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Journal of Environmental Science and Health, Part A: Toxic/Hazardous Substances and Environmental Engineering Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/lesa20

Electrochemical oxidation of textile industry wastewater by graphite electrodes a

a

b

b

Rajendra Bhatnagar , Himanshu Joshi , Indra D. Mall & Vimal C. Srivastava a

Department of Hydrology, Indian Institute of Technology Roorkee, Roorkee, Uttarakhand, India b

Department of Chemical Engineering, Indian Institute of Technology Roorkee, Roorkee, India Published online: 25 Apr 2014.

To cite this article: Rajendra Bhatnagar, Himanshu Joshi, Indra D. Mall & Vimal C. Srivastava (2014) Electrochemical oxidation of textile industry wastewater by graphite electrodes, Journal of Environmental Science and Health, Part A: Toxic/Hazardous Substances and Environmental Engineering, 49:8, 955-966, DOI: 10.1080/10934529.2014.894320 To link to this article: http://dx.doi.org/10.1080/10934529.2014.894320

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Journal of Environmental Science and Health, Part A (2014) 49, 955–966 Copyright Ó Taylor & Francis Group, LLC ISSN: 1093-4529 (Print); 1532-4117 (Online) DOI: 10.1080/10934529.2014.894320

Electrochemical oxidation of textile industry wastewater by graphite electrodes RAJENDRA BHATNAGAR1, HIMANSHU JOSHI1, INDRA D. MALL2 and VIMAL C. SRIVASTAVA2 1

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2

Department of Hydrology, Indian Institute of Technology Roorkee, Roorkee, Uttarakhand, India Department of Chemical Engineering, Indian Institute of Technology Roorkee, Roorkee, India

In the present article, studies have been performed on the electrochemical (EC) oxidation of actual textile industry wastewater by graphite electrodes. Multi-response optimization of four independent parameters namely initial pH (pHo): 4–10, current density (j): 27.78–138.89 A/m2, NaCl concentration (w): 0–2 g/L and electrolysis time (t): 10–130 min have been performed using BoxBehnken (BB) experimental design. It was aimed to simultaneously maximize the chemical oxygen demand (COD) and color removal efficiencies and minimize specific energy consumption using desirability function approach. Pareto analysis of variance (ANOVA) showed a high coefficient of determination value for COD (R2 ¼ 0.8418), color (R2 ¼ 0.7010) and specific energy (R2 ¼ 0.9125) between the experimental values and the predicted values by a second-order regression model. Maximum COD and color removal and minimum specific energy consumed was 90.78%, 96.27% and 23.58 kWh/kg COD removed, respectively, were observed at optimum conditions. The wastewater, sludge and scum obtained after treatment at optimum condition have been characterized by various techniques. UV-visible study showed that all azo bonds of the dyes present in the wastewater were totally broken and most of the aromatic rings were mineralized during EC oxidation with graphite electrode. Carbon balance showed that out of the total carbon eroded from the graphite electrodes, 27–29.2% goes to the scum, 71.1–73.3% goes into the sludge and rest goes to the treated wastewater. Thermogravimetric analysis showed that the generated sludge and scum can be dried and used as a fuel in the boilers/incinerators. Keywords: Electrochemical techniques, graphite, electrode, textile industry wastewater, carbon balance.

Introduction Industrial revolution has influenced the quality of available fresh water. Many industries like textile, refineries, chemical, plastic and food-processing plants produce wastewater characterized by a perceptible content of organics with strong color.[1] For example, a typical textile dyeing processing unit consisting of desizing, scouring, bleaching, dyeing, finishing and drying operations produce wastewater contain high amount of pollution load along with huge quantity of dyes.[2] With the increasing demand for textile products, the wastewater of textile industry is rising proportionally, making it one of the main sources of severe pollution problems globally. In particular, the release of colored effluents into the environment is undesirable, not only because of their color, but also because

Address correspondence to Indra D. Mall, Department of Chemical Engineering, Indian Institute of Technology Roorkee, Roorkee 247667, India; E-mail: [email protected] Received June 25, 2013. Color versions of one or more of the figures in this article can be found online at www.tandfonline.com/lesa.

many dyes and their breakdown products are toxic and/or mutagenic to aquatic life.[3,4] As environmental regulations become stringent, novel processes for efficient treatment of various kinds of wastewater are needed, which require lower operating cost and remove refractory organic materials like dyes. In this context, researchers are exploring the feasibility of using various alternative processes such as electrochemical technique, wet oxidation, ozonation, photo-catalytic methods, etc. for the degradation of organic compounds. Among these advanced oxidation processes, the electrochemical treatment has received greater attention in recent years due to its versatility, energy efficiency, automation and cost effectiveness.[5] The main reagent responsible for oxidation of organic compounds by electrochemical method is the electron along with some monomeric metallic ions depending upon the type of electrode used. Electrochemical methods have been widely used for industrial wastewater treatment, including Kraft mill, textile, tannery and dye waste.[3,6] Several authors have reported that it is possible to obtain the degradation of different toxic compounds by electrolysis.[7,8] The main advantages of electrochemical techniques to treat pollutants are (1) the elimination of redox chemicals, thus avoiding the need to

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Bhatnagar et al.

Table 1. Literature on treatment of textile industry wastewaters by graphite electrode. Wastewater (ww) Synthetic ww C.I. Reactive blue-19, C.I. Acid red-266, and C.I. Disperse yellow -218

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Cibacron Navy W-B

Electrode Type

% COD reduction

% Color reduction

Reference

95

[20]

I ¼ 1.6 A; pH ¼ 6.2; w ¼ 2 g/L



Graphite

j ¼ 100 A/m2; pH ¼ 3.0; w ¼ 5 g/L

93

I ¼ 200 mA; pH ¼ 13; NaCl ¼ 4 g

43

75

[22]

58.5

98.6

[23]

Exfoliated graphite anode and copper cathode

Fast Blue B Salt Graphite as containing ww anode and SS as cathode Acid brown, Graphite as reactive blue anode and SS dyes containing as cathode ww Azo dye ww Fe as anode and graphite as cathode Textile ww Graphite Actual textile ww Industrial azo dye ww

Parameter values

Al as anode and graphite as cathode

Synthetic textile Graphite as ww containing anode and SS indigo dye as cathode Methyl orange dye ww

Initial COD (mg/L)

Glassy carbon as anode and SS as cathode Textile ww Graphite Printing and Expanded dyeing mill ww graphite/ attapulgite anode and copper cathode Textile ww Graphite (Acrylic)

17540



j ¼ 130 A/m2; pH ¼ 2; t ¼ 20 min; w ¼ 4 g/L

[21]

2980

Vap ¼ 16 V; pH ¼ 4.5; t ¼ 20 min; w ¼ 0.5 g/L

75

88

[24]



j ¼ 66.3 A/m2; pH ¼ 7; t ¼ 80 min; w ¼ 15 g/L

89

100

[25]

84.7

83.4

[26]

71



[27]

81

85–90

[28]

1200

I ¼ 49 mA

1316

j ¼ 45 A/m2

5957 j ¼ 399 A/m2; pH ¼ 2.8 (50% dilute) 530 443

j ¼ 45 A/m2; pH ¼ 7.9 j ¼ 50 A/m2; pH ¼ 7.0; t ¼ 20 min; NaCl ¼ 0.2 g

93 43.5

— 90.6

[29] [30]

347

j ¼ 27.8 A/m2, w ¼ 2.0 g/L; t ¼ 110 min; pHo ¼ 4.0

90. 8

96.3

Present Study

J: current density; w: NaCl dosage; t: time; I: Current; Vap: applied voltage.

treat spent redox streams, (2) close control of the desired reactions using the applied potential or current, and (3) the increased possibility of on-site treatment.[9] In addition, an electrochemical reactor requires less space and can treat a variety of pollutants. Recently, anodic oxidation, either direct or indirect, has been employed for removing the color or COD of dyestuffs in simple labmade wastewater.[10–12]

Electrochemical degradation of pollutants proceeds by either direct or indirect oxidation process. In direct anodic oxidation process, the pollutants are first adsorbed on the anode surface and then destroyed by the anodic electron transfer reaction. In indirect oxidation process, strong oxidants such as hypochlorite/chlorine and hydrogen peroxide are electrochemically generated. The pollutants are then destroyed in the bulk solution by oxidation

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Electrochemical oxidation of textile industry wastewater reaction of the generated oxidant. All the oxidants are generated in-situ and are utilized immediately.[3,13] In recent years, it has been shown that direct oxidation and redox couple mediated electro-oxidation can be competitive technologies for the treatment of textile wastewater. However, despite proven efficiency and cost effectiveness, optimization of the electrochemical reactor for this kind of application is still needed before the process can be implemented at an industrial scale.[3,14,15] Several researchers have investigated the feasibility of electrochemical oxidation of dyes with various electrode materials such as titanium-based DSA electrodes,[16] platinum electrode,[17] diamond and metal alloy electrodes,[18] boron-doped diamond electrodes,[19] etc. Previously, graphite electrode has been used for the treatment of textile waste containing various types of dye solution (Table 1).[20–30] It may be seen in Table 1 that most of the researchers have used synthetic wastewater in EC studies [20–27] and only a few investigators have also applied EC method using graphite electrode for the treatment of actual textile industry wastewater.[28–30] A deeper analysis of previous studies shows that none of the previous investigators have used multiresponse optimization of parameters. Also previous investigators did not characterize the sludge and scum obtained during the EC by graphite electrodes. Also, to the best of the knowledge of the authors, no study is reported on carbon balance during EC by graphite electrodes. Based on these research gaps, it was aimed to treat actual textile industry wastewater by graphite electrodes and optimize the operating parameters. The aim of the research was to maximize COD and color removal efficiencies and simultaneously minimize specific energy consumption. The wastewater, sludge and scum obtained after treatment at optimum condition have been characterized by various techniques. In addition, carbon balance has been done so as to understand the removal mechanism.

Materials and methods Chemicals, electrode materials and reactor All the chemicals used in the study were of analytic reagent (AR) grade. Potassium dichromate, sulfuric acid, hydrochloric acid and NaOH were obtained from Ranbaxy Chemicals Ltd. (New Delhi, India). Silver sulfate and mercuric sulfate were obtained from Himedia laboratories (Mumbai, India). Graphite plates (length ¼ 10 cm, breadth ¼ 8 cm and thickness ¼ 4 mm) were used as the anode and cathode material. The dimensional characteristics of the experimental setup and the electrical assembly are shown in Table 2.

Table 2. Characteristics of the EC cell. Electrodes Material (anode and cathode) Shape Size Thickness (mm) Plate arrangement Effective electrode surface area (cm2) Reactor Characteristics Shape and material of construction Reactor operation Volume (L) Electrode gap (mm) Stirring mechanism Power Supply Voltage (V) Current (A)

Graphite Rectangular plate 9 cm  10 cm 4 parallel 180 Circular, glass Batch mode 1.0 10 Magnetic bar Direct current (DC) 0–15 0–6

Characterization of wastewater The wastewater used in this study was collected from a textile industry in Panipat, India which manufactured a variety of textile products such as carpet, blanket, knitting goods, decorative fabrics, sarees, etc. The wastewater sample was stored at 4 C in a deep freezer and used in the experiments without any dilution. The wastewater was characterized for COD, pH, total solids, suspended solids, dissolved solids, turbidity, conductivity and color by using the standard methods.[31] The characteristics of the wastewater are shown in Table 3.

Analytical The analysis of different parameters was carried out using different equipment/ instruments. pH and conductivity was measured using a multi-parameter digital meter (HACH, Loveland, CO, USA). Color was measured by a Table 3. Characteristics of wastewater used for electrochemical treatment. Characteristics COD (mg/L) TOC (mg/L) Color (platinum cobalt units) pH Conductivity (mmho/cm) Fluoride (mg/L) Chloride (mg/L) Nitrate (mg/L) Sulfate (mg/L) Sodium (mg/L) Potassium (mg/L) Calcium (mg/L) Magnesium (mg/L)

Before Treatment After Treatment 347 109 1100 8.7 12 1 296 18.7 603 524 24 234 109

32 19 41 2.6 16.6 0.4 195 1.9 547 321 18 50 35

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958 colorimeter (Aqualytic, Dortmund, Germany). The COD of the solution was determined by using a COD/TOC analysis system DR 5000 (HACH).[31,32] The TOC of the solution was determined by using TOC analyser (TOC-V, Shimadzu, Singapore). Voltammetric measurements were carried out using an Autolab potentiostat/galvanostat, model PGSTAT101 (M/s Metrohm, Utrecht, The Netharlands) controlled by electrochemical software. The cell used for cyclic voltammetric experiments was a three electrode type. The working electrode was a glassy carbon disc of 0.3 mm diameter.[33] A Pt auxiliary electrode and a saturated calomel electrode (SCE) were used as the counter and reference electrodes, respectively. Experiments were carried out at room temperature. Nitrogen was diffused into the electrolyte solution at least for 5 min between the analysis so as to ensure no presence of oxygen in the solution. During the analysis also, measurements were done in nitrogen atmosphere so as to maintain non-oxygen conditions.

Experimental setup and procedure All test runs were performed at 30  2 C. In each run, 1 L of wastewater was fed into the EC reactor. The initial pH of the solution was adjusted to the desired value by sodium hydroxide (0.1 N) or HCl (0.1 N) aliquots. The initial conductivity was adjusted by adding sodium chloride. Experiments were conducted for four plate configuration at desired current density. To optimize the parameters for reactor performance, the experiments were carried out at varying current density, pH, NaCl concentration and function of electrolysis time. At the end of the EC process, sample was centrifuged, filtered and analyzed for pH, COD and color. The electrode plates were cleaned manually by abrasion with 15% HCl followed by washing with distilled water prior to their reuse. The energy consumption for the removal of 1 kg of COD was calculated in kWh.

Experimental design In the present study, the 4-factor and 5-level Box-Behnken experimental design has been applied to investigate the effect of various variables. Percent COD and color removal (Y1 and Y2), and specific energy consumption (Y3) have been taken as three responses of the system, while four process parameters, namely, current density (j): 27.78–138.89 A/m2, pH: 4–10, time (t): 10–130 min and NaCl (w): 0–2 g/L are variable input parameters. The factor levels were coded as -2 (low), 0 (central point or middle) and 2 (high).[32,34] The variables and their levels are given in Table 4. A total of 27 experiments have been employed in this work. The actual experimental design

Bhatnagar et al. Table 4. Process parameters and their levels for EC treatment of textile wastewater using Box–Behnken design. Level Variable, unit Current density, j (A/m2) NaCl concentration, w (g/L) Time, t (min) pH

Factors X X1

2

1

0

1

2

27.78 55.56 83.33 111.11 138.89

X2

0

0.5

1

1.5

2

X3 X4

10 4

40 5.5

70 7

100 8.5

130 10

matrix for the treatment with graphite electrodes are given in Table 5. The results were analyzed using the coefficient of determination (R2), pareto analysis of variance (ANOVA) and statistical and response plots.[34] A nonlinear regression method was used to fit the secondorder polynomial (Eq. (1)) to the experimental data and to identify the relevant model terms. Considering all the linear terms, square terms and linear by linear interaction items, the quadratic response model can be described as: Y ¼ bo þ

X

bi x i þ

X

bii x2 þ

X

bij xi xj þe

ð1Þ

where, bo is the offset term, bi is the slope or linear effect of the input factor xi, bii is the quadratic effect of input factor xi and bij is the linear by linear interaction effect between the input factor xi and xj.[35]

Results and discussion Box-Behnken analysis To study the combined effect of operating factors such as j, pH, t and w, experiments were performed for different combinations of the physical parameters using statistically designed experiments.[36] COD removal (Y1,%), color removal (Y2,%) and specific energy consumed (Y3, kWh/kg COD removed) during electrochemical treatment of textile industry wastewater by graphite electrodes were measured according to design matrix, and the measured responses are listed in Table 5. The measured responses were correlated with the four design factors using the second-order polynomial (Eq. (1)).[37] The quadratic regression model for COD removal (Y1,%), color removal (Y2,%) and specific energy consumed (Y3, kWh/kg COD removed) by graphite electrode in terms of coded factors are given

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Electrochemical oxidation of textile industry wastewater Table 5. Full factorial design used for the EC treatment of textile wastewater by Graphite electrodes.

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% COD reduction

% Color reduction

Specific Energy, kWh/kg COD removed

Std

j (X1)

w (X2)

t (X3)

pH (X4)

Yexp(%)

Ypre(%)

Yexp(%)

Ypre(%)

Yexp(%)

Ypre(%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

138.89 83.33 111.11 111.11 111.11 111.11 55.56 83.33 83.33 83.33 83.33 55.56 55.56 83.33 55.56 55.56 55.56 55.56 27.78 83.33 83.33 111.11 83.33 111.11 111.11 111.11 83.33 83.33 55.56 83.33

1 1 0.5 1.5 1.5 1.5 1.5 0 1 1 1 1.5 1.5 1 0.5 0.5 0.5 1.5 1 1 1 1.5 2 0.5 0.5 0.5 1 1 0.5 1

70 70 120 40 120 120 40 70 10 70 70 40 120 70 40 120 120 120 70 70 130 40 70 40 40 120 70 70 40 70

7 7 8.5 8.5 8.5 5.5 5.5 7 7 7 10 8.5 8.5 7 8.5 5.5 8.5 5.5 7 7 7 5.5 7 8.5 5.5 5.5 4 7 5.5 7

88.47 74.64 90.78 34.00 66.86 69.74 51.58 80.40 5.47 74.64 50.14 36.89 59.08 74.64 19.60 81.27 39.77 90.20 31.12 74.64 88.18 56.19 43.23 48.12 64.84 91.07 58.21 74.64 68.01 74.64

79.08 74.93 90.17 36.94 70.45 74.91 50.34 75.16 18.06 74.93 41.64 22.47 53.97 74.93 21.35 77.27 49.41 81.84 47.76 74.93 81.12 41.40 55.71 60.10 64.55 94.62 73.96 74.93 49.22 74.93

93.91 63.54 92.45 88.45 91.45 91.64 89.64 93.09 10.90 63.54 91.36 90.18 91.27 63.54 93.36 94.73 98.54 92.73 90.54 63.54 95.09 91.64 92.54 85.27 90.73 93.54 93.64 63.54 88.82 63.54

95.38 61.84 97.43 80.66 96.47 97.06 81.86 97.54 49.99 61.84 95.68 81.27 97.08 61.84 82.23 98.63 98.04 97.67 96.60 61.84 73.70 81.25 95.62 81.62 82.21 98.02 96.85 61.84 82.82 61.84

211.15 89.07 231.01 231.14 363.40 324.13 43.91 92.91 149.10 89.07 128.17 66.87 142.04 89.07 103.95 59.08 151.88 80.89 66.76 89.07 141.02 132.47 146.06 123.19 88.80 190.31 129.21 89.07 27.58 89.07

234.14 85.90 228.51 197.74 343.71 307.43 47.37 74.68 114.49 85.90 161.34 83.74 137.25 85.90 115.27 76.80 128.69 90.90 33.37 85.90 184.24 171.44 153.89 122.62 90.76 186.67 85.64 85.90 73.35 85.90

as follows: Y1 ¼ 74:93 þ 15:66 j  9:73 w þ 31:53 t  16:16 pH ð2Þ  11:51 j2  9:49 w2  25:33 t2  17:13 pH2  24:28 j  w þ 1:51 j  t þ 23:41 j  pH þ 2:58w  t ð3Þ Y2 ¼ 61:84  0:61 j  0:96 w þ 11:86 t  0:59 pH þ 34:14 j2 þ34:73 w2 þ34:42 pH2 Y3 ¼ 85:90 þ 100:38j þ 39:61w þ 34:88t þ 37:85pH þ 47:85 j2 þ 28:38 w2 þ 63:46 t2 þ 37:59 pH2 ð4Þ þ 106:65 j  w þ 69:35 j  t  10:06 j  pH þ 30:06 w  t  5:55 w  pH þ 7:48 t  pH Statistical testing of the model was performed using Fishers’ statistical test (F-test) ANOVA, which was conducted to determine the fitness of the second-order polynomial equation with the experimental results. The results of the ANOVA for COD, color and specific energy consumption by graphite electrodes are shown in Table 6. The ANOVA results for the COD, color and specific energy consumption showed a F-value of 7.54, 7.37 and 11.18,

respectively. The large value of F indicates that most of the variation in the response can be explained by the regression equation, and the terms in the model have a significant effect on the response.[37] Three responses gave R2 values of 0.8418, 0.7010 and 0.9125, respectively. These values express a satisfactory correlation between the observed and the predicted valules. Electrochemical oxidation mechanism Electrochemical treatment using graphite electrodes occurs via combination of direct oxidation on the surface of the electrode and indirect oxidation by the oxidizing agent generated in situ.[29,38] Electrochemical oxidation takes place with the help of various oxidants such as nascent oxygen, ozone, hydrogen peroxide, free chlorine and free radicals such as OH, ClO and Cl. At the anode surface, generation of hydroxyl radicals takes place according to the following reactions:[29] 2 H2 O!2 OHþ2 Hþ þ2 e OH !OHþe

ð5Þ ð6Þ

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Table 6. Analysis of variance (ANOVA) for the second-order polynomial model of COD and color removal and specific energy consumption by graphite electrode. Source

Sum of squares

Degrees of freedom

Mean square

17 12 5

133.63 189.31 0.000

22 17 5

128.50 166.29 0.000

15 10 5

998.69 1498.03 0.000

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COD removal Residual 2271.69 Lack of fit 2271.69 Pure error 0.000 Color removal Residual 2826.96 Lack of fit 2826.96 Pure error 0.000 Specific energy consumption Residual 14980.30 Lack of fit 14980.30 Pure error 0.000

Hydroxyl radicals are highly reactive components which oxidize the organic pollutants present in wastewater.[29,38] However, at the anode surface, anodic oxygen evolution occurs as a primary reaction in dilute chloride solutions by following reaction:[3,39] 2 H2 O!O2 þ4 Hþ þ4 e

ð7Þ

F-value

P

R2- values R2 ¼ 0.8418 ¼ 0.7301

0.0001

R2adjust

7.37

0.0001

R2adjust

11.18

< 0.0001

R2adjust

7.54

R2 ¼ 0.7010 ¼ 0.6058 R2 ¼ 0.9125 ¼ 0.8309

Graphite electrodes have small values of over-potential for oxygen evolution indicating that the effective oxidation of pollutants on graphite anodes occurs only at very low current densities. At high current density, current efficiency becomes less because of the production of oxygen.[3,40] Presence of chloride in the wastewater during the electrooxidation process not only increases the concentration

Fig. 1. Effect of various variables on EC treatment of textile industry wastewater by graphite electrode. (a) Effect of time and current density on COD removal efficiency; (b) effect of time and current density on color removal efficiency; (c) effect of time and current density on specific energy consumption; (d) effect of pH and NaCl on COD removal efficiency; (e) effect of pH and NaCl on color removal efficiency; and (f) effect of pH and NaCl on specific energy consumption.

961

Electrochemical oxidation of textile industry wastewater of the hypochlorite ions but also the cell conductivity.[3] It may be noted that the hypochlorite ions are strong oxidants and oxidize the organic compounds present in the wastewater. An increase the cell conductivity results in an increase in current density at the same voltage and thus enhances rate of chlorine production, enhances the degradation of organic compounds and decreases the specific energy consumption.[3,41] Indirect oxidation of organic compounds present in wastewater occurs when the chlorine or hypochlorite produced gets reduced to chlorite ions by the following expressions:[23] 2 Cl !Cl2 þ2 e ð8Þ Cl2 þH2 O!Hþ þCl þHOCl þ



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HOCl!H þOCl

ð9Þ ð10Þ

The oxidation of pollutants may also occur by oxidation of chloride involving active chlorine (HOCl and OCl).[29] Effects of process parameters Three-dimensional response surface graphs for all responses by various operational parameters j, w, t and pHi are shown in Figure 1. Effect of j and t on the three responses is shown in Figures 1a, 1b, and 1c. Parameters j and t were found to have no effect on color removal efficiency in the studied parameter range (Fig. 1a). It may be seen in these figures that the COD removal and specific energy consumed increase continuously with an increase in j and t. An increase in j increases the rate of production of electrons, which increase the rate of oxidation of organics and dyes present in textile industry wastewater. At high j, H2O2 produced from cathodic reduction of molecular oxygen helps in increasing the COD removal efficiency.[3,27] Specific energy consumption was found to increase with an increase in j and t. This is due to the depletion of organic compound concentration in the vicinity of electrode surface with an increase in t, so that more energy is required for same quantity of COD removal.[29] Also the conversion of organic compounds to stable intermediates that resist further oxidation after certain time of treatment increase the value of specific energy consumption. Passive film formation on anode surface can also increase specific energy consumption, though its contribution is likely to be less owing to the continuous generation of hypochlorite ions in aqueous solution.[29] Effect of pH and w (NaCl dose) on the COD removal, color removal and specific energy consumed is shown in Figures 1d, 1e, and 1f. Figure shows that these factors have marginal effect on COD and color removal efficiency. It may be seen in the figure that the COD removal continuously increased and the specific energy consumption continuously decreased with an increase in pHo from 4 to 10. At pH 4, hypochlorous acid is dominant chlorine

species present in the solution which has higher oxidation potential than hypochlorite and helps in increasing the COD removal efficiency.[3,35] OCl þH2 O!HOCl þ OH

ð11Þ

At neutral pH, free chlorine oxidizes to form chlorate and perchlorate which is considered as undesirable in electrolytic oxidation process. Moreover, HOCl and hypochlorite ions also combine to form chlorate.[3,41] These factors decrease the removal efficiency and increase the specific energy consumption at neutral pH range. At basic pH, the only removal that happens is because of the less potent hypochlorite ions.[3] Though w (NaCl dose) was found to have only a marginal effect on removal efficiency, chlorine concentration was found to decrease from 296 mg/L to 195 mg/L (Table 3) indicating the discharge of chlorine at the anode.[29] Optimization Since, there are three responses in this study, therefore, multi-response processes optimization by desirability function approach was used to optimize the EC treatment of textile industry wastewater. One-sided desirability di is used in the study given by: 8 " 0 > #r > if Yi  Yimin < Yi  Yimin di ¼ If Yimin < Yi < Yimax ð12Þ Y  Yimin > > if Yi  Yimax : imax 1 where Yi is response values, Yi-min and Yi-max is minimum and maximum acceptable values of response i, and r is a weight and a positive constant, used to determine scale of desirability. For % COD removal (Y1), the minimum and maximum acceptable values are considered as 5.47% (the minimum experimental value) and 91.07% (maximum experimental value), respectively.[41] Following equations show the desirabilities (d1, d2 and d3) of responses (Y1, Y2 and Y3): 8 "0 > # > if Y1  5:47 < Y1  5:47 if 5:47 < Y1 < 91:07 d1 ¼ 91:07  5:47 > > if Y 1  91:07 : 1 8 " 0 > # > if Y2  10:90 < Y2  10:90 d2 ¼ if 10:90 < Y2 < 98:54 98:54  10:90 > > ifY2  98:54 : 1 8 " 0 > # > ifY3  27:58 < 363:40  Y3 d3 ¼ if 27:58 < Y3 < 363:40 363:40  27:58 > > if Y3  363:40 : 1

ð13Þ

ð14Þ

ð15Þ

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In the above equations, value of r was taken as 1. The overall desirability D was calculated by the following equation:

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pffiffiffiffiffiffiffiffiffiffiffiffiffi d1 d2 d3

ð16Þ

By using D as a new desirability, the optimum values of operational parameters were found to be j ¼ 27.78 A/m2, w ¼ 2.0 g/L, t ¼ 110 min and pHo ¼ 4.0, which produced overall D was 0.980. It may be seen in Table 1 that many of the previous investigators have reported higher optimum j as compared to that obtained in the present study and few have reported lower optimum j values also. It may be mentioned that the optimum conditions depend upon characteristics of wastewater, dimensions of electrode and the reactor, mixing mechanism, etc. Therefore, optimum conditions reported by various investigators are different. To verify the optimization results, three verification runs were conducted with the optimized set of operational parameters. The average value of responses Y1, Y2 and Y3 were found to be 90.78%, 96.27% and 23.58 kWh/kg COD removed, respectively.[41]

untreated textile wastewater which corresponds to the direct oxidation of pollutants present in the textile wastewater. It is important to note that, when CV was applied to EC treated textile wastewater, many of these peaks disappear in the voltammogram indicating that the pollutants have been reduced/oxidized.[42] Reduction peaks are observed at a potential of 1.1 V, which can be attributed to the presence of inorganic ions of the untreated textile wastewater. The peaks in the reduction side increase in the treated wastewater because of the presence of anions (Cl, or the other species formed during EC) in the wastewater.[43]

UV-Visible study

Cyclic voltammetric (CV) studies are conducted to determine the presence of the oxidizable/reducible species in the wastewater. To obtain further information on the electrochemical processes occurring at the electrodes, CV experiments were performed using pyrolytic graphite as working electrodes. Figure 2 shows a CV of textile wastewater before and after of ECT. CV of textile wastewater before ECT at a sweep rate of 100 mV/s exhibited some chemically irreversible oxidation small peaks in the

EC degradation before and during treatment (at various treatment time) was studied using UV–visible analysis (200–800 nm) (Fig. 3). A peak was observed at 220 nm, which may be assigned to the aromatic rings of the dyes present in the wastewater. Broad peak between 500– 550 nm (inset figure) may be attributed to the azo structure of dyes present in the wastewater.[44] This broad peak totally disappeared in the treated water. Similarly, the peak which appeared in the range of 240–280 nm (representing mono-aromatic rings) in the untreated wastewater, faded after the treatment. This study shows that all azo bonds were totally broken and most of the aromatic rings were mineralized during EC treatment with graphite electrode. However, it seems that mineralization of poly-aromatic rings was marginal. In the present study, maximum COD and color removal were found to be 90.78% and 96.27%, respectively, emphasizing the mineralization of various compounds present in the wastewater. Similar trends were observed by Singh et al.[45,46] for the treatment of

Fig. 2. Cyclic voltagram of textile industry wastewater before and after electrochemical treatment.

Fig. 3. UV-visible spectra before and after electrochemical treatment with graphite electrodes.

Cyclic voltameter studies

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Electrochemical oxidation of textile industry wastewater

Fig. 4. Scanning electron micrograph of (a) unused graphite plate, (b) used graphite plate, (c) sludge, and (d) scum.

malachite green using aluminum electrode in which maximum COD, TOC and color removal efficiencies were found to be 82.4%, 63.5% and 99.4%, respectively. Disposal of residues At optimum conditions, electrochemical treatment of textile industry wastewater was found to generate 0.330 g of sludge and 0.012 g of scum per litre of treated wastewater. Kushawaha et al.[35] have reported generation of 1.787 g/ L of sludge and 0.74 g/L of scum during treatment of dairy wastewater by aluminum electrode. Similarly, Kabdasli et al.[47] reported generation of 2.1 g and 2.6 g of sludge by SS and aluminum electrode for treatment of one litre of simulated reactive dye bath effluent. Arslan-Alaton et al.[48] reported generation of 8.2 g/L sludge for treatment of simulated acid dye bath effluent with Al electrode. It may be seen that the amount of sludge or scum generated with graphite electrode is about less than 20% of that reported with Al or SS electrode for treatment of various type of wastewaters. The amount of scum generated with graphite electrode is very marginal.

SEM of unused and used graphite electrodes is shown in Figures 4a and 4b, respectively. Unused electrode shows plain surface while used anode electrode shows dents of varying size on its surface. The size of the dents is between 10–500 mm. These dents are due to erosion of carbon from the graphite electrode surface. SEM of sludge and scum is shown in Figures 4c and 4d, respectively. Scum particles were found to be smaller in size as compared to sludge particles. Carbon mass balance was also performed for sludge and scum generated at optimum condition and the calculations are shown in Table 7. Out of the total carbon eroded from the electrodes into the reactor, 27–29% goes to the scum, 71–73% goes into the sludge and rest goes to the treated textile industry wastewater. Treated textile industry wastewater contained 32 mg/L of COD, which is within the COD standard set by CPCB.[49] TGA/DTA curves of the generated residues in air environment are shown in Figure 5. The TGA patterns of sludge and scum show loss of 10.0% and 3.1% bound moisture upto 100 C. The degradation temperature range for very rapid mass loss were found to be 100–700 and

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Table 7. Carbon mass balance calculation at optimum condition (j ¼ 27.78 A/m2, w ¼ 2 g/L, t ¼ 110 min, pHo ¼ 4) (Basis: 1 L of ww).

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Total carbon introduced Estimated carbon in ww (mg/L)

Carbon eroded (mg) (Experimentally determined) Total carbon going in to the system (ww feed þ eroded) (mg) Total carbon in residues and treated ww Sludge generated (mg) Total carbon in sludge (mg)# Scum generated (mg) Total carbon in scum (mg)# Estimated carbon in treated ww (mg/L)

Total carbon (residues þ treated SDW) (g) % Error

130 180 310

330 224 12 7.5 12 243.8 21.39

ww: textile industry wastewater.

Estimated using TC/TOC analyser. # Estimated using TGA data.

800–1025 C; and the degradation percentage of mass within these temperature ranges were 32.2 and 33.6% for sludge and scum, respectively. The temperature at which maximum rate of weight loss occurred were found to be 477 and 1025 C; and the corresponding maximum rate of weight loss were found to be 0.112 and 0.249 mg/min for sludge and scum, respectively. Overall, sludge and scum showed total weight loss of 45.2 and 51.6%, respectively. The little higher oxidation of scum may be due to its higher carbon content. The strong exothermic peak associated with oxidation of sludge and scum confirms the energy evolution during their oxidation in air atmosphere. Thus, the generated sludge and scum by EC treatment of textile industry

wastewater can be dried and used as a fuel in the boilers/ incinerators, or can be used for the production of fuelbriquettes. The bottom ash may be blended with clay to make fire bricks.[49]

Conclusion The EC oxidation of organic matter present in the actual textile industry wastewater was maximized in terms of COD and color removal efficiencies using graphite electrodes with minimum specific energy consumption. Box– Behnken design was used to examine the role of 4-factors namely initial pH (pHo): 4–10, current density (j): 27.78– 138.89 A/m2, NaCl concentration (w): 0–2 g/L and electrolysis time (t): 10–130 min on COD, color and specific energy consumption efficiency. F-values (COD: 7.54, color: 7.37 and specific energy consumption: 11.18) and coefficient of determination (R2) values (COD: 0.8418, color: 0.7010 and specific energy consumption: 0.9125) showed that a second-order polynomial regression model could properly interpret the experimental data. Under optimized conditions, maximum COD and color removal and minimum specific energy consumption were found to be 90.78%, 96.27% and 23.58%, respectively. Out of the total carbon eroded from the electrodes, majority of the carbon was found to go into the scum and the sludge. Final COD of treated wastewater was found to be within the prescribed limit.

Funding Authors are thankful to Council of Scientific & Industrial Research (CSIR), India, for providing financial help for carrying out this work.

References

Fig. 5. TG/DTA of sludge and scum generated by the EC treatment of textile industry wastewater with graphite electrodes at optimum conditions.

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Electrochemical oxidation of textile industry wastewater by graphite electrodes.

In the present article, studies have been performed on the electrochemical (EC) oxidation of actual textile industry wastewater by graphite electrodes...
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