Electrochemical treatment of benzoic acid (BA) from aqueous solution and optimization of parameters by response surface methodology (RSM)

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Electrochemical treatment of benzoic acid (BA) from aqueous solution and optimization of parameters by response surface methodology (RSM) Krishan Kishor Garg∗, Basheshwer Prasad Department of Chemical Engineering, Indian Institute of Technology Roorkee, Uttarakhand, 247667 India

a r t i c l e

i n f o

Article history: Received 28 January 2015 Revised 27 March 2015 Accepted 7 April 2015 Available online xxx Keywords: Benzoic acid wastewater Electro coagulation Optimization Residue analysis Al and Fe electrodes

a b s t r a c t The present research work is based on the degradation of benzoic acid (BA) and chemical oxygen demand (COD) from laboratory prepared aqueous solution. An electrocoagulation approach using aluminum (Al) and iron (Fe) as electrode material is applied in this study. A full factorial (4-factors, 5-level, and 3-response) rotatable central composite design in response surface methodology was developed for optimization of process parameters. Operating parameters such as pH (4–12), current density (A/m2 )—(76.00–230.00), electrolysis time (min)—(15–90) and electrolyte concentration (NaCl) (g/L)—(0.5–1.5) are chosen for the study. Maximum % removal efficiency of BA—70.52, 64.42; COD—65.65, 60.70 and E. consumption (kWh/kgCODremoved )—102.89, 133.52 are achieved at optimal operating conditions by Al and Fe electrodes respectively. Sludge samples generated after EC treatment at optimum treatment conditions are also analyzed by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy, TGA/DTA (thermo gravimetric analysis and differential thermo gravimetric analysis, FTIR (Fourier transform infrared spectroscopy), XRD (X-ray diffraction). Settling characteristics of sludge samples are also studied for both Al and Fe electrodes. © 2015 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

1. Introduction Petrochemical industries generate large amount of wastewater with a very high COD. Around 3–4 m3 wastewater (COD—5–20 g/L) is generated during manufacturing of 1-ton of terephthalic acid (TPA) from purified terephthalic acid (PTA) unit [1]. PTA wastewater mainly consists of aromatic compounds in very high concentration [2]. Benzoic acid (BA) is counted as one of the major toxic pollutants of PTA wastewater and occupies almost 30% of total COD of PTA wastewater. It is a major component of PTA wastewater which is formed as an intermediate during the manufacturing process of terephthalic acid (TPA) [3,4]. Wastewater generated from PTA plant contains various types of pollutants like: phthalic acid (PA), para-toluic acid (p-TA), acetic acid (AA), CTA (crude terephthalic acid), benzoic acid (BA) in a very high concentration. BA is also a major toxic pollutant of PTA wastewater and found in a higher concentration [5]. It is used as a chemical additive or preservative in agro-based or food industries. Also used for preservation of different fruit products, i.e. orange juice, jams, sauces and jellies to retard their enzymatic, microbiological and chemical degradation [6], preservation of beverages, condiments and chemically leavened baked goods [7]. Due to its toxic nature, BA is

∗

Corresponding author. Tel.: +91 8979528557. E-mail address: [email protected], [email protected] (K.K. Garg).

completely banned as a food preservative in several countries like China, Japan and European Union [8]. It may induce acute toxicity to living beings in water bodies [9], dangerous for animals and human being, able to alter hormone secretion in animals [10]. Removal of BA from wastewater is a really big challenge today. According to World Health Organization (WHO) maximum tolerable intake of BA in a human body is less than 5 mg/kg of body weight per day [11]. Therefore, removal of BA from wastewater needs much more public attention. Since last few decades, BA wastewater was treated by various treatment methods such as: adsorption [7,12,13], chemical oxidation [14,15], solvent extraction [16], membrane process [17], liquid–liquid extraction (LLE) [18], advance oxidation processes – photo Fenton oxidation (UV/O3 /H2 H2 /Fe) [19,20], photo-catalysis (Fe+3 /O3 /UV) [19] and electrochemical treatment- electrocoagulation [14,21]. In recent years, electrocoagulation (EC) technology is effectively used for treatment of wastewater. Electrocoagulation occurs via serial steps: (i) Electrolytic reactions at electrode surfaces (ii) In-situ oxidation of metal ions and subsequent precipitation of metal hydroxides in aqueous phase, and (iii) Adsorption of soluble or colloidal pollutants on coagulants surface [22]. The main advantage of EC process is, it generates lower quantity of sludge in comparison to other treatment process like coagulation–flocculation process, absorption, filtration with membrane process and advanced oxidation processes (Ozonation, Fenton, Photo-Fenton, and photolysis) [23–26]. In this study, Al and Fe are selected as electrode material. Both metals can form multivalent ions, Al3+ , Fe2+ and Fe3+ , and various

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Please cite this article as: K.K. Garg, B. Prasad, Electrochemical treatment of benzoic acid (BA) from aqueous solution and optimization of parameters by response surface methodology (RSM), Journal of the Taiwan Institute of Chemical Engineers (2015), http://dx.doi.org/10.1016/j.jtice.2015.04.005

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hydrolysis products [27]. Metal cations go through a series of hydrolytic reactions depending on the pH of the solution and mononuclear and poly-nuclear hydroxides form in the solution. Neutral amorphous metal hydroxides, Al(OH)3 and Fe(OH)3 , are poorly soluble species. Poly-nuclear species, e.g., Al13 [AlO4 Al12 (OH)7+ 24 ] are effective in destabilization of colloids due to their large size and high positive charge [27,28]. Mechanism of EC process by using both Al and Fe electrode is given below: 1.1. Reaction mechanism by using Al electrode A current is passed through a metal electrode, oxidizing the metal (Al) to its cation (Al3+ ) (Eq. (1); thus EC introduces metal cations in situ, using sacrificial anodes. Oxygen development is also possible at the anode (Eq. (2)). Anode:

Al(S) 0 → Al(aq) 3+ + 3e−

(1)

4OH− → O2 + 2H2 O + 4e−

(2)

Cathode: The reaction occurring at the cathode is dependent on pH. At neutral or alkaline pH, hydrogen is produced via Eq. (3):

3H2 O + 3e− → 3/2H2(g) + 3OH(aq) −

(3)

While under acidic conditions, hydrogen is produced via Eq. (4): +

2H + 2e− →H2

(4)

Al3+ and OH− ions generated by electrode reactions (1) and (2) react to form different monomeric and polymeric species, which transform finally into Al(OH)3(S). Chemical dissolution of aluminum at both electrodes can be described by Eq. (5) [29]: Solution reaction: Alkaline conditions

Al(aq) 3 + + 3H2 O ⇒ Al(OH)3(S) + 3H(aq) +

(5)

Freshly formed amorphous Al(OH)3(S) “sweep flocs” have large surface areas which is beneficial for a rapid adsorption of soluble organic compounds and trapping of colloidal particles. Finally, these flocs are removed easily from aqueous solution by sedimentation or floatation [30–32]. On the other hand, at high pH values, further hydrolysis of aluminum is presented by Eq. (6). Acidic conditions

2Al(S) 0 + 6H2 O + 2OH− (aq) ⇒ 2Al(OH)− 4(aq) + 3H2(g)

(6)

1.2. Reaction mechanism by using Fe electrode Mechanism 1 (Acidic Conditions) Anode Eq. (7) and Eq. (8) are showing reactions at anode by using Fe electrodes

2Fe(s) → 2Fe2+ (aq) + 4e

(7)

2Fe+2 (aq) + 4H2 O(l) + O2 (g) → 2Fe(OH)3 (s) + 2H+

(8)

Cathode Under acidic conditions, hydrogen is produced via Eq. (4): Overall Under acidic conditions, Fe formed ferric (III) hydroxide according to Eq. (9)

4Fe(s) + 4H2 O(l) + 4O2(g) + 2H+ → 2Fe(OH)3(s) + H2(g)

(9)

Fig. 1. Schematic diagram of experimental setup.

Mechanism 2 (Alkaline conditions) Anode In alkali conditions iron converted into Fe2+ cations and formed ferrous (II) hydroxide as per Eq. (10)) and Eq. (11)

Fe(s) → Fe2+ (aq) + 2e

(10)

Fe+2 (aq) + 2HO((aq) → Fe(OH)2(s)

(11)

Cathode The reaction occurring at the cathode is dependent on pH. At neutral or alkaline pH, hydrogen is produced via Eq. (12): Overall

Fe(s) + 2H2 O(l) → Fe(OH)2(s) + H2(g)

(12)

2. Materials and method 2.1. Chemicals Analytical reagent (AR) grade chemicals have been used for the entire process. All chemicals are supplied by MERCK (Germany). BA— C7 H6 O2 (99% purity, MW—122.12 g/mol) was supplied by Himedia Laborites Pvt. Ltd. Mumbai, India. NaOH was supplied by RFCL limited, Delhi, India. All other chemicals such as NaCl, K2 Cr2 O7 , H2 SO4 , HgSO4 , acetic acid, Propane-2-OL and methanol are purchased from “Viswani Chemicals Pvt Ltd”, Civil Line, Roorkee, Uttarakhand, India. 2.2. Wastewater sample preparation and characteristics Standard solution of BA (1000 mg/L) was prepared synthetically at laboratory scale. Water used for experimental study is purified by Millipore milliQ system. All samples and reagents are always preserved at 4 °C to reduce microorganism growth and unwanted biodegradation. Initial concentration of BA (400 mg/L) is chosen according to previous study [33–35]. Initial COD if wastewater solution was estimated 706 mg/L. 3. Experimental procedure Schematic diagram of the experimental setup is given in Fig. 1. Experimental conditions, electrode characteristics, and range of operating parameters in their coded levels are given in Table 1. Direct current (DC) power supply was used to generate current between

Please cite this article as: K.K. Garg, B. Prasad, Electrochemical treatment of benzoic acid (BA) from aqueous solution and optimization of parameters by response surface methodology (RSM), Journal of the Taiwan Institute of Chemical Engineers (2015), http://dx.doi.org/10.1016/j.jtice.2015.04.005

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Table 1 Reactor, electrode and central composite design characteristics. Reactor characteristics

Electrode characteristics

Central composite design characteristics Parameter (range)

Reactor type Shape and material Dimensions Volume (m3 ) Stirring mechanism Electrode gap (cm) Magnetic stirrer speed (RPM)

Batch

Material (anode and cathode) Plexiglass (rectangular) Shape 13 cm × 12 cm × 0.5 cm Size of electrode 0.0016 Electrode area into solution Magnetic bar No. of electrode 1 cm (constant) Effective area 500–700 RPM Electrode (constant) arrangement

Rectangular 11 cm × 6 cm × 0.1 cm 7 cm × 6 cm × 0.1 cm

Level –2(–α ) –1

X1 4.00 6.00

CD (A/m2 ) (76→230) X2 76.00 114.50

6 259.80 cm2 Parallel connection

0 +1 +2(α )

8.00 10.00 12.00

153.00 191.50 230.00

electrodes. Entire experimental process was completed at room temperature (25 ± 2 °C). NaCl was used as a supporting electrolyte to improve the ionic strength of the BA wastewater solution. Solution pH was adjusted by (1 N, 2 N, 5 N) solutions of NaOH and H2 SO4 . Later on each run, samples were drawn from supernatant liquid for further analysis for measurement of concentration of COD and BA. A central composite design (4-factor, 5-levels, 3-responses) was developed by using design expert software (6.08, 2002, stat-ease inc. Minnepolis) in response surface methodology. All experimental run was conducted according to design given by CCD (Table 2). 3.1. Analysis of samples and estimation of response All samples are analyzed by using high pressure liquid chromatography (HPLC) (Waters 1525 binary HPLC pump, Waters 2414 refractive index detector, USA) equipped with UV detector (Waters 2487 absorbance detector, USA) System. COD of all samples were analyzed by using COD analyzer (Aqualytic, Germany). Before HPLC analysis samples were filtered with Nylon syringe filter (0.22 μm, Millimax syringe driven filter unit, USA). The detector wavelength was set on 240 nm [36–39]. HPLC was operated at isocratic mode using C-18 column at ambient temperature. A solution of 91% Millipore water, 2% acetic acid, 7% propane -2-OL was used as a mobile phase [40] with 1.2 mL/min. flow rate. Removal efficiencies of BA and COD are calculated according to Eq. 13

% Removal of BA or COD =

CBAi or CODi − CBAf CBAi or CODi

or CODf

(13)

Where: CBAi or CODi and CBAf or CODf are initial and final concentrations of BA and COD respectively. Consumption of energy is calculated by Eq. 14

  Energy consumption kWh/kg COD removed VIt  =  × 1000 % Removal of COD CCOD,i V R

PH (4–12)

Al, Fe

(14)

Where: V = applied voltage, I = applied current (Amp), VR = volume of treated wastewater (m3 ), t = is operating time (h), CCOD,i = initial COD of wastewater (mg/L). 4. Results and discussion 4.1. Effect of pH, CD, time (t), and NaCl concentration on % removal BA and COD for Al and Fe electrode The removal efficiency of BA and COD is increased from pH 4. Optimal removal of BA—70.52%, 64.42% and COD: 65.65%, 60.70% are achieved at optimal operating conditions (Table 3). The removal efficiency of BA and COD are increased up to pH value—8.48, 7.99 for Al and Fe electrodes respectively (Fig. 2). Removal efficiency was decreased because of Al(OH)4− and Fe(OH)4− ions are formed at highly

NaCl conc. (mg/L) (0.5→1.5) X3 0.50 0.75

Time (min) (15→90) X4 15.00 33.75

1.00 1.25 1.50

52.50 71.25 90

alkaline pH. Al(OH)4− and Fe(OH)4− ions show poor binding nature with pollutants at higher pH or repulsion between metal and benzoate anions was increased at higher values of pH due to removal efficiency of BA and COD decreased [41]. Rate of formation of Fe+3 and Al+3 ions are increased in the electrolytic solution above pH-5. Therefore, rate of formation of metal hydroxide flocs is increased, hence large number of benzoate ions get neutralize in the solution. The removal efficiency of BA and COD increases with increasing value of the CD—76 A/m2 , time (t)—15 min, CNaCl —0.5 g/L and reaches maximum at CD—150.26, 155.30, time—53.92, 57.18 and concentration of NaCl—0.97, 1.00 g/L. The removal efficiency of BA and COD decreases after optimum values of CD, time, and NaCl concentration. CD of solution determines the metal dissolution rate and size of bubble formation (affected the growth of flocs) [42–43]. Propagation of the metal ions increases in the resolution with increasing value of current density and the modest size of flocs aggregated together and formed bigger size of flocs. Concentration of metal ions increases in the solution with time. At a higher value of time (after 50–60 min), metal ions concentration decreases or found in dissolving form, so removal efficiency of BA, COD decreases (Fig. 2). Dosages of (NaCl) in the electrolytic solution increase the electrical conductivity of the electrolytic solution. Conductivity of the electrolytic solution was very low at beginning of the experimental run and increased with the addition of NaCl dosages in the solution. Therefore, removal efficiency of BA and COD is increased. At higher dosages of NaCl, corrosion of electrodes get started therefore removal efficiency of BA and COD is decreased (Fig. 2). 4.2. Effect of pH, CD, time (t), and NaCl concentration on E. consumption (kWh/kgCODremoved ) for Al and Fe electrode Consumption of energy (kWh/kgCODremoved ) strongly depends upon the CD, time, and voltage, COD removal efficiency. E. consumption increase with increasing pH value and reaches optimum at pH—8.48 (Al), 7.99 (Fe) as shown in Fig. 3. Current density is an important factor for consumption of electrical energy. As the value of current density increases removal efficiency of COD increases due to fall of electrical energy consumption. After optimum values (Table 3) of CD—150.26 (Al), 155.30 (Fe)% removal of COD decreases as the energy consumption increases. E. consumption decreases up to time— 53.92 min (Al), 57.18 min (Fe), because removal of COD increases due to E. consumption decrease (Fig. 3). Consumption of energy decreases with increase in NaCl concentration up to 0.97 g/L (Al), 1.00 g/L (Fe) and after that % removal of COD increases as shown in Fig. 3. 5. Response surface methodology (RSM) study 5.1. Optimization Criteria for optimization – operating variables like pH, CD, time and electrolyte concentration are used for optimization of various

Please cite this article as: K.K. Garg, B. Prasad, Electrochemical treatment of benzoic acid (BA) from aqueous solution and optimization of parameters by response surface methodology (RSM), Journal of the Taiwan Institute of Chemical Engineers (2015), http://dx.doi.org/10.1016/j.jtice.2015.04.005

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Operating variables

% BA removal (mg/L): (Y1 )

% COD removal (mg/L): (Y2 )

E. consumption (kWh/kg CODremoved ): (Y3 )

Al

Al

Al

Fe

Fe

Fe

X1 : (PH)

X2 : (j) (A/m2 )

X3 : (C) (Mg/L)

X4 : (t) (Min.)

Actual

CCD Pre.

Actual

CCD Pre.

Actual

CCD Pre.

Actual

CCD Pre.

Actual

CCD Pre.

Actual

CCD Pre.

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

6 10 6 10 6 10 6 10 8 8 6 10 6 12 6 10 6 10 8 8 4 10 8 8 8 8 8 8 10 8

76 114.5 191.5 191.5 114.5 114.5 191.5 191.5 153 191.5 114.5 114.5 191.5 191.5 114.5 114.5 230 191.5 153 153 153 153 114.5 191.5 153 191.5 153 114.5 153 114.5

0.75 0.75 0.75 0.75 1.25 1.25 1.5 1.25 1.00 1.00 0.5 0.75 0.75 0.75 1.25 1.25 1.25 1.25 1.00 1.25 1.00 1.00 1.00 1.00 0.75 1.25 1.00 1.00 0.75 0.75

71.25 15(–α ) 33.75 71.25 33.75 71.25 71.25 33.75 52.5 71.25 33.75 90(+α ) 71.25 33.75 71.25 33.75 33.75 71.25 33.75 52.5 52.5 52.5 52.5 52.5 52.5 52.5 71.25 71.25 52.5V 52.5

18.9 17.37 24.37 29.47 21.29 33.37 28.36 24 76.37 71 18.37 27.87 30.55 21 21 26.36 22.5 43.83 53 65.47 24.46 61.37 45.48 72 60 66.37 73.46 40.35 57.98 38.6

14.99 18.49 27.03 33.74 16.05 32.38 30.46 31.93 71.23 67.21 15.27 24.24 34.25 19.55 22.82 21.19 23.71 44.09 59.42 60.42 26.87 61.08 50.98 61.57 58.73 59.41 65.86 43.27 55.41 41.89

26.77 21.85 27 24 24 31.46 40.5 22.47 73.92 66.39 27.4 30.9 35 21.2 25 28 35 46.9 48.27 61.28 37.38 57.48 42.9 64.36 49.15 61.28 66.46 31.1 50.8 25

23.77 22.79 28.94 27.16 19.29 30.34 42.08 24.86 66.23 62.90 25.25 28.57 38.25 15.88 30.65 23.50 36.04 52.01 56.09 57.70 40.95 53.70 47.73 53.48 48.72 59.42 59.19 42.27 46.30 27.16

14 14.4 20.3 26.5 17.6 30 24.5 19.6 70.7 63.5 16.8 25.8 26.1 19 16 23.4 17.9 40.6 50.8 61.6 19.8 56.1 41 67.3 57 61 65.8 35.4 54.5 32.4

9.43 15.22 23.16 27.13 17.69 26.64 26.04 21.73 65.93 56.13 12.28 22.14 29.56 12.96 23.45 18.57 19.59 45.27 55.61 56.57 21.82 56.49 49.74 62.85 54.27 51.08 56.16 40.75 51.80 33.35

22 19.2 24.2 22.4 21.2 26.9 35.3 19.3 68.9 63.2 23.3 26.4 30.5 19 23 24.3 31.4 42.5 45.6 59.6 30.5 51.7 40.3 62.8 43.3 55 58 29 47.4 22

19.86 19.94 25.85 23.82 21.21 26.29 36.64 30.16 62.60 56.24 17.19 21.76 33.83 14.40 28.38 19.28 33.15 49.68 55.50 54.44 33.30 48.89 44.15 59.78 44.60 49.71 49.21 33.04 42.29 23.04

166 67.6 285.4 472.5 126.1 158.5 508.1 293.8 79.9 192.8 132.1 234.4 471.7 299.7 297.1 94 406.5 301.5 69.2 90.4 279.2 97.8 85.8 133.7 98.4 146.7 116.9 135.5 102.9 107.6

174.0 56.9 277.6 422.4 146.2 176.6 494.1 266.8 82.8 203.5 140.3 239.9 467.9 302.3 260.9 112.9 389.9 288.3 57.5 106.6 269.0 78.3 70.4 144.8 121.4 170.1 120.0 132.2 103.3 99.9

135.8 62.5 223.1 524.6 98.2 169 216 278 82.6 186 89.3 215.1 383 284.1 197.7 84.8 235.4 276.5 79.4 96.9 187.9 110.9 81.1 136.7 131.4 145.8 135.6 156.8 119.2 151.4

144.1 57.1 230.8 497.2 125.4 163.9 223.0 278.6 95.2 210.5 108.0 228.0 366.0 303.7 194.6 109.8 235.1 279.1 74.0 93.7 171.8 112.4 75.9 155.0 144.7 142.4 144.4 154.9 111.7 143.6

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Table 2 Actual and model predicted values of removal efficiencies of BA, COD and consumption of energy by Al and Fe electrodes based on CCD matrix.

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Table 3 Optimum operating conditions given by the CCD for Al and Fe electrodes. pH

Al Fe

8.48 7.99

C.D. (A/m2 )

145.26 155.30

NaCl Conc. (mg/L)

0.97 1.00

Time (min.)

53.92 57.18

% Removal of BA (mg/L)

% COD (mg/L)

RSM suggested

Test runs

RSM suggested

Test runs

RSM suggested

Energy KWh/kg COD removed Test runs

70.52 64.42

70.13 62.45

65.65 60.70

64.00 57.33

102.89 133.52

108.88 144.66

Fig. 2. Removal efficiencies of BA and COD for Al and Fe electrodes.

Fig. 3. Consumption of energy for Al and Fe electrodes.

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Fig. 4. Settling characteristics, XRD spectra and SEM/EDX images of sludge for (a, c, e and f) Al, and (b, d, g and h) Fe electrodes.

responses for maximum removal of BA, COD and minimum consumption of energy by Al and Fe electrodes. Central composite design (Table 2) was developed for this study. It was found that values of responses given by the model at optimum operating conditions were near the test run values. It shows good correlation between actual and model predicted values.

COD

Y2A1 = 65.08 + 3.37 × pH + 3.05 × j − 2.19 × C + 3.61 × t − 8.19 × pH2 − 8.52 × j2 − 9.23 × C 2 − 8.77 × t2 − 1.26 × pH × j − 2.36 × pH × C + 0.36 × pH × t + 6.65 × j × C + 0.83 × j × t + 1.93 × C × t

(18)

5.2. Model response equations based on ANOVA results Second order polynomial response equation in terms of independent variables generally follows Eq. (15)

2

2

× j + b33 × C + b44 × t + b12 × pH × j + b13 × pH × C + b14 × pH × t + b23 × j × C + b24 × j × t + b34 × C × t

(15)

Regression model in terms of independent variables has been expressed by a second order polynomial equation for both Al and Fe electrodes for different responses Y1 , Y2 , Y3 are given below: BA

2

2

− 8.50 × pH − 8.40 × j − 9.98 × C − 8.91 × t

+ 1.76 × j × t + 2.55 × C × t

(19)

Energy consumption

Y3A1 = 73.51 + 6.40 × pH + 69.54 × j − 15.15 × C + 44.81 × t + 19.53 × pH2 + 28.95 × j2 + 33.07 × C 2 + 23.09 × t2 + 19.44 × pH × j + 8.96 × pH × C − 1.03 × pH × t − 40.48 × j × C + 8.84 × j × t − 22.66 × C × t

Y1Al = 70.11 + 2.91 × pH + 3.30 × j − 2.04 × C + 4.10 × t 2

× pH2 − 06.59 × j2 − 9.93 × C 2 − 8.10 × t2 − 2.09 × pH × j − 2.36 × pH × C + 0.33 × pH × t + 7.84 × j × C

Y = b0 + b1 × pH + b2 × j + b3 × C + b4 × t + b11 × pH2 + b22 2

Y2Fe = 60.50 + 0.26 × pH + 2.82 × j − 0.98 × C + 3.75 × t − 4.93

(20)

2

Y3Fe = 89.16 + 13.62 × pH + 137.59 × j − 29.35 × C + 89.98 × t

− 1.29 × pH × j − 2.27 × pH × C − 0.044 × pH × t + 6.85 × j × C + 0.89 × j × t + 2.03 × C × t

(16)

+ 39.95 × pH2 + 57.56 × j2 + 65.71 × C 2 + 45.71 × t2 + 39.52 × pH × j + 17.16 × pH × C − 2.81 × pH × t

Y1Fe = 64.16 + 0.032 × pH + 2.87 × j − 0.75 × C + 4.29 × t

− 78.93 × j × C + 18.63 × j × t − 46.70 × C × t

− 4.22 × pH2 − 7.28 × j2 − 9.91 × C 2 − 7.98 × t2 − 2.46 × pH × j − 1.76 × pH × C + 0.18 × pH × t + 8.59 × j × C + 1.77 × j × t + 2.28 × C × t

(17)

(21)

Where, Y1 Al , Y1 Fe , Y2 Al , Y2 Fe , and Y3 Al , Y3 Fe are denoted the removal efficiencies of BA, COD and consumption of energy by Al and Fe electrodes respectively.

Please cite this article as: K.K. Garg, B. Prasad, Electrochemical treatment of benzoic acid (BA) from aqueous solution and optimization of parameters by response surface methodology (RSM), Journal of the Taiwan Institute of Chemical Engineers (2015), http://dx.doi.org/10.1016/j.jtice.2015.04.005

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Table 4 Elemental composition of sludge based on EDAX results for both Al and Fe electrodes. Element Weight %

Al (sludge) Fe (sludge)

CK

OK

Na K

Al K

Cl K

Fe K

Zn L

Total

9.99 7.92

50.12 35.66

10.25 16.41

28.91 —

0.74 2.49

— 33.59

— 3.93

100.00 100.00

Fig. 5. FTIR and DTA/TGA spectra for sludge (a and c) Al and (b and d) Fe electrode.

6. Analysis of sludge generated after EC treatment by Al and Fe electrodes 6.1. Settling characteristics, XRD and SEM/EDX analysis of sludge generate by Al and Fe electrodes Response of sludge height (cm) with respect to time (min) is measured by using graduated glass cylinder. A solution of 500 mL slurry generated after EC treatment by both Al and Fe electrodes was used for this study. Change in sludge height with respect to time for both Al and Fe electrodes is shown in Fig. 4(a and b). After time (t = 1 min) the level (9.5 cm) of Al electrodes generated sludge was higher than the sludge generated by Fe electrodes (9.0 cm). The settling height of Al (1.8 cm) and Fe (0.85 cm) electrodes generated sludge was near about constant after 50 min and 25 min respectively. It shows that the settling height of Fe generated sludge was higher

than the Al generated sludge. It occurs due to the higher density of iron hydroxides flocks or harder nature of iron hydroxide flocs to the aluminum hydroxide flocs. XRD spectra’s of sludge samples generated after EC process using Fe and Al electrodes is given in Fig. 4(c and d). XRD spectrum of samples gives information about their morphological structure and extent of the crystallinity. Samples of sludge were dried at ambient temperature because drying at high temperature can lead to experimental artifacts such as increased crystallinity [44]. XRD spectra of filtered sludge generated by Al electrodes shows very broad and shallow diffraction peaks (Fig. 4c). Bragg reflection having very broad and lower intensity peaks indicate that the analyzed phase possesses an order of short range, i.e., poorly crystalline and most likely amorphous phase for aluminum hydroxide/oxy-hydroxide. XRD of Fe electrode generated sludge does not show any peak.

Please cite this article as: K.K. Garg, B. Prasad, Electrochemical treatment of benzoic acid (BA) from aqueous solution and optimization of parameters by response surface methodology (RSM), Journal of the Taiwan Institute of Chemical Engineers (2015), http://dx.doi.org/10.1016/j.jtice.2015.04.005

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K.K. Garg, B. Prasad / Journal of the Taiwan Institute of Chemical Engineers 000 (2015) 1–9 Table 5 Operating costs of the process at optimum operating conditions and their test run values for both Al and Fe electrodes. Type of electrode

Results

Energy consumed Rs. 5/kwh

Electrode consumed (kg) (Al→ Rs. 115/kg & Fe→ Rs. 50/kg)

Operating cost ($) 1 $ = 60 Rs

Al

CCD Test run CCD Test run

102.89 108.88 133.52 144.66

2.0 2.1 3.6 3.8

12.47 13.16 14.19 15.28

Fe

SEM pictures of samples give information about the surface structure of the sludge (crystalline or amorphous) and EDX spectra gives information about the elemental composition of samples. SEM images of sludge samples obtained by Al and Fe electrodes are given in Fig. 4(e and g). It was found that sludge generated from Fe electrodes was much porous in comparison to sludge generated by Al electrodes. Therefore, particle size of Fe generated sludge was much lower than the sludge generated by Al electrodes. SEM micrograph of Fe generated sludge and scum shows the dense spherical structure in comparison to Al electrode generated sludge. It has happened due to the better settling characteristics of Fe generated sludge. EDX spectra of sludge samples for both types of electrodes (Al and Fe) are given in Fig. 4(f and h). Al and Fe are found in dominant nature in sludge residues. Composition (weight %) of sludge samples (obtained by Al and Fe electrodes) after EDX analysis is given in Table 4. 6.2. FTIR and DTA/TGA analysis of sludge samples FTIR spectra of the sludge samples are given in Fig. 5(a and b). It provides detailed information about surface chemistry and the presence of functional groups in sludge sample. The presence of different functional groups in sludge sample shows electrolyte interaction between flocks and cations, which plays an important role for the removal of colloids during EC experiment. According to Anad et al. [45] wavelength of carbonyl group lies between the regions of 1070 and 1820 cm−1 . Peaks in the region of 1600 cm−1 – 400 cm−1 depicts the aromatic C=C stretching. Wavelengths (1385.19 cm−1 , 1037.37 cm−1 , 1636.58 cm−1 ) of Al generated sludge and 1634.24 cm−1 , 1386.48 cm−1 for Fe generated sludge lie between the ranges of carbonyl groups which confirm the C–O–H in plane bending. Wavelengths 3697.15 cm−1 , 3684.87 cm−1 (Al sludge) and low intensity peaks 3273.69 cm−1 , 3584.15 cm−1 (Fe sludge) shows the presence of X(OH) or hydroxyl group in the wastewater solution. Peak at 3684.87 cm−1 (Al sludge), 3584.15 cm−1 (Fe sludge) shows the characteristics of hydrogen atoms in the ring. Wavelength between 800 and 400 cm−1 shows characteristics of infrared bands of organic halogen compounds [46]. TGA graph for Al and Fe generated sludge shows three and four phases of weight losses respectively and is shown in Fig. 5(c and d). TGA graphs show that the oxidizing capacity of Fe generated sludge was much more eminent than the Al generated sludge. Loss in weight of sludge samples below 100 °C shows the evaporation of the pore water or solution that was not removed during drying at ambient temperature. Rate of weight loss for Al electrode generated sludge reaches maximum 0.198 mg/min at 100 °C and minimum 0.276 mg/min at 350 °C (where the combustion of volatiles traces place endothermic). In the case of Fe electrode generated sludge rate of weight loss reaches maximum 0.153 mg/min at 480 °C and minimum 0.019 mg/min at above 450 °C. Reason of the weight loss of the sludge sample in mentioned range is formation CO and CO2 . About 35.74% (Al), 97.12% (Fe) sludge samples become oxidized at temperature up to 1000 °C. A DTA graph of Al electrodes generated sludge shows endothermic and exothermic nature at different temperatures. Fe electrodes generated sludge shows the exothermic nature and release 568 MJ/mg energy at 400 °C. Oxidation of sludge and scum sample shows the exothermic nature of reaction due to fragmentation of carbon chain

and endothermic nature due to decomposition with early oxidation of different fragments, condensation of some functional groups that are usually removed in the EC process. 7. Operating cost (OC)

OC = Energyconsumption(kg/kgCODremoved ) × EEC + Electrodeconsumption(kg/kgCODremoved ) × EMP + Sludgedisposalandtransportation

(22)

Where, EEC = electrical energy cost, EMP = electrode material cost and $60 10−3 kg-1 for sludge disposal and transportation [47]. Operating cost of the process based on energy consumption, electrode consumption and sludge disposal and transportation is given in Table 5.

8. Conclusion •

•

•

•

•

EC process is strongly enhanced at the Al electrode surface however, Fe electrodes generate less amount of sludge in comparison to Al electrodes. CCD developed for this study shows high correlation between actual (experimental) and the model predicted responses. Maximum removal of BA and COD are achieved at optimum pH—8.48(Al), 7.99(Fe). Process shows maximum removal at alkaline conditions. Alkaline conditions are not so much harmful for aquatic microorganisms or human being. Therefore, this process is environmental friendly and does not affect human being and microorganisms any more. Al and Fe electrodes generated sludge shows crystalline and porous structure respectively. Based on all results EC process shows higher efficiency with Al electrodes in comparison to Fe electrodes and low operating cost in case of Al electrodes.

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Please cite this article as: K.K. Garg, B. Prasad, Electrochemical treatment of benzoic acid (BA) from aqueous solution and optimization of parameters by response surface methodology (RSM), Journal of the Taiwan Institute of Chemical Engineers (2015), http://dx.doi.org/10.1016/j.jtice.2015.04.005

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Please cite this article as: K.K. Garg, B. Prasad, Electrochemical treatment of benzoic acid (BA) from aqueous solution and optimization of parameters by response surface methodology (RSM), Journal of the Taiwan Institute of Chemical Engineers (2015), http://dx.doi.org/10.1016/j.jtice.2015.04.005