Treating dye wastewater by TiO2 coated on coal cinder

Vol 8 No. 3Joumal of Chongqing University(English Edition)[ISSN 1671-82241September 2009Article|D:1671-8224200903016505Toce this article: LIU Jian-hua, WANG Huai-jun Treating dye wastewater by TiOz coated on coal cinder J J Chongqing Univ. Eng Ed [IssN 1671-82241 2009. 8(3): 165-Treating dye wastewater by tiO, coated on coal cinderLIU Jian-hua 4T, WANG Hai-jun?1 Key La 1Department of Chemical and Material Engineering, Jiangnan University,oratory of Yangtze River Water Environment, Ministry of Education, Yibinity, Sichuan 644000, P R Chinau 214122 P.R. chinaReceived 21 April 2009: received in revised form 5 May 2009Abstract: We investigated the photocatalytic degradation of dye wastewater by using titanium dioxide(tiO2)coated on a coalcinder. The coal cinder was used as the carrier, with a thin film of TiO2 coated on it by using the sol-gel method. Using theCongo red as the model pollutant for dye wastewater, we studied the decolorization efficiency, and effects of Tio film thicknessand roasting temperature on the efficiency. We also evaluated the recycling and regeneration of the immobilized TiO 2(TiO/cinder). Results show that the decolorization rate of Congo red solution was more than 98% after 2 h treatment when wused TiO,/cinder calcined at 500C for 2 h and coated four times as the photocatalyst. At the same time, the TiO//cinderemained high catalytic activity after being reused and regenerated for many timesKeywords: titanium dioxide; coal cinder, Congo red; dye wastewater; photocatalysiCLC number: o643 36Document code: A1 Introductionmethods to reduce biological oxygen demand(BOD),chemical oxygen demand(COD), and suspended solidThe majority of dyes used in the textile industry are are generally satisfactory, except the removal of dyethe azo dyes, accounting for more than 50% of all color [3]. Therefore, the decolorizing treatment of dyecommercial dyes. These dyes include azo groups(N=N) wastewater is a difficult problem to solve. Manymainly bound to substituted benzene or naphthalene researchers have studied on the semiconductor as arings [1]. Dyes are visible even of small quantities photocatalyst in decolorization of dye wastewater [4](0.005 mg), and the color of dyes can interfere with Among a variety of photo catalysts, titanium dioxidetransmission of sunlight into natural streams. (TiO2) is the most preferable due to its non-toxic,Furthermore, many of the azo dyes and their insoluble, stability, high photoactivity and inexpensiveintermediate products, such as aromatic amines, are nature [5-6]. However, the technology oftoxic to aquatic life, carcinogenic and mutagenicphotocatalysis has not been industrially applied tohumans. Consequently, dyes have to be removed from treating wastewater. The main problem of thetextile wastewater before discharge [2]pplication is the separation and reuse of powderDye wastewater is characterized by high photocatalyst. Such difficulty is more serious withconcentration,complicated component, deep color, and nanometer-scale ultrafine powder [7-8difficult to biodegrade. Treatment of wastewater byWe used the coal cinder as a photocatalyst carrier toconventional chemical coagulation and biological solve this problem. Its main compositions are SiozAl2O,, Fe2 O3, and Cao [9-10]. TiO2 attached on it canact as a composite photocatalyst, which shows higherLIU Jian-hua(刘建华} Liujianhua418@126cmcatalytic activity for facilitating the separation ofFunded by the Youth Fund Project of Yibin University(No QJ05- electron and hole during the reaction of photocatalysisThe coal cinder has high adsorbability because it has中国煤化工CNMHGJ. H. Liu, et al.Dye wastewater treatmentmany pores on the surface and inner parts. Therefore, different temperature for 2 h and cooling it at roomwhen used as the photocatalyst carrier, it improves the temperature. Then the TiOz films on the cinder ofeffect of photodegradation of wastewater and changes different layers and roasting temperature were formedrom solid waste into valuables. At present, althoughmany researchers have studied on the wastewater 2.2 Photocatalysis experimentsphotocatalyst carriers, relatively less research has beerThe reactor was a glass beaker(250 mL), in whichcarried out on treating wastewater by using5 g cinder coated with TiO2 film were settled near thephotocatalyst fixed on cinder.surface of solution (about 1 cm). Then, 200 mLCongo red, which is a stable basic azo dye, is a solution of Congo red(20 mg/L)was added into thecompound that contains azo groups(N=N-) Due tobeaker and stirred by a magnetic stirrer. For indoorits high stability, Congo red is commonly used as a experiments, thelight source was a lamp(Philips tLDtitration indicator and a staining agent [ 11]. Moreover, 15 w) emitting between 300 nm and 450 nm. Durings a typical direct azo dye of biphenyl amine, Congothe reaction, the solution was sampled every 30 mined is commonly used in textile industries. Therefore,The decolorization rate (X was determined bywe chose it as a model pollutant in this study.measuring the absorbance at 504 nm using an UV-visible spectrophotometer.2 Experimental detailsX=(4-A)4×100%2.1 Preparation of TiO2 on cinderwhere Ao and A were the initial absorbance and thereacting absorbance of Congo red, respectively2.1. I Pretreatment of cinder3 Results and discussionCoal cinder was pretreated to remove the chlorineion and other organic compounds on it. Loose and 3. I Influence of preparing condition of TiO2 film onporous cinder from the stokehold of Yibin Universitydecolorizing efficiencywas used, which was broken into granule with thediameter of about 1.0 cm. The granules were soaked in 3. 1. I Calcination temperature of TiO.a solution of hydrochloric acid (H20: HCI= 1: 1)for24 h. then taken out and washed for three times withWe performed a set of tests to study the influence of thedistilled water and anhydrous ethanol, respectively. TiO /cinder calcination temperature on the decolorizationAfter dried in an oven the cinder was put into muffle of Congo red. In these studies, we only changed theto be calcined at 500C for 2 h, and cooled off for calcination temperature of TiOy cinder while keeping thefurther useating times of TiO2 on cinder for four times. The totaldecolorizing reaction time was 2h.2.2.2 Preparation ofTio, film on cinderThe calcination temperature was an important factorffecting the photocatalysis efficiency. Fig. I showsThe TiO2 film on cinder was prepared by the sol-gel that the decolorization rate increased as the calcinationmethod. In 100 mL anhydrous ethanol, 20 mL butyl temperature of TiOz-cinder increasing. It reached thetitanate and 16 mL acetic acid were dissolved under peak at 500Cvigorous stirring for 1 h(solution A). Under stirring, a It has been proved that the content of anatase in Tiozmixture of 2 mL triethanolamine 60 mL ethanol, and arrays increases with increasing heat treatment16 mL water was dropwise added into the solution A. temperature. Near 500C, the rutile phase emerges,Then, 0.2 mL polyglycol was added in the solution and and the content of rutile increases as the temperaturestirred for 30 min to form TiOz-sol. The resulting increasing [12]. It is widely accepted that the anatasealkoxide solution was remained in the dark for 16 h to phase of titania is a relatively ideal photocatalyticproduce TiOr-gel. The TiO2 film was coated on the material among its three crystalline phases [13]. Incinder as follows: 1)dipping the cinder in the TiOz-sol addition, the anatase with a small fraction of rutilefor 30 min and drying at room temperature; 2) shows enhanced photocatalytic activity compared torepeating the operation for different times; 3)heating to the pure anatase due to the electron and hole transfer中国煤化工J Chongqing Univ. Eng. Ed [SSN 1671-82CNMHGJ H. Liu, et al.between the two phases [14]. Beckley et al. [15] (UV) irradiation only; 2)using the cinder only; 3)presented that the mixed crystal has higher catalytic using TiOz under UV irradiation; 4)using TiO/cinderactivity compared to the pure anatase and rutile. The under UV irradiationcatalyst treated at 500C has better photocatalyticefficiency. The reason is that the mixed crystal canfacilitate the separation and prevent the recombinationof cavities with electronsT23Fig. 2 Relationship between coating times and the300350400450500550decolorization rateFig. 1 Influence of calcination temperature of TiOx-cinderFig. 3 shows that the Congo red coulddecolorized partly using UV irradiation or cinderwithout any catalyst. However, the decolorizationefficiency was significantly improved by using3. 1.2 Coating times of tiO, on cinderTiO/cinder as the photocatalyst under UV irradiation.that TiO2 couldthe decompositiWe kept the calcination temperature of Tio/cinder of Congo red with the exposure of UV radiation.at 450C, and changed the coating times only to studyRecently, Strataki et al. [18]investigated thethe influence of the coating times of TiOz on cinder. photodegradation mechanism of several dyes under UVThe total reaction time of decolorization was 2 hirradiation. The photocatalytic degradation of organicThe coating times of TiO on cinder was an dyes in wastewater using TiO2(animportant factor affecting the efficiency of the semiconductor)is initiated by light of wavephotocatalysis. Fig. 2 shows that the decolorization rate as 390 nm(3.2 eV). Then, electrons are excited fromincreased as more coating times, and it reached the the valence band to the conduction band, generatingmaximum value at four times. When the coating timespositive holes and free electrons. The producedwere zero, the decolorization rate was 42%. This means electron-hole pairs can recombine or interact with otherthat the cinder has high adsorbability, and it can partlyorganic substrates on the surface of TiO2 particles viadecolorize the Congo red without TiO2 coating becauseoxidation and reduction reactions. In an aqueousit has manypores on its surface and inner parts, solution, positive holes are scavenged by surfaceIncreasing the number of the catalyst layers, the hydroxyl groups to produce very reactive oxidizingamount of TiO2 coated on the cinder is increased and hydroxyl radicals OH), which promotes thethe activity of photocatalysis reaction is improved. degradation process and subsequently leads to the totalHowever, the cinder can not be coated for too many mineralization of the organic substrate [6, 19)times. Otherwise, the coating film would be uneven,The first step of organic oxidative decomposition isand will be easily crazed and separated from the cinderthe oxidation of organic molecules with hydroxylsurface [16-17radicals produced on the photocatalyst surfaceTherefore, the adsorption process of organic pollutant3.2 Contrast experimenton photocatalyst is very important for heterogeneousphotocatalytic reactions [20]. Coating TiO2 on cinder,e decolorized the Congo red solution underwhich has strong adsorption capacity, can improve thefollowing conditions respectively: I)using ultravidecolorization efficiency of the Congo red. MoreoverJ Chongqing Univ. Eng. Ed [ISSN 1671-8YH中国煤化工CNMHGJ. H. Liu, et alDye wastewater treatmentit can also solve the problem of TiO2 particles recovery. 4 ConclusionWe presented a new method combining the effect of±adsorption and photodegradation in treating dyewastewater. Results show that the coating times ofV-TiO-UVTiO, on cinder and the calcination temperature ofTiO/cinder have considerable impact on thedecolorization efficienoated for four timescalcined at 500C for 2 h, TiO,/cinder had the bestphotocatalytic efficiency. The TiO2 coated on cindercan be recycled and the method of regeneration may beused to maintain high efficiency of the photocatalyst.The TiO2 coated on cinder can solve the problem ofdecolorization efficiency of dye wastewater. However,100150200250full-scale experiments in future research are needed toensure the application of this method. The degradationFig. 3 Decolorization rate of Congo red under differentmechanism should be researched by studying the endproducts and intermediates produced during thephotodegradation process of the dye wastewater.3.3 Recycling use of the TiO/cinder filmWe compared the results of reusing TiO/cinder todecolorize the Congo red for eight times directly and We greatly acknowledge the Youth Fund Project ofultrasonicating the TiO /cinder in distilled water for 30 Yibin University(No. QJ05-28)minutes after each reaction( Fig. 4)Fig. 4 shows that the decrease of decolorization Referencesefficiency was inconspicuous after the TiOz film wasused for eight times directly. 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