Interaction between pH and Conductivity in the Indirect Electro

Article pubs.acs.org/IECR

Interaction between pH and Conductivity in the Indirect Electro-oxidation of Azo Dyes C. Gutiérrez-Bouzán*,† and M. Pepió‡ †

INTEXTER, Institut d’Investigació Tèxtil i Cooperació Industrial de Terrassa; UPC, Universitat Politècnica de Catalunya-BarcelonaTech; Colom 15, 08222, Terrassa, Spain ‡ Department of Statistics and Operations Research; ETSEIAT, Escola Tècnica Superior d’Enginyeries Industrial i Aeronàutica de Terrassa; UPC, Universitat Politècnica de Catalunya-BarcelonaTech; Colom 11, 08222, Terrassa, Spain ABSTRACT: The indirect electro-oxidation of dyes results in a rapid break of the dye molecules. The influence of the dye chromophore in this process has already been demonstrated. In this work, an electrochemical treatment in a batch cell with Ti/Pt anode is applied to degrade two reactive dyes (Color Index Reactive Oranges 4 and 13) in the presence of chloride ions, usually used as dyeing electrolyte. For the two dyes, kinetic constants were modeled. A clear influence of the reactive group in the response surfaces was found despite the unique difference between both dyes being only one substituent (OH or NH2). Models are strongly dependent on pH and conductivity. Also, an important interaction of both factors was evidenced, which can be attributed to different mechanisms of oxidizing species generation.

1. INTRODUCTION The textile processing industry is one of the major water consumers. As much as 230−270 m3 of water are required to process one ton of textile, which represents about 1500−2000 m3 per day for a medium size mill. The dyeing process contributes to 15−20% of the total wastewater flow1 and requires huge quantity of additives (mainly dyes and salts).2 In general 10−25% of textile dyes used in the dyeing process are discharged to effluents.3 Since the estimated annual dye production is 7 × 105 tones,4 dye-containing wastewater constitutes one of the main sources of severe pollution problems worldwide.5 For these reasons, stringent environmental standards have been imposed by many countries6 as water scarcity has become an issue worldwide.7 Biological treatments with activated sludge are the most suitable treatment for textile wastewater because they are generally cheap and simple and provide high efficiency of organic matter removal.8 However, they are incapable of obtaining satisfactory dye elimination as most commercial dyes are toxic to the organisms used in the process, which often leads to sludge bulking.9,10 The recalcitrant character of many organic dyes is due to their complex chemical structure.11 In particular azo12 and anthraquinone13 dyes are not totally biodegradable and half-life values up to 46 years have been reported.14 Azo dyes constitute 60−70% of the organic dyes.15 Thereby, the toxicity effects of azo dyes,16 their breakdown products,17 and mechanism of interaction18,19 have been widely investigated. The Zahn Wellens test can be used to evaluate the biodegradation of dyes in aqueous solution under aerobic conditions.20 However, in this test only the removal of soluble organic matter is considered, but the persistence of the dye degradation in the sludge is not evaluated. Different tertiary treatments such as coagulation−flocculation with resins or adsorption with active carbon are used to remove color although these methods produce in general a residual sludge or concentrate in which dyes are retained.17 For © 2014 American Chemical Society

this reason, the advanced oxidation processes (AOP), based on the dye molecules destruction, have become especially interesting as color removal methods.21 Diverse AOP wastewater treatments such TiO2 photocatalysis,22 Fenton processes,23 and electrochemical methods24 are gaining attention. Electro-oxidation is probably the most popular electrochemical AOP for wastewater remediation,25 and it has been particularly studied for the treatment of dye containing wastewaters.26,27 With this purpose, modeling studies have been increasingly applied28,29 as they constitute an important tool to optimize the experimental conditions.30 Among the different classes of dyes, reactive ones are the most unfavorable from the ecological point of view, as they produce effluents with high salinity and heavy color.31 Dyeing 1 kg of cotton with reactive dyes requires 70−150 L water, 0.6−0.8 kg NaCl, and 30−60 g of dyestuff.32,13 For this reason, these types of dyes are the subject of our research. Our previous studies demonstrated that, because of the high salinity of reactive dye effluents, the oxido-reduction with Ti/Pt oxide electrodes is an attractive option which enables the reuse of the treated effluents to diminish water and salt consumption.33−35 By means of a factorial design we also demonstrated that the chromophore group played an important role in the decoloration yield, both in a continuous flow cell36,37 and in a batch reactor.38 However, the influence of other groups of the dye molecule, such as the reactive group, has not been investigated. This work aims to evaluate the possible influence of the reactive group on the electrochemical treatment of reactive dyes. Two reactive dyes with exactly the same chromophore were selected. Once hydrolyzed, as they are found in real effluents, Received: Revised: Accepted: Published: 18993

June 18, 2014 October 22, 2014 November 16, 2014 November 17, 2014 dx.doi.org/10.1021/ie502460e | Ind. Eng. Chem. Res. 2014, 53, 18993−19000

Industrial & Engineering Chemistry Research

Article

Figure 1. Chemical structure of the selected dyes, in both parent and hydrolyzed forms.

cool to room temperature and finally, pH and conductivity were adjusted according to conditions fixed in the experimental design (section 2.3). The conductivity was adjusted by the addition of NaCl, and the pH was adjusted with NaOH or HCl. All reagents were of analytical grade (Scharlau, Spain). 2.2. Electrochemical Conditions. The electrolyses of the hydrolyzed dye aqueous solutions were carried out under oxido-reduction conditions. The volume treated was 10 L. The electrochemical treatment was performed in an undivided batch electrolytic cell with a poly(methyl methacrylate) reactor containing the electrodes located at the bottom. The cathode was constituted by two stainless steel hollow cylinders and the anode by two bars of Ti coated by Pt placed inside them (anodic surface: 150 cm2). Among the extensive range of available electrode materials, Ti/Pt was selected because of its stability in a strong oxidizing medium and also because this material provides a high dye degradation kinetic rate, according to our previous study.25 All the experiments were conducted under potentiostatic conditions. The electrical source voltage was fixed at 7 V and, consequently, the intensity (I) was variable (10 A ≤ I ≤ 25 A) according to the solution conductivity (10, 30, or 50 mS·cm−1). Experiments were carried out up to almost full decoloration (color removal ≥ 98%), which required a treatment time from 8 to 45 min. Thus, the total loaded charge was within the range of 0.13 to 1.88 A·h·L−1. 2.3. Experimental Trials. In addition to the parameters related to the reactor design, the efficiency of the electrochemical method in the degradation of organic compounds is dependent on temperature, pH, and salt-concentration.41 From the applied point of view, it is more advantageous to treat the stocked colored solutions at room temperature. For this reason, in this work the modeling study is focused on the effect of two variables, pH, and conductivity, in the color removal. The study was carried out for the dyes PX and MX, as they have a rather simple chemical structure and they are widely used in the dyeing processes. Also because, once hydrolyzed they have

their only difference is one substituent in the dye molecule (OH or NH2). The main objective of this work is to establish the effect of chloride concentration and pH on the indirect electro-oxidation of both dyes by means of models and surface responses, paying special attention to the combination effects of the two factors. As far as we know, no study has been published about the combined effect of pH and electrolyte content on the electrochemical process.

2. EXPERIMENTAL SECTION 2.1. Reagents and Hydrolysis of Dyes. Dye solutions were prepared with two dyestuffs supplied by DyStar (Spain): (1) C.I. Reactive Orange 4 with commercial name Procion Orange MX-2R (hereinafter MX) and C.I. Chemical Structure 18260,39 and (2) C.I. Reactive Orange 13 with commercial name Procion Orange PX-2R (hereinafter PX) and C.I. Chemical Structure 18270.39 Both dyes have the same monoazo-chromophoric structure (Figure 1). Their only difference is their reactive group, which is dichlorotriazine in the case of MX and monochlorotriazine for the PX. Consequently, their UV−visible spectra are nearly the same and they also have the same maximum wavelength absorption in the visible region (488 nm). Dyes, provided in the parent form, were hydrolyzed previously to the electrochemical treatments because this is their actual form in textile effluents, that is to say, the Cl atoms of the dye molecule have been replaced by OH groups. Thus, the only difference between the two hydrolyzed dyes will be one substituent in the triazine group: OH in the case of MX and NH2 in the PX. According to published studies,40 dye concentrations in textile effluents are generally in the range 0.01 to 0.25 g·L−1. Consequently, in this work, the initial dye concentration was fixed at 0.1 g·L−1. The dye hydrolysis was carried out by heating to boiling solutions containing the required amount of dye at pH 12 (adjusted with NaOH). After 1 h boiling, they were allowed to 18994

dx.doi.org/10.1021/ie502460e | Ind. Eng. Chem. Res. 2014, 53, 18993−19000


Article

Table 1. Experimental Trials, Kinetic Constants, R2 and Significance Level for MX and PX MX exp. num. 1 2 3 4 5

pH 5 5 9 9 9

C mS·cm−1 10 50 10 50 30

kD 0.1043 0.4050 0.1049 0.1995 0.0967

PX

R2

sig. level

0.9973 0.9991 0.9982 0.9960 0.9991

8.19 6.94 2.79 2.18 4.85

× × × × ×

−08

10 10−06 10−08 10−07 10−09

kD

R2

0.1172 0.4634 0.1200 0.2245 0.1540

0.9993 0.9979 0.9980 0.9992 0.9991

sig. level 2.44 2.71 4.05 1.49 1.93

× × × × ×

10−09 10−05 10−08 10−07 10−07

Figure 2. Kinetic decoloration rates for dyes MX and PX corresponding to the trials in Table 1.

MX:

exactly the same structure except for one substituent. For both dyes, a factorial design 22 (experiments 1−4 in Table 1) with one additional point (experiment 5 in Table 1) was selected to study the influence of the two main system parameters, namely, pH and conductivity. For the pH, 5 and 9 were the minimum and maximum values selected, whereas for conductivity (C) 10 and 50 mS·cm−1 were set. On the basis of our previous works,36,38 the conductivity must have a high influence on color removal. For this reason, one additional point at a medium value of C (30 mS·cm−1) was also studied. For this additional point, the pH value was 9, as reactive dyes effluents are generally alkaline. The temperature and dye concentration were fixed at 20 °C and 0.1 g·L−1, respectively. The electrolyte most commonly used in the industrial dyeing process is NaCl. For this reason, this salt has been selected as supporting electrolyte for the experiments. Then, the conductivity and pH were adjusted with NaCl and HCl. 2.4. Spectrophotometric Analysis. The decoloration evolution was studied by UV−visible spectroscopy analysis. The absorbance was measured at the visible maximum absorption wavelength (488 nm for both dyes). Several samples were collected and analyzed during the electrolysis experiments at different time intervals, depending on the trial kinetic rate. Absorbance (abs) measurements were carried out with a UV−vis spectrophotometer (Shimadzu UV-2401 PC). Dye absorbance has a linear behavior versus the dye concentration as it is shown in Lambert−Beer equations (eqs 1a and 1b), where c is the dye concentration (mg·L−1). PX: abs = 0.00050 + 27.00897c

(R2 = 0.99950)

abs = 0.00019 + 28.54338c

(R2 = 0.99981)

(1b)

2.5. Kinetics Studies. Kinetics studies of each experiment were performed. As it is known, the organic pollutant abatement can be treated following pseudo-first order kinetics.42 In our previous studies25,43 it was verified that the electrochemical dye degradation follows a pseudo-first order reaction. The decoloration rate constants (kD) were calculated according to (eq 2).

⎛C ⎞ −ln⎜ t ⎟ = kD·t ⎝ C0 ⎠

(2)

where ct is the dye concentration at time t and c0 the initial dye concentration. These decoloration rate constants kD were modeled with respect to pH, conductivity, and type of dye by means of regression analysis and stepwise regression. A single model for both dyes was proposed with a dummy variable in order to evaluate whether models have significant differences which can be attributed to the type of dye. Any coefficient with p-value > 0.05 was considered not significant and it was removed from the model in a stepwise backward regression. Calculations were made with Excel (Microsoft). To confirm the validity of the obtained models, two additional experiments were carried out as follows: (1) dye MX, pH = 8 and conductivity 20 mS·cm−1; and (2) dye PX, pH = 6 and conductivity 40 mS·cm−1. Both experiments were included into the range selected for this study.

(1a) 18995



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3. RESULTS OF THE MODELING STUDY 3.1. Decoloration Kinetic Rate. The decoloration kinetic constants (kD) corresponding to the different electrochemical treatments were calculed from the slope of the curves shown in Figure 2, for which −ln(ct/c0) is plotted versus the time (t). All kD values can be seen in Table 1. The R squared coeficients are higher than 0.9960 and the significance levels of adjustment are lower than 2.71 × 10−05, which demonstrates that the dye decoloration kinetics follow a pseudo-first-order reaction, that is, a first-order law with respect to the dye concentration. For MX, kD values are close to 0.10 in trials 1, 3, and 5 which correspond to conductivity values 10 and 30 mS·cm−1; whereas the kinetic rate is about twice in trial 4 (kD = 0.20) and aproximately quadruple in trial 2 (kD = 0.41), both with high conductivity value (50 mS·cm−1). This behavior evidences the important effect of conductivity on kD. In addition, a comparison of the kinetics of trials 2 and 4 (Figure 2) shows that at high conductiviy values, pH 5 results much faster kinetic rates than pH 9. The same behavior is not observed in kinetics 1 and 3 with low conductivy values (10 mS·cm−1) which provide the same kD. For PX, trials 1 and 3 (conductivity 10 mS·cm−1) also exhibit a slow kinetic rate, closer to 0.12. The slope of kinetics for trials 2 and 4 is also about quadruple (kD = 0.46) and twice this value (kD = 0.23). A slight difference between MX and PX is observed in trial 5, which exhibits the lower slope for MX and slightly higher slope for PX than that of trials 1 and 3. Although trial 5 lies in an intermediate conductivity value, kD is in the lower range in both cases, probably because pH 9 is not a favorable option. These results agree with those of del Rio et al.27 which established a pseudo-first order kinetic for MX decoloration with Ti/SnO2−Sb-Pt anode and stainless steel cathode, both in oxidation and reduction treatments. 3.2. Combined study of MX and PX. The influence of conductivity and pH on the decoloration rate has been demonstrated in the previous section. To establish their possible interactions a modeling statistical study is required. But prior to the study, it is important to elucidate whether MX and PX differences are significant in order to carry out the modeling study for both dyes together or separately. In the previous section it was verified that kD values are very different under the 5 different experimental conditions, which shows that the selected factors (pH and conductivity) have a major influence on the decoloration yield. It can be seen that, systematically, PX values are slightly higher than MX. A statistical combined treatment of both dyes was carried out to establish whether this small difference is significant. Also, the combined statistical treatment provides information on whether the studied factors affect equally both dyes. A special dummy variable, MP, is defined for the combined dyes study in which the value is −1 for MX and +1 for PX. The initial model is set according to eq 3.

Table 2. Results of Regression multiple R R square adjusted R square standard error observations ANOVA df regression residual total intercept pH C pH·C C2 C·MP

SS

MS

F

significance F

0.0311 0.0002

169.0714

9.66 × 10−05

5 0.1555 4 0.0007 9 0.1562 coefficients

standard error

t stat

P-value

0.00387 0.01443 0.00955 −0.00140 9.22 × 10−05 5.06 × 10−04

0.0326 0.0043 0.0020 0.0001 0.0000 0.0001

0.1186 3.3373 4.8577 −11.6743 3.1403 4.1198

0.9113 0.0289 0.0083 0.0003 0.0348 0.0146

Figure 3. Regression residual plots.

The residuals of the model are plotted versus predicted kD (Figure 3a) and evidence that the variance of the original response (kD) is constant for all factor levels. The normal probability plot of residuals is shown in Figure 3b. This figure confirms the normality assumption for the response variable It can be seen in Figure 4 that, for each dye, there is a high similarity between the kD values obtained from both the

Figure 4. Decoloration kinetic constants of MX and PX: experimental and predicted values for trials 1−5 (working conditions are detailed in Table 1).

kD = β0 + βpH ·pH + βC ·C + βMP ·MP + βpH·C · pH·C + βC2 ·C 2 + βC·MP ·C·MP + βpH·MP · pH ·MP

0.9976 0.9953 0.9894 0.0136 10

experimental kinetics and the adjusted models, according to the adjustment parameters (R2 and significance level). The adjusted model confirms that kD values are almost equal in points 1, 3, and 5 for dye MX, whereas for PX the modeled constant is clearly higher in point 5 than in points 1 and 3. This predicted behavior agrees with the kinetics shown in Figure 2.

(3)

The results of stepwise regression, calculated by Microsoft Office Excel 2007, are exhibited in Table 2. As can be seen, the model has a very good adjustment (R2 = 0.9953 and significance level = 9.66 × 10−05). 18996



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Figure 6. Values of the predicted decoloration kinetic constants. Figure 5. Response surfaces of decoloration kinetic constants with respect to the factors (conductivity and pH) and to the dye.

For each dye, the results of Table 2 can be analyzed by means of response surfaces (Figure 5) in which kD values are plotted versus pH and conductivity. In the model, pH and conductivity (linear and quadratic, C and C2) and their interaction (pH·C) are involved. The influence of conductivity on kD is different for each dye, according to the term C·MP which provides two different response surfaces (MX and PX). 3.3. Effect of pH and Conductivity on the Dye Decoloration Rate. From the previous results, it can be stated that each dye follows a different kD model (eqs 4 and 5). MX: Figure 7. Verification of models: Experimental and calculated kinetics corresponding to the additional points.

kD = 0.00387 + 0.01443· pH + 0.00904·C − 0.00140·pH ·C + 9.22 × 10−05·C 2

(4)

PX:

4. DISCUSSION OF RESULTS 4.1. Influence of Chloride Ions. Chloride ions content has a double influence in the experiments results. First, it must be considered that experiments were conducted under potentiostatic conditions (electrical source at fixed voltage). Consequently, an increase of sodium and chloride ions implies an increment of the solution conductivity which therefore produces an increase of current intensity from 10 to 25 A (at 10 mS·cm−1 and 50 mS·cm−1). Figure 6 shows evidence that kinetic constants are always higher for the higher conductivity values. Second, Figure 6 also shows that kD values are not only dependent on the conductivity but also on the pH, mainly in the case of higher conductivity values, which indicates that chloride ions are involved in the mechanism of dyes decoloration. In our previous studies38 it was already stated that the chloride ion plays an important role in the mechanism of dye degradation. In fact, when the support electrolyte is sodium sulfate, the kinetic of decoloration is always much slower than in the presence of chloride ions. In this sense, del Rio et al.40 also studied different supporting electrolytes and highlighted the role of chloride ions. They stated that the electrochemical degradation efficiency of azo dyes was improved using chloridemediated wastewaters. This phenomenon may be explained considering that the indirect electrochemical oxidation of the organic matter involves the chlorine/hypochlorite generated from chloride ions (eqs 6−8). Anode: 2Cl− → Cl 2 + 2e− (6)

kD = 0.00387 + 0.01443· pH + 0.01006·C − 0.00140·pH ·C + 9.22 × 10−05C 2

(5)

For any pH and conductivity values, the decoloration rate is systematically higher for PX because its C coefficient is slightly higher. Consequently, the higher the conductivity value, the higher the difference between the decoloration kinetic constants of both dyes. For both dyes, there is a direct effect of conductivity on kD. In addition, this effect is accelerated by the positive coefficient of term C2. There is also a direct influence of pH on decoloration. Moreover, the interaction of pH with conductivity shows an influence. For both studied dyes, this effect is clearly observed in Figure 6. It can be seen that the pH influence is not significant at lower conductivities, whereas higher conductivities exhibit a marked effect of pH. In fact, kD decreases dramatically at high conductivity values when changing from acid to alkaline pH. In addition, it can also be stated that this rate is lower for MX dye than for PX. The validity of models (eqs 4 and 5) was verified by means of two additional experiments, as indicated in section 2.5 (dye = MX, pH = 8, C = 20 and dye = PX, pH = 6, C = 40). These values were selected to obtain a slow kinetic rate and a fast one, both into the experimental range. The concordance of the experimental kinetic rates corresponding to the additional points and the models, eqs 4 and 5, is shown is Figure 7. Thus, it can be stated that the obtained models are able to predict the decoloration kinetic into the studied range of pH and conductivity. 18997



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quickly dismutated into the different species and consequently, the oxidant power will be highly dependent on the pH. On the other hand, the more acidic the pH is, the more stable is the specie S−OH2+ (eqs 9 and 10) and consequently, the lower is the generation of Cl2. This fact is less evident for higher chloride concentration because the formation of Cl2 is clearly favored by reactions shown in (eqs 11−13). Finally, it must be taken into account that the experiments were performed under potentiostatic conditions and consequently, the lower the concentration of chloride is, the lower is the intensity which also causes a lower chlorine evolution. In the case of a slower reaction rate, the chlorine must be more retained in the electrode surface, and parameters that influence the equilibrium in the solution exhibit a smaller influence. 4.2. Influence of Dye Structure. Electrochemical performance depends on the special relation between dye, oxidant species and medium. del Rio et al.40 studied the electrochemical degradation of dye MX by an oxido-reduction process and concluded that this technique degraded both azo group and aromatic structures. In these experiences a high number of intermediates were generated as HPLC analysis demonstrated. Some of these intermediates had a very high oxidation state, as carbonyl groups were detected after the treatment. The same authors45 found by means of HPLC technique and UV−vis spectroscopy that a major intermediate was generated during the initial steps of the process which by nature was similar to 2amino-1,5-naphthalenedisulfonic acid. In the oxido-reduction process (as it was our case), this intermediate was degraded after being formed, and a pathway for the generation and degradation of the intermediate was proposed. This intermediate is detected when the supporting electrolyte is either chloride or sulfate.27 Spectroscopic studies also indicated that the electrochemical treatment was able to degrade both triazinic and azo groups. Also, the electrochemical oxidation degraded the aromatic groups present in the dye molecule. They also reported the presence of an adsorbed species on the electrode surface as a consequence of a degradation process of the dye molecule during the electrochemical treatment, and no spontaneous irreversible adsorption of the MX molecule was found. All these studies evidence that electrochemical treatment of MX (and also of PX or any other dye) is a complicated process in which lots of reactions are involved. Probably, there is a competence between azo-bound breakage and both aromatic and triazine rings destruction. For the same amount of electric current passing through the solution (that is, for the same number of electrons to react), the easier is the oxidation of triazine group, the lower is the azo or aromatic degradation. In general, the decoloration kinetic rate is increased when the molecular structure is simpler as the competitive reactions are diminished. The different substituents and their position also strongly affect the coloration process maybe because of the different estereochemical and resonance inductive effects.25 Consequently, the whole dye structure must be considered to evaluate the dye decoloration, as the decoloration process usually involves many types of reactions. In this work, it has been demonstrated that the active chlorine mediated electrolysis is a complex process and that a difference of only one substituent in the molecular structure can produce significant differences in the decoloration kinetic rate. To evaluate the effect of the different substituents, additional studies on reaction mechanisms are required.

Bulk solution: Cl 2 + H 2O → HOCl + H+ + Cl−

(7)

HOCl → H+ + ClO−

(8)

In the first step, the anodic oxidation of chloride ions generates chlorine gas (eq 6), which is very soluble in water. Subsequently, chlorine is quickly dismutated into hypochlorous acid (HOCl) and hydrochloric acid (HCl) that is fully dissociated into H+ and Cl− (eq 7). Finally, the hypochlorous acid, which is a weak acid, is partly dissociated (eq 8) into H+ and hypochlorite ion (ClO−). According to Aquino et al.41 the electrolyses of chloride containing solutions at different pH values lead to distinct chloro-oxidant species (mainly Cl2, HOCl, and ClO−), whose stabilities depend also on the chloride ion concentration, the ionic strength, and the temperature. Cl2 can only be found at very acid pH values, which are not the current work range. The predominant species from the usual acid pH to slightly alkaline solutions (pH < 7.5) is HClO, whereas ClO is the predominant species for pH ≥ 7.5 (pKa(HOCl) = 7.58 at 20 °C). The oxidation potential of the ClO− species is smaller than that of HClO; for this reason, the system should be more efficient at acid pH than under alkaline conditions. Also, during the operation at alkaline conditions, the generation of ClO3− and ClO4− ions is possible, which can lead to an additional decrease in the color removal rate.44 In Figure 6, it can be verified that higher conductivity solutions follow the expected behavior with respect to the effect of pH on the kD. Better results were obtained at acidic pH, which is partially consistent with studies of Aquino et al.41 on synthetic soluble dyes. These authors stated that among the investigated variables (pH, current density, chloride concentration, and temperature), the pH is the most important one when using chloride ions, due to the different pH-dependent predominance regions of electrogenerated active chlorine species. In our studies, this behavior was only observed for higher conductivity solutions, but in the case of the lower conductivity values, the decoloration rate seems not to be dependent on pH (Figure 6). This different behavior at lower and higher conductivity can be attributed to mechanism of chlorine generation (eqs 6−8). Sala et al.25 stated that the chlorine was formed in the electrode surface (S) following the five consecutive steps indicated in eqs 9−13: S − OH 2+ ⇄ S − (OH)ads + H+

(9)

S − (OH)ads ⇄ S − Oads + H+ + e−

(10)

S−Oads + Cl− → S−(OCl)ads + e−

(11)

S−(OCl)ads + Cl− + H+ ⇄ S−(OH)ads + Cl 2

(12)

S−(OCl)ads + Cl− ⇄ S−(O)ads + Cl 2 + e−

(13)

Thus, Cl2 is generated through the hydroxyl radical (OH•) formed previously by water electrolysis. Two chloride ions have to reach the electrode surface to obtain a Cl2 molecule (eqs 11 and 12 or eqs 11 and 13). Then, for higher chloride concentration, the formation of chlorine is much easier. Reactions indicated in (eqs 11−13) are probably the limiting steps for solutions with lower Cl− content, whereas in the case of high Cl− concentration, a high amount of Cl2 is produced which is 18998



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5. CONCLUSIONS Fast decoloration of MX and PX solutions is obtained with the oxido-reduction process in the presence of chloride ions (used as dyeing electrolyte). The electrochemical decoloration yield was found to be strongly dependent on pH and conductivity within the interval selected. Models for the decoloration kinetic constants were established. The kinetic constants are increased with the increase of conductivity. Both dyes also exhibit an interaction between pH and conductivity. As a result of this interaction, the decoloration is almost independent of pH at the low conductivity value. However, at the high conductivity value, the dye degradation is much more efficient at acid pH. Results show that both dyes have a similar trend, but the chemical structure is a very important factor in the decoloration process. A difference of only one substituent in the dye molecule causes an appreciable difference in the surface responses: The group NH2 provides a faster decoloration kinetic rate than the OH.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel. +34 937398008. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Spanish Ministry of Economy and Competitiveness (MINECO, project CTM2012-31461). REFERENCES

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Interaction between pH and Conductivity in the Indirect Electro

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