm (0.1, 0.5, 1.0, and 2.0 ppm) to cover indoor air levels, and the range of SFRs investigated was 1.0 to 4.0 L min−1 (1.0, 2.0, 3.0, and 4.0 L min−1). The RH range for these experiments was 20−90% (20%, 45%, 70%, and 90%) to cover both dried and humidified environments. The LST included an 8-W 4476
η = 100 × (C in − Cout)/C in
(1)
r = f (C in − Cout)Q /A
(2)
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Figure 1. Photocatalytic decomposition efficiency (PDE) of BTEX determined using PAN/TiO2 nanofibers prepared at four different weight ratios of PAN, TiO2, and DMF (1:0.05:9, 1:0.1:9, 1:0.5:9, or 1:1.0:9): (a) benzene; (b) toluene; (c) ethyl benzene; and (d) xylene.
where η indicates decomposition efficiency (%); r indicates reaction rate (μmol m−2 s−1); Cin and Cout indicate the inlet and outlet concentrations of each target compound (ppm), respectively; Q indicates the air flow rate (m3 s−1); A indicates the photocatalyst-coated area (m2); and f indicates the conversion factor (40.9 μmol m−3 ppm−1).
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RESULTS AND DISCUSSION The application of as-prepared PAN-TiO2 fibers for the photocatalytic decomposition of indoor air quality level VOCs was examined under a variety of experimental conditions by varying the RPTD, IC, SFR, RT, and LST. Figure 1 shows the photocatalytic decomposition efficiencies of PAN-TiO2 fibers prepared with four different weight ratios of PAN, TiO2, and DMF over a 3-h photocatalytic process. The photocatalytic activity reached rapidly to maximum value in 1 h. This was ascribed to short photocatalytic reaction times (generally, 0.87−5.48 s) in plug-flow reactor systems.23 For all target compounds, the decomposition efficiencies of PANTiO2 fibers increased as the TiO2 ratio increased. Similarly, Prahsarn et al.18 reported that PAN-TiO2 fiber with 2 and 3% (wt) showed more rapid degradation of aqueous methylene blue than PAN-TiO2 fiber with 1% (wt). These findings were attributed to the higher amounts of TiO2 particles on the surface of the PAN-TiO2 fibers with higher TiO2 ratios. For the highest TiO2 ratio (1:1:9), the decomposition efficiencies of the PAN-TiO2 fiber were close to 100% for the target compounds over the 3-h photocatalytic process, whereas for the lowest TiO2 ratio (1:0.05:9), they were initially less than 30% and then gradually decreased to close to zero at the end of the 3-h photocatalytic process. These results suggest that an RPTD of
Figure 2. Scanning electron microscopy of PAN/TiO2 composite fibers prepared at four different weight ratios of PAN, TiO2, and DMF: (a) 1:0.05:9; (b) 1:0.1:9; (c) 1:0.5:9; and (d) 1:1:9.
1:1:9 was the optimum value for the preparation of PAN-TiO2 fiber using the preparation process applied in this study. In contrast, the decomposition efficiencies of PAN-TiO2 fibers with the two lowest TiO2 ratios (1:0.1:9 and 1:0.05:9) decreased significantly as time passed over the 3-h process. This gradual decreasing trend over the 3-h process might have been due to a gradual deactivation of the PAN-TiO2 fibers prepared using such low TiO2 contents. The SEM and XRD images confirmed that the PAN-TiO2 fibers prepared using high TiO2 ratios contained higher 4477
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Table 1. Photocatalytic Reaction Rates (μmol m−2 s−1) of BTEX According to Initial Concentration input concentration (ppm) compound benzene toluene ethyl benzene xylene
0.1 1.8 1.9 2.0 2.0
× × × ×
10−3 10−3 10−3 10−3
0.5 3.9 6.5 7.1 8.0
× × × ×
10−3 10−3 10−3 10−3
0.7 3.0 7.1 8.4 10.1
× × × ×
1.0 10−3 10−3 10−3 10−3
1.9 4.6 6.4 10.9
× × × ×
10−3 10−3 10−3 10−3
both TiO2 particles and fibers have positive charges.17 Moreover, the XRD patterns (Figure 3) demonstrated that the aggregated masses on the surface of the fibers were TiO2 anatase crystal phase. For all PAN-TiO2 fibers prepared in the current study, one major anatase crystal phase peak was observed at 2θ = 25.7°. However, the intensities of anatase peaks that appeared at different 2θ values decreased as the RPTD values decreased. Moreover, the XRD peak observed at 2θ = 22.5° was assigned to carbon.17 Figure 4 shows the decomposition efficiencies of the PANTiO 2 fibers prepared using the ratio of 1:0.5:9 for PAN:TiO2:DMF over a 3-h photocatalytic process according to ICs, which included typical indoor air levels. For all target compounds, the decomposition efficiencies revealed a decreasing trend with increasing IC. At the lowest IC (0.1 ppm), average decomposition efficiencies were 85%, 92%, 94%, and 94% for BTEX, respectively, whereas at the highest IC (1.0 ppm) they were 9%, 22%, 31%, and 53%, respectively. The adsorption of contaminants on the surface of the catalyst was a
Figure 3. X-ray diffraction pattern of PAN/TiO2 composite fibers prepared at four different weight ratios of PAN, TiO2, and DMF: (a) 1:0.05:9; (b) 1:0.1:9; (c) 1:0.5:9; and (d) 1:1:9.
amounts of TiO2 on the surface of the fiber composites. The SEM image (Figure 2) shows that the PAN-TiO2 fibers prepared using high RPTD values had higher masses, larger fiber diameters, and rougher fiber surfaces. This pattern was consistent with that reported by Prahsarn et al.18 For the present study, the fiber diameters of the PAN-TiO2 fibers prepared using RPTD values of 1:0.05:9, 1:0.1:9, 1:0.5:9, and 1:1:9 were about 600−1000, 830−1200, 1000−1300, and 1200−1600 nm, respectively. The aggregation of TiO2 masses on the fibers was attributed to electrostatic repulsion, because
Figure 4. Photocatalytic decomposition efficiency (PDE) of BTEX determined using PAN/TiO2 fibers prepared at the ratio of 1:0.5:9 for PAN:TiO2:DMF according to the initial concentration. 4478
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Figure 5. Photocatalytic decomposition efficiency (PDE) of BTEX determined using PAN/TiO2 fibers prepared at the ratio of 1:0.5:9 for PAN:TiO2:DMF according to the stream flow rate.
Table 2. Photocatalytic Reaction Rates (μmol m−2 s−1) of BTEX According to Stream Flow Rate
tubular reactor coated with Degussa P25 TiO2 powder, have reported that photocatalytic reaction rates of vaporous trichloroethylene decreased as IC increased, whereas the decomposition efficiencies increased. According to the Langmuir−Hinshelwood (LH) kinetic model, reaction rate increment and decomposition efficiency decrement with IC increases suggested that the IC range tested corresponded to an intermediate range among three concentration regimes.20 Moreover, it was notable that the reaction rates decreased as the IC increased further from 0.5 to 1 ppm for benzene and from 0.7 to 1.0 ppm for toluene and ethyl benzene. These results were ascribed to the presence of photocatalyst deactivation at the high ICs. Similarly, previous studies27,28 have reported that the LH model did not fit to photocatalytic decomposition kinetics for certain aromatic hydrocarbons, due to potential photocatalyst deactivation. Moreover, in most cases, the photocatalytic reaction rate increased in the order benzene < toluene < ethyl benzene < xylene, likely due to the higher bond energy of benzene ring as compared to the bond energy of aliphatic chains, which was consistent with the results reported by Strini et al.29 The decomposition efficiencies of PAN-TiO2 fibers prepared using the ratio of 1:0.5:9 for PAN:TiO2:DMF over a 3-h photocatalytic process according to SFRs are shown in Figure 5. The decomposition efficiencies decreased as the SFR increased. As the SFR increased from 1 to 4 L min−1, the average decomposition efficiencies decreased from 85% to 42%, 92% to 56%, 94% to 57%, and 94% to 60% for BTEX, respectively. Consistently, Yu and Brouwers25 also reported that NO photocatalytic decomposition efficiencies decreased
stream flow rate (L min−1) compound benzene toluene ethyl benzene xylene
1 1.8 1.9 2.0 2.0
× × × ×
2 10−3 10−3 10−3 10−3
2.2 2.9 2.9 3.1
× × × ×
3 10−3 10−3 10−3 10−3
2.7 3.6 3.7 3.9
× × × ×
4 10−3 10−3 10−3 10−3
3.5 4.6 4.7 4.9
× × × ×
10−3 10−3 10−3 10−3
major parameter influencing the photocatalytic decomposition of chemical species.19 Therefore, the increasing trend in decomposition efficiency with the decrease of IC was likely due to competitive adsorption between pollutant molecules on the photocatalyst surface. Specifically, at high ICs a limited amount of the reactive sites on the surface of the PAN-TiO2 fibers would be available for the adsorption of BTEX molecules prior to photocatalytic decomposition. Similarly, Devahasdin et al.24 reported that the NO photocatalytic decomposition efficiencies of an UV-irradiated TiO2 system decreased from 70% to 15% as IC increased from 5 to 60 ppm. In addition, Yu and Brouwers25 tested a photocatalytic reactor with carbondoped TiO2 under visible-light irradiation and found that NO decomposition efficiencies decreased from 61 to 16 as IC increased from 0.1 to 1.0 ppm. In contrast to the decomposition efficiencies, the reaction rates increased as the IC increased over certain ranges depending on target compound (benzene, 0.1−0.5 ppm; toluene and ethyl benzene, 0.1−0.7 ppm; and xylene, 0.1−1.0 ppm) (Table 1). Similarly, Ku et al.,26 who used a plug-flow 4479
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Figure 6. Photocatalytic decomposition efficiency (PDE) of BTEX determined using PAN/TiO2 fibers prepared at the ratio of 1:0.5:9 for PAN:TiO2:DMF according to the relative humidity.
from to 62% to 13% as SFR increased from 1 to 5 L min−1. The reaction times for the SFRs of 1, 2, 3, and 4 L min−1, which were calculated by dividing the reactor volume by the SFRs, were 13.4, 6.7, 5.1, and 3.4 s, respectively. Accordingly, the lower BTEX decomposition efficiencies for high SFRs were assigned to an insufficient reaction time in the photocatalytic reactor. In contrast to the decomposition efficiencies, the photocatalytic reaction rates increased as FR increased from 1−4 L min−1 (Table 2). Yu et al.,30 who reported the same pattern at a low flow rate range (0−0.6 L min−1), suggested that, under the flow rate conditions, the photocatalytic reaction rate corresponded to the gas-phase mass transfer rate, which was an important factor involved in the photocatalytic reaction mechanism. Other studies19,20 also suggested that if the mass transfer rate was a determinant factor for the photocatalytic decomposition kinetics, photocatalytic reaction rates of BTEX would increase as SFR increased due to increased mass transfer. Accordingly, the photocatalytic reaction rates obtained at the flow rate range in the present study were ascribed to mass transfer rates, rather than surface reaction rates. The effects of a broad range of RH values that cover dry and humid environmental conditions on the decomposition efficiencies were investigated via the PAN-TiO2 fibers prepared using the ratio of 1:0.5:9 for PAN:TiO2:DMF over a 3-h photocatalytic process. As shown in Figure 6, the decomposition efficiencies decreased with increasing RH for all target compounds. As the RH increased from 20% to 90%, the average decomposition efficiencies decreased from 90% to 54%, 94% to 81%, 95% to 85%, and 96% to 88% for BTEX,
respectively. This RH dependence was consistent with that reported by Ao et al.31 on a UV-irradiated TiO2 system. Under conditions with a shortage or absence of water molecules, photocatalytic reactions could be retarded due to a lack of hydroxyl groups that could oxidize BTEX on the surface of the PAN-TiO2 fibers. Moreover, excessive water vapor might compete with BTEX molecules for the active reaction sites on the surface of the PAN-TiO2 fibers. Therefore, the decrease in the photocatalytic decomposition efficiency with increasing RH was attributed to the competitive adsorption between BTEX and water molecules on the surface of the PAN-TiO2 fibers. However, Jeong et al.32 found that toluene photocatalytic decomposition efficiencies of a UV-irradiated TiO2 system increased as RH increased from less than 1% to 50% and then remained constant as RH increased from 50% to 90%. In their study, an increased population of hydroxyl radicals due to increased water vapor was given as an explanation for the increase in toluene photocatalytic decomposition efficiencies. Accordingly, these contrasting RH effects on photocatalytic efficiency appeared to depend on the amount of water vapor as well as the type and input concentration of aromatic hydrocarbons applied to those studies. TiO2 applications require UV-light sources because TiO2 only exhibits a high activity level under UV light, which exceeds the band gap energy of 3.0 or 3.2 eV in its rutile or anatase crystalline phases.33 Black-light fluorescent lamps are one of the most popularly used UV-light sources for photocatalytic processes of indoor air pollutants.14,34,35 In recent years, LEDs have received increasing interest due to their advantages over conventional light sources.36,37 Therefore, the present 4480
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Figure 7. Photocatalytic decomposition efficiency (PDE) of BTEX determined using PAN/TiO2 fibers prepared at the ratio of 1:0.5:9 for PAN:TiO2:DMF according to the light source type.
decomposition efficiencies obtained from the 380-nm UVLED indicated that the light intensity effect would outweigh the light energy effect on the decomposition efficiencies of these pollutants.
study examined the effects of UV LEDs as well as a conventional UV lamp on the photocatalytic decomposition of PAN-TiO2 fibers. Figure 7 shows the decomposition performance of the PAN-TiO2 fibers units using three commercially available light sources (8-W black-light lamp and two types of UV LEDs). The average efficiencies of BTEX obtained from the black-light lamp with a maximum light intensity at 352 nm/PAN-TiO2 fiber system were 8%, 92%, 94%, and 94%, respectively, which were substantially higher than those of the UV LED with 380 and 365 nm/PAN-TiO2 fiber systems. The black-light lamp showed higher light energy (lower wavelength, 352 nm) than those (380 and 365 nm, respectively) of the two types of UV LEDs. In addition, the black-light lamp showed higher light intensity (2.3 mW cm−2) than the UV LEDs (0.98 and 0.35 mW cm−2 for 365 and 380 nm, respectively). Previous studies25,32,38 reported that the activity of a photocatalyst was highly dependent on the light intensity per unit area (photon flux) on the surface of the catalyst as well as the light energy per unit area (light wavelength). Therefore, the higher decomposition efficiency for the black-light lamp/PAN-TiO2 fiber system was assigned to the synergistic interaction of the higher light energy and higher light intensity. In addition, the UV LED with lower light energy (a maximum light intensity at 380 nm) exhibited higher photocatalytic decomposition efficiencies than those of the UV LED with lower light energy (a maximum light intensity at 365 nm). For this study, the light intensity of the 380-nm UVLED (0.98 mW cm−2) was much higher than that of the 365nm UV-LED (0.35 mW cm −2 ). As such, the high
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CONCLUSIONS
The current study examined the application of PAN-TiO2 fibers prepared using RPTD for the photocatalytic decomposition of airborne aromatic compounds (BTEX). Within the RPTD range specified in this study, the decomposition efficiencies of the PAN-TiO2 fibers increased as the TiO2 ratio increased. These results were attributed to the higher amounts of TiO2 particles on the surface of the PAN-TiO2 fibers with higher TiO2 ratios, which was confirmed by the SEM and XRD images. In contrast to decomposition efficiencies, photocatalytic reaction rates revealed an ascending trend with increasing IC and SFR. Moreover, the findings presented herein indicated that indoor air quality concentration levels, hourly air treatment volume, and indoor air humidity should be considered to enable effective operation of the PAN-TiO2 fibers for the photocatalytic decomposition of indoor VOCs. Similar to other photocatalysts, light sources were another important factor influencing the photocatalytic process of PAN-TiO2 fibers, which was likely due to differences in light energy and intensity. Overall, the results presented herein indicate that the PANTiO2 fibers would be effectively used for remediation of indoor aromatic compounds, when operational conditions are optimized. 4481
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AUTHOR INFORMATION
Corresponding Author
*Tel.: +82-53-950-6584. Fax: +82-53-950-6579. E-mail: wkjo@ knu.ac.kr. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (MEST) (no. 2011-0027916). We appreciate the reviewers for their thoughtful and valuable suggestions.
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