Anal. Methods Environ. Chem. J. 5 (2) (2022) 39-50

Research Article, Issue 2

Analytical Methods in Environmental Chemistry Journal

Journal home page: www.amecj.com/ir

AMECJ

Response surface modeling for the treatment of methylene

blue from aqueous media using electro-Fenton process

before determination by UV-Vis spectrometer: Kinetic and

degradation mechanism

Sara Zahedi a, Ali Asadipour b, Maryam Dolatabadi c, Saeid Ahmadzadeh d, e, *

a Student Research Committee, Kerman University of Medical Sciences, Kerman, Iran.

b Department of Medicinal Chemistry, Faculty of Pharmacy, Kerman University of Medical Sciences, Kerman, Iran.

c Environmental Science and Technology Research Center, Department of Environmental Health Engineering, School of

Public Health, Shahid Sadoughi University of Medical Sciences, Yazd, Iran.

d Pharmaceutics Research Center, Institute of Neuropharmacology, Kerman University of Medical Sciences, Kerman, Iran.

e Pharmaceutical Sciences and Cosmetic Products Research Center, Kerman University of Medical Sciences, Kerman, Iran

ABSTRACT

In the present study, response surface methodology was employed

to investigate the effects of main variables, including the initial

MB concentration, hydrogen peroxide dosage, current density, and

electrolysis time on the removal efciency of MB using the electro-

Fenton (EF) process. The MB concentration determination by UV-

Vis spectrometer. The EF process degrades the MB contaminant

molecules by the highly oxidizing species of the •OH. A quadratic

regression model was developed to predict the removal of MB, where

the R2 value was found to be 0.9970, which indicates the satisfactory

accuracy of the proposed model. ANOVA analysis showed a non-

signicant lack of t value (0.0840). Moreover, the predicted

correlation coefcient values (R2=0.9915) were reasonably in

agreement with the adjusted correlation coefcient value (R2=0.9958),

demonstrating a highly signicant model for MB dye removal. In

addition, the obtained results showed 95.8% MB was removed in the

optimum removal efciency, including the initial MB concentration

of 20 mg L-1, H2O2 dosage of 400 μL, and the current density of 7.0

mA cm-2, and electrolysis time of 10 min which was agreed with the

predicted removal efciency of 98.3%. Electrical energy consumption

was found to be 0.163 kWh m-3. The constant rate value of Kapp at the

optimum operating condition was 0.3753 min-1.

Keywords:

Electro-Fenton process,

UV-Vis spectrometer,

Methylene blue,

Response surface methodology,

Degradation,

Kinetic

ARTICLE INFO:

Received 3 Mar 2022

Revised form 29 Apr 2022

Accepted 16 May 2022

Available online 29 Jun 2022

*Corresponding Author: Saeid Ahmadzadeh

Email: saeid.ahmadzadeh@kmu.ac.ir

https://doi.org/10.24200/amecj.v5.i02.178

------------------------

1. Introduction

Due to the increasing use of tens of thousands of

various types of dyes, which are employed in plastic

products, cosmetics, leather, and food industries,

and their release of about over 100 tons into the

aqueous environments, a severe global concern has

been raised for efcient removal of such organic

micropollutants before releasing the industrial

wastewater into the environmental medium [1,

2]. The adverse effects of toxic dyes on human

beings, biota, microorganisms, and environmental

health, even at decient concentrations on the one

hand, and their high resistance to photochemical

and biological degradation, on the other hand, the

necessity has required to eliminate the synthetic

dyes contaminant from efuents [3-5]. Among the

high-consumption dyes, methylene blue (MB),

a derivative of phenothiazine, is widely used as

a redox indicator, biological staining reagent,

medication to treat methemoglobinemia, and

adjunct employed in endoscopic polypectomy.

Besides, it is used extensively in textiles dyeing

industries of cotton, wool, and silk. Exposure of

methylene blue to the eyes causes irreversible

damage of partial blindness [6, 7]. Moreover,

its inhalation causes respiratory problems. Its

ingestion orally causes a burning sensation in

the gastrointestinal tract and induces nausea,

vomiting, profuse sweating, as well as mental

confusion, and methemoglobinemia [8-11].

There are many techniques such as UV-VIS

spectrometer, gas chromatography, and liquid

chromatography for determining MB, dye,

tyrosine kinase, toluene, benzene, and organic

materials in different matrixes [12-17]. During

the last decades, several treatments techniques,

including coagulation, adsorption, and ltration

processes, have been developed for the removal

of dye contaminants which suffered from the

disadvantages such as high operational costs

due to the production of a massive amount of

sludge or solid waste and transferring the dyes

to another phase by concentrating them rather

than eliminating them [18, 19]. The advanced

oxidation processes (AOPs) as an economical

and high efcient approach provide a valuable

opportunity to overcome the limitations of the

conventional treatment process by generating

a robust oxidative agent of hydroxyl radicals,

which degrade and mineralize the dye molecules

into their less-toxic forms and harmless inorganic

products such as water or carbon dioxide [20-22].

As seen from the proposed reaction in Equation

1, Fenton’s reagent of Fe2+ and H2O2 generates

the oxidative agents of hydroxyl radicals under

the acidic medium of solution pH less than 3.

The Fe2+ species are generated from the oxidation

of iron anode electrode and reduction of Fe3+

ions through cathodic reaction as described by

Equation 2 [23, 24].

(Fenton’s reaction) (Eq. 1)

(Eq. 2)

In the current work, response surface methodology

(RSM) was employed as a computational approach

to optimize the removal efciency of MB using

the EF process. The effect of main operational

variables, including electrolysis time, MB initial

concentration, H2O2 dosage, and current density

throughout the treatment process, were evaluated,

and the kinetics of the process were studied.

2. Material and methods

2.1. Chemical

All chemicals used in the MB removal study are

analytical (high purity) grade without further

purication. C16H18ClN3S (Methylene blue),

H2O2 (30%, v/v), Na2SO4, H2SO4, and NaOH

were obtained from Merck® Co. (CH3)3COH,

and CH3OH were obtained from Sigma Aldrich®

Co. The investigation samples in this study were

prepared in double-distilled water (DDW).

2.2. EF process system

The Pyrex cylindrical reactor with dimensions

of 9.0 cm (height) × 7.0 cm (diameter) and a

volume capacity of 300 mL was used to carry out

MB removal studies using the EF process. The

iron electrodes with a thickness of 0.1 cm and an

immersed area of 12 cm2 were used as cathode and

anode electrodes. The distance between electrodes

(anode and cathode) was considered constant and

equal to 3.0 cm. In a typical run, MB solution was

degraded in the reactor with 50 mM Na2SO4 as the

supporting electrolyte. The pH of the MB solution

was adjusted to 3.0 by 0.01 M of H2SO4. The rotation

speed was kept at 200 rpm during the experiment.

2.3. Analytical methods

The MB concentration of samples before and after

the treatment process was measured by a UV-

Anal. Methods Environ. Chem. J. 5 (2) (2022) 39-50

Vis spectrometer (OPTIZEN 3220 model) at 670

nm [1]. Evaluations for the degradation and MB

removal efciency and total amount of electrical

energy consumption (kWh m-3) were calculated

from the Equation (3) and (4) [25-27]:

(Eq. 3)

(Eq. 4)

where C0 (MB concentration before EF process;

mg L–1), Ct (MB concentration after EF process;

mg L–1), U (applied voltage; V), I (electrical

current; A), t (electrolysis time; h), and V (volume

of sample;L).

2.4. Application of Response Surface

Methodology

Response Surface Methodology (RSM) is a

widely used mathematical and statistical approach

employed to model, design, and evaluate the

relationship between several independent variables

and responses of the proposed model. The goal

is to optimize the applied reaction in a short

time and reduce the costs of the process. Unlike

conventional methods for data analysis, RSM

analyzes data using simple techniques based on the

mathematical model. In the RSM, to optimize the

studied variables, a polynomial function (often a

quadratic polynomial model) can be used, as given

in Equation 5 [27, 28]

(Eq. 5)

where Y is the predicted response (MB removal

%), k is the number of independent factors, β0,

βi, βii, and βij are the constant, linear, quadratic,

and interaction model coefcients, respectively,

also, Xi, Xj, and ɛ are the independent factors and

the error. Analysis of variance (ANOVA) with a

95% condence interval was used to determine

the signicance of the parameters. In the current

study, the RSM approach was used to investigate

four variables’ effects (initial MB concentration,

H2O2 dosage, current density, and electrolysis time)

and identify the optimal condition for MB removal

using the EF process employing Design-Expert

Software Version 11.0.4.0.

3. Result and discussion

3.1. Development of models and analysis of

variance (ANOVA) using RSM

Table 1 describes the experimental condition of 30

runs designed by RSM and the obtained responses

of the developed model in each run summarized.

The mentioned removal efciencies of MB are the

averages of duplicate runs in each experimental

condition.

Some valuable parameters, including lack of t of

the model, the signicance of linear and interaction

effects of operating variables, and coefcient

of determination, were evaluated by analysis of

variance (ANOVA). As seen in Table 2, the p-value

less than 0.0001 corresponds to the signicant

coefcients.

As seen, the obtained results from ANOVA

analysis conrmed that the developed quadratic

regression model was satisfactorily tted to the

removal efciency of the treatment process with

the F-value of 858. Since the calculated p-value

of the Lack of Fit was less than 0.05, it conrmed

that the lack of t was insignicant, corresponding

to the pure error. The predicted and adjusted R2 of

0.9915 and 0.9958 are in reasonable agreement.

Furthermore, the tted model’s prediction

ability over the experimental condition range is

acceptable due to the obtained predicted R2 of

about 100%. The normality of the residuals was

analyzed and demonstrated in Figure 1(a). As

seen, all the demonstrated data are relatively near

to a straight line with the R2 of 0.9970. Moreover,

the association between the obtained values of

actual removal efciencies and predicted removal

efciencies were revealed in Figure 1(b). As seen,

since the residual results were distributed near

the diagonal line, it can be concluded that the

proposed treatment model successfully predicted

the removal efciencies.

Modeling and determination of methylene blue by UV-Vis Sara Zahedi et al

42 Anal. Methods Environ. Chem. J. 5 (2) (2022) 39-50

Table 1. Experimental values for the removal efciency of MB from the CCD.

Run

order

Actual values Coded values

Removal

(%)

(mg L-1)

(µL)

(mA cm-2)

(min) X1X2X3X4

140 387 8.2 5.2 +1 +1 +1 -1 70.6

230 275 6.5 8.5 0 0 0 0 82.9

3 30 275 6.5 8.5 0 0 0 0 84.1

420 387 4.7 11.7 -1 +1 -1 +1 85.2

5 20 162 8.2 11.7 -1 -1 +1 +1 71.0

640 387 4.7 11.7 +1 +1 -1 +1 64.0

7 20 162 8.2 5.2 -1 -1 +1 -1 66.6

8 30 275 10.0 8.5 0 0 +2 0 67.2

9 20 387 4.7 5.2 -1 +1 -1 -1 82.3

10 10 275 6.5 8.5 -2 0 0 0 99.8

11 30 500 6.5 8.5 0 +2 0 0 77.1

12 30 50 6.5 8.5 0 -2 0 0 41.3

13 30 275 6.5 8.5 0 0 0 0 84.5

14 40 162 8.2 11.7 +1 -1 +1 +1 59.3

15 20 162 4.7 11.7 -1 -1 -1 +1 63.3

16 40 387 8.2 11.7 +1 +1 +1 +1 75.2

17 50 275 6.5 8.5 +2 0 0 0 69.8

18 30 275 3.0 8.5 0 0 -2 0 47.1

19 40 162 4.7 11.7 +1 -1 -1 +1 49.7

20 20 387 8.2 11.7 -1 +1 +1 +1 95.7

21 40 162 4.7 5.2 +1 -1 -1 -1 45.5

22 30 275 6.5 8.5 0 0 0 0 84.1

23 40 162 8.2 5.2 +1 -1 +1 -1 56.1

24 20 162 4.7 5.2 -1 -1 -1 -1 58.6

25 20 387 8.2 5.2 -1 +1 +1 -1 90.9

26 30 275 6.5 8.5 0 0 0 0 83.7

27 30 275 6.5 2.0 0 0 0 -2 67.5

28 30 275 6.5 15.0 0 0 0 +2 79.2

29 40 387 4.7 5.2 +1 +1 -1 -1 61.3

30 30 275 6.5 8.5 0 0 0 0 84.4

The regression model developed for the removal

efciency of MB is represented in Equation 6. The

proposed second-order model in the term of coded

factors by eliminating the insignicant terms is

expressed as follows (6):

(Eq. 6)

Here, X1 represents initial MB concentration, X2

is H2O2 dosage, X3 is current density, and X4 is

electrolysis time. As seen, the intensity of each

particular variable on the removal efciency as the

response of the proposed model is identied by the

related magnitude of each variable’s coefcient. Each

coefcient’s positive or negative value indicates

the synergistic or antagonistic effect of the related

variable on the response. As seen, the coefcients

Inte rnally Stude ntiz ed Re s iduals

Normal % Probability

Actual (%)

Predict (%)

100

30 40 50 60 70 80 90 100

-3.0 -2.0 -1.0 0.0 +1.0 +2.0 +3.0

(a)

(b)

Table 2. ANOVA of the tted polynomial model.

Source Sum of

Squares

Degree of

freedom (df)

Mean

square F-value p-value

Model 6580 8 822 858 < 0.0001

X11535 11535 1602 < 0.0001

X22142 12142 2237 < 0.0001

X3555 1 555 579 < 0.0001

X4125 1 125 130 < 0.0001

X1X272 1 72 76 < 0.0001

21087 11087 1135 < 0.0001

21276 1 1276 1332 < 0.0001

2204 1204 214 < 0.0001

Residual 20 21 0.96 - -

Lack of Fit 18 16 1.15 3.54 0.0840

Pure Error 1.6 5 0.33 - -

Core Total 6600 29 - - -

Mean: 71.65 R2: 0.9970

Coefcient of Variance: 1.37% Adj.R2: 0.9958

Standard Deviation: 0.98 Pred. R2: 0.9915

Fig. 1. (a) Normal probability plot of studentized residuals,

(b) Predicted removal efciencies vs. experimental removal efciencies.

Modeling and determination of methylene blue by UV-Vis Sara Zahedi et al

of X2 (H2O2 dosage), X3 (current density), and X4

(electrolysis time) are all positive. Moreover, it can

be concluded that the variables of X1 and X2 with

much larger coefcients play more signicant roles

in MB’s removal efciency as the model’s response.

3.2. Effect of parameter on the MB removal

efciency

In the present study, the effect of initial MB dye

concentration on removal efciency using the EF

process was investigated. As Figure 2 shows, the

initial MB dye concentration is inversely related to

the MB removal efciency, when increasing the initial

MB dye concentration from 10 mg L-1 to 500 mg

L-1, and removal efciency decreases from 99.8% to

68.2%. This phenomenon can be due to the decrease

in the ratio of oxidant agents to MB dye molecules.

For example, if all the parameters studied in the EF

process, are constant (at high MB dye concentrations),

the number of dye molecules is greater than the

oxidizing agents. Therefore, the number of oxidizing

agents is insufcient to remove high concentrations

of the dye molecule, and consequently, the removal

efciency decreases [29, 30].

Optimizing the amount of H2O2 in all treatment

methods in which H2O2 plays a key role is

very important because it affects the removal

efficiency, the management of the treated

effluent, and the cost. The effects of H2O2 dosage

in the range of 50-500 µL on MB removal were

investigated at the initial MB concentration of 30

mg L-1, the current density of 6.5 mA cm-2, and

electrolysis time of 8.5 min, the result is shown

in Figure 3 the obtained results show that the

increase in degradation percentage from 40.7%

to 78.1% whenever H2O2 increased from 50 to

500 µL, at the initial MB concentration of 30

mg L-1, the current density of 6.5 mA cm-2, and

electrolysis time of 8.5 min.

Excess concentration of H2O2 causes the

spontaneous auto composition of H2O2to H2O

and O2 molecules through Equation(7), as well

as the decomposition of •OH, produced during

the EF reaction to radicals with lower oxidation

strength (HO2

• ions) according to Equation (8)

and (9) which reduces the removal efficiency

[31, 32].

Anal. Methods Environ. Chem. J. 5 (2) (2022) 39-50

10.0

8.3

6.5

4.8

3.0

10 20 30 40 50

Curre nt De nsity (mA cm

-2

)

Initial MB Concentration (mg L

-1

)

Fig. 2. Contour plots of the initial MB concentration and current density.

(Eq. 7)

(Eq. 8)

(Eq. 9)

Current density is one of the most effective

parameters that affects the kinetic rate and removal

efciency. Increasing the current density more than

the optimal value causes more electrical energy

consumption and heat generation and consequently

adverse effects on the removal process, so its value

must be optimized. Due to the importance and

inuence of current density in the EF process, the

effect of applied current density was studied in the

range of 3-10 mA cm-2. The removal of MB dye

depended on the current density.

The increase in current density from 2 to 5.5 mA

cm-2 offered faster MB dye degradation (MB

removal efciency of 47.5% to 66.9%). Increasing

the dye removal efciency with increasing current

density can be attributed to the fact that increasing

the current density increases the amount of iron

generated by the anode electrode. The increase in

removal and degradation rates from 3 mA cm-2 to

the optimum value of 7.0 mA cm-2 can be related to

the acceleration of Fenton’s Reaction (Equation1).

Therefore, increasing the concentration of Fe2+

produced could lead to an increase in •OH generated

through (Equation 1). These •OH generated reacts

immediately with the MB dye, increasing the MB

removal efciency.

However, more than the optimal value of current

density leads to a decrease in the MB removal

efciency in the EF reactor. This negative effect

is due to the role of Fe2+ as the scavenger of •OH

(Equation 10) [32, 33]. Hence, 7.0 mA cm-2 was

selected as the optimal value of current density.

(Eq. 10)

The effect of electrolysis time in the range of 2-15

min on MB removal was studied (Fig. 4). According

to the results; MB removal efciency was directly

related to electrolysis time, so with increasing

electrolysis time, the removal efciency increases.

From the start EF process until 9 min after electrolysis

time, due to in presence of sufcient H2O2 and

Fe2+, a large number of hydroxyl radicals (•OH) are

generated according to the Fenton’s reaction, which

Removal (%)

Fig. 3. 3Dplot of the initial MB concentration and H2O2 dosage.

Modeling and determination of methylene blue by UV-Vis Sara Zahedi et al

subsequently leads to the MB dye degradation and

increased removal efciency. As the electrolysis

time increases, the concentration of H2O2 in the

electrochemical reactor decreases until the reactor

is free of H2O2, consequently, the Fenton’s reaction

slows down and eventually stops [34].

3.3. Optimization process

The optimization process was carried out over the

upper to lower limits of each operating parameter’s

value to determine the best treatment condition

for maximum MB removal at a reasonable cost.

The highest MB removal efciency of 98.3% was

predicted at the optimum condition of initial MB

concentration of 20 mg L-1, H2O2 dosage of 400 μL,

the current density of 7.0 mA cm-2, and electrolysis

time of 10 min, which was in good accordance with

the obtained experimental MB removal efciency.

3.4. Kinetic model of hydroxide radical assisted

EF process

The kinetic studies of the treatment process

revealed that hydroxyl radical reaction with

MB contaminant follows the pseudo-rst-order

reaction, which indicates that •OH directly attacked

the contaminant molecules. The hydroxyl radicals

are generated and consumed continuously at a

similar rate to provide a steady-state concentration

of active radicals throughout the treatment process.

The apparent constant rate of Kapp could be evaluated

by the pseudo-rst-order reaction equation and

corresponds to the slope of demonstrated plot

in Figure 5, with the value of 0.3753 min-1 [35].

Figure 5 revealed the variation of Ln (C0/Ct) in

the function of time during the 15 min treatment

process with the initial concentration of MB 20 mg

L-1, the current density of 7.0 mA cm-2, and H2O2

dosage of 400 µL L-1.

4. Conclusion

Herein, to achieve an economical treatment

process for the efcient removal of MB, the

response surface methodology was employed. The

analysis of variance (ANOVA) at the condence

level of 95% was carried out to evaluate the

signicance of the independent variables and their

interactions. The obtained results revealed that the

initial MB concentration and H2O2 dosage affected

the removal efciency signicantly. The obtained

experimental removal efciency of 95.8% by the

EF process was in satisfactory agreement with

the predicted removal efciency of 98.3% by the

Anal. Methods Environ. Chem. J. 5 (2) (2022) 39-50

Removal (%)

Fig. 4. The 3D plot of the current density and electrolysis time effects on the MB removal efciency

developed quadratic regression treatment model.

The kinetic analysis showed that the applied

treatment process followed a pseudo-rst-order

model.

5. Acknowledgements

The authors would like to express their appreciation

to the student research committee of Kerman

University of Medical Sciences [Grant number

400000747] for supporting the current work.

Funding: This work received a grant from the

Kerman University of Medical Sciences [Grant

number 400000747].

Conict of interest: The authors declare that they

have no conict of interest regarding the publication

of the current paper.

Ethical approval: The Ethics Committee of

Kerman University of Medical Sciences approved

the study (IR.KMU.REC.1400.510).

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