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Enhanced photocatalytic activity of CeO2 using β-cyclodextrin on visible light assisted decoloration of methylene blue Sakthivel Pitchaimuthu and Ponnusamy Velusamy
ABSTRACT An attempt has been made to enhance the photocatalytic activity of CeO2 for visible light assisted decoloration of methylene blue (MB) dye in aqueous solutions by β-cyclodextrin (β-CD). The inclusion complexation patterns between host and guest (i.e., β-CD and MB) have been confirmed with UV–visible spectral data. The interaction between CeO2 and β-CD has also been characterized by field emission scanning electron microscopy analysis. The photocatalytic activity of the catalyst
Sakthivel Pitchaimuthu Ponnusamy Velusamy (corresponding author) Centre for Research and Post-graduate Studies in Chemistry, Ayya Nadar Janaki Ammal College, Sivakasi – 626 124, Tamilnadu, India E-mail: [email protected]
under visible light was investigated by measuring the photodegradation of MB in aqueous solution. The effects of key operational parameters such as initial dye concentration, initial pH, CeO2 concentration as well as illumination time on the decolorization extents were investigated. Among the processing parameters, the pH of the reaction solution played an important role in tuning the photocatalytic activity of CeO2. The maximum photodecoloration rate was achieved at basic pH (pH 11). Under the optimum operational conditions, approximately 99.6% dye removal was achieved within 120 min. The observed results indicate that the decolorization of the MB followed a pseudofirst order kinetics. Key words
| β-CD, CeO2, methylene blue dye, photolytic decoloration
INTRODUCTION Large quantities of dyes are manufactured worldwide and used in a variety of applications. It is estimated that about 15% of the total world production is lost during the synthesis and processing. Therefore dyes can be found in wastewater released by the textile and dyestuff industries. Such colored dye effluents pose a major threat to the surrounding ecosystems owning to their non-biodegradability, toxicity and potential carcinogenic nature (Ji et al. ). Therefore, various chemical and physical processes such as precipitation, adsorption by activated carbon, air stripping, coagulation, reverse osmosis and membrane nanofiltration have been employed for color removal from textile effluents (Mo et al. ). However, these techniques are non-destructive and they transport organic compounds from one phase to another, which requires further treatment to avoid secondary pollution. Hence, it is necessary to develop treatment methods that are more effective in destroying dyes from wastewater (Natarajan et al. ). Among several proposed techniques for wastewater doi: 10.2166/wst.2013.553
treatment, the photocatalytic oxidation process provides an alternative interesting route for the detoxification of a variety of toxic and hazardous pollutants (Konstantinou & Albanis ). Many semiconductor photocatalysts (such as TiO2, ZnO, Fe2O3, CdS, CeO2 and ZnS) have been used to degrade organic pollutants. These semiconductors can act as sensitizers for light-induced redox processes due to their electronic structure, which is characterized by a filled valence band and an empty conduction band (Sakthivel et al. ; He et al. ; Ji et al. ; Li et al. ; Rego et al. ). It is reported by Ji et al. () that the properties of CeO2 are similar to the titania features such as wide band gap, non-toxicity, and high stability (Hu et al. ). Because of its unique 4f electron configuration, CeO2 has been frequently selected as a component to prepare complex oxides or as a dopant to improve titania-based catalysts. Zhai et al. () and Borker & Salker () recently
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Enhanced photocatalytic activity of CeO2 using β-CD
reported the photocatalytic behaviors of CeO2 under sunlight irradiation to degrade dyes. Cyclodextrin (CD)-modified semiconductor nanocomposites have attracted renewed interest since, β-CD could stabilize TiO2 colloids and facilitate interfacial electron transfer processes (Willner & Eichen ). Excellent studies have been published to prove the effects of β-CD on metal oxide photochemical properties (Lu et al. ; Tachikawa et al. ; Wang et al. ; Du et al. ; Velusamy et al. ; Pitchaimuthu et al. ; Rajalakshmi et al. ; Zhang et al. , ). CDs can form inclusion complexes with organic pollutants and organic pesticides to reduce the environmental impact of the chemical pollutants. In this study, the activity of CeO2 and the effect of addition of β-CD with CeO2 on photocatalytic decoloration of methylene blue (MB) dye solutions under visible light radiation have been studied and the results are documented. A suitable mechanism has also been proposed to explain the photocatalytic activity of CeO2.
EXPERIMENTAL Materials The commercial organic dye MB (molecular formula C16H18ClN3SxH2O, λmax ¼ 661 nm) obtained from Loba Chemie was used as received. The semiconductor photocatalyst CeO2 was purchased from Merck Chemicals. β-CD was received from HiMedia. All other chemicals were of analytical grade, received from Merck and used without further purification. Double distilled water was used throughout this study for the preparation of all the experimental solutions.
Characterization Field emission scanning electron microscopy (FE-SEM) was used to investigate the morphology of the samples of β-CD, CeO2 and CeO2–β-CD. FE-SEM images were obtained on a Carl Zeiss (ΣIGMA Series, Germany) microscope taken at an accelerated voltage of 2 kV. X-ray diffraction patterns of powder samples were recorded with a high-resolution powder X-ray diffractometer, model RICH SIERT & Co with CuKα radiation as the X-ray source (λ ¼ 1.5406 × 1010 m). UV-visible spectra were recorded by a UV-visible spectrophotometer (Shimadzu UV-1700) and the scan range was from 200 to 700 nm.
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Experimental conditions Photocatalytic decoloration experiments were carried out under visible light irradiation. MB dye solutions containing the photocatalysts of either CeO2 or CeO2–β-CD were prepared. The pH values of dye solutions were adjusted using a digital pen pH meter (Hanna Instruments, Portugal) depending on desired values with HCl and NaOH solution. Prior to the irradiation experiment, CeO2 suspensions were kept in the dark for 10 min. The dye is adsorbed onto the CeO2–β-CD surface and adsorption–desorption equilibrium is attained between the dye and the CeO2/visible light system. After adsorption the equilibrium concentration of the dye solution is determined and it is considered as the initial dye concentration for kinetic analyses. An annulartype visible (500 W, OSRAM, Heber Scientific, India) photoreactor was used as light source in the central axis. The reaction vessels were taken out at different intervals of time and the solutions were centrifuged. The supernatant liquid was collected for the determination of concentrations for the remaining dye by measuring its absorbance (at λmax ¼ 661 nm) with a visible spectrophotometer (ELICO, Model No. SL207). In all the cases, exactly 50 mL of the reactant solution was irradiated with the required amount of photocatalysts. The molar ratio concentrations of MB dye and β-CD were kept constant at 1:1 and the effects of the other experimental parameters on the rate of photocatalytic decoloration of MB dye solutions were investigated. The natural pH of MB dye solution was 7.1 and the irradiation time was fixed as 120 min. Preparation of CeO2–β-CD system for characterization In order to study the interaction of β-CD on the CeO2 surface, a suspension containing 2.0 g/L CeO2 and 10.0 g/L β-CD was magnetically stirred for 24 h, centrifuged, and then the solid phase was collected. After being centrifuged, the solid phase of the suspension was carefully washed with double distilled water. Eventually, the CeO2–β-CD system was dried at 50 C. The sample prepared in this way was used for FE-SEM analysis. W
RESULTS AND DISCUSSION Field emission scanning electron microscopy Figure 1 depicts FE-SEM micrographs of the bare β-CD, bare CeO2 and CeO2–β-CD systems respectively. Bare CD shows an amorphous surface. The surfaces of bare CeO2 and
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Figure 1
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FE-SEM analysis of (a) β-CD (b) CeO2 and (c) CeO2–β-CD.
CeO2–β-CD exhibit a similar morphology, which indicates that there is no change in the lattice structure of CeO2. However, the outer boundary of the CeO2–β-CD system was slightly different from CeO2. This may due to the aggregation of CeO2 and β-CD particles as the surfaces of the particles are very loose. This kind of surface structure can provide a better adsorption environment and more active site for the photocatalytic reaction (Pitchaimuthu et al. ; Rajalakshmi et al. ). UV-visible spectral analysis The molecular structure of β-CD allows various guest molecules with suitable dimensions to accommodate into its torous cavity and form host/guest inclusion complexes. In this study, the inclusion complex between β-CD and MB dye was characterized with UV-visible spectra as given in Figure 2. Figure 2 depicts that the absorbance of the inclusion complex increases with increasing concentration of β-CD (Wang et al. ). In this work, the optimum molar ratio between β-CD and MB dye was fixed as 1:1. Dissociation constant measurements The dissociation constant (KD) value for the complexation between β-CD and MB dye can be calculated using the Benesi–Hildebrand equation (Velusamy et al. , ; Rajalakshmi et al. ): ([C][S]=ΔOD) ¼ ([C] þ [S]=Δε) þ (KD =Δε)
(1)
where [C ] and [S] represent the concentrations of the β-CD and MB dye molecules respectively at equilibrium. ΔOD ¼
Figure 2
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UV-visible spectral analysis for the complexation pattern between β-CD and
MB dye. (a) MB dye, (b) 1:1 β-CD/MB, (c) 2:1 β-CD/MB, (d) 3:1 β-CD/MB, (e) 4:1
β-CD/MB, (f) 5:1 β-CD/MB, (g) 6:1 β-CD/MB.
the increase in absorption upon addition of β-CD. Δε ¼ the difference in molar extinction coefficients between the bound and the free MB dye. KD¼ dissociation constant. KD can be obtained from the ratio of the intercept (KD/Δε) and the slope (1/Δε) from the linear plot of [C] [S]/ΔOD versus {[C ] þ [S]} (Figure (3a)). The determined KD value is 5.44 × 103 M. This confirms the effective complexation between MB and β-CD. Effect of operational parameters The effect of initial MB dye concentration on the photocatalytic efficiency has been explored by varying the initial dye concentration from 1.563 × 105 to 9.379 × 105 M with catalyst loading of 2 g/L. It can be seen that the percentage decoloration decreases with the
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Figure 3
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Enhanced photocatalytic activity of CeO2 using β-CD
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(a) {[C ] [S]/ΔOD} × 107 vs {[C ] þ [S]} × 105. (b) Effect of initial concentration of MB dye. (c) Effect of initial pH. (d) Effect of CeO2 concentration. (e) Effect of illumination time. (f) Decoloration kinetics.
increase in the MB dye concentration (Figure 3(b)). The possible explanation for this phenomenon is that the amount of dye adsorbed on the catalyst surface increases with the increase of dye concentration, but the intensity of light and irradiation time are constant; so the path length of incident photons entering the solution decreases
and consequently the hydroxyl radicals formed on the surface of CeO2 decreases, which results in the decrease of removal efficiency (Zhang & Zeng ). Since photocatalysis is a surface reaction, the photocatalytic performance of any semiconductor catalyst can be highly influenced by the pH of dye solution, the type of
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the dye, and the ability to be absorbed onto the photocatalyst surface. The pH value dependence can be explained on the basis of the metal oxide point of zero charge (pzc). The pHpzc of CeO2 is 6.8 ( Ji et al. ). For pH values less than pHpzc the surface becomes positively charged, and for pH values greater than pHpzc the CeO2 surface will be negatively charged. The effect of pH on the efficiency of the photocatalytic degradation process can be explained on the basis of the acid base property of the metal oxide surface and the ionization state of the ionisable organic molecule (Bansal & Sud ). CeOH þ Hþ ! CeOHþ 2 (pH < pzc)
þ
CeOH ! CeO þ H (pH > pzc)
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Decoloration kinetics The experimental results indicate that the degradation rates of the photocatalytic decoloration process of MB dye over illuminated CeO2 and CeO2–β-CD systems fit the pseudofirst order kinetic model (Krishnakumar et al. ). The regression curve of the natural logarithm of Co/Ct versus illumination time gives a straight line in both cases (i.e. CeO2/ visible light system and CeO2–β-CD/visible light system) (Figure 3(f)). The linearity of the plot suggests that the photodecoloration reaction approximately follows the pseudo-first order kinetics using the formula:
(2) ln (Co =Ct ) ¼ kt (3)
The percentage decoloration of MB dye for CeO2 and CeO2–β-CD systems at pH 11 are 79.8 and 99.6%, respectively. The observed results indicate that the decoloration of MB dye is easier and quicker with CeO2–β-CD systems than that of the CeO2 system (Figure 3(c)). An increase in decoloration efficiency, for pH > pHzpc, can be ascribed to a negatively charged photocatalyst surface being more available for the absorption of the MB molecules. For the sake of avoiding the use of excess catalyst it is essential to determine the optimum loading for efficient removal of MB dye. The effect of catalyst loading on the removal efficiency of the dye was explored in Figure 3(d). As can be seen, when the concentration of the catalyst increases from 0.5 to 3.0 g/L, the photocatalytic decoloration efficiency increases with an increase in the amount of CeO2. This is due to the fact that increase in the number of MB dye molecules adsorbed on the CeO2 surface leads to increase in rate of decoloration (Chen ). As CeO2 concentration increases, the availability of the CeO2 surface for the adsorption of MB dye increased. The minimum percentage removal of MB dye at lower CeO2 concentration can be attributed to the fact that the transmitted light is not utilised in the photocatalytic reaction. Illumination time also plays a vital role in the decoloration process of the pollutants. In this way, the illumination time was increased from 30 min to 180 min. The remaining concentration of MB dye is decreased with an increase in the illumination time (Figure 3(e)). It is observed that nearly 92.5% decoloration of MB dye solution is achieved within 180 min.
(4)
where Co and Ct represent the initial concentration of the corresponding MB dye in solution and that of illumination time of t respectively and kt represents the apparent rate constant (min1). Mechanism of the effect of β-CD on photodecoloration The proposed mechanism for the photocatalytic decoloration of MB dye by excitation of the CeO2–β-CD system is shown in Figure 4. Since β-CD has higher affinity on metal oxide surface than dye molecules, β-CD molecules could be adsorbed on the CeO2 surface and engage the active sites. β-CD would capture holes on the active CeO2 surface, resulting in the formation of a stable CeO2–β-CD complex. Thus the inclusion complex reaction of β-CD with MB dye molecules should be the key step in photocatalytic decoloration in CeO2 suspension containing β-CD. Dye molecules form an inclusion complex, resulting in the indirect photodecoloration being the main reaction channel (Zhang et al. ). MB dye molecules enter into the cavity of β-CD,
Figure 4
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Proposed mechanism for the photodecoloration of MB dye with CeO2–β-CD.
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which is linked to the CeO2 surface in the equilibrium stage and they absorb light radiation followed by excitation. An electron is rapidly injected from the excited dye to the conduction band of CeO2 (Velusamy et al. ; Pitchaimuthu et al. ). Another important radical in illumination of CeO2–β-CD is the superoxide anion radical (•O2). The dye and dye cation radical then undergo degradation. CeO2–β-CD could show significant photocatalytic activity mainly because β-CD could trap the photo-generated þ holes resulting in the lower e CB/hVB recombination (Zhang et al. ). In general, the lifetimes for the excited states of unreacted guests is prolonged when incorporated inside the cavity of cyclodextrins. Therefore, cyclodextrin facilitates the electron injection from the excited dyes to the CeO2 conduction band and thereby enhances the decoloration (Zhang et al. ; Velusamy et al. ).
CONCLUSION In this work, we studied the effect of β-CD on the photodegradation of MB dye in CeO2 aqueous solution. FE-SEM exhibits similar morphology, which indicates that there is no change in the lattice structure of CeO2. However, the outer boundary of the CeO2–β-CD system was slightly different from CeO2. Photodecoloration of MB dye in the CeO2–β-CD/visible light system exhibits better photocatalytic decoloration efficiency than that of the CeO2/visible light system. This work provides essential information on the promotion effects of β-CD on the photodegradability of CeO2 on dye in aqueous solution.
ACKNOWLEDGEMENTS The authors thank the management and the Principal of Ayya Nadar Janaki Ammal College, Sivakasi, India, for providing necessary facilities. The authors also thank the University Grants Commission, New Delhi, for the financial support through UGC-Major Research Project Ref. [UGC Ref. No. F. No. 38-22/2009 (SR) dated: 19.12.2009]. The Instrumentation Centre, Ayya Nadar Janaki Ammal College, Sivakasi, and the Centre for Nanoscience and Nanotechnology, Bharathidasan University, Tiruchirappalli, are highly appreciated for recording the UV-visible and FE-SEM analyses respectively.
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REFERENCES Bansal, P. & Sud, D. Photodegradation of commercial dye, Procion Blue HERD from real textile wastewater using nanocatalysts. Desalination 267, 244–249. Borker, P. & Salker, A. V. Solar assisted photocatalytic degradation of Naphthol Blue Black dye using Ce1xMnxO2. Materials Chemistry and Physics 103, 366–370. Chen, C. Y. Photocatalytic degradation of azo dye reactive orange 16 by TiO2. Water, Air, & Soil Pollution 202, 335–342. Du, M., Feng, J. & Zhang, S. B. Photo-oxidation of polyhydroxyl molecules on TiO2 surfaces: from hole scavenging to light-induced self-assembly of TiO2-cyclodextrin wires. Physics Review Letter 98, art. 066102, 4 pp. He, T., Ma, H., Zhou, Z., Xu, W., Ren, F., Shi, Z. & Wang, J. Preparation of ZnS–Fluoropolymer nanocomposites and its photocatalytic degradation of methylene blue. Journal of Polymer Degradation and Stability 94, 2251–2256. Hu, S., Zhou, F., Wang, L. & Zhang, J. Preparation of Cu2O/ CeO2 heterojunction photocatalyst for the degradation of Acid Orange 7 under visible light irradiation. Catalysis Communications 12, 794–797. Ji, P., Zhang, J., Chen, F. & Anpo, M. Study of adsorption and degradation of acid orange 7 on the surface of CeO2 under visible light irradiation. Applied Catalysis B: Environmental 85, 148–154. Konstantinou, I. K. & Albanis, T. A. Photocatalytic transformations of pesticides in aqueous titanium dioxide suspensions using artificial and solar light: intermediates and degradation pathways. Applied Catalysis B: Environmental 42, 319–335. Krishnakumar, B., Subash, B. & Swaminathan, M. AgBr– ZnO – An efficient nano-photocatalyst for the mineralization of Acid Black 1 with UV light. Separation and Purification Technology 85, 35–44. Li, J., Ni, Y., Liu, J. & Hong, J. Preparation, conversion, and comparison of the photocatalytic property of Cd(OH)2, CdO, CdS and CdSe. Journal of Physics and Chemistry of Solids 70, 1285–1289. Lu, P., Wu, F. & Deng, N. S. Enhancement of TiO2 photocatalytic redox ability by β-cyclodextrin in suspended solutions. Applied Catalysis B: Environmental 53, 87–93. Mo, J., Hwang, J. E., Jegal, J. & Kim, J. Pretreatment of a dyeing wastewater using chemical coagulants. Dyes and Pigments 72, 240–245. Natarajan, T. S., Thomas, M., Natarajan, K., Bajaj, H. C. & Tayade, R. J. Study on UV-LED/TiO2 process for degradation of Rhodamine B dye. Chemical Engineering Journal 169, 126–134. Pitchaimuthu, S., Velusamy, P., Rajalakshmi, S. & Kannan, N. Enhanced photocatalytic activity of titanium dioxide by βcyclodextrin in decoloration of Acid Yellow 99 dye. Desalination and Water Treatment (in press), DOI: 10.1080/ 19443994.2013.799049. Rajalakshmi, S., Pitchaimuthu, S., Velusamy, P. & Kannan, N. Photocatalytic effect of β-cyclodextrin on semiconductors for the removal of Acid Violet dye under UV light irradiation.
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Desalination and Water Treatment (in press), DOI: 10.1080/ 19443994.2013.809024. Rego, E., Marto, J., Marcos, P. S. & Labrincha, J. A. Decolouration of Orange II solutions by TiO2 and ZnO active layers screen-printed on ceramic tiles under sunlight irradiation. Applied Catalysis A: General 355, 109–114. Sakthivel, S., Geissen, S. U., Bahnemann, D. W., Murugesan, V. & Vogelpohl, A. Enhancement of photocatalytic activity by semiconductor heterojunctions: α-Fe2O3, WO3 and CdS deposited on ZnO. Journal of Photochemistry and Photobiology A: Chemistry 148, 283–293. Tachikawa, T., Tojo, S., Fujitsuka, M. & Majima, T. Oneelectron oxidation pathways during β-cyclodextrin-modified TiO2 photocatalytic reactions. Chemistry A. European Journal 12, 7585–7594. Velusamy, P., Pitchumani, K. & Srinivasan, C. Selectivity in the bromination of aniline and N-substituted anilines encapsulated in β-cyclodextrin. Tetrahedron 52, 3487–3496. Velusamy, P., Pitchaimuthu, S., Rajalakshmi, S. & Kannan, N. Modification of the photocatalytic activity of TiO2 by bcyclodextrin in decoloration of ethyl violet dye. Journal of Advanced Research (in press), http://dx.doi.org/10.1016/j. jare.2012.10.001. Wang, G. H., Wu, F., Zhang, X., Luo, M. D. & Deng, N. S. Enhanced TiO2 photocatalytic degradation of
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bisphenol A by β-cyclodextrin in suspended solutions. Journal of Photochemistry and Photobiology A: Chemistry 179, 49–56. Willner, Y. & Eichen, A. J. Titanium dioxide and cadmium sulfide colloids stabilized by β-cyclodextrins: Tailored semiconductor-receptor systems as a means to control interfacial electron-transfer processes. Journal of American Chemical Society 109, 6862–6863. Zhai, Y. Q., Zhang, S. Y. & Pang, H. Preparation, characterization and photocatalytic activity of CeO2 nanocrystalline using ammonium bicarbonate as precipitant. Materials Letter 61, 1863–1866. Zhang, D. & Zeng, F. Visible light-activated cadmium-doped ZnO nanostructured photocatalyst for the treatment of methylene blue dye. Journal of Materials Science 47, 2155–2161. Zhang, X., Wu, F., Wang, Z., Guo, Y. & Deng, N. S. Photocatalytic degradation of 4,40 -biphenol in TiO2 suspension in the presence of cyclodextrins: A trinity integrated mechanism. Journal of Molecular Catalysis A: Chemical 301, 134–139. Zhang, X., Li, X. & Deng, N. Enhanced and selective degradation of pollutants over cyclodextrin/TiO2 under visible light irradiation. Industrial & Engineering Chemistry Research 51, 704–709.
First received 29 July 2013; accepted in revised form 11 September 2013. Available online 25 October 2013
© IWA Publishing 2014 Water Science & Technology
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Enhanced photocatalytic activity of CeO2 using β-cyclodextrin on visible light assisted decoloration of methylene blue Sakthivel Pitchaimuthu and Ponnusamy Velusamy
ABSTRACT An attempt has been made to enhance the photocatalytic activity of CeO2 for visible light assisted decoloration of methylene blue (MB) dye in aqueous solutions by β-cyclodextrin (β-CD). The inclusion complexation patterns between host and guest (i.e., β-CD and MB) have been confirmed with UV–visible spectral data. The interaction between CeO2 and β-CD has also been characterized by field emission scanning electron microscopy analysis. The photocatalytic activity of the catalyst
Sakthivel Pitchaimuthu Ponnusamy Velusamy (corresponding author) Centre for Research and Post-graduate Studies in Chemistry, Ayya Nadar Janaki Ammal College, Sivakasi – 626 124, Tamilnadu, India E-mail: [email protected]
under visible light was investigated by measuring the photodegradation of MB in aqueous solution. The effects of key operational parameters such as initial dye concentration, initial pH, CeO2 concentration as well as illumination time on the decolorization extents were investigated. Among the processing parameters, the pH of the reaction solution played an important role in tuning the photocatalytic activity of CeO2. The maximum photodecoloration rate was achieved at basic pH (pH 11). Under the optimum operational conditions, approximately 99.6% dye removal was achieved within 120 min. The observed results indicate that the decolorization of the MB followed a pseudofirst order kinetics. Key words
| β-CD, CeO2, methylene blue dye, photolytic decoloration
INTRODUCTION Large quantities of dyes are manufactured worldwide and used in a variety of applications. It is estimated that about 15% of the total world production is lost during the synthesis and processing. Therefore dyes can be found in wastewater released by the textile and dyestuff industries. Such colored dye effluents pose a major threat to the surrounding ecosystems owning to their non-biodegradability, toxicity and potential carcinogenic nature (Ji et al. ). Therefore, various chemical and physical processes such as precipitation, adsorption by activated carbon, air stripping, coagulation, reverse osmosis and membrane nanofiltration have been employed for color removal from textile effluents (Mo et al. ). However, these techniques are non-destructive and they transport organic compounds from one phase to another, which requires further treatment to avoid secondary pollution. Hence, it is necessary to develop treatment methods that are more effective in destroying dyes from wastewater (Natarajan et al. ). Among several proposed techniques for wastewater doi: 10.2166/wst.2013.553
treatment, the photocatalytic oxidation process provides an alternative interesting route for the detoxification of a variety of toxic and hazardous pollutants (Konstantinou & Albanis ). Many semiconductor photocatalysts (such as TiO2, ZnO, Fe2O3, CdS, CeO2 and ZnS) have been used to degrade organic pollutants. These semiconductors can act as sensitizers for light-induced redox processes due to their electronic structure, which is characterized by a filled valence band and an empty conduction band (Sakthivel et al. ; He et al. ; Ji et al. ; Li et al. ; Rego et al. ). It is reported by Ji et al. () that the properties of CeO2 are similar to the titania features such as wide band gap, non-toxicity, and high stability (Hu et al. ). Because of its unique 4f electron configuration, CeO2 has been frequently selected as a component to prepare complex oxides or as a dopant to improve titania-based catalysts. Zhai et al. () and Borker & Salker () recently
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Enhanced photocatalytic activity of CeO2 using β-CD
reported the photocatalytic behaviors of CeO2 under sunlight irradiation to degrade dyes. Cyclodextrin (CD)-modified semiconductor nanocomposites have attracted renewed interest since, β-CD could stabilize TiO2 colloids and facilitate interfacial electron transfer processes (Willner & Eichen ). Excellent studies have been published to prove the effects of β-CD on metal oxide photochemical properties (Lu et al. ; Tachikawa et al. ; Wang et al. ; Du et al. ; Velusamy et al. ; Pitchaimuthu et al. ; Rajalakshmi et al. ; Zhang et al. , ). CDs can form inclusion complexes with organic pollutants and organic pesticides to reduce the environmental impact of the chemical pollutants. In this study, the activity of CeO2 and the effect of addition of β-CD with CeO2 on photocatalytic decoloration of methylene blue (MB) dye solutions under visible light radiation have been studied and the results are documented. A suitable mechanism has also been proposed to explain the photocatalytic activity of CeO2.
EXPERIMENTAL Materials The commercial organic dye MB (molecular formula C16H18ClN3SxH2O, λmax ¼ 661 nm) obtained from Loba Chemie was used as received. The semiconductor photocatalyst CeO2 was purchased from Merck Chemicals. β-CD was received from HiMedia. All other chemicals were of analytical grade, received from Merck and used without further purification. Double distilled water was used throughout this study for the preparation of all the experimental solutions.
Characterization Field emission scanning electron microscopy (FE-SEM) was used to investigate the morphology of the samples of β-CD, CeO2 and CeO2–β-CD. FE-SEM images were obtained on a Carl Zeiss (ΣIGMA Series, Germany) microscope taken at an accelerated voltage of 2 kV. X-ray diffraction patterns of powder samples were recorded with a high-resolution powder X-ray diffractometer, model RICH SIERT & Co with CuKα radiation as the X-ray source (λ ¼ 1.5406 × 1010 m). UV-visible spectra were recorded by a UV-visible spectrophotometer (Shimadzu UV-1700) and the scan range was from 200 to 700 nm.
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Experimental conditions Photocatalytic decoloration experiments were carried out under visible light irradiation. MB dye solutions containing the photocatalysts of either CeO2 or CeO2–β-CD were prepared. The pH values of dye solutions were adjusted using a digital pen pH meter (Hanna Instruments, Portugal) depending on desired values with HCl and NaOH solution. Prior to the irradiation experiment, CeO2 suspensions were kept in the dark for 10 min. The dye is adsorbed onto the CeO2–β-CD surface and adsorption–desorption equilibrium is attained between the dye and the CeO2/visible light system. After adsorption the equilibrium concentration of the dye solution is determined and it is considered as the initial dye concentration for kinetic analyses. An annulartype visible (500 W, OSRAM, Heber Scientific, India) photoreactor was used as light source in the central axis. The reaction vessels were taken out at different intervals of time and the solutions were centrifuged. The supernatant liquid was collected for the determination of concentrations for the remaining dye by measuring its absorbance (at λmax ¼ 661 nm) with a visible spectrophotometer (ELICO, Model No. SL207). In all the cases, exactly 50 mL of the reactant solution was irradiated with the required amount of photocatalysts. The molar ratio concentrations of MB dye and β-CD were kept constant at 1:1 and the effects of the other experimental parameters on the rate of photocatalytic decoloration of MB dye solutions were investigated. The natural pH of MB dye solution was 7.1 and the irradiation time was fixed as 120 min. Preparation of CeO2–β-CD system for characterization In order to study the interaction of β-CD on the CeO2 surface, a suspension containing 2.0 g/L CeO2 and 10.0 g/L β-CD was magnetically stirred for 24 h, centrifuged, and then the solid phase was collected. After being centrifuged, the solid phase of the suspension was carefully washed with double distilled water. Eventually, the CeO2–β-CD system was dried at 50 C. The sample prepared in this way was used for FE-SEM analysis. W
RESULTS AND DISCUSSION Field emission scanning electron microscopy Figure 1 depicts FE-SEM micrographs of the bare β-CD, bare CeO2 and CeO2–β-CD systems respectively. Bare CD shows an amorphous surface. The surfaces of bare CeO2 and
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FE-SEM analysis of (a) β-CD (b) CeO2 and (c) CeO2–β-CD.
CeO2–β-CD exhibit a similar morphology, which indicates that there is no change in the lattice structure of CeO2. However, the outer boundary of the CeO2–β-CD system was slightly different from CeO2. This may due to the aggregation of CeO2 and β-CD particles as the surfaces of the particles are very loose. This kind of surface structure can provide a better adsorption environment and more active site for the photocatalytic reaction (Pitchaimuthu et al. ; Rajalakshmi et al. ). UV-visible spectral analysis The molecular structure of β-CD allows various guest molecules with suitable dimensions to accommodate into its torous cavity and form host/guest inclusion complexes. In this study, the inclusion complex between β-CD and MB dye was characterized with UV-visible spectra as given in Figure 2. Figure 2 depicts that the absorbance of the inclusion complex increases with increasing concentration of β-CD (Wang et al. ). In this work, the optimum molar ratio between β-CD and MB dye was fixed as 1:1. Dissociation constant measurements The dissociation constant (KD) value for the complexation between β-CD and MB dye can be calculated using the Benesi–Hildebrand equation (Velusamy et al. , ; Rajalakshmi et al. ): ([C][S]=ΔOD) ¼ ([C] þ [S]=Δε) þ (KD =Δε)
(1)
where [C ] and [S] represent the concentrations of the β-CD and MB dye molecules respectively at equilibrium. ΔOD ¼
Figure 2
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UV-visible spectral analysis for the complexation pattern between β-CD and
MB dye. (a) MB dye, (b) 1:1 β-CD/MB, (c) 2:1 β-CD/MB, (d) 3:1 β-CD/MB, (e) 4:1
β-CD/MB, (f) 5:1 β-CD/MB, (g) 6:1 β-CD/MB.
the increase in absorption upon addition of β-CD. Δε ¼ the difference in molar extinction coefficients between the bound and the free MB dye. KD¼ dissociation constant. KD can be obtained from the ratio of the intercept (KD/Δε) and the slope (1/Δε) from the linear plot of [C] [S]/ΔOD versus {[C ] þ [S]} (Figure (3a)). The determined KD value is 5.44 × 103 M. This confirms the effective complexation between MB and β-CD. Effect of operational parameters The effect of initial MB dye concentration on the photocatalytic efficiency has been explored by varying the initial dye concentration from 1.563 × 105 to 9.379 × 105 M with catalyst loading of 2 g/L. It can be seen that the percentage decoloration decreases with the
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(a) {[C ] [S]/ΔOD} × 107 vs {[C ] þ [S]} × 105. (b) Effect of initial concentration of MB dye. (c) Effect of initial pH. (d) Effect of CeO2 concentration. (e) Effect of illumination time. (f) Decoloration kinetics.
increase in the MB dye concentration (Figure 3(b)). The possible explanation for this phenomenon is that the amount of dye adsorbed on the catalyst surface increases with the increase of dye concentration, but the intensity of light and irradiation time are constant; so the path length of incident photons entering the solution decreases
and consequently the hydroxyl radicals formed on the surface of CeO2 decreases, which results in the decrease of removal efficiency (Zhang & Zeng ). Since photocatalysis is a surface reaction, the photocatalytic performance of any semiconductor catalyst can be highly influenced by the pH of dye solution, the type of
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the dye, and the ability to be absorbed onto the photocatalyst surface. The pH value dependence can be explained on the basis of the metal oxide point of zero charge (pzc). The pHpzc of CeO2 is 6.8 ( Ji et al. ). For pH values less than pHpzc the surface becomes positively charged, and for pH values greater than pHpzc the CeO2 surface will be negatively charged. The effect of pH on the efficiency of the photocatalytic degradation process can be explained on the basis of the acid base property of the metal oxide surface and the ionization state of the ionisable organic molecule (Bansal & Sud ). CeOH þ Hþ ! CeOHþ 2 (pH < pzc)
þ
CeOH ! CeO þ H (pH > pzc)
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Decoloration kinetics The experimental results indicate that the degradation rates of the photocatalytic decoloration process of MB dye over illuminated CeO2 and CeO2–β-CD systems fit the pseudofirst order kinetic model (Krishnakumar et al. ). The regression curve of the natural logarithm of Co/Ct versus illumination time gives a straight line in both cases (i.e. CeO2/ visible light system and CeO2–β-CD/visible light system) (Figure 3(f)). The linearity of the plot suggests that the photodecoloration reaction approximately follows the pseudo-first order kinetics using the formula:
(2) ln (Co =Ct ) ¼ kt (3)
The percentage decoloration of MB dye for CeO2 and CeO2–β-CD systems at pH 11 are 79.8 and 99.6%, respectively. The observed results indicate that the decoloration of MB dye is easier and quicker with CeO2–β-CD systems than that of the CeO2 system (Figure 3(c)). An increase in decoloration efficiency, for pH > pHzpc, can be ascribed to a negatively charged photocatalyst surface being more available for the absorption of the MB molecules. For the sake of avoiding the use of excess catalyst it is essential to determine the optimum loading for efficient removal of MB dye. The effect of catalyst loading on the removal efficiency of the dye was explored in Figure 3(d). As can be seen, when the concentration of the catalyst increases from 0.5 to 3.0 g/L, the photocatalytic decoloration efficiency increases with an increase in the amount of CeO2. This is due to the fact that increase in the number of MB dye molecules adsorbed on the CeO2 surface leads to increase in rate of decoloration (Chen ). As CeO2 concentration increases, the availability of the CeO2 surface for the adsorption of MB dye increased. The minimum percentage removal of MB dye at lower CeO2 concentration can be attributed to the fact that the transmitted light is not utilised in the photocatalytic reaction. Illumination time also plays a vital role in the decoloration process of the pollutants. In this way, the illumination time was increased from 30 min to 180 min. The remaining concentration of MB dye is decreased with an increase in the illumination time (Figure 3(e)). It is observed that nearly 92.5% decoloration of MB dye solution is achieved within 180 min.
(4)
where Co and Ct represent the initial concentration of the corresponding MB dye in solution and that of illumination time of t respectively and kt represents the apparent rate constant (min1). Mechanism of the effect of β-CD on photodecoloration The proposed mechanism for the photocatalytic decoloration of MB dye by excitation of the CeO2–β-CD system is shown in Figure 4. Since β-CD has higher affinity on metal oxide surface than dye molecules, β-CD molecules could be adsorbed on the CeO2 surface and engage the active sites. β-CD would capture holes on the active CeO2 surface, resulting in the formation of a stable CeO2–β-CD complex. Thus the inclusion complex reaction of β-CD with MB dye molecules should be the key step in photocatalytic decoloration in CeO2 suspension containing β-CD. Dye molecules form an inclusion complex, resulting in the indirect photodecoloration being the main reaction channel (Zhang et al. ). MB dye molecules enter into the cavity of β-CD,
Figure 4
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Proposed mechanism for the photodecoloration of MB dye with CeO2–β-CD.
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which is linked to the CeO2 surface in the equilibrium stage and they absorb light radiation followed by excitation. An electron is rapidly injected from the excited dye to the conduction band of CeO2 (Velusamy et al. ; Pitchaimuthu et al. ). Another important radical in illumination of CeO2–β-CD is the superoxide anion radical (•O2). The dye and dye cation radical then undergo degradation. CeO2–β-CD could show significant photocatalytic activity mainly because β-CD could trap the photo-generated þ holes resulting in the lower e CB/hVB recombination (Zhang et al. ). In general, the lifetimes for the excited states of unreacted guests is prolonged when incorporated inside the cavity of cyclodextrins. Therefore, cyclodextrin facilitates the electron injection from the excited dyes to the CeO2 conduction band and thereby enhances the decoloration (Zhang et al. ; Velusamy et al. ).
CONCLUSION In this work, we studied the effect of β-CD on the photodegradation of MB dye in CeO2 aqueous solution. FE-SEM exhibits similar morphology, which indicates that there is no change in the lattice structure of CeO2. However, the outer boundary of the CeO2–β-CD system was slightly different from CeO2. Photodecoloration of MB dye in the CeO2–β-CD/visible light system exhibits better photocatalytic decoloration efficiency than that of the CeO2/visible light system. This work provides essential information on the promotion effects of β-CD on the photodegradability of CeO2 on dye in aqueous solution.
ACKNOWLEDGEMENTS The authors thank the management and the Principal of Ayya Nadar Janaki Ammal College, Sivakasi, India, for providing necessary facilities. The authors also thank the University Grants Commission, New Delhi, for the financial support through UGC-Major Research Project Ref. [UGC Ref. No. F. No. 38-22/2009 (SR) dated: 19.12.2009]. The Instrumentation Centre, Ayya Nadar Janaki Ammal College, Sivakasi, and the Centre for Nanoscience and Nanotechnology, Bharathidasan University, Tiruchirappalli, are highly appreciated for recording the UV-visible and FE-SEM analyses respectively.
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First received 29 July 2013; accepted in revised form 11 September 2013. Available online 25 October 2013
