Zirconia and silica mixed titanium oxides were synthe-sized by modified sol-gel method in presence of a surf-actant (CETAB). Materials were characterized by surface area, XRD, SEM  and UV-Vis spectroscopy. Materials are found to be in uniform size and mostly spherical when synthesized in presence of CETAB. Effect of sum factant concentration  on  the  particle  size, shape  and the catalytic activity is also evaluated. Calcination temp-erature plays an important role on the phase stability 

and catalytic  activity  of  the  materials. It is well observed that without any hole scavenger the efficiency 

of photocatalytic metal removal is quite low for  both 

the metal ions within 30minuites of reaction.  However, addition of hole scavenger  increases the activity many folds and hence complete removal of Se and Cu was possible in  30 minutes of reaction. Among all  the  organic  hole scavengers used, sodium formate is found  to be the most active one for selenium whereas for  copper  EDTA is the most suitable one. Reaction pH also plays important role in the efficiency of the metal removal. Above all synth-esized materials are found to be efficient  catalyst for visible light reaction. The same catalyst is found to be reusable at least for 5-6 times with 4-5% decrease in the activity

Aman, Noor

Hait, Jhumki

Jana, R K

Mishra, T

English

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PHOTOCATALYTIC REMOVAL OF COPPER AND SELENIUM FROM WASTEWATER UNDER VISIBLE LIGHT T. Mishra, J. Halt, Noor Aman and R. K. Jana National Metallurgical Laboratory, Jamshedpur Abstract Zirconia and silica mixed titanium oxides were synthesized by modified sol-gel method in presence of a surfactant (CETAB). Materials were characterized by surface area, XRD, SEM and UV-Vis spectroscopy. Materials are found to be in uniform size and mostly spherical when synthesized in presence of CETAB. Effect of sum factant concentration on the particle size, shape and the catalytic activity is also evaluated. Calcination temperature plays an important role on the phase stability and catalytic activity of the materials. It is well observed that without any hole scavenger the efficiency of photocatalytic metal removal is quite low for both the metal ions within 30minuites of reaction. However, addition of hole scavenger increases the activity many folds and hence complete removal of Se and Cu was possible in 30 minutes of reaction. Among all the organic hole scavengers used, sodium formate is found to be the most active one for selenium whereas for copper EDTA is the most suitable one. Reaction pH also plays a4mp.ortant role in the efficiency of the metal removal. Above all synthesized materials are found to be efficient catalyst for visible light reaction. The same catalyst is found to be reusable at least for 5-6 times with 4-5% decrease in the activity. Introduction Since the discovery of photo-induced water cleavage on titanium dioxide electrodes by Fujishima and Honda [1] in the early 70s, the interest in applying TiO2 as heterogeneous photocatalyst to purify water and wastewater has been increasing rapidly. Compared with conventional water treatment approaches, such as chlorination, ozonation, UV radiation and advanced filtration processes; TiO2 as a durable semiconductor has a number of advantages including high oxidation efficiency for complete decomposition and mineralization of organic waste, reductive removal of toxic metal ions, cost effectiveness and minimal disinfection byproducts [2, 3]. In particular solar light induced photocatalytic process can be comparatively low cost and hence much attractive for larger use. In this context so many modified catalysts were developed in recent years for potential use as photocat• st in visible light. The incorporation of transition metal into the anatase phase of TiO2 has been frequently studied as a way to extend the spectral response to the visible range [4]. In addition, certain Ti l,M,02 phases show an enhancement of the photoactivity due to structural and electronic modifications induced by the dopant. In this respect, the inclusion of Sn4+ in the rutile and the anatase structures leads to a significant increment of the rate for the photocatalytic oxidation of acetone [5] or methylcyclohexane vapor. In the recent years many mixed oxide systems were reported as potential photocatalyst due to the different nature of the binary oxide systems over pure titania [6, 7]. In particular addition of Zr02 was found to be beneficial as it inhibited the anatase to rutile :phase transformation [8, 9]. Similarly addition of silica also increases the catalytic activity particularly in presence of UV light. Anderson and Bard [10, Tech-4 / 1 11] reported that a mixed oxide powder of '1402 and Si02 can be more efficient photocatalyst than pure TiO2 powder. On the other hand nonmetal doping of TiO2 rekindled a great,interest in visible light catalysis since the report of the work of Asahi et al [12]. It is observed that doping with nonmetals like N[12], C[13], F[14] etc can increase the visible light absorption of the photocatalyst. Here we report the visible light mediated photocatalytic removal of selenium and copper over synthesized silica and zirconia mixed titana materials. Experimental Material synthesis Titanium isopropoxide, zirconium butoxide and tetraethyl orthosilicate (Aldrich) were used as starting material. Materials were synthesized according to the earlier reported method [15]. Pure titania and silica, zirconia mixed titania were prepared by controlled hydrolysis of corresponding isopropoxide in presence of surfactant (cetyltrimethylammonium bromide, CETAB) in n-propanol water mixture. Silica and zirconia (10 and 20 wt%) mixed titania samples were prepared by using calculated amount of corresponding alcoxide mixture in n-propanol so as to maintain the ratio of 10:90 and 20:80 in the binary oxides. Required amount of surfactant (2mol%) was added to the reacting mixture before hydrolysis. Hydrolysis was carried out by drop wise addition of n-propanol and water mixture from a burette under magnetic stirring condition in 30min time period. The gel so obtained was maintained in the same stirring condition for another 4h. Subsequently the gel was filtered and washed with distilled water several times, dried at 100°C, calcined at 500, 700 and 900°C for 4h and kept for further use. Silica mixed titania and zirconia mixed titania are denoted as TiSi and TiZr, respectively. NuMber after the sample indicates the weight percentage of silica or zirconia mixed with titania. Material characterisation XRD patterns of all the synthesized samples calcined at different temperatures were recorded on a Siemens (model: D-500) semiautomatic diffractometer using Cu-Ka radiation source and Ni filter in the range of 10 to 70 degree. BET surface area was determined by N2 adsorption desorption method at liquid nitrogen temperature using Quantasorb (Quantachrome, USA). Prior to adsorption desorption measurements, the samples were degassed at 393K at 10-4 Ton for 5h. SEM micrographs of powdered samples were recorded on scanning electron microscope (JEOL 840A, Japan) after gold coating. FT-IR spectra of different samples were recorded with Nicolet 750 FT-IR spectrometer using KBr palate at room temperature in the range of 400 to 4000cm-I with scanning rate of 4cm-I /min. Photocatalytic activity Prepared materials were used as photocatalyst for the reductive removal of selenium and copper ions in acidic pH. Stock solution of selenium and copper was prepared by dissolving required amount of sodium selertite and cupric chloride • respectively, in double distilled water. All the OhdtOcatalytic experiments were carried out in a 200m1 capacity three necked glass reactor fitted with l'25w visible lamp under magnetic stirring condition. Constant reaction temperature was maintained through water circulation in the outer jacket of the glass reactor. All the experiments were performed in presence of air at atmospheric pressure. Initial Tech-4 / 2 concentration of metal ions was maintained at 100pprn..in each experiment. Sodium formate (2000prn) was used as the hole scavenger for all the experitneht. After' the reaction, metal deposited solid catalyst was separated by centrifuge. Metal ion concentration in the solution after and before reaction was estimated by AAS. Result and Discussion Materials calcined at 500°C shows low crystallinity of anatase phase. The crystallinity on the materials increases with the increasing calcination ternperature. It is observed that pure titania completely transform to rutile phase even at . 700°C calcination. However, the other four binary oxide materials show the presence of clear anatase phase after calcination at 700°C. This justifies the phase stabiliiation effect of silica and zirconia even at low concentrations. Broad peak of 101 reflection conforms the nanocrystalline nature of the anatase phase. However, with the increase in the calcination temperature the peak became sharper indicating the increase in the crystallite size. Peak sharpening is clearly marked incase of pure titania with the increase in calcination temperature. Crystallite size calculated from the XRD line broadening technique is tabulated in Table-1 which also supports our argument. Table-1: Textural properties of synthesized TiO2 and binary oxides. Sample code. Titania (Wt%) Silica (wt%) Zirconia (wt%) BET Surface area (m2/g) CS (nm) , 500°C 700°C 900°C 500°C TiO2 100 - - 91 58 43 14.8 TiSi-10 90 10 - 126 106 85 10.2 TiSi-20 80 20 - 138 108 88 9.98 TiZr-10 90 - 10 105 89 76 12.6 TiZr-20 80 - 20 96 84 71 15.3 SEM micrographs of all the materials calcined at 500°C temperature are presented in Fig-1. At 500°C calcination all the materials maintain spherical shape with average particle diameter within 0.8-1.2pm. Increasing silica content in the material favored the smaller particle formation as observed in Fig-5. Unlike silica mixed samples zirconia mixed samples shows mostly uniform particle size. The average particle size increases to 1100nm due to zirconia addition. All the materials show maximum BET surface area at 500°C calcination. Subsequently the surface area decreases (Table-1) with the increasing calcination temperature up to 700°C. However a remarkable decrease is noticed in case of pure titania. Probably the phase transformation to rutile also decreases the surface area through particle aggregation forming larger crystallite size as observed in the Table-1. Moreover, the decrease in surface area is less in case of silica and zirconia mixed titania samples. Therefore, the presence of Tech-4 / 3 9424 .15KU 	 XS,000 	 1Pm 14D39 silica or zirconia not only helps in anatase phase stabilization but also controls the sharp decrease in surface area by restricting the particle aggregation. Fig-1:SEM micrographs of (A) Ti, (B) TiSi-10 and (C) TiZr-10 after calcination at 500°C. Nitrogen doping UV-vis absorption spectroscopy had been carried out to investigate the optical property of various synthesized materials. There is a clear red shift in the absorption band of all the materials (Fig-8) prepared in presence of CETAB in comparison to the TiO2 prepared without CETAB. Even with the silica and zirconia mixed materials the absorption band is extended to the visible range. Visible light absorption is highest in case of TiZr-10 sample. Since the TiO2 prepared without CETAB absorbs only UV light, the visible light absorption in case of other samples must originate from doping of nitrogen or carbon from CETAB into the materials during heat treatment. Therefore, it is understood that the presence. of CETAB also favours the nonmetal doping of the samples and hence the enhanced visible light absorption. Tech-4 / 4  i. • "1/ : .1 ;111:;1 5)) .*. 200 	 300 	 400 	 500 Wavelength (nm) 600 	 700 Fig-2: Diffuse reflectance spectra of (a) TiO2 with out CETAB, (b) Ti and (c) TiZr-10. Photocatalytic activity It is well observed that without any hole scavenger the efficiency of photocatalytic metal removal is quite low for both the metal ions within 30minuites of reaction. flowever,.addition of hole scavenger increases the activity many folds and hence complete removal of Se andCu was possible in 30 minutes of reaction. • Tech-4 / 5 Removal (mol%)  120 100 80 60 40 20 0 • EDTA Formate #c• 	 (15) 	 ,\C) 	 (15) •• 	 •• 	 •• Sample .Fig-3: Photocatalyti c removal of selenium in presence of EDTA and formate. 80 0••nn o 60 :E co 40 E 20 cc 0 ,\O 20 ,,\O 20 	 gip EDTA m Formate Ph otocatalyst Fig-4: Photocatalytic removal of copper in presence of EDTA and formate. Among all the organic hole scavenger§ used, sodium formate is found to be the most active one for selenium, whereasfor. „copper .EDTA is the most suitable one Probably. EDTA attached with titania.surfaOe helps in the complex formation with cupper ions and hence the higher phOtoreduction. keaCticin pH also plays an important role in the efficiency of the metal removal as observed in the Fig-5. At lOwer pH selenium removal is higher whereas copper removal is highest at higher pH." It is understood that at pH within 1-3 copper has a tendency to .remain in the soluble form. However selenium metal is not soluble in that pH. So pH Tech-4 / 6 70 ° 60 o 50 • 40 z; 30 No cu 20 Cck 10 1 2 3 adjustment can help in the separation of pure selenium and copper from the mixed solution. Above all synthesized materials are found to be efficient catalyst for visible light. reaction. The same catalyst is found to be reusable at least for 5-6 times with 4-5% decrease in the activity. -r° pH - : A . Fig-5: Effect of pH on the selenium and copper photoreduction. Conclusion Spherical and uniform sized binary oxide materials were synthesized in presence of CETAB through sol-gel method. Synthesized materials showed visible light absorption and hence can be used as photocatalyst for solar light reaction. Catalytic removal of selenium. and copper depends on the hole scavenger and pH of the solution. EDTA is found to be effective for copper removal whereas formate favors the high selenium removal in a similar reaction condition. Similarly low pH favors the high selenium removal. Copper removal increases with the increase in pH. So' copper and selenium can be separated from a mixture solution only through pH adjustment. . Acknowledgment Authors are thankful to Prof. S. P. Mehrotra, Director, NML for his constant encouragement and permission to publish this work. Tech-4 / 7 References 1) A. Fujishima and K. Honda (1972) Nature. 238 (1972) 37. 2) M. R. Hoffmann, S. T. Martin, W. Choi and D. W. Bahnemann, Chem. Rev. 95 (1995) 69. 3) A. Mills and S. Le-Hunte, 3. Photochem. Photobiol. A: Chem. 108 (1997) 1. 4) A. Fuerte, M. D. Henia'ndez-Alonso, A. J. Maira, A. Marti'nez-Arias, M. Ferna'ndez-Garci'a, J. C. Conesa, J. Soria, G. Munuera, J. Catal. 212 (2002) 1. 5) J. Lin, J. C. Yu, D. Lo, S. K. Lam, 3. Catal. 183(1999) 368. 6) A. S. Deshpande, D. G. Shchukin, E. Ustinovich, M. Antonietti and R. A. Caruso, Adv. Funct. Mater. 15(2005) 239. 7) X. Z. Fu, L. A. Clark, Q. Yang, M. A. Anderson; Env. Sci. Technol. 30(1996) 8) X. Wang,..1. C. Yu, Y. Chen, L. Wu, X. Fu, Env. Sci. Technol. 40 (2006) 2369. 9) J. Lukac, M. Klementova, P. Bezdicka, S. Bakardjieva, J. Subrt, L. Szatmary, Z. Bastl, J. Jirkovsky, Appl. Catal. B 174 (2007) 83. 10) C. Anderson and A. J. Bard, J. Phys. Chem., 1995, 99, 9882. 11) C. Anderson and A. J. Bard, J. Phys. Chem. B 1997, 101, 2611. 12) R. Ashi, T. Morikawa, T. Ohwaki, K. Aoki and Y. Taga, Science 293(2001) 269. 13) S. Sakthivel, H. Kisch, Angew. Chem. Int. Ed. 42(2003) 4908. 14) W. Ho, J. C. Yu, S. Lee, Chem. 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Photocatalytic Removal of Copper and Selenium from Waste water under Visible Light

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Photocatalytic Removal of Copper and Selenium from Waste water under Visible Light

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