Article pubs.acs.org/IECR
Electrochemical Decoloration of Synthetic Wastewater Containing Rhodamine 6G: Behaviors and Mechanism Yu-Ming Zheng, Rita Farida Yunus, K.G. Nadeeshani Nanayakkara, and J. Paul Chen* Department of Civil and Environmental Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260 ABSTRACT: Dye wastewater has posed a great threat to our aqueous environment. In this study, the treatment of synthetic wastewater containing Rhodamine 6G by electrochemical technology using RuO2-coated Ti mesh as anode was investigated. The effects of solution pH, temperature, and dye auxiliaries on the performance were investigated. Carbon and nitrogen mass balance analyses, UV−vis spectroscopy, Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and cyclic voltammetry (CV) were used to elucidate the working mechanism. It was found that lower solution pH and lower temperature facilitated the decoloration of the wastewater. The addition of dye auxiliaries did not significantly affect the decoloration. Under optimal condition, complete decoloration of the synthetic wastewater was obtained within 5 min, and 42.3% of the dye was mineralized. The amine and methyl groups were first detached from the dye molecule, leading to a change in the dye structure from polar into nonpolar to form a water insoluble substance. The insoluble substances were floated by the hydrogen bubbles that were generated from the cathode to produce foam products. On the other hand, the soluble substances that remained in the solution were mineralized via indirect electro-oxidation by active chlorine generated by the anode. A conceptual model for the electrochemical treatment of Rhodamine 6G containing water was proposed to illustrate the mechanism of decoloration.
1. INTRODUCTION Synthetic dye-containing wastewater from colorant manufacturing industries such as textile industries, paper mills, and leather industries presents serious problems to our water environment.1,2 The nonaesthetic properties and the toxicity, the mutagenicity, and the carcinogenicity of synthetic dyes have long been recognized.3 Conventional biological technology, catalytic oxidation, membrane filtration, flocculation, coagulation, and adsorption are available for the treatment of dyecontaining water.4−8 Especially, the combination of anaerobic and aerobic biological treatment for the removal of dyes from wastewater, which is effective but requires longer reaction times ranging from a half day to several days,9 has been studied by a large number of researchers. Most of the commercially available technologies are often ineffective and/or less cost-effective due to such drawbacks and limitations as being time-consuming, high disposal or regeneration costs, a need of additives, and the production of secondary pollution. Electrochemical technologies such as electro-coagulation, electro-flotation, electro-oxidation, and electro-deposition have attracted a growing interest in the past several decades as they are cost-effective in the treatment of different types of industrial wastewater.10−15 Electro-coagulation treatment devices equipped by aluminum, iron, or the hybrid Al/Fe electrodes have demonstrated superior performances in treatment of industrial effluents containing suspended solids, oil and grease, and other organic/inorganic pollutants. Electro-flotation is effective in removing colloidal particles, grease, oil, and organic pollutants, and has been demonstrated to outperform dissolved air flotation and sedimentation. Electro-oxidation technology is proven to be effective in degradation of the biorefractory pollutants. Electro-deposition appears to be effective in recovering heavy metals from wastewater streams. © 2012 American Chemical Society
The electrochemical technology has recently demonstrated to be a better option for treatment of dye-containing wastewater, which has higher decoloration and total organic carbon removal efficiency.16−21 Many researchers have focused mostly on the improvement of stability and electro-catalytic activity of electrodes, the treatment efficiency of electrodes, and the effects of operation parameters. The nature of electrode material significantly influences the electro-catalytic activity and behavior. Platinum anode, lead dioxide anode, dimensionally stable anode (DSA), and synthetic boron-doped diamond anode are commonly used for the electrochemical treatment of dye-containing wastewater. Compared with other anodes, DSA has the advantage of perfect mechanical and chemical resistance even under strongly acid condition and high current density. It has been demonstrated that DSA is effective in the electrochemical oxidation of a wide range of synthetic dyes. However, limited information is available on the mechanism of decolorization of dye-containing wastewater by electrochemical catalysis technology using DSA. A better understanding of the mechanisms in the process is essential to further scale-up laboratory-based technology to full-scale design/operation in the treatment of industrial wastewater. In this study, Rhodamine 6G, a cationic dye, was chosen as a model of a dye pollutant because of its rigid structure, remarkable photostability, and biorefractory.22 The aims of the present work were to evaluate the performance of electrochemical treatment of Rhodamine 6G by an RuO2-coated titanium mesh DSA, and explore its working mechanisms. The Received: Revised: Accepted: Published: 5953
August 28, 2011 February 12, 2012 March 17, 2012 April 17, 2012 dx.doi.org/10.1021/ie2019273 | Ind. Eng. Chem. Res. 2012, 51, 5953−5960
Industrial & Engineering Chemistry Research
Article
effects of solution pH, temperature, and dye auxiliaries on the electrochemical treatment were investigated. UV−vis spectroscopy, Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and cyclic voltammetry (CV) were used to elucidate the mechanisms.
The foam product sample was obtained as follows. The solution after the electrochemical treatment was centrifuged for 5 min and, the supernatant was discarded. The solid residue was rinsed for several times with DI water, and was dried at 60 °C until the weight of the solids was constant. The dried solid was subsequently used for FTIR, XPS, TOC, and TN analysis. A Fourier transform infrared spectrometer (FTS-135, BioRad) was used to determine the characteristic and changes of functional groups of dye during the electrochemical treatment process. The spectra were recorded from 500 to 3500 cm−1 using pure potassium bromide (KBr) as background. The samples were first mixed with KBr, then ground in an agate mortar at an approximate mass ratio of 1:10 (sample:KBr) in the preparation of pellets. The resulting mixture was pressed at 10 tons for 5 min; 32 scans were collected and coadded, and a step size of 4 cm−1 was applied in recording the spectra. The background obtained from the scan of pure KBr was automatically subtracted from the sample spectra. All the spectra were recorded and plotted in the same scale on the transmittance axis. X-ray photoelectron spectroscopy analysis was carried out using a VG ESCALAB MkII XPS spectrometer, with monochromatized Mg Kα X-ray source (photon energy of 1253.6 eV) working at 300 W, 15 kV and 20 mA and a base pressure of 3 × 10−8 Torr in the analytical chamber. To compensate for the charging effect, all spectra were calibrated with graphitic carbon as the reference at a binding energy (BE) of 284.5 eV. The high resolution C 1s and N 1s scans were acquired in the constant analyzer energy mode at a pass energy of 20 eV and step size of 0.05 eV. The XPS results were collected in binding energy forms and fitted using a nonlinear least-squares curve fitting program (XPSPEAK41 Software). The spectra were deconvolved with the subtraction of a linear background and a Gaussian (80%)−Lorentzian (20%) mixed function. The peak’s full width at half-maximum (fwhm) was maintained constant at ∼1.5 eV for all components. The cyclic voltammetry spectra were recorded using a potentiostat−galvanostat instrument (Autolab PGSTAT302N, Eco Chemi, Netherlands) with a standard three-compartment cell consisting of a Ti mesh as a counter-electrode, an Ag/AgCl electrode as a reference, and the RuO2-coated Ti mesh electrode as a working electrode. Scan rate was selected as 100 mV/s. The range of voltage scan was from 0.2 to 2.5 V.
2. EXPERIMENTAL SECTION 2.1. Materials and Reagents. Rhodamine 6G (C28H31N2O3Cl), benzalkonium chloride (C6H5N(CH3)2RCl (R = C8H17 − C18H37) and Brij 35 (polyoxyethylene 23 lauryl ether) were purchased from Sigma-Aldrich. Sodium chloride, ethanol, and hydrochloric acid were purchased from Merck. Sodium hydroxide was purchased from Sino Chemical Co. Total nitrogen reagent was purchased from Hach. All chemicals were of analytical reagent grade and used without further purification. The RuO2-coated Ti mesh electrode and Ti plate were purchased from Beijing Hengli Titanium Industry & Trading Pte Ltd. and Techmaster Engineering & Trading Pte Ltd., respectively. The Ti plate was grade II with thickness of 1.5 mm; while RuO2-coated Ti mesh was diamond shape with mesh size of 4 mm × 4 mm and thickness of 1.5 mm. All experimental solutions were prepared using deionized (DI) water. 2.2. Experimental Setup and Procedure. The electrochemical treatment of Rhodamine 6G containing wastewater was conducted in a Pyrex jacketed reactor with a working volume of 300 mL, in which the temperature was maintained at 25 °C. A Ti plate (90 mm × 45 mm) and RuO2-coated Ti mesh (90 mm × 45 mm) were used as cathode and anode, respectively. The information about experimental setup and the operation of the electrochemical reactor had been described in detail in our previous study.21 A series of experiments were conducted to optimize the experimental conditions for the removal of Rhodamine 6G.21 The results presented in this paper were obtained from the experiments conducted under the optimized conditions described as follows. A 300 mL portion of synthetic wastewater solution containing 200 mg/L Rhodamine 6G and 0.2 M NaCl was prepared, and the water pH was subsequently adjusted to 2 by the addition of HCl unless otherwise stated. The electrodes were vertically placed in the water with a space of 5 mm, and a desired constant current intensity of 1.9 A was applied across the electrodes by using a rectifier (TDK-lambda UP 36−24 Sanyo Denki), while the voltage was varied between 2.5 and 3.0 V. During the reaction, the water was magnetically stirred and a 2-mL sample was collected at given time intervals. The foam product from the experiment was collected and used to study the process mechanisms. In the effect of dye auxiliaries study, the same experimental procedure and conditions were used, except that 100 mg of benzalkonium chloride and 15 mg of Brij 35 were added into the Rhodamine 6G-containing water solution. 2.3. Analysis. The pH and the conductivity of the solution were monitored by an Inolab pH meter and a Schott instruments Lab 970 conductivity meter, respectively. Dye solution was analyzed by recording the respective spectrum from 190 to 600 nm using a UV−vis spectrophotometer (Thermo-scientific Genesys). The total organic carbon was measured by a Shimadzu TOC analyzer; while the total nitrogen (TN) was determined by TNT persulfate digestion method using a HACH DR/2010 spectrophotometer.
3. RESULTS AND DISCUSSION 3.1. Overall Performance. Batch experiments were conducted to investigate the electrochemical decolorization efficiency of Rhodamine 6G by RuO2-coated Ti mesh DSA under the optimized condition.21 During the experiments, it was observed that a certain amount of red color foam was produced and accumulated on the top of the dye solution. According to the electrochemical reactions involved, a proposed schematic diagram for the formation of the foam is illustrated in Figure 1. The formation of foam may be initiated via the generation of water-insoluble dye particulates, which may be caused by the attack of active chlorine. The produced water-insoluble dye particulates may coagulate together to form bigger ones. At the same time, fine hydrogen bubbles, which were produced by the cathode, lifted the particulates up to the liquid surface, resulting in the formation of foam product. This process is similar to the electro-coagulation and electroflotation. The difference is that no additional coagulant was added or produced when the foam product was formed. The 5954
dx.doi.org/10.1021/ie2019273 | Ind. Eng. Chem. Res. 2012, 51, 5953−5960
Industrial & Engineering Chemistry Research
Article
removed by electrolysis with a three-phase three-dimensional electrode in 30 min.23 Nearly complete discoloration of the Acid Red 14 solution was observed after 360 min electrolysis in electro-oxidation process by Wang et al. (2004).24 LopezGrimau and Gutierrez (2006) reported 95% of decoloration of simulated reactive dyebath effluent can be obtained within 1 h by UV light -assisted electrochemical oxidation.25 The mineralization extent of Rhodamine 6G can be determined by performing TOC analysis. The TOC removal can be used to indicate the mineralization extent of the dye, which can be represented as the below equation. mineralization = TOC removal (%) TOCinitial − (TOCfoam + TOCsupernatant) = × 100% TOCinitial
(1)
where TOCinitial is the amount of total organic carbon in the dye solution before treatment, TOCfoam is the amount of total organic carbon transferred into foam after the treatment, and TOCsupernatant is the amount of total organic carbon remained in the solution after treatment. It was found that after 5 min reaction, the treated Rhodamine 6G solution contained 7.5 mg/L TOC (∼5.6% initial value); while as much as 69.3 mg/L TOC (∼ 52.1% initial value) was retained in the foam product. On the basis of eq 1, it was estimated that approximately 42.3% of initial TOC, that is, 56.2 mg/L, was removed during the process. The mineralization percentage could have been higher with the increase of reaction time. Oleivera et al. (2007) reported that acid red 29 could be completely decolorized by active chlorine produced in an electrochemical reactor equipped with Ti/Sn0.99Ir0.1O2 as electrode; 70% of TOC was removed after 240 min reaction time at a current density of 25 mA/cm2.20 3.2. Effect of pH. The pH of dye wastewater may vary from 2 to 12 depending on the dyeing process used. In the study, batch experiments were conducted at five different initial pH values (2, 4, 6, 10, and 12) to evaluate the influence of pH on the decoloration efficiency. As shown in Figure 3a, under acidic conditions the reaction time needed for the same decoloration rate is shorter than that of under alkaline conditions. Faster decoloration in acidic conditions is probably due to the more
Figure 1. Schematic representation of the formation of water-insoluble foam.
resulting foam product was water-insoluble with red color. The formation of foam product was responsible for the efficient removal of Rhodamine 6G. The residual Rhodamine 6G may be further oxidized by the active chlorine or/and free radicals. The foam product formation and the mineralization of the dye were involved in the decoloration of the dye. It was found that the color of the dye solution almost disappeared in 5 min. Figure 2a shows the changes in UV−vis absorption spectra of Rhodamine 6G solution during the electrochemical process. The absorbance at 526 nm (a typical characteristic of Rhodamine 6G) decreased markedly with time. After 3 min reaction, the maximum absorption peak of Rhodamine 6G almost disappeared. The typical trend in time of concentration of residual Rhodamine 6G and the dye removal efficiency are depicted in Figure 2b. The results showed that the dye solution was almost fully decolorized in a very short time of 5 min. Xiong et al. (2001) reported 87% of the color of wastewater containing acid orange II can be
Figure 2. (a) Trend in time of UV−vis spectra of the Rhodamine 6G solution during the electrochemical process, and (b) trend in time of residual Rhodamine 6G and dye removal efficiency. 5955
dx.doi.org/10.1021/ie2019273 | Ind. Eng. Chem. Res. 2012, 51, 5953−5960
Industrial & Engineering Chemistry Research
Article
Figure 3. (a) Effect of initial pH on the reaction time needed for 99.5% dye removal efficiency, (b) distribution of chlorine species as a function of solution pH.
produced at the cathode. Thereafter, chlorine undergoes two important reactions, namely, redox reaction (eq 5) and disproportionating reaction (eq 6) in the solution. According to the relative equilibrium constants of chlorine in water,26 the chlorine species distribution as a function of solution pH was calculated using MINIQL+ and illustrated in Figure 3b. Under acidic conditions (pH < 4), Cl2 and HClO are the predominant species in the water solution, while ClO− is minor. With an increase in pH, the formation of ClO− increases (eq 7), while the disproportionation reaction of Cl2 will be continued until the exhausting of Cl2 (eq 6). At a high pH (pH > 10), almost all the HClO can be converted into ClO−, and ClO−becomes a major oxidizing agent. During the electrochemical treatment process, the production of hydrogen leaves hydroxyl ion at cathode area (eqs 3−4), resulting in the increase of pH from initial value to a higher level.27 The variation of pH during the treatment process is shown in Table 1. According to the Pourbaix diagram for chlorine−
powerful oxidizability of the oxidants generated in the electrochemical process. To better understand the effect of solution pH on the decoloration efficiency, the main reactions that occurred at the electrodes and solution were analyzed. In the presence of chloride ions, the main electrochemical reactions occurring in the solution during the electrochemical process are listed as follows:26,27
Anode Reaction: 2 Cl 2 − e− → Cl 2
[E(Cl 2/Cl−) 0 = 1.36 V]
(2)
[E 0 = 0V]
(3)
Cathode Reactions: 2H+ + 2e− → H 2
2H 2O + 2e− → H 2 + 2OH−
[E 0 = − 0.83V]
(4)
Bulk Solution Reactions: Cl 2 + 2e−* ↔ 2Cl−
[E 0 = 1.36V]
Cl 2 + H 2O ↔ HClO + Cl− + H+
(5)
[E 0 = 0.47V]
HClO ↔ H+ + ClO− +
Table 1. Variation of pH and Redox Potential during the Electrochemical Process
(6) (7)
−
2HClO + 2H + 2e * ↔ Cl 2 + 2H 2O
initial pH
0
[E = 1.63V] (8)
ClO− + 2H+ + 2e−* ↔ Cl− + H 2O
[E 0 = 1.72V] (9)
−
−
−
ClO + H 2O + 2e * ↔ Cl + 2OH
−
final pH
redox potential E (V) estimated from Pourbaix diagram E(HClO/Cl2) = 1.329−1.476 V E(Cl2/Cl−) = 1.395 V E(HClO/Cl2) = 1.062−1.358 V E(ClO−/Cl−) = 1.183−1.479 V E(HClO/Cl2) = 1.021−1.239 V E(ClO−/Cl−) = 1.142−1.360 V E(HClO/Cl2) = 1.003−1.145 V E(ClO−/Cl−) = 1.124−1.266 V E(ClO−/Cl−) = 1.006−1.089 V
2.0
2.0−4.5
4.5
4.0
4.0−9.0
9.0
6.0
6.0−9.7
9.7
10.0
7.6−10.0
9.8
12.0
10.6−12.0
10.8
0
[E = 0.89V] (10)
where E0 is the standard redox potential and the asterisk (∗) means the electron in the bulk solution may come from pollutant in the solution, for example, dye.
water,28 the redox potentials of active chlorine under different pH ranges were summarized in Table 1. It can be seen that the redox potentials of the available oxidizing agents in acidic conditions are higher than that of the available oxidizing agents in alkaline conditions. Hence the performance of electrochemical treatment of Rhodamine 6G was better at pH 2. 3.3. Effect of Temperature. In general, the temperature of textile dye wastewater is much higher than room temperature.29 For instance, the temperature of cationic dye wastewater from the dyeing process for acrylic fiber can reach as high as 60 °C. Thus, the electrochemical treatment of Rhodamine 6G were
The overall reaction is as follows: dye + oxidizing agents → intermediates → CO2 + H 2O + Cl−
pH variation during the process
(11)
Oxidizing agents include active chlorine such as Cl2, HClO, and ClO−. As shown in eqs 2−4, chloride ions in the solution are oxidized to chlorine at the anode, while hydrogen gas is 5956
dx.doi.org/10.1021/ie2019273 | Ind. Eng. Chem. Res. 2012, 51, 5953−5960
Industrial & Engineering Chemistry Research
Article
conducted at two different temperatures, room temperature, and 60 °C, to evaluate the influence of temperature on the performance. As shown in Figure 4, the dye removal efficiency decreases with the increase of solution temperature. The decrease of
Figure 5. Effect of dye auxiliaries on decolorization of Rhodamine 6G.
in the presence and the absence of auxiliaries. In the presence of dye auxiliaries, full decoloration can also be achieved within 5 min. As the mixture of dye auxiliaries and the salt-containing dye solution can be considered as a synthetic textile wastewater, a conclusion can be drawn that the electrochemical treatment using RuO2-coated Ti mesh as DSA has a greater potential for the treatment of industrial textile wastewater. 3.5. Mechanism Study. As aforementioned, some waterinsoluble foam was produced during the electrochemical process (Figure 1). UV−vis analysis was first used to identify the characteristics of the foam product. As the foam product is water-insoluble, ethanol was used to dissolve the foam and Rhodamine 6G, and the UV−vis spectra of the foam and dye in ethanol are depicted in Figure 6. As shown, the foam has a
Figure 4. Effect of temperature on electrochemical removal of Rhodamine 6G.
decoloration efficiency could be caused by the lower solubility and stability of chlorine gas at higher temperature. A lower solubility of chlorine may lead to a lower concentration of active chlorine in the aqueous phase. Solubility of chlorine gas in the aqueous phase follows Henry’s Law as follows.30 Pgas KH = Ca (12) ⎛ dln K ⎛ 1 1 ⎞⎞ H KH = KHo exp⎜⎜ ⎜ − ⎟⎟⎟ T0 ⎠⎠ ⎝ d(1/T ) ⎝ T
(13)
where KH is the Henry’s law constant (atm/M), Pgas is the partial pressure of the gas (atm), and Ca is the concentration of gas in aqueous phase (M). According to eqs 12 and 13, the Henry’s law constant generally increases with an increase in temperature. The larger Henry’s Law constant would contribute to the decrease in gas solubility. At a pressure of 1 atm, the chlorine solubility in water decreases from 6.3 to 3.1 g/L when the temperature is increased from 25 to 60 °C.31 In addition, Pourbaix reported that at higher temperature, some of Cl2 would be converted into chlorate (ClO3−) via hydrolysis, resulting in the decrease of production of predominant HClO from the hydrolysis of Cl2.28 The generation of chlorate, the oxidizing ability of which is less than that of chlorine (Cl2) or hypochlorous acid (HClO), may also attribute to the decrease in dye removal efficiency. 3.4. Effect of Auxiliary Additive. Dye auxiliaries, such as cationic retarder and nonionic dispersant, are commonly added into a dye bath during the dyeing process using cationic dyes. The cationic retarder mainly acts as a leveling agent which controls the penetration of dye into the fiber, while the nonionic dispersant plays a role to maintain a stable dispersion. In the study, benzalkonium chloride and Brij 35 were used as cationic retarder and nonionic dispersant, respectively. The effect of cationic retarder and nonionic dispersant on the electrochemical process is illustrated in Figure 5. It can be seen that there are no obvious difference in the decoloration of dye
Figure 6. UV−vis spectra of Rhodamine 6G and the foam product in ethanol.
strong peak at ∼210 nm and a small peak at ∼532 nm; while Rhodamine 6G has a main peak at ∼526 nm and small peaks at 205, 250, and 350 nm. This observation clearly indicates that the molecular structure of foam is different from Rhodamine 6G. Rhodamine 6G, the molecular structure of which is shown as follows,21 is a polar molecule and can be easily dissolved in water. The water-insoluble foam may possess nonpolar molecular structures. It is deduced that some of the amine groups could be detached from the dye, which decreases the polarity of the dye. To confirm the hypothesis, total nitrogen (TN) analysis was then carried out. 5957
dx.doi.org/10.1021/ie2019273 | Ind. Eng. Chem. Res. 2012, 51, 5953−5960
Industrial & Engineering Chemistry Research
Article
If there was no detachment of amine groups, the treated solution would contain no more than 47.9% of initial TN, which should be proportional to TOC content (100% − 52.1% = 45.6%) in the treated solution. However, the total nitrogen analysis demonstrated that the treated solution had 61.5% initial TN, which was much more than 45.6%. This confirms the detachment of amine group from the Rhodamine 6G during the treatment. Furthermore, FTIR analysis was conducted to further explore the formation mechanism of the foam, and the obtained results were shown in Figure 7. The peaks for foam at a wavenumber
Figure 8. XPS spectra of Rhodamine 6G and the foam product: (a) C 1s of Rhodamine 6G, (b) C 1s of foam product, (c) N 1s of Rhodamine 6G, and (d) N 1s of foam product.
decrease in the C−C and CC peak may imply that demethylating and aromatic opening occurred during the electrochemical treatment of Rhodamine 6G. It can be concluded from the TN, FTIR, and XPS analyses that partial detachment of the amine group from the main structure of dye occurred during the electrochemical treatment process. The detachment of the amine group from the main structure of Rhodamine 6G could lead to a decrease in the polarity of the compound and result in the generation of waterinsoluble foam product. The above analyses show that the indirect oxidation by the active chlorine plays an important role in the decoloration process. However, direct oxidation by the electrode might occur during the decoloration process. To determine whether the direct oxidation is involved in the process, cyclic voltammetry analysis was carried out, and the result was shown in Figure 9. The voltammogram recorded in the potential region below chlorine evolution (E < 1.3 V versus Ag/ AgCl) is nearly featureless, and no anodic current peak is observed. As the potential is in the range of 1.3 to 2.5, the portion of the plot of current versus potential is linear and smooth, indicating the evolution of chlorine. This indicates the decoloration of Rhodamine 6G mainly due to the indirect oxidation by active chlorine rather than direct oxidation by the electrode under the experimental condition. However, there is a possibility of direct oxidation of Rhodamine 6G on the anode if more positive potential is applied to the electrode, at which the evolution of chlorine would also take place. To better understand the mechanism of electrochemical decoloration of wastewater containing Rhodamine 6G, a conceptual model was proposed and shown in Figure 10. The amine groups are first detached from the dye, and then water-insoluble products are formed. The water-insoluble products are floated. The soluble substances are further oxidized and some are completely mineralized. As the electrochemical removal of Rhodamine 6G is a complicated process, further research should be carried out.
Figure 7. FTIR spectra of Rhodamine 6G dye and the foam product.
of 1289 and 1717 cm−1, which can be assigned to C−O−C and CO (aryl esters), respectively, become more prominent than the characteristic peak of secondary aromatic amine at 1536 cm−1. Compared with Rhodamine 6G, the weaker characteristic peak intensity of the amine group of the foam could result from the detachment of the amine group. This is consistent with the results of TN analysis. The detachment of the amine group was further investigated by XPS analysis. High resolution C 1s and N 1s spectra of Rhodamine 6G and the foam are shown in Figure 8. The C 1s spectra can be deconvoluted into four peaks representing four types of functional groups C−C and CC, C−O and C−N, CN, and OC−O with the binding energies of 284.2, 285.2, 286.5, and 288.4 eV (for Rhodamine 6G), and 284.3, 285.5, 286.9, and 288.4 eV (for foam product). The deconvolution of N 1s spectra produces CN−C and C− N−C peaks with binding energies of 398.5 and 399.2 eV (for Rhodamine 6G), 398.2 and 399.3 eV (for foam).32 The relative contents of component peaks are summarized in Table 2. It can be seen from Table 2 and Figure 8c,d that the content of CN−C single bond decreases from 60.8 to 39.3% due to electrochemical process, indicating the possible breakage of the tertiary nitrogen bond in Rhodamine 6G dye and resulting in the detachment of the amine group. Furthermore, Figure 8 a and 8b show that there is also a decrease in the content of the C−C and CC peak at 284.2−284.3 eV (from 57 to 47.6%) as a result of electrochemical treatment. The 5958
dx.doi.org/10.1021/ie2019273 | Ind. Eng. Chem. Res. 2012, 51, 5953−5960
Industrial & Engineering Chemistry Research
Article
Table 2. Binding Energy and Relative Content of C and N in Rhodamine 6G and the Foam Product valence state
sample
binding energy (eV)
intensity (counts/s)
proposed component
relative content (%)
C 1s
Rhodamine 6G
284.2 285.2 286.5 288.4 284.3 285.5 286.9 288.4 398.5 399.2 398.2 399.3
3918.2 2105.1 1132.3 787.1 3684.2 2860.4 1923.2 1373.8 1379.4 1250.2 1722.7 1852.2
C−C, CC C−N, C−O CN OC−O C−C, CC C−N, C−O CN OC−O CN−C C−N−C CN−C C−N−C
57.0 28.2 11.0 3.9 47.6 31.7 15.3 5.4 60.8 39.2 39.3 60.7
foam product
N 1s
Rhodamine 6G foam product
effect of the residual chlorine, and reduce the formation of chlorinated organic compounds that may be toxic.
4. CONCLUSIONS The behaviors and mechanism of electrochemical decoloration of synthetic-dye water containing Rhodamine 6G were investigated. The effects of solution pH, temperature, and the addition of auxiliaries were explored. Carbon, nitrogen mass balance analyses, UV−vis, FTIR, XPS, and CV spectra were used to systematically study the decoloration mechanism. The obtained results are summarized as follows: (1) pH and temperature played important roles for the electrochemical treatment of wastewater containing Rhodamine 6G. Lower solution pH and lower temperature facilitated the decoloration of the Rhodamine 6G containing water. (2) The addition of auxiliaries did not obviously influence the decoloration efficiency. (3) Complete decoloration of the dye-containing water was obtained within 5 min. (4) TOC analysis implied that 42.3% of Rhodamine 6G was mineralized by the electrochemical treatment within 5 min. (5) Formation of water-insoluble foam and mineralization of the Rhodamine 6G by indirect oxidation were involved during the electrochemical process. UV−vis spectra analysis showed the foam had a different molecular structure from Rhodamine 6G. FTIR, XPS, TOC, and TN analyses indicated that the amine and methyl groups were detached, which changed the dye from a polar into a nonpolar molecule and resulted in formation of a waterinsoluble substance; then the water-insoluble substance was floated by the hydrogen bubble generated on the cathode, and the others in the solution were further mineralized via aromatic opening. CV spectra showed that indirect oxidation by the active chlorine played an important role in the decoloration process. (6) A conceptual model for the electrochemical
Figure 9. Cyclic voltammogram of 200 mg/L of Rhodamin 6G in 0.2 M NaCl at pH of 2.
The foam products may be more toxic or recalcitrant than Rhodamine 6G. Hence, the exact structure and properties, especially the eco-toxicity of the foam products must be identified in the future study. On the basis of the properties of the foam products, a proper procedure for the safety disposal of the foam should be developed. Furthermore, there is a possibility of formation of a low concentration of chlorinated organic compounds in the treated water. The concentration and composition of the generated chlorinated organic compounds should be identified. More studies need to be conducted to reduce the potential production of the chlorinated organic compounds to make sure the effluent can be safely discharged into the receiving water. Before discharging, the effluent should undergo dechlorination to remove the residual chlorine by adding sodium thiosulfate or sodium sulfite. The dechlorination can minimize the negative
Figure 10. Schematic diagram of electrochemical decoloration of wastewater containing Rhodamine 6G. 5959
dx.doi.org/10.1021/ie2019273 | Ind. Eng. Chem. Res. 2012, 51, 5953−5960
Industrial & Engineering Chemistry Research
Article
(17) Szpyrkowicz, L.; Juzzolino, C.; Kaul, S. N. A comparative study on oxidation of disperse dyes by electrochemical process, ozone, hypochlorite and fenton reagent. Water Res. 2001, 35, 2129−2136. (18) Jain, R.; Bhargava, M.; Sharma, N. Electrochemical studies on a pharmaceutical azo dye: Tartrazine. Ind. Eng. Chem. Res. 2003, 42, 243−247. (19) Li, G. T.; Qu, J. H.; Zhang, X. W.; Ge, J. T. Electrochemically assisted photocatalytic degradation of Acid Orange 7 with β-PbO2 electrodes modified by TiO2. Water Res. 2006, 40, 213−220. (20) Oliveira, F. H.; Osugi, M. E.; Paschoal, F. M. M.; Profeti, D.; Olivi, P.; Zanoni, M. V. B. Electrochemical oxidation of an acid dye by active chlorine generated using Ti/Sn(1‑x)IrxO2 electrodes. J. Appl. Electrochem. 2007, 37, 538−592. (21) Yunus, R. F.; Zheng, Y. M.; Nanayakkara, K. G. N.; Chen, J. P. Electrochemical removal of rhodanie 6G by using RuO2-Coated Ti DSA. Ind. Eng. Chem. Res. 2009, 48, 7466−7473. (22) Zollinger, H. Color Chemistry: Syntheses, Properties, and Applications of Organic Dyes and Pigments; Wiley-VCH: Switzerland, 2003. (23) Xiong, Y.; Strunk, P. J.; Xia, H. Y.; Zhu, X. H.; Karlsson, H. T. Treatment of dye wastewater containing Acid Orange II using a cell with three-phase three-dimensional electrode. Water Res. 2001, 35, 4226−4230. (24) Wang, A. M.; Qu, J. H.; Liu, H. J.; Ge, J. T. Degradation of azo dye Acid Red 14 in aqueous solution by electrokinetic and electrooxidation process. Chemosphere 2004, 55, 1189−1196. (25) Lopez-Grimau, V.; Gutierrez, M. C. Decolorisation of simulated reactive dyebath effluents by electrochemical oxidation assisted by UV light. Chemosphere 2006, 62, 106−112. (26) Deborde, M.; von Gunten, U. Reactions of chlorine with inorganic and organic compounds during water treatmentKinetics and mechanisms: A critical review. Water Res. 2008, 42, 13−51. (27) White, G. C. 1999. Handbook of Chlorination; John Wiley & Sons: New York, 1999. (28) Pourbaix, M. Atlas of Electrochemical Equilibria in Aqueous Solutions; Pergamon Press: Brussels, Belgium, 1966. (29) Hunger, K. Industrial Dyes: Chemistry, Properties, Application; Wiley-VCH: Germany, 2003. (30) Leaist, D. G. Absorption of chlorine into water. J. Solution Chem. 1986, 15, 827−837. (31) McKetta, J. J. Inorganic Chemicals Handbook; Marcel Dekker: New York, USA, 1993. (32) Kundu, S.; Xia, W.; Busser, W.; Becker, M.; Schmidt, D. A.; Havenith, M.; Muhler, M. The formation of nitrogen-containing functional groups on carbon nanotube surfaces: A quantitative XPS and TPD study. Phys. Chem. Chem. Phys. 2010, 12, 4351−4359.
treatment of Rhodamine 6G-containing water was proposed to illustrate the decoloration mechanism.
■
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS The authors would like to express their appreciation to the National University of Singapore, Maritime and Port Authority of Singapore, (R-288-000-074-490 and R-288-000-050-490) for financial support of this study. R. F. Yunus is grateful to AUN/ SEE-Net, JICA for a scholarship for her master study in National University of Singapore.
■
REFERENCES
(1) Zhang, S. J.; Yu, H. Q.; Zhao, Y. Kinetic modeling of the radiolytic degradation of Acid Orange 7 in aqueous solutions. Water Res. 2005, 39, 839−846. (2) Pepió, M.; Gutiérrez-Bouzán, M. C. Empirical models for the decoloration of dyes in an elctrochemical batch cell. Ind. Eng. Chem. Res. 2011, 50, 8965−8972. (3) Can, O. T.; Kobya, M.; Demirbas, E.; Bayramoglu., M. Treatment of the textile wastewater by combined electrocoagulation. Chemosphere 2006, 62, 181−187. (4) Pala, A.; Tokat, E. Color removal from cotton textile industry wastewater in an activated sludge system with various additives. Water Res. 2002, 36, 2920−2925. (5) Chen, F.; Ma, W. H.; He, J. J.; Zhao, J. C. Fenton degradation of malachite green catalyzed by aromatic additives. J. Phys. Chem. A 2002, 106, 9485−9490. (6) Simonic, M.; Lobnic, A. The efficiency of a hybrid flocculation/ UF process for a real dye-house effluent using hydrophilic and hydrophobic membranes. Desalination 2011, 271, 219−224. (7) Cañizares, P.; Martínez, F.; Jiménez, C.; Lobato, J.; Rodrigo, M. A. Coagulation and electrocoagulation of wastes polluted with dyes. Environ. Sci. Technol. 2006, 40, 6418−6424. (8) Wang, Y. R.; Chu, W. Adsorption and removal of a xanthene dye from aqueous solution using two solid wastes as adsorbents. Ind. Eng. Chem. Res. 2011, 50, 8734−8741. (9) van der Zee, F. P.; Villaverde, S. Combined anaerobic−aerobic treatment of azo dyesA short review of bioreactor studies. Water Res. 2005, 8, 1425−1440. (10) Chen, G. H. Electrochemical technologies in wastewater treatment. Sep. Purif. Technol. 2004, 38, 11−41. (11) Feng, Y. J.; Li, X. Y. Electro-catalytic oxidation of phenol on several metal-oxide electrodes in aqueous solution. Water Res. 2003, 37, 2399−2407. (12) Chen, J. P.; Lim, L. L. Recovery of precious metals by an electrochemical deposition method. Chemosphere 2005, 60, 1384− 1392. (13) Li, X. Y.; Cui, Y. H.; Feng, Y. J.; Xie, Z. M.; Gu, J. D. Reaction pathways and mechanisms of the electrochemical degradation of phenol on different electrodes. Water Res. 2005, 39, 1972−1981. (14) Zhao, X.; Zhu, Y. F. Synergetic degradation of Rhodamin B at a porous ZnWO4 film electrode by combined electro-oxidation and photocatalysis. Environ. Sci. Technol. 2006, 40, 3367−3372. (15) Zhao, X.; Qu, J. H.; Liu, H. J.; Hu, C. Photoelectrocatalytic degradation of Triazine-containing azo dyes at γ-Bi2MoO6 film electrode under visible light irradiation (λ > 420 nm). Environ. Sci. Technol. 2007, 41, 6802−6807. (16) Yang, C. H.; Lee, C. C.; Wen, T. C. Hypochlorite generation on Ru−Pt binary oxide for treatment of dye wastewater. J. Appl. Electrochem. 2000, 30, 1043−1051. 5960
dx.doi.org/10.1021/ie2019273 | Ind. Eng. Chem. Res. 2012, 51, 5953−5960
