Ind. Eng. Chem. Res. 2003, 42, 243-247
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APPLIED CHEMISTRY Electrochemical Studies on a Pharmaceutical Azo Dye: Tartrazine Rajeev Jain,* Meenakshi Bhargava, and Nidhi Sharma Department of Environmental Chemsitry, Jiwaji University, Gwalior-474011, India
Tartrazine (trisodium salt of 3-carboxy-5-hydroxy-1-p-sulfophenyl-4-p-sulfophenylazopyrazole) was selected as the model compound for this study because of its wide application in various pharmaceutical formulations either as such or in combination with some other dyes as color additives. A well-defined single cathodic peak was obtained at -0.535 V. In the acidic pH region, two cathodic peaks were obtained that merged into a single peak at higher pH. Controlledpotential electrolysis (CPE) at -1.20 V reduced peak current from -90.00 to -2.00 µA with a considerable decrease in color and absorbance. The rate of decrease of the current and absorbance was found to exhibit a first-order dependence. The COD of the solution showed a decrease from 1080 to 560 mg/L. No peak could be observed in the voltammograms after CPE, indicating the absence of any electroactive substance left in the solution. Electrochemical reduction of tartrazine results in the formation of the hydrazono derivative in a two-electron reduction pathway. Introduction Wastewater from the textile, pharmaceutical, paper, cosmetic, and food industries is characterized by high chemical and biological oxygen demands (COD and BOD, respectively), suspended solids, and intense colors as a result of the extensive use of synthetic dyes. These dyes are highly visible even at low concentration ( bR), the wave is displaced toward negative potentials, beyond the position where the reversible wave of a diffusing species would occur. For this reason, it is termed a postwave. If the species R is adsorbed more strongly (bR > bO), then the wave occurs at potentials more positive than E0′ and is called a prewave.22 With increasing dye concentration, the cathodic peak potential shows a positive shift, which supports the diffusion-controlled nature of the electrode process. Electrochemical Behavior at Various pH’s. Cyclic voltammograms of tartrazine (2 × 10-4 M) were recorded in the pH range 2.5-10.5. Significant cathodic peaks were observed in the acidic range (Figure 3), and as the pH of the medium increased, these peaks merged into one and were not as visible in the alkaline range (Figure 4). Data pertaining to the electrochemical characteristics of tartrazine at various pH’s are presented in Table 2. At acidic pH’s, the peak potential shows a negative shift, and as the pH of the medium is increased, the cathodic peak potential (Ep,c) shows a positive shift indicating more facile electroreduction at
higher pH. The peak current shows a linear decreasing trend with increasing pH. In the acidic range (pH 2.5-5.6), additional reduction peaks were observed that had corresponding oxidation peaks, i.e., the peak on the reverse scan of the cyclic voltammogram was at the same potential as that on the forward scan. It can be said that, in the acidic range, Nernstian charge transfer takes place as the voltammetric response satisfy the following criteria:25 (a) The red peak potential separation (Ep ) Eox p - Ep ) is nearly equal to 57.0 mV and is independent of v. (b) The peak red current ratio (Iox p /Ip ) is equal to 1.0 ( 0.2 and is independent of v. (c) The peak current, most conveniently measured on the forward sweep, increases linearly with v1/2. On the other hand, in the alkaline range, the process becomes irreversible, i.e., irreversibility increases with increasing pH. On this basis, it can be proposed that the reduction pathway is H-e-H-e (proton-electronproton-electron) at low pH and e-H-e-H at higher pH.26 Controlled-Potential Electrolysis and Coulometry. The controlled-potential electrolysis of a 2 × 10-4 M solution of tartrazine dye was carried out at an electrolysis potential of -1.20 V. With platinum foil as the working electrode, the reaction took approximately 4 h for complete decolorization and reduction of the cathodic peak current. With steel foil, the reaction time was reduced to approximately 2 h, indicating the higher efficiency of steel foil electrode for decolorization (Figure 5). Coulometric measurements were carried out to
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Table 2. Electrochemical Characteristics of Tartrazine at Varying pH at a Platinum Working Electrodea ip,c (µA) pHb
ip,a (µA)
Ep,a (V)
(i)
(ii)
(i)
(ii)
(i)
(ii)
(i)
(ii)
(1) A (2) B
-99.40 -50.00
-125.00 -
-0.505 -0.676
-0.633 -
96.28 45.00
47.89 -
-0.445 -0.402
-0.772 -
(1) A (2) B
-38.29 -20.00
-85.89 -48.00
-0.719 -0.719
-0.872 -0.872
74.93 43.62
80.63 -
-0.730 -0.641
-0.890 -
(1) A (2) B
-45.36 -6.93
-
-0.295 -0.249
-
72.67 83.65
-
-0.797 -0.705
-
(1) A (2) B
-25.51 -15.75
-
-0.235 -0.327
-
101.00 116.00
-
-0.747 -0.861
-
2.5 5.6 8.8 10.5
a
Ep,c (V)
Concentration ) 2 × 10-4 M, scan rate ) 50 mV/s. b A ) Before electrochemical treatment, B ) After electrochemical treatment.
calculate the number of electrons taking part in the overall electrode process, n, and it was found that n ) 2. After exhaustive electrolysis at both the working electrodes, no cathodic peak could be observed in the voltammograms recorded, indicating the absence of any electroactive species present in the electrolyzed solution. Redox Mechanism. Keeping in view the feasibilities of the sites of reduction, it was concluded that, of the possible reduction sites sCdCs, sCdNs, and sNdNs, the latter, i.e., sNdNs, is more susceptible to reduction, as its reduction occurs at lower potential than those of sCdNs and sCdCs. As evidenced from the coulometric studies, the number of electrons involved in the reduction is 2, and therefore, it can be proposed that the reduction of tartrazine follows a well-defined two-step reversible two-electron reduction, as supported by the work of others.27-34
Figure 5. Plots of ip,c (mA) vs time (min) for tartrazine (2 × 10-4 M in distilled water) at (I) platinum foil (II) steel foil working electrodes. Scan rate ) 50 mV/s.
Spectral and Chromatographic Studies. UVvisible spectra of tartrazine were recorded in the region 300-600 nm. To monitor the progress of electrolysis and subsequent decolorization, the absorbance of the solution was monitored at regular time intervals at λmax ) 425 nm. The decrease in the absorbance of the intermediate species with time was used to evaluate the kinetics of the reaction. There was gradual decrease in absorbance with time. The absorbance later became essentially parallel to the baseline. A linear plot of the logarithm of absorbance vs time was observed, indicat-
ing that the decrease in color and absorbance obeys a first-order rate expression. Attempts to separate the electroreduction products were carried out following conventional controlledpotential electrolysis (CPE). After CPE, the electrolyzed solution was subjected to thin-layer chromatography using dioxane/water (8:2) as the mobile phase for separation. Four spots were obtained on the silica gel chromatographic plate, along with one colored spot of the initial colored tartrazine solution. As evidenced by the reaction mechanism, one broad band was assigned to the presence of the hydrazono derivative, whereas the other three spots were attributed to the formation of some minor intermediates, which could not be completely identified. Chemical Oxygen Demand. After electrolysis and end product identification, both colored and electrolyzed solutions were subjected to toxicity tests. As stated earlier, many azo dye compounds have very high COD values along with intense colors. Therefore, tests of the initial and final COD values were necessary to obtain an idea of the toxicity and COD of the electrolyzed products formed. The electrolyzed solution showed a decrease in COD value relative to the initial colored solution (2 × 10-4 M) from the initial value of 1080 mg/L to a final value of 560 mg/L.
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Acknowledgment The authors are thankful to Ministry of Environment and Forests, New Delhi, India, for financial support that made this study possible Literature Cited (1) Robinson, T.; Chandran, B.; Nigam, P. Removal of dyes from a synthetic textile dye effluent by biosorption on apple pomace and wheat straw. Water Res. 2002, 36, 2824. (2) Pala, A.; Tokat, E. Color removal from cotton textile industry wastewater in an activated sludge system with various additives. Water Res. 2002, 36, 2920. (3) Nyanhongo, G. S.; Gomes, J.; Gubitz, G. M.; Zvauya, R.; Read, J.; Steiner, W. Decolorization of textile dyes by laccases from a newly isolated strain of Trametes modesta. Water Res. 2002, 36, 1449. (4) Reife, A.; Freeman, H. S. Environmental Chemistry of Dyes and Pigments; John Wiley & Sons: New York, 1996. (5) Venkataram, K. The Chemistry of Synthetic Dyes; Academic Press: New York, 1978; Vol. 7. (6) Waring, D. R.; Hallas, G. The Chemistry and Application of Dyes; Plenum Press: New York, 1990. (7) Helz, G.; Zepp, R.; Crosby, D. Aquatic and Surface Chemistry; Lewis Publishing Co.: Boca Raton, FL, 1995. (8) Morrison, C.; Bandra, J.; Kiwi, J. Sunlight Induced Decolouration/Degradation of Non-Biodegradable Orange II Dye by Advanced Oxidation Technologies in Homogeneous and Heterogeneous Media. J. Adv. Oxid. Technol. 1996, 1, 160. (9) Bandra, J.; Nadtochenko, V.; Kiwi, J.; Pulgarin, C. Dynamics of Oxidant Addition as a Parameter in the Modelling of Dye Mineralization (Orange II) via Advanced Oxidation Technologies. Water Sci. Technol. 1997, 35, 87. (10) Liakou, S.; Pavlov, S.; Lyberatos, G. Ozonation of Azo Dyes. Water Sci. Technol. 1997, 35, 279. (11) Zwiener, C.; Frimmel, F. H. Oxidative Treatment of Pharmaceuticals in Water. Water Res. 2000, 34, 1881. (12) Chagas, E. P.; Durrant, L. R. Decolourisation of Azo Dyes by Phanerochaete chrysosporium and Pleurotus sajorcaju. Enzyme Microb. Technol. 2001, 29, 829. (13) Fukatsu, K.; Kokot, S. Bleaching of Cotton Fabric by Electrogenated SpeciessDecoloration of Coloring Matter by Electrolysis. Text. Res. J. 2000, 70, 340. (14) Fukatsu, K.; Kokot, S. Effects of Halogen Ions on Degradation of Azo Dyes with Electrolysis. Sen’i Gakkaishi 1997, 53, 15. (15) Gregory, P.; Stead, C. V. The Degradation of Water-Soluble Azo Compounds by Dilute Sodium Hypochlorite Solution. J. Soc. Dyers Colour. 1978, 94, 402. (16) Karunditu, K. W.; Carr, C. M.; Dodd, K. Activated Hydrogen Peroxide Bleaching of Wool. Text. Res. J. 1994, 64, 570. (17) Thompson, K. M.; Griffith, W. P.; Spiro, M. Mechanism of Peroxide Bleaching at High pH. J. Chem. Soc., Chem. Commum. 1992, 21, 1600.
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Received for review March 25, 2002 Revised manuscript received October 26, 2002 Accepted October 26, 2002 IE020228Q
