DOI 10.1515/reveh-2014-0027 Rev Environ Health 2014; 29(1-2): 109–112
Rengaraj Selvaraj*, Salma M.Z. Al-Kindy, Mika Silanpaa and Younghun Kim
Nanotechnology in environmental remediation: degradation of volatile organic compounds (VOCs) over visible-light-active nanostructured materials Abstract: Volatile organic compounds (VOCs) are major pollutants and are considered to be one of the most important contaminants generated by human beings living in urban and industrial areas. Methyl tert-butyl ether (MTBE) is a VOC that has been widely used as a gasoline additive to reduce VOC emissions from motor vehicles. However, new gasoline additives like MTBE are having negative environmental impacts. Recent survey reports clearly show that groundwater is often polluted owing to leakage of petroleum products from underground storage tanks. MTBE is highly soluble in water (e.g., 0.35–0.71 M) and has been detected at high concentrations in groundwater. The presence of MTBE in groundwater poses a potential health problem. The documented effects of MTBE exposure are headaches, vomiting, diarrhea, fever, cough, muscle aches, sleepiness, disorientation, dizziness, and skin and eye irritation. To address these problems, photocatalytic treatment is the preferred treatment for polluted water. In the present work, a simple and template-free solution phase synthesis method has been developed for the preparation of novel cadmium sulfide (CdS) hollow microspheres using cadmium nitrate and thioacetamide precursors. The synthesized products have been characterized by a variety of methods, including X-ray powder diffraction, high-resolution scanning electron microscopy (HR-SEM), X-ray photoelectron spectroscopy, and UV-visible diffused reflectance spectroscopy. The HR-SEM measurements revealed the spherical morphology of the CdS microspheres, which evolved by the oriented aggregation of the primary CdS nanocrystals. Furthermore, studies of photocatalytic activity revealed that the synthesized CdS hollow microspheres exhibit an excellent photocatalytic performance in rapidly degrading MTBE in aqueous solution under visible light illumination. These results suggest that CdS microspheres will be an interesting candidate for photocatalytic detoxification studies under visible light radiation. Keywords: degradation; methyl tert-butyl ether; nanotechnology; photocatalysis; volatile organic compounds.
*Corresponding author: Rengaraj Selvaraj, Department of Chemistry, Sultan Qaboos University, P.C. 123, Al-Khoudh, Muscat, Sultanate of Oman, E-mail: [email protected]; [email protected] Salma M.Z. Al-Kindy: Department of Chemistry, Sultan Qaboos University, P.C. 123, Al-Khoudh, Muscat, Sultanate of Oman Mika Silanpaa: Faculty of Technology, Laboratory of Green Chemistry, Lappeenranta University of Technology, Mikkeli FI-50100, Finland Younghun Kim: Department of Chemical Engineering, Kwangwoon University, Seoul 139-701, Korea
Introduction In recent years, hollow-structured micro- and nanostructured materials have attracted special interest because of their higher specific surface area, lower density, better permeability, and special optical/electrical/magnetic properties, which are distinctively different from those of their solid counterparts. Materials of this kind can enable many technologically important applications, like catalysis, nanoscale chemical reactors, encapsulation and controlled release of bioactive agents, sensors, drug delivery, chemical/biological separation, photonic devices, lightweight fillers, and various new application fields (1, 2). Until now, much effort has been devoted to generating inorganic hollow structures, including conventional templating methods as well as newly emerging templatefree methods. Template-assisted syntheses have proven to be a versatile approach to fabricating hollow structures. To prepare hollow structures, a range of templates have been extensively employed, including hard templates like polymer latex particles (1), silica spheres, and carbon spheres, and soft templates like emulsion droplets, micelles, and gas bubbles (3). However, templateassisted methods usually require tedious procedures, including surface modification, precursor attachment, and core removal. It is still a great challenge in material science to understand the factors governing the creation of nanocrystal assemblies and to develop simple, reliable methods of fabricating various materials using simple
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110 Selvaraj et al.: Nanotechnology in environmental remediation
Crystal structure analysis Figure 1 shows the XRD patterns of the CdS microspheres prepared with different ratios of cadmium nitrate to TAA. The observed intense diffraction peaks are perfectly matched to the cubic phase of CdS (JCPDS card no. 80-0019). Sample CdS 3:1 showed diffraction peaks corresponding to the pure cubic crystalline phase. For samples CdS 3:2 to CdS 3:4, two weak diffraction peaks at 24.9° and 28.3° were observed, indicating the existence of some hexagonal CdS crystals in the final product. The grain sizes of
100
0 50
(222)
(220)
50
(311)
CdS: Cubic phase JCPDS card No: 80-0019 (200)
The CdS microspheres were synthesized by using analytical-grade cadmium nitrate [Cd(NO3)2·4H2O] (99%; Sigma-Aldrich, Finland) and thioacetamide (TAA, C2H5NS) (99%, Sigma-Aldrich, Finland) without further purification. In a typical synthesis, 3.00 g of Cd(NO3)2·4H2O and 1.00–4.00 g of TAA were dissolved in 100 mL of deionized water and continuously stirred for 30 min to form a clear solution. The TAA serve as an S2– source to form CdS. The solution was then transferred to a flask and refluxed at ∼105°C for 30 min, yielding a yellow precipitate. The precipitate was harvested by centrifugation and washed several times using deionized water and ethanol to remove possible remaining cations and anions, and then dried in an oven at 70°C for 24 h. Finally, the products were calcined at 350°C for 2 h for further characterization. The crystalline properties of the synthesized CdS microspheres were studied by X-ray power diffraction (XRD) using a Bruker (D5005) X-ray diffractometer with graphite monochromatized CuKα radiation (λ = 1.54056 Å). The scanning electron microscopic (SEM) measurements were performed using a Hitachi S-4800 high-resolution (HR) field emission scanning electron microscope. The chemical states and relative compositions of the samples were studied by X-ray photoelectron spectroscopy (XPS), a highly surface-sensitive technique with a typical information depth of a few nanometers. The X-ray source used AlKα radiation (1.486.7 eV) from an Al anode. Photo emitted electrons from the sample were analyzed in a hemispherical energy analyzer at a pass energy of Ep = 20 eV. All spectra were obtained with an energy step of 0.1 eV and a dwell time of 50 ms. A software package (Avantage Thermo VG) was used to analyze the XPS data and fit the curves. Absorption spectra of the samples in the diffused reflectance spectrum mode were recorded in the wavelength range of 200–1000 nm using a spectrophotometer (Jasco V670), with BaSO4 as a reference. From the absorption edge, the band gap values were calculated by extrapolation. Visible light photocatalytic activity studies were performed to study MTBE degradation in an aqueous solution. For each run, reaction suspensions were freshly prepared by adding 0.25 g of catalyst to 250 mL of aqueous MTBE solution with an initial concentration of 25 mg/L. Before photoreaction, the suspension was magnetically
Results and discussion
(111)
Experimental
stirred in the dark for 30 min to attain adsorption/desorption equilibrium. The aqueous suspension containing MTBE and photocatalyst was then irradiated by visible light with constant aeration. At specific time intervals, the analytical samples were taken from the suspension and immediately centrifuged at 4000 rpm for 15 min and then filtered to remove the catalyst. The filtrate was analyzed by a gas chromatography (GC) instrument to examine the MTBE degradation. The degree of degradation was derived from the GC peak intensity, which was integrated at a retention time of 8.77 min.
D
3:4
C
3:3
25 0 Intensity Cps.
chemical components to achieve controlled morphologies (4, 5). As an important II-IV compound semiconductor, cadmium sulfide (CdS), with a direct band gap energy of 2.4 eV at room temperature, has attracted much attention because of its unique properties and potential application in light-emitting diodes, flat panel displays, solar cells, photocatalysis, and thin-film transistors (3). As a material’s properties depend greatly on its morphological features, nanostructured CdS with different sizes, shapes, and dimensionalities has been fabricated and characterized (6). In this study, we present a facile one-pot templatefree route to preparing monodisperse CdS hollow spheres assembled from nanocrystals. Methyl tert-butyl ether (MTBE) degradation experiments have also been done to evaluate the catalytic properties of the CdS microspheres.
50
H
25 0
H
B
3:2
50 25 0
A
3:1
50 25 0 20
25
30
35
40 2θ (°)
45
50
55
60
Figure 1 XRD pattern of CdS microspheres prepared with different cadmium nitrate and thioacetamide ratios.
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Selvaraj et al.: Nanotechnology in environmental remediation 111
the synthesized CdS microspheres have been calculated from the full width at half maximum of the diffraction peaks using the Debye-Scherrer formula (7). The grain size values calculated from XRD analysis reveal that the average grain size of the CdS microspheres was initially ∼20 nm (for sample CdS 3:1) and increased to ∼35 nm (for sample CdS 3:4). That is, the grain size increased with the ratio of cadmium nitrate to TAA.
A
Morphology SEM analysis was employed to examine the detailed microstructure and morphology of the CdS microspheres. Figure 2 depicts a typical SEM image recorded for sample CdS 3:2. Similar images were also recorded for other samples (not shown here). From the SEM images, it is clear that the synthesized CdS products are composed of micro- and submicrospheres varying in diameter between 2 and 3 μm. A high-magnification image (Figure 2B) of the microspheres reveals that the maximum diameter is around 2.5 μm. A further magnified image of these microspheres reveals that a microsphere is constructed of thousands of small nanoparticles (Figure 2C), the size of which tends to be very uniform at approximately 30–40 nm. This value agrees very closely with the grain size values calculated from XRD measurements. Similar observations have also been reported by Zhang et al. (6) for CdS microspheres prepared by a self-assembly method.
B
C
Photocatalytic activity studies Volatile organic compounds (VOCs) are major pollutants and are considered to be one of the most important contaminants generated by human beings living in urban and industrial areas (3). MTBE is a VOC that has been widely used as a gasoline additive to reduce VOC emissions from motor vehicles. However, new gasoline additives like MTBE are having negative environmental impacts. Recent survey reports clearly show that groundwater is often polluted owing to leakage of petroleum products from underground storage tanks. MTBE is highly soluble in water (e.g., 0.35–0.71 M) and has been detected at high concentrations in groundwater. The presence of MTBE in groundwater poses a potential health problem. The documented effects of MTBE exposure are headaches, vomiting, diarrhea, fever, cough, muscle aches, sleepiness, disorientation, dizziness, and skin and eye irritation. To address these problems, photocatalytic treatment is the preferred treatment for polluted water (8).
Figure 2 SEM images of CdS microspheres (sample CdS 3:2). (A) overall view, (B) individual microsphere, and (C) surface of a microsphere.
Although several authors have reported that CdS itself was not an effective photocatalyst for degrading organic pollutants (9), the present investigation clearly demonstrates that the hollow CdS microspheres were active and can be efficient photocatalysts for the degradation of MTBE under visible-light irradiation. Figure 3
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112 Selvaraj et al.: Nanotechnology in environmental remediation
GC peak intensity, a.u
8.0x105
collected at suitable time intervals for up to 60 min and then analyzed for total organic carbon (TOC) concentration. From the photocatalytic reaction, it has been observed that the initial TOC concentration decreased significantly in the presence of CdS microspheres. The result demonstrated that MTBE completely degraded within 1 h, but 25% of the TOC was left after 1 h. These results confirmed that in the presence of CdS microspheres, MTBE degradation is faster than its mineralization.
0 min 5 min 10 min 15 min 20 min 30 min 40 min 50 min 60 min
6.0x105
4.0x105
2.0x105
0.0 8.70
8.72
8.74 8.76 8.78 8.80 Retention time, min.
8.82
8.84
8.86
Figure 3 Time-dependent GC peak intensity of the MTBE solution (25 mg/L, 250 mL) in the presence of CdS 3:2 microspheres under visible light.
displays the time-dependent GC peak intensity of the MTBE solution during photodegradation in the presence of CdS microspheres. The peak at around 8.77 min decreased gradually with irradiation time. At the end of the degradation, no peak could be detected, implying that complete oxidation of MTBE occurred owing to the presence of hollow CdS microspheres under visible light. Finally, to study the mineralization of MTBE in the photocatalytic reaction, experiments were performed with an initial MTBE concentration of 25 mg/L (TOC = 5.4 mg/L) in the presence of CdS microspheres. The samples were
Conclusion In this study, we demonstrated a simple, versatile solution phase method for synthesizing high-quality CdS microspheres. High-magnification HR-SEM measurements showed that the hollow shell wall was constructed by the aggregation of several thousands of nanoparticles 30–40 nm in size. The hollow interior of these microspheres formed by Ostwald ripening. The CdS microspheres exhibited good photocatalytic activity for the degradation of MTBE aqueous solution under visible-light illumination. Because of their facile synthesis and controlled morphology, the obtained CdS microspheres may find potential applications in catalysis. Received January 17, 2014; accepted January 17, 2014; previously published online February 24, 2014
References 1. Caruso F, Caruso RA, Mohwald H. Nanoengineering of inorganic and hybrid hollow spheres by colloidal templating. Science 1998;282:1111–4. 2. Dai Z, Dahne L, Mohwald H, Tiersch B. Novel capsules with high stability and controlled permeability by hierarchic templating. Angew Chem Int Ed 2002;41:4019–22. 3. Rengaraj S, Jee SH, Venkataraj S, Kim Y, Vijayalakshmi S, et al. CdS microspheres composed of nanocrystals and their photocatalytic activity. J Nanosci Nanotech 2011;11:2090–9. 4. Rengaraj S, Venkataraj S, Jee SH, Kim Y, Tai C-W, et al. Cauliflower-like CdS microspheres composed of nanocrystals and their physicochemical properties. Langmuir 2011;27:352–8. 5. Rengaraj S, Venkataraj S, Tai C-W, Kim Y, Repo E, et al. Self-assembled mesoporous hierarchical-like In2S3 hollow
microspheres composed of nanofibers and nanosheets and their photocatalytic activity. Langmuir 2011;27:5534–41. 6. Zhang B, Jian JK, Zheng Y, Sun Y, Chen Y, et al. Low temperature hydrothermal synthesis of CdS submicro- and microspheres self-assembled from nanoparticles. Mater Lett 2008;62:1827–30. 7. Klug HP, Alexander LE. X-ray diffraction procedures. New York: Wiley, 1974:687. 8. Yang X, Wang Y, Xu L, Yu X, Guo Y. Aromaticity: an ab initio evaluation of the properly cyclic delocalization energy and the π-delocalization energy distortivity of benzene. J Phys Chem C 2008;112:11481–6. 9. Wu CZ, Xie Y, Lei LY, Hu SQ, Ou Yang CZ. Synthesis of new-phased VOOH hollow “dandelions” and their application in lithium-ion batteries. Adv Mater 2006;18:1727–32.
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Rengaraj Selvaraj*, Salma M.Z. Al-Kindy, Mika Silanpaa and Younghun Kim
Nanotechnology in environmental remediation: degradation of volatile organic compounds (VOCs) over visible-light-active nanostructured materials Abstract: Volatile organic compounds (VOCs) are major pollutants and are considered to be one of the most important contaminants generated by human beings living in urban and industrial areas. Methyl tert-butyl ether (MTBE) is a VOC that has been widely used as a gasoline additive to reduce VOC emissions from motor vehicles. However, new gasoline additives like MTBE are having negative environmental impacts. Recent survey reports clearly show that groundwater is often polluted owing to leakage of petroleum products from underground storage tanks. MTBE is highly soluble in water (e.g., 0.35–0.71 M) and has been detected at high concentrations in groundwater. The presence of MTBE in groundwater poses a potential health problem. The documented effects of MTBE exposure are headaches, vomiting, diarrhea, fever, cough, muscle aches, sleepiness, disorientation, dizziness, and skin and eye irritation. To address these problems, photocatalytic treatment is the preferred treatment for polluted water. In the present work, a simple and template-free solution phase synthesis method has been developed for the preparation of novel cadmium sulfide (CdS) hollow microspheres using cadmium nitrate and thioacetamide precursors. The synthesized products have been characterized by a variety of methods, including X-ray powder diffraction, high-resolution scanning electron microscopy (HR-SEM), X-ray photoelectron spectroscopy, and UV-visible diffused reflectance spectroscopy. The HR-SEM measurements revealed the spherical morphology of the CdS microspheres, which evolved by the oriented aggregation of the primary CdS nanocrystals. Furthermore, studies of photocatalytic activity revealed that the synthesized CdS hollow microspheres exhibit an excellent photocatalytic performance in rapidly degrading MTBE in aqueous solution under visible light illumination. These results suggest that CdS microspheres will be an interesting candidate for photocatalytic detoxification studies under visible light radiation. Keywords: degradation; methyl tert-butyl ether; nanotechnology; photocatalysis; volatile organic compounds.
*Corresponding author: Rengaraj Selvaraj, Department of Chemistry, Sultan Qaboos University, P.C. 123, Al-Khoudh, Muscat, Sultanate of Oman, E-mail: [email protected]; [email protected] Salma M.Z. Al-Kindy: Department of Chemistry, Sultan Qaboos University, P.C. 123, Al-Khoudh, Muscat, Sultanate of Oman Mika Silanpaa: Faculty of Technology, Laboratory of Green Chemistry, Lappeenranta University of Technology, Mikkeli FI-50100, Finland Younghun Kim: Department of Chemical Engineering, Kwangwoon University, Seoul 139-701, Korea
Introduction In recent years, hollow-structured micro- and nanostructured materials have attracted special interest because of their higher specific surface area, lower density, better permeability, and special optical/electrical/magnetic properties, which are distinctively different from those of their solid counterparts. Materials of this kind can enable many technologically important applications, like catalysis, nanoscale chemical reactors, encapsulation and controlled release of bioactive agents, sensors, drug delivery, chemical/biological separation, photonic devices, lightweight fillers, and various new application fields (1, 2). Until now, much effort has been devoted to generating inorganic hollow structures, including conventional templating methods as well as newly emerging templatefree methods. Template-assisted syntheses have proven to be a versatile approach to fabricating hollow structures. To prepare hollow structures, a range of templates have been extensively employed, including hard templates like polymer latex particles (1), silica spheres, and carbon spheres, and soft templates like emulsion droplets, micelles, and gas bubbles (3). However, templateassisted methods usually require tedious procedures, including surface modification, precursor attachment, and core removal. It is still a great challenge in material science to understand the factors governing the creation of nanocrystal assemblies and to develop simple, reliable methods of fabricating various materials using simple
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110 Selvaraj et al.: Nanotechnology in environmental remediation
Crystal structure analysis Figure 1 shows the XRD patterns of the CdS microspheres prepared with different ratios of cadmium nitrate to TAA. The observed intense diffraction peaks are perfectly matched to the cubic phase of CdS (JCPDS card no. 80-0019). Sample CdS 3:1 showed diffraction peaks corresponding to the pure cubic crystalline phase. For samples CdS 3:2 to CdS 3:4, two weak diffraction peaks at 24.9° and 28.3° were observed, indicating the existence of some hexagonal CdS crystals in the final product. The grain sizes of
100
0 50
(222)
(220)
50
(311)
CdS: Cubic phase JCPDS card No: 80-0019 (200)
The CdS microspheres were synthesized by using analytical-grade cadmium nitrate [Cd(NO3)2·4H2O] (99%; Sigma-Aldrich, Finland) and thioacetamide (TAA, C2H5NS) (99%, Sigma-Aldrich, Finland) without further purification. In a typical synthesis, 3.00 g of Cd(NO3)2·4H2O and 1.00–4.00 g of TAA were dissolved in 100 mL of deionized water and continuously stirred for 30 min to form a clear solution. The TAA serve as an S2– source to form CdS. The solution was then transferred to a flask and refluxed at ∼105°C for 30 min, yielding a yellow precipitate. The precipitate was harvested by centrifugation and washed several times using deionized water and ethanol to remove possible remaining cations and anions, and then dried in an oven at 70°C for 24 h. Finally, the products were calcined at 350°C for 2 h for further characterization. The crystalline properties of the synthesized CdS microspheres were studied by X-ray power diffraction (XRD) using a Bruker (D5005) X-ray diffractometer with graphite monochromatized CuKα radiation (λ = 1.54056 Å). The scanning electron microscopic (SEM) measurements were performed using a Hitachi S-4800 high-resolution (HR) field emission scanning electron microscope. The chemical states and relative compositions of the samples were studied by X-ray photoelectron spectroscopy (XPS), a highly surface-sensitive technique with a typical information depth of a few nanometers. The X-ray source used AlKα radiation (1.486.7 eV) from an Al anode. Photo emitted electrons from the sample were analyzed in a hemispherical energy analyzer at a pass energy of Ep = 20 eV. All spectra were obtained with an energy step of 0.1 eV and a dwell time of 50 ms. A software package (Avantage Thermo VG) was used to analyze the XPS data and fit the curves. Absorption spectra of the samples in the diffused reflectance spectrum mode were recorded in the wavelength range of 200–1000 nm using a spectrophotometer (Jasco V670), with BaSO4 as a reference. From the absorption edge, the band gap values were calculated by extrapolation. Visible light photocatalytic activity studies were performed to study MTBE degradation in an aqueous solution. For each run, reaction suspensions were freshly prepared by adding 0.25 g of catalyst to 250 mL of aqueous MTBE solution with an initial concentration of 25 mg/L. Before photoreaction, the suspension was magnetically
Results and discussion
(111)
Experimental
stirred in the dark for 30 min to attain adsorption/desorption equilibrium. The aqueous suspension containing MTBE and photocatalyst was then irradiated by visible light with constant aeration. At specific time intervals, the analytical samples were taken from the suspension and immediately centrifuged at 4000 rpm for 15 min and then filtered to remove the catalyst. The filtrate was analyzed by a gas chromatography (GC) instrument to examine the MTBE degradation. The degree of degradation was derived from the GC peak intensity, which was integrated at a retention time of 8.77 min.
D
3:4
C
3:3
25 0 Intensity Cps.
chemical components to achieve controlled morphologies (4, 5). As an important II-IV compound semiconductor, cadmium sulfide (CdS), with a direct band gap energy of 2.4 eV at room temperature, has attracted much attention because of its unique properties and potential application in light-emitting diodes, flat panel displays, solar cells, photocatalysis, and thin-film transistors (3). As a material’s properties depend greatly on its morphological features, nanostructured CdS with different sizes, shapes, and dimensionalities has been fabricated and characterized (6). In this study, we present a facile one-pot templatefree route to preparing monodisperse CdS hollow spheres assembled from nanocrystals. Methyl tert-butyl ether (MTBE) degradation experiments have also been done to evaluate the catalytic properties of the CdS microspheres.
50
H
25 0
H
B
3:2
50 25 0
A
3:1
50 25 0 20
25
30
35
40 2θ (°)
45
50
55
60
Figure 1 XRD pattern of CdS microspheres prepared with different cadmium nitrate and thioacetamide ratios.
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Selvaraj et al.: Nanotechnology in environmental remediation 111
the synthesized CdS microspheres have been calculated from the full width at half maximum of the diffraction peaks using the Debye-Scherrer formula (7). The grain size values calculated from XRD analysis reveal that the average grain size of the CdS microspheres was initially ∼20 nm (for sample CdS 3:1) and increased to ∼35 nm (for sample CdS 3:4). That is, the grain size increased with the ratio of cadmium nitrate to TAA.
A
Morphology SEM analysis was employed to examine the detailed microstructure and morphology of the CdS microspheres. Figure 2 depicts a typical SEM image recorded for sample CdS 3:2. Similar images were also recorded for other samples (not shown here). From the SEM images, it is clear that the synthesized CdS products are composed of micro- and submicrospheres varying in diameter between 2 and 3 μm. A high-magnification image (Figure 2B) of the microspheres reveals that the maximum diameter is around 2.5 μm. A further magnified image of these microspheres reveals that a microsphere is constructed of thousands of small nanoparticles (Figure 2C), the size of which tends to be very uniform at approximately 30–40 nm. This value agrees very closely with the grain size values calculated from XRD measurements. Similar observations have also been reported by Zhang et al. (6) for CdS microspheres prepared by a self-assembly method.
B
C
Photocatalytic activity studies Volatile organic compounds (VOCs) are major pollutants and are considered to be one of the most important contaminants generated by human beings living in urban and industrial areas (3). MTBE is a VOC that has been widely used as a gasoline additive to reduce VOC emissions from motor vehicles. However, new gasoline additives like MTBE are having negative environmental impacts. Recent survey reports clearly show that groundwater is often polluted owing to leakage of petroleum products from underground storage tanks. MTBE is highly soluble in water (e.g., 0.35–0.71 M) and has been detected at high concentrations in groundwater. The presence of MTBE in groundwater poses a potential health problem. The documented effects of MTBE exposure are headaches, vomiting, diarrhea, fever, cough, muscle aches, sleepiness, disorientation, dizziness, and skin and eye irritation. To address these problems, photocatalytic treatment is the preferred treatment for polluted water (8).
Figure 2 SEM images of CdS microspheres (sample CdS 3:2). (A) overall view, (B) individual microsphere, and (C) surface of a microsphere.
Although several authors have reported that CdS itself was not an effective photocatalyst for degrading organic pollutants (9), the present investigation clearly demonstrates that the hollow CdS microspheres were active and can be efficient photocatalysts for the degradation of MTBE under visible-light irradiation. Figure 3
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112 Selvaraj et al.: Nanotechnology in environmental remediation
GC peak intensity, a.u
8.0x105
collected at suitable time intervals for up to 60 min and then analyzed for total organic carbon (TOC) concentration. From the photocatalytic reaction, it has been observed that the initial TOC concentration decreased significantly in the presence of CdS microspheres. The result demonstrated that MTBE completely degraded within 1 h, but 25% of the TOC was left after 1 h. These results confirmed that in the presence of CdS microspheres, MTBE degradation is faster than its mineralization.
0 min 5 min 10 min 15 min 20 min 30 min 40 min 50 min 60 min
6.0x105
4.0x105
2.0x105
0.0 8.70
8.72
8.74 8.76 8.78 8.80 Retention time, min.
8.82
8.84
8.86
Figure 3 Time-dependent GC peak intensity of the MTBE solution (25 mg/L, 250 mL) in the presence of CdS 3:2 microspheres under visible light.
displays the time-dependent GC peak intensity of the MTBE solution during photodegradation in the presence of CdS microspheres. The peak at around 8.77 min decreased gradually with irradiation time. At the end of the degradation, no peak could be detected, implying that complete oxidation of MTBE occurred owing to the presence of hollow CdS microspheres under visible light. Finally, to study the mineralization of MTBE in the photocatalytic reaction, experiments were performed with an initial MTBE concentration of 25 mg/L (TOC = 5.4 mg/L) in the presence of CdS microspheres. The samples were
Conclusion In this study, we demonstrated a simple, versatile solution phase method for synthesizing high-quality CdS microspheres. High-magnification HR-SEM measurements showed that the hollow shell wall was constructed by the aggregation of several thousands of nanoparticles 30–40 nm in size. The hollow interior of these microspheres formed by Ostwald ripening. The CdS microspheres exhibited good photocatalytic activity for the degradation of MTBE aqueous solution under visible-light illumination. Because of their facile synthesis and controlled morphology, the obtained CdS microspheres may find potential applications in catalysis. Received January 17, 2014; accepted January 17, 2014; previously published online February 24, 2014
References 1. Caruso F, Caruso RA, Mohwald H. Nanoengineering of inorganic and hybrid hollow spheres by colloidal templating. Science 1998;282:1111–4. 2. Dai Z, Dahne L, Mohwald H, Tiersch B. Novel capsules with high stability and controlled permeability by hierarchic templating. Angew Chem Int Ed 2002;41:4019–22. 3. Rengaraj S, Jee SH, Venkataraj S, Kim Y, Vijayalakshmi S, et al. CdS microspheres composed of nanocrystals and their photocatalytic activity. J Nanosci Nanotech 2011;11:2090–9. 4. Rengaraj S, Venkataraj S, Jee SH, Kim Y, Tai C-W, et al. Cauliflower-like CdS microspheres composed of nanocrystals and their physicochemical properties. Langmuir 2011;27:352–8. 5. Rengaraj S, Venkataraj S, Tai C-W, Kim Y, Repo E, et al. Self-assembled mesoporous hierarchical-like In2S3 hollow
microspheres composed of nanofibers and nanosheets and their photocatalytic activity. Langmuir 2011;27:5534–41. 6. Zhang B, Jian JK, Zheng Y, Sun Y, Chen Y, et al. Low temperature hydrothermal synthesis of CdS submicro- and microspheres self-assembled from nanoparticles. Mater Lett 2008;62:1827–30. 7. Klug HP, Alexander LE. X-ray diffraction procedures. New York: Wiley, 1974:687. 8. Yang X, Wang Y, Xu L, Yu X, Guo Y. Aromaticity: an ab initio evaluation of the properly cyclic delocalization energy and the π-delocalization energy distortivity of benzene. J Phys Chem C 2008;112:11481–6. 9. Wu CZ, Xie Y, Lei LY, Hu SQ, Ou Yang CZ. Synthesis of new-phased VOOH hollow “dandelions” and their application in lithium-ion batteries. Adv Mater 2006;18:1727–32.
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