КИНЕТИКА И КАТАЛИЗ, 2011, том 52, № 1, с. 42-49
УДК 541.145
PHOTO-FENTON DEGRADATION OF PHENOL RED CATALYZED BY INORGANIC ADDITIVES: A TECHNIQUE FOR WASTEWATER TREATMENT
© 2011 A. Jain*, D. Vaya, V. K. Sharma, S. C. Ameta
Photochemistry and Solar Energy Laboratory, Department of Chemistry, University College of Science, M.L. Sukhadia University, Udaipur, Rajasthan, India *E-mail: jainabhilasha5@gmail.com Received 08.01.2010
Oxidation by photo-Fenton like reaction is an economically feasible process for degradation of a variety of hazardous pollutants in wastewater from dyeing and printing industries. In present study, the progress of the reaction has been monitored spectrophotometrically. An effort has been made to observe the effect of various inorganic additives like sodium thiosulphate and potassium bromate. The effect of variation of different parameters such as pH, concentrations of dye, Fe3+ ion and additives, amount of H2O2, and light intensity on the rate of photodegradation was also observed. A tentative mechanism for the reaction has been proposed.
The world is facing an ever increasing pace of water pollution with various pollutants like acids, alkalis, detergents, phenols, fungicides etc. which are released from different chemical industries pollute water resources. Dyes used in textile, and other industries are difficult to remove by conventional methods.
The known technical and economical drawbacks of conventional treatment and biological processes have fuelled the development of new, more effective and economically viable methods for wastewater treatment. In this context, the advanced oxidation processes have enormous potential for becoming viable alternatives for the remediation of polluted water which involve the production of highly reactive hydroxyl radicals and these are of current interest for the destruction of organic pollutants in surface and ground water as well industrial wastewater. The Fenton reagent consisting of H2O2 and Fe(II) has been shown to be effective in the degradation of wide spectrum of organic and inorganic pollutants. The mechanism and kinetics of Fenton reaction have been studied by many researchers [1—3]. In the reaction, ferrous ions react with hydrogen peroxide and oxidized to ferric ions with the generation of hydroxyl radicals. These hy-droxyl radicals are the active oxidizing species of the degradation of organic compounds. But the main drawback is that this reaction stops after complete consumption of Fe2+ ions. The Fe2+ ions can be regenerated from Fe3+ ions under the present conditions with additional requirement of light. This makes the process cyclic and photodegradation will proceed more smoothly.
A comparative study of degradation of 2-cholorophenol by Fenton and photo-Fenton processes has been done by Kavitha et al. [4]. Decolorization
of synthetic dyes by the Fenton reagent and Cu/Pyri-dine/H2O2 system has been investigated by Nerud etal. [5]. Chen et al. [6] studied the photo-Fenton degradation of dye in methanolic solution under both UV and visible irradiation. Matthews [7] reported the photocatalytic oxidation of aromatic compounds generally takes place through the hydroxylation mechanism by 'OH radicals. Fernadez et al. [8] studied photoassisted Fenton degradation of non-biodegradable azo dyes (Orange II) in the iron free solution mediated by cation transfer members. The catalytic oxidation of various organic compounds by H2O2 has also been reported by many researchers [9—12]. The utility ofFenton reagent for the oxidation of various inorganic compounds has been studied by many groups of researchers [13—15]. A comparative study of Fenton's, photo-Fenton's and other related reagents on the degradation of various types of compounds, e.g. dyes, re-sorcinol etc has been done [16, 17]. Huang et al. [18] reported the removal of citrate and hypophosphite binary components using Fenton, photo-Fenton and electro-Fenton processes. Methomyl degradation in aqueous solutions by Fenton's reagent and the photo-Fenton system is observed by Tamimi et al. [19]. Optimizing the treatment of landfill leachate by conventional Fenton and photo-Fenton processes has been carried out by Hermosilla et al. [20]. Papi et al. [21] studied the decolourization and mineralization of commercial reactive dyes by using homogeneous and heterogeneous Fenton and UV/Fenton processes, whereas the treatment of wastewater by Fenton and photo-Fenton processes has been investigated by different groups of researchers [22].
Although photo-Fenton reaction is a useful tool for the degradation of dyes as well as organic compounds,
the major drawback is slow rate of degradation. Thus it is important and necessary to find out some better and suitable modifications to increase the efficiency and applications of this reagent. In this context, the rate of
photo-Fenton degradation of dye Phenol Red (A) is accelerated in the presence of inorganic additives like thiosulphate and bromate ions. Phenol Red can also exist in a zwitter ion form (B).
In the present work, the effect of inorganic additives on the rate of photo-Fenton degradation of Phenol Red was investigated and the main emphasis is given to determine the conditions where these additives show the maximum rate of reaction. An attempt is also made to explain their catalytic behavior.
EXPERIMENTAL DETAILS
All solutions were prepared in doubly distilled water. Irradiation has been carried out with a 200 W tungsten lamp. The light intensity was measured with the help of a solarimeter (CEL, Model SM 201). A water filter has been used to cut off thermal radiations. A digital pH meter (Systronics, Model 335) is used to adjust the pH of the solutions by the addition of previously standardized 0.1 N sulfuric acid and 0.1 N sodium hydroxide solutions. The progress of the photocat-alytic reactions was observed by measuring the absor-bance at regular time intervals using UV-visible spectrophotometer (Systronics Model 106). Sodium thiosulphate (s.d. Fine), potassium bromate (EM), FeCl3 (CDH) and Phenol Red (Hi media) have been used as received. Hydrogen peroxide (EM) (concentration 30%, w/v) has been used. An aliquot of 3.0 ml was taken out from the reaction mixture at regular time intervals and the absorbance was measured at ^max = 430 nm. After measuring the optical density, this solution is further added to reaction mixture, so that the volume of reaction mixture is not changed.
RESULTS AND DISCUSSION
General
It was observed that absorbance of the solution at ^max = 430 nm decreases with increasing time of exposure, indicating a decrease in concentration of dye Phenol Red. Plots of log of absorbance versus time was
linear and follows pseudo-first order kinetics. The rate constant was calculated with the expression
k = 2.303 x slope.
The results are represented in Tables 1 and 2 and by graphs also (Fig. 1). It was observed that reaction is completed in two stages. The first stage has an induction period, may be for the generation of the ' OH radicals but during this time a small decrease in absor-bance has been observed, which may be attributed as a result of simple photochemical degradation of dye. After this slow stage, a major decrease in absorbance has been observed, which seems to be the second stage of photo-Fenton degradation, where 'OH radicals act as oxidizing species for the degradation of dye. All these reactions are shown below.
Fe3+ + H2O Fe2+ + 'OH + H+, (I)
Fe3+ + H2O2 -Shbr Fe2+ + HO^ + H+, (II)
Fe2+ + H2O2 —Fe3+ + 'OH + OH, (III)
OH + H2O2 — HO^ + H2O, (IV)
Fe2+ + 'OH — Fe3+ + OH, (V)
Fe3+ + HO^ — Fe2+ + O2 + H+, (VI)
Dye + ' OH —► Colourless degradation products. (VII)
The effect of various parameters on the rate of degradation is studied by comparing the rate of the second stage of the photo-Fenton reaction. Moreover, when the experimental conditions approach to the optimum conditions, then the time taken by first stage (slow step, induction period) was found to be reduced.
The inorganic additives accelerate the rate of reaction either by reducing the time taken in first stage, i.e. 'OH radical generation becomes faster, and/or increasing the rate of second stage.
Table 1. A typical run in Thiosulphate system
Table 2. A typical run in Bromate system
Time, min
log of absorbance [a.u.]
Treated (with additive)
0.3176 0.3151 0.3148 0.3132 0.3100 0.3042 0.2999 0.2949 0.2844 0.2732 0.2535 0.2660 0.1892 0.1316 0.0481 0.0360 0.1993 0.3392 0.5004
kx = 4.66 x 10-5 s-1, k2 = 1.21 x 10-3 s-1, time taken by first stage 71 min.
Untreated (without additive)
0.3199 0.3197 0.3195 0.3193 0.3186 0.3140 0.3094 0.3072 0.3036 0.2975 0.2946 0.2880 0.2862 0.2734 0.2632 0.2540 0.2405 0.2240 0.2052 0.1622 0.1152 0.0732 0.0253 0.0380 0.0681 0.1512 0.2558
-5 „-1
kx = 2.36 x 10-5 s k2 = 4.49 x 10-4 s-1, time taken by first stage 90 min.
Note. [Phenol Red] = 1.33 x 10-4 mole/l, amount of H2O2
0.12 ml, [FeCl3] = 3.00 x 10-5 mole/l, [Thiosulphate] = 1.33 x 10-5 mole/l, light intensity 60.0 mW/cm, pH 3.50.
Time, min
0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0 40.0 45.0 50.0 55.0 60.0 65.0 70.0
log of absorbance [a.u.]
Treated (with additive)
0.2714
0.2645
0.2593
0.2535
0.2464
0.2375
0.2250
0.2089
0.1832
0.1464
0.0831
-0.0561
-0.4318
-1.0968
-1.3468
6.74 x 10-5 i 3.35 x 10-3 i
time taken by first stage 52 min.
Untreated (without additive)
0.2650
0.2583
0.2540
0.2484
0.2417
0.2289
0.2172
0.1994
0.1717
0.1296
0.0648
-0.0625
-0.3915
-0.8631
-1.1986
6.72 x 10-5 i 2.86 x 10-3 i
time taken by first stage 57 min.
Effect of pH
The effect ofpH on additive catalyzed photochemical degradation was investigated in the pH range 2.5-4.0. The results are reported in Fig. 2
The rate of photo-Fenton degradation of Phenol Red was maximum at pH 3.50, and 3.00 for thiosulphate and bromate system, respectively. The observations indicate that the rate of photo-Fenton reaction strongly depends on the pH of the system
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