Carbofuran removal kinetics

The pseudo-first order reaction model as shown in Eq. 3-4 was used to study the carbofuran removal kinetics, where, C0 and Ct are the concentrations of carbofuran (mg L^-1) at reaction times zero and t, respectively. kapp is the apparent rate constant (min^-1) and t is the reaction time (min). The rate constants were estimated based on the linear plot of ln(C0/Ct) versus t.

ln(C₀/C_t) = k_appt (3-4)

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Chapter 4

Results and discussions

There were numbers of well chosen experiments carried out in this study. Based on the results obtained, this chapter is divided into ten major sections listed as follows:

4.1 Effect of irradiation time on carbofuran degradation

4.2 Effect of carbofuran concentration on carbofuran degradation 4.3 Effect of initial pH on carbofuran degradation

4.4 Effect of H2O2 dosage rate on carbofuran degradation 4.5 Effect of Fe³⁺ dosage on carbofuran degradation 4.6 Chemical degradability of carbofuran

4.7 CCD of experiments for the photo-Fenton process 4.8 Degradation pathway

4.9 Toxicity and oxidation state assessment 4.10 Biodegradability assessment

Many studies have shown that the performance of the photo-Fenton process is highly related to the operating parameters. In order to achieve high removal efficiency, the optimum operation conditions must be investigated. Therefore, experiments were carried out to study the effects of operating parameters including irradiation time, carbofuran concentration, pH, H2O2 dosage rate and Fe³⁺ dosage on the degradation and mineralization of carbofuran and their results are described in sections 4.1-4.5.

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Control experiments such as H₂O₂, Fe³⁺, UV, Fe³⁺ + H₂O₂, UV + Fe³⁺and UV + H₂O₂ were also carried out to evaluate the efficiency of each process and their results are summarized in section 4.6. The favorable reagent dosages obtained by CCD along with RSM were discussed and reported in section 4.7. The degradation pathway of carbofuran degradation under favorable reagent dosages is proposed and discussed in section 4.8 based on the intermediates identified. Finally, the appropriate time for coupling the photo-Fenton process with biological treatment has been discussed according to the variations of toxicity and biodegradability at favorable reagent dosages in sections 4.8 and 4.9, respectively.

4.1 Effect of irradiation time on carbofuran degradation

The effect of irradiation time was evaluated under an initial carbofuran concentration of 50 mg L^-1 and pH 3 with the Fe³⁺ dosage at 5 mg L^-1 and H2O2 dosage rate at 0.1 mg L^-1 min^-1. The profiles of carbofuran and DOC removals are shown in Fig.

4-1. After 240 min, carbofuran and DOC removals were reached around 97% and 42%, respectively. These results indicate that even with low dosage of Fe³⁺ and dosage rate of H2O2, it is possible to obtain higher carbofuran degradation as well as its mineralization.

However, no significant increase in the carbofuran removal was observed between 120-240 min (less than 10%). The DOC removal profile indicates that carbofuran mineralization rate could be enhanced by increasing the UV irradiation time.

4.2 Effect of carbofuran concentration on carbofuran degradation

In order to investigate the effect of carbofuran concentration for the photo-Fenton degradation of carbofuran, the experiments were repeated at 21 and 80 mg L^-1

49

(a)

0 40 80 120 160 200 240

0 20 40 60 80 100

C ar bof ur an rem ov al ( % )

Time (min)

(b)

0 40 80 120 160 200 240

0 20 40 60 80 100

D O C r em ov al ( % )

Time (min)

Fig. 4-1. The profiles of (a) carbofuran (b) DOC removals with time (Fe³⁺ dosage at 5 mg L^-1, H2O2 dosage rate at 0.1 mg L^-1 min^-1 and pH 3).

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carbofuran concentration with two different reagent dosages at pH 3, i.e. H₂O₂ dosage rate of 1.3 mg L^-1 min^-1, Fe³⁺ dosage of 10 mg L^-1 and H2O2 dosage rate of 1.3 mg L^-1 min^-1, Fe³⁺ dosage of 40 mg L^-1. Fig. 4-2 shows the trend of degradation and mineralization of carbofuran with the reaction time. It can be noticed in Figs. 4-2a and b that carbofuran concentration has great negative effect on carbofuran degradation and mineralization, regardless of the amount of H₂O₂ dosage rate and Fe³⁺ dosage employed.

The reduction of carbofuran removal accompanies with the increase of carbofuran concentration. In addition, carbofuran removal increased with increasing of limiting factor Fe³⁺ dosage at high carbofuran concentration (Fig. 4-2a). After 30 min of reaction, carbofuran were completely removed in cases of A and C. However, the carbofuran removal has reached only less than 74% in cases of B and D. Similar variations were also determined for DOC removal (Fig. 4-2b). Mineralization efficiencies obtained at low carbofuran concentration (A and C) were greater than those at high carbofuran concentration (B and D).

4.3 Effect of initial pH on carbofuran degradation

The photo-Fenton reaction is strongly pH dependent as solution pH will affect the generation of ‧OH, the oxidation performance and the species concentration of Fe³⁺

complex in aqueous solution. The experiments were conducted using 50 mg L^-1 carbofuran concentration at various pH values (2-4) with Fe³⁺ dosage of 35 mg L^-1 and H2O2 dosage rate of 4 mg L^-1 min^-1. The effect of initial pH on the carbofuran degradation in the photo-Fenton process is shown in Fig. 4-3 and Table 4-1. As shown in Fig. 4-3, the profiles demonstrate that the removals of carbofuran and DOC are greatly affected by the initial pH level. Almost 100% degradation of carbofuran was

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Fig. 4-2. The profiles of (a) carbofuran (b) DOC removals with time at pH 3, using H2O2: 1.3 mg L^-1 min^-1, Fe³⁺: 10 mg L^-1 (A and B) and H2O2: 1.3 mg L^-1 min^-1, Fe³⁺:

40 mg L^-1 (C and D) (Solid points represent carbofuran 21 mg L^-1 and hollow points represent carbofuran 81 mg L^-1).

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Fig. 4-3. The profiles of (a) carbofuran (b) DOC removals with time at various pH (Fe³⁺ dosage at 35 mg L^-1, H2O2 dosage rate at 4 mg L^-1 min^-1 and carbofuran 50 mg

L^-1).

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Table 4-1. Effect of various pH on carbofuran and DOC removals (Fe³⁺ dosage at 35 mg L^-1, H2O2 dosage rate at 4 mg L^-1 min^-1 and carbofuran 50 mg L^-1).

pH

Carbofuran

removal (%)

DOC

removal (%)

Pseudo-first order carbofuran removal kinetics

30 min 120 min 30 min 120 min k_app (×10^-2 min^-1) R²

2 70 100 7 82 3.5 0.92

2.5 95 100 22 92 8.1 0.88

3 100 100 31 93 8.6 0.94

3.5 61 100 9 63 3.2 0.96

4 37 100 7 49 2.1 0.85

achieved in between 100-120 minutes of the photo-Fenton process under all initial pH values (Fig. 4-3a). At the end of 30 minutes, 37% degradation of carbofuran was observed at pH 4, whereas it was 61% at pH 3.5 and almost 100% at pH 3. This indicates that the lower pH values are better for the photo-Fenton decomposition of carbofuran. However, the decomposition of carbofuran was reduced to 95% and 70%

under pH 2.5 and 2, respectively at the end of 30 minutes of photo-Fenton reaction.

Similar trends can be seen in Fig. 4-3b, where the DOC removal at the end of 120 minutes increased from 49% at pH 4 to 93% at pH 3, and then decreased to 82% at pH 2.

It can also be seen in Table 4-1 that increasing solution pH from 2 to 3 enhances the removal of carbofuran at 30 min reaction from 70% to 100%; however, further

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increase in pH (above 3) decreases the removal efficiency, and at pH 4 the removal efficiency decreased to 37%. The reduced efficiency at higher pH values (pH > 3) is due to the precipitation of Fe³⁺ as inactive Fe(OH)3 thus hindering the reaction between Fe²⁺

and H₂O₂. Furthermore, H₂O₂ can also be hydrolyzed to water and oxygen (Eq. 4-1) at higher pH values [14]. The decomposition performance under low pH condition (pH < 3) decreases since excess H⁺ will react with ‧OH via Eq. 2-17 and produce water [33].

The consumption of H2O2 and ‧OH can decrease the oxidation performance of target compounds. Moreover, the dominant Fe(OH)²⁺ reaches a maximum at pH around 3 [14].

The lower quantity of Fe(OH)²⁺ at pH below 3 causes lower amount of Fe²⁺ reduction and ‧OH production in Eq. 2-12 resulted in poor performance of carbofuran removal.

2H₂O₂ → 2H₂O + O2 (4-1)

The variations of DOC removal at 30 min and 120 min are similar with the results of carbofuran removal. At pH value of 3, the DOC removals at 30 min and 120 min were found to be 31% and 93%, respectively. Furthermore, the carbofuran removals were fitted well with the pseudo-first order reaction model and the R² values ranged between 0.85 to 0.96. The apparent rate constants of carbofuran removal were increased with the decrease in pH value from 4 to 3. Further decrease in pH (below 3) decreased the apparent rate constant of carbofuran removal. The highest apparent rate constant (8.6 × 10^-2 min^-1) was obtained at pH 3. Base on the results obtained, the optimal pH for the oxidation of carbofuran using photo-Fenton process is 3, which is in good agreement with the previous photo-Fenton studies conducted for the destruction of various organic compounds [27,74].

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4.4 Effect of H2O2 dosage rate on carbofuran degradation

In order to identify a better dosage rate of H2O2 required for the photo-Fenton reaction, the experiments were repeated at 50 mg L^-1 carbofuran concentration and pH 3, with a Fe³⁺ dosage of 35 mg L^-1 and varying H₂O₂ dosage rates (0-6 mg L^-1 min^-1).

Carbofuran and DOC removals under various dosage rates of H2O2 are shown in Fig.

4-4 and Table 4-2. It can be noticed in Fig. 4-4a that the carbofuran degradation increases under the higher dosage rates of H2O2, which is mainly due to the generation of ‧OH with extra H2O2 addition. In the absence of H2O2, i.e. UV/Fe³⁺ process, the degradation and mineralization of carbofuran were reached around 77% and 13%, respectively, within 120 minutes of reaction. The result proves that dosage of H₂O₂ is necessary to enhance the mineralization of carbofuran. At a H2O2 dosage of 0.25 mg L^-1 min^-1, more than 90% carbofuran removal was observed corresponding to a DOC removal of 33% at the end of 100 minutes of photo-Fenton reaction. Both carbofuran and DOC removals distinctly amplified with the increased additions of H2O2. When the H₂O₂ dosage was increased to 4 mg L^-1 min^-1, almost 100% of carbofuran was decomposed within 30 minutes reaction and a maximum carbofuran mineralization of 94% was observed after 120 minutes of reaction (Fig. 4-4b). At higher H2O2 dosage conditions, the DOC removal reveals a sharp increase within 20-60 minutes (10-60%), and then followed a gradual increase after 60 minutes (60-94%).

On the other hand, carbofuran removals have improved greatly, i.e. 32 to 73%, 39 to 90% and 47 to 100% within 15, 20 and 30 min reaction, respectively, when the H2O2

dosage rate was increased from 0 to 4 mg L^-1 min^-1 (Table 4-2). At the same condition, the DOC removal efficiency has increased from 13% to 93% after 120 min reaction.

However, no improvement in the carbofuran and DOC removals were observed when

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Fig. 4-4. The profiles of (a) carbofuran (b) DOC removals with time at various H₂O₂ dosage rates (Fe³⁺ dosage at 35 mg L^-1, pH 3 and carbofuran 50 mg L^-1).

57

the H₂O₂ was overdosed, i.e. beyond 4 mg L^-1 min^-1. Several researchers have also reported the negative effect of H2O2 dosage under H2O2 overdosed photo-Fenton systems for the degradation of target compound [27,66]. Under this overdosed rate, H₂O₂ could react with ‧OH, and as a result, less powerful HO₂‧ is formed as shown in Eq. 2-14. Moreover, HO₂‧ could further react with ‧OH and form water and oxygen as per Eq. 2-15 [27,42]. Therefore, the H2O2 dosage ratebeyond 4 mg L^-1 min^-1 can reduce the oxidative capacity of the photo-Fenton reaction by decreasing the amount of ‧OH and oxidant in the system [74].

The variation of residual H2O2 concentration as shown in Fig. 4-5 is in good agreement with the DOC profile. The residual H2O2 concentration under H2O2 dosage rates of 4 and 5 mg L^-1 min^-1 decrease at reaction time 15-40 min, and then, increases gradually after 40 min (Fig. 4-5). The carbofuran removals were fitted using the pseudo-first order reaction model; the highest apparent rate constant of 8.8 × 10^-2 min^-1 was observed at the H2O2 dosage rate of 4 mg L^-1 min^-1 (Table 4-2). This reveals that H₂O₂ dosage rate at 4 mg L^-1 min^-1 has accelerated the oxidation rate of carbofuran. The experimental outcomes, i.e. carbofuran degradation, mineralization and ratio of DOC removal/carbofuran degradation, at the end of 120 minutes of the photo-Fenton process are shown in Table 4-3. The ratio of DOC removal/carbofuran degradation increases with increasing H2O2 dosage rate, which demonstrates that the addition of H2O2 is useful for enhancing the mineralization of carbofuran in the photo-Fenton process.

Throughout the experiments, the solution pH remains relatively unchanged (2.8-3.4) as shown in Fig. 4-6. The increase of pH in the initial stage of reaction is due to OH -generated through Eq. 2-1. Also, it can be noted that at the end of experiment the pH value decreased (Fig. 4-6). The carbamic acid formed during the reaction might be the reason for this decrease in pH.

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Table 4-2. Effect of various H2O2 dosage rates on carbofuran and DOC removals (Fe³⁺ dosage at 35 mg L^-1, pH 3 and carbofuran 50 mg L^-1).

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Table 4-3. Summarization of carbofuran degradation, DOC removal and the ratio of DOC removal/carbofuran degradation by the photo-Fenton reaction at various

H₂O₂ dosage rates (Fe³⁺ 35 mg L^-1, pH 3 and carbofuran 50 mg L^-1).

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4.5 Effect of Fe³⁺ dosage on carbofuran degradation

The photo-Fenton experiments were repeated under 50 mg L^-1 carbofuran concentration at different Fe³⁺ dosages (5-50 mg L^-1) and H2O2 dosage rates (0.8 and 4 mg L^-1 min^-1), and at fixed pH, i.e. pH 3. The experimental outcomes are shown in Fig.

4-7 and Table 4-4. The profiles of H2O2 consumption are shown in Fig. 4-8. Throughout the study not much variation in pH (2.9-3.3) was observed as shown in Fig. 4-9. The carbofuran removal increases with increasing Fe³⁺ dosages due to the production of more ‧OH through Fenton reaction. It can be seen from Fig. 4-7a that even at the Fe³⁺

dosage of 5 mg L^-1 carbofuran was completely oxidized after 100 and 60 min of reaction time under H₂O₂ dosage rates of 0.8 and 4 mg L^-1 min^-1, respectively. At higher Fe³⁺ dosages (> 35 mg L^-1), the reaction times to reach 100% carbofuran removal were reduced to 60 and 30 min with around 30 and 80% of DOC mineralized. But there were no obvious further improvements on carbofuran degradation and mineralization for Fe³⁺

dosage higher than 35 mg L^-1 (Figs. 4-7a and b). This observation is in good agreement with the results reported in the previous studies [27,33,78]. The main reason for the limited degradation of carbofuran at H2O2 dosage rate of 0.8 mg L^-1 min^-1 is due to the insufficient residual H2O2 concentration in the system (Fig. 4-8). The higher removal at a H₂O₂ dosage rate of 4 mg L^-1 min^-1 could be attributed to the increased residual H₂O₂ concentration in the system; however, the increase in Fe³⁺ dosage beyond 35 mg L^-1 at this condition has produced similar carbofuran removal owing to the photostationary equilibrium between Fe²⁺ and Fe³⁺ [5].

Table 4-4 also demonstrates that carbofuran and DOC removal efficiencies could be improved by increasing the Fe³⁺ dosage. The increasing in Fe³⁺ dosage facilitates the

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Fig. 4-7. The profiles of (a) carbofuran (b) DOC removals with time under various Fe³⁺ dosages at pH 3 and carbofuran 50 mg L^-1 (Solid points represent H₂O₂ dosage rate 0.8 mg L^-1 min^-1 and hollow points represent H2O2 dosage rate 4 mg L^-1 min^-1).

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Table 4-4. Effect of various Fe³⁺ dosages on carbofuran and DOC removals under H2O2 dosage rates 0.8 and 4 mg L^-1 min^-1 at pH 3 and carbofuran 50 mg L^-1.

H₂O₂ dosage rate (mg L^-1 min^-1)

Fe³⁺ dosage (mg L^-1)

Carbofuran removal (%)

DOC removal (%)

Pseudo-first order carbofuran removal kinetics 15 min 20 min 30 min 30 min 120 min kapp (×10^-2 min^-1) R²

0.8

5 6 17 40 0 46 2.6 0.95

20 34 41 64 12 59 4.4 0.92

35 54 68 81 16 62 6.5 0.98

50 58 72 85 19 63 6.9 0.99

4

5 42 54 79 9 79 5.2 0.96

20 71 85 100 32 91 8.3 0.96

35 73 90 100 31 93 8.6 0.94

50 80 97 100 34 93 10.5 0.94

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Fig. 4-9. Profiles of pH with time under different Fe³⁺ dosages at pH 3 and carbofuran 50 mg L^-1 (Solid points represent H2O2 dosage rate 0.8 mg L^-1 min^-1 and

hollow points represent H₂O₂ dosage rate 4 mg L^-1 min^-1).

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higher carbofuran and DOC removal efficiencies. The carbofuran removals shown in Table 4-4 were fitted using the pseudo-first order reaction model. The apparent rate constants are shown in Table 4-4, which indicate that increase in Fe³⁺ dosages and the dosage rates of H₂O₂ have the capability of increasing the carbofuran removal rate to a certain extent. The highest carbofuran removal rate constant of 10.5 × 10^-2 min^-1 was observed at 50 mg L^-1 Fe³⁺ dosage with 4 mg L^-1 min^-1 ofH2O2 dosage rate.

4.6 Chemical degradability of carbofuran

A summary of different experiments and their main outcomes are presented in Table 4-5. This could be useful to evaluate the efficiency of each process in carbofuran degradation/mineralization. It can be seen in Table 4-5 that no carbofuran volatilization was observed in 120 min reaction; therefore, the carbofuran removal in the present study was solely by the treatment technique adopted. The simple oxidation techniques, i.e. H2O2, Fe³⁺, UV irradiation and the combination of H2O2 and Fe³⁺, have produced low carbofuran removal and insignificant DOC removal efficiencies. While H₂O₂ and Fe³⁺ were combined with UV irradiation, i.e. UV + H2O2 and UV + Fe³⁺, the carbofuran removal was increased to 84% and 77%, respectively, after 120 min reaction. However, the DOC removal was reached only less than 17% in these combinations. In these two cases, H2O2 had an oxidation potential of 1.78 V/SHE and high valence iron was responsible for the direct attack to organic matter justifying the results obtained [26,68,93]. These results reveal that even in the absence of Fe³⁺ and H2O2, the degradation and mineralization of carbofuran occur.

On the other hand, the photo-Fenton reaction has produced complete carbofuran removal (100%) in 30 min and up to 93% DOC removal in 120 min. These results

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Table 4-5. Carbofuran and DOC removals under various experimental conditions.

Experiment

H2O2

(mg L^-1 min^-1)

Fe³⁺

(mg L^-1)

Carbofuran

(mg L^-1)

pH

Removals after 120 min

Carbofuran (%)

DOC (%)

Control 0 0 50 3 0 0

UV 0 0 50 3 7 0

H2O2 4 0 50 3 10 0

Fe³⁺ 0 35 50 3 0 0

Fe³⁺ + H2O2 4 35 50 3 18 5

UV + H2O2 4 0 50 3 84 17

UV + Fe³⁺ 0 35 50 3 77 13

Photo-Fenton¹ 4 35 50 3 100 93

1UV + Fe³⁺ + H2O2

indicate that the photo-Fenton treatment is highly effective for the rapid degradation and mineralization of carbofuran. Also, in literature, the photo-Fenton system was reported as efficient process for the degradation of several pollutants [24,26,27]. In this study, H2O2 was supplied at a constant flow rate during the treatment, which could minimize the reagent cost. As a whole, the application of the photo-Fenton reaction with constant supply of H2O2 can generate rapid removal of carbofuran from aqueous systems.

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4.7 CCD of experiments for the photo-Fenton process

As mentioned in the introduction, several parameters i.e. pH, dosages of H2O2 and Fe³⁺ and carbofuran concentration affect the photo-Fenton degradation and mineralization efficiency of carbofuran. From Eqs. 2-1 to 2-3, it is expected the complex sequential reactions are pH dependent. Furthermore, the generation and consumption of ‧OH and iron species (Fe²⁺, Fe³⁺) all strongly influenced by pH [37].

The optimum pH value varies with different target compounds treated by photo-Fenton reaction. In addition, carbofuran concentration shows great negative effect on the oxidation of carbofuran. Therefore, plenty of experiments need to be conducted with the application of a four factor CCD. For this reason, pH and carbofuran concentration were selected to fix at 3 and 100 mg L^-1 according to the previous investigation, respectively.

The CCD is the most popular response surface design methodologies for fitting second-order polynomial equations in the design of experiment [36]. With the aim to find out favorable reagent dosages in the carbofuran degradation by estimating the coefficients of the second-order polynomial equation and constructing the response surface and contour plots, a two factor CCD was carried out using H2O2 dosage rate ranging from 1 to 10 mg L^-1 min^-1 and Fe³⁺ dosage from 1 to 100 mg L^-1. The responses (carbofuran and DOC removals and BOD₅/DOC ratio) at different reaction times are shown in Table 4-6. The influence of H2O2 dosage rate and Fe³⁺ dosage on carbofuran and DOC removals were determined at 45 and 60 min, while their influence on BOD5/DOC ratio were determined at 60 min. The central point was repeated three times (Runs 4, 5, 6) in Table 4-6 and the similar results were obtained indicating the reproducibility of experiment was quite well. The carbofuran removals were fitted well with the pseudo-first order reaction model (R² value greater than 0.96) and the highest

67

apparent rate constant of 6.3 × 10^-2 min^-1 was observed. However, this table can only provide the results of carbofuran and DOC removals and BOD5/DOC ration at certain experimental conditions. Therefore, CCD along with RSM is necessary to estimate the maximum responses at different H2O2 dosage rate and Fe³⁺ dosage.

4.7.1 The regression model coefficients

The coefficients of the second-order polynomial equation corresponding to each dependent variable were developed by multiple regression analysis using the MINITAB^® 14 statistical software. Carbofuran removals (Y1, 45 min, Y2, 60 min), DOC removals (Y_{3, 45 min}, Y_{4, 60 min}) and BOD₅/DOC ratio (Y_{5, 60 min}) were expressed as a function of H2O2 dosage rate (X1) and Fe³⁺ dosage (X2) as per Eqs. 4-2 to 4-6. The responses can be estimated from these empirical equations; moreover, the surface and contour plots constructed based on these equations can be used to find out the optimal reaction condition.

The regression coefficients and the probability P-values for all linear, quadratic and interaction effects of the variable on responses are shown in Table 4-7. The coefficient of variable in the equation represents the weight of itself to individual response, i.e. the contribution of first-order, quadratic and cross effects, the trend of response and interaction among variable. A positive sign for the coefficients of H2O2 dosage rate and Fe³⁺ dosage in the fitted model for all responses indicated that the responses increased with increased levels of reagent dosages.

The P-value was used to estimate the statistical significance and its value less than 0.05 in analysis of variance (ANOVA) indicates that the component is considered as

The P-value was used to estimate the statistical significance and its value less than 0.05 in analysis of variance (ANOVA) indicates that the component is considered as

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