Effective Photocatalytic Reduction of Cr(VI) by Carbon Modified (CM)-n-TiO2 Nanoparticles under Solar Irradiation

Yasser A. Shaban

Marine Chemistry Department, Faculty of Marine Sciences, King Abdulaziz University, Jeddah, KSA; National Institute of Oceanography & Fisheries, Qayet Bay, Alexandria, Egypt.

DOI: 10.4236/wjnse.2013.34018                

Abstract

Photocatalytic reduction of toxic Cr(VI) was successfully achieved using carbon modified titanium oxide (CM-n-TiO2) nanoparticles under natural sunlight illumination. Modification of titanium oxide by carbon significantly enhanced the photocatalytic reduction of Cr(VI) under natural sunlight irradiation. The effects of various experimental parameters such as catalyst dose, initial concentration of Cr(VI), and solution pH on the reduction rate of Cr(VI) were investigated. The highest reduction rate of Cr(VI) was obtained at the optimal conditions of pH 5 and 2.0 g·L?1 of CM-n-TiO2. Interestingly, in the presence of phenol, as a sacrificial electron donor, the rate of Cr(VI) reduction was nearly 1.7 times higher than in its absence. The solar photoreduction of Cr(VI) in aqueous solution using CM-n-TiO2 obeyed a pseudo-first order kinetics according to the Langmuir-Hinshelwood model.

Keywords

Photocatalytic Reduction; Chromium; Titanium Oxide; Carbon Modification

1. Introduction

The discharge of toxic heavy metals into aquatic environment has been known to cause serious pollution problems. Among these pollutants, chromium possesses the most severe environmental concern due to its high toxicity, potential carcinogenicity, and high mobility in water [1,2]. The major sources of chromium pollution are electrochemical, steel manufacturing industries and leather tanning [3,4]. In aquatic environments, chromium exists in hexavalent (Cr(VI)) and trivalent (Cr(III)) forms, of which hexavalent form is more toxic than the trivalent one, and is known to be human carcinogen [5]. Different techniques have been reported for the treatments of Cr (VI), such as ion exchange [6], membrane separation [7], physical and biological adsorption [8-10], and electrocoagulation [11,12]. However, most of these techniques have several limitations and drawbacks, and they require either high energy or massive use of reducing agents.

Semiconductor photocatalysis has attracted considerable interest as an effective and economical technique for detoxification of polluted waters [13-22]. It can effectively reduce highly toxic Cr(VI) into the less harmful Cr(III), which can then be precipitated as Cr(OH)₃ in neutral or alkaline solutions [23]. Titanium dioxide photocatalyst was considered as one of the most practical candidates due to its optical and electronic properties, low cost, high level of photocatalytic activity, chemical stability and non-toxicity. However, its wide band gap (3.0 - 3.2 eV) limits its photoresponse in the ultraviolet region which is only a small fraction (~5%). Therefore, much attention has been devoted to enhancing its catalytic efficiency or expanding its applicability under solar irradiation. Recently, carbon modification of n-TiO₂ has been proved to be an effective approach to enhance its catalytic efficiency [21,22,24-26].

Most of the studies on heterogeneous photocatalytic reduction of Cr(VI) using n-TiO₂ were performed under illumination of UV light. Therefore, in this study, visible light active carbon-modified (CM)-n-TiO₂ nanoparticles were prepared via sol-gel method. The photocatalytic performance of CM-n-TiO₂ was examined for the photoreduction of Cr(VI) in aqueous solution under illuminetion of natural sunlight. The photocatalytic activity of CM-n-TiO₂ was compared with regular n-TiO₂. The effects of various experimental parameters such as photocatalyst loading, Cr(VI) concentration, and pH on the photocatalytic removal rate of Cr(VI) were studied. Additionally, the effect of presence of phenol, as a sacrificial electron donor, on the photocatalytic reduction of Cr(VI) was also investigated.

2. Experimental

2.1. Synthesis and Characterization of n-TiO₂ and CM-n-TiO₂ Nanoparticles

Visible light active carbon modified titanium dioxide (CM-n-TiO₂) nanoparticals were synthesized by a sol-gel method using titanium butoxide (Ti[O(CH₂)₃CH₃]₄, Fluka, 97%), a carbon-containing precursor, as a molecular precursor of TiO₂ as well as a carbon source. Regular (unmodified) titanium dioxide (n-TiO₂) nanoparticles were synthesized by hydrolysis and oxidation of titanium trichloride (TiCl₃ 12% in hydrochloric acid (5% - 12%), Sigma-Aldrich) in an aqueous medium. Details on the procedures used for catalysts preparation and characterization can be found elsewhere [22].

2.2. Photocatalytic Experiments

All solar photocatalytic experiments were carried out at the Faculty of Marine sciences, Obhur, Jeddah, KSA, in the daytime between 11:00 am to 15:00 pm, during MayJune, 2013. Experimental set up consisted of a magnetically stirred 500 mL glass photoreactor loaded with the aqueous solution containing different concentrations of Cr(VI) ranging from 1 to 10 ppm, then the synthesized photocatalyst (n-TiO₂ or CM-n-TiO₂) was added. The photocatalytic reactor was then directly exposed to natural sunlight. The average solar intensity was about 1200 W m⁻², measured by Field Scout Light Sensor Reader (Spectrum Technologies, Inc.) equipped with 3670i Silicon Pyranometer Sensor. Prior to analysis, aliquots of treated samples were regularly withdrawn from the reactor and centrifuged immediately to remove the catalyst. The Cr(VI) was determined colorimetrically at 540 nm using a Shimadzu UV-VIS Spectrophotometer (Model PharmaSpec UV-1700) according to the diphenylcarbazide colorimetric method [27].

3. Results and Discussion

3.1. Photocatalytic Activity of n-TiO₂ and CM-n-TiO₂

Figure 1 compares the photoreduction efficiency of 3.0 ppm of Cr(VI) under illumination of real sunlight in the presence of 2.0 g·L⁻¹ of CM-n-TiO₂ and n-TiO₂, respectively. It is clearly observed that the photocatalytic activity of CM-n-TiO₂ is higher than that of n-TiO₂ under sunlight irradiation. Complete photoreduction of 3.0 ppm of Cr (VI) was achieved after only 10 min using 2.0 g/L

Conflicts of Interest

The authors declare no conflicts of interest.

References

[1]	C. Lee and P. Linneman, S. Rengaraj, S. Venkataraj, J. Yeon, Y. Kim, X. Z. Li and G. K. H. Pang, “Preparation, Characterization and Application of Nd-TiO2 Photocatalyst for the Reduction of Cr(VI) under UV Light Illumination,” Applied Catalysis B: Environmental, Vol. 77, No. 1-2, 2007, pp. 157-165. http://dx.doi.org/10.1016/j.apcatb.2007.07.016
[2]	S. Deng and R. Bai, “Removal of Trivalent and Hexavalent Chromium with Aminated Polyacrylonitrile Fibers: Performance and Mechanisms,” Water Research, Vol. 38, No. 9, 2004, pp. 2424-2432. http://dx.doi.org/10.1016/j.watres.2004.02.024
[3]	R. J. Kieber, J. D. Willey and S. D. Zvalaren, “Chromium Speciation in Rainwater: Temporal Variability and Atmospheric Deposition,” Environmental Science and Technology, Vol. 36, No. 24, 2002, pp. 5321-5327. http://dx.doi.org/10.1021/es020777n
[4]	J. J. Testa, M. A. Grela and M. I. Litter, “Heterogeneous Photocatalytic Reduction of Chromium(VI) over TiO2 Particles in the Presence of Oxalate: Involvement of Cr(V) Species,” Environmental Science and Technology, Vol. 38, No. 5, 2004, pp. 1589-1594. http://dx.doi.org/10.1021/es0346532
[5]	H. J. Gibb, P. S. Lecs, P. F. Pinsky and B. C. Rooney, “Lung Cancer among Workers in Chromium Chemical Production,” American Journal of Industrial Medicine, Vol. 38, No. 2, 2000, pp. 115-126. http://dx.doi.org/10.1002/1097-0274(200008)38:2<115::AID-AJIM1>3.0.CO;2-Y
[6]	Y. Xing, X. Chen and D. Wang, “Electrically Regenerated Ion Exchange for Removal and Recovery of Cr(VI) from Wastewater,” Environmental Science and Technology, Vol. 41, No. 4, 2007, pp. 1439-1443. http://dx.doi.org/10.1021/es061499l
[7]	R. Guell, E. Antico, V. Salvado and C. Fontas, “Efficient Hollow Fiber Supported Liquid Membrane System for the Removal and Preconcentration of Cr(VI) at Trace Levels,” Separation and Purification Technology, Vol. 62, No. 2, 2008, pp. 389-393. http://dx.doi.org/10.1016/j.seppur.2008.02.015
[8]	S. Mor, K. Ravindra and N. R. Bishnoi, “Adsorption of Chromium from Aqueous Solution by Activated Alumina and Activated Charcoal,” Bioresource Technology, Vol. 98, No. 4, 2007, pp. 954-957. http://dx.doi.org/10.1016/j.biortech.2006.03.018
[9]	M. E. R. Carmona, M. A. P. da Silva and S. G. F. Leite, “Biosorption of Chromium Using Factorial Experimental Design,” Process Biochemistry, Vol. 40, No. 2, 2005, pp. 779-788. http://dx.doi.org/10.1016/j.procbio.2004.02.024
[10]	M. Kobya, “Removal of Cr(VI) from Aqueous Solutions by Adsorption onto Hazelnut Shell Activated Carbon: Kinetic and Equilibrium Studies,” Bioresource Technology, Vol. 91, No. 3, 2004, pp. 317-321. http://dx.doi.org/10.1016/j.biortech.2003.07.001
[11]	I. Heidmann and W. Calmano, “Removal of Zn(II), Cu(II), Ni(II), Ag(I) and Cr(VI) Present in Aqueous Solutions by Aluminium Electrocoagulation,” Journal of Hazardous Materials, Vol. 152, No. 3, 2008, pp. 934-941. http://dx.doi.org/10.1016/j.jhazmat.2007.07.068
[12]	P. Gao, X. Chen, F. Shen and G. Chen, “Removal of Chromium (VI) from Wastewater by Combined Electrocoagulation-Electroflotation without a Filter,” Separation and Purification Technology, Vol. 43, No. 2, 2005, pp. 117-123. http://dx.doi.org/10.1016/j.seppur.2004.10.008
[13]	J.-K. Yang and S.-M. Lee, “Removal of Cr(VI) and Humic Acid by Using TiO2 Photocatalysis,” Chemosphere, Vol. 63, No.10, 2006, pp. 1677-1684. http://dx.doi.org/10.1016/j.chemosphere.2005.10.005
[14]	C. Burda, Y. Lou, X. Chen, A. C. S. Samia, J. Stout and J. L. Gole, “Enhanced Nitrogen Doping in TiO2 Nanoparticles,” Nano Letters, Vol. 3, No. 8, 2003, pp. 1049-1051. http://dx.doi.org/10.1021/nl034332o
[15]	T. Oppenlander, “Photochemical Purification of Water and Air,” Wiley-VCH, Weinheim, 2003.
[16]	A. A. C. Magalh?es, D. L. Nunes, P. A. Robles-Dutenhefner and E. M. B. De Sousa, “Catalytic Activity of Porous TiO2 Obtained by Sol-Gel Process in the Degradation of Phenol,” Journal of Non-Crystalline Solids, Vol. 348, No. 1-2, 2004, pp. 185-189. http://dx.doi.org/10.1016/j.jnoncrysol.2004.08.166
[17]	S. Parsons, “Advanced Oxidation Processes for Water and Wastewater Treatment,” IWA Publishing, Cornwall, 2004.
[18]	R. M. Mohamed, A. A. Ismail, I. Othman and I. A. Ibrahim, “Preparation of TiO2-ZSM-5 Zeolite for Photodegradation of EDTA,” Journal of Molecular Catalysis A: Chemical, Vol. 238, No. 1-2, 2005, pp. 151-157. http://dx.doi.org/10.1016/j.molcata.2005.05.023
[19]	K. Demeestere, J. Dewulf, T. Ohno, P. H. Salgado and H. Van Langenhove, “Visible Light Mediated Photocatalytic Degradation of Gaseous Trichloroethylene and Dimethyl Sulfide on Modified Titanium Dioxide,” Applied Catalysis B: Environmental, Vol. 61, No. 1-2, 2005, pp. 140-149. http://dx.doi.org/10.1016/j.apcatb.2005.04.017
[20]	H. Park and W. Choi, “Photocatalytic Reactivities of Nafion-Coated TiO2 for the Degradation of Charged Organic Compounds under UV or Visible Light,” The Journal of Physical Chemistry B, Vol. 109, No. 23, 2005, pp. 11667-11674. http://dx.doi.org/10.1021/jp051222s
[21]	C. Xu, R. Killmeyer, M. L. Gray and S. U. M. Khan, “Photocatalytic Effect of Carbon-Modified n-TiO2 Nanoparticles under Visible Light Illumination,” Applied Catalysis B: Environmental, Vol. 64, No. 3-4, 2006, pp. 312- 317. http://dx.doi.org/10.1016/j.apcatb.2005.11.008
[22]	Y. A. Shaban, M. A. El Sayed, A. A. El Maradny, R. Kh. Al Farawati and M. I. Al Zobidi, “Photocatalytic Degradation of Phenol in Natural Seawater Using Visible Light Active Carbon Modified (CM)-n-TiO2 Nanoparticles under UV Light and Natural Sunlight Illuminations,” Chemosphere, Vol. 91, No. 3, 2013, pp. 307-313. http://dx.doi.org/10.1016/j.chemosphere.2012.11.035
[23]	D. P. Das, K. Parida and B. R. De, “Photocatalytic Reduction of Hexavalent Chromium in Aqueous Solution over Titania Pillared Zirconium Phosphate and Titanium Phosphate under Solar Radiation,” Journal of Molecular Catalysis A: Chemical, Vol. 245, No. 1-2, 2006, pp. 217-224. http://dx.doi.org/10.1016/j.molcata.2005.10.001
[24]	S. U. M. Khan, M. Al-Shahry and W. B. Ingler Jr., “Efficient Photochemical Water Splitting by a Chemically Modified n-TiO2,” Science, Vol. 297, No. 5590, 2002, pp. 2243-2245. http://dx.doi.org/10.1126/science.1075035
[25]	Y. A. Shaban and S. U. M. Khan, “Carbon Modified (CM)-n-TiO2 Thin Films for Efficient Water Splitting to H2 and O2 under Xenon Lamp Light and Natural Sunlight Illuminations,” Journal of Solid State Electrochemistry, Vol. 13, No. 7, 2009, pp. 1025-1036. http://dx.doi.org/10.1007/s10008-009-0823-4
[26]	Y. A. Shaban and S. U. M. Khan, “Efficient Photoelectrochemical Splitting of Water to H2 and O2 at Nanocrystalline Carbon Modified (CM)-n-TiO2 and (CM)-n-Fe2O3 Thin Films,” International Journal of Nanotechnology, Vol. 7, No. 1, 2010, pp. 69-98. http://dx.doi.org/10.1504/IJNT.2010.029549
[27]	APHA, “Standard Methods for the Examination of Water and Wastewater,” 20th Edition, American Public Health Association, Washington DC, 1998.
[28]	M. C. Lu, G. D. Roam, J. N. Chen and C. P. Huang, “Factors Affecting the Photocatalytic Degradation of Dichlorovos over Titanium Dioxide Supported on Glass,” Journal of Photochemistry and Photobiology A: Chemistry, Vol. 76, No. 1-2, 1993, pp. 103-109. http://dx.doi.org/10.1016/1010-6030(93)80180-H
[29]	N. Wang, Z. Chen, L. Zhu, X. Jiang, B. Lv and H. Tang, “Synergistic Effects of Cupric and Fluoride Ions on Photocatalytic Degradation of Phenol,” Journal of Photochemistry and Photobiology A: Chemistry, Vol. 191, No. 2-3, 2007, pp. 193-200. http://dx.doi.org/10.1016/j.jphotochem.2007.04.023
[30]	S. Chen and G. Cao, “Study on the Photocatalytic Reduction of Dichromate and Photocatalytic Oxidation of Dichlorvos,” Chemosphere, Vol. 60, No. 9, 2005, pp. 1308- 1315. http://dx.doi.org/10.1016/j.chemosphere.2005.01.056
[31]	R. Wang, D. Ren, S. Xia, Y. Zhang and J. Zhao, “Photocatalytic Degradation of Bisphenol A (BPA) Using Immobilized TiO2 and UV Illumination in a Horizontal Circulating Bed Photocatalytic Reactor (HCBPR),” Journal of Hazardous Materials, Vol. 169, No. 1-3, 2009, pp. 926- 932. http://dx.doi.org/10.1016/j.jhazmat.2009.04.036
[32]	M. L. Chin, A. R. Mohamed and S. Bhatia, “Performance of Photocatalytic Reactors Using Immobilized TiO2 Film for the Degradation of Phenol and Methylene Blue Dye Present in Water Stream,” Chemosphere, Vol. 57, No. 7, 2004, pp. 547-554. http://dx.doi.org/10.1016/j.chemosphere.2004.07.011
[33]	H. Chen, Y. Shao, Z. Xu, H. Wan, Y. Wan, S. Zheng and D. Zhu, “Effective Catalytic Reduction of Cr(VI) over TiO2 Nanotube Supported Pd Catalysts,” Applied Catalysis B: Environmental, Vol. 105, No. 3-4, 2011, pp. 255- 262. http://dx.doi.org/10.1016/j.apcatb.2011.04.004
[34]	H. Mekatel, S. Amokran, B. Bellal, M. Trari and D. Nibou, “Photocatalytic Reduction of Cr(VI) on Nanosized Fe2O3 Supported on Natural Algerian Clay: Characteristics, Kinetic and Thermodynamic Study,” Chemical Engineering Journal, Vol. 200-202, 2012, pp. 611-618. http://dx.doi.org/10.1016/j.cej.2012.06.121
[35]	M. A. Fox and M. T. Dulay, “Heterogeneous Photocatalysis,” Chemical Reviews, Vol. 93, No. 1, 1993, pp. 341- 357. http://dx.doi.org/10.1021/cr00017a016
[36]	P. Zamostny and Z. Belohlav, “Identification of Kinetic Models of Heterogeneously Catalyzed Reactions,” Applied Catalysis A: General, Vol. 225, No. 1-2, 2002, pp. 291-299. http://dx.doi.org/10.1016/S0926-860X(01)00875-4
[37]	N. J. Peill and M. R. Hoffmann, “Mathematical Model of a Photocatalytic Fiber-Optic Cable Reactor for Heterogeneous Photocatalysis,” Environmental Science and Technology, Vol. 32, No. 3, 1998, pp. 398-404. http://dx.doi.org/10.1021/es960874e
[38]	A. Houas, H. Lachheb, M. Ksibi, E. Elaloui, C. Guillard and J.-M. Hermann, “Photocatalytic Degradation Pathway of Methylene Blue in Water,” Applied Catalysis B: Environmental, Vol. 31, No. 2, 2001, pp. 145-157. http://dx.doi.org/10.1016/S0926-3373(00)00276-9
[39]	Y. Ku and I. Jung, “Photocatalytic Reduction of Cr(VI) in Aqueous Solutions by UV Irradiation with the Presence of Titanium Dioxide,” Water Research, Vol. 35, No. 1, 2001, pp. 135-142. http://dx.doi.org/10.1016/S0043-1354(00)00098-1
[40]	J. J. Testa, M. A. Grela and M. I. Litter, “Experimental Evidence in Favor of an Initial One-Electron-Transfer Process in the Heterogeneous Photocatalytic Reduction of Chromium(VI) over TiO2,” Langmuir, Vol. 17, No. 12, 2001, pp. 3515-3517. http://dx.doi.org/10.1021/la010100y
[41]	L. A. G. Rodenas, A. D. Weisz, G. E. Magaz and M. A. Blesa, “Effect of Light on the Electrokinetic Behavior of TiO2 Particles in Contact with Cr(VI) Aqueous Solutions,” Journal of Colloid Interface Science, Vol. 230, No. 1, 2000, pp. 181-185. http://dx.doi.org/10.1006/jcis.2000.7053
[42]	S. G. Schrank, H. J. José and R. F. P. M. Moreira, “Simultaneous Photocatalytic Cr(VI) Reduction and Dye Oxidation in a TiO2 Slurry Reactor,” Journal of Photochemistry and Photobiology A: Chemistry, Vol. 147, No. 1, 2002, pp. 71-76. http://dx.doi.org/10.1016/S1010-6030(01)00626-8
[43]	J. R. Bolton, “Solar Photoproduction of Hydrogen: A Review,” Solar Energy, Vol. 57, No. 1, 1996, pp. 37-50. http://dx.doi.org/10.1016/0038-092X(96)00032-1
[44]	Y. Li, G. Lu and S. Li, “Photocatalytic Hydrogen Generation and Decomposition of Oxalic Acid over Platinized TiO2,” Applied Catalysis A: General, Vol. 214, No. 2, 2001, pp. 179-185. http://dx.doi.org/10.1016/S0926-860X(01)00491-4
[45]	S. Mu, Y. Z. Long and S. Z. Kang. “Surface Modification of TiO2 Nanoparticles with a C60 Derivative and Enhanced Photocatalytic Activity for the Reduction of Aqueous Cr(VI) Ions,” Catalysis Communications, Vol. 11, No. 8, 2010, pp. 741-744. http://dx.doi.org/10.1016/j.catcom.2010.02.006
[46]	B. Sun, E. P. Reddy and P. G. Smirniotis, “Visible Light Cr(VI) Reduction and Organic Chemical Oxidation by TiO2 Photocatalysis,” Environmental Science and Technology, Vol. 39, No. 16, 2005, pp. 6251-6259. http://dx.doi.org/10.1021/es0480872
[47]	H. Fu, G. Lu and S. Li, “Adsorption and Photo-Induced Reduction of Cr(VI) Ion in Cr(VI)-4CP (4-Chlorophenol) Aqueous System in the Presence of TiO2 as Photocatalyst,” Journal of Photochemistry and Photobiology A: Chemistry, Vol. 114, No. 1, 1998, pp. 81-88. http://dx.doi.org/10.1016/S1010-6030(98)00205-6