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Modeling and Optimization of Electrocoagulation Process for the Removal of Yellow145 dye Based on Central Composite Design.

Byline: Abdul Rauf Shah, Hajira Tahir and Sonia Sadiq

Keywords: Electrocoagulation process, Response surface methodology, Central composite design, Yellow145, FTIR, Cost analysis.

Introduction

The textile industrial sector consumes large volume of water during dyeing and finishing operations. This sector also discharges large portion of effluent into the natural resources of water. The textile effluent contains many dyes which cause environmental and health problems. Azo dyes are used in the textile, paint, ink and plastic industries. These dyes show stability against bio-degradation [1-4]. Electrochemical-based systems are controlled and rapid process. They need only electrons to treat wastewater. They do not require chemicals and micro-organisms [5, 6]. Among the electrochemical techniques, (EC) process has a wide applications for the treatment of industrial effluents.

The theory of EC and chemical coagulation is almost similar. In EC, the coagulant of Al3+, Fe3+ ions are supplied by applying an electric current to the metal anodes kept in a EC reactor [7, 8]. Firstly, the metal anode is oxidized as follows:

M(S) a M(aq)n+ + ne-

Additionally, the electrolysis of water takes place at the cathode and anode as follows:

2H2O(l) + 2e- a H2 (g) + 2OH- at cathode

2H2O(l) a 4H+ +O2(g) +4e- at anode

The formed chemical species of dissolved metal cations destabilize the contaminants, particulate suspension, and also break emulsions. The generation of hydrogen gas is also promote the floatation process to remove the flocculated particles out of the water.

Furthermore, the hydroxide ions formed at the cathode increase the pH of the EC system and induce the formation of precipitates of metal ions with the hydroxides as follows:

M(aq)n+ + nOH- a M(OH)n(s)

Eventually, the destabilized particles in the EC system undergo aggregation to form flocs [9].

In case of using iron electrodes, the following two mechanisms have been proposed for the formation of the metal hydroxide [10]:

Mechanism-1

The reactions at anode:

4Fe(S) + 8 e- a 4Fe+2

4Fe+2 + 10H2O + O2 a 4Fe(OH)3(S)+8H+

The reactions at cathode:

8H+ + 8e- a 4H2(g)

Overall reactions:

4Fe (S) +10H2O + O2 a 4Fe(OH) 3(S) + 4H 2(g) (8)

nFe(OH) 3(S) a Fe n (OH) 3n(s)

Mechanism-2

The reaction at anode:

Fe (S) - 2e- a Fe +2(aq)

Fe +2(aq) + 2OH- a Fe(OH) 2

The reaction at cathode:

2H 2 O + 2 e- a 2OH - + 4H 2(g)

Overall reactions:

Fe (S) + 2H2O a Fe(OH) 2 + H 2(g)

nFe(OH) 2(S) a Fe n (OH) 2n(s)

The way of removing dye from wastewater.

Precipitation:

Dye+[Monomeric Fe ] a [Dye-Monomeric Fe ]

Dye + Polymeric-Fe a [Dye-Polymeric Fe]

Adsorption:

Dye + Fe n (OH) n a Sludge(s)

[Dye-Polymeric Fe ] + Fe n (OH) n a Sludge(s)

The main reactions at the electrodes in case of Al-Al electrodes combination are:

Al a Al +3 at anode (19)

3H 2 O + 3e- a 3/2H 2 + 3OH - at cathode

At high pH, the formation of hydrogen gas at the cathode generates OH - ions [11].

2Al + 6H 2 O + 2OH - a 2Al(OH) 4- +3H 2

The formed aluminium and hydroxide ions react to form various monomeric species which are finally changed into Al(OH) 3 according to complex precipitation kinetics. The following interaction mechanisms are possible between the molecules of a dye and hydrolysis by-products [12, 13].

Precipitation:

Dye + [Monomeric Al] a [Dye-Monomeric Al]s, pH = 4-5

Dye+PolymericAl a [Dye-Polymeric Al]s, pH=5-6

Adsorption:

Dye + Al(OH)3 a a Particle (24)

[Dye-Polymeric Al]s + Al(OH) 3(s) a Particle

These flocs polymerise as:

nAl(OH)3 a Aln(OH) 3n

In EC process the excessive amount of coagulants is not added owing to their direct generation by electro-oxidation of a sacrificial anode. EC equipment is simple and easy to operate and also require short reaction time and low sludge production [14].

The EC process is optimized to determine the best levels of the factors to get maximum efficiency of the process in the treatment of aqueous discharges. Recently, the experiments based on statistical design are used to determine interactions among the factors to optimize the operating parameters in multivariable analysis. They are also used to obtain statistically significant models by performing the minimum number of experiments. RSM is used for modeling and optimizing a response of a process. It is also analyzed the ways in which several variables influence a response of interest [15]. The CCD, the design under RSM, is an efficient method. It provides more detailed information regarding the data obtained from a treatment process in fewer number of experiments.

This article present the results of the laboratory scale studies on the removal of Y145 dye from simulated wastewater using coupled iron-iron (Fe-Fe) and aluminium-aluminium (Al-Al) electrodes by EC process. The study will provide guidance in operating and designing reactors for the treatment of large volume of wastewater.

Materials and methods

Experimental setup and procedure

A laboratory-scale glass EC setup was used. For each test, a 1000 mL of Y145 dye sample was used. Before each run, acetone was used to wash the electrodes and the impurities were removed by dipping in a solution of HCl solution [16]. All the chemicals used to be of analytical-reagent grade. The EC experiments were conducted by applying a voltage of 15V by means of a DC power supply (Yaxun 1502DD; 15V, 2A). At the end of each run, the sample was collected and filtered to remove floated and precipitated materials. After that, the filtrate was analyzed with T80 UV/VIS spectrophotometer. The dye and the obtained sludge were characterized by FTIR model NICOLET 6700 [17].

Experimental design and model development

The optimum levels of the factors of EC process were found in three analytical steps; adequacy of various model tests (sequential model sum of squares and model summary statistics), ANOVA, and the response surface plotting for the removal of Y145 from simulated wastewater. The data were analyzed with the Minitab Software 17. A total of 31 experiments were conducted in this study. The levels of the variables are given in Table 1. Table 2 is representing the treatments and corresponding the removal of Y145 by using two separate electrodes systems. The linear, quadratic, and combined effects of the operational parameters on the removal of the dye were described by a second-order polynomial as given below:

(Equation)

Where Y is the predicted response and considered as a dependent variable. The X i and X j show the independent variables. While, [beta]0 is the constant coefficient, and [beta]i, [beta]ij and [beta]ii are the coefficients of linear, interaction and quadratic terms, respectively [18].

Table 1. The levels of the factors of EC process.

Independent###-oe###-1###0###+1###+oe

Variables

[A] [dye]###100###150###200###250###300

(mg/L)

[B] NaCl (g/L)###1###1.25###1.5###1.75###2

[C] pH###3###5###7###9###11

[D]###10###15###20###25###30

Electrolysis

Time (min)

Table 2. The full-factorial design used for the removal of Y145 dye using EC.

[dye]###NaCl###pH###Electrolysis###Fe-Fe###Al-Al

(mg/L)###(g/L)###Time(min)

250###1.25###9###15###100###83

250###1.75###5###25###100###92

150###1.25###9###25###100###87

250###1.25###9###25###100###82

100###1.5###7###20###96###87

250###1.75###9###25###100###90

150###1.75###5###25###96###87

200###1.5###7###10###100###99

200###1.5###7###20###99###80

300###1.5###7###20###100###52

250###1.25###5###15###96###87

150###1.25###5###15###100###92

250###1.25###5###25###96###87

250###1.75###5###15###100###95

200###1.5###7###20###100###96

200###2###7###20###93###75

150###1.75###9###15###89###94

150###1.25###9###15###97###78

200###1.5###7###30###100###81

200###1.5###7###20###99###87

150###1.25###5###25###100###92

200###1.5###11###20###100###89

150###1.75###9###25###94###83

200###1.5###7###20###96###86

250###1.75###9###15###99###76

200###1.5###7###20###96###86

200###1.5###3###20###95###99

200###1.5###7###20###100###76

200###1.5###7###20###96###86

200###1###7###20###100###81

150###1.75###5###15###94###93

Table 3. ANOVA for the removal of Y145 dye using Fe-Fe and Al-Al electrode combinations.

###Fe-Fe###Al-Al

Source###DF###Adj SS###AdjMS###F-Value###P-Value###DF###Adj SS###AdjMS###F-Value###P-Value

Model###14###194.003###13.8573###5.11###0.001###14###1383.34###98.81###1.51###0.213

Linear###4###82.167###20.5417###7.57###0.001###4###562.33###140.5###2.15###0.122

A###1###35.042###35.0417###12.91###0.002###1###294.00###294.0###4.49###0.050

B###1###40.042###40.0417###14.76###0.001###1###4.17###4.167###0.06###0.804

C###1###2.042###2.0417###0.75###0.399###1###216.00###216.0###3.30###0.088

D###1###5.042###5.0417###1.86###0.192###1###48.17###48.16###0.74###0.404

Square###4###13.461###3.3653###1.24###0.334###4###756.26###189.0###2.89###0.056

AA###1###0.152###0.1520###0.06###0.816###1###292.17###292.2###4.47###0.051

BB###1###5.737###5.7371###2.11###0.165###1###32.83###32.82###0.50###0.489

CC###1###1.120###1.1201###0.41###0.530###1###245.25###245.2###3.75###0.071

DD###1###5.216###5.2159###1.92###0.185###1###106.36###106.3###1.63###0.220

2-Way Interaction###6###98.375###16.3958###6.04###0.002###6###64.75###10.79###0.16###0.983

AB###1###60.062###60.0625###22.13###0.000###1###2.25###2.250###0.03###0.855

AC###1###18.063###18.0625###6.66###0.020###1###4.00###4.000###0.06###0.808

AD###1###5.062###5.0625###1.87###0.191###1###20.25###20.25###0.31###0.586

BC###1###10.563###10.5625###3.89###0.066###1###1.00###1.000###0.02###0.903

BD###1###1.562###1.5625###0.58###0.459###1###12.25###12.250###0.19###0.671

CD###1###3.062###3.0625###1.13###0.304###1###25.00###25.000###0.38###0.545

Error###16###43.417###2.7135###*###*###16###1046###65.41###*###*

Lack-of-Fit###10###21.417###2.1417###0.58###0.784###10###813.17###81.32###2.09###0.190

Pure Error###6###22.000###3.6667###*###*###6###233.43###38.905###*###*

Total###30###237.419###*###*###*###30###2429.94###*###*###*

Table 4. Student t and p-values for the removal of Y145 dye using Fe-Fe and Al-Al electrode combinations.

###Fe-Fe###Al-Al

Term###Effect###Coef###SECoef###T-Value###P-Value###VIF###Effect###Coef###SECoef###T-Value###P-Value###VIF

Constant###98.00###0.623###157.40###0.000###85.29###3.06###27.90###0.000

A###2.417###1.208###0.336###3.590###0.002###1.00###-7.00###-3.50###1.65###-2.12###0.050###1.00

B###-2.583###-1.292###0.336###-3.840###0.001###1.00###0.83###0.42###1.65###0.25###0.804###1.00

C###0.583###0.292###0.336###0.870###0.399###1.00###-6.00###-3.00###1.65###-1.82###0.088###1.00

D###0.917###0.458###0.336###1.360###0.192###1.00###-2.83###-1.42###1.65###-0.86###0.404###1.00

AA###-0.146###-0.073###0.308###-0.240###0.816###1.03###-6.39###-3.20###1.51###-2.11###0.051###1.03

BB###-0.896###-0.448###0.308###-1.450###0.165###1.03###-2.14###-1.07###1.51###-0.71###0.489###1.03

CC###-0.396###-0.198###0.308###-0.640###0.530###1.03###5.86###2.93###1.51###1.94###0.071###1.03

DD###0.854###0.427###0.308###1.390###0.185###1.03###3.86###1.93###1.51###1.28###0.220###1.03

AB###3.875###1.937###0.412###4.700###0.000###1.00###0.75###0.37###2.02###0.19###0.855###1.00

AC###2.125###1.062###0.412###2.580###0.020###1.00###-1.00###-0.50###2.02###-0.25###0.808###1.00

AD###-1.125###-0.562###0.412###-1.370###0.191###1.00###2.25###1.12###2.02###0.56###0.586###1.00

BC###-1.625###-0.812###0.412###-1.970###0.066###1.00###0.50###0.25###2.02###0.12###0.903###1.00

BD###0.625###0.313###0.412###0.760###0.459###1.00###-1.75###-0.87###2.02###-0.43###0.671###1.00

CD###0.875###0.437###0.412###1.060###0.304###1.00###2.50###1.25###2.02###0.62###0.545###1.00

Table 5. Model summary of EC process for the removal of Y145 dye.

Statistical parameters###Fe-Fe###Al-Al

S(square mean of error)###1.647###8.088

R2###81.71%###56.93%

R2(adj)###65.71%###19.24%

R2 (pred)###35.43%###0.000%

Table 6. The IR absorption bands for the given functional groups in the dye.

Functional groups###Wave number

###(cm-1)

NH###3450

CH, Arom###3096

C=C-###1481

N=N###1435

-C=N-###1587

C-N###1221

Sulfonates###1365-1340/1200-1100

Disulfides (CS stretch)###705-570

Sulfate ion###1130-1080/680-610

Results and discussion

Statistical analysis and optimization

EC process using Fe-Fe and Al-Al electrodes

The experimental results obtained by CCD were fitted to the second-order (quadratic) polynomial response surface model. The following regression equations with the coded variables were obtained using Fe-Fe and Al-Al electrodes for the removal of Y145 dye:

Fe-Fe=134.1-0.2260A-8.3 B+ 0.28C - 0.823 D-0.000029 AA-7.17BB - 0.0495 CC+0.0171 DD+ 0.1550 AB+ 0.01062AC - 0.00225AD - 1.625 BC+ 0.250BD+ 0.0437 CD

Al-Al = 112 + 0.341 A + 57.6 B - 14.00 C -4.09D - 0.001279 AA - 17.1 BB + 0.732 CC+ 0.0771 DD+ 0.030AB - 0.0050 AC + 0.00450AD + 0.50 BC - 0.70BD + 0.125 CD

The synergistic effect of the factors is shown by the positive sign of the coefficients in equation (28,29), whereas the negative sign suggests antagonistic effect [18].

Statistical testing of the model was evaluated by the ANOVA. The data are tabulated in Table 3. It can be noted that the regression model was significant for Fe-Fe electrodes, while insignificant for Al-Al electrodes. The significance of the parameters is given in Table 4. It was also encountered that the concentration of the dye (A), and amount of electrolyte (B) were showing linearly and combined effects on the removal of the dye in Fe-Fe electrodes system. In the second electrode system, the concentration of the dye (A) has a linear and quadratic effect on the removal of the dye. The significance of the model can be analyzed in Table 5 [19-24].

Residual Plots

In the normal probability plots the data were around the normal line it mean the second order polynomial model was well fitted to the data. The residuals were lying between 1 to -1 in Fe-Fe electrode system while in Al-Al electrode system the residuals were between 10 to -10. The residual plots are shown in Fig. 2-3.

Main interaction plots

The main interaction plots were showed that as the levels of the factors were changing the removal of Y145 dye was also changing. It was more prominent in Fe-Fe electrode system as compared to Al-Al electrode system as shown in Fig 5a-b.

Full interaction plots

The lines in the full interaction plots were not parallel to both X-axis and Y-axis. This indicated that the levels of the factors were varying the removal of Y145 dye during the EC process using Fe-Fe and Al-Al electrodes systems as shown in Fig. 6a-b. The effect of factors on the percent removal of Y145 dye using Fe-Fe and Al-Al electrode combinations.

The effect of dye concentration

To observe the effect of initial dye concentration on the removal of the dye using two electrode systems by EC, the experiments were carried out for five different dye concentrations (100,150, 200, 250 and 300 mg/L). The higher removals of the dye were obtained using Fe-Fe electrodes as compared to Al-Al electrodes. Furthermore, at the low concentration of the dye the removal was greater as compared to presence of the high concentration of the dye. Because, the high concentration requires the high quantity of coagulant for coagulation in a fixed period of time [25]. The effect of concentration of the dye combined with other factors on the removal efficiency of the dye is shown in Fig. 7, and Fig. 8.

The effect of NaCl

The effect of amount of supporting electrolyte (NaCl) on the removal of Y145 dye is given in Fig. 7 and 8. It can be ascertained from Fig. 7a-d that the dye removal efficiency was not increasing with an increase in the supporting electrolyte concentration in Fe-Fe electrode system while it was increasing using Al-Al electrode system as shown in Fig. 8a-d. The formation of chlorine gas and OCl- owing to the oxidation of chloride ions at the anode, additionally, provides support for the removal of contaminants in the EC system. Thus, the supporting electrolyte not only increases the conductivity but also contributes as a strong oxidizing agent [26].

The effect of pH

The pH of the EC system significantly affects the efficiency of EC process. The pH determines the type of hydroxide of metal cations in the EC system and also determines the interaction between the formed coagulants and the molecules of dye in the solution. Consequently, It influences the mechanism of the removal of dye from the aqueous system. In order to investigate the effect of the pH of the solution on the removal of the dye, the experiments were performed by adjusting the initial pH such as 3, 5, 7, 9 and 11. Fig. 7 and Fig. 8 are showing that the removal of the dye was higher in the acidic condition as compared to the basic environment of the EC system. Because, the acidic condition increases the rate of formation of Fe+3 cations in the system [27]. The effect of pH of the dye solution combined with other factors on the removal efficiency of the dye is shown in Fig. 7 and 8.

The effect of electrolysis time

The removal efficiency of the dye was increasing with the increase of electrolysis time for both the electrode combinations. It was also in agreement with the reported data [28, 29]. The effect of electrolysis time combined with other factors on the removal efficiency of the dye is shown in Fig. 7 - 8.

The effect of nature of electrode system

In EC process, the electrodes of Al and Fe were used. They are easily available, low cost, and undergo better dissolution [30]. The published data of some authors pointed out to the benefits of Fe electrodes, while others pointed out to the advantages of using Al anodes. It is also pointed out that Fe is relatively cheaper [31]. Fe forms more denser flocks of coagulated contaminants as compared to Al because it is heavy in mass [32]. Furthermore, Fe has better complex formation nature with inorganic/organic pollutants than Al [33]. However, it is also reported that Al/Al electrode combinations effectively utilized for the removal of dyes and Fe/Fe electrode worked better for COD and phenol removal, while Al/Fe and Fe/Al electrode combinations were successfully applied for the treatment of paper mill wastewater [34]. It is found that around 9 carbon atoms are complexed by one Fe atom, whereas 3 carbon atoms are allowed for complexation by an Al atom.

It is also determined that "coordination" numbers of Fe or Al largely depend on the wastewater to be treated. Therefore, the efficiency of the type of electrode is depend upon the type of effluent [35]. In this work Fe-Fe electrode system provided the data of higher removal of Y145 dye as compared to Al-Al electrode system. Therefore, Fe-Fe electrode system will be proved more effective for the removal of the dye in the real effluent.

FT-IR analysis

Fig. 9 shows the FT-IR spectrum of Y145 dye. The IR absorption bands of Y145 dye are given in Table 6.

Conclusion

In the present study, CCD was successfully applied to optimize the parameters of the EC process for the removal of Y154 dye from the simulated wastewater using Fe-Fe and Al-Al electrodes. The optimum operating parameters for the Fe-Fe electrode system were 100 mgL-1 of dye concentration, 1.0 gL-1 of NaCl, pH of 5.0 and 30 min of electrolysis time. The maximum removal of the dye was found to be 99.21%. On the other hand, the optimum operating parameters for the Al-Al electrode system were 120 mgL-1 of dye concentration, 1.66 gL-1 of NaCl, pH of 3.0 and 10 min of electrolysis time. The percent removal of the dye was found to be 98.24%. Furthermore, It was also encountered that the concentration of the dye (A), amount of electrolyte (B) were showing linearly and combined effect on the removal of the dye in Fe-Fe electrodes system.

In the second electrode system, the concentration of the dye (A) has a linear and quadratic effect on the removal of the dye. It is concluded that the process can be effectively applied for the removal of Y145 dye from wastewater at large scale.

Acknowledgment

The author acknowledges the financial assistance from the University of Karachi, Karachi, Pakistan.

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Author:Abdul Rauf Shah, Hajira Tahir and Sonia Sadiq
Publication:Pakistan Journal of Analytical and Environmental Chemistry
Date:Dec 31, 2019
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