Indirect Electrochemical Reduction of Indigo on Carbon Felt: Process

Jun 5, 2014 - A carbon felt was used as the cathode material for the indirect electrochemical reduction of indigo in alkaline aqueous solution employi...
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Indirect Electrochemical Reduction of Indigo on Carbon Felt: Process Optimization and Reaction Mechanism Ying’hua Xu, Hai’feng Li, Cheng’pu Chu, Ping Huang, and Chun’an Ma* State Key Laboratory Breeding Base of Green Chemistry−Synthesis Technology, College of Chemical Engineering, Zhejiang University of Technology, Hangzhou 310032, People’s Republic of China ABSTRACT: A carbon felt was used as the cathode material for the indirect electrochemical reduction of indigo in alkaline aqueous solution employing iron-triethanolamine (Fe-TEA) complex as a redox mediator. Surface structure and specific surface area of the carbon felt electrode and its electrochemical performance toward the reduction of indigo were investigated by SEM, BET, and cyclic voltammetry, respectively. Moreover, an optimization of different operating conditions for the reduction of indigo was performed with an H-cell. In particular, the dependence of current efficiency (CE) on electrolysis time was chosen to analyze the indirect electrochemical reduction mechanism of indigo. The results showed that the carbon felt, having a high specific surface area of approximately 7289 cm2·g−1, is an excellent electrode material for the indirect electrochemical reduction of indigo and that a moderate current density, temperature, and concentration of indigo has the most beneficial effects on the reduction process. indirect electrochemical reduction.11 In autocatalysis electrochemical reduction of vat dyes, chemical reducing agents are replaced by electrons, which is in line with purposes of economy and ecology. The mechanism of autocatalysis electrochemical reduction of indigo is exemplified by Scheme 1, in which two indigo radicals are formed by a comproportio-

1. INTRODUCTION Vat dyes, especially indigo, still play an important role in our today’s dyeing industry due to the popularity of blue jeans.1,2 They are insoluble in water, but can be reduced in the presence of an alkali and a reducing agent to soluble form known as leuco dye with an excellent affinity for cellulosic fibers. Then the reduced dyestuff is absorbed into the fiber and is reoxidized on the fiber back to the insoluble form which remains fixed there.3 At present, in most industrial dyeing processes, vat dyes are still been reduced using mainly sodium dithionite.4,5 Unfortunately, its use leads to high costs (e.g., sodium dithionite price amounts to approximately 9800 RMB for the reduction of 1 ton indigo in China) and various problems with the effluent (high salts load, depletion of dissolved oxygen, problems with nasal nuisance, toxicity of sulfide, etc.), which has nearly failed to meet demands for modern economy and ecology. In the past, many studies had been carried out to search for the replacement of sodium dithionite by various ecologically attractive alternatives, such as various reducing agents or catalytic hydrogenation.6−10 Although some reducing agents (such as borohydride7,8 and α-hydroxycarbonyls9) can meet requirements in terms of reductive efficiency and biodegradability, these compounds are more expensive and are restricted to use in closed systems due to the formation of strong smelling condensation products in alkaline solution. Moreover, the catalytic hydrogenation is recognized as an ecological and environmental method for preparing prereduced dyes, however, it is not suitable for dyeing on-site in a factory on account of high explosion and fire risk.6,10 Compared with the above methods, electrochemical techniques would be of wide prospects in the industrial applications because it minimizes the consumption of chemicals, reduces the cost of production, and diminishes the discharge of wastewater. There are three types of electrochemical processes for the dye reduction: autocatalysis electrochemical reduction, electrocatalytic hydrogenation, and © 2014 American Chemical Society

Scheme 1. Autocatalysis Mechanism of Electrochemical Reduction of Indigo

nation reaction between an indigo and a leuco indigo, followed by the electrochemical reduction of these radical to leuco indigos.12 Nevertheless, the slow diffusion rate of the indigo radicals to the electrode surface and the low concentration of the indigo radicals in the system will limit the rate of this electrochemical process.13−15 Received: Revised: Accepted: Published: 10637

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reactions. Therefore, the second purpose of this work is to optimize the operational conditions of indirect electrochemical reduction of indigo (such as current density, temperature, concentration of indigo, and thickness of carbon felt). In addition, the indirect electrochemical reduction mechanism of high concentration indigo on a carbon felt electrode, and the economy of this reduction process in a scale-up electrolysis experiment is the third purpose of this work.

Electrocatalytic hydrogenation is a promising alternative in terms of economical and environmental aspects.16−19 It is a process in which adsorbed hydrogen, produced in situ by the electrochemical reduction of water, chemically reacts with vat dyes at the electrode surface (e.g., Raney Ni or platinum black) to obtain the leuco dyes. However, this method is still in need of improvement, with respect to low CE and a limiting factor which is originated from the competing reaction of the adsorbed hydrogen forming molecular hydrogen.20 Additionally, an indirect electrochemical process employing soluble redox mediators (such as anthraquinone 21 or substituted anthraquinone,22 Fe(III)/Fe(II)−TEA23−25 and Fe(III)/Fe(II)−gluconate26) has been developed for the reduction of vat dyes. Scheme 2 illustrates the indirect

2. MATERIALS AND METHODS 2.1. Chemicals and Materials. Indigo (98%) was supplied from the Aladdin Reagent Co., China. TEA, sodium hydroxide, ferric sulfate, sodium dithionite, and potassium ferricyanide were analytical grade chemicals, obtained from the Sinopharm Chemical Reagent Co., China. Carbon felt and glass carbon electrode (GC, φ = 3 mm), were obtained from Cells Electrochemistry Experiment Equipment Co., China (www. hzcell.com). All these materials were used as received without further purification. All solutions were prepared using H2O with a resistivity of 18.2 MΩ cm obtained from a Millipore Milli-Q system. 2.2. Characterization of Carbon Felt and Indigo Microparticles. The surface morphology of carbon felt was determined by SEM (Hitachi S-4700 II). The apparent surface area of carbon felt was detected from N2 (77K) adsorption isotherms by using BET (ASAP 2020).40 The average size and distribution of the indigo microparticles dispersed in the electrolyte solutions was measured with a Malvern Mastersizer apparatus (Mastersizer 2000, U.K.) based on laser diffraction. 2.3. Cyclic Voltammetry (CV) Experiments. CV experiments were carried out by an electrochemical workstation (CHI660C). The testing electrolytes were deoxygenated for at least 30 min before the CV experiments and maintained under nitrogen atmosphere during measurements. A three-electrode cell composed of an aqueous saturated calomel electrode (SCE) used as the reference electrode, a platinum sheet (0.5 × 0.5 cm2) used as the counter electrode and a piece of carbon felt (geometric area 0.5 × 0.5 × 0.5 cm3) used as the working electrode, was used for the CV experiments. Only one surface of the carbon felt electrode was exposed to the testing electrolyte in the CV experiments. A GC electrode (φ= 3 mm) was also used as working electrode. The working electrodes were pretreated before the CV experiments. The GC electrode was carefully polished with alumina (30−50 nm), and then washed ultrasonically in distilled water for 2 min. The carbon felt was washed ultrasonically by ethanol for 15 min, then dried for 2 h under 80 °C. All CV experiments were carried out at 30 °C. 2.4. Electrolysis Experiments. A conventional twocompartment glass H-cell, separated by a Nafion-324 membrane, was used for the electrolysis experiments to obtain the optimized electrolysis conditions. The cathode and anode were manufactured from a piece of carbon felt (thickness 10 mm, geometric area 25 cm2) and a platinum sheet (2 × 2 cm2), respectively. Both the volume of catholyte and anolyte were 170 mL. The catholyte was stirred during the electrolysis process. A plate and frame cell consisted of a cathode, an anode and a Nafion-324 membrane was chosen for a scale-up electrolysis experiment. A piece of carbon felt (thickness 10 mm, geometric area 7 × 11 cm2) and a Pt/Ti plate (11 × 15 cm2) were used as the cathode and anode, respectively. The schematic construction of electrolysis devices using the plate and frame cell is

Scheme 2. Mechanism of Indirect Electrochemical Reduction of Indigo

electrochemical process for reduction of indigo using Fe(III)−TEA as mediator. Fe(III) is reduced on the surfaces of the electrode first, and the product (Fe(II)) then reduces indigo to leuco indigo,27−29 at the same time Fe(III) is regenerated. This technique can enhance the rate of electrontransfer between the dyes and cathode, and hence improve reduction efficiency. In addition, a small amount of reduced mediators existing in the dyebath can prevent leuco dyes from being reoxidized by the air in the dying process, and thus ensure the dying quality. Very limited electrode materials, such as silver mesh,30 stainless steel fabric,31−33 and copper mesh4 have been attempted for the indirect electrochemical reduction of vat dyes. Due to the limitation of specific surface area, CE and current density obtained by using these materials are not very satisfactory. Using multicathode membrane electrolyzers27,28 or increasing mediator concentration31 can further improve the CE and current density of indirect electrochemical reduction of vat dyes; however, there are problems with complex electrolyzer structure and serious mediator loss in the dyeing process, respectively. Therefore, it is urgent to find a new cathode material which has high specific surface area to be used for indirect electrochemical reduction of vat dyes. As known to us, the carbon felt acts as a typical electrode material applied to redox flow batteries, which exhibits a property of wide operation potential range, satisfactory stability, and availability in high surface area at reasonable cost.34−37 Surprisingly, the application study of carbon felt in the indirect electrochemical reduction of indigo has been seldom discussed in literature.38 So, the primary purpose of this work is to evaluate the performance of carbon felt in the indirect electrochemical reduction of indigo. Furthermore, there are lacking the systematic studies toward two important operational conditions (concentration of indigo and temperature) in indirect electrochemical reduction of indigo. In fact, the electrolysis efficiency is strongly dependent on reactant concentration39 and temperature31 in some electrochemical 10638

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given in Figure 1. Both the total volume of catholyte and anolyte were 600 mL.

Figure 2. Titration curves of different current densities at the end of electrolysis. All conditions were the same as those in Table 1.

Figure 1. Schematic construction of electrolysis device using a plate and frame cell.

All electrolysis experiments were performed at 50 °C (except for temperature experiments) and 1.0 M NaOH aqueous solution was used as anolyte in all those experiments. 2.5. Potentiometric Titration. The yield of leuco indigo (Y) and CE in the electrolysis experiments were calculated by a potentiometric titration serving potassium ferricyanide as redoxtitration.41The potentiometric titration experiments were carried out with a ZD-2 Automatic Potential Titrator (Shanghai Precision & Scientific Instrument Co.). A Pt electrode was used as the indicator electrode. The titration procedure was described as follow: Take 2 mL of catholyte with a syringe and dilute with deionized water to 10 mL, and then titrate with definite concentration (0.012 M) of potassium ferricyanide solution. The reduced catholyte is sensitive to oxygen, thus the processes of titration were also carried out under nitrogen atmosphere. The Y and CE were calculated by the two following expressions, respectively: Y = [n × C × ΔV /(2ml × 2e × C0)] × 100%

(1)

CE = (n × F × C × R × ΔV ′ /It ) × 100%

(2)

Figure 3. SEM images of a carbon felt taken at various magnifications, (a) × 50, (b) × 100, (c) × 200, and (d) × 500.

carbon fibers. Such a porosity surface structure would provide much more electrochemical active sites and enhance the electrochemical performance of the carbon felt in the indirect electrochemical reduction of indigo. In addition, the specific surface area of carbon felt was determined by BET as 7289 cm2· g−1. Figure 4 shows the average size and distribution of the indigo microparticles dispersed in the electrolyte solutions. As shown

where n = 1e, which is the number of electrons transferred in the redox process; C = 0.012 M, is the concentration of potassium ferricyanide; C0 is the initial concentration of indigo in the electrolysis experiments; ΔV and ΔV′ are the volume of consumed potassium ferricyanide for the potential plateau at −700 mV and the two potential plateaus (−700 and −1000 mV), respectively, as showed in Figure 2; F is Faraday constant; R is the volume conversion coefficient, is 85 for the electrolysis data obtained in the conventional two-compartment glass Hcell; I is the current applied in electrolysis process; t is the electrolysis time.

Figure 4. Particle size distribution of dispersed indigo.

3. RESULTS AND DISCUSSION 3.1. Surface Structure of Carbon Felt and Characterization of Indigo Microparticles. The surface structure of a piece of carbon felt was observed by SEM. As showed in Figure 3, the carbon felt is composed of numerous carbon fibers with an average diameter of approximate 20 μm and the interspaces between the fibers are 2−5 times as larger as the diameter of

in this Figure, the dispersed indigo particles sizes range from 2 to 20 μm, with an average particle size of 5.6 μm. 3.2. Cyclic Voltammograms. Figure 5 shows the typical cyclic voltammograms (two cycles) for the reduction of indigo in the presence of Fe(III)−TEA at a glassy carbon electrode. As can be seen, reversible redox peaks (I and II, at around −1.0 V) 10639

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Figure 6. Cyclic voltammograms (second cycle) of (a) a glassy carbon electrode and (b) a carbon felt electrode in the aqueous solution of 0.5 M NaOH + 0.0118 M Fe2(SO4)3 + 0.23 M TEA+ 0.01 M indigo, at the scan rate of 10 mV·s−1, scanning from −0.4 to −1.3 V.

Figure 5. Cyclic voltammograms of a glassy carbon electrode in the aqueous solution of 0.5 M NaOH + 0.0118 M Fe2(SO4)3 + 0.23 M TEA+ 0.01 M indigo, at the scan rate of 100 mV·s−1, scanning from −0.4 V to −1.3 V.

3.3. Electrolysis Experiments. To investigate the effect of different operating parameters on the reduction efficiency and obtain the optimal conditions for the indirect electrochemical reduction of indigo, a series of galvanostatic electrolysis experiments using carbon felts were performed. 3.3.1. Effect of Current Density. Table 1 shows the effect of current density on the reduction efficiency of indigo, as

corresponding to the reduction−oxidation of Fe(III)/Fe(II)42 are observed in the CV curves of two cycles, however, complete reversible redox peaks (IV and III, at around −0.70 V) for the reduction−oxidation of indigo/leuco indigo43 only appear in the CV curve of second cycle. From this result, the reaction processes occurred on the surface of gassy carbon electrode can be described as follows. Initially, the dispersed indigo could not be reduced in the potential region between −0.4 and −1.0 V because of the poor contact between the dispersed indigo and the electrode. Subsequently, the dispersed indigo were reduced to soluble leuco indigo by a type Catalytic (EC′) Reaction:44 Fe(III) was first reduced to Fe(II) when the potential was shifted to around −1.1 V, at the same time, one part of the produced Fe(II) reacted with indigo to produce the soluble leuco indigo, and the remainders were then reoxidized to Fe(III) at −1.0 V during the reverse scan. Finally, the soluble leuco indigo produced by the Catalytic (EC′) Reaction were reoxidized to insoluble indigo particles which deposited on the electrode surface when the potential shifted positively to −0.63 V, and the deposited indigo particles were reduced to the soluble leuco indigo again at −0.75 V in the second CV curve. According to the above electrochemical processes, two important conclusions can be obtained. (1) The direct electrochemical reduction of indigo is thermodynamically feasible, and the main reason that the effective direct electrochemical reduction cannot be accomplished lies in the kinetics (poor contact between dispersed indigo and electrode). (2) Fe-TEA is able to establish a reduction potential sufficiently negative to reduce indigo, and therefore it can solve the greater resistance of the electrochemical reduction of indigo in the kinetics. Figure 6 shows cyclic voltammograms of quasi-steady-state cycle recorded on a glassy carbon and a carbon felt electrode, respectively, for the indirect electrochemical reduction of indigo. As can be seen, it is found that the magnitude of reductive peak current density of Fe(III) at the carbon felt electrode has increased significantly (Figure 6b), which has a 20- to 30-fold enhancement compared to the voltammetric responses generated on the gassy carbon electrode (Figure 6a). This result suggests that considerable inner surface of the carbon felt electrode is accessible for the reduction of Fe(III) even in a motionless solution. Obviously, this is due to that the micropores on the carbon felt electrode are much larger than the diffusion layer thickness (normally few micrometers).45 This characteristic of the carbon felt is helpful to improve the current density of the indirect electrochemical reduction of indigo.

Table 1. Effect of Current Density on the Reduction of 0.068 M Indigo in the Aqueous Solution of 0.5 M NaOH + 0.0118 M Fe2(SO4)3 + 0.23 M TEA, at 50°C, Using an H-Cella current density

1st stage

2nd stage

3rd stage

whole reaction

yield of leuco indigo

(A dm−2)

η1 (%)

η2 (%)

η3 (%)

η1−3 (%)

Y (%)

0.8 1.0 1.2 1.6 2.0

51.9 73.3 62.0 54.0 51.9

70.7 88.1 88.8 86.3 73.9

53.7 74.1 61.8 30.2 19.3

58.8 78.5 70.8 56.8 48.4

60.4 88.2 94.9 97.1 105.9

ηi is the CE for every electrolysis stage (one hour for a stage), η1‑3 is the CE of whole reaction, total electrolysis time t = 3 h.

a

evaluated from the Y and CE, during the electrolysis experiments. The Y is steadily on the increase with the current density, while the best CE is achieved in the current density range of 1.0−1.2 A·dm−2. This current density is similar to the maximum instantaneous current density of three-dimensional copper wire electrode under similar reaction conditions,4 which suggests that the carbon felt can assuredly provide high current density for the indirect electrochemical reduction of indigo. In addition, it is remarkable that the CE shows a trend of first increase and then decrease with the increase of electrolysis time under all different current densities. This experimental finding is obviously opposed to the conventional relationship between CE and electrolysis time, in which CE would decreases steadily with the prolongation of electrolysis time if the current density was greater than the limiting current density.46 Similar discrepancy was observed for the indirect electrochemical reduction of indigo31 and indanthrene brilliant green FFB32 on stainless steel cathode in our previous works. The considerable existence of electrochemical reduction of indigo by the means of autocatalysis mechanism can explain such a discrepancy. According to the autocatalysis mechanism (Scheme 1), the rate of electrochemical reduction of indigo depends on the concentration of indigo radical. Therefore, high concentration of indigo radical would results in high rate of electrochemical reduction of indigo. Obviously, a high 10640

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Table 2. Results of an Electrolysis Reduction Experiment of Indigo Excluding Fe(III)−TEAa current density

1st stage

2nd stage

3rd stage

whole reaction

yield of leuco indigo

(A dm−2)

Y1 (%)

η1 (%)

η2 (%)

η1−2 (%)

Y (%)

1.0

25.5 ± 2.1

68.6 ± 2.5

28.7 ± 3.7

48.6 ± 2.9

72.2 ± 2.6

Current density: 1.0 A dm−2, catholyte: 0.068 M indigo +0.5 M NaOH + 0.05 M Na2S2O4, all other conditions were the same as those in Table 1. Y1 is yield of leuco indigo before the electrolysis experiment, ηi is the CE of every electrolysis stage (one hour for a stage), η1−2 is the CE of whole reaction. The electrolysis reduction experiments were repeated for 3 times. a

increase of temperature. Generally, increasing temperature would accelerates mass transport of the dissolved electroactive species such as Fe (III) and indigo radical, that is, these active substances spread to the electrode surface more quickly under higher temperature, and therefore the reduction rate of indigo can be accelerated by enhanced electrode kinetics at higher temperature. Besides, the influence of temperature on kinetics of the chemical redox reaction between indigo and Fe(II) as well as the comproportionation reaction between indigo and leuco-indigo should be also taken into account. Because the rate of those two reactions would increase with the rise of temperature, which means that the concentration of products (Fe (III) and indigo radical) of the two reactions would be replenished more quickly at higher temperature on the surface of carbon felt cathode. Obviously, it favors the indigo reduction. Therefore, the higher temperature can resulted in the higher rate of indigo reduction. Unfortunately, a serious evaporation of electrolyte was observed at 60 °C, which caused an imbalance of the catholyte. In addition, only a minor improvement was obtained when the temperature increased from 50 to 60 °C. So, 50 °C was chosen as the optimal value for the following electrolysis experiments. 3.3.3. Effect of Indigo Concentration. The effect of indigo concentration on the CE is showed in Figure 8. As can be seen,

concentration of indigo radical would appear in the middle stage of electrolysis experiment where the concentration of indigo and leuco indigo is proportionate, and they could produce indigo radical rapidly via a comproportionation reaction between them. In order to confirm this hypothesis further, an electrochemical reduction of indigo excluding Fe(III)−TEA was carried out in a similar condition (Table 2), in which 0.05 M Na2S2O4 was added into a catholyte before the electrolysis started, aiming to produce enough indigo radical. As showed in Table 2(1st stage), approximate 1/4 of indigo was reduced by the added Na2S2O4. It indicates that considerable indigo radical probably were produced in the catholyte before the electrolysis. The results of the electrolysis experiment are showed in Table 2. According to the electrolysis experiment, it is found that the autocatalysis electrochemical reduction of indigo actually occurred, and that quite high CE (68.6%) can be obtained in the initial stage of the electrolysis (2nd stage). On basis of this finding, it is confirmed that the autocatalysis electrochemical reduction is main reduction path in the middle stage of indirect electrochemical reduction of indigo, and this is the reason why the CE is higher in that stage compared with the initial and the last stage of the reduction of indigo (Table 1). Figure 2 shows the titration curves of different current densities at the end of electrolysis. As shown in this figure, there are two potential platforms in the titration curves of all current densities except for 0.8 A·dm−2. According to the CV curves of Figure 5, the potential platform approximate at −1000 mV is corresponding to the oxidation of Fe(II), while the potential platform about at −700 mV is mainly attributed to the oxidation of leuco indigo. Because Fe(II) would immediately consume indigo,47 it can be believed that all indigo had been converted into leuco indigo and indigo radical at the end of electrolysis with all different current densities except for 0.8 A· dm−2. 3.3.2. Effect of Temperature. Figure 7 shows the effect of temperature on the CE of electrolysis reduction of indigo. As can be seen, the CE is logarithmically increased with the

Figure 8. Effect of indigo concentration on the reduction of indigo at a current density of 1 A·dm−2. Electrolysis time of different indigo concentrations (0.034, 0.051, 0.068, 0.085, 0.102 M) are 2, 3, 4, 5, 6 h, respectively. All other conditions were the same as those in Table 1.

the CE shows a trend of first increase and then decrease with the increase of indigo concentration, and the best CE is achieved in the indigo concentration range of 0.051−0.068 M. This concentration of indigo is much larger than that in the conventional dye bath,4 fortunately it is compatible with the practical industrial process of indigo electrochemical dyeing.29 An increase to a higher indigo concentration (>0.068 M) cause a rapid drop in the CE. In fact, an obvious agglomeration of indigo on the surface of catholyte could be observed when the indigo concentration increased to 0.085 M and the agglomeration did not disappear at the end of electrolysis experiment. Obviously, the agglomeration of indigo probably is the main reason for the rapid drop in the CE. In addition, the acceleration of hydrogen evolution at the higher indigo

Figure 7. Effect of temperature on the reduction of indigo at a current density of 1 A·dm−2. All other conditions were the same as those in Table 1. 10641

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concentration is the other possible reason which caused the rapid drop in the CE. Because partial cathode surface possibly was covered with indigo at the higher indigo concentration, which resulted in the increase of the real current density. On basis of the results above, we chose 0.068 M as the optimal value for the following electrolysis experiments. 3.3.4. Effect of Thickness of Carbon Felt. In addition, the effect of thickness of carbon felt on the CE was also investigated. As showed in Figure 9, with the same electrolysis

(1) The direct electrochemical reduction of indigo is thermodynamically feasible but kinetically restricted, Fe-TEA is able to establish a reduction potential sufficiently negative to overcome the greater resistance in kinetics. (2) The porosity characteristic of carbon felt is helpful to improve the current density for indirect electrochemical reduction of indigo, and under optimized experimental conditions (current density = 1 A·dm−2, temperature = 50 °C, thickness of carbon felt = 10 mm), 0.068 M indigo can completely reduced with a CE of 78.5%. (3) The autocatalysis electrochemical reduction is the main reduction path in the middle stage of the indirect electrochemical reduction of indigo, and it is very helpful to improve the reduction efficiency of indigo. (4) A preliminary scale-up electrolysis with a plate and frame cell showed that the indirect electrochemical reduction is more economic than the conventional reduction with sodium dithionite for the reduction of indigo.

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Figure 9. Effect of thickness of carbon felt on the reduction of indigo at a current density of 1 A·dm−2. All other conditions were the same as those in Table 1.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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time, the CE of 3 different carbon felts increased with the rise of thickness of carbon felt. It suggests that thicker carbon felt favors the reduction of indigo. In view of the facts that the price of carbon felt is very low and the carbon felt of 10 mm has an advantage in mechanical strength, the carbon felt of 10 mm was used for the following electrolysis experiment. 3.3.5. A Scale-Up Electrolysis Experiment. Using the optimized experimental conditions above (Current density = 1 A·dm−2, Temperature = 50 °C, Concentration of indigo = 0.068 M, Thickness of carbon felt = 10 mm), a scale-up electrolysis experiment with a plate and frame cell was carried out at a flow rate of 160 L·h−1 to assess the economy of indirect electrochemical reduction for indigo. After electrolysis of 5 h, the following results were obtained: The total CE is 57.2%, the average cell voltage is 3.2 V, and the direct current power consumption is 1.15 kW·h·kg−1 indigo. The last economic data are very competitive compared to the costs of sodium dithionite required in the conventional reduction of indigo (sodium dithionite price amounts to approximately 9.8 RMB for the reduction of 1 kg indigo) in view of the fact that the price of commercial power is 0.7−1.0 RMB·(kW·h)−1 in China. Unfortunately, a serious block of the plate and frame cell was observed at the end of experiment. Moreover, it was also observed that the indigo agglomeration phenomenon occurred in the upper solution of liquid storage tank in the electrolysis process. It indicates that the hydrodynamics of the plate and frame cell is more complex than that of the H-cell, and the technical and economic data would be better if the structure of the plate and frame cell could be improved.

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21106133), the National Basic Research Program of China (973 Program) (2012CB722604), Key Innovation Group of Zhejiang Province, China (2009R5000222), and Fresh Talent Program for Science and Technology Department of Zhejiang Province.

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REFERENCES

(1) Roessler, A.; Crettenand, D. Direct electrochemical reduction of vat dyes in a fixed bed of graphite granules. Dyes Pigm. 2004, 63, 29. (2) Ben Ticha, M.; Meksi, N.; Kechida, M.; Farouk-Mhenni, M. A promising route to dye cotton by indigo with an ecological exhaustion process: A dyeing process optimization based on a response surface methodology. Ind. Crop. Prod. 2013, 46, 350. (3) Roessler, A.; Dossenbach, O.; Mayer, U.; Marte, W.; Rys, P. Direct electrochemical reduction of indigo. CHIMIA I. J. Chem. 2001, 55, 879. (4) Bechtold, T.; Burtscher, E.; Turcanu, A.; Bobleter, O. Indirect electrochemical reduction of dispersed indigo dyestuff. J. Electrochem. Soc. 1996, 143, 2411. (5) Bechtold, T.; Burtscher, E.; Turcanu, A.; Bobleter, O. Dyeing behavior of indigo reduced by indirect electrolysis. Text. Res. J. 1997, 67, 635. (6) Schnitzer, G.; Suetsch, F.; Schmitt, M.; Kromm, E.; Schlueter, H.; Krueger, R.; Weiper-Idelmann, A. Dyeing cellulose-containing textile material with hydrogenated indigo. U.S. Patent 5,586,992, Dec. 24, 1996. (7) Meksi, N.; Ben Ticha, M.; Kechida, M.; Mhenni, M. F. New catalysts for the borohydride dyeing process. Ind. Eng. Chem. Res. 2010, 49, 12333. (8) Meksi, N.; Kechida, M.; Mhenni, M. F. Cotton dyeing by indigo with the borohydride process: Effect of some experimental conditions on indigo reduction and dyeing quality. Chem. Eng. J. 2007, 131, 187. (9) Meksi, N.; Ben-Ticha, M.; Kechida, M.; Farouk-Mhenni, M. Using of ecofriendly α-hydroxycarbonyls as reducing agents to replace sodium dithionite in indigo dyeing processes. J. Clean. Prod. 2012, 24, 149.

4. CONCLUSIONS The carbon felt, having a specific surface area of approximately 7289 cm2·g−1, was used for the indirect electrochemical reduction of indigo using the Fe(III)−TEA complex as a mediator. The main findings are as follows: 10642

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dx.doi.org/10.1021/ie500603y | Ind. Eng. Chem. Res. 2014, 53, 10637−10643