Degradation of Orange II by UV-Assisted Advanced Fenton Process

Article pubs.acs.org/IECR

Degradation of Orange II by UV-Assisted Advanced Fenton Process: Response Surface Approach, Degradation Pathway, and Biodegradability Huiyuan Li, Yanhua Gong, Qianqian Huang, and Hui Zhang* Department of Environmental Engineering, Hubei Biomass-Resource Chemistry and Environmental Biotechnology Key Laboratory, Wuhan University, Wuhan 430079, China S Supporting Information *

ABSTRACT: Orange II in aqueous solution was degraded with hydroxyl radicals generated from zerovalent iron (ZVI) and hydrogen peroxide (H2O2) under UV irradiation. Results showed that the introduction of UV light could remarkably enhance the decolorization efficiency of Orange II in the ZVI/H2O2 process (advanced Fenton process, AFP). Central composite design (CCD) and the response surface methodology (RSM) were applied to investigate operating condition effects, such as initial pH, ZVI loading, and H2O2 concentration, on the decolorization efficiency at 30 min. The RSM model was derived and the response surface plots were developed based on the results. Moreover, the intermediates during the oxidation process were identified and the degradation pathway was proposed accordingly. After UV-assisted AFP, 68.3% of COD was removed and BOD5/COD ratio increased from 0.019 to 0.41, indicating the effluent was more suitable for biodegradation. Fe0 + 2H+ → Fe 2 + + H 2

1. INTRODUCTION Azo dyes, which contain nitrogen to nitrogen double bonds (−NN−), contribute more than 70% of all textile dyes produced, and are the most common chromophore in all kinds of reactive dyes.1 Large quantities of azo dyes are widely used in a variety of products. Their release into the environment is a major problem for life and a threat to the environment.2 Dye wastewaters are chemically and photolytically stable. Generally, they are resistant to weather, light, water, and detergent. They may be degraded slowly via conventional biological processes, generating colored treated effluents.3 In recent years, much research has been conducted on treating azo dye wastewater using advanced oxidation processes (AOPs), including Fenton,4,5 ozonation,6,7 photoassisted AOPs,8,9 and so on. Fenton’s reagent is a mixture of hydrogen peroxide (H2O2) and ferrous ions, which generates hydroxyl radicals (•OH) according to the following reaction.10 Fe 2 + + H 2O2 → Fe3 + + •OH + OH−

The ferrous ion reacts rapidly with H2O2 to produce hydroxyl radicals via eq 1, and in the meantime generate ferric ions. These ferric ions are then reduced to ferrous ions by further interaction with the ZVI surface at a faster rate compared to the homogeneous process.15,16 Fe 0 + 2Fe3 + → 3Fe 2 +

(3)

This process is known as the advanced Fenton process (AFP).11,17−19 The advantage of the implementation of ZVI powder instead of iron salts is the avoidance of unnecessary loading of aquatic system with counteranions. In addition, the amount of iron ions present in wastewater treated by the AFP is significantly lower than those by the classical Fenton process.20,21 Moreover, in the presence of dissolved oxygen, H2O2 can also be generated through iron corrosion (eq 4).22,23 Fe 0 + O2 + 2H+ → H 2O2 + Fe 2 +

(1)

(4)

In the AFP, H2O2 may be consumed slowly during the process. However, in the presence of both H2O2 and dissolved oxygen, the continuous production of ferrous ions and H2O2 by ZVI corrosion can generate hydroxyl radicals and facilitate the degradation of contaminants.23 UV-assisted AOPs have been successfully used in the degradation of many contaminants.24−26 In the case of the AFP, the presence of UV irradiation will accelerate the reactions in different ways: (i) iron corrosion enhancement (eq 5);27−29 (ii) enhancing the reduction of ferric ions to

As is well-known, hydroxyl radicals are strong oxidants (oxidation potential: 2.8 V) which can nonspecifically oxidize target compounds at high reaction rates of the order of 109 M−1 s−1. The main advantage of Fenton reaction is the complete destruction of contaminants to harmless compounds, e.g. carbon dioxide, water, and inorganic salts.10 For efficient reaction, stoichiometric amounts of ferrous ions and H2O2 are required and this usually means that large quantities of ferric salts need to be disposed of after the reactions are complete.11 In the last decades, it has been reported that zerovalent iron (ZVI) can also drive oxidative reactions through hydroxyl radicals in the presence of H2O2 or oxygen.12−14 In acidic conditions, the corrosion of ZVI would give rise to the generation of ferrous ion and hydrogen gas (eq 2). © 2013 American Chemical Society

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Received: Revised: Accepted: Published: 15560

May 11, 2013 September 23, 2013 October 4, 2013 October 4, 2013 dx.doi.org/10.1021/ie401503u | Ind. Eng. Chem. Res. 2013, 52, 15560−15567

Industrial & Engineering Chemistry Research

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ferrous ions (eq 6);30,31 and (iii) direct photolysis of H2O2 (eq 7).32,33 Fe0 + hν → Fe2 + + 2e−

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Fe3 + + H 2O + hν → Fe 2 + + •OH + H+

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H 2O2 + hν → 2•OH

(7)

initial pH was adjusted by sulfuric acid or sodium hydroxide. Before the experiments started, air was bubbled into the solution for 30 min in the dark to ensure the saturation of oxygen in the aqueous solution. In this way, H2O2 can be generated via eq 4 to enhance the Fenton reaction. Then the required amount of H2O2 and ZVI was added to a 200-mL stirred dye solution and the UV light was switched on. The mixing was provided by a mechanical stirrer. At selected time intervals, samples were withdrawn and filtered through 0.22 μm membranes prior to analysis. 2.2. Analytical Methods. Specific surface area (SBET) of ZVI was determined by the N2 Brunauer−Emmett−Teller (BET) method using a Quantachrome NOVA2000e analyzer (USA) at 77 K. Prior to N2 physisorption, the sample was degassed under vacuum at 500 °C for 3 h. The total pore volume (Vpore) was determined on the plateau of the adsorption branch at P/P0 = 0.98. The pore size, Dp, was evaluated using the Barret−Joyner−Halenda (BJH) method applied to the adsorption branch. The physical properties of ZVI used in the experiments are collected in Table 1.

Based on the above mechanisms, the particular advantages of the AFP are the cost savings due to the use of metal iron instead of iron salts and the faster recycling of ferric iron at the iron surface.15 The presence of UV light and dissolved oxygen also enhances continuous production of hydroxyl radicals to mineralize the contaminants. Therefore, in the present study, UV-assisted AFP in the presence of dissolved oxygen was employed to degrade a reactive azo dye, Orang II. Usually experiments are carried out using the traditional onefactor-at-a-time approach. Experimental factors are varied one at a time, with the remaining factors being held at constant. This method assumes that various treatment parameters do not interact and the response variables are only functions of the single varied parameters. It is not only time-consuming, but also usually incapable of reaching the true optimum due to ignoring the interactions among variables.34−36 Therefore, response surface methodology (RSM), a multivariate technique which mathematically fits the experimental domain studied in the theoretical design through a response function, is then proposed to solve these problems. Using RSM, it is possible to estimate linear, interaction and quadratic effects of the factors and a prediction model for the response. In this way, RSM designs can be used to find improved or optimal process settings in an efficient use of resources.37−39 Different types of RSM designs include three-level factorial design, central composite design (CCD), Box-Behnken design (BBD), and D-optimal design.38−40 Among all these experimental designs, the CCD gives almost as much information as a multilevel factorial, requires much fewer experiments than a full factorial, and allows a reasonable amount of information for testing lack of fit while not involving an unusually large number of design points.37,41 Therefore, in this study, CCD was performed in order to investigate the effects of initial solution pH, ZVI loading, and H 2 O 2 concentration on the decolorization efficiency of Orange II. To further investigate the mechanism, gas chromatography− mass spectrometry (GC-MS) was applied to determine the intermediates, and a plausible degradation pathway of Orange II is proposed accordingly. Finally, the biodegradability in terms of BOD5/COD was also monitored during the reaction.

Table 1. Physical Properties of ZVI sample

SBET (m2/g)

Vpore (cm3/g)

Dp (nm)

ZVI

1.426

5.512 × 10−3

15.46

The solution pH was measured by a Mettler-Toledo FE20 pH meter (Mettler-Toledo Instruments Co., Ltd., Shanghai). A Rayleigh UV-9100 spectrophotometer (Rayleigh Co., China) was employed to determine the absorbency of Orange II at the maximum absorption wavelength 485 nm. UV−vis spectra of each sample were recorded between 200 and 800 nm on a UV1700 spectrophotometer (Shimadzu). COD was determined using a closed reflux spectrophotometric method based on the Standard of the People’s Republic of China for Environmental Protection.42 BOD5 was measured using Oxi-top BOD analyzer (WTW Oxitop IS6, Germany). For GC-MS analyses, the samples at different time intervals were centrifuged and the aqueous phase was extracted with CH2Cl2 (10 times, 20 mL each time). The extraction liquid was then concentrated to 1 mL on a rotatory evaporator, and was kept at 4 °C prior to GC analyses. The identification of intermediates in the degradation was performed on a GC-MS system (GCMS-QP2010, Shimadzu) equipped with a HP-5MS capillary column (30 m × 0.25 mm × 0.25 μm) under the following conditions: carrier gas (He), 5.5 mL/min; electron ionization, 70 eV; and mass scan range, 45−500 m/z. The temperature was held at 40 °C for 10 min and ramped at 12 °C/min to 100 °C, then ramped to 200 °C at a rate of 5 °C/ min, and finally ramped at 20 °C/min to 270 °C and held at 270 °C for 5 min. 2.3. Central Composite Design (CCD). The central composite design was applied to investigate the effects of the three independent variables on the response functions. The independent variables were initial pH (X1), ZVI loading (X2), and concentration of H2O2 (X3). Considering this work focused on the color removal and the improvement of the biodegradability of azo dye, and based on the preliminary results, the experimental levels for each variable were selected and are summarized in Table 2. The notations (−1) and (+1) refer to the low level and the high level of the two-level-factorial design, respectively. The notations of (−1.414), (+1.414), and (0) are those levels of star points and center point used. The

2. MATERIALS AND METHODS 2.1. Chemicals and Experimental Setup. Orange II (C16H11N2NaO4S) from Shanghai No. 3 Reagent Factory (China) was used without further purification. ZVI powder was obtained from Shanghai No. 2 Metallurgical Plant. The powder was immersed in 1 M hydrochloric acid for 10 min to remove the oxide on the surface and then washed with deionized water for more than three times. H2O2 (analytical grade, 30% w/w) was obtained from Shanghai Sinopharm Chemicals Reagent Co. Ltd. (China). A stock solution of 100 mg/L Orange II was freshly prepared with deionized water before each run. A UV lamp (15 W, 254 nm, Xinghui Lamp Co. Ltd., Hunan) was placed vertically in the center of a beaker and immersed into the solution. The 15561

dx.doi.org/10.1021/ie401503u | Ind. Eng. Chem. Res. 2013, 52, 15560−15567


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Table 2. Experimental Range and Levels of the Independent Variables

Table 3. Design Matrix in Coded Units and the Experimental Responses

variables

symbol

−1.414

−1

0

+1

+1.414

run

X1

X2

X3

decolorization efficiency (%)

initial pH, A ZVI (g/L), B H2O2 (mM), C

X1 X2 X3

1.6 0.6 0.2

2.0 1.0 0.4

3.0 2.0 0.8

4.0 3.0 1.2

4.4 3.4 1.4

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

−1 1 −1 1 −1 1 −1 1 −1.41 1.41 0 0 0 0 0 0 0 0 0 0

−1 −1 1 1 −1 −1 1 1 0 0 −1.41 1.41 0 0 0 0 0 0 0 0

−1 −1 −1 −1 1 1 1 1 0 0 0 0 −1.41 1.41 0 0 0 0 0 0

94.3 30.5 97.8 29.1 97.6 46.8 98.0 57.5 98.9 36.5 65.9 79.2 52.9 74.1 70.2 69.7 69.6 69.8 71.6 71.2

coded values of the independent variables were coded as Xi according to eq 8 x − x0 Xi = i Δxi (8) where Xi is the dimensionless value of an independent variable, xi represents the real value of the independent variable, x0 is the real value of the independent variable at the center point, and Δxi is the step change.35 The dependent variable or objective function was the decolorization efficiency at 30 min, which was determined by the following equation: A −A Decolorization efficiency(%) = 0 × 100 A0 (9) where A0 and A were the absorbance of Orange II at time 0 and t, respectively. The three independent variables (X1, X2, and X3) and the mathematical relationship describing the response of these variables can be approximated by the following quadratic polynomial equation Y = β0 + β1X1 + β2X 2 + β3X3 + β12X1X 2 + β13X1X3 + β23X 2X3 + β11X12 + β22X 2 2 + β33X32

(10)

where Y is the response value, Xi are the coded values of factors, β0 is the constant, and βi, βii, and βij are the linear, quadratic, and interaction coefficients, respectively.43 As described in our previous research,44 eight (= 23) runs of two-level-factorial-design experiments for three parameters were performed randomly (Runs 1−8 in Table 3). To check the assumption of linearity in the factor effects, further experiments to the star points and center point were performed randomly based on the conditions illustrated in Table 3.

Figure 1. Degradation of Orange II in different processes (C0 = 100 mg/L, initial pH 3.0, ZVI loading 2.0 g/L, H2O2 concentration 0.8 mM, UV 15 W, 254 nm).

3. RESULTS AND DISCUSSION 3.1. Degradation of Orange II in Different Processes. Experiments were conducted under different conditions (UV, UV/ZVI, UV/H2O2, ZVI/H2O2, and UV/ZVI/H2O2) and the results are depicted in Figure 1. The photodegradation of Orange II was negligible due to the fact that most commercially dyes are usually designed to be light resistant. Therefore, they are rather difficult to be degraded under UV light without addition of other agents45 (data not shown). In the ZVI/H2O2 process (AFP), the produced hydroxyl radicals could attack the molecules of Orange II, causing 49.5% of color removal. This indicates that the decolorization capacity in the AFP is still limited. The decolorization efficiencies of Orange II were similar in UV/ZVI and UV/H2O2 processes (66.1% and 64.4%, respectively), which were higher compared to that in the AFP.This suggests that the introduction of UV light can significantly enhance the degradation of the contaminants, which is in agreement with several studies.46−48 In the UV/ H2O2 process, it was mainly the hydroxyl radicals generated through eq 7 that contributed to the decolorization. In the UV/

Figure 2. Predicted vs actual plot for decolorization efficiency.

ZVI process, the decolorization was caused by the reducing ability of ZVI. The introduction of UV light can accelerate the oxidation rate of ZVI to ferrous ion, as shown in eq 5.28 As a result, ZVI loses electrons, which will be accepted by dye molecules. Then they combine with H+ and result in the cleavage of azo bonds (eq 11).49 −NN−+2H+ + 2e− → −NH + HN− 15562

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Figure 4. UV−vis spetra changes with reaction time (C0 = 100 mg/L, initial pH 2.0, ZVI loading 3.0 g/L, H2O2 concentration 0.4 mM, UV 15 W, 254 nm).

Table 4. GC-MS identified reaction intermediates during Orange II degradation

Y = 70.00 − 26.01X1 + 2.67X 2 + 6.52X3 − 0.68X1X 2 + 5.15X1X3 + 1.13X 2X3 − 0.63X12 + 1.79X 2 2 − 2.73X32

(12)

The analysis of variance (ANOVA) results of the established model show that the p-value of the model is less than 0.0001. The p-value less than 0.0500 indicates that the model terms are significant, while the value greater than 0.1000 indicates terms are not significant. In the present work, the very low p-value (< 0.0001) demonstrates the model is significant.39,50 The determination coefficients R2 of the quadratic regression model were determined to be 0.9820. The R2 value indicates how much of the variability in the data is accounted for by the model. This implies that 98.20% of the variations for the decolorization efficiency are explained by the independent variables and this also means only about 1.80% of the variations are not explained by the model. Adjusted R2 (Adj-R2) is also a measure of goodness of fit, but it is more suitable for comparing models with different numbers of independent variables. It modified the R2 value by taking into account the number of covariates or predictors in the model. Here, Adj-R2 value (0.9658) was close to the corresponding R2 value. These higher

Figure 3. Response surface plots showing decolorization efficiency as a function of two independent variables (other variables were held at their respective center levels). (a) Initial pH (X1) and ZVI loading (X2), (b) initial pH (X1) and H2O2 concentration (X3), (c) ZVI loading (X2) and H2O2 concentration (X3).

Among all the processes observed, the UV-assisted AFP achieved the highest color removal. It can be clearly seen in Figure 1 that the decolorization rate was the highest and the decolorization efficiency reached 83.2% after 60 min reaction. The results showed that UV irradiation and the AFP had a significant synergetic effect on the degradation of Orange II. 3.2. Response Surface Evaluation. On the basis of the results obtained in each run (Table 3), RSM model of second order polynomial equations was determined as follows: 15563



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Figure 6. Changes of COD, BOD5, and biodegradability with reaction time (C0 = 100 mg/L, initial pH 3.0, ZVI loading 2.0 g/L, H2O2 concentration 0.8 mM, UV 15 W, 254 nm).

the residuals follow a normal distribution, in which case the points will follow a straight line. S-shaped curve was not formed, indicating that there is no apparent problem with normality and no need for transformation of response.39,50 Furthermore, the studentized residues versus predicted value plot (SI Figure S2) shows that the residues appear to be a random scatter, suggesting the variance of original observations is constant for all values of the response.50,52 The comparison between the predicted values of the decolorization efficiency and the experimental points is presented in Figure 2. All the points are distributed relatively near to the regression line. The graph confirms that the proposed predicted values are in good agreement with the observed ones.39,53 The corresponding response surface plots obtained from the RSM equation are presented in Figure 3. The response surface in Figure 3a and Figure 3b clearly shows that acidic condition is beneficial for the decolorization of Orange II. Most studies have reported the same results that Fenton process favors lower pH (usually 2 and 3) because iron powder is easily dissolved under acidic conditions, producing ferrous ions based on reaction 2.4,20,54 In addition, the amount of hydroxyl radicals is also dependent on ZVI loading and H2O2 concentration. Based on reactions 1, 5, and 7, higher ZVI and H2O2 concentration will generate more hydroxyl radicals and accelerate the reaction. Therefore, the decolorization efficiency gets higher with more ZVI and H2O2 addition, as shown in Figure 3b and Figure 3c. 3.3. Degradation Pathway. To clarify the changes of molecular and structural characteristics of Orange II during the oxidation process in UV-assisted AFP, samples were taken at time intervals of 0, 5, 10, 20, and 30 min and representative UV−vis spectra changes as a function of reaction time were observed. The corresponding spectra are shown in Figure 4. The azo dye, Orange II, basically consists of an azo linkage (−NN−), a benzene ring, and a naphthalene ring, all of which exhibit different absorbance peaks. The maximum absorption wavelength for Orange II was determined to be 485 nm, which accounts for the orange color of solutions and can be attributed to the azo linkage. The two other bands in the ultraviolet region located at 229 and 310 nm are related to the naphthalene and benzene rings, respectively.55−57 As the reaction proceeded, the visible band remarkably disappeared and this was mainly due to the fragmentation of the azo links by oxidation. In addition, the decay of the absorbance at 229 and 310 nm was considered as evidence of aromatic fragment degradation in the dye molecule and its intermediates.20 At the same time, a new peak at 248 nm appeared and its absorbance

Figure 5. Proposed pathways for degradation of Orange II (C0 = 100 mg/L, initial pH 3.0, ZVI loading 2.0 g/L, H2O2 concentration 0.8 mM, UV 15 W, 254 nm).

R2 coefficients ensured satisfactory adjustment of the quadratic models to the experimental data.37,43 Moreover, adequate precision measures the signal-to-noise ratio and a ratio greater than 4 is generally desirable.41,50,51 Therefore, the ratio of 26.751 indicates adequate signals for the models to be used to navigate the design space. The checking of model adequacy is an important part of the data analysis procedure, as it will give poor or misleading results if it is an inadequate fit. The normal probability plot (Supporting Information (SI) Figure S1) indicates whether 15564



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reached maximum at 10 min, and then gradually decreased. It is speculated that this peak is associated with some intermediates formed during the reaction. These intermediates are then degraded by hydroxyl radicals generated in the process. To further identify the intermediate products, GC-MS analysis was employed and the products identified are shown in Table 4. A plausible degradation pathway is proposed accordingly (Figure 5). Based on the products detected, it is assumed that the degradation process is initiated by cleavage of the C−N bond due to the oxidative attack of hydroxyl radicals, which leads to the formation of 4-diazenylbenzosulfonate and 1,2-naphthoquinone (G).58 The intermediates coumarin (D) and nenadione (F) may be downstream products derived from G. Then they are further oxidized, leading to cleavage of the naphthoquinone ring, thereby generating phthalic acid (E) and later become ring-opening products.59,60 4-Diazenylbenzosulfonate is unstable and it is oxidized to phenol with the attack of hydroxyl radicals. At the same time, nitrite, nitrate, and N2 are produced from nitrogen of the azo group of Orange II. Benzoquinone (B) is formed as a consequence of phenol oxidation, which then undergoes ring-opening reactions leading to the formation of short-chain aliphatic acids (A, C). Finally, these organics are mineralized into CO2 and H2O. 3.4. Changes of Biodegradability with Reaction Time. As illustrated in Figure 6, the initial COD of Orange II was 119.3 mg/L, and it dropped to 37.9 mg/L after 30 min of treatment, reaching 68.2% removal efficiency. This result suggests that the UV-assisted AFP is effective in the degradation of Orange II. On the other hand, BOD5 was first increased from 2.26 to 18.60 mg/L, and then decreased to 15.72 mg/L. This may be due to nonbiocompatible organics first transformed to biocompatible organics such as aliphatic acids (Table 4), while these intermediates were ultimately oxidized to CO2, accounting for decrease of BOD5.61 A ratio of BOD5/COD is normally used to express the biodegradability of the wastewater. When the value of BOD5/ COD is more than 0.3, the wastewater has a better degradability. Otherwise, the wastewater is difficult to be biodegraded.62 As shown in Figure 6, the relatively low initial BOD5/COD value (0.019) indicated that Orange II solution was rather recalcitrant to biodegradation. As the reaction proceeded, the ratio of BOD5 and COD gradually increased and achieved 0.41 after 30 min treatment. The results suggest that the UV-assisted AFP is effective in both removing COD and enhancing the biodegradability of Orange II.

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• According to the products identified, it is suggested that the degradation process of Orange II is initiated by cleavage of the C−N bond and further undergoes ringopening reaction to cause mineralization. • The COD removal efficiency reached 68.2% after 30 min reaction, and the ratio of BOD5 and COD increased to 0.41 after treatment, suggesting the effluent is biodegradable.

ASSOCIATED CONTENT

S Supporting Information *

The internally studentized residues and normal % probability plot for decolorization efficiency of Orange II, and residues vs actual plot for decolorization efficiency. This information is available free of charge via the Internet at http://pubs.acs.org/.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: 86-27-68775837; fax: 86-27-68778893; e-mail: eeng@ whu.edu.cn. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Natural Science Foundation of Hubei Province, China (Grant 2012FFA089). We appreciate the valuable comments and suggestions from the anonymous reviewers.

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REFERENCES

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4. CONCLUSIONS In this study, response surface methodology was applied to the degradation of Orange II in the UV-assisted advanced Fenton process based on central composite design. The intermediates during the reaction were identified by GC-MS and a plausible degradation pathway is proposed. Furthermore, the variations of biodegradability were also investigated. Based on the obtained results, the following conclusions are drawn: • Orange II can be decolorized rapidly in the UV-assisted advanced Fenton process. Lower initial pH and higher ZVI and H2O2 concentration are beneficial for Orange II decolorization. ANVOA indicates that the proposed regression model is in agreement with the experimental data with the high determination coefficient and high adjusted determination coefficient. 15565



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Degradation of Orange II by UV-Assisted Advanced Fenton Process

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