ARTICLE pubs.acs.org/IECR
Empirical Models for the Decoloration of Dyes in an Electrochemical Batch Cell Montserrat Pepio† and M. Carmen Gutierrez-Bouzan*,‡ †
Department of Statistics and Operations Research (EIO) and ‡INTEXTER (Institute of Textile Research and Industrial Cooperation of Terrassa), Universitat Politecnica de Catalunya, Barcelona, Spain ABSTRACT: Among several types of textiles colorants, reactive dyes represent about 3050% of the total market share, and the most common group used as chromophore is the azo group (70%), followed by anthraquinone. The removal of such dyes from wastewater is a problem in textile mills. Electrochemical treatment has proven to be an interesting technique for removing dyes from wastewater, as it does not require any added chemical reagents. Generally, this technique is applied to obtain partial dye degradation (until color removal). In this work, electrochemical treatment was performed in a batch cell. The influence of conductivity, temperature, pH, dye concentration, and dyeing electrolyte on the decoloration was studied. The time required to reach a fixed level of decoloration and the corresponding electricity consumption were also evaluated. Six reactive dyes with different chromophores and reactive groups were selected. The main statistical study was carried out on CI Reactive Orange 13, a textile dye with the commercial name Procion Orange PX-2R (OrPX). Subsequently, the behavior of the other dyes was studied in comparison with the OrPX models.
1. INTRODUCTION From the synthesis of the first synthetic dye to today, hundreds of dyes have been commercialized for textile applications. All of these dyes are reported in the Colour Index1 (denoted CI), and they are classified both by their chemical structure (azo, anthraquinone, phthalocyanine, etc.) and by their fixation on the textile substrate (dyeing classes: direct, reactive, vat, sulfur, disperse, etc.).2 Reactive dyes are one of the most common dyeing classes, whose main advantage is that they can react with the cotton fiber (or other kind of fibers) through a covalent bond. For this reason, dyeing performed with reactive dyes has better fastness properties. However, the main drawback of reactive dyes is that, simultaneously with the dyefiber reaction, an irreversible hydrolysis reaction also occurs. The amount of hydrolyzed dye is in the range 1030%, depending on the dye constitution and on the dyeing method selected. The dye, once hydrolyzed, cannot react with fibers and is discharged in the residual effluent.3 Among several types of textile colorants, reactive dyes represent about 3050% of the total market share, and the most common group used as chromophore is the azo group (70%), followed by anthraquinone.4,5 The removal of such dyes from wastewater is a problem in textile mills. Social concern about the environmental impact caused by industry is growing, and new laws demanding stricter environmental protection are being approved. For this reason, the search for “greener” and more efficient methods for wastewater treatment is increasing.6 Among the diverse technologies employed to approach color removal, electrochemical treatment has proven to be an interesting technique to remove dyes from wastewater.711 Electrochemical treatment processes for pollutants are quite unique in the sense that they do not involve any hazardous materials and do not require any added chemical reagents to carry out oxidation or reduction reactions, as the electrolyte medium is already contained in the spent dyeing baths.12 r 2011 American Chemical Society
However, this technique cannot compete in terms of cost with conventional biological treatments in the removal of easily biodegradable organic compounds.13 Consequently, electrochemical treatments are used as complementary to biological plants. In many cases, before the biological treatment, it is convenient to separate the colored effluents to treat them in an electrochemical cell until they are completely or partially decolored. Generally, partial dye degradation (until 8090% decoloration) is sufficient, as full dye mineralization is too expensive. The residual organic compounds can be further removed in a biological plant. The main electrochemical procedures utilized for the remediation of dyestuff wastewaters are electrocoagulation and electrochemical oxidation (direct oxidation with different anodes and indirect electrooxidation with active chlorine). Electrocoagulation uses a current to dissolve Fe (or steel) or Al sacrificial anodes immersed in the polluted water, giving rise to different Fe(II) [and/or Fe(III)] or Al(III) species that act as coagulants or destabilization agents. They bring about charge neutralization for dye separation from the wastewater. Sludge formed by the coagulated particles can be separated by sedimentation or electroflotation.14 An interesting study to optimize the electrocoagulation of CI Acid Blue 193 in the wastewater of dyestuff production was carried out by Arslan-Alaton et al.15 Over the past 10 years, electrochemical oxidation techniques have been of special interest for wastewater remediation, as they do not produce sludge. They also show facility and precision in process control (because the main reagent is electrons) and compact design. An extensive range of electrode materials [e.g., graphite; boron-doped diamond; ruthenium, platinum, or tin Received: December 21, 2010 Accepted: June 10, 2011 Revised: May 30, 2011 Published: June 10, 2011 8965
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Table 1. Description of Selected Reactive Dyes abbreviation
commercial name
CI name
CI chemical structure
chromophore
λmax (nm)
reactive group
OrPX
Procion Orange PX-2R
React Orange 13
18270
monoazo
monochlorotriazine
488
Or3R
Remazol Brilliant Orange 3R React Orange 16
17757
monoazo
vinyl sulfone
485
Red
Reactol Red SF-2B
React Red 240
18215
monoazo
monochlorotriazine and vinyl sulfone
Viol
Cibacron Violet P-2R
React Violet 2
18157
monoazo metal complex monochlorotriazine
557
Blue
Basilen Bright Blue P-BR
React Blue 5
61205:1
anthraquinone
monochlorotriazine
576
Turq
Cibacron Turquoise G-E
React Blue 7
74460
Cu phthalocyanine
monochlorotriazine
622
oxide coated titanium (dimensionally stable anodes, DSAs)] have been employed, as well as different supporting electrolytes, such as H2SO4 or Na2SO4 and NaCl or KCl. In this sense, sulfate provides an inert supporting medium that does not produce any reactive species during the electrolysis (except under special conditions where it can generate persulfate), whereas chloride can have a significant effect on the color removal and degree of degradation of azo dyes through indirect oxidation.16 Krishna Prasad and Srivastava17 conducted electrochemical degradation experiments to degrade spent distillery wash. They applied a catalytic anode to evaluate the influence of different factors on color removal and chemical oxygen demand (COD) removal. From the factorial design of the experiments results, they found that increasing the chloride concentration and increasing the current density increased the color removal. In our previous studies,10,11 a factorial design was also established to evaluate the main parameters affecting the cost and yield of electrochemical treatment in the degradation of effluents containing synthetic reactive dyes. We found that electrochemical treatment of reactive dyes in a continuous cell provides good yields in removing wastewater color, although the information obtained about the rate of color removal was poor. For this reason, in this work, electrochemical treatment was carried out in a batch cell from which samples were collected throughout the color removal process. The most important factors affecting the color removal during electrochemical treatment and also the electricity consumption (directly related to the time required to reach a fixed level of decoloration) were evaluated. The main statistical study was carried out with the reactive dye CI Orange 13 with the commercial name Procion Orange PX-2R (denoted OrPX in this work). Subsequently, the behaviors of five other reactive dyes with different reactive groups and chromophores were also studied and compared with the OrPX models. Similar behaviors by these dyes would imply that chemical structure does not play an important role. On the contrary, if the behaviors are very different, this would imply a need for an individual study to establish the decoloration parameters for each dye.
2. EXPERIMENTAL SECTION 2.1. Dye Hydrolysis and Electrochemical Treatment. Solutions of six reactive dyes were prepared and hydrolyzed at alkaline pH to simulate industrial effluent discharged after the dyeing process. The studied dyes are listed in Table 1. Their CI numbers, chromophores, reactive groups, and CI structures are also indicated.1 The hydrolysis was carried out by heating solutions containing the required amount of dye at pH 12 (adjusted with NaOH) to boiling. After 1 h of boiling, the solutions were allowed to cool to
521
Table 2. Experimental Design factor values expt
conductivity
temperature
dye concentration
no.
pH
(mS 3 cm1)
(°C)
(g 3 L1)
1
5
10
20
0.1
2
9
10
20
2
3
5
50
20
2
4
9
50
20
0.1
5
5
10
50
2
6
9
10
50
0.1
7
5
50
50
0.1
8 9
9 7
50 30
50 35
2 1.05
room temperature and finally, the pH and conductivity were adjusted according to conditions fixed for each experiment (see Table 2). Reagents were of analytical grade (Scharlau, Barcelona, Spain), and the studied reactive dyes were kindly supplied by DyStar (L'Hospitalet de Llobregat, Spain) and Colorcenter (Terrassa, Spain). The electrochemical treatment was performed in a batch electrochemical cell with a poly(methyl methacrylate) vessel (15 L) containing the electrodes located at the bottom. The cathode consisted of two stainless steel hollow cylinders, and the anode of two bars of Ti coated by platinum oxide placed inside them (anodic surface = 150 cm2). The electrical source voltage was fixed at 12 V, and consequently, the intensity was variable (e55 A) depending on the solution conductivity. 2.2. Experimental Design. The main statistical study was carried out for the dye OrPX, as this dye has a rather simple chemical structure and is widely used in the dyeing process. A fractional factorial design 241 with one central point was selected to study the influence of the main system parameters, namely, pH, conductivity, temperature, and dye concentration (Table 2). The following minimum, medium, and maximum values were set: for pH, 5, 7, and 9; for conductivity (C), 10, 30, and 50 mS 3 cm1; for temperature (T), 20, 35, and 50 °C; and for dye concentration (d), 0.1, 1.05, and 2 g 3 L1. Two electrolytes are commonly used in the industrial dyeing process: NaCl and Na2SO4. For this reason, the experimental design (Table 2) was applied to the treatment with both electrolytes. When the electrolyte selected was the chloride salt, the conductivity and pH were adjusted with NaCl and HCl. When the sulfate salt was used as the electrolyte, these parameters were adjusted with Na2SO4 and H2SO4. To evaluate the evolution of the decoloration, samples were taken at different time intervals during the electrochemical 8966
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Table 3. Coefficients of OrPX Decoloration Models with Na2SO4 as the Electrolyte D (%)
a0
aC
ad
R2
significance level
10 20
33.52 69.84
0.54 1.13
7.22 14.71
0.917 0.925
5.6 104 4.1 104
30
109.97
1.76
22.42
0.935
2.7 104
40
155.91
2.47
30.24
0.947
1.5 104
50
214.35
3.32
37.24
0.953
1.0 104
factors codified and the saturated initial model t½Dð%Þ ¼ β0 + β1 X1 + β2 X2 + β3 X3 + β4 X4 + β12 X1 X2 + β13 X1 X3 + β14 X1 X4 + ε
Figure 1. Decoloration kinetics of the dye OrPX with Na2SO4.
treatments. The decoloration, expressed in percentages, corresponds to the color removal at the maximum dye absorbance wavelength. Also, the electricity consumption (EC) was measured during the experiments. Experiments were carried out to full decoloration or were terminated after 150 min even if total decoloration was not achieved. However, in cases of very low color removal, the experiment was continued up to 50% decoloration. Consequently, the responses studied in the regression model were the treatment time (minutes), denoted t[D(%)], which is the time required to reach a fixed percentage decoloration, and the average electricity consumption (W 3 h 3 L1 3 min1), expressed as ECu, that corresponds to a mean of the experimentally measured consumption values per unit volume and per unit time (for each of the nine design points). Although the main statistical study was carried out for the dye OrPX, because of its simple chemical structure, the other five dyes (Table 1) were subsequently used to compare their behavior with that of OrPX. All dyes were tested with NaCl and with Na2SO4 as the electrolyte, and the experimental conditions of point 4 were selected: pH 9, conductivity 50 mS 3 cm1, temperature 20 °C and dye concentration 0.1 g 3 L1. This point was chosen for its similarity to an industrial effluent constituted by a mixture of the concentrated residual dyeing liquor and the first washing bath.
3. RESULTS AND DISCUSSION 3.1. Decoloration Models for OrPX with Na2SO4. The kinetics shown in Figure 1 correspond to OrPx decoloration experiments carried out with Na2SO4 as the electrolyte, according to the nine sets of experimental conditions listed in Table 2. Polynomial interpolation, from the kinetics in Figure 1, provides the time t[D(%)] required to achieve decoloration values from 10% to 50%, with increases of 10%. Time values were modeled on the basis of the four factors and their interactions. Decoloration values higher than 50% were not considered in this study as they require, in most cases, excessive treatment times. The statistical analysis was performed using the Microsoft Office Excel 2007 program. The results of the 241 factorial design (experiments 18, Table 2) were analyzed with the
ð1Þ
where X1 = (pH 7)/2, X2 = (C 30)/20, X3 = (T 35)/15, and X4 = (d 1.05)/0.95. The coefficients of the model were estimated, and their significance was analyzed using the half-normal probability plot. In all cases, factors X2 and X4 were found to be the only significant factors. Consequently, all double interactions X1X2, X1X3, and X1X4 and their respective aliases X3X4, X2X4, and X2X3 were not significant. The experimental central point (experiment 9, Table 2) was incorporated into the previous analysis, and the influence of the quadratic term was studied (all of the quadratic terms were aliased) using linear regression techniques. The proposed model then becomes t½Dð%Þ ¼ β0 + β2 X2 + β4 X4 + β11 X12 + ε
ð2Þ
The term β11 X12 has not been significant (R = 0.05) in any of the D (%) studied. These results confirm that it is not necessary to perform new experiments to separate significant groups of aliases. In all cases, after this study, the models expressed in terms of the direct factors have the structure t½Dð%Þ ¼ a0 + aC C + ad d
ð3Þ
In any case, no significant influence of pH or temperature was found within the ranges studied. This is a very important result, because it implies that effluents do not require an adjustment of these parameters before treatment. These results are in agreement with the findings of Catanho et al,18 who also reported that temperature has very little effect on the rate of color removal. The coefficients of eq 3 estimated for each selected decoloration time are listed in Table 3. The values of the coefficient of determination (R2) and the significance levels of the models are also included in the table. Figure 2 shows the response surfaces of the time required to achieve 10% and 50% decolorization versus the conductivity and concentration of residual dye bath. In any decoloration process, when all of the treatment conditions are fixed, the time required depends only on the decoloration selected. In the obtained model, this effect is mainly reflected by the intercept, a0. The coefficient a0 is higher when the required decoloration level is increased (Table 3). Coefficient aC, associated with the conductivity, is always negative, indicating that the higher the conductivity, the shorter the time needed to remove color, as the reaction is favored by an increase in ion mobility. This implies that the higher the fixed decoloration, the greater the influence of conductivity on the 8967
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Table 4. Coefficients of OrPX Decoloration Models with NaCl as the Electrolyte D (%)
a0
aC
ad
R2
significance level
10
2.26
0.04
0.97
0.940
2.2 104
20 30
3.91 5.71
0.08 0.11
2.24 3.63
0.979 0.991
9.6 106 7.3 107
40
7.75
0.15
5.10
0.996
9.0 108
50
10.12
0.19
6.59
0.997
2.3 108
60
12.92
0.23
8.04
0.997
1.8 108
70
16.31
0.28
9.34
0.996
5.6 108
80
20.55
0.33
10.28
0.991
7.7 107
90
26.15
0.37
10.30
0.963
5.1 105
95
29.83
0.37
9.43
0.906
8.4 104
Figure 2. OrPX response surfaces for 10% and 50% decoloration with Na2SO4 as the electrolyte.
Figure 4. OrPX response surfaces corresponding to 10% and 95% decoloration (electrolyte NaCl).
Figure 3. OrPX decoloration kinetics with NaCl as the electrolyte (experiments 19, detailed in Table 2).
treatment time, because of the increase of the reaction rate (Figure 2). Coefficient ad, corresponding to the dye concentration, is always positive, which indicates that the higher the dye concentration in the wastewater, the longer the time required to remove the color. This increase is greater when the decoloration is higher, as coefficient ad increases with the decoloration (Figure 2). As expected, the treatment time increases when the amount of dye to be removed is greater. 3.2. Decoloration Models for OrPX with NaCl. The kinetics of decoloration for the dye OrPX using NaCl as the electrolyte are shown in Figure 3 (corresponding to experiments 19 in Table 2). It should be highlighted that treatments 2 and 5 behave differently from the others, as well as from the Na2SO4 kinetics (Figure 1). The decoloration kinetics are much slower for treatments 2 and 5 than for the rest of the treatments. These two tests correspond to treatments performed at low conductivity and high dye concentration. This different behavior is discussed in section 3.3. Because the experimental conditions corresponding to points 2 and 5 are not common in industrial practice and because they exhibited a discrepancy with the rest of the experiments, the
kinetics for experiments 2 and 5 were not included in modeling the time on the basis of process control factors. The treatment times required to obtain a set of decoloration values (from 10% to 95%) were obtained by parabolic interpolation of the other kinetics plotted in Figure 3. Table 4 lists the estimated coefficients, the coefficients of determination (R2), and the significance levels of the models. These coefficients have the same interpretation and similar behavior to those obtained with Na2SO4, although in this case, the treatment time was much shorter. In the same way, in color removal with NaCl, the time was found not to depend significantly on pH or temperature within the experimental range. Figure 4 shows the times required to decolorize 10% and 95% of OrPx with NaCl, as functions of the conductivity and dye concentration in the residual bath. 3.3. Influence of the Electrolyte. In the dye decoloration process, the breakage of the azo bond (—NdN—) leads to the elimination of color.3 Simultaneously to the azo group removal, aromatic ring breakage also occurs.19 The electrooxidation mechanism of a reactive dye with Na2SO4 as the electrolyte was studied by Del Rio et al.20,21 They concluded that the azo group and the aromatic structures were degraded following pseudo-first-order kinetics. Electrochemical oxidation processes can be classified in two types: those that take place on the electrode surface, known as 8968
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direct oxidation, and those that take place through the action of molecules formed in the anode, known as indirect oxidation. The two processes can occur simultaneously. The most common species in indirect oxidation are hydroxyl radical (OH•) and also chlorine and hypochlorite ion.19 That is, the same direct oxidation reactions occur in the presence of both electrolytes, but different species are involved in the indirect oxidations with the two electrolytes. When the electrolyte is sulfate, mainly hydroxyl radical is produced from water oxidation, whereas chloride solutions in undivided cells involve the oxidation of chloride ion to yield soluble chlorine, which is rapidly hydrolyzed and disproportioned to hypochlorite.14 Consequently, the rate of indirect decoloration process is increased in the presence of chloride ion, according to the reactions Anode 2Cl f Cl2 + 2e
ð4Þ
Cl2 + H2 O f HOCl + H+ + Cl
ð5Þ
HClO f H+ + ClO
ð6Þ
Bulk solution
dye + ClO f dye oxidation intermediates + H2 O + Cl f CO2 + N2 + H2 O + Cl ð7Þ By comparing the experimental results presented in Figures 1 and 3, it can be seen that the NaCl electrolyte provides much better results than Na2SO4, as higher decoloration is achieved in a shorter treatment time. As expected, the dye oxidation rate is increased by the chlorine and hypochlorite species obtained from the chloride ion. Despite the difference in decoloration efficiencies, the two electrolytes have the same type of model with respect to significant factors. In fact, the treatment time necessary to obtain a fixed decoloration depends only on the conductivity and the dye concentration. For this reason, experiments 2 and 5 (low conductivity, high dye concentration) exhibited extreme behaviors. Probably, the kinetics of chloride and hypochlorite generation is too slow under these conditions, and the small amount of these species generated is unable to accomplish the indirect oxidation of a high concentration of dye. Figure 5 shows the coefficients of models in terms of decoloration for treatments performed with each electrolyte. Although the evolutions of the coefficients are similar, it can be seen that the Na2SO4 intercepts (a0) are much higher than the corresponding NaCl intercepts. That is, under the same experimental conditions, the time required to reach a fixed decoloration was much longer with Na2SO4. Also, the effects of conductivity (aC) are similar, both in their action on the decoloration time and also in their evolution with the fixed decoloration level, although its effect is much more pronounced in the case of Na2SO4. Coefficients ad, which measure the influence of the initial dye concentration, are much higher in treatments performed with Na2SO4. This indicates that, in this case, it takes much longer to reach the same decoloration. That is, Na2SO4 can be used in dilute solutions, but in highly concentrated solutions, the method
Figure 5. Comparison of coefficients corresponding to decoloration time models.
Figure 6. OrPX response surfaces to obtain 50% decoloration, with NaCl or with Na2SO4, versus conductivity and dye concentration.
is viable only with NaCl. For any initial concentration of residual dye, the same increase in the degree of decoloration requires a much higher increase in treatment time in the case of Na2SO4 than NaCl. Treatment with NaCl is more efficient, as the a0 values and decoloration times are much lower. The NaCl system is less sensitive to the residual dye concentration and conductivity than the Na2SO4 system, as can be seen in Figure 6. These results agree with the published studies22 carried out with Ti/SnO2, Ti/SnO2Sb, and deactivated Ti/SnO2SbPt anodes. They demonstrate that the use of NaCl gives better 8969
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Figure 7. Decoloration kinetics of the six reactive dyes considered in this work.
results than Na2SO4 for the decoloration rate, although the oxidation efficiency was practically the same independently of what type of electrolyte was used. They also found that treated solutions presented oxidation-resistant intermediates as final products in all cases. In this study, in addition to color removal, partial organic matter removal was also achieved with the electrochemical treatment. The total organic carbon (TOC) evolution was measured for all experiments. In the case of sulfate solutions, TOC removal values between 1.5% and 10% were obtained, and for the chloride, the values were slightly increased to 315%. Attempts to adjust a TOC model did not provide satisfactory results. In a previous work,23 we carried out a study of dye mineralization. TOC removal of 81% was obtained after a prolonged treatment at high current density. This treatment required a high electricity consumption, which implies that, although the technique is very effective for color removal, it is uncompetitive for organic matter removal. At the industrial scale, complete degradation of textile effluents can be achieved at lower cost by other methods, such as conventional activated sludge biological treatments. 3.4. Decoloration of Other Reactive Dyes. To compare other reactive dyes with OrPX electrochemical decoloration, one point of the design was selected to test all dyes listed in Table 1. The conditions of point 4 (Table 2) were selected, because it provided good decoloration results with both electrolytes and because alkaline pH is most common in industrial dyeing processes. Figure 7 shows the decoloration kinetics obtained under these conditions for all dyes. The figure also includes the kinetics estimated from the OrPX models. In all cases, NaCl treatment was more efficient than Na2SO4 treatment (Figure 7a). In this figure, it can be seen that the dyes have different behaviors in the treatment with Na2SO4. The kinetics corresponding to the NaCl treatments are displayed in Figure 7b. Judging from the two figures, it can be stated that the individual behaviors of the dyes are different depending on whether the electrolyte is Na2SO4 or NaCl. Models obtained for OrPX are not directly applicable to the rest of the dyes, although most of them show similar trends. When the electrooxidation was performed with Na2SO4 (Figure 7a), the dye Or3R was more difficult to decolor than
Table 5. Electricity Consumption, ECu (W 3 h 3 L1 3 min1), Corresponding to OrPX Treatments expt
Na2SO4
NaCl
expt
Na2SO4
NaCl
1
0.33
0.30
2
0.38
0.32
6
0.31
0.30
7
0.66
3
0.60
0.69
0.60
8
0.62
4
0.59
0.70
0.66
9
0.55
5
0.50
0.32
0.31
the model compound, although its qualitative behavior was very similar. Viol also exhibited a behavior qualitatively similar to that of the model, but in this case, its decoloration was faster. Blue and Turq followed the model until 30% and 50% color removals, respectively. From these values, they were more difficult to degrade than OrPx. The very different behavior of Blue in the treatments with NaCl must be highlighted (Figure 7b). The decoloration of this dye was dramatically faster than those of the rest of the dyes in the NaCl treatments. In only 1 min, it reached a color removal of 95%. Probably, its anthraquinone structure is easier to degrade than the chromophores of the other dyes, and consequently, its decoloration was faster. The rest of the dyes were similar to the model up to 60% color removal. From this point, their decoloration was faster than that of OrPX. 3.5. Electricity Consumption. Table 5 shows the consumption values, ECu, expressed in W 3 h 3 L1 3 min1, for the nine experimental points (listed in Table 2), corresponding to OrPX treatments with NaCl and Na2SO4. To verify the possible influence of the electrolyte on electricity consumption, in addition to the four process control factors, the electrolyte was introduced as categorical variable at level 1 for Na2SO4 and level +1 for NaCl. Stepwise regression indicated that consumption was significantly affected only by the conductivity, giving EC u ¼ 0:24649 + 0:00797C
ð8Þ
where R = 0.9477 and the significance level of the model is 1.13 1011. 2
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Figure 9 shows the electricity consumption, ECu, obtained in the 12 decoloration kinetics experiments plotted in Figure 7. The consumption model for OrPx (eq 8) and the 95% prediction interval for individual consumption expected under these experimental conditions are also included. It can be seen that all dyes are within the 95% range, with any electrolyte.
Figure 8. Total electricity consumption to remove up to 50% and up to 95% of the color using NaCl as the electrolyte.
4. CONCLUSIONS Models for the treatment time required to degrade the dye OrPX to a fixed decoloration level and the corresponding electricity consumption were obtained. The results show that the electrolyte has a significant influence on the decoloration: the treatment is much faster with chloride than with sulfate. The time required to decolorize was modeled separately for each electrolyte on the basis of the same factors. In both cases, the pH and temperature did not have any significant influence on electrochemical treatments within the interval selected. The empirical equations obtained depend only on dye concentration and conductivity. However, these factors have different degrees of influence depending on the electrolyte. In addition, five dyes with different chromophore and reactive group were compared with OrPx. In all cases, they showed similar trends: NaCl was found to be much more effective than Na2SO4. However, under the same treatment conditions, the decoloration level was different for each dye. Models obtained for OrPX are not applicable to the rest of dyes. Consumption per unit time and per unit volume was found to depend only on the conductivity. For all dyes, this value was included in the 95% interval defined for the OrPX consumption model. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected] Figure 9. Electricity consumption in the decoloration treatment of the six dyes considered in this work.
This model shows that the higher the conductivity, the greater the consumption required per minute. The total consumption, EC[D(%)], expressed in (W 3 h 3 L1) is given by EC½Dð%Þ ¼ EC u t½Dð%Þ
ð9Þ
That is, the total consumption to achieve a certain degree of decoloration depends on the conductivity and the concentration of dye in the wastewater, as shown in eq 10, which was obtained by combining the models (eqs 3, 8, and 9) EC½Dð%Þ ¼ ð0:24649 + 0:00797CÞða0 + aC C + ad dÞ
ð10Þ
Figure 8 shows the total consumption required, using NaCl as the electrolyte, to decolorize the effluent up to 50% and 95%. It can be highlighted that, for low residual dye concentrations, the influence of conductivity on consumption is almost negligible, regardless of the desired decoloration. However, for high residual dye concentrations, the total electricity consumption increases with the conductivity, and this increase fades for high conductivities. Simultaneously, consumption is less sensitive to the increase of the residual dye concentration at low conductivities than at high conductivities.
’ ACKNOWLEDGMENT This work was supported by the Spanish Ministry of Science and Innovation (MICINN, Projects CTM2007-66570-C02-01 and CTM2010-18842-C02-01). The authors thank Mr. Joan Vi~nas for his collaboration in the experimental part. ’ REFERENCES (1) Colour Index International; Society of Dyers and Colourists (SDC)/American Association of Textile Chemists and Colorists (AATCC): Bradford, U.K., 1992. (2) Zollinger, H. Color Chemistry: Syntheses, Properties, and Applications of Organic Dyes and Pigments, 3rd ed.; VCH: Weinheim, Germany, 2004. (3) Gutierrez, M. C.; Crespi, M. A review of electrochemical treatments for color elimination. J. Soc. Dyers Colour. 1999, 115, 342. (4) Carneiro, P. A.; Osugi, M. E.; Fugivara, C. S.; Boralle, N.; Furlan, M.; Zanoni, M. V. B. Evaluation of different electrochemical methods on the oxidation and degradation of Reactive Blue 4 in aqueous solution. Chemosphere 2005, 59, 431. (5) Rajkumar, D.; Kim, J. G. Oxidation of various reactive dyes with in situ electro-generated active chlorine for textile dyeing industry wastewater treatment. J. Hazard. Mater. 2006, 136, 203. (6) Valero, D.; Ortiz, J. M.; Exposito, E.; Montiel, V.; Aldaz, A. Electrochemical Wasterwater Treatment Directly Powered by Photovoltaic Panels: Electrooxidation of a Dye-Containing Wastewater. Environ. Sci. Technol. 2010, 44, 5182. 8971
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(7) Shen, Z.; Wang, W.; Jia, J.; Feng, X.; Hu, W.; Peng, A. Catalytically assisted electrochemical oxidation of dye Acid Red B. Water Environ. Res. 2002, 74, 117. (8) Vlyssides, A. V. G.; Looizidou, M.; Karlis, P. K.; Zorpas, A. A.; Papaioannou, D. Electrochemical oxidation of a textile dye wastewater using a Pt/Ti electrode. J. Hazard. Mater. 1999, B70, 41. (9) Carvalho, G.; Delee, W.; Novais, J. M.; Pinheiro, H. M. A factorially-designed study of physico-chemical reactive dye color removal from simulated cotton textile processing wastewaters. Color. Technol. 2000, 118, 215. (10) Gutierrez, M. C.; Pepio, M.; Crespi, M.; Mayor, N. Control factors in the electrochemical oxidation of reactive dyes. Color. Technol. 2001, 117, 356. (11) Gutierrez, M. C.; Pepio, M.; Crespi, M. Electrochemical oxidation of reactive dyes: Method validation and application. Color. Technol. 2002, 118, 1. (12) Bayram, E.; Ayranci, E. Electrochemically Enhanced Removal of Polycyclic Aromatic Basic Dyes from Dilute Aqueous Solutions by Activated Carbon Cloth Electrodes. Environ. Sci. Technol. 2010, 44, 6331. (13) Vilaseca, M.; Gutierrez, M. C.; Lopez-Grimau, V.; Lopez-Mesas, M.; Crespi, M. Biological treatment of a textile effluent after electrochemical oxidation of reactive dyes. Water Environ. Res. 2009, 82, 176. (14) Martinez-Huitle, C. A.; Brillas, E. Decontamination of wastewaters containing synthetic organic dyes by electrochemical methods: A general review. Appl. Catal. B: Environ. 2009, 87, 105. (15) Arslan-Alaton, I.; Kobya, M.; Akyol, A.; Bayramoglu, M. Electrocoagulation of azo dye production wastewater with iron electrodes: Process evaluation by multi-response central composite design. Color. Technol. 2009, 125, 234. (16) Del Rio, A. I.; Molina, J.; Bonastre, J.; Cases, F. Influence of electrochemical reduction and oxidation processes on the decoloration and degradation of C.I. Reactive Orange 4 solutions. Chemosphere 2009, 75, 1329. (17) Krishna Prasad, R.; Srivastava, S. N. Electrochemical degradation of distillery spent wash using catalytic anode: Factorial design of experiments. Chem. Eng. J. 2009, 146, 22. (18) Catanho, M.; Malpass, G. R. P.; Motheo, A. J. Photoelectrochemical treatment of the dye reactive red 198 using DSA electrodes. Appl. Catal. B: Environ. 2006, 62, 193. (19) Riera-Torres, M.; Gutierrez, M. C. Color removal of three reactive dyes by UV light exposure after electrochemical treatment. Chem. Eng. J. 2010, 156, 114. (20) Del Rio, A. I.; Molina, J.; Bonastre, J.; Cases, F. Study of the electrochemical oxidation and reduction of C.I. Reactive Orange 4 in sodium sulphate alkaline solutions. J. Hazard. Mater. 2009, 172, 187. (21) Del Rio, A. I.; Fernandez, J.; Molina, J.; Bonastre, J.; Cases, F. Electrochemical treatment of synthetic wastewater containing a sulphonated azo dye. Determination of naphthalenesulphonic compounds produced as main byproducts. Desalination 2011, 273, 428. (22) Del Rio, A. I.; Fernandez, J.; Molina, J.; Bonastre, J.; Cases, F. On the behavior of doped SnO2 anodes stabilized with platinum in the electrochemical degradation of reactive dyes. Electrochim. Acta 2010, 55, 7282. (23) Lopez-Grimau, V.; Gutierrez, M. C. Decolorisation of simulated reactive dyebath effluents by electrochemical oxidation assisted by UV light. Chemosphere 2006, 62, 106.
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dx.doi.org/10.1021/ie102549f |Ind. Eng. Chem. Res. 2011, 50, 8965–8972
