Electrochemical Removal of Rhodamine 6G by Using RuO2 Coated Ti

Jul 14, 2009 - of NaCl concentration of 0.2 M, applied current of 1.0-1.9 A, pH < 6, and distance between electrode of 5 mm. The RuO2-coated Ti mesh a...
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APPLIED CHEMISTRY Electrochemical Removal of Rhodamine 6G by Using RuO2 Coated Ti DSA Rita FaridaYunus, Yu-Ming Zheng,* K. G. Nadeeshani Nanayakkara, and J. Paul Chen* DiVision of EnVironmental Science and Engineering, National UniVersity of Singapore, 10 Kent Ridge Crescent, Singapore 119260

In this study, electrochemical removal of rhodamine 6G containing wastewater using RuO2-coated Ti mesh as dimensionally stable anode is investigated. The effects of pH, supporting electrolyte, electrolyte concentration, applied current, and distance between electrodes on the removal efficiency were examined in a batch reactor. It is shown that NaCl is the best supporting electrolyte among NaCl, Na2SO4, NaClO4, and NaNO3. Lower solution pH and higher concentration of supporting electrolyte facilitate the treatment process. Small distance between electrodes greatly decreases the energy consumption. The decolorization efficiency of above 99.5% and the low energy consumption of 1.58 kWh/m3 can be achieved under the optimized experimental condition of NaCl concentration of 0.2 M, applied current of 1.0-1.9 A, pH < 6, and distance between electrode of 5 mm. The RuO2-coated Ti mesh anode is shown to have good recyclability and lifespan. A greater dye removal of 99.2% can be obtained by a continuous stirred-tank reactor with a HRT of 4 min. It is anticipated that the developed technology from this study can be further scaled up for full-scale treatment of industrial dye containing wastewater. 1. Introduction Generally, the textile industry generates 700-1500 L of dyecontaining wastewater per ton of cloths produced.1,2 The volume of wastewater has greatly increased as a result of the increasing demand in polyester and cellulosic fabrics.3,4 Textile wastewater typically contains 0.01-7 g/L dye, depending on the dyes and processes used.5,6 Due to its intense color, large volume, and adverse effects on human health, the wastewater presents a serious environmental problem.7 Biological technologies are able to treat textile wastewater; the success however relies on types of microorganisms and operational conditions.8 Startup and maintenance of biological treatment operation, and sludge disposal are often costly and require operators with good experiences. Physicochemical methods such as membrane filtration, coagulation and flocculation, oxidation (e.g., by Fenton agents, UV, H2O2, and O3), and adsorption are available for treatment of dye-containing wastewater.9 Among them, ozonation is reportedly one of the better technologies for the treatment. Due to its strong oxidative abilities, ozone can simultaneously degrade/mineralize organic pollutants and destroy microbes. The combination of ozonation with UV irradiation, H2O2, and/or Fenton agents can greatly improve the efficiency. A major disadvantage of ozonation is the short half-life of ozone.10 The half-life of ozone can be further shortened if dyes are present. The stability is affected by the presence of salts, pH, and temperature. Thus, a continuous ozone supply in the treatment is required, resulting in higher operation cost.11 Photocatalytic oxidation is one of the emerging technologies for treatment of dyes.12-15 It has a relatively high degradation efficiency; however, there are several limitations. The fine catalysts are effective in the degradation kinetics. They must be separated from the treated effluent after the reactions. The separation after * To whom correspondence should be addressed. E-mail: esecjp@ nus.edu.sg, [email protected] (J.P.C.); [email protected], [email protected] (Y.-M.Z.); Fax: +1-831-303-8636 (J.P.C.); +656872-5483 (Y.-M.Z.).

the treatment makes the large-scale operations of the technology more difficult. In addition, the treatment efficiency will reduce significantly if the color and turbidity of wastewater are high. Membrane filtration, coagulation, and flocculation, and adsorption technologies are able to decontaminate dye wastewater; however, they have several drawbacks such as disposal of concentrated stream or sludge, chemical additives needed, and high regeneration costs.5,16,17 There has been an increasing interest in development and application of electrochemical technology for the dye wastewater treatment because it is highly efficient, has relatively low operation cost, and is easy to operate.18-20 To achieve better treatment efficiency, it is crucial that the conductivity of wastewater should be high. In textile dyeing, a large amount of salts such as sodium sulfate and/or sodium chloride is generally added into dye bath.3 For instance, in the dyeing of acrylic fibers with cationic dyes, sodium sulfate with a concentration of approximately 5 g/L is required to be added, and the dyeing of cotton with reactive dyes needs 5-80 g/L of salts.2,3,5,21 Therefore, the electrochemical process would be more suitable for treatment of dye-containing wastewater from textile industry than other types of wastewater (e.g., domestic wastewater). Since chemically inert electrodes (e.g., RuO2 in this study) are used in the process, operational problems such as corrosion of electrode are of less concern even in a solution with high salt contents (e.g., 70 g/L). There could be a series of intermediate products in the oxidation process. Their presence in the treated effluent is less of a concern as the treated effluent is normally not used for drinking purpose. In the case in which the effluent is used for drinking, it would have to be further treated by other advanced physiochemical technologies such as adsorption and membrane filtration in order to remove the salts and trace organic compounds. Several types of electrodes such as instant boron doped diamond, carbon, and titanium coated with metal oxides (e.g., PbO2, TiO2, and SnO2) were developed for the treatment of textile wastewater.4,20,22,23 Compared to other metal oxide films,

10.1021/ie801719b CCC: $40.75  2009 American Chemical Society Published on Web 07/14/2009

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2. Experimental Section

Figure 1. Chemical structure of rhodamine 6G (C28H31N2O3Cl).

RuO2 has higher electrocatalytic activity toward the production of powerful oxidants such as chlorine, ozone, and free radicals, which are responsible for the oxidization of organic pollutants.18,19,24,25 It also has a great advantage of stronger resistance to corrosion. A few papers appeared in the literature on the electrochemical treatment of organic dyes by using ruthenium-based DSA.1,23,26 However, their studies were limited to the electrochemical treatment of reactive dyes in various medium pHs, electrolyte amounts, and current densities. Key parameters such as electrolyte type and distance between electrodes were not covered in the studies. Most of the studies published in the literature were on reactive azo dyes, which account for about 50% of all the dyestuffs produced,27 and less research works were on decolorization of cationic dyes. Rhodamine 6G, highly colored with a molar extinction coefficient of 116000 M-1 cm-1, is one of cationic dyes that are usually applied to polyester fibers, wool, silk, and acrylic fibers.3 Although anionic dyes and reactive dyes are more common in the textile dyeing industries, the cationic dyes in the textile dyeing become increasingly important due to the development of synthetic fibers, particularly polyester fibers. Rhodamine 6G being a fluorescent dye has a more rigid structure than other organics because of its ring closure, as demonstrated in Figure 1. One of its interesting chemical properties is that it is photostable. Thus, it cannot be easily treated by using such technologies as adsorption, advanced oxidation process (AOP), and biological treatment.28 Among limited reported studies, it is found that the decontamination kinetics of rhodamine 6G is varied. A trichoderma harzianum based adsorbent was used for the removal; an equilibrium time of 2 h was reported.29 The thermally treated waste biological sludge was able to remove the dye in 30 h.30 One-third of the dye can be oxidized within 1 h when a photocatalytic process by a TiO2-HAD-NT was used.31 No obvious removal was observed in 0.5 h when an electrochemical system equipped with a carbon electrode as working electrode and Pt net as counter electrode was used.32 To our best knowledge, electrochemical degradation of rhodamine 6G by ruthenium-based DSA has not been studied and reported in the literature. The aim of the study was to investigate the influence of important parameters on the electrochemical treatment of rhodamine 6G by using a ruthenium-based anode. The type and concentration of supporting electrolyte, the solution pH, the applied current, the space between electrodes, and the initial dye concentration were studied. The optimal operational condition was obtained. Finally, the recyclability of the anode and the operation of continuous stirred-tank reactor were investigated.

2.1. Materials and Reagents. Rhodamine 6G (C28H31 N2O3Cl, 95% dye content) was purchased from Sigma Aldrich (Singapore). The chemical structure of rhodamine 6G is shown in Figure 1. Sodium chloride, sodium sulfate, sodium nitrate, sodium perchlorate, and hydrochloric acid (37%) were purchased from Merck (Singapore). Sodium hydroxide was purchased from Sino Chemical (Singapore). Chemical oxygen demand (COD) reagent was purchased from the HACH (USA). All chemicals were used without further purification. The dye solutions used in this study were prepared by dissolving rhodamine 6G in the deionized water in our laboratory. The RuO2-coated Ti mesh and the Ti plate were purchased from Beijing Hengli Titanium Industry & Trading Pte Ltd. (China) and Techmaster Engineering & Trading Pte Ltd. (Singapore), respectively. The Ti plate was grade II and with a thickness of 1.5 mm, while RuO2 coated Ti mesh was diamond shaped with a mesh size of 4 × 4 mm and a thickness of 1.5 mm. The effective surface area of the RuO2-coated Ti mesh is 115 cm2. 2.2. Experimental Setup and Procedure. The batch electrochemical experiments of rhodamine 6G containing wastewater treatment were conducted in a reactor with water jacket demonstrated in Figure 2a. The continuous stirred tank reactor is shown in Figure 2b. Both reactors have an inner diameter of 55 mm and a height of 280 mm. A reactor with higher volume was employed only for the study of effect of dye working volume. Prior to each experiment, a certain amount of electrolyte salt was added into the solution and the pH of solution was subsequently adjusted to a desired value by addition of 1 M HCl or 1 M NaOH. RuO2-coated Ti mesh (9 × 4.5 cm2) with an effective surface area of 115 cm2 and Ti plate (9 × 4.5 cm2) were used as anode and cathode, respectively. The electrodes were vertically placed in the reactor with a space of 5-30 mm. A constant direct current was applied across the electrodes by using a rectifier (TDKlambda UP 36-24 Sanyo Denki). The applied current was ranged from 0.6 to 1.9 A, while the voltage changed according to the electrolyte concentration and the distance between electrodes. During the experiment, the dye solution was magnetically stirred and 2 mL samples were collected at given time intervals. The temperature was maintained at 25 °C. 2.3. Measurements. The dye concentration was determined by measuring the absorbance at the wavelength of 526 nm using a UV-vis spectrophotometer (Thermo-scientific Genesys 10 UV scanning spectrophotometer Thermo, USA). The pH and the conductivity of samples were also monitored by an Inolab pH meter and a Schott instruments Lab 970 conductivity meter. The COD was measured by digestion method using a HACH DR/2010 spectrophotometer (Hach, USA). 3. Results and Discussion 3.1. Effect of Supporting Electrolyte. Electrolyte salts are usually added into the wastewater to increase its conductivity, which can lead to a decrease in energy consumption and enhance the treatment efficiency. Besides acting as a conductivity medium, supporting electrolyte can also serve as a source of oxidant(s) in the mineralization. To explore the effect of the type of supporting electrolyte on the treatment efficiency, NaCl, Na2SO4, NaClO4, and NaNO3 were selected as supporting electrolyte salts, in which NaCl and Na2SO4 are the most commonly used salts in the dyeing process.3 Amounts of 0.2

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Figure 2. Schematic diagram of experimental setup: (a) batch reactor; (b) continuously stirred-tank reactor. Table 1. List of Redox and Solution Reactions E0

half redox reacn

Figure 3. Effect of electrolyte types on the electrochemical treatment process: (a) removal efficiency; (b) solution pH. Conditions: Co ) 200 ppm; V ) 300 mL; pHo ) 2; I ) 1.9 A; d ) 30 mm; σ ) (19 mS/cm.

M NaCl, 0.1 M Na2SO4, 0.2 M NaClO4, and 0.2 M NaNO3 were added into the dye solutions, respectively, so that the conductivities were 19 mS/cm. As shown in Figure 3a, the performance of rhodamine 6G removal decreases in the descending sequence of NaCl > NaClO4 > NaNO3 > Na2SO4. The decolorization of the dye is mainly due to the electrochemical oxidation reactions resulting from oxidants (e.g., chlorine, ozone, and/or free radicals). The difference in the

a

(V)

2Cl- - 2e- T Cl2 (at the anode)

(1)

1.36

4OH- - 4e- T O2 + 2H2O (at the anode)

(2)

1.23

2H+ + 2e- T H2 (at the cathode)

(3)

0

2HClO + 2H+ + 2e- T Cl2 + 2H2O

(4)

1.63

ClO- + 2H+ + 2e- T Cl- + H2O

(5)

1.72

ClO- + H2O + 2e- T Cl- + 2OH-

(6)

0.89

ClO4- + 8H+ + 8e- T Cl- + 4H2O

(7)

1.37

NO3- + 4H+ + 3e- T NO + 2H2O

(8)

0.96

NO3- + 2H+ + e- T NO2- + H2O

(9)

0.80

Cl2 + H2O T HClO + Cl- + H+

(10)

HClO T H+ + ClO-

(11)

a

Standard redox potential.

treatment performance results from different effects of oxidants in the process. As shown in Table 1, the chloride ions are oxidized to chlorine gas by the electrode, which subsequently forms HClO and ClO-. As the standard redox potentials (E0) follows a descending order of ClO- > HClO > ClO4- > NO3-, the dye removal in the electrolyte solutions has a sequence of NaCl > NaClO4 > NaNO3. Because free radicals (e.g., hydroxyl radical) are produced during the reactions, one can see that some dye can be removed in the Na2SO4 solution. However, the removal is much lower than the other three supporting electrolyte salts.

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Figure 5. Effect of initial pH on the electrochemical removal of rhodamine 6G. Conditions: Co ) 200 mg/L; V ) 300 mL; I ) 1.9 A; d ) 30 mm; [NaCl] ) 0.2 M; σ ) 19 mS/cm.

Figure 4. Effect of electrolyte concentration on (a) electrochemical removal efficiency of rhodamine 6G and (b) energy consumption. Conditions: Co ) 200 mg/L; pHo ) 2; V ) 300 mL; I ) 1.9 A; σ ) 2.5-19 mS/cm; d ) 30 mm.

The change in pH during the dye removal is illustrated in Figure 3b. NaCl solution increases to about pH 7 when the oxidation is completed. This is due to the consumption of hydrogen ions in cathode. In the NaClO4 and Na2SO4 solutions, only water electrolysis reactions happen. Hydroxide ions are consumed according to eq 2, which leads to production of hydrogen ions at the anode. Hydroxide ions are generated at the cathode, which results from consumption of hydrogen ions (eq 3). The hydrogen ions at the anode neutralize the hydroxide ions at the cathode in the well mixed batch reactor. As a result, the pH in both NaClO4 and Na2SO4 solutions does not change. In the NaNO3 solution, hydrogen ions are consumed according to eqs 8. Thus, the solution pH increases. 3.2. Effect of Electrolyte Concentration. NaCl is used as a supporting electrolyte and serves as a conductivity medium and a source of chloride ions that can be further oxidized into active chlorine used for the electrooxidation. An experiment was conducted at pH 2 under an applied current of 1.9 A to optimize electrolyte concentration. As shown in Figure 4a, rhodamine 6G can be nearly completely removed (above 99.5%) in the presence of 0.05-0.2 M NaCl within 5 min. When the NaCl concentration is lowed to 0.02 M, it takes a longer time to achieve the complete dye removal. It indicates the salt content in the dye solution must be maintained above 0.05 M NaCl in order to allow the electrochemical process to perform well. Higher concentration of supporting electrolyte can lead to higher conductivity of aqueous solution and thus facilitate the passage of electrical current. Tran et al. demonstrated electrochemical degradation of polycyclic aromatic hydrocarbons in creosote solution.33

As the concentration of supporting electrolyte was increased from 0 to 4000 mg/L, the treatment efficiencies increased. A simulated beet sugar factory wastewater was treated by an electrochemical technology.34 The electrolyte concentration ranged from 0 to 50 g/L. It was shown that higher electrolyte concentration greatly improved the COD removal efficiency. The effect of electrolyte concentration on the C. I. acid orange 7 electrochemical degradation rate was studied by Fernandes et al.35 At the concentration > 0.02 M, there is no influence of the electrolyte concentration on the rate of color removal. However, the degradation rate greatly decreased at concentration of 0.01 M. Figure 4b demonstrates that the concentration of electrolyte considerably influences the energy consumption. When the electrolyte concentration is increased from 0.02 to 0.2 M, the energy consumption reduces from 134.49 kWh/kg of dye (26.9 kWh/m3 of wastewater or 72.7 kWh/kg of COD) to 15.83 kWh/kg of dye (3.17 kWh/m3 of wastewater or 8.5 kWh/kg of COD). It is thus suggested that the concentration of supporting electrolyte be maintained at 0.2 M of NaCl (∼12 mS/cm) or higher in order to save energy in the operation. Our finding in Figure 4b is in agreement with those from other researchers. Mohan et al. observed the energy consumption decreased from 95.6 to 61.3 kWh/kg of COD when the NaCl concentration was increased from 0.01 to 0.03 M during the electrochemical process.1 The RuO2 electrodes can be used in the sodium chloride concentration that is as high as 3 M; neither corrosion nor passivation was found.36 It was also reported that chloride was able to reduce the passivation of electrodes.37,38 As high concentration of sodium chloride was used in our study, the passivation was prevented with an evidence that the electrodes were used for more than 6 months with constant treatment efficiency. It is anticipated that the technology developed in this study be directly used in the treatment of wastewater from the dyeing bath, of which has an electrolyte concentration ranging from 0.08 to 1.2 M. The energy consumption could further decrease as the electrolyte concentration is much higher than that in our laboratory study. 3.3. Effect of pH. The pH of dye wastewater varies between 2 and 12.4 In this study, the effect of pH was conducted at initial pHs of 2, 4, 6, 10, and 12 with 0.2 M NaCl as the supporting electrolyte. As shown in Figure 5, almost complete removal (>99.5%) is achieved in all the cases when the treatment is completed. However, the kinetics is highly dependent upon the initial pH. The rate of dye removal under acid conditions initially was slightly faster than in alkaline conditions initially. Better dye removal can be achieved at initial pH < 6. Acidic condition is in favor of dye decolorization probably due to the more powerful

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Figure 6. pH profiles during the electrochemical treatment process at different initial pH. Conditions: Co ) 200 mg/L; V ) 300 mL; I ) 1.9 A; d ) 30 mm; [NaCl] ) 0.2 M; σ ) 19 mS/cm.

Figure 7. Effect of applied current on the electrochemical removal of rhodamine 6G. Conditions: Co ) 200 mg/L; V ) 300 mL; pHo ) 2; d ) 30 mm; [NaCl] ) 0.1 M; σ ) 11 mS/cm.

oxidizability of the oxidant under acidic conditions as shown in Table 1. The overall expected reaction under the electrochemical process in the presence of chloride ion is shown below: dye + oxidizing agents* f intermediates f CO2+H2O + Cl(12)

Noted that the oxidizing agents include Cl2, HClO, or ClOand free radicals. The Nernst equation that can be depicted in Pourbaix diagram reveals a significant decrease in the redox potentials (E) of HClO and ClO- with the increase of pH; the E of Cl2 is not affected by pH.39 The similar effect of pH on the removal efficiency of dye pollutant by in situ generated active chlorine was also reported by Oliveira and co-workers.40 Cathodic hydrogen production and CO2 release from the solution are affected by the pH. According to eq 3 of Table 1, the formation of hydrogen gas at cathode is enhanced if the solution is acidic. CO2 is an acidic gas and exists only in acidic condition. Figure 6 shows the pH profiles during the electrochemical treatment under different initial pHs. pHs with initial values of 2, 4, and 6 increase certain levels, caused by hydrogen production at cathode.41 The pH with initial value of 10 or 12 initially decreases and then increases up to a certain level. This could be due to the formation of acids (e.g., HClO) from chlorine dissolution. The subsequent increase in the pH may result from the continuous formation of hydroxyl ion from the hydrogen production. 3.4. Effect of Applied Current. The applied current is one of the important factors that must be considered for scaling up the electrochemical processes, since it corresponds to both material and operational costs. To determine the effect of applied current on the electrochemical removal of dye, four experiments with current ranging from 0.6 to 1.9 A were performed with an initial pH 2 and 0.1 M NaCl. As shown in Figure 7, the dye removal efficiency of above 99.5% within 5 min can be achieved at the applied current of 1.0-1.9 A or current density of 8.7-16.5 mA/cm2. It takes more than 10 min to reach the dye removal of 99.5% when the applied current is 0.6 A (current density of 5.2 mA/cm2). Higher applied current enhances the rate of oxidation reactions. Similar result was reported on the treatment of synthetic textile wastewater using Ti/Pt-Ir electrode by Muthukumar et al.42

Figure 8. Effect of distance between electrodes on energy consumption. Conditions: Co ) 200 mg/L; V ) 300 mL; pHo ) 2; I ) 1.9 A; [NaCl] ) 0.2 M; σ ) 19 mS/cm.

3.5. Effect of Distance between Electrodes. The effect of distance between electrodes on energy consumption was studied. Figure 8 shows that the distance between electrodes in the treatment for an efficiency of above 99.5% can significantly affect the energy consumption. Shorter distance between the electrodes would lead to a lower energy consumption. The energy consumption is 15.83 kWh/kg of dye (3.17 kWh/m3 of wastewater) when the distance is 30 mm. However, it significantly reduces to 7.92 kWh/kg of dye (1.58 kWh/m3 of wastewater) when the distance is 5 mm. As the distance between electrodes reflects resistance of the system, the voltage and thus the energy consumption of the system can change. 3.6. Effect of Initial Dye Concentration and Dye Working Volume. Textile industries generally discharged a considerable amount of wastewater which contains dyes with higher concentrations. For this particular reason, the effect of initial dye concentration (200-800 mg/L) and dye working volume (300-1500 mL) were investigated, in order to evaluate the performance of the process and the potential for the process scaleup. The experiments were conducted at pH 2, with applied current of 1.9 A, and in the presence of 0.2 M NaCl. As shown in Figure 9, a slightly longer reaction time is needed to treat a higher initial concentration of dye so that the higher removal efficiency is reached. The concentration of rhodamine 6G dye can be reduced from 800 to less than

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Figure 9. Effect of initial dye concentration on electrochemical removal of rhodamine 6G. Conditions: V ) 300 mL; pHo ) 2; I ) 1.9 A; [NaCl] ) 0.2 M; σ ) 19 mS/cm.

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Figure 11. COD removal under optimal experimental condition. Conditions: V ) 300 mL; pHo ) 2; I ) 1.9 A; Initial dye concentration ) 200 mg/L; initial COD ) 410 mg/L; [NaCl] ) 0.2 M; d ) 5 mm; σ ) 19 mS/cm. Table 2. Recyclability Study of Ti Mesh/RuO2 at High Current Density of the Electrodea cycle

dye removal (%)

reacn time (min)

1 2 3 4 5

99.89 99.93 99.91 99.90 99.90

6 5 5 5 5

cycle

dye removal (%)

reacn time (min)

7 8 9 10

99.83 99.93 99.88 99.88

5 5 5 5

a Conditions: Co ) 200 mg/L; V ) 300 mL; pHo ) 2; I ) 1.9 A (16.5 mA/m2); d ) 5 mm; [NaCl] ) 0.2 M; anode-cathode ) Ti mesh/ RuO2-Ti plate (9 × 4.5 cm2).

Table 3. Recyclability Study of Ti Mesh/RuO2 at Low Current Density of the Electrodea Figure 10. Effect of dye working volume on electrochemical removal of rhodamine 6G. Conditions: Co ) 200 mg/L; pHo ) 2; I ) 1.9 A; [NaCl] ) 0.2 M; σ ) 19 mS/cm.

0.5 mg/L (>99.5% removal of dye) within 7 min. These results show that the amount of oxidizing agents generated from 0.2 M of NaCl is powerful enough to remove 200-800 mg/L rhodamine 6G dye. On the other hand, Figure 10 shows that a larger volume of dye wastewater needs a longer reaction time to achieve the same level of dye removal. The reaction rate decreases due to the lower ratio of electrode surface area to dye working volume. Nevertheless, full decolorization of 1500 mL of dye-containing wastewater with initial concentration of 200 mg/L can be achieved within 11 min. Furthermore, the energy consumption required for treating 1500 mL of dye-containing wastewater (>99.5% of dye removal) is 1.16 kWh/m3 of wastewater. It was reported that 80% decolorization of a textile wastewater was accomplished by 2 kWh/m3.5 Therefore, it can be anticipated that the RuO2-based electrochemical technology be scaled-up for larger pilot-scale or full-scale operations. 3.7. COD Removal Efficiency. Because this study aimed at the decolorization of dye, we used the dye concentration to evaluate the treatment. The efficiency in terms of dye decolorization under the optimized condition is above 99.5%. The oxidation by the electrodes has a series of reactions, leading to various immediate products. The COD of the treated solution can indicate the overall oxidation of the dye. A batch experiment was carried out under the optimal experimental condition (pH 2, I ) 1.9 A, [NaCl] ) 0.2 M, and d ) 5 mm). As illustrated in Figure 11, a COD removal efficiency of approximately 80% can be achieved after a reaction time of 5 min. Comparison of Figure 11 with Figures

cycle

dye removal (%)

reacn time (min)

1 2 3

99.79 99.75 99.81

40 40 40

cycle

dye removal (%)

reacn time (min)

4 5

99.81 99.78

40 40

a Conditions: Co ) 200 mg/L; pHo ) 12; V ) 300 mL; I ) 0.6 A (5.22 mA/cm2); [NaCl] ) 0.02 M; d ) 30 mm; anode-cathode ) Ti mesh/RuO2-Ti plate (9 × 4.5 cm2).

3-10 shows that the removal percentages (in terms of color and COD removal) are inconsistent and thus further indicates that there exists some byproduct in the electrolysis. These left organics which are from the partial oxidation of dye can be easily removed by other physicochemical technologies such as adsorption if the treated solution will be reused. 3.8. Recyclability of RuO2-Coated Ti Mesh. The recyclability of electrode is essential to the stability of the electrochemical treatment system. The lifespan of the electrodes can be affected by the used current density on the electrode. Thus, a study was conducted at two different situations: high and low current densities (16.5 and 5.22 mA/cm2) of the electrode. Table 2 summarizes the recyclability of the anode at conditions where the high current density of electrode and short duration of reaction occur in the system; while Table 3 summarizes the recyclability of the electrode at conditions where the low current density of electrodes and long duration of reaction occur in the system. From the tables, we can see that there are no decreases in the efficiency of treatment after the anode is used for 5 times at low current density of the anode or 10 times at high current density. Therefore, this further demonstrates that RuO2-coated Ti mesh can be a promising anode in the electrochemical treatment process in which active

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chlorine and free radicals are generated for decolorization and mineralization of dyes. 3.9. Continuous Stirred-Tank Reactor Experiment. In general, batch reactor has its limitations while continuously stirred-tank reactor (CSTR) is more applicable in industrial scale applications. An experiment with a CSTR was conducted to investigate the system performance at an initial pH of 2, with an applied current of 1.9 A, and in the presence of 0.2 M NaCl. As shown in Figure 2, the experimental setup of CSTR is similar to the batch reactor except that the dye wastewater is continuously filled in from the bottom of the reactor by a peristaltic pump and flow out through an outlet located at the upper end of the reactor. The working volume of the reactor is around 420 mL. The experimental results show 99.2% removal of dye can be obtained using the continuous reactor with a flow rate of 130 mL/min (HRT ) 4 min), which indicates that the electrochemical technology is suitable for industrial-scale wastewater treatment. 4. Summary A series of important parameters on the performance of electrochemical treatment process for decontamination of rhodamine 6G containing wastewater using RuO2-coated Ti DSA was studied. The results are summarized as follows: (1) The electrochemical treatment process is affected by the supporting electrolyte. Among NaCl, Na2SO4, NaClO4, and NaNO3, NaCl is the best supporting electrolyte for the dye wastewater treatment. Higher electrolyte concentration decreases the energy consumption and enhances the treatment efficiency. (2) Lower pH and higher applied current accelerate the electrochemical removal. (3) Complete decolorization can be achieved within 5 min in the presence of 0.1-0.2 M of NaCl and by using the applied current of 1.0-1.9 A at pH 2-6. A relatively low energy consumption of 1.58 kWh/m3 of wastewater can be achieved. (4) Small distance between electrodes greatly decreases the energy consumption. A distance of 5 mm is recommended. (5) The RuO2-coated Ti mesh anode has good recyclability. (6) A CSTR with a HRT of 4 min can successfully treat the dye wastewater with an efficiency of 99.2%. Acknowledgment This work was funded by Academic Research Funds (Grants R-288-000-050-490 and R-288-000-050-123) from Maritime and Port Authority of Singapore and the National University of Singapore. Literature Cited (1) Mohan, N.; Balasubramanian, N.; Subramanian, V. Electrochemical Treatment of Simulated Textile Effluent. Chem. Eng. Technol. 2001, 24, 749. (2) Alle`gre, C.; Moulin, P.; Maisseu, M.; Charbit, F. Treatment and Reuse of Reactive Dyeing Effluents. J. Membr. Sci. 2006, 269, 15. (3) Hunger, K. Industrial Dyes: Chemistry, Properties, Application; Wiley-VCH: Weinheim, Germany, 2003. (4) Vaghela, S. S.; Jethva, A. D.; Mehta, B. B.; Dave, S. P.; Adimurthy, S.; Ramachandraiah, G. Laboratory Studies of Electrochemical Treatment of Industrial Azo Dye Effluent. EnViron. Sci. Technol. 2005, 39, 2848. (5) Vandevivere, P. C.; Bianchi, R.; Verstraete, W. Treatment and Reuse of Wastewater Textile Wet-Processing Industry: Review of Emerging Technologies. J. Chem. Technol. Biotechnol. 1998, 72, 289. (6) O’Neill, C.; Hawkes, F. R.; Hawkes, D. L.; Lourenco, N. D.; Pinheiro, H. M.; Dele´e, W. Colour in Textile Effluents-Sources, Measure-

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ReceiVed for reView November 11, 2008 ReVised manuscript receiVed June 10, 2009 Accepted June 16, 2009 IE801719B