Ind. Eng. Chem. Res. 2000, 39, 3241-3248
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Electrochemical Oxidation of Dyeing Baths Bearing Disperse Dyes Lidia Szpyrkowicz,†,* Claudia Juzzolino,† Santosh N. Kaul,‡ Salvatore Daniele,§ and Marco D. De Faveri| Environmental Sciences Department, University of Venice, Dorsoduro 2137, 30123 Venice, Italy, National Environmental Engineering Research Institute, 440020 Nagpur, India, Department of Physical Chemistry, University of Venice, Dorsoduro 2137, 30123 Venice, Italy, and Faculty of Agriculture, Catholic University of Piacenza, v. Emilia Parmense 84, 29100 Piacenza, Italy
This paper presents the results of the electro-oxidation of pollutants in synthetic textile wastewater containing partially soluble disperse dyes. The experiments were performed in an electrochemical undivided cell reactor using seven different anode materials and 0.1 M NaCl as the supporting electrolyte. With the Ti/Pt-Ir anode, which showed the best performance among all the tested materials, additional experiments were also carried out using 0.05 M Na2SO4 as the supporting electrolyte. Experimental results obtained in the electrochemical reactor, supported by the data obtained during cyclic voltammetry studies, showed that under the conditions of the present research the removal of pollutants was mediated by active chlorine generated by electro-oxidation of chloride ions or by other mediators generated in situ and not by a direct discharge of pollutants at the anode. Under the conditions of free pH evolution 39% removal of chemical oxygen demand was obtained after 40 min of electrolysis. The apparent pseudo-first-order rate constant for the removal of color was equal to 2.54 × 10-4 s-1 and it increased to 8.23 × 10-4 s-1 under pH control at the value of 4.5, resulting in 90% removal of color after the passage of 1.9 A h dm-3. In comparative studies on the chemical oxidation of pollutants by hypochlorite far lower efficiency was obtained. Introduction
(3) dyes characterized by the presence of an azo group:
The process of dyeing synthetic fabrics such as alkantara, polyester, acetate of cellulose, acrylic, and polyamide materials is mostly accomplished by the application of disperse dyes. The class of disperse dyes comprises organic nonionic compounds generally characterized by low solubility in water. However, the presence of the OH group in the dye structure makes the molecule slightly soluble1 as, for example, (1) dyes derived from 1-aminoanthraquinone and 1,4diaminoanthraquinone:
(2) nitrodiphenylamino (mostly yellow and orange) dyes:
* To whom correspondence should be addressed. † Environmental Sciences Department, University of Venice. ‡ National Environmental Engineering Research Institute. § Department of Physical Chemistry, University of Venice. | Catholic University of Piacenza.
Other compounds in this class, which have an amino group, are almost totally insoluble in water. Wastewaters that are generated at various stages of dyeing differ in strength and temperature. Their pollution load is high and it is caused mainly by spent dyeing baths, composed principally of unreacted dyeing compounds, dispersing agents (e.g., poly(vinyl alcohol), PVA), surfactants, salts, and organics, washed out from the material undergoing dyeing. The wastewaters are characterized by high color, high chemical oxygen demand (COD) level and pH varying from 2 to 12.2-4 The conventional treatment of wastewaters generated in the process of dyeing consists of biochemical oxidative destruction, which often produces a final effluent still exceeding standards for color and sometimes for nitrogen if azo dyes or those containing amino groups are used in the factory. Pretreatment of separated streams of wastewater can lead to a significant decrease in organic load and color and is thus a common practice. The various processes used for the pretreatment of dyebearing wastewater, including chemical oxidation with different reagents such as hypochlorite, ozone, ozone + UV, hydrogen peroxide, hydrogen peroxide + UV, and hydrogen peroxide + ferrous ions (Fenton’s reagent), have been widely studied.5-9 Ozonation combined with chemical coagulation also proved efficient as a pretreatment step3 before the biological process.
10.1021/ie9908480 CCC: $19.00 © 2000 American Chemical Society Published on Web 07/29/2000
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Relatively fewer studies have been conducted on the application of methods based on electrochemical oxidation. The research of Lin and Chen5 and Lin and Peng10 was conducted using steel electrodes; thus, the coupled effect of electro-coagulation caused by Fe ions from the corroded anode and a possible direct oxidation on the anode and reduction at the cathode has been observed. Under the best conditions of current density (92.5 A m-2) the efficiency of COD removal was 60% when a certain amount of polyaluminum chloride (PAC) was added and around 50% for electrolysis without PAC addition. In the study on the application of electrochemical oxidation to the treatment of full-scale textile plant wastewater, containing a mixture of direct, reactive, and disperse dyes,11 92% removal of COD was obtained while using a Ti/RuO2 anode. It appeared challenging to investigate the possibility of applying electrochemical oxidation to treat a solution bearing disperse dyes. The partial solubility of these dyes means that solid-liquid separation processes, e.g., coagulation, are not fully efficient for their removal12 and thus makes oxidative methods an alternative. The present work was aimed at verifying the effectiveness of indirect (mediated by the products of chloride oxidation) and direct electro-oxidation of disperse dye solutions, using seven different anode materials, considering that the application of different types of anode material can govern the type of the electrochemical reaction, with the possibility of influencing the selectivity, conversion, and the type of the final product of electrolysis.13 Experimental Section A synthetic dyeing bath wastewater was obtained by mixing 3 dyes: 0.181 g dm-3 of Disperse Yellow 126 (Dispersol D-7G), 0.034 g dm-3 of Disperse Red 74 (Foron S-BWFL), and 0.158 g dm-3 of Disperse Blue 139 (Navy Blue Sumikaron S-2GL), following the preparation procedure for a green dyeing bath.1 PVA (0.444 g dm-3) was added as the dispersing agent, together with 0.055 g dm-3 of Nicca Sunsolt 7000 (anionic surfactant). The dyeing bath proved to have a significant electrical resistance (conductivity was equal to 134 µS cm-1), resulting in the need to add a supporting electrolyte. Chloride solution (0.1 M NaCl) was preferentially used, while sulfate solution (0.05 M Na2SO4) was also used in the experiments with the Ti/Pt-Ir anode. The concentration of chloride ions was calculated considering a subsequent 1:9 dilution of the wastewater during rinsing of the fabrics and discharge standards for chloride in the effluent (1200 mg dm-3). Experiments with sodium sulfate as a supporting electrolyte were carried out using the Ti/Pt-Ir anode as it gave the best performance in the preliminary tests (see later). A schematic view of the experimental setup is depicted in Figure 1. Electrochemical oxidation of wastewater was conducted in an undivided cell reactor of 0.7 dm3 volume, equipped with a 10 × 10 cm plate anode and a stainless steel (SS) plate cathode of the same dimensions. Several anodic materials were used: Ti/ PdO-Co3O4 (30 layers of PdO-Co3O4 (70%/30%) obtained by brush painting using a water-methanol solution of cobalt and palladium chlorides and air-drying at 430 °C), Ti/RhOx-TiO2 (brush painting of a 2-propanol solution of RhCl3 and TiCl4 and drying at 520 °C), Ti/MnO2-RuO2 (brush painting of a solution containing
Figure 1. Schematic view of the experimental setup (1, dc power source; 2, anode; 3, cathode; 4, stirring bar; 5, electrochemical cell; 6, magnetic mixer; 7, SCE; 8, voltammeter).
MnCl2 and RuCl3 and drying at 450 °C), Ti/Pt-Ir (30 layers obtained by brush painting of a water-methanol solution of H2PtCl6 and IrCl4 and final drying at 530 °C), Ti/SnO2-Sb2O5 (spraying of an aerosol of a solution of SnCl4 and SbCl5 and final drying at 500 °C), Ti/ RuO2-TiO2 (brush painting of a 2-propanol solution of RuCl3 and TiCl4 and final drying at 475 °C), and Ti/Pt (brush painting of a water-ethanol solution of H2PtCl6 and final drying at 530 °C).14 Electrochemical oxidation was conducted under galvanostatic conditions at a 2 A dm-2 current density, using a dc stabilized power source with voltage monitoring and control over the range between 1 and 10 V. The anode potential was monitored during the experiments using a homemade saturated calomel electrode (SCE) as a reference electrode. It was connected to the working electrode by a high-impedance voltmeter. To enhance the mass transport to the electrodes, high mixing was assured by a magnetic stirrer. The duration of all the experiments of electrolysis was 40 min. Electrochemical oxidation experiments were run under isoperibolic conditions at 25 ( 1 °C. Cyclic voltammetric measurements using various concentrations of the bath in the chloride and sulfate media were performed using a standard three-electrode cell.15 The waveforms were generated by a 283 model potentiostat/galvanostat (EG&G PAR), which was controlled by a PC via EG&G PAR 270 software. The working electrodes were either a Ti/Pt-Ir or Ti/Pt electrodes having an almost square geometrical shape and surface area of about 4 mm2. Unless otherwise stated the scan rate employed was 100 mV s-1. The reference electrode was an Ag/AgCl saturated with KCl and the counter electrode was a platinum gauze. The results of electro-oxidation in NaCl solution were compared with those obtained by chemical oxidation using sodium hypochlorite, the oxidizing reagent that is commonly used in the pretreatment of spent dyeing solutions.4 The amount of NaClO was varied over the concentration range from 750 to 5900 mg dm-3, expressed as active chlorine. Unless otherwise stated, the pH of the solutions was kept constant at 8.5 by the addition of an apprioriate amount of sulfuric acid. Although not optimal for the hypochlorite oxidation, this pH was chosen in order to maintain the same conditions during chemical and electrochemical processes (the increase of the pH to the value of 8.5 was observed from 5 min on during the electrolysis).
Ind. Eng. Chem. Res., Vol. 39, No. 9, 2000 3243 Table 1. Results of Electro-oxidation of the Dyeing Bath Using Various Anodes Obtained after 40 min of Electrolysis color removal
anode material
anode potential (V)
cell potential (V)
degree of removal (%)
kobs × 10-4 (s-1)
Ti/Pt Ti/RuO2-TiO2 Ti/SnO2-Sb2O5 Ti/Pt-Ir Ti/MnO2-RuO2 Ti/RhOx-TiO2 Ti/PdO-Co3O4
4.36 4.33 4.59 4.56 4.44 4.41 4.32
8.0-7.6 7.8-7.4 8.6-7.4 8.6-7.8 8.2-7.4 8.6-7.6 8.6-7.8
40 42 45 50 46 47 48
2.01 2.03 2.24 2.54 2.36 2.43 2.43
COD removal
r2
degree of removal (%)
Faraday efficiency η (%)
0.82 0.86 0.80 0.89 0.90 0.89 0.90
9 26 23 39 10 29 25
21.4 60.4 61.7 104.8 23.9 77.6 57.9
Figure 2. Calibration curve for determination of color (9, EtOH solution, y ) 0.0294x, r2 ) 0.9992; 2, H2O solution, y ) 0.0221x, r2 ) 0.9996).
Figure 3. UV-vis spectra of single dyes and of the dyeing bath (s, Dispersed Red 74; - - - , Dispersed Yellow 125; - - -, Dispersed Blue 139; s, synthetic wastewater before treatment).
The effect of treatment by electro-oxidation and by chemical oxidation was followed by the analysis of a global, nonspecific parameter of COD and color, at different time intervals. Determination of the color of the wastewater was carried out by measuring the absorbance at a fixed wavelength (590 nm) using a Hach DR 2000 singlebeam spectrophotometer (extinction coefficient equal to 10.45 × 10-3 dm3 mg-1 cm-1), after sample dilution (1:16). As the dyes were only partially soluble in water, this measurement gave the sum of both the turbidity and color. To check if the method was suitable as a measure of color, a correlation curve was prepared for various wastewater concentrations and compared with a curve obtained after dissolution of the samples in ethanol (Figure 2). As can be seen, the results of the analysis of water solution and of water-ethanol solution (1:16 volumetric ratio) are well correlated. Although the extinction coefficient for the water-ethanol solution of the dyes was higher (14.7 × 10-3 dm3 mg-1 cm-1) than that for the water only solution, the use of water solutions was chosen for the determination of color due to the simplicity of this procedure. A similar approach was applied by Do and Chen12 in their study on dye removal by electrocoagulation. For the samples of raw and treated wastewater UVvis spectra were performed using a double-beam PerkinElmer 554 spectrophotometer. Other parameters followed during the study were COD, determined by oxidation with dichromate and subsequent colorimetric determination using Hach DR 2000, pH (Orion pH-meter), and conductivity (Crison Micro CM 2200).
the dyeing bath influenced the spectrum to a far lesser extent. UV-vis spectra were also performed using wastewater samples containing only the chloride or sulfate solution at a time. The main difference regarding the spectra depicted in Figure 3 was a strong absorption peak at 210 nm observed in the presence of sulfates. Table 1 shows the initial anode potentials (vs SCE), recorded at the beginning of each series of experiments, together with the range of the cell potential variation during electrolysis. In all the runs the anodes were working at a very high potential (above 4 V), which exceeded the minimum values for chlorine and oxygen evolution typical for solid electrode materials.16 It followed that both the gases were continuously generated. Differences in the potential of the anodes as a function of anodic material can be considered negligible. Variation of the degree of the removal of color (the ratio between the value of absorbance measured after 40 min of treatment and its initial value) obtained using different anode materials was relatively small and varied between 40 and 50%, respectively, for Ti/Pt and Ti/Pt-Ir, as reported in Table 1. The anode that showed the best performance (Ti/Pt-Ir) was chosen for further investigations. Considering color removal as the main objective, it can be concluded that the type of anode material had little influence on the final effect of the treatment, contrary to what had been expected. In fact, according to the literature, two different processes can occur at the electrode as a function of the anode material employed. On the anodes having high electrocatalytic activity, such as platinum, direct oxidation can take place. On metal oxide electrodes indirect electrochemical oxidation can occur via surface mediators that remain fixed on the anode surface, where they are continuously generated.13,17 Although in the case of the present study the tested anode materials included those at which a nonactive type of oxidation could be expected and those that could exhibit active behavior, only small differences were observed during the removal of color. To explain
Results and Discussion In Figure 3 the UV-vis spectra of the synthetic wastewater and of single dyes contained in it are shown. The spectrum of raw wastewater is the sum of the spectra of the single dyes. Other additives present in
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Other loss reactions could also occur in the bulk solution and at the cathode.20-22 The possible cathode loss reaction is
ClO- + H2O + 2e- f Cl- + 2OH-
(7)
which occurs simultaneously with the cathode primary reaction of hydrogen evolution. The bulk solution loss reactions are Figure 4. Changes of pH during electrolysis using different anode materials; supporting electrolyte 0.1 N NaCl (+, Ti/PdO-Co3O4; [, Ti/Pt; 0, Ti/RuO2-TiO2; 2, Ti/SnO2-Sb2O5; ×, Ti/Pt-Ir; f, Ti/ MnO2-RuO2; O, Ti/RhOx-TiO2) or 0.05 N Na2SO4 (s, Ti/Pt-Ir).
these results, it was hypothesized that the indirect electroxidation, involving various forms of chlorine, was the predominating process during color removal. The production of this highly oxidative reagent occurs after the surface reaction of electrochemical oxidation of chloride ions at the anode. The reaction mechanism can be sketched18 as
2Cl- a Cl2(el) + 2e-
(1)
Cl2(el) f Cl2(sol)
(2)
Chlorine formed at the electrode (Cl2(el)) can undergo a dismutation reaction in the bulk solution (Cl2(sol)) to form hypochlorous acid and hypochlorite ion, depending on whether the pH is low or high, respectively:
Cl2(sol) + H2O a HClO + H+ + Cl-
(3)
HClO a H+ + OCl-
(4)
Chlorine, hypochlorous acid, and hypochlorite ions are strong oxidizing species and are often referred to as “active chlorine”. During the removal of color under the conditions of NaCl as the supporting electrolyte, two different regions were evident: the first (the initial 5 min of the reaction) characterized by high rates of color removal and the second, characterized by much slower reactions. This behavior can be explained taking into account the pH changes of the solution during the electrolysis. The pH variation observed in the experiments using different anode materials is shown in Figure 4. From this figure it can be seen that within 5 min the pH increased sharply from the initial value of 4.5 up to 8.5-9.5. Afterward, it remained nearly constant. The pH change affects the nature of chlorine forms. At the acidic pH chlorine is present in the solution in the form of a hypochlorous acid, which has a higher oxidation potential (E° ) 1.50 V) than that of hypochloric ions (E° ) 0.89 V), the latter being prevalent at alkaline pH. Moreover, at a high pH the following parasite reactions of chlorate and perchlorate production, leading to the depletion of hypochlorite concentration, could also occur at the anode:19,20
6HClO + 3H2O f 2ClO3- + 4Cl- + 12H+ + 3/2O2 + 6e- (5) ClO3- + H2O f ClO4- + 2H+ + 2e-
(6)
2ClO- f O2 + 2Cl-
(8)
2HClO + ClO- f ClO3- + 2Cl- + 2H+
(9)
Under the conditions of the present study the occurrence of the loss reactions is very probable as the reactor was an undivided cell. This, associated with the lower oxidation potential of hypochlorite ions, could further slow the rate of dye destruction under alkaline conditions. It is likely that at the beginning of electrolysis oxidation could have been mediated by hypochlorous acid, while later on, the oxidation reaction occurred through hypochloric ions, thus leading to a lower oxidation effect. This leads to the “recovery” of chloride ions, which are, in turn, continuously oxidized at the anode to form chlorine/hypochlorite again. It follows that, in solutions containing chloride ions, the pollutant removal rate due to electrochemical oxidation is a function of the pollutant concentration ([c], mg dm-3) and also of the active chlorine concentration ([Cl2], mg dm-3) since the indirect oxidation is mediated by chlorine/hypochlorite. Thus, for the kinetics of pollutant removal in the chloride medium the following equation should apply:
-
d[c] ) k[c][Cl2] dt
(10)
Assuming stationary conditions under which there is no accumulation of chlorine in the solution and the rate of its production (which is proportional to the applied current) and rate of the consumption are equal, the concentration of active chlorine during electrolysis can be assumed to be constant and the observed constant kobs can be measured experimentally. Accordingly, eq 10 can be written
-
d[c] ) kobs[c] dt
(11)
The values of the apparent pseudo-first-order kinetic constant kobs obtained for the removal of color under the conditions without pH control are reported in Table 1 for different anodes. These values are calculated for the alkaline pH region only. The values of r2, although not very high (between 0.8 and 0.9), are however higher than those calculated considering a zero or a secondorder kinetics. In fact for the latter cases r2 values lower than 0.7 were always obtained. As the present study used a mixture containing dyeing substances of an unknown structure covered by patent rights and other dyeing bath additives (disperdants and a surfactant), the electro-chemical process was evaluated through a global, nonspecific parameter of COD. Consequently, the theoretical current efficiency
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is related to the latter parameter, as shown by the equation23
η)
F∆cV 8Q
(12)
where F is Faraday’s constant (96487 C equiv-1), ∆c is the decrease in COD (g dm-3), V is the volume of the solution (dm3), and Q is the quantity of electricity (C), calculated as the product of applied current (I) and the time of electrolysis (t) in seconds. Current efficiency data reported in Table 1 indicate the excellent performance of the Ti/Pt-Ir anode. The very high current efficiency obtained with this electrode cannot be attributed only to the indirect electro-oxidation mediated by active chlorine. As already stated above, water decomposition and evolution of oxygen occurred simultaneously with the generation of chlorine. Consequently, the effective current used for chlorine production should have been lower than the total current applied in the reactor. To explain the high efficiencies obtained, it must be considered that during oxygen evolution the formation of hydroxyl radicals (‚OH) is likely to occur and may be an additional source of oxidizing species.13 Thus, the kobs values reported in Table 1 include the effects due to both active chlorine and oxygen-containing mediators. The current efficiency slightly exceeding 100% may be related to errors in measurement of the current or may indicate that other processes, concomitant to electro-oxidation, might also have concurred in the removal of pollutants. Considering the only partial solubility of disperse dyes, a certain contribution of electroflotation may reasonably be presumed. However, the entity of this process was not significant as the foam that floated to the surface of the reactor was not visible. To verify the effect of pH on the overall process, a series of experiments was also carried out with the pH control at the value of 4.5 with the addition of appropriate amounts of sulfuric acid. As expected, under these conditions the process displayed a better performance. Decolorization of the wastewater reached 90% in 40 min. The pseudo-firs-order kinetic constant increased to 8.23 × 10-4 s-1. The overall removal degree of COD, calculated as a ratio between quantity removed and the initial value, achieved after a 40-min treatment, is reported in Table 1. In contrast to the color removal, the elimination of COD was influenced to a higher degree by the kind of the anode used. The degree of COD removal (being a measure of the removal of organic substances) was the lowest for the Ti/Pt and Ti/MnO2 + RuO2 electrodes: 9 and 10%, respectively, while the Ti/Pt-Ir anode again proved to be the best one (39% removal degree). The far lower degree of removal obtained in the present study with respect to previous results is probably due to the different composition of the investigated textile bath. In the present case only disperse dyes were used, while in the earlier work the wastewater cocktail contained both direct and disperse dyes. During indirect electro-oxidation mediated by electrogenerated chlorine, the rate of removal of the pollutants under conditions of vigorous stirring had previously been found to be controlled by surface kinetics of chlorine evolution.24 This observation is consistent with the achievement in the present study of much better results using the Ti/Pt-Ir anode by comparison with the Ti/Pt effect. In fact, the chlorine evolution can
Figure 5. Color and COD removal with the Ti/Pt-Ir anode under conditions of different supporting electrolytes: 0.1 N NaCl (O, COD; 4, color) and 0.05 N Na2SO4 (b, COD; 2, color).
Figure 6. Removal of color by the chemical oxidation as a function of hypochlorite dose and reaction time ([, 10 min; 9, 30 min; 4, 60 min; ×, 150 min; O, 210 min).
be considered a fast reaction at a Ti/Pt-Ir electrode.25 The presence of Ir in the coating prevents the oxidation of platinum,25 the latter being a better catalyst for chloride oxidation than the oxidized surface. To verify the hypothesis regarding the predominance of indirect oxidation involving chlorine species in the overall electrochemical process, experiments were performed using sodium sulfate as the supporting electrolyte. In the experiments using the latter medium the initial potential of the anode was 3.6 V, while the cell potential during the electrolysis varied between 8.2 and 7.4 V. Figure 5 compares the results for color and COD removal obtained with a Ti/Pt-Ir electrode in chloride and sulfate solutions. It can be seen that when sulfate was used as the electrolyte, the removal of COD was rather low (15% after 40 min of electrolysis) while it reached 40% for the removal of color. These results suggest that the process leads to conversion rather than to a complete combustion of organics to H2O and CO2, which is consistent with the literature on the electrocatalytic behavior of noble metal anodes.13,17 Comparative chemical oxidation, carried out using sodium hypochlorite, gave the results depicted in Figure 6, in which the absorbance, determined at various time intervals, is plotted against the hypochlorite dose, expressed as active chlorine. Although the reaction leading to a decrease in color was very quick (it was essentially completed in the first 5 min), levels of decolorization comparable with those obtained by electrochemical treatment could be achieved only by using a very high hypochlorite dose, equal to 6 g dm-3 (referred to as active chlorine). To achieve the same treatment effect by electro-oxidation, a 40-min elec-
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Figure 7. Spectra of the wastewater after chemical (s) and electrochemical oxidation with 0.05 N Na2SO4 as the supporting electrolyte (s) and 0.1 N NaCl (- - -).
trolysis was necessary. Assuming a 100% Faraday efficiency of chlorine evolution (no oxygen evolution and no losses due to reactions (5)-(10)), the quantity of evolved chlorine is 0.875 g dm-3, which is far less than the amount of active chlorine used during chemical oxidation. It must also be considered that the effect of the removal of color by chemical oxidation was not proportional to the reagent dose: only a small improvement is associated with doses of hypochlorite higher than 2.2 g dm-3 of active chlorine (see Figure 6). The rate of color elimination reached a plateau, thus indicating that the chemical method based on hypochlorite addition did not allow complete color removal, even when a large excess of the reagent was used. Figure 7 compares the UV-vis spectra of wastewater after electro-oxidation in a reactor equipped with a Ti/ Pt-Ir anode, using sulfate and chloride as supporting electrolytes, and after hypochlorite oxidation. The analysis was performed after dissolution of the samples in EtOH. The spectrum of the sample treated using the sulfate solution shows a non-zero baseline, due to the formation of compounds that are not soluble in EtOH. A large absorption around 210 nm is the same as the one obtained for the wastewater sample before treatment in the presence of sulfates only. Some peroxides, which absorb in the range between 300 and 200 nm, might also have been produced by the cathodic reduction of oxygen, as the reactor was an undivided cell.26 Furthermore, in contrast to the spectrum of a sample obtained after electrolysis in the presence of chloride, the spectrum with sulfate shows no decrease in the absorbance at the wavelength region corresponding to yellow and red. These results may suggest that the dyes were removed by indirect electrolysis when chloride was present and that the Ti/Pt-Ir anode has no selectivity for any of the three disperse dyes studied. From Figure 7 it seems that the mechanism of chlorination of the dyeing bath may be similar to indirect electro-oxidation. In fact, as with the electrochemical process, peaks relative to the wavelength of yellow and red are almost completely removed. To obtain further information on the electrochemical processes occurring at the anodes, a series of cyclic voltammetric experiments was performed, using both Ti/Pt-Ir and Ti/Pt materials as the working electrodes. Figure 8 shows typical cyclic voltammograms obtained in aqueous solutions containing either NaCl (Figure 8a) or Na2SO4 (Figure 8b) as the supporting electrolyte. The potential range explored varied between -1.5 and 2 V, to include respectively the hydrogen and oxygen evolution processes.16 From these voltammograms it is evident that the cathodic zone is substantially the same,
Figure 8. Cyclic voltammograms recorded at the 4 mm2 Ti/PtIr electrode over the potential window from -1.5 to 1.8 V without the wastewater (s) and with the addition of the wastewater (- - -). Synthetic wastewater contained the following: 0.09 g dm-3 of Disperse Yellow 126 (Dispersol D-7G), 0.017 g dm-3 of Disperse Red 74 (Foron S-BWFL), 0.079 g dm-3 of Disperse Blue 139 (Navy Blue Sumikaron S-2GL), 0.222 g dm-3 of PVA, and 0.027 g dm-3 of Nicca Sunsolt 7000 (anionic surfactant). Supporting electrolyte: (a) 0.1 N NaCl; (b) 0.05 N Na2SO4. Scan rate, 100 mV s-1.
regardless of the nature of the supporting electrolyte employed, whereas a shoulder (indicated with S in Figure 8a), conceivably due to the chlorine evolution process,27 appears before the oxygen evolution reaction when the solution contains NaCl. Figure 8 also includes the voltammograms obtained in the presence of 186 mg dm-3 of the dyes and other organic compounds of the bath. These voltammograms clearly indicate that no process attributable to direct oxidation of dyes or other organic substances present in the bath is evident in the different electrolyte solutions investigated. This result is in agreement with the above-formulated hypothesis about the involvement of indirect electro-oxidation in the destruction of the organic matter present in the solution. Since, as already stated, the experimental measurements carried out in the electrochemical reactor were run at an applied potential of about 4 V, a series of voltammetric experiments was also run over an anodic potential zone to include the latter limit. Figure 9 shows typical cyclic voltammograms obtained in the solutions containing NaCl as the supporting electrolyte and using Ti/Pt and Ti/Pt-Ir as working electrodes. Although the voltammograms obtained under the latter conditions are
Ind. Eng. Chem. Res., Vol. 39, No. 9, 2000 3247
reported above regarding the higher efficiency of the Ti/ Pt-Ir electrode during both color and COD removal. Conclusions
Figure 9. Cyclic voltammograms recorded over the potential window from 0 to 4 V with 0.1 N NaCl as the supporting electrolyte without the wastewater (s) and with the addition of the wastewater (s). Synthetic wastewater contained: 0.09 g dm-3 of Disperse Yellow 126 (Dispersol D-7G), 0.017 g dm-3 of Disperse Red 74 (Foron S-BWFL), 0.079 g dm-3 of Disperse Blue 139 (Navy Blue Sumikaron S-2GL), 0.222 g dm-3 of PVA, and 0.027 g dm-3 of Nicca Sunsolt 7000 (anionic surfactant). Anode materials: (a) Ti/Pt, 4 mm2; (b) Ti/Pt-Ir, 4 mm2. Scan rate, 100 mV s-1.
in general affected by a large Ohmic drop, they can give useful information. In the absence of the dyes the voltammograms display large oscillations when the anode potential applied was greater than about 2.3 V. These oscillations are conceivably due to bubbles of oxygen delivered from the electrode surface. In the voltammograms recorded in the presence of dyes the oscillations disappear, while the current is lower. The latter effect is even more marked when the voltammograms were taken with the Ti/Pt electrode. These results once again indicate that over a rather high potential window no process due to the direct oxidation of dyes or any other organic compound is evident. This also supports the hypothesis that the indirect oxidation process involving the various forms of chlorine was the prevailing process in the color and COD removal. The current decrease and elimination of the oscillation in the voltammograms in dyeing bath solutions can be due either to a passivation of the electrode surface, owing to adsorption of the organic substances, or to the fact that oxygen bubbles were formed to a lower extent, because of the interaction between the physioadsorbed ‚OH radicals with the organic matter. The less marked decrease of the current at the Ti/Pt-Ir electrode suggests that the latter electrode material may become less passivated, owing to a larger number of active sites containing physioadsorbed ‚OH radicals, which may be co-responsible for oxidation of the organic material. This view is in agreement with the experimental results
The results of the present study prove the feasibility of application of electro-oxidation for the destruction of the pollutants present in dyeing baths containing partially soluble disperse dyes and indicate that electrochemical oxidation, which can lead to substantial decolorization, is promising for the treatment of this kind of wastewater. The efficiency of the treatment depended on the nature of the supporting electrolyte and the bulk pH in the reactor and, to a lesser degree, on the type of the anode material. The best results were obtained in a chloride-rich medium under acidic pH using the Ti/Pt-Ir anode. Since a cyclic voltammetry study showed no direct discharge of pollutants at the Ti/Pt-Ir anode, it was concluded that the process was mediated by chlorine-hypochlorite species obtained via electro-oxidation of chlorides at the anode and (‚OH) radicals generated during water discharge. The apparent pseudo-first-order rate constant for the removal of color was equal to 2.54 × 10-4 s-1 under conditions of free pH evolution. The control of pH at the acidic level at the value of 4.5 resulted in an over 3-fold increase of the reaction rate. Less encouraging results obtained during the comparative chemical oxidation of pollutants by hypochlorite ions indicate that electrochemical oxidation is preferred to the commonly applied chemical treatment. Electro-oxidation also proved feasible in the sulfate solution medium as the supporting electrolyte, but the process efficiency was lower. In this case electrooxidation was probably mediated by ‚OH radicals adsorbed at the anode surface or by persulfates. Nomenclature [Cl2] ) concentration of active chlorine, mg dm-3 [c] ) pollutant concentration measured as chemical oxygen demand, g dm-3 F ) Faraday’s constant, 96487 C equiv-1 I ) applied current, A k ) pseudo-first-order kinetic constant, min-1 kobs ) apparent pseudo-first-order kinetic constant, min-1 Q ) charge, C t ) reaction time, min V ) reactor volume, dm3 Greek Symbol η ) current efficiency, %
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Received for review November 22, 1999 Revised manuscript received May 17, 2000 Accepted June 8, 2000 IE9908480
