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International Journal of Enviornmental Science and Technology, Vol. 2, No. 4, Winter 2006, pp. 365-371 Treatment of dye solution containing colored index acid yellow 36 by electrocoagulation using iron electrodes 1*M. Kashefialasl, 2M. Khosravi, 1R. Marandi and 2,3 K. Seyyedi 1Department of Environmental Engineering, Islamic Azad University, Tehran-North Branch, Tehran, Iran 2Department of Applied Chemistry, Islamic Azad University, Tehran-North Branch, Tehran, Iran 3Department of Applied Chemistry, Islamic Azad University, Shiraz Branch, Shiraz, Iran Received 20 October 2005; revised 15 November 2005; accepted 27 November 2005 available online 22September 2005 Code Number: st05049 ABSTRACT: The removal of pollutants from effluents by electrocoagulation has become an attractive method in recent years. This paper deals with the batch removal of the reactive textile dye Colored Index (C. I.). Acid yellow 36 from an aqueous medium by the electrocoagulation method using iron electrodes. The effects of electrolyte concentration, initial pH, current density, electrode area, interelectrode distance, dye concentration, and treatment time on the decolorization efficiency have been investigated. Iron hydroxypolymeric species formed during an earlier stage of the operation efficiently remove dye molecules by adsorption and precipitation, and in a subsequent stage, Fe(OH)3 flocs trap colloidal precipitates and make solid liquid separation easier during the flotation stage. These stages of electrocoagulation must be optimized to design an economically feasible electrocoagulation process. The increase of current density up to 127.8 A/m2 enhanced the color removal efficiency. Our results showed that the optimum electrolysis time was 6 min. The optimum pH was determined 8. It was also found that the color removal percent (R.P.%) with increasing of dye concentration, decrease. The optimum amount of electrolyte (NaCl) was found to be 8 g/l when the dye concentration was 50 mg/l. Key words: Electrocoagulation, wastewater treatment, textile dye, C. I. acid yellow 36, color removal INTRODUCTION The pollution induced by dyestuff losses and discharge during dying and finishing processes in the textile industry has been a serious environmental problem for years. Dyes in the wastewater undergo chemical as well as biological changes, consume dissolved oxygen from the stream, and destroy aquatic life because of their toxicity (Ahmed, et al., 1992). It is therefore necessary to treat textile effluents prior to their discharge into the receiving water. Various physical-chemical techniques are also available for the treatment of aqueous streams to eliminate dyes; chemical coagulation followed by sedimentation (Lin, et al.,1993), and adsorption are the widely used ones (Mckey, 1990), but other advanced techniques are often applied, e.g., UV (Arslan, et al., 1999 and Hachem, et al., 2001), ozonation (Liakou, et al., 1997), ultrasonic decomposition, or combined oxidation processes (Arslan, et al., 1999 and Lorimer, et al., 2000 and Fung, et al., 1999). Meanwhile, high treatment costs of these methods have stimulated, in recent years, the search for more cost-effective treatment methods. Electrocoagulation is a process consisting of creating metallic hydroxide flocs within the wastewater by electrodissolution of soluble anodes, usually made of iron or aluminum. This method has been practiced for most of the 20th century with limited success. Recently, however, there has been renewed interest in the use of electrocoagulation owing to the increase in environmental restrictions on effluent wastewater. In the past decade, this technology has been increasingly used in developed countries for the treatment of industrial wastewaters, by allowing the particles to react with: (i) an ion having an opposite charge; or (ii) a floc of metallic hydroxides generated within the effluent (Scott, 1995 and rajeshwar et al., 1994 and Mollah, et al., 2001). The EC process is highly dependent on the chemistry of the wastewater, especially its conductivity. In addition, other characteristics such as pH, particle size, and chemical constituent influence the process. The mechanism of removal of pollutants by EC process with iron electrodes is shown below (Mollah, et al., 2001). Iron upon oxidation in an electrolytic system produces iron hydroxide, Fe(OH)n where n = 2 or 3. Two mechanisms have been proposed for the production of Fe(OH)n: The insoluble metal hydroxides of iron can remove pollutants by surface complexation or electrostatic attraction. The prehydrolysis of Fe3+cations also leads to the formation of reactive clusters for wastewater treatment (Mollah, et al., 2001). The prehydrolysis of Fe3+cations also leads to the formation of reactive clusters for astewater treatment (Mollah, et al., 2001). The EC process is characterized by a fast rate of pollutant removal, compact size of the equipment, simplicity in operation, and low operating and equipment costs (Chen, et al., 2000). The EC is a simple and efficient method for the treatment of water and many kinds of wastewater. It has been tested successfully in the separation of pollutants from restaurant wastewater (Chen, et al., 2000), treatment of urban wastewater (Pouet, et al., 1995), degradation and decolorization of dye solution (Shen, et al., 2001 and Daneshvar, et al., 2004), defluoridation of water (Mameri, et al., 2001 and 1998), separation of aqueous suspensions of ultrafine particles (Matteson, et al., 1995), and removal of nitrate, sulfide, sulfate from water (Koparal, et al., 2002 and Murugananthan, et al., 2004). Hence, it is expected that the electrocoagulation would be an ideal choice for decolorization of dye solutions (Chen, et al., 2000). The purpose of this study is to conduct an experimental investigation on the removal of a reactive textile dye (C.I.Acid yellow 36) from the wastewater using the electrocoagulation method. Several fundamental aspects regarding the effects of wastewater conductivity, initial pH, current density, stirring rate, dye concentration, and time on the dye removal efficiency are explored. This reasreach have been done at Islamic Azad University,Shiraz Branch in 2003. MATERIALS AND METHODS The experimental equipment schematically is shown in Fig. 1. The electrocoagulationunit consisted of an 250 ml electrochemical reactor with iron (ST 37-2) anode and cathode. The electrodes were 30mm×50mm and interelectrodes distance was 2.5cm. The current density was maintained constant by means of a precision DC power supply (ADAK-PS 808). The dye solution was prepared using Acid yellow 36 provided by Alvan Sabet company (Iran). The structure of C.I. Acid yellow 36 is shown in Fig. 2. All samples were allowed to settle for 5 min in a 250-ml vessel and after filtration were analysed. The dye concentration was determined using a UV-Vis spectrophotometer (jenway6505) at 414 nm. The equation used to calculate the color removal efficiency in the treatment experiments was: R%= {(C0-C)/C0}×100 (5) Where C0 and C were the initial and present concentrations of the dye in solution (mg/l), respectively. RESULTS The exact range oftheoptimum current density will depend on the geographical as well as the economic situation where the EC process is utilized. As shown in Fig. 3, an increase in current density from 32 to 127.8 A/m2 yields an increase in the efficiency of color removal from 21 to 83.5% because when the current density increases, the efficiency of ion production on the anode and cathode increases. Therefore, there is an increase in floc production in the solution and hence an improvement in the efficiency of color removal. For a solution with a dye concentration of 50 ppm, the optimum current density was 127.8 A/m2. During electrolysis, the positive electrode undergoes anodic reactions while cathodic reactions occur on the negative electrode. The released ions neutralize the particle charges and thereby initiate coagulation. The color-removal efficiency depends directly on the concentration of ions produced by the electrodes. When the electrolysis period increases, an increase occurs in concentration of ions and their hydroxide flocs. Accordingly, as shown in Fig. 4, an increase in the time of electrolysis from 3 to 6 min yields an increase in the efficiency of color removal from 60.7 to 83%. For a solution having a dye concentration of 50 ppm, and a treatment unit with current density of approximately 127.8A/m2, the optimum time of electrolysis was 6 min. After 6 min color removal percent decrease because dye molecules desorption from flocs to solution probably. As shown in Table 1 type of electrolyte don’t have any important effect against color removal percent, but in contrast of applied voltage and conductivity of solutions contains each of electrolytes (10 g/l), it can be shown that NaCl with higher color removal percent (83%) is the best electrolyte because it is cheap and the solution contains it has high conductivity (19.13 ms/cm) thus it need low voltage for electrocoagulation (2.9V) and so it is economical in industrial scale. Therefore NaCl is selected as electrolyte in this investigation. Due to the chemical substances added at a high concentration from dyeing and finishing processes in the textile industry, the textile wastewaters have a broad variation in ionic strength. The greater ionic strength will generally cause an increase in current density at the same cell voltage, or the cell voltage decreases with increasing wastewater conductivity at constant current density. Therefore, it is necessary to investigate the effect of wastewater conductivity on electrocoagulation in terms of color removal. The conductivity of the wastewater is adjusted to the desired levels by adding an appropriate amount of NaCl. When the concentration of NaCl salt in solution increases, solution conductivity increase. Consequently, with respect to: V = EC - EA-ζA-ζC - IRcell - IRcircuit (6) the necessary voltage for access to a certain current density will be diminished, so the consumed electrical energy is decreased. The effect of NaCl concentration on removal efficiency is shown in Fig. 5. It can be seen that there is an increase in removal efficiency up to 83% when electrolyte concentration was 8 g/l. It has been established that the influent pH is an important operating factor influencing the performance of electrochemical process (Daneshvar, et al., 2004). To examine its effect, the dye solution was adjusted to the desired pH for each experiment by adding sodium hydroxide or hydrochloric acid. Fig. 6 demonstrates the efficiency of color removal as a function of the solution pH. The maximum efficiency of color removal was observed at pH in the range 7–9 as expected considering the nature of the reaction between ferrous and hydroxide ions. As shown in figure 7 When the pH of solution is lower than 6, Fe(OH)3 is in soluble form (Fe+3) and when it is higher than 9, Fe(OH)3 isinsoluble form{Fe(OH)4-} and because Fe(OH)3 has major role in removing of color, thus when pH of solution is 8 color removal is the highest. The dye solution with different initial concentrations in the range of 20-60 mg/l was treated by ECin optimized current density and time of electrolysis values. According to the results in Fig. 8, with increase of dye concentration color removal percent decrease, because in constant condition, production of flocs and adsorption of dye to them is constant value. Up to the 9 min, in all of concentrations the adsorption capacity of flocs was not exhausted and because of desorption of dye to solution, color removal decrease, especially in higher concentrations . DISCUSSION AND CONCLUSION The decolorization of dye solution (Acid yellow 36) by means of electrocoagulation was affected by the current density, initial pH of the solution, electrolyte concentration, and time of electrolysis. The results showed that when the initial concentration of the dye was 50 ppm, the dye was effectively removed (83%) at pH ranging from 7 to 9, time of electrolysis of approximately 6 min, current density of approximately 127.8 A/m2, temperature of approximately 298 K, and interelectrode distance of 2.5 cm. Fig. 8 illustrates the absorption spectrum of dye solution before and after EC process. It can be seen that EC process in the optimized condition causes to near 85% removal of color from dye solution. ACKNOWLEDGEMENTS The authors are grateful to the IslamicAzad University of Shiraz for laboratory and other supports provided. REFERENCES:
© 2006 Center for Environment and Energy Research and Studies (CEERS)
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