# Degradation of Pesticide Chlorothalonil by Visible Light-Responsive Photocatalyst Ferrioxalate andH2O2 under Solar Irradiation

Authors: Malay Chaudhuri; Hafizi Zuhali; Augustine Chioma Affam
Journal: International Journal of Photoenergy
 (2013)

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## Abstract

Ferrioxalate is a visible light-responsive photocatalyst. The solar ferrioxalate/H2O2 process has high degradation efficiency because ferrioxalate is able to absorb light strongly at longer wavelength and generates hydroxyl radical with high quantum yield. Degradation of pesticide chlorothalonil in aqueous solution by ferrioxalate/H2O2 under solar irradiation was examined. The optimum operating conditions for treatment of a 300 mg/L chlorothalonil aqueous solution were obtained by using the central composite design of the response surface methodology. Under the optimum operating conditions (H2O2/COD molar ratio 2.75, H2O2/Fe3+ molar ratio 75, H2O2/C2H2O4 molar ratio 37.5, reaction time 90 min, and pH 3),  COD, NH3-N, and TOC removal of 75.71, 47.11, and 54.33%, respectively, was achieved and the biodegradability (BOD5/COD ratio) improved from zero to 0.42. Model prediction and actual removal were in close agreement (<4% error). The solar ferrioxalate/H2O2 process is effective in pretreatment of the chlorothalonil aqueous solution for biological treatment.


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## Body

## 1. Introduction

Chlorothalonil is a broad-spectrum of organochlorine pesticide (fungicide) used to prevent foliar diseases in vegetable and ornamental crops in agricultural fields [1] and belongs to class IV of the World Health Organization classification of pesticides [2]. It is classified as a probable carcinogen, and the 4-hydroxy chlorothalonil transformation product is more soluble, more stable, and, for some species, more toxic than its parent compound [3]. Chlorothalonil has the potential to contaminate water bodies adjacent to its point of use by spray drift, runoff, or sediment transport. Chlorothalonil has been detected in surface water [4, 5], inland waterways [6], natural water [7], rainfall [8], and air samples [9] generally adjacent to agricultural areas where it was applied. Chlorothalonil is sufficiently persistent to undergo long-range atmospheric transport on a regional scale [10] and can also exhibit chemical stability and resistance to biodegradation [11–13]. Reports have indicated that a minimum of 50 μg/L concentration of pesticide is toxic to guppy fish [14].Advanced oxidation processes (AOPs) constitute a promising technology for the treatment of water and wastewater containing recalcitrant organic compounds with high toxicity and low biodegradability [15]. Oxidation technologies have shown that a partial oxidation of toxic water may increase its biodegradability [16, 17]. Oxidation with Fenton’s reagent is based on hydroxyl radical (OH∙) produced by catalytic decomposition of hydrogen peroxide (H2O2) in reaction with ferrous ion (Fe2+) [18]. In the photo-Fenton process, the rate of OH∙ radical formation is increased by photoreactions of H2O2 and/or Fe3+ that produce OH∙ radical directly or regenerate Fe2+ [19], thus increase the efficiency of the process. Ferrioxalate is a visible light-responsive photocatalyst. The solar ferrioxalate/H2O2 process has high degradation efficiency because ferrioxalate is able to absorb light strongly at longer wavelength and generates OH∙ radical with high quantum yield [20]. The quantum yield of Fe2+ regeneration greatly increases when Fe2+ complexes with a carboxylic anion, such as oxalate [21]. The ferrioxalate complex, FeIII(C2O4)33-, is highly photosensitive, and reduction of Fe3+ to Fe2+, through a photo-induced ligand to metal charge transfer, can occur over the ultraviolet and into the visible (out to λ ~ 550 nm):
(1)FeIII(C2O4)33-+hν⟶Fe2++2C2O42-+C2O4∙-(2)C2O4∙-⟶CO2∙-+CO2(3)CO2∙-+FeIII(C2O4)33-⟶Fe2++CO2+3C2O42-The reactions can be collapsed into one reaction, since the short lifetime of the oxyl radical,C2O4∙-, should preclude it from participation in other reactions, and its decarboxylation product, CO2∙-, is not involved in any other significant reactions:
(4)FeIII(C2O4)33-+hν⟶Fe2++CO2+2.5C2O42-There are no other significant photochemical reactions (e.g.,H2O2 photolysis) because the molar extinction coefficients of the reactants are such that ferrioxalate is the predominant absorber. The Fe2+ produced then generates OH∙ radical via the Fenton reaction:
(5)Fe2+H2O2+3C2O42-⟶FeIII(C2O4)33-+OH-+OH∙In the presence of a sufficient excess of oxalate,Fe3+ will coordinate with either two or three oxalate ligands. As with the photo-Fenton reaction, iron cycles between oxidation states and the production of hydroxyl radical is limited only by the availability of light, H2O2, and oxalate, the latter two of which are depleted during the reaction. UV-vis/ferrioxalate/H2O2 treatment of aniline wastewater [22], dyehouse waste [23], reactive dyes [24], orange II [25], and phenolic pollutants [26] have been reported. There is no report on degradation of pesticide chlorothalonil by solar ferrioxalate/H2O2 process.The present study examined degradation of pesticide chlorothalonil in aqueous solution by solar ferrioxalate/H2O2 process in terms of chemical oxygen demand (COD), ammonia nitrogen (NH3-N) and total organic carbon (TOC) removal, and biodegradability (BOD5/COD ratio) improvement. The treatment was optimized by using the central composite design (CCD) of the response surface methodology (RSM).


## 2. Materials and Methods

### 2.1. Chemicals and Pesticide

Hydrogen peroxide (30%, w/w), oxalic acid(C2H2O4·2H2O), and ferric sulfate (Fe2(SO4)3) were purchased from R&M Marketing, Essex, UK. The pesticide chlorothalonil used to prepare aqueous solution was obtained from a commercial source and was used as received. Figure 1 shows the chemical structure of chlorothalonil.Figure 1
Chemical structure of chlorothalonil.


### 2.2. Analytical Methods

Chemical oxygen demand (COD) was determined according to Method 5220D (closed reflux, colorimetric method) of the Standard Methods [27], where the sample contained hydrogen peroxide (H2O2), to reduce interference in COD determination, pH was increased to above 10 so as to decompose hydrogen peroxide to oxygen and water [28, 29]. TOC analyzer (Model 1010, O & I Analytical) was used for determining total organic carbon (TOC). The pH was measured by a pH meter (HACH sension 4) and a pH electrode (HACH platinum series pH electrode model 51910, HACH Company, USA). Biodegradability was measured by 5-day biochemical oxygen demand (BOD5) test according to Method 5210B (seeding procedure) of the Standard Methods [27]. The treated pesticide aqueous solution was adjusted to pH 7 before the BOD5 test. Ammonia nitrogen (NH3-N) was measured by the Nessler method [30]. DO was measured using YSI 5000 dissolved oxygen meter. The seed for the BOD5 test was obtained from a municipal wastewater treatment plant.


### 2.3. Chlorothalonil Aqueous Solution

Chlorothalonil aqueous solution was 300 mg/L of chlorothalonil in distilled water. It was prepared weekly and stored at 4°C. The characteristics of the aqueous solution were COD 350 mg/L,NH3-N 1.58 mg/L, and TOC 94.49 mg/L. The high chlorothalonil concentration was chosen to reflect the concentration in pesticide-manufacturing wastewater and to explore the oxidation potential of the ferrioxalate/H2O2 process.


### 2.4. Experimental Procedure

Batch experiments were conducted with 200 mL of chlorothalonil aqueous solution in a 250 Pyrex beaker, placed in a SolSim solar simulator photoreactor (Luzchem Research Inc., Gloucester, ON, Canada), with solar intensity 0.85 kW/m2. The required amount of C2H2O4 and Fe3+ was added to the aqueous solution and mixed by a magnetic stirrer to ensure complete homogeneity during reaction. Thereafter, necessary amount of H2O2 was added to the mixture with simultaneous adjustment to pH 3 by using H2SO4. The time at which H2O2 was added to the mixture was considered as the beginning of the experiment. Samples were taken at preselected time intervals and filtered through 0.45 μm membrane filter for determination of COD, NH3-N, and TOC, and when required BOD5.


### 2.5. Optimization and Response Surface Modeling

Design expert software Version 6.0.7 [31] was used for statistical design of experiment and data analysis. Central composite design (CCD) of the response surface methodology (RSM) was used to optimize the operating conditions (variables) of the treatment because it is well suited for fitting a quadratic surface, which usually works well for process optimization, and it is the experimental design mostly utilized for the development of analytical procedure as against three-level factorial design which is not frequently used and has been limited to the optimization of two variables [32]. The variables were simultaneously changed in a central composite circumscribed design. The values of the variables H2O2/COD molar ratio, H2O2/Fe3+ molar ratio, H2O2/C2H2O4 molar ratio, and reaction time were set at three levels: −1 (low), 0 (central) and +1 (high), and the total number of experiments with three factors was obtained as 30 (2k+2k+6), where k is the number of factors (which equals 4 in this case). Twenty four experiments were augmented with six replications at the design center to evaluate the pure error and carried in randomized order as required in the circumscribed composite design. The variables H2O2/COD molar ratio, H2O2/Fe2+ molar ratio, H2O2/C2H2O4 molar ratio, and reaction time were studied in the range 1.5–4.0, 25–50, 50–100, and 60–120 min, respectively. Chosen response parameters for the process were removal of COD, NH3-N, and TOC. Table 1 shows the experimental design and the predicted response (removal). Regression models and statistical analysis, contour plots normal probability and plots were made. Model terms were evaluated by the P value (probability) with 95% confidence level. The quality of fit of the polynomial model was expressed by the coefficient of determination R2. The optimum operating conditions (variables) were identified from the contour plots and response equation simultaneously. The following response equation describing an empirical second-order polynomial model was used to assess the predicted results:
(6)Y=βo+∑i=1kβixi+∑i=1kβiix12+∑i=1k∑j=1kβijxixj+ε,
where Y is the dependent response; βo  is the constant coefficient; i, ii, and ij are the coefficients for the linear, quadratic, and interaction effect; xi and xj are the factors (i.e., H2O2/COD,H2O2/Fe3+, and H2O2/C2H2O4 molar ratio and reaction time); k signifies the number of independent variables, and ε is the random error [32]. The result (Y) was calculated as the sum of a constant (βo), four first-order effects (A, B, C, and D), four second-order effects (A2,B2,C2,andD2) and four interaction effects (AB, AC, BD, and CD).Table 1
Experimental design and predicted and actual removal.
Experimental design
COD
Removal (%)   NH3-N
TOC
A: H2O2/COD(molar ratio)
B: H2O2/Fe3+  (molar ratio)
C: H2O2/C2H2O4 (molar ratio)
D: Time(min)
Predict
Actual
Predict
Actual
Predict
Actual
4.00
100.00
25.00
120.00
67.89
71.14
46.11
52.11
46.10
50.00
2.75
25.00
37.50
90.00
69.45
67.14
49.00
43.23
35.16
28.00
1.50
50.00
25.00
120.00
70.85
68.57
46.26
44.00
40.54
41.16
2.75
75.00
37.50
150.00
73.14
73.54
41.17
40.47
63.49
60.11
4.00
100.00
50.00
60.00
67.18
69.15
40.60
41.81
44.25
45.08
4.00
100.00
50.00
120.00
67.04
60.00
48.29
49.11
42.79
44.44
2.75
75.00
37.50
90.00
72.71
75.14
44.81
43.29
52.20
49.11
1.50
100.00
25.00
60.00
61.42
66.93
58.30
62.00
34.23
40.44
1.50
100.00
25.00
120.00
69.36
67.42
47.57
43.00
61.57
57.31
2.75
75.00
37.50
90.00
72.71
72.00
44.81
49.60
52.20
53.44
2.75
75.00
62.50
90.00
71.69
70.85
45.93
44.44
52.33
51.16
4.00
50.00
50.00
60.00
64.08
65.71
35.33
35.00
61.77
70.11
2.75
75.00
37.50
90.00
72.71
69.71
44.81
40.00
52.20
55.50
1.50
100.00
50.00
60.00
61.37
61.14
51.39
52.40
34.23
30.11
4.00
50.00
25.00
60.00
64.93
61.42
38.82
34.50
56.82
57.11
2.75
125.00
37.50
90.00
71.07
74.00
54.81
50.00
38.67
40.30
4.00
50.00
25.00
120.00
63.22
63.14
40.91
40.00
46.81
55.00
1.50
50.00
50.00
60.00
64.41
60.85
45.00
37.95
30.73
29.00
1.50
50.00
50.00
120.00
78.18
78.00
46.25
49.11
47.53
54.00
2.75
75.00
37.50
90.00
72.71
75.14
44.81
43.11
52.20
50.00
4.00
50.00
50.00
120.00
68.80
74.00
41.20
36.44
50.76
46.00
5.25
75.00
37.50
90.00
57.78
56.85
35.45
35.00
47.91
43.00
4.00
100.00
25.00
60.00
74.46
74.28
47.21
44.44
47.28
44.88
1.50
100.00
50.00
120.00
70.22
73.42
49.44
52.70
60.58
61.75
0.25
75.00
37.50
90.00
59.59
61.14
53.00
51.39
31.63
31.00
2.75
75.00
12.50
90.00
71.68
73.14
52.55
55.00
48.37
44.00
2.75
75.00
37.50
30.00
66.40
67.42
41.02
42.67
47.16
45.00
2.75
75.00
37.50
90.00
72.71
70.00
44.81
44.11
52.20
56.12
1.50
50.00
25.00
60.00
63.55
70.28
53.80
53.07
22.75
25.17
2.75
75.00
37.50
90.00
72.71
74.28
44.81
48.77
49.00
52.20


## 2.1. Chemicals and Pesticide

Hydrogen peroxide (30%, w/w), oxalic acid(C2H2O4·2H2O), and ferric sulfate (Fe2(SO4)3) were purchased from R&M Marketing, Essex, UK. The pesticide chlorothalonil used to prepare aqueous solution was obtained from a commercial source and was used as received. Figure 1 shows the chemical structure of chlorothalonil.Figure 1
Chemical structure of chlorothalonil.


## 2.2. Analytical Methods

Chemical oxygen demand (COD) was determined according to Method 5220D (closed reflux, colorimetric method) of the Standard Methods [27], where the sample contained hydrogen peroxide (H2O2), to reduce interference in COD determination, pH was increased to above 10 so as to decompose hydrogen peroxide to oxygen and water [28, 29]. TOC analyzer (Model 1010, O & I Analytical) was used for determining total organic carbon (TOC). The pH was measured by a pH meter (HACH sension 4) and a pH electrode (HACH platinum series pH electrode model 51910, HACH Company, USA). Biodegradability was measured by 5-day biochemical oxygen demand (BOD5) test according to Method 5210B (seeding procedure) of the Standard Methods [27]. The treated pesticide aqueous solution was adjusted to pH 7 before the BOD5 test. Ammonia nitrogen (NH3-N) was measured by the Nessler method [30]. DO was measured using YSI 5000 dissolved oxygen meter. The seed for the BOD5 test was obtained from a municipal wastewater treatment plant.


## 2.3. Chlorothalonil Aqueous Solution

Chlorothalonil aqueous solution was 300 mg/L of chlorothalonil in distilled water. It was prepared weekly and stored at 4°C. The characteristics of the aqueous solution were COD 350 mg/L,NH3-N 1.58 mg/L, and TOC 94.49 mg/L. The high chlorothalonil concentration was chosen to reflect the concentration in pesticide-manufacturing wastewater and to explore the oxidation potential of the ferrioxalate/H2O2 process.


## 2.4. Experimental Procedure

Batch experiments were conducted with 200 mL of chlorothalonil aqueous solution in a 250 Pyrex beaker, placed in a SolSim solar simulator photoreactor (Luzchem Research Inc., Gloucester, ON, Canada), with solar intensity 0.85 kW/m2. The required amount of C2H2O4 and Fe3+ was added to the aqueous solution and mixed by a magnetic stirrer to ensure complete homogeneity during reaction. Thereafter, necessary amount of H2O2 was added to the mixture with simultaneous adjustment to pH 3 by using H2SO4. The time at which H2O2 was added to the mixture was considered as the beginning of the experiment. Samples were taken at preselected time intervals and filtered through 0.45 μm membrane filter for determination of COD, NH3-N, and TOC, and when required BOD5.


## 2.5. Optimization and Response Surface Modeling

Design expert software Version 6.0.7 [31] was used for statistical design of experiment and data analysis. Central composite design (CCD) of the response surface methodology (RSM) was used to optimize the operating conditions (variables) of the treatment because it is well suited for fitting a quadratic surface, which usually works well for process optimization, and it is the experimental design mostly utilized for the development of analytical procedure as against three-level factorial design which is not frequently used and has been limited to the optimization of two variables [32]. The variables were simultaneously changed in a central composite circumscribed design. The values of the variables H2O2/COD molar ratio, H2O2/Fe3+ molar ratio, H2O2/C2H2O4 molar ratio, and reaction time were set at three levels: −1 (low), 0 (central) and +1 (high), and the total number of experiments with three factors was obtained as 30 (2k+2k+6), where k is the number of factors (which equals 4 in this case). Twenty four experiments were augmented with six replications at the design center to evaluate the pure error and carried in randomized order as required in the circumscribed composite design. The variables H2O2/COD molar ratio, H2O2/Fe2+ molar ratio, H2O2/C2H2O4 molar ratio, and reaction time were studied in the range 1.5–4.0, 25–50, 50–100, and 60–120 min, respectively. Chosen response parameters for the process were removal of COD, NH3-N, and TOC. Table 1 shows the experimental design and the predicted response (removal). Regression models and statistical analysis, contour plots normal probability and plots were made. Model terms were evaluated by the P value (probability) with 95% confidence level. The quality of fit of the polynomial model was expressed by the coefficient of determination R2. The optimum operating conditions (variables) were identified from the contour plots and response equation simultaneously. The following response equation describing an empirical second-order polynomial model was used to assess the predicted results:
(6)Y=βo+∑i=1kβixi+∑i=1kβiix12+∑i=1k∑j=1kβijxixj+ε,
where Y is the dependent response; βo  is the constant coefficient; i, ii, and ij are the coefficients for the linear, quadratic, and interaction effect; xi and xj are the factors (i.e., H2O2/COD,H2O2/Fe3+, and H2O2/C2H2O4 molar ratio and reaction time); k signifies the number of independent variables, and ε is the random error [32]. The result (Y) was calculated as the sum of a constant (βo), four first-order effects (A, B, C, and D), four second-order effects (A2,B2,C2,andD2) and four interaction effects (AB, AC, BD, and CD).Table 1
Experimental design and predicted and actual removal.
Experimental design
COD
Removal (%)   NH3-N
TOC
A: H2O2/COD(molar ratio)
B: H2O2/Fe3+  (molar ratio)
C: H2O2/C2H2O4 (molar ratio)
D: Time(min)
Predict
Actual
Predict
Actual
Predict
Actual
4.00
100.00
25.00
120.00
67.89
71.14
46.11
52.11
46.10
50.00
2.75
25.00
37.50
90.00
69.45
67.14
49.00
43.23
35.16
28.00
1.50
50.00
25.00
120.00
70.85
68.57
46.26
44.00
40.54
41.16
2.75
75.00
37.50
150.00
73.14
73.54
41.17
40.47
63.49
60.11
4.00
100.00
50.00
60.00
67.18
69.15
40.60
41.81
44.25
45.08
4.00
100.00
50.00
120.00
67.04
60.00
48.29
49.11
42.79
44.44
2.75
75.00
37.50
90.00
72.71
75.14
44.81
43.29
52.20
49.11
1.50
100.00
25.00
60.00
61.42
66.93
58.30
62.00
34.23
40.44
1.50
100.00
25.00
120.00
69.36
67.42
47.57
43.00
61.57
57.31
2.75
75.00
37.50
90.00
72.71
72.00
44.81
49.60
52.20
53.44
2.75
75.00
62.50
90.00
71.69
70.85
45.93
44.44
52.33
51.16
4.00
50.00
50.00
60.00
64.08
65.71
35.33
35.00
61.77
70.11
2.75
75.00
37.50
90.00
72.71
69.71
44.81
40.00
52.20
55.50
1.50
100.00
50.00
60.00
61.37
61.14
51.39
52.40
34.23
30.11
4.00
50.00
25.00
60.00
64.93
61.42
38.82
34.50
56.82
57.11
2.75
125.00
37.50
90.00
71.07
74.00
54.81
50.00
38.67
40.30
4.00
50.00
25.00
120.00
63.22
63.14
40.91
40.00
46.81
55.00
1.50
50.00
50.00
60.00
64.41
60.85
45.00
37.95
30.73
29.00
1.50
50.00
50.00
120.00
78.18
78.00
46.25
49.11
47.53
54.00
2.75
75.00
37.50
90.00
72.71
75.14
44.81
43.11
52.20
50.00
4.00
50.00
50.00
120.00
68.80
74.00
41.20
36.44
50.76
46.00
5.25
75.00
37.50
90.00
57.78
56.85
35.45
35.00
47.91
43.00
4.00
100.00
25.00
60.00
74.46
74.28
47.21
44.44
47.28
44.88
1.50
100.00
50.00
120.00
70.22
73.42
49.44
52.70
60.58
61.75
0.25
75.00
37.50
90.00
59.59
61.14
53.00
51.39
31.63
31.00
2.75
75.00
12.50
90.00
71.68
73.14
52.55
55.00
48.37
44.00
2.75
75.00
37.50
30.00
66.40
67.42
41.02
42.67
47.16
45.00
2.75
75.00
37.50
90.00
72.71
70.00
44.81
44.11
52.20
56.12
1.50
50.00
25.00
60.00
63.55
70.28
53.80
53.07
22.75
25.17
2.75
75.00
37.50
90.00
72.71
74.28
44.81
48.77
49.00
52.20


## 3. Results and Discussion

Based on the experimental design, predicted and actual removal (average of triplicate experimental results) are shown in Table1.


### 3.1. Regression Models and Statistical Analysis

To ascertain the suitability of the regression model, assess the interaction between the independent variables (operating conditions) and the dependent variables (responses), and subsequently obtain the “goodness of fit”, analysis of variance (ANOVA) was performed. FisherF-test value, P-value, coefficient of determination R2, and adequate precision (A.P) are shown in Table 2. F-test value is a measure of variation of the data about the mean [26]. A P-value less than 0.05 indicates the suitability of the proposed models for treatment as there is no lack-of-fit. The models for COD, NH3-N, and TOC removal (Y1,Y2,andY3) were significant by the F-test at 95% confidence level employed as all responses had a P-value < 0.05, and therefore the removal fits the data well. The coefficient of determination (R2) is the proportion of variability in a data set which indicates whether the empirical model is good enough for the quadratic fit to navigate the design space defined by the CCD [27]. The R2 value gives the proportion of the total variation in the response predicted by the model to the actual data. The R2 values were 0.8031 (COD), 0.7579 (NH3-N), and 0.8196 (TOC). Adequate precision (A.P.) ratio compares the range of the predicted value at the design points to the average prediction error. Ratios greater than 4 indicate adequate model discrimination and can be used to navigate the design space defined by the CCD [27]. The A.P. for all the responses was greater than 4. The ANOVA results indicate adequate agreement between the model prediction and actual removal. The following fitted regression models were obtained to quantitatively investigate the effects of A: H2O2/COD molar ratio, B: H2O2/Fe3+ molar ratio, C: H2O2/C2H2O4 molar ratio, and D: reaction time on COD, NH3-N, and TOC removal, respectively.COD removal,(7)Y1=74.68-1.03A+0.34B-0.19C+3.17D-4.16A2-1.22B2-0.79C2±0.43D2+2.36AB+0.68AC-0.11BD+3.35CD.NH3-N removal,
(8)Y2=43.88-3.73A+3.52B-2.12C-0.75D-0.43A2+0.96B2+2.12C2-8.437E-003D2+1.40AB+0.40AC+1.98AD+0.18BC-1.01BD+3.49CD.TOC removal,(9)Y3=51.19+4.81A-1.76B+1.08C+3.84D-3.59A2-1.52B2-0.98C2+0.35D2-6.35AB-0.73AC-6.85AD-2.76BC+2.65BD+0.51CD.Table 2
ANOVA of the response parameters.
Parameter
F-test
P value
R
2
Adequate precision (A.P)
COD
2.63
0.0001
0.8031
11.992
NH3-N
2.88
0.0001
0.7579
11.320
TOC
6.54
0.0001
0.8196
15.546In (7), (8), and (9), the values of the sum of a constant (βo), (74.68, 43.88, and 51.19) represent the percentage removal of COD, NH3-N, and TOC, respectively. The positive sign indicates that the variable is directly proportional to the response (COD, NH3-N, and TOC removal), and the negative sign indicates that the variable is inversely proportional to the response.


### 3.2. Process Analysis

Visualization of the predicted model equation can be obtained from the contour plot [25]. A contour plot is a two-dimensional display of the surface plot, and, in the contour plot, lines of constant response are drawn in the plane of the variables. The contour plot helps to visualize the shape of a response surface. When the contour plot displays ellipses or circles, the center of the system refers to a point of maximum or minimum response. Sometimes, contour plot may display hyperbolic or parabolic system of the contours [28]. Figures 2, 3, and 4 show the contour plots for COD, NH3-N, and TOC removal. Decreasing oxalate (increasing H2O2/C2H2O4 molar ratio) and increasing H2O2 (increasing H2O2/COD molar ratio) will reduce COD removal, increasing oxalate (decreasing H2O2/C2H2O4 molar ratio) and increasing H2O2 (increasing H2O2/COD molar ratio) will reduce NH3-N removal, and increasing oxalate (decreasing H2O2/C2H2O4 molar ratio) and decreasing H2O2 (decreasing H2O2/COD molar ratio) simultaneously or one at a time will reduce TOC removal at H2O2/Fe3+ molar ratio 75.0 and reaction time 90 min in all three cases. The adequacy of the models was also evaluated by the residuals that is, difference between the predicted and the actual response value. Plot of predicted versus actual removal (Figures 5, 6, and 7) indicates that there is no abnormalities in the model as all data were found around the line of “best fit”.Figure 2
Contour plot for COD removal.Figure 3
Contour plot forNH3-N removal.Figure 4
Contour plot for TOC removal.Figure 5
Plot of the predicted versus actual COD removal.Figure 6
Plot of predicted versus actual NH3-N removal.Figure 7
Plot of predicted versus actual TOC removal.


### 3.3. Optimization and Model Verification

Numerical optimization was used to determine the optimum operating conditions for COD,NH3-N, and TOC removal. Based on the response surface and desirability functions (figure not shown), the optimum operating conditions were obtained. In this case, all responses were targeted to be in range and were goaled to be maximized. The optimum conditions were obtained for highest desirability at H2O2/COD molar ratio 2.75, H2O2/Fe3+ molar ratio 75, H2O2/C2H2O4 molar ratio 37.5, and reaction time 90 min at pH 3. Under the operating conditions, 72.71, 45.47, and 52.20% removal of COD, NH3-N, and TOC, respectively, was predicted based on desirability function of 1.00 (Table 3). Actual removal under the optimum operating conditions is shown in Table 3. The model prediction and actual removal were in close agreement (<4% error).Table 3
Model prediction and actual removal.
Parameter
Prediction
Actual
%  Error
COD removal (%)
72.71
75.71
3.96
NH3-N removal (%)
45.47
47.11
3.48
TOC removal (%)
52.20
54.33
3.90


### 3.4. Biodegradability

Under the optimum operating conditions (H2O2/COD molar ratio 2.75, H2O2/Fe3+ molar ratio 75, H2O2/C2H2O4 molar ratio 37.5, reaction time 90 min, and pH 3),        solar ferrioxalate/H2O2 treatment of the chlorothalonil aqueous solution improved the biodegradability (BOD5/COD ratio) from zero to 0.42, indicating that the treated pesticide aqueous solution was amenable to biological treatment [33].


### 3.5. Prospective Application

Complete depollution of pesticide (herbicide) aqueous solution by photoelectro-Fenton or electro-Fenton using boron-doped diamond electrode has been reported [34, 35]. However, solar ferrioxalate/H2O2 is a simple energy-efficient process and can be applied as pretreatment of pesticide wastewater to improve biodgradability for subsequent biological treatment.


## 3.1. Regression Models and Statistical Analysis

To ascertain the suitability of the regression model, assess the interaction between the independent variables (operating conditions) and the dependent variables (responses), and subsequently obtain the “goodness of fit”, analysis of variance (ANOVA) was performed. FisherF-test value, P-value, coefficient of determination R2, and adequate precision (A.P) are shown in Table 2. F-test value is a measure of variation of the data about the mean [26]. A P-value less than 0.05 indicates the suitability of the proposed models for treatment as there is no lack-of-fit. The models for COD, NH3-N, and TOC removal (Y1,Y2,andY3) were significant by the F-test at 95% confidence level employed as all responses had a P-value < 0.05, and therefore the removal fits the data well. The coefficient of determination (R2) is the proportion of variability in a data set which indicates whether the empirical model is good enough for the quadratic fit to navigate the design space defined by the CCD [27]. The R2 value gives the proportion of the total variation in the response predicted by the model to the actual data. The R2 values were 0.8031 (COD), 0.7579 (NH3-N), and 0.8196 (TOC). Adequate precision (A.P.) ratio compares the range of the predicted value at the design points to the average prediction error. Ratios greater than 4 indicate adequate model discrimination and can be used to navigate the design space defined by the CCD [27]. The A.P. for all the responses was greater than 4. The ANOVA results indicate adequate agreement between the model prediction and actual removal. The following fitted regression models were obtained to quantitatively investigate the effects of A: H2O2/COD molar ratio, B: H2O2/Fe3+ molar ratio, C: H2O2/C2H2O4 molar ratio, and D: reaction time on COD, NH3-N, and TOC removal, respectively.COD removal,(7)Y1=74.68-1.03A+0.34B-0.19C+3.17D-4.16A2-1.22B2-0.79C2±0.43D2+2.36AB+0.68AC-0.11BD+3.35CD.NH3-N removal,
(8)Y2=43.88-3.73A+3.52B-2.12C-0.75D-0.43A2+0.96B2+2.12C2-8.437E-003D2+1.40AB+0.40AC+1.98AD+0.18BC-1.01BD+3.49CD.TOC removal,(9)Y3=51.19+4.81A-1.76B+1.08C+3.84D-3.59A2-1.52B2-0.98C2+0.35D2-6.35AB-0.73AC-6.85AD-2.76BC+2.65BD+0.51CD.Table 2
ANOVA of the response parameters.
Parameter
F-test
P value
R
2
Adequate precision (A.P)
COD
2.63
0.0001
0.8031
11.992
NH3-N
2.88
0.0001
0.7579
11.320
TOC
6.54
0.0001
0.8196
15.546In (7), (8), and (9), the values of the sum of a constant (βo), (74.68, 43.88, and 51.19) represent the percentage removal of COD, NH3-N, and TOC, respectively. The positive sign indicates that the variable is directly proportional to the response (COD, NH3-N, and TOC removal), and the negative sign indicates that the variable is inversely proportional to the response.


## 3.2. Process Analysis

Visualization of the predicted model equation can be obtained from the contour plot [25]. A contour plot is a two-dimensional display of the surface plot, and, in the contour plot, lines of constant response are drawn in the plane of the variables. The contour plot helps to visualize the shape of a response surface. When the contour plot displays ellipses or circles, the center of the system refers to a point of maximum or minimum response. Sometimes, contour plot may display hyperbolic or parabolic system of the contours [28]. Figures 2, 3, and 4 show the contour plots for COD, NH3-N, and TOC removal. Decreasing oxalate (increasing H2O2/C2H2O4 molar ratio) and increasing H2O2 (increasing H2O2/COD molar ratio) will reduce COD removal, increasing oxalate (decreasing H2O2/C2H2O4 molar ratio) and increasing H2O2 (increasing H2O2/COD molar ratio) will reduce NH3-N removal, and increasing oxalate (decreasing H2O2/C2H2O4 molar ratio) and decreasing H2O2 (decreasing H2O2/COD molar ratio) simultaneously or one at a time will reduce TOC removal at H2O2/Fe3+ molar ratio 75.0 and reaction time 90 min in all three cases. The adequacy of the models was also evaluated by the residuals that is, difference between the predicted and the actual response value. Plot of predicted versus actual removal (Figures 5, 6, and 7) indicates that there is no abnormalities in the model as all data were found around the line of “best fit”.Figure 2
Contour plot for COD removal.Figure 3
Contour plot forNH3-N removal.Figure 4
Contour plot for TOC removal.Figure 5
Plot of the predicted versus actual COD removal.Figure 6
Plot of predicted versus actual NH3-N removal.Figure 7
Plot of predicted versus actual TOC removal.


## 3.3. Optimization and Model Verification

Numerical optimization was used to determine the optimum operating conditions for COD,NH3-N, and TOC removal. Based on the response surface and desirability functions (figure not shown), the optimum operating conditions were obtained. In this case, all responses were targeted to be in range and were goaled to be maximized. The optimum conditions were obtained for highest desirability at H2O2/COD molar ratio 2.75, H2O2/Fe3+ molar ratio 75, H2O2/C2H2O4 molar ratio 37.5, and reaction time 90 min at pH 3. Under the operating conditions, 72.71, 45.47, and 52.20% removal of COD, NH3-N, and TOC, respectively, was predicted based on desirability function of 1.00 (Table 3). Actual removal under the optimum operating conditions is shown in Table 3. The model prediction and actual removal were in close agreement (<4% error).Table 3
Model prediction and actual removal.
Parameter
Prediction
Actual
%  Error
COD removal (%)
72.71
75.71
3.96
NH3-N removal (%)
45.47
47.11
3.48
TOC removal (%)
52.20
54.33
3.90


## 3.4. Biodegradability

Under the optimum operating conditions (H2O2/COD molar ratio 2.75, H2O2/Fe3+ molar ratio 75, H2O2/C2H2O4 molar ratio 37.5, reaction time 90 min, and pH 3),        solar ferrioxalate/H2O2 treatment of the chlorothalonil aqueous solution improved the biodegradability (BOD5/COD ratio) from zero to 0.42, indicating that the treated pesticide aqueous solution was amenable to biological treatment [33].


## 3.5. Prospective Application

Complete depollution of pesticide (herbicide) aqueous solution by photoelectro-Fenton or electro-Fenton using boron-doped diamond electrode has been reported [34, 35]. However, solar ferrioxalate/H2O2 is a simple energy-efficient process and can be applied as pretreatment of pesticide wastewater to improve biodgradability for subsequent biological treatment.


## 4. Conclusions

Visible light-responsive photocatalyst ferrioxalate andH2O2 under solar irradiation is effective in degradation of the pesticide chlorothalonil. The optimum operating conditions for solar ferrioxalate/H2O2 treatment of a 300 mg/L chlorothalonil aqueous solution obtained by using the central composite design of the response surface methodology were H2O2/COD molar ratio 2.75, H2O2/Fe3+ molar ratio 75, H2O2/C2H2O4 molar ratio 37.5, reaction time 90 min, and pH 3. Under optimum operating conditions, 75.71, 47.11, and 54.33% removal of COD, NH3-N, and TOC, respectively was achieved and the biodegradability (BOD5/COD ratio) improved from zero to 0.42. Model prediction and actual removal were in close agreement. The solar ferrioxalate/H2O2 process is effective in pretreatment of chlorothalonil aqueous solution for biological treatment.



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*Source: 435017-2013-05-14.xml*