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A comparative study of electrochemical degradation of benzoic acid and terephthalic acid from aqueous solution of purified terephthalic acid (PTA) wastewater
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A comparative study of electrochemical degradation of benzoic acid and terephthalic acid from aqueous solution of purified terephthalic acid (PTA) wastewater Vishal Kumar Sandhwar ∗ , Basheshwar Prasad Department of Chemical Engineering, Indian Institute of Technology Roorkee, Roorkee 247667, Uttarakhand, India
a r t i c l e
i n f o
Article history: Received 12 August 2016 Received in revised form 21 March 2017 Accepted 21 March 2017 Available online xxx Keywords: Benzoic acid Terephthalic acid Electrochemical oxidation Electro-Fenton Graphite electrode
a b s t r a c t In this article, studies have been performed on the treatment of benzoic acid (BA), terephthalic acid (TPA) and COD from synthetic binary solution of PTA wastewater. Acid precipitation pretreatment of aqueous solution was performed initially at different pH (2–5) and temperature (15–60 ◦ C). The acid treated solution was re-treated by electrochemical oxidation (EO) and electro-Fenton (EF) techniques using graphite electrodes. Independent operating parameters namely initial pH: (1–9), current density (A/m2 ): (30.48–91.45), electrolyte concentration (g/L): (0.5–1.5) and electrolysis time (min): (10–90) for EO process and pH (1–5), current density (A/m2 ): (30.48–91.45), Fe2+ concentration (mmol/L): (0.5–1.5) and electrolysis time (min): (10–90) for EF process were modeled and optimized by central composite design (CCD) in response surface methodology (RSM). The maximum removal efficiencies of the process during EF treatment were 80.45% of BA, 76.83% of TPA and 73.70% of COD with energy consumption (kWh/kg COD removed) – 19.39 at optimum operating conditions. During EC treatment removal capacities were 70.76%, 68.52% and 67.27% with 31.01 (kWh/kg COD removed) respectively. It was observed that EF process was more efficient than EO based on removals of BA, TPA and COD with minimum energy consumption. © 2017 Elsevier Ltd. All rights reserved.
1. Introduction Purified terephthalic acid (PTA) wastewater contains high concentrations of various aromatic compounds like benzoic acid (BA), para-toluic acid (p-TA), terephthalic acid (TPA), phthalic acid (PA), and 4-carboxybenzaldehyde (4-CBA) with low concentration of pxylene, methyl acetate and 4-formylbenzoic acid. Approximately 70–75% of total COD of PTA wastewater is contributed by these aromatic compounds [1–7]. United States Environmental Protection Agency (USEPA) has added these compounds in the priority pollutants list [8–10]. There is no specific discharge limit has been proposed for these aromatic compounds by any pollution regulating agencies in India. For petrochemical wastewater the permissible discharge limit of COD into surface waters has been prescribed by Central Pollution Control Board of India is less than 250 mg/L [11]. Various physico-chemical treatment techniques have been applied for PTA wastewater remediation in recent years. Among these treatment methods electrochemical technologies
∗ Corresponding author. E-mail address: [email protected] (V.K. Sandhwar).
have received much more attention due to its automation, high efficiency, versatility and cost effectiveness features [12–14]. Some of the previous studies on PTA wastewater treatment are discussed in Table 1 [15–22]. During electrochemical treatment, pollutants degradation is done by either indirect or direct oxidation processes. In indirect oxidation, electrochemically generated strong oxidants (like hydrogen peroxide and hypochlorite/chlorine) are used to degrade the pollutants through oxidation. In direct oxidation, degradation of pollutants is done through adsorption on the surface of anode and anodic electron transfer reaction [23,24]. Electrochemical treatment with graphite electrodes takes place by combination of direct oxidation on electrode surface as well as indirect oxidation of in situ generated oxidizing agents [25,26]. Electrochemical oxidation (EO) occurs through various oxidants such as hydrogen peroxide, nascent oxygen, free chlorine and radicals like Cl, ClO and hydroxyl radicals (OH• ). Generation of OH• takes place at anode by following reactions [24]. H2 O → H+ + OH• + e−
(1)
OH− → OH• + e−
(2)
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Please cite this article in press as: V.K. Sandhwar, B. Prasad, A comparative study of electrochemical degradation of benzoic acid and terephthalic acid from aqueous solution of purified terephthalic acid (PTA) wastewater, J. Water Process Eng. (2017), http://dx.doi.org/10.1016/j.jwpe.2017.03.006
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Table 1 Literature on treatment of purified terephthalic acid wastewaters by electrochemical methods. Wastewater (ww)
Electrode Type
Pollutants concentration
Synthetic ww (single component)
Al (as anode and cathode) and Fe (as anode and cathode)
BA – 400 mg/L COD – 706 mg/L
Boron-doped diamond Synthetic ww (single component) on silicon (as anode and cathode)
BA – 150 mg/L
Synthetic ww (single component)
Al (as anode and cathode) and Fe (as anode and cathode)
TPA – 400 mg/L COD –567 mg/L
Synthetic ww (single component)
Fe (as anode) Graphite (as cathode)
BA – 400 mg/L COD – 751 mg/L
Synthetic ww (multi component)
Al (as anode and cathode) and Fe (as anode and cathode)
BA – 400 mg/L TPA – 400 mg/L p-TA – 500 mg/L COD – 2055 mg/L
Synthetic ww (single component)
Fe (as anode) graphite (as cathode)
TPA – 400 mg/L COD – 584 mg/L
Industrial ww
Iron (as anode and cathode)
TPA – 400 mg/L COD – 560 mg/L
Industrial ww
SS (as anode and cathode)
COD – 1155 mg/L
Synthetic ww (binary component)
Graphite (as anode and cathode)
BA – 400 mg/L TPA – 400 mg/L COD – 1220 mg/L
Parameter values (optimum) Al Fe pH – 7.99 pH – 8.48 j – 145.26 A/m2 j – 155.30 A/m2 ec – 0.97 g/L ec – 1 g/L t – 53.92 min t – 57.18 min pH – 3.8 Current intensity – 18 A ec – 0.05 mol/L t – 120 min Fe Al pH – 7 pH – 7 j –84.33 A/m2 j – 91.47 A/m2 ec – 0.97 g/L ec – 0.97 g/L t – 37.57 min t – 53.12 min EC EF pH – 2.99 pH – 7.34 j – 50.97 A/m2 j – 44.87 A/m2 ec – 0.05 mol/L H2 O2 – 307 mg/L t – 58.86 min t – 55.10 min Al Fe pH – 8.0 pH – 8.18 j – 172.97 A/m2 j – 180.04 A/m2 ec – 1.69 g/L ec – 1.74 g/L t – 63.47 min t – 65.55 min EC EF pH – 3.20 pH – 7.44 j – 32.71 A/m2 j – 27.89 A/m2 ec – 0.04 mol/L H2 O2 – 142.7 mg/L t – 43.49 min t – 38.55 min pH – 7.44 j – 118 A/m2 ec – 400 mg/L gap – 10 mm pH – 8.2 j – 125 A/m2 gap – 1 cm t – 180 min EO EF pH – 3.1 pH – 4.6 j – 65.15 A/m2 j – 54.39 A/m2 ec – 1 g/L Fe2+ – 1 mmol/L t – 58.02 min t – 50.11 min
Removal (%)
Ref.
Al BA – 70.5 COD – 65.6
Fe 64.4 60.7
[15]
BA – 42.7
[16]
Al TPA – 89.7 COD – 83.8
Fe 73.8 67.6
[17]
EC TPA – 76.8 COD – 69.2
EF 88.5 82.2
[18]
Al BA – 59.5 p-TA – 45.7 TPA – 56.2
Fe 53.8 39.9 42.9
[19]
EC TPA – 82.7 COD – 78.3
EF 91.8 85.1
[20]
TPA – 77 COD – 72
[21]
COD – 66.40
EO BA – 70.7 TPA – 68.5 COD – 67.2
EF 80.4 76.8 73.7
[22]
Present study
j – current density, ec – electrolyte concentration, t – time.
However in dilute chloride solution, generation of anodic oxygen takes place as a primary reaction by following reaction [23,27]. 2H2 O → 4H+ + O2 + 4e−
(3)
Indirect oxidation in wastewater containing organic pollutants takes place, when the produced chlorine or hypochlorite gets reduced into chlorite ions, represented by following reactions [28]. 2Cl− → Cl2 + e−
(4)
+
−
Cl2 + H2 O → H + Cl + HOCl +
HOCl → H + OCl
−
(5) (6)
Among the electrochemical advanced oxidation processes (EAOPs), electro-Fenton (EF) process has got more attention for water treatment. EF is one of the most popular and eco-friendly treatment process based on Fenton’s reaction chemistry [29,30]. In EF process electrogeneration of H2 O2 takes place by reduction of oxygen at cathode in acidic solution with addition of Fe2+ ions according to Eq. (8) [31,32]. H2 O2 + Fe2+ → Fe3+ + OH− + • OH +
−
O2 + 2H + 2e → H2 O2
(7) (8)
PTA wastewater exerts acute, chronic and molecular toxicities and also responsible for damage of bladder, kidneys, liver and histopathological abnormalities in human beings. This study is based on the treatment of major constituent of PTA wastewater viz., BA and TPA. Acid precipitation has been explored to reduce the concentrations of BA and TPA in aqueous solution in this study. Acid precipitation is a comparatively economical process, which does not require much electricity. The acid pretreated solution was further subjected to electrochemical treatment process. Both electrochemical processes were optimized through central composite design (CCD) of response surface methodology (RSM) under Design Expert Software (DES). RSM is an excellent and effective statistical tool for the optimization of various processes, widely used for experimental designs. It allows less number of experiments with rapid interpretation [33–36]. In this study operating parameters such as pH, current density (CD), electrolyte concentration (ec) and electrolysis time (t) for EO process and pH, current density, Fe2+ concentration and electrolysis time for EF process were optimized for the removal of BA, TPA and COD with minimum energy consumption (E.consumption). The results of the present work have shown that this combined treatment process (acid precipitation and electrochemical) was very effective in removing/degrading BA and TPA from aqueous solution.
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2. Materials and methods 2.1. Chemicals All chemicals used in entire treatment were of analytical (AR) grade. TPA was procured from Himedia Lab Pvt. Ltd., Mumbai, India. Sodium chloride (NaCl) and BA were supplied by Loba Chemie Pvt. Ltd., Mumbai, India. Ferrous sulfate heptahydrate (FeSO4 ·7H2 O) was obtained from Merck. Sodium hydroxide (NaOH), potassium dichromate (K2 Cr2 O7 ), sulfuric acid (H2 SO4 ), methanol (CH3 OH), isopropyl alcohol (C3 H8 O), mercury (II) sulphate (HgSO4 ), silver sulphate (Ag2 SO4) and acetic acid (CH3 COOH) were obtained from Ranbaxy Fine Chemicals Limited, New Delhi (India). 2.2. Preparation of synthetic solution and sample analysis Stock solution of BA (1000 mg/L) and TPA (1000 mg/L) with distilled water was synthetically prepared. Entire reagents as well as samples were preserved at 4 ◦ C to avoid microorganism’s growth and biodegradation. Based on previous studies [21,37–39] working concentration of BA (400 mg/L) and TPA (400 mg/L) was taken. Initial COD of solution was obtained 1220 mg/L. Characterization of aqueous solution was done by standard methods [40]. TPA and BA concentrations were analyzed by High Performance Liquid Chromatography (HPLC) (Waters, USA) with UV detector (Waters 2487 absorbance detector, USA) at 240 nm wavelength [42–44]. HPLC was operated at ambient temperature in C18 column and mobile phase solution of 91% Milli-pore water, 7% isopropyl alcohol and 2% acetic acid with a flow rate of 1.2 mL/min in isocratic mode [45]. COD of solution was determined by COD analyzer (Aqualytic, Germany). All the samples were filtered through nylon syringe filter (0.21 m) before analysis. Removal of BA, TPA, COD and E.consumption (kWh/kg CODremoved ) were calculated by the following equations [41]. % Removal of BA, TPA and COD =
Ci − Cf Ci
× 100
(9)
where Ci and Cf are the initial and final concentrations of BA, TPA and COD. E.consumption (kWh/kgCODremoved ) =
VIT × 100 × 1000 (% Removal of COD)CCODi × VR
(10)
where V, I, T, and Vs are voltage, current (amp), time (hour) and solution volume (liter) respectively. 3. Experimental setup and procedure Initially, acid precipitation was carried out by mixing of H2 SO4 (1 N) to the solution at different temperatures (15–60 ◦ C) and pH (2–5). Then, the solution was allowed to settle for 4 h. After that the supernatant was filtered and retreated by electrochemical processes. Both electrochemical experiments (EO and EF) were done in a rectangular plexiglas batch cell of 1.6 L capacity. Plain graphite plates (100 mm × 80 mm × 3 mm) were used as anode and cathode to treat 1 L of solution during EO and EF treatments. The effective electrode area into the solution was 131.2 cm2 . Distance between parallel electrodes was set at 4 cm. Fig. 1 shows the schematic diagram of electrochemical setup. Direct current (0–4 A) and voltage (0–35 V) were used during electrochemical treatments. The supply mode of electric current was continuous for each individual run. After each and every experimental run, electrodes were cleaned and washed by HCl solution (15%, v/v) and water respectively. Both electrochemical experiments were performed at room temperature (25 ± 2 ◦ C) and atmospheric pressure. In EF treatment, prior to
Fig. 1. Schematic diagram of experimental set up for electrochemical treatment.
electrolysis air was bubbled continuously through fish aerator to saturate the solution with oxygen (O2 ) till the end of experiments. This dissolved oxygen leads to in situ generation of H2 O2 at the surface of cathode as in Eq. (8). A different concentration of ferrous ions was added during EF process. To determine the range of operating parameters during electrochemical treatment, some random experimental runs were conducted. Tables 2 and 3 show operating parameters ranges for EO and EF processes respectively. Both EO and EF experiments were conducted at operating conditions predicted by CCD as shown in Tables 4 and 5 respectively. 4. Results and discussion 4.1. Effect of acid precipitation on removal of BA, TPA and COD BA and TPA present in the ionized state in aqueous solution, as the pKa value for TPA are 3.51 and 4.82 and for BA it is 4.2. The initial pH (pHo = 6.9) of solution was decreased by the addition of 1 N H2 SO4 then TPA and BA get deionized. Hydrogen ions concentration also increases by lowering the pH of solution through which is responsible for the conversion of dissociated weak acid ions to undissociated acid molecules. And finally, due to common-ion (hydrogen ions) effect, ionic product values of TPA and BA surpass its solubility product value resulting precipitation [46]. Acids (TPA and BA) present in the wastewater facilitate agglomeration by reduction in pH and then settling is allowed for 4 h. After that the supernatant was filtered and used for further treatment. Precipitation of TPA and BA in the aqueous solution results concentration reduction of BA, TPA and COD in solution approximately 48.7%, 83.4% and 57% respectively at temperature 15 ◦ C and pH 2. Effect of temperature and pH were also studied as shown in Fig. 2a–c. The new concentrations in supernatant are found, BA – 205.2 mg/L, TPA – 66.4 mg/L and COD – 524.6 mg/L. The supernatant was re-treated by of EO and EF processes separately. 4.2. Effect of pH on removal of BA, TPA, COD and E.consumption Effect of pH on percent removal of BA, TPA, COD and E.consumption is shown in Figs. 3a, c, e, g and 4a, c, e, g by EO and EF methods respectively. pH is an important tool to influence the performance of EO and EF processes. Both EO and EF treatments were completed according to the CCD predicted sets as given in Tables 1 and 2. During electrochemical treatment, acidic medium favors generation of hydroxide radicals (• OH). In EO, formation of hypochlorous acid (HOCl) also takes place in the aqueous solution
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Table 2 Operating parameters and their levels obtained from the statistical software for EO process. Central composite design characteristics Levels
Parameter (range)
−2(−˛) −1 0 +1 +2(˛)
X1 pH(3–11)
X2 CD (A/m2 )(30.48 → 91.44)
X3 Electrolyte concentration (g/L)(0.5 → 1.5)
X4 Time (min)(10 → 90)
1 3 5 7 9
30.48 45.72 60.96 76.20 91.44
0.5 0.75 1 1.25 1.5
10 30 50 70 90
Table 3 Operating parameters and their levels obtained from the statistical software for EF process. Central composite design characteristics Levels
Parameter (range)
−2(−˛) −1 0 +1 +2(˛)
X1 pH(1–5)
X2 CD (A/m2 )(30.48 → 91.44)
X3 Fe2+ concentration (mmol/L)(0.5 → 1.5)
X4 Time (min)(10 → 90)
1 2 3 4 5
30.48 45.72 60.96 76.20 91.44
0.5 0.75 1 1.25 1.5
10 30 50 70 90
at low pH. HOCl is a dominant chlorine compound having high oxidation potential favors high removal with low energy consumption. OCl− + H2 O → HOCl + OH−
(11)
At neutral pH, formation of chlorate and perchlorate takes place by the oxidation of free chlorines as well as through combination of HOCl and hypochlorite which is undesirable in EO process
resulting lower removal efficiencies with higher energy consumption. At high pH, removal occurs due to less potent hypochlorite ions [23,47]. It is clearly shown in Fig. 3a, c and e that removal efficiencies of BA, TPA and COD were increased with pH and beyond optimum pH (i.e. 4.6) removal decreased. In EF process, air was bubbled continuously through the fish aerator before electrolysis to saturate the solution with oxygen that leads to in situ generation
Table 4 Actual and CCD predicted removal efficiencies of BA, TPA, COD and energy consumption for EO process. Run no.
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
Independent variables
% removal of BA: (R1 )
% removal of TPA: (R2 )
% removal of COD: (R3 )
E.consumption (kWh/kg CODremoved ): (R4 )
X1 : (pH)
X2 : (CD)
X3 : (ec)
X4 : (t)
Actual (Exp.)
CCD (Pre.)
Actual (Exp.)
CCD (Pre.)
Actual (Exp.)
CCD (Pre.)
Actual (Exp.)
CCD (Pre.)
3 7 3 7 3 7 3 7 5 5 3 9 3 7 3 7 3 7 5 5 1 7 5 5 5 5 5 5 5 5
45.72 45.72 76.20 76.20 45.72 45.72 76.20 76.20 60.96 60.96 45.72 45.72 76.20 76.20 45.72 30.48 76.20 76.20 60.96 60.96 60.96 60.96 45.72 76.20 60.97 91.44 60.96 45.72 45.72 76.20
0.75 0.75 0.5 1.5 1.25 1.25 1.25 1.25 1 1.25 0.75 0.75 0.75 1.25 1.25 1.25 1.25 1.25 0.75 1 1 1 1 1 1 1 1 1 1 0.75
70 30 30 70 30 70 70 30 50 50 30 70 70 10 90 30 50 70 50 70 50 50 50 50 30 50 30 70 30 50
37.07 35.31 46.59 48.09 36.32 52.09 66.29 54.14 65.32 63.69 29.21 31.52 57.53 32.12 48.68 37.38 49.23 47.08 64.69 66.62 33.42 60.32 56.42 67.82 57.96 64.31 62.02 56.61 47.35 53.07
38.86 40.67 44.46 45.9 35.83 56.38 60.11 48.36 64.43 63.83 28.81 28.25 57.64 35.72 47.47 36.17 58.39 53.41 59.42 64.83 34.28 58.82 56.01 66.06 57.28 60.91 57.28 57.49 47.78 63.31
33.57 28.91 42.19 42.69 33.92 51.24 60.89 51.74 61.92 60.29 26.82 29.12 56.13 29.72 46.28 34.38 45.83 44.68 60.29 62.32 33.01 55.92 52.12 65.42 55.56 59.92 57.62 52.23 49.97 49.97
35.53 34.05 40.93 41.06 33.75 54.41 55.43 45.34 60.84 59.92 25.18 25.73 55.43 33.58 44.58 33.22 54.21 50.66 54.81 61.73 34.32 55.75 52.01 62.54 53.9 57.16 53.89 54.14 59.39 59.39
35.62 33.83 44.77 46.85 34.85 50.15 63.82 52.67 62.85 62.23 27.74 30.05 53.06 30.65 47.21 35.91 46.76 45.61 63.24 63.15 31.93 58.85 54.95 63.35 56.49 62.84 60.55 55.13 51.62 51.62
37.21 39.65 42.69 44.72 34.72 54.66 57.17 47.32 62.31 62.01 27.74 26.79 53.51 34.14 46.16 34.61 56.08 51.41 57.41 61.98 32.51 57.16 54.54 63.78 55.2 58.99 55.21 55.58 61.14 61.14
37.46 16.90 29.80 66.45 16.40 26.60 48.78 25.33 24.26 24.50 24.73 53.28 58.67 14.51 36.33 08.49 47.56 68.26 24.11 33.80 47.75 25.91 17.34 32.53 16.19 48.53 15.11 24.20 43.08 43.08
36.51 15.99 31.35 66.7 19.45 27.49 55.26 27.63 23.97 23.39 25.74 53.38 57.65 14.53 34.69 7.87 40.73 63.35 27.2 37.18 47.14 29.14 16.34 35.39 13.94 50.59 13.93 24.89 11.02 36.75
Please cite this article in press as: V.K. Sandhwar, B. Prasad, A comparative study of electrochemical degradation of benzoic acid and terephthalic acid from aqueous solution of purified terephthalic acid (PTA) wastewater, J. Water Process Eng. (2017), http://dx.doi.org/10.1016/j.jwpe.2017.03.006
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Table 5 Actual and CCD predicted removal efficiencies of BA, TPA, COD and energy consumption for EF process. Run no.
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
Independent variables
% removal of BA: (R1 )
% removal of TPA: (R2 )
% removal of COD: (R3 )
E.consumption (kWh/kg CODremoved ): (R4 )
X1 : (pH)
X2 : (CD)
X3 : (Fe2+ concn )
X4 : (t)
Actual (Exp.)
CCD (Pre.)
Actual (Exp.)
CCD (Pre.)
Actual (Exp.)
CCD (Pre.)
Actual (Exp.)
CCD (Pre.)
2 4 2 4 2 4 2 4 3 3 2 5 2 4 2 4 2 4 3 3 1 4 3 3 3 3 3 3 3 3
45.72 45.72 76.20 76.20 45.72 45.72 76.20 76.20 60.96 60.96 45.72 45.72 76.20 76.20 45.72 30.48 76.20 76.20 60.96 60.96 60.96 60.96 45.72 76.20 60.97 91.44 60.96 45.72 45.72 76.20
0.75 0.75 0.5 1.5 1.25 1.25 1.25 1.25 1 1.25 0.75 0.75 0.75 1.25 1.25 1.25 1.25 1.25 0.75 1 1 1 1 1 1 1 1 1 1 0.75
70 30 30 70 30 70 70 30 50 50 30 70 70 10 90 30 50 70 50 70 50 50 50 50 30 50 30 70 30 50
46.63 44.76 56.15 57.65 45.68 62.18 73.85 63.71 72.88 71.25 38.77 41.08 67.19 41.68 58.24 46.94 58.79 56.64 72.25 74.18 42.96 69.58 65.98 74.31 67.52 72.87 71.58 66.16 56.91 62.63
48.34 49.77 54.32 55.32 45.12 66.07 68.38 57.59 72.89 72.24 38.82 38.12 66.42 45.69 57.02 46.28 66.71 62.63 67.89 73.39 43.98 67.60 64.87 74.45 66.12 69.54 66.12 66.51 56.96 71.70
41.85 40.18 50.27 52.81 41.12 57.43 71.11 58.92 69.17 69.47 33.99 36.32 62.33 36.92 53.46 42.16 54.01 53.86 69.47 71.41 38.18 65.12 62.23 70.64 61.74 69.09 66.82 62.38 52.13 59.85
44.05 44.28 45.51 52.08 38.69 63.4 65.88 52.07 68.19 67.74 35.52 33.19 60.29 41.51 51.8 41.81 64.12 58.32 61.12 69.4 39.98 61.59 60.74 68.94 62.08 63.01 62.01 63.27 53.26 62.56
40.08 39.19 50.57 52.07 40.31 56.07 70.27 58.12 69.37 67.67 33.19 35.52 61.51 36.13 52.66 43.36 53.21 51.06 68.67 70.62 37.40 64.32 60.41 73.89 62.94 68.29 66.21 60.59 51.33 57.05
41.93 44.88 48.48 49.61 40.04 60.91 64.26 52.41 68.6 67.98 32.58 32.01 61.84 39.83 51.19 41.65 62.44 57.58 63.24 68.9 38.23 63.12 60.16 70.35 61.64 65.39 61.64 61.4 52.24 67.43
33.29 14.59 26.38 59.79 14.18 23.79 44.30 22.95 21.98 22.53 20.67 45.07 50.61 12.31 32.57 07.03 41.79 60.97 22.20 30.23 40.77 23.70 15.77 30.09 14.53 44.66 13.81 22.02 11.14 38.98
32.08 14.03 27.54 60.05 16.73 24.61 49.22 25.19 21.95 21.69 21.96 45.14 50.25 12.27 31.37 06.39 36.41 56.62 24.72 33.26 40.16 26.31 15.25 32.37 12.51 46.42 12.51 22.52 09.68 33.55
of H2 O2 and enhances hydroxyl radical (• OH) formation at acidic pH resulting higher removal. At very low pH, generation of oxonium ions (H3 O2 + ) occur [48] and at very high pH, decomposition of H2 O2 into water and oxygen take place resulting lower removal. In Fig. 4a, c, e and g, it is shown that removal of BA, TPA and COD increased and E.consumption decreased with pH and beyond optimum pH (i.e. 3.1), removals were decreased with high E.consumption.
4.3. Effect of current density and electrolysis time on removal of BA, TPA, COD and E. consumption Effects of current density and time on removal of BA, TPA, COD and E.consumption are shown in Figs. 3a–h and 4a–h by EO and EF processes respectively. Current density and electrolysis plays vital role during electrochemical treatment. Current densities (30.48–91.44 A/m2 ) and electrolysis time (10–90 min) were studied during electrolysis. Rate of generation of electrons and hydroxyl radicals increases with current density and time during electrochemical reactions resulting higher removal. It was found that after a while efficiencies decrease due to presence of excess amount of H2 O2 leading to the scavenging effect on • OH radicals [23,49,50]. H2 O2 + • OH → HO2 • + H2 O
(12)
Beyond optimum values (CD – 65.15 A/m2 , t – 58.02 min for EO and 54.39 A/m2 and 50.11 min for EF), percent removal decreases and E.consumption increases due to low generation of metal ions and • OH radicals as well as due to consumption of considerable amount of charges during electrolysis through some side reactions. Very high current density causes temperature rise of electrolytic solution and enhancement in E.consumption [51,52].
4.4. Effect of electrolyte and Fe2+ concentration on removal of BA, TPA, COD and E.consumption Effect of supporting electrolyte is an important parameter in electrochemical process. It is basically used for the enhancement of conductivity and electron transfer rate during electrolysis. In EO process, NaCl with different concentrations (0.5–1.5 g/L) was used as supporting electrolyte and obtained an optimum value (1 g/L). It was observed that removals were higher and E.consumption was lower at this optimum value during EO treatment as shown in Fig. 3b, d, e and h. This optimum value for electrolyte (1 g/L) was also used during EF treatment. Fe2+ ions concentration plays a big role in EF process. During EF treatment, Fe2+ ions accelerates hydroxyl radicals (• OH) generation, the main oxidizing agent and responsible for high removal as shown in Eq. (7) [53]. However, excess amount of Fe2+ ions causes • OH consumption during treatment resulting lower removal [54]. It can be clearly seen in Fig. 4b, d, f and h that initially removal increases and E.consumption decreases with Fe2+ concentration and beyond optimum value (i.e. 1 mmol/L) removal starts to decrease with higher E.consumption.
5. RSM study 5.1. Optimization Both EO and EF techniques were optimized to get maximum removal and minimum E.consumption. Operating conditions for both the processes with their experimental and CCD results are given in Tables 4 and 5. Optimum operating conditions was reconfirmed by performing experimental runs and obtained the optimized results as shown in Table 6. The model shows good adequacy due to closeness of CCD predicted values and experimental run results.
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Fig. 2. Effect of pH at different temperatures during acid precipitation treatment (a) on removal of BA (b), on removal of TPA, (c) on removal of COD.
5.2. ANOVA analysis 5.2.1. Coefficient of regression (R2 ), predicted R2 , adjusted R2 and adequacy of precision R2 is an important tool to check adequacy of the model. For a good fitting model R2 should be greater than 0.80 [55]. In this study R2 values for removal of BA: 0.93 (EO), 0.96 (EF); TPA: 0.89 (EO), 0.92 (EF); COD: 0.90 (EO), 0.91 (EF) and for E.consumption: 0.96 (EO), 0.97 (EF) as given in Table 7 indicate that good adequacy of the model. Predicted R2 value measures variation in the model predicted data. High adjusted R2 value shows the highly significant model and adequacy of precision indicates signal to
noise ratio which should be greater than 4 [56,57]. Values of predicted R2 , adjusted R2 and adequacy of precision are shown in Table 7.
5.2.2. PRESS (predicted residual error sum of squares), CV (coefficient of variation) F-value and P-value PRESS is sum of the squared differences of estimated and actual values over all points that measures fitting quality of model at each point. For a good model PRESS value should be less. In this study PRESS values for removal of BA, TPA, COD and E.consumption are not so high which shows a good model. CV values
Table 6 Optimum operating conditions predicted by CCD and experimental test run by EO and EF processes. pH
EO EF
4.6 3.1
CD (A/m2 )
65.15 54.39
Electrolyte Fe2+ concn concn (g/L) (mmol/L)
1 –
– 1
Time (min)
58.02 50.11
% removal of BA
% removal of TPA
% removal of COD
E.consumption (kwh/kg CODremoved )
CCD Pre.
Test run
CCD Pre.
Test Run
CCD Pre.
TEST run
CCD Pre.
Test run
70.76 80.45
67.21 75.67
68.52 76.83
64.01 70.91
67.27 73.70
64.34 71.73
31.01 19.39
33.51 21.23
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Fig. 3. Effect of pH, CD, electrolyte concentration and time on removal of BA, TPA, COD and E.consumption for EO process.
indicate about variation in actual and model predicted values and it should not more than 10. F-value tells about distribution of actual data around the fitted model and P-value explains significance of model terms. Lower values of F and P values are
desirable for a good model. P-value less than 0.05 indicate the significant model and greater than 0.1000 shows insignificant model term [58]. Values of PRESS, CV, F-values and P-values are given in Table 7.
Fig. 4. Effect of pH, CD, Fe2+ concentration and time on removal of BA, TPA, COD and E.consumption for EF process.
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Table 7 Quadratic model ANOVA analysis for removal of BA, TPA, COD and E.consumption. Variable
BA
Standard deviation Mean Coefficient of variance R2 Adjusted R2 Predicted R2 Press Adequacy of precision F-value Prob > F
TPA
COD
EF
EO
EF
EO
EF
EO
EF
5.54 50.94 7.87 0.93 0.88 0.85 1990.76 21.65 18.39 <0.0001
5.15 60.03 6.57 0.96 0.93 0.91 1471.06 20.82 14.18 <0.0001
5.26 47.59 8.05 0.89 0.86 0.82 1924.81 20.04 18.55 <0.0001
4.38 55.21 7.18 0.92 0.88 0.81 1557.56 24.56 16.16 <0.0001
5.51 49.09 10.08 0.90 0.88 0.81 2004.71 19.44 17.34 <0.0001
5.19 55.07 9.78 0.91 0.90 0.84 1889.98 17.53 14.76 <0.0001
3.94 32.31 6.91 0.96 0.94 0.90 1405.13 21.09 16.52 <0.0001
3.27 35.91 5.72 0.97 0.93 0.89 1131.77 19.16 15.70 <0.0001
5.2.3. Model equations based on ANOVA analysis Second-order polynomial equations in terms of independent variables were used for both electrochemical processes to express regression model. Generalized equation: Ri = b0 + b1 × pH + b2 × CD + b3 × Ci + b4 × t + b11 × pH2
(13)
(18)
R3 EF = 68.61 + 1.40 × pH + 5.10 × CD + 2.37 × C2 + 3.63
× t − 2.45 × CD × C2 − 0.95 × CD × t + 1.19 × C2 × t
(19)
E.consumption
BA
R4 EO = 29.14 + 7.32 × pH + 11.29 × CD − 1.68 × C1 + 14.08
R1 EO = 58.90 + 3.1 × pH + 1.84 × CD + 2.04 × C1 + 2.49 × t − 6.90 × pH2 − 3.40 × CD2 − 2.81 × C1 2 − 3.37 × t 2 + 3.1 × pH × CD + 0.17 × pH × C1 − 1.29 × pH × t − 2.25 × CD × C1 − 1.08 × CD × t + 1.11 × C1 × t
(14)
× t 2 + 1.76 × pH × CD + 0.24 × pH × C1 + 2.46 × pH × t + 1.92 × CD × C1 + 4.72 × CD × t − 0.94 × C1 × t
× t 2 − 3.13 × pH × CD + 0.10 × pH × C2 − 1.15 × pH × t − 2.25 (15)
× t 2 + 1.86 × pH × CD + 0.052 × pH × C2 + 1.92 × pH × t + 1.59 × CD × C2 + 3.97 × CD × t − 0.54 × C2 × t where R – response, C1 – electrolyte concentration, C2 – centration.
TPA R2 EO = 55.76 − 6.4 × pH + 2.20 × CD + 2.98 × C1 + 3.13 2
2
(20)
× t + 4.49 × pH2 + 1.82 × CD2 + 1.26 × C2 2 − 0.94
× t − 6.58 × pH2 − 3.23 × CD2 − 2.82 × C2 2 − 3.13
× CD × C2 − 1.14 × CD × t + 1.16 × C2 × t
× t + 5.59 × pH2 + 1.89 × CD2 + 1.35 × C1 2 − 1.59
R4 EF = 21.95 + 6.21 × pH + 8.60 × CD − 1.51 × C2 + 10.38
R1 EF = 72.89 + 1.29 × pH + 4.79 × CD + 2.17 × C2 + 3.63
(21) Fe2+
con-
6. Conclusion
2
× t − 6.12 × pH − 3.54 × CD − 3.48 × C1 − 3.03 × t 2 − 3.04 × pH × CD + 0.43 × pH × C1 − 0.79 × pH × t − 2.88 × CD × C1 − 1.19 × CD × t + 0.72 × C1 × t
(16)
R2 EF = 68.19 − 5.3 × pH + 4.10 × CD + 3.31 × C2 + 3.69 × t − 6.90 × pH2 − 3.35 × CD2 − 3.76 × C2 2 − 2.48 × t 2 − 4.64 × pH × CD + 1.12 × pH × C2 − 0.56 × pH × t − 0.69 × CD × C2 − 1.31 × CD × t + 1.29 × C2 × t COD R3
× t − 2.22 × CD × C1 − 1.36 × CD × t + 1.39 × C1 × t
× t 2 − 3.36 × pH × CD − 0.10 × pH × C2 − 1.28 × pH
+ b13 × pH × Ci + b14 × pH × t + b23 × CD × Ci + b24 × CD × t + b34 × Ci × t
× t 2 − 2.66 × pH × CD − 0.38 × pH × C1 − 1.38 × pH
× t − 6.89 × pH2 − 3.35 × CD2 − 2.99 × C2 2 − 3.33
+ b22 × CD2 + b33 × Ci 2 + b44 × t 2 + b12 × pH × CD
EO
E.consumption
EO
= 57.17 + 6.1 × pH + 1.96 × CD + 1.90 × C1 + 2.01
× t − 6.68 × pH2 − 3.14 × CD2 − 2.59 × C1 2 − 3.71
(17)
In the present study, comparison of EO and EF treatment was investigated for the removal of BA, TPA and COD with E.consumption. Initially acid precipitation was performed at different pH and temperature and obtained 48.7%, 83.4% and 57% of BA, TPA and COD removals respectively. The aqueous solution was re-treated by electrochemical (EO and EF) techniques. Effect of various parameters such as pH, CD, electrolyte concentration and time was studied for EO treatment and obtained maximum removal of BA, TPA and COD 70.76%, 68.52% and 67.27% with E.consumption – 31.01 kWh/kgCODremoved at optimum conditions (pH – 4.6, CD – 65.15 A/m2 , ec – 1 g/L, t – 58.02 min). In EF treatment operating parameters like pH, CD, Fe2+ concentration and time was investigated and obtained maximum removal of BA – 80.45%, TPA – 76.83% and COD – 73.70% with E.consumption – 19.39 kWh/kgCODremoved at optimum conditions (pH – 3.1, CD – 54.39 A/m2 , Fe2+ concentration – 1 mmol/L, t – 50.11 min). Closeness of CCD predicted values and experimental run results indicates good adequacy of the model.
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EF method was found more effective than EO process based on high removal efficiency and low E.consumption.
Acknowledgements Authors are grateful to the Department of Chemical Engineering, Indian Institute of Technology, Roorkee, for providing technical facilities and Ministry of Human Resource Development (Grant No. MHR02-23/41-103-427/429), New Delhi, India for financial support.
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A comparative study of electrochemical degradation of benzoic acid and terephthalic acid from aqueous solution of purified terephthalic acid (PTA) wastewater Vishal Kumar Sandhwar ∗ , Basheshwar Prasad Department of Chemical Engineering, Indian Institute of Technology Roorkee, Roorkee 247667, Uttarakhand, India
a r t i c l e
i n f o
Article history: Received 12 August 2016 Received in revised form 21 March 2017 Accepted 21 March 2017 Available online xxx Keywords: Benzoic acid Terephthalic acid Electrochemical oxidation Electro-Fenton Graphite electrode
a b s t r a c t In this article, studies have been performed on the treatment of benzoic acid (BA), terephthalic acid (TPA) and COD from synthetic binary solution of PTA wastewater. Acid precipitation pretreatment of aqueous solution was performed initially at different pH (2–5) and temperature (15–60 ◦ C). The acid treated solution was re-treated by electrochemical oxidation (EO) and electro-Fenton (EF) techniques using graphite electrodes. Independent operating parameters namely initial pH: (1–9), current density (A/m2 ): (30.48–91.45), electrolyte concentration (g/L): (0.5–1.5) and electrolysis time (min): (10–90) for EO process and pH (1–5), current density (A/m2 ): (30.48–91.45), Fe2+ concentration (mmol/L): (0.5–1.5) and electrolysis time (min): (10–90) for EF process were modeled and optimized by central composite design (CCD) in response surface methodology (RSM). The maximum removal efficiencies of the process during EF treatment were 80.45% of BA, 76.83% of TPA and 73.70% of COD with energy consumption (kWh/kg COD removed) – 19.39 at optimum operating conditions. During EC treatment removal capacities were 70.76%, 68.52% and 67.27% with 31.01 (kWh/kg COD removed) respectively. It was observed that EF process was more efficient than EO based on removals of BA, TPA and COD with minimum energy consumption. © 2017 Elsevier Ltd. All rights reserved.
1. Introduction Purified terephthalic acid (PTA) wastewater contains high concentrations of various aromatic compounds like benzoic acid (BA), para-toluic acid (p-TA), terephthalic acid (TPA), phthalic acid (PA), and 4-carboxybenzaldehyde (4-CBA) with low concentration of pxylene, methyl acetate and 4-formylbenzoic acid. Approximately 70–75% of total COD of PTA wastewater is contributed by these aromatic compounds [1–7]. United States Environmental Protection Agency (USEPA) has added these compounds in the priority pollutants list [8–10]. There is no specific discharge limit has been proposed for these aromatic compounds by any pollution regulating agencies in India. For petrochemical wastewater the permissible discharge limit of COD into surface waters has been prescribed by Central Pollution Control Board of India is less than 250 mg/L [11]. Various physico-chemical treatment techniques have been applied for PTA wastewater remediation in recent years. Among these treatment methods electrochemical technologies
∗ Corresponding author. E-mail address: [email protected] (V.K. Sandhwar).
have received much more attention due to its automation, high efficiency, versatility and cost effectiveness features [12–14]. Some of the previous studies on PTA wastewater treatment are discussed in Table 1 [15–22]. During electrochemical treatment, pollutants degradation is done by either indirect or direct oxidation processes. In indirect oxidation, electrochemically generated strong oxidants (like hydrogen peroxide and hypochlorite/chlorine) are used to degrade the pollutants through oxidation. In direct oxidation, degradation of pollutants is done through adsorption on the surface of anode and anodic electron transfer reaction [23,24]. Electrochemical treatment with graphite electrodes takes place by combination of direct oxidation on electrode surface as well as indirect oxidation of in situ generated oxidizing agents [25,26]. Electrochemical oxidation (EO) occurs through various oxidants such as hydrogen peroxide, nascent oxygen, free chlorine and radicals like Cl, ClO and hydroxyl radicals (OH• ). Generation of OH• takes place at anode by following reactions [24]. H2 O → H+ + OH• + e−
(1)
OH− → OH• + e−
(2)
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Please cite this article in press as: V.K. Sandhwar, B. Prasad, A comparative study of electrochemical degradation of benzoic acid and terephthalic acid from aqueous solution of purified terephthalic acid (PTA) wastewater, J. Water Process Eng. (2017), http://dx.doi.org/10.1016/j.jwpe.2017.03.006
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Table 1 Literature on treatment of purified terephthalic acid wastewaters by electrochemical methods. Wastewater (ww)
Electrode Type
Pollutants concentration
Synthetic ww (single component)
Al (as anode and cathode) and Fe (as anode and cathode)
BA – 400 mg/L COD – 706 mg/L
Boron-doped diamond Synthetic ww (single component) on silicon (as anode and cathode)
BA – 150 mg/L
Synthetic ww (single component)
Al (as anode and cathode) and Fe (as anode and cathode)
TPA – 400 mg/L COD –567 mg/L
Synthetic ww (single component)
Fe (as anode) Graphite (as cathode)
BA – 400 mg/L COD – 751 mg/L
Synthetic ww (multi component)
Al (as anode and cathode) and Fe (as anode and cathode)
BA – 400 mg/L TPA – 400 mg/L p-TA – 500 mg/L COD – 2055 mg/L
Synthetic ww (single component)
Fe (as anode) graphite (as cathode)
TPA – 400 mg/L COD – 584 mg/L
Industrial ww
Iron (as anode and cathode)
TPA – 400 mg/L COD – 560 mg/L
Industrial ww
SS (as anode and cathode)
COD – 1155 mg/L
Synthetic ww (binary component)
Graphite (as anode and cathode)
BA – 400 mg/L TPA – 400 mg/L COD – 1220 mg/L
Parameter values (optimum) Al Fe pH – 7.99 pH – 8.48 j – 145.26 A/m2 j – 155.30 A/m2 ec – 0.97 g/L ec – 1 g/L t – 53.92 min t – 57.18 min pH – 3.8 Current intensity – 18 A ec – 0.05 mol/L t – 120 min Fe Al pH – 7 pH – 7 j –84.33 A/m2 j – 91.47 A/m2 ec – 0.97 g/L ec – 0.97 g/L t – 37.57 min t – 53.12 min EC EF pH – 2.99 pH – 7.34 j – 50.97 A/m2 j – 44.87 A/m2 ec – 0.05 mol/L H2 O2 – 307 mg/L t – 58.86 min t – 55.10 min Al Fe pH – 8.0 pH – 8.18 j – 172.97 A/m2 j – 180.04 A/m2 ec – 1.69 g/L ec – 1.74 g/L t – 63.47 min t – 65.55 min EC EF pH – 3.20 pH – 7.44 j – 32.71 A/m2 j – 27.89 A/m2 ec – 0.04 mol/L H2 O2 – 142.7 mg/L t – 43.49 min t – 38.55 min pH – 7.44 j – 118 A/m2 ec – 400 mg/L gap – 10 mm pH – 8.2 j – 125 A/m2 gap – 1 cm t – 180 min EO EF pH – 3.1 pH – 4.6 j – 65.15 A/m2 j – 54.39 A/m2 ec – 1 g/L Fe2+ – 1 mmol/L t – 58.02 min t – 50.11 min
Removal (%)
Ref.
Al BA – 70.5 COD – 65.6
Fe 64.4 60.7
[15]
BA – 42.7
[16]
Al TPA – 89.7 COD – 83.8
Fe 73.8 67.6
[17]
EC TPA – 76.8 COD – 69.2
EF 88.5 82.2
[18]
Al BA – 59.5 p-TA – 45.7 TPA – 56.2
Fe 53.8 39.9 42.9
[19]
EC TPA – 82.7 COD – 78.3
EF 91.8 85.1
[20]
TPA – 77 COD – 72
[21]
COD – 66.40
EO BA – 70.7 TPA – 68.5 COD – 67.2
EF 80.4 76.8 73.7
[22]
Present study
j – current density, ec – electrolyte concentration, t – time.
However in dilute chloride solution, generation of anodic oxygen takes place as a primary reaction by following reaction [23,27]. 2H2 O → 4H+ + O2 + 4e−
(3)
Indirect oxidation in wastewater containing organic pollutants takes place, when the produced chlorine or hypochlorite gets reduced into chlorite ions, represented by following reactions [28]. 2Cl− → Cl2 + e−
(4)
+
−
Cl2 + H2 O → H + Cl + HOCl +
HOCl → H + OCl
−
(5) (6)
Among the electrochemical advanced oxidation processes (EAOPs), electro-Fenton (EF) process has got more attention for water treatment. EF is one of the most popular and eco-friendly treatment process based on Fenton’s reaction chemistry [29,30]. In EF process electrogeneration of H2 O2 takes place by reduction of oxygen at cathode in acidic solution with addition of Fe2+ ions according to Eq. (8) [31,32]. H2 O2 + Fe2+ → Fe3+ + OH− + • OH +
−
O2 + 2H + 2e → H2 O2
(7) (8)
PTA wastewater exerts acute, chronic and molecular toxicities and also responsible for damage of bladder, kidneys, liver and histopathological abnormalities in human beings. This study is based on the treatment of major constituent of PTA wastewater viz., BA and TPA. Acid precipitation has been explored to reduce the concentrations of BA and TPA in aqueous solution in this study. Acid precipitation is a comparatively economical process, which does not require much electricity. The acid pretreated solution was further subjected to electrochemical treatment process. Both electrochemical processes were optimized through central composite design (CCD) of response surface methodology (RSM) under Design Expert Software (DES). RSM is an excellent and effective statistical tool for the optimization of various processes, widely used for experimental designs. It allows less number of experiments with rapid interpretation [33–36]. In this study operating parameters such as pH, current density (CD), electrolyte concentration (ec) and electrolysis time (t) for EO process and pH, current density, Fe2+ concentration and electrolysis time for EF process were optimized for the removal of BA, TPA and COD with minimum energy consumption (E.consumption). The results of the present work have shown that this combined treatment process (acid precipitation and electrochemical) was very effective in removing/degrading BA and TPA from aqueous solution.
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2. Materials and methods 2.1. Chemicals All chemicals used in entire treatment were of analytical (AR) grade. TPA was procured from Himedia Lab Pvt. Ltd., Mumbai, India. Sodium chloride (NaCl) and BA were supplied by Loba Chemie Pvt. Ltd., Mumbai, India. Ferrous sulfate heptahydrate (FeSO4 ·7H2 O) was obtained from Merck. Sodium hydroxide (NaOH), potassium dichromate (K2 Cr2 O7 ), sulfuric acid (H2 SO4 ), methanol (CH3 OH), isopropyl alcohol (C3 H8 O), mercury (II) sulphate (HgSO4 ), silver sulphate (Ag2 SO4) and acetic acid (CH3 COOH) were obtained from Ranbaxy Fine Chemicals Limited, New Delhi (India). 2.2. Preparation of synthetic solution and sample analysis Stock solution of BA (1000 mg/L) and TPA (1000 mg/L) with distilled water was synthetically prepared. Entire reagents as well as samples were preserved at 4 ◦ C to avoid microorganism’s growth and biodegradation. Based on previous studies [21,37–39] working concentration of BA (400 mg/L) and TPA (400 mg/L) was taken. Initial COD of solution was obtained 1220 mg/L. Characterization of aqueous solution was done by standard methods [40]. TPA and BA concentrations were analyzed by High Performance Liquid Chromatography (HPLC) (Waters, USA) with UV detector (Waters 2487 absorbance detector, USA) at 240 nm wavelength [42–44]. HPLC was operated at ambient temperature in C18 column and mobile phase solution of 91% Milli-pore water, 7% isopropyl alcohol and 2% acetic acid with a flow rate of 1.2 mL/min in isocratic mode [45]. COD of solution was determined by COD analyzer (Aqualytic, Germany). All the samples were filtered through nylon syringe filter (0.21 m) before analysis. Removal of BA, TPA, COD and E.consumption (kWh/kg CODremoved ) were calculated by the following equations [41]. % Removal of BA, TPA and COD =
Ci − Cf Ci
× 100
(9)
where Ci and Cf are the initial and final concentrations of BA, TPA and COD. E.consumption (kWh/kgCODremoved ) =
VIT × 100 × 1000 (% Removal of COD)CCODi × VR
(10)
where V, I, T, and Vs are voltage, current (amp), time (hour) and solution volume (liter) respectively. 3. Experimental setup and procedure Initially, acid precipitation was carried out by mixing of H2 SO4 (1 N) to the solution at different temperatures (15–60 ◦ C) and pH (2–5). Then, the solution was allowed to settle for 4 h. After that the supernatant was filtered and retreated by electrochemical processes. Both electrochemical experiments (EO and EF) were done in a rectangular plexiglas batch cell of 1.6 L capacity. Plain graphite plates (100 mm × 80 mm × 3 mm) were used as anode and cathode to treat 1 L of solution during EO and EF treatments. The effective electrode area into the solution was 131.2 cm2 . Distance between parallel electrodes was set at 4 cm. Fig. 1 shows the schematic diagram of electrochemical setup. Direct current (0–4 A) and voltage (0–35 V) were used during electrochemical treatments. The supply mode of electric current was continuous for each individual run. After each and every experimental run, electrodes were cleaned and washed by HCl solution (15%, v/v) and water respectively. Both electrochemical experiments were performed at room temperature (25 ± 2 ◦ C) and atmospheric pressure. In EF treatment, prior to
Fig. 1. Schematic diagram of experimental set up for electrochemical treatment.
electrolysis air was bubbled continuously through fish aerator to saturate the solution with oxygen (O2 ) till the end of experiments. This dissolved oxygen leads to in situ generation of H2 O2 at the surface of cathode as in Eq. (8). A different concentration of ferrous ions was added during EF process. To determine the range of operating parameters during electrochemical treatment, some random experimental runs were conducted. Tables 2 and 3 show operating parameters ranges for EO and EF processes respectively. Both EO and EF experiments were conducted at operating conditions predicted by CCD as shown in Tables 4 and 5 respectively. 4. Results and discussion 4.1. Effect of acid precipitation on removal of BA, TPA and COD BA and TPA present in the ionized state in aqueous solution, as the pKa value for TPA are 3.51 and 4.82 and for BA it is 4.2. The initial pH (pHo = 6.9) of solution was decreased by the addition of 1 N H2 SO4 then TPA and BA get deionized. Hydrogen ions concentration also increases by lowering the pH of solution through which is responsible for the conversion of dissociated weak acid ions to undissociated acid molecules. And finally, due to common-ion (hydrogen ions) effect, ionic product values of TPA and BA surpass its solubility product value resulting precipitation [46]. Acids (TPA and BA) present in the wastewater facilitate agglomeration by reduction in pH and then settling is allowed for 4 h. After that the supernatant was filtered and used for further treatment. Precipitation of TPA and BA in the aqueous solution results concentration reduction of BA, TPA and COD in solution approximately 48.7%, 83.4% and 57% respectively at temperature 15 ◦ C and pH 2. Effect of temperature and pH were also studied as shown in Fig. 2a–c. The new concentrations in supernatant are found, BA – 205.2 mg/L, TPA – 66.4 mg/L and COD – 524.6 mg/L. The supernatant was re-treated by of EO and EF processes separately. 4.2. Effect of pH on removal of BA, TPA, COD and E.consumption Effect of pH on percent removal of BA, TPA, COD and E.consumption is shown in Figs. 3a, c, e, g and 4a, c, e, g by EO and EF methods respectively. pH is an important tool to influence the performance of EO and EF processes. Both EO and EF treatments were completed according to the CCD predicted sets as given in Tables 1 and 2. During electrochemical treatment, acidic medium favors generation of hydroxide radicals (• OH). In EO, formation of hypochlorous acid (HOCl) also takes place in the aqueous solution
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Table 2 Operating parameters and their levels obtained from the statistical software for EO process. Central composite design characteristics Levels
Parameter (range)
−2(−˛) −1 0 +1 +2(˛)
X1 pH(3–11)
X2 CD (A/m2 )(30.48 → 91.44)
X3 Electrolyte concentration (g/L)(0.5 → 1.5)
X4 Time (min)(10 → 90)
1 3 5 7 9
30.48 45.72 60.96 76.20 91.44
0.5 0.75 1 1.25 1.5
10 30 50 70 90
Table 3 Operating parameters and their levels obtained from the statistical software for EF process. Central composite design characteristics Levels
Parameter (range)
−2(−˛) −1 0 +1 +2(˛)
X1 pH(1–5)
X2 CD (A/m2 )(30.48 → 91.44)
X3 Fe2+ concentration (mmol/L)(0.5 → 1.5)
X4 Time (min)(10 → 90)
1 2 3 4 5
30.48 45.72 60.96 76.20 91.44
0.5 0.75 1 1.25 1.5
10 30 50 70 90
at low pH. HOCl is a dominant chlorine compound having high oxidation potential favors high removal with low energy consumption. OCl− + H2 O → HOCl + OH−
(11)
At neutral pH, formation of chlorate and perchlorate takes place by the oxidation of free chlorines as well as through combination of HOCl and hypochlorite which is undesirable in EO process
resulting lower removal efficiencies with higher energy consumption. At high pH, removal occurs due to less potent hypochlorite ions [23,47]. It is clearly shown in Fig. 3a, c and e that removal efficiencies of BA, TPA and COD were increased with pH and beyond optimum pH (i.e. 4.6) removal decreased. In EF process, air was bubbled continuously through the fish aerator before electrolysis to saturate the solution with oxygen that leads to in situ generation
Table 4 Actual and CCD predicted removal efficiencies of BA, TPA, COD and energy consumption for EO process. Run no.
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
Independent variables
% removal of BA: (R1 )
% removal of TPA: (R2 )
% removal of COD: (R3 )
E.consumption (kWh/kg CODremoved ): (R4 )
X1 : (pH)
X2 : (CD)
X3 : (ec)
X4 : (t)
Actual (Exp.)
CCD (Pre.)
Actual (Exp.)
CCD (Pre.)
Actual (Exp.)
CCD (Pre.)
Actual (Exp.)
CCD (Pre.)
3 7 3 7 3 7 3 7 5 5 3 9 3 7 3 7 3 7 5 5 1 7 5 5 5 5 5 5 5 5
45.72 45.72 76.20 76.20 45.72 45.72 76.20 76.20 60.96 60.96 45.72 45.72 76.20 76.20 45.72 30.48 76.20 76.20 60.96 60.96 60.96 60.96 45.72 76.20 60.97 91.44 60.96 45.72 45.72 76.20
0.75 0.75 0.5 1.5 1.25 1.25 1.25 1.25 1 1.25 0.75 0.75 0.75 1.25 1.25 1.25 1.25 1.25 0.75 1 1 1 1 1 1 1 1 1 1 0.75
70 30 30 70 30 70 70 30 50 50 30 70 70 10 90 30 50 70 50 70 50 50 50 50 30 50 30 70 30 50
37.07 35.31 46.59 48.09 36.32 52.09 66.29 54.14 65.32 63.69 29.21 31.52 57.53 32.12 48.68 37.38 49.23 47.08 64.69 66.62 33.42 60.32 56.42 67.82 57.96 64.31 62.02 56.61 47.35 53.07
38.86 40.67 44.46 45.9 35.83 56.38 60.11 48.36 64.43 63.83 28.81 28.25 57.64 35.72 47.47 36.17 58.39 53.41 59.42 64.83 34.28 58.82 56.01 66.06 57.28 60.91 57.28 57.49 47.78 63.31
33.57 28.91 42.19 42.69 33.92 51.24 60.89 51.74 61.92 60.29 26.82 29.12 56.13 29.72 46.28 34.38 45.83 44.68 60.29 62.32 33.01 55.92 52.12 65.42 55.56 59.92 57.62 52.23 49.97 49.97
35.53 34.05 40.93 41.06 33.75 54.41 55.43 45.34 60.84 59.92 25.18 25.73 55.43 33.58 44.58 33.22 54.21 50.66 54.81 61.73 34.32 55.75 52.01 62.54 53.9 57.16 53.89 54.14 59.39 59.39
35.62 33.83 44.77 46.85 34.85 50.15 63.82 52.67 62.85 62.23 27.74 30.05 53.06 30.65 47.21 35.91 46.76 45.61 63.24 63.15 31.93 58.85 54.95 63.35 56.49 62.84 60.55 55.13 51.62 51.62
37.21 39.65 42.69 44.72 34.72 54.66 57.17 47.32 62.31 62.01 27.74 26.79 53.51 34.14 46.16 34.61 56.08 51.41 57.41 61.98 32.51 57.16 54.54 63.78 55.2 58.99 55.21 55.58 61.14 61.14
37.46 16.90 29.80 66.45 16.40 26.60 48.78 25.33 24.26 24.50 24.73 53.28 58.67 14.51 36.33 08.49 47.56 68.26 24.11 33.80 47.75 25.91 17.34 32.53 16.19 48.53 15.11 24.20 43.08 43.08
36.51 15.99 31.35 66.7 19.45 27.49 55.26 27.63 23.97 23.39 25.74 53.38 57.65 14.53 34.69 7.87 40.73 63.35 27.2 37.18 47.14 29.14 16.34 35.39 13.94 50.59 13.93 24.89 11.02 36.75
Please cite this article in press as: V.K. Sandhwar, B. Prasad, A comparative study of electrochemical degradation of benzoic acid and terephthalic acid from aqueous solution of purified terephthalic acid (PTA) wastewater, J. Water Process Eng. (2017), http://dx.doi.org/10.1016/j.jwpe.2017.03.006
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Table 5 Actual and CCD predicted removal efficiencies of BA, TPA, COD and energy consumption for EF process. Run no.
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
Independent variables
% removal of BA: (R1 )
% removal of TPA: (R2 )
% removal of COD: (R3 )
E.consumption (kWh/kg CODremoved ): (R4 )
X1 : (pH)
X2 : (CD)
X3 : (Fe2+ concn )
X4 : (t)
Actual (Exp.)
CCD (Pre.)
Actual (Exp.)
CCD (Pre.)
Actual (Exp.)
CCD (Pre.)
Actual (Exp.)
CCD (Pre.)
2 4 2 4 2 4 2 4 3 3 2 5 2 4 2 4 2 4 3 3 1 4 3 3 3 3 3 3 3 3
45.72 45.72 76.20 76.20 45.72 45.72 76.20 76.20 60.96 60.96 45.72 45.72 76.20 76.20 45.72 30.48 76.20 76.20 60.96 60.96 60.96 60.96 45.72 76.20 60.97 91.44 60.96 45.72 45.72 76.20
0.75 0.75 0.5 1.5 1.25 1.25 1.25 1.25 1 1.25 0.75 0.75 0.75 1.25 1.25 1.25 1.25 1.25 0.75 1 1 1 1 1 1 1 1 1 1 0.75
70 30 30 70 30 70 70 30 50 50 30 70 70 10 90 30 50 70 50 70 50 50 50 50 30 50 30 70 30 50
46.63 44.76 56.15 57.65 45.68 62.18 73.85 63.71 72.88 71.25 38.77 41.08 67.19 41.68 58.24 46.94 58.79 56.64 72.25 74.18 42.96 69.58 65.98 74.31 67.52 72.87 71.58 66.16 56.91 62.63
48.34 49.77 54.32 55.32 45.12 66.07 68.38 57.59 72.89 72.24 38.82 38.12 66.42 45.69 57.02 46.28 66.71 62.63 67.89 73.39 43.98 67.60 64.87 74.45 66.12 69.54 66.12 66.51 56.96 71.70
41.85 40.18 50.27 52.81 41.12 57.43 71.11 58.92 69.17 69.47 33.99 36.32 62.33 36.92 53.46 42.16 54.01 53.86 69.47 71.41 38.18 65.12 62.23 70.64 61.74 69.09 66.82 62.38 52.13 59.85
44.05 44.28 45.51 52.08 38.69 63.4 65.88 52.07 68.19 67.74 35.52 33.19 60.29 41.51 51.8 41.81 64.12 58.32 61.12 69.4 39.98 61.59 60.74 68.94 62.08 63.01 62.01 63.27 53.26 62.56
40.08 39.19 50.57 52.07 40.31 56.07 70.27 58.12 69.37 67.67 33.19 35.52 61.51 36.13 52.66 43.36 53.21 51.06 68.67 70.62 37.40 64.32 60.41 73.89 62.94 68.29 66.21 60.59 51.33 57.05
41.93 44.88 48.48 49.61 40.04 60.91 64.26 52.41 68.6 67.98 32.58 32.01 61.84 39.83 51.19 41.65 62.44 57.58 63.24 68.9 38.23 63.12 60.16 70.35 61.64 65.39 61.64 61.4 52.24 67.43
33.29 14.59 26.38 59.79 14.18 23.79 44.30 22.95 21.98 22.53 20.67 45.07 50.61 12.31 32.57 07.03 41.79 60.97 22.20 30.23 40.77 23.70 15.77 30.09 14.53 44.66 13.81 22.02 11.14 38.98
32.08 14.03 27.54 60.05 16.73 24.61 49.22 25.19 21.95 21.69 21.96 45.14 50.25 12.27 31.37 06.39 36.41 56.62 24.72 33.26 40.16 26.31 15.25 32.37 12.51 46.42 12.51 22.52 09.68 33.55
of H2 O2 and enhances hydroxyl radical (• OH) formation at acidic pH resulting higher removal. At very low pH, generation of oxonium ions (H3 O2 + ) occur [48] and at very high pH, decomposition of H2 O2 into water and oxygen take place resulting lower removal. In Fig. 4a, c, e and g, it is shown that removal of BA, TPA and COD increased and E.consumption decreased with pH and beyond optimum pH (i.e. 3.1), removals were decreased with high E.consumption.
4.3. Effect of current density and electrolysis time on removal of BA, TPA, COD and E. consumption Effects of current density and time on removal of BA, TPA, COD and E.consumption are shown in Figs. 3a–h and 4a–h by EO and EF processes respectively. Current density and electrolysis plays vital role during electrochemical treatment. Current densities (30.48–91.44 A/m2 ) and electrolysis time (10–90 min) were studied during electrolysis. Rate of generation of electrons and hydroxyl radicals increases with current density and time during electrochemical reactions resulting higher removal. It was found that after a while efficiencies decrease due to presence of excess amount of H2 O2 leading to the scavenging effect on • OH radicals [23,49,50]. H2 O2 + • OH → HO2 • + H2 O
(12)
Beyond optimum values (CD – 65.15 A/m2 , t – 58.02 min for EO and 54.39 A/m2 and 50.11 min for EF), percent removal decreases and E.consumption increases due to low generation of metal ions and • OH radicals as well as due to consumption of considerable amount of charges during electrolysis through some side reactions. Very high current density causes temperature rise of electrolytic solution and enhancement in E.consumption [51,52].
4.4. Effect of electrolyte and Fe2+ concentration on removal of BA, TPA, COD and E.consumption Effect of supporting electrolyte is an important parameter in electrochemical process. It is basically used for the enhancement of conductivity and electron transfer rate during electrolysis. In EO process, NaCl with different concentrations (0.5–1.5 g/L) was used as supporting electrolyte and obtained an optimum value (1 g/L). It was observed that removals were higher and E.consumption was lower at this optimum value during EO treatment as shown in Fig. 3b, d, e and h. This optimum value for electrolyte (1 g/L) was also used during EF treatment. Fe2+ ions concentration plays a big role in EF process. During EF treatment, Fe2+ ions accelerates hydroxyl radicals (• OH) generation, the main oxidizing agent and responsible for high removal as shown in Eq. (7) [53]. However, excess amount of Fe2+ ions causes • OH consumption during treatment resulting lower removal [54]. It can be clearly seen in Fig. 4b, d, f and h that initially removal increases and E.consumption decreases with Fe2+ concentration and beyond optimum value (i.e. 1 mmol/L) removal starts to decrease with higher E.consumption.
5. RSM study 5.1. Optimization Both EO and EF techniques were optimized to get maximum removal and minimum E.consumption. Operating conditions for both the processes with their experimental and CCD results are given in Tables 4 and 5. Optimum operating conditions was reconfirmed by performing experimental runs and obtained the optimized results as shown in Table 6. The model shows good adequacy due to closeness of CCD predicted values and experimental run results.
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Fig. 2. Effect of pH at different temperatures during acid precipitation treatment (a) on removal of BA (b), on removal of TPA, (c) on removal of COD.
5.2. ANOVA analysis 5.2.1. Coefficient of regression (R2 ), predicted R2 , adjusted R2 and adequacy of precision R2 is an important tool to check adequacy of the model. For a good fitting model R2 should be greater than 0.80 [55]. In this study R2 values for removal of BA: 0.93 (EO), 0.96 (EF); TPA: 0.89 (EO), 0.92 (EF); COD: 0.90 (EO), 0.91 (EF) and for E.consumption: 0.96 (EO), 0.97 (EF) as given in Table 7 indicate that good adequacy of the model. Predicted R2 value measures variation in the model predicted data. High adjusted R2 value shows the highly significant model and adequacy of precision indicates signal to
noise ratio which should be greater than 4 [56,57]. Values of predicted R2 , adjusted R2 and adequacy of precision are shown in Table 7.
5.2.2. PRESS (predicted residual error sum of squares), CV (coefficient of variation) F-value and P-value PRESS is sum of the squared differences of estimated and actual values over all points that measures fitting quality of model at each point. For a good model PRESS value should be less. In this study PRESS values for removal of BA, TPA, COD and E.consumption are not so high which shows a good model. CV values
Table 6 Optimum operating conditions predicted by CCD and experimental test run by EO and EF processes. pH
EO EF
4.6 3.1
CD (A/m2 )
65.15 54.39
Electrolyte Fe2+ concn concn (g/L) (mmol/L)
1 –
– 1
Time (min)
58.02 50.11
% removal of BA
% removal of TPA
% removal of COD
E.consumption (kwh/kg CODremoved )
CCD Pre.
Test run
CCD Pre.
Test Run
CCD Pre.
TEST run
CCD Pre.
Test run
70.76 80.45
67.21 75.67
68.52 76.83
64.01 70.91
67.27 73.70
64.34 71.73
31.01 19.39
33.51 21.23
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Fig. 3. Effect of pH, CD, electrolyte concentration and time on removal of BA, TPA, COD and E.consumption for EO process.
indicate about variation in actual and model predicted values and it should not more than 10. F-value tells about distribution of actual data around the fitted model and P-value explains significance of model terms. Lower values of F and P values are
desirable for a good model. P-value less than 0.05 indicate the significant model and greater than 0.1000 shows insignificant model term [58]. Values of PRESS, CV, F-values and P-values are given in Table 7.
Fig. 4. Effect of pH, CD, Fe2+ concentration and time on removal of BA, TPA, COD and E.consumption for EF process.
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Table 7 Quadratic model ANOVA analysis for removal of BA, TPA, COD and E.consumption. Variable
BA
Standard deviation Mean Coefficient of variance R2 Adjusted R2 Predicted R2 Press Adequacy of precision F-value Prob > F
TPA
COD
EF
EO
EF
EO
EF
EO
EF
5.54 50.94 7.87 0.93 0.88 0.85 1990.76 21.65 18.39 <0.0001
5.15 60.03 6.57 0.96 0.93 0.91 1471.06 20.82 14.18 <0.0001
5.26 47.59 8.05 0.89 0.86 0.82 1924.81 20.04 18.55 <0.0001
4.38 55.21 7.18 0.92 0.88 0.81 1557.56 24.56 16.16 <0.0001
5.51 49.09 10.08 0.90 0.88 0.81 2004.71 19.44 17.34 <0.0001
5.19 55.07 9.78 0.91 0.90 0.84 1889.98 17.53 14.76 <0.0001
3.94 32.31 6.91 0.96 0.94 0.90 1405.13 21.09 16.52 <0.0001
3.27 35.91 5.72 0.97 0.93 0.89 1131.77 19.16 15.70 <0.0001
5.2.3. Model equations based on ANOVA analysis Second-order polynomial equations in terms of independent variables were used for both electrochemical processes to express regression model. Generalized equation: Ri = b0 + b1 × pH + b2 × CD + b3 × Ci + b4 × t + b11 × pH2
(13)
(18)
R3 EF = 68.61 + 1.40 × pH + 5.10 × CD + 2.37 × C2 + 3.63
× t − 2.45 × CD × C2 − 0.95 × CD × t + 1.19 × C2 × t
(19)
E.consumption
BA
R4 EO = 29.14 + 7.32 × pH + 11.29 × CD − 1.68 × C1 + 14.08
R1 EO = 58.90 + 3.1 × pH + 1.84 × CD + 2.04 × C1 + 2.49 × t − 6.90 × pH2 − 3.40 × CD2 − 2.81 × C1 2 − 3.37 × t 2 + 3.1 × pH × CD + 0.17 × pH × C1 − 1.29 × pH × t − 2.25 × CD × C1 − 1.08 × CD × t + 1.11 × C1 × t
(14)
× t 2 + 1.76 × pH × CD + 0.24 × pH × C1 + 2.46 × pH × t + 1.92 × CD × C1 + 4.72 × CD × t − 0.94 × C1 × t
× t 2 − 3.13 × pH × CD + 0.10 × pH × C2 − 1.15 × pH × t − 2.25 (15)
× t 2 + 1.86 × pH × CD + 0.052 × pH × C2 + 1.92 × pH × t + 1.59 × CD × C2 + 3.97 × CD × t − 0.54 × C2 × t where R – response, C1 – electrolyte concentration, C2 – centration.
TPA R2 EO = 55.76 − 6.4 × pH + 2.20 × CD + 2.98 × C1 + 3.13 2
2
(20)
× t + 4.49 × pH2 + 1.82 × CD2 + 1.26 × C2 2 − 0.94
× t − 6.58 × pH2 − 3.23 × CD2 − 2.82 × C2 2 − 3.13
× CD × C2 − 1.14 × CD × t + 1.16 × C2 × t
× t + 5.59 × pH2 + 1.89 × CD2 + 1.35 × C1 2 − 1.59
R4 EF = 21.95 + 6.21 × pH + 8.60 × CD − 1.51 × C2 + 10.38
R1 EF = 72.89 + 1.29 × pH + 4.79 × CD + 2.17 × C2 + 3.63
(21) Fe2+
con-
6. Conclusion
2
× t − 6.12 × pH − 3.54 × CD − 3.48 × C1 − 3.03 × t 2 − 3.04 × pH × CD + 0.43 × pH × C1 − 0.79 × pH × t − 2.88 × CD × C1 − 1.19 × CD × t + 0.72 × C1 × t
(16)
R2 EF = 68.19 − 5.3 × pH + 4.10 × CD + 3.31 × C2 + 3.69 × t − 6.90 × pH2 − 3.35 × CD2 − 3.76 × C2 2 − 2.48 × t 2 − 4.64 × pH × CD + 1.12 × pH × C2 − 0.56 × pH × t − 0.69 × CD × C2 − 1.31 × CD × t + 1.29 × C2 × t COD R3
× t − 2.22 × CD × C1 − 1.36 × CD × t + 1.39 × C1 × t
× t 2 − 3.36 × pH × CD − 0.10 × pH × C2 − 1.28 × pH
+ b13 × pH × Ci + b14 × pH × t + b23 × CD × Ci + b24 × CD × t + b34 × Ci × t
× t 2 − 2.66 × pH × CD − 0.38 × pH × C1 − 1.38 × pH
× t − 6.89 × pH2 − 3.35 × CD2 − 2.99 × C2 2 − 3.33
+ b22 × CD2 + b33 × Ci 2 + b44 × t 2 + b12 × pH × CD
EO
E.consumption
EO
= 57.17 + 6.1 × pH + 1.96 × CD + 1.90 × C1 + 2.01
× t − 6.68 × pH2 − 3.14 × CD2 − 2.59 × C1 2 − 3.71
(17)
In the present study, comparison of EO and EF treatment was investigated for the removal of BA, TPA and COD with E.consumption. Initially acid precipitation was performed at different pH and temperature and obtained 48.7%, 83.4% and 57% of BA, TPA and COD removals respectively. The aqueous solution was re-treated by electrochemical (EO and EF) techniques. Effect of various parameters such as pH, CD, electrolyte concentration and time was studied for EO treatment and obtained maximum removal of BA, TPA and COD 70.76%, 68.52% and 67.27% with E.consumption – 31.01 kWh/kgCODremoved at optimum conditions (pH – 4.6, CD – 65.15 A/m2 , ec – 1 g/L, t – 58.02 min). In EF treatment operating parameters like pH, CD, Fe2+ concentration and time was investigated and obtained maximum removal of BA – 80.45%, TPA – 76.83% and COD – 73.70% with E.consumption – 19.39 kWh/kgCODremoved at optimum conditions (pH – 3.1, CD – 54.39 A/m2 , Fe2+ concentration – 1 mmol/L, t – 50.11 min). Closeness of CCD predicted values and experimental run results indicates good adequacy of the model.
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EF method was found more effective than EO process based on high removal efficiency and low E.consumption.
Acknowledgements Authors are grateful to the Department of Chemical Engineering, Indian Institute of Technology, Roorkee, for providing technical facilities and Ministry of Human Resource Development (Grant No. MHR02-23/41-103-427/429), New Delhi, India for financial support.
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