Decolorization of Synthetic Dye by Photocatalytic Oxidation
R Manoj Prabakaran*, V Niranjan, VM Seenivasan and S Thenesh Kumar
Dept. of Chemical Engineering, St Peters Engineering College, Avadi, Chennai-600054 India
*Corresponding Author E-mail: firstname.lastname@example.org
Wastewater effluent from textile plants using various dyestuffs is one of the major water pollution sources to environment. Except for coloring receiving water bodies such as rivers and lakes, dyes also undergo chemical and biological changes that consume dissolved oxygen in the water bodies; some dyes even possess toxicity that is hazardous to aquatic life. Traditional methods for treating the textile dye wastewaters consist of various chemical, physical, and biological processes. More advanced combined chemical and physical treatment techniques include adsorption, electro coagulation, ultrasonic decomposition, advanced chemical oxidation, nanofiltration, chemical coagulation followed by sedimentation and so forth. Most of the treatment methods have disadvantages such as high cost, high energy waste, and generating secondary pollution during the treatment process. The photo catalytic process involving TiO2 semiconductor particles under UV light illumination has been shown to be potentially advantageous and applicable in the treatment of wastewater pollutants for the last two decades. It is now considered as a promising technology for the treatment of organic contaminants in aquatic and atmospheric environments. It has a several advantages compared to the traditional treatment methods, it wont leave any residue to the environment and complete mineralization has been reported in most of the study. In order to treat the wastewater effluents from dyeing and finishing industries by an advanced oxidation process, the decomposition kinetics of organic dye by TiO2/UV was systematically studied in batch slurry reactors. The factors of study include agitation speed, effect of dye concentration and effect of TiO2 dose and reuse potential of used TiO2.
KEYWORDS: Dye, Titanium oxide, Textile, Wastewater.
The removal of non-biodegradable organic chemicals is a crucial ecological problem. Dyes are an important class of synthetic organic compounds used in the textile industry and are therefore common industrial pollutants. Due to the stability of modern dyes, conventional biological treatment methods for industrial waste water are ineffective resulting often in an intensively colored discharge from the treatment facilities. Heterogeneous photo catalysis by semiconductor particles is a promising technology for the reduction of global environmental pollutants. Inorganic photo catalysts, such as TiO2 have shown to be relatively cheap and effective way of removing organic compounds and pollutant gases1. Oxidation processes with TiO2 have been shown to be an effective alternative regard2, the vital snag of TiO2 semiconductor is that it absorbs UV light (band energy gap of TiO2 is 3.2 eV) Light excitation of TiO2 semiconductor generates electrons and holes in the valence and conduction bands respectively.
Dye sensitization, a technique reported for degradation of colorants in visible light illuminated dye modified TiO2 dispersion, could also be workable for degradation of colorless water pound pollutants3 recently a number of researches have dealt with the heterogeneous photocatalytic decomposition of dye in the presence of UV4.Also, oxidative process has been also used to decolorize and mineralize many kinds of azo-dyes in a bench scale by using both artificial irradiation5
The present work deals with photocatalytic (UV/ TiO2) degradation of color removal from aqueous solution containing methyl blue dye (MeB).The dependence of dye photo-oxidation rate were studied on the basis of following parameters: initial dye concentration; irradiation time; irradiation intensity were also investigated.
MATERIALS AND METHODS:
Material and Reagents:
The commercially used Methyl blue dye was used without further purification. The photo catalyst used in this work was nonionized TiO2, The photo catalyst was used without further treatment. Methyl Blue structure (MeB) is reported in Figure 1. Different concentrations of MeB dye (30ppm, 40ppm, 50ppm, 60ppm and 70ppm) were prepared using deionized water with solid dye.
Figure 2: Photo degradation of methylene blue (30 ppm conc.)
Photocatalytic reactor with UV light source:
The reactor was a laboratory – built mixed flow reactor (batch type) system consisting of UV light source (16W). The light source was covered by a quartz casing and it was immersed into the sample. 500mL of dye sample solution and 0.1gm of TiO2 were used in this method. The suspension was constantly stirred with an electric stirrer at a moderate speed. The stirrer was used in order to have a homogeneous suspension, to promote the adsorption on the surface of TiO2 and the oxygen was supplied with the aid of the Oxygen motor. The degraded solution was taken for a colorimeter measurement at various time intervals after filtration.
Conditions for irradiation experiments:
The UV light source was completely immersed into the specimen in order to have the maximum penetration of light. The suspension was electrically stirred at a moderate speed through out the experiment. Three concentrations of methyl blue solution (30ppm, 40ppm, 50ppm) were used to find the effect of initial dye concentration on color removal. In order to find the effectiveness of the catalyst 0.2gm of TiO2 was added to the 500mL of the sample solution and the experiment was performed, it resulted in the formation of soapy substance and when less than 0.1gm of catalyst was added to the sample the effect of degradation was very slow and hence 0.1gm of TiO2 was added as a catalyst for every 500mL of the sample dye solution.
Temporal changes in the concentration of Methyl Blue were monitored by examining the variation in the
Figure 3: Photo degradation of methylene blue (40 ppm conc.)
maximal absorption. At different time intervals (every 15mins) the sample was taken out with the help of a pipette and it was double filtered by using the Wattmann No 1 filter paper and the measurements were taken by using the colorimeter with the aid of glass cell. The percentage of degradation was calculated from the following equation.( Fig-5)
Degradation % = [1-At/A0] x 100------------------------ (1)
Where, At, is the absorbance after time t.
A0, is the initial dye concentration before degradation.
The results revealed that, the absorbance value was decreasing for the time intervals of every 15mins with simultaneous increase in the degradation percentage.
PHOTO CATALYTIC DEGRADATION MECHANISMS AND KINETICS:
Photo catalytic oxidation:
The detailed mechanism of MeB dye catalyzed degradation states that conduction band electron ( e . ) and the valence band holes (h+) are generated when aqueous TiO2 suspension is irradiated with light energy greater than its band gap energy (Eg = 3.2eV) . The Photo-generated electrons could reduce the dye or react with electron acceptors such as O2 adsorbed on the Ti (II)-surface or dissolved in water reducing it to superoxide radical anion O2 (Fig- 2, 3 and 4). The Photo-generated holes can oxidize the Organic molecule to form R+, or react with OH or H2O oxidizing them to form OH. radicals. Together with other highly oxidant species (peroxide radicals) they are reported to be responsible for the heterogeneous TiO2 photodecomposition of organic substrates as dyes. According to this, the relevant reactions at the semiconductor surface causing the degradation of dyes can be expressed as follows,
TiO2 + hv (UV) TiO2 - (eCB + hVB+)
TiO2 (hVB+) + H2O TiO2 + H+ + OH-
TiO2 (hVB+) + OH - TiO2 + OH .
TiO2 (e CB -) + O2 TiO2 + O2. -
O2. - + H+ HO2
Dye + OH. Degraded products
Figure 4: Photo degradation of methylene blue (70 ppm conc.)
Figure 5: Change in methylene blue concentration with respect to time
The resulting OH radical, being a very strong oxidizing agent (standard redox potential +2.8eV) can oxidize most of Methyl Blue Dye the mineral end products .Substrates not reactive towards hydroxyl radicals are degraded toward hydroxyl radicals are degraded employing TiO2 photocatalysis with rates of decay highly influenced by the semiconductor valence band edge position6
Photo catalytic degradation kinetics:
Previous results of photocatalytic degradation kinetics indicated that the destruction rates of photocatalytic oxidation of various dyes over illuminated TiO2 fitted the Langmuir-Hinshelwood (L-H) kinetics model7, as it can be seen that concentration of one of the reactants remains constant (because it is a catalyst and the concentration is less with respect to the other reactants) its concentration can be included in the rate constant, obtaining a pseudo constant: if B is the reactant whose concentration is constant then r = k[A][B] = k'[A]. The second order rate equation has been reduced to a pseudo first order rate equation. This makes the treatment to obtain an integrated rate equation much easier8. Hence the kinetics of the reaction can be expressed by the following rate equation.
Ln (Co/C) = kt ----------------------------------------------- (2)
Co - initial concentration (mol/lit)
C - Final concentration (mol/lit)
K - Rate constant
t - Time (sec)
Figure 6: Pseudo first order kinetic
The photocatalytic degradation of methyl blue in TiO2 suspension was carried out using UV lamp. Five concentrations of methyl blue dye were used with UV lamp. The Photo-catalytic processes were influenced by the initial concentrations of methyl blue dye. Color change from blue to colorless is irreversible and the degradation rate of methyl blue dye followed the pseudo-first order kinetics (Fig-6). It is found that the photocatalytic degradation was completely based upon the intensity of the UV lamp and the concentration of catalyst. In order to implement the mentioned process in the large scale the intensity of the light and the concentration of the catalyst must be varied accordingly.
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Received on 27.03.2009 Modified on 25.05.2009
Accepted on 16.06.2009 © AJRC All right reserved