Fenton degradation of 4-chlorophenol contaminated water promoted by solar irradiation

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Solar Energy 84 (2010) 59–65 www.elsevier.com/locate/solener

Fenton degradation of 4-chlorophenol contaminated water promoted by solar irradiation Wen S. Kuo *, Li N. Wu Department of Safety, Health, and Environmental Engineering, National United University, 1 Lien-Da, Kung-Ching Li, Miao-Li 360, Taiwan Received 19 May 2009; received in revised form 6 September 2009; accepted 2 October 2009 Available online 31 October 2009 Communicated by: Associate Editor Gion Calzaferri

Abstract The degradation of 4-chlorophenol (4-CP) contaminated water by Fenton process with or without solar irradiation assistance were investigated. It was found that the COD degradation and mineralization efficiency of 4-CP were more than 90% when a 30 min treatment of solar photo-Fenton oxidation process was applied and under an optimum [H2O2]0/[Fe2+]0 ratio of 40, the COD degradation and mineralization efficiency increased 65% as compared to Fenton oxidation. Meanwhile, the AOS values increased from 0.33 to 2.13 in solar photo-Fenton process while no significant improvement for AOS values was found in Fenton process, implying a higher degree of oxidation for 4-CP in solar photo-Fenton process. In addition, increasing the intensity of solar irradiation seemed to be beneficial for treatment of 4-CP contaminated water. Formation of chloride ion as a result of mineralization of organically bounded chlorine was identified during the treatment of 4-CP solution. Near-stoichiometric accumulation of chlorine was observed during the degradation of 4-CP in both Fenton and solar photo-Fenton processes. However, accumulation rate of chloride ions were much faster in solar photo-Fenton process. The degradation of 4-CP was found to obey a pseudo-first-order reaction kinetics. As compared to Fenton process, the presence of solar light in photo-Fenton process increases the reaction rate by a factor of 6.5 and 15.8 for COD and TOC degradation, respectively. In other words, during the treatment of 4-CP contaminated water, solar photo-Fenton process possesses notably higher mineralization efficiency in a relatively short radiation time as compared to Fenton process, and could enhance the degradation treatment of refractory organic wastewater such as 4-CP in a cost-effective approach. Ó 2009 Elsevier Ltd. All rights reserved. Keywords: Solar photo-Fenton process; 4-Chlorophenol; Mineralization efficiency; AOS; Chloride

1. Introduction Chlorophenols (CPs) have been notified as potential toxic compounds by United States Environmental Protection Agency (USEPA) and constitute an important category of organic water pollutants that are not readily biodegradable (Abe and Tanaka, 1997). Consequently, conventional biological treatment is not very effective and activated carbon adsorption is commonly used for removing CPs from chemical effluents. However, the need of frequent carbon reactivation renders this process both *

Corresponding author. Tel.: +886 37 381763; fax: +886 37 333187. E-mail address: [email protected] (W.S. Kuo).

0038-092X/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.solener.2009.10.006

inconvenient and costly. Advanced oxidation processes (AOPs) have been previously described as a promising option to remove persistent pollutants from contaminated water (Al Momani et al., 2004; Pera-Titus et al., 2004) when conventional water treatment processes are not efficient enough. AOPs are able to produce a highly reactive, nonspecific oxidant, mainly hydroxyl radicals (OH). The hydroxyl radical possesses inherent properties that enable it to attack refractory organic pollutants in water to possibly obtain a complete mineralization. Fenton process which is one of AOPs, using a mixture of ferrous ion (Fe2+) and hydrogen peroxide as reagents, has been used extensively for oxidation of organics in water such as phenols (Kavitha and Palanivelu, 2004), chlorinated

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phenol (Kavitha and Palanivelu, 2003; Ghaly et al., 2001; Raja et al., 2005) and pesticides (Huston and Pignatello, 1999), and to reduce the chemical oxygen demand (COD) and total organic carbon (TOC) content. Since normally less than 50% of the organic carbon can be converted to CO2 (Balcioglu and Arslan, 1999), Fenton’s reaction cannot completely mineralize organic pollutants. Furthermore, substantial Fe3+-containing sludge generated from this process requires further treatment and then increases operation cost. To reduce the amount of Fe3+-containing sludge and increase treatment efficiency, Fenton process assisted by light irradiation, especially using natural solar UV–vis light as a light source, could be a potentially powerful method due to a lower Fe2+ dosage and energy cost needed. When the process uses ultraviolet (UV) radiation, visible light with wave length (k) less than 450 nm (Sagawe et al., 2001) or a combination of both, the process is known as the photo-Fenton process. The photo-Fenton process has several advantages, mainly an increase of the degradation rate and a lower sludge generation (Malato et al., 2002; Salvadori et al., 2002). The photo-Fenton process starts with the combination of H2O2 with Fe2+ (Eq. (1)): Fe2þ þ H2 O2 ! Fe3þ þ OH þ OH

ð1Þ 3+

When irradiation (k < 450 nm) is involved, the Fe generated by Fenton’s reaction (Eq. (1)) is continuously reduced to Fe2+ as described by Eqs. (2) and (3): 2þ

Fe3þ þ H2 O ! FeðOHÞ FeðOHÞ

2þ

þ hm ! Fe

2þ

þ Hþ

þ OH

ð2Þ ð3Þ

Most of the photo-Fenton oxidation studies on CPs have focused on combining artificial UV light with Fenton process. Theoretically, ferrous ions can be regenerated through the use of solar irradiation because there is 20% of solar irradiation with k < 450 nm (Fallmann et al., 1999). The possibility of applying natural solar light in this study to take the place of artificial UV light would pave a new way for industrial wastewater treatment. Although it is believed that the solar assisted photo-Fenton process may have a higher efficiency for treating organic wastewater, the promoted effects of solar irradiation on Fenton process and related parameters or influences are still not clear nowadays. In this study, the effect of solar irradiation on Fenton degradation of one of chlorophenols, 4-chlorophenol (4-CP) was investigated. Specifically, the degradation efficiency, mineralization efficiency, average oxidation state (AOS) variation and kinetics of 4-CP towards both Fenton and solar photo-Fenton processes were compared. In addition to solar irradiation intensity, effect of other parameters including H2O2 dosage and Fe2+ dosage were examined as well.

peroxide (H2O2) with a concentration of 35% was supplied by Panreac Co., USA. Analytical-grade FeSO47H2O (Panreac Co., USA) was used as a source of ferrous ion. HPLCgrade methanol (J.T. Baker Co., USA) was employed as eluent. All other chemicals used in this study were analytical grade and used as received. 2.2. Procedures All experiments were carried out in a batch mode. A 1-L glass beaker containing 500 mL of 1 mM 4-CP solution was used and maintained at a temperature of 30 °C by using a water bath during the experiments at room and solar conditions. The initial pH of the solution was adjusted to pH 3.0 ± 0.1 using 0.1 M H2SO4. The concentration of hydrogen peroxide adding to 4-CP solution was in the range of 2.5–20.0 mM. The amount of Fe2+ ion used for each run was 0.2, 0.3, or 1.0 mM to make a initial ratio of [H2O2]/[Fe2+] being 2.5–100. For solar assisted photoFenton process, experiments were conducted at National United University, Taiwan (N24°110 3300 , E120°340 4800 ), under clear sky conditions. The solar intensity was measured by using a pyranometer (Li-200SA) with a Li-250 radiation indicator (Li-COR Co., USA). Two solar intensities in the range of 750–800 W/m2 (namely S800) and 375– 425 W/m2 (namely S400), respectively, were applied during the experimental runs. The pH values of the solutions were monitored using a pH meter (SP-701LI 120, Suntex Co., Taiwan) equipped with a glass electrode during the experiments. Samples were withdrawn from the reactor at preset time intervals, tested for H2O2 consumption according to the method reported by Seller (1980) and concentration of iron as Fe2+ ion determined by Fe(II)/1,10-phenanthroline complex at 510 nm (APHA et al., 2000) using a Hitachi (Japan) U-2001 spectrophotometer instantly, and then quenched with sodium hydrogen sulfite to avoid further reactions. The samples were then stored at 4 °C for the following HPLC, TOC, COD and IC analysis. 2.3. Analytical methods 2.3.1. Analytical measurement of 4-CP Residual 4-CP in solution was analyzed by HPLC using a Jasco system (Japan). This system was equipped with two PU1580 pumps and an UV1575 detector setting at a wavelength of 275 nm for 4-CP analysis. A Supelco C-18 reversed phase column (L: 25 cm, ID: 4.6 mm, particle size: 5 lm) was used. The mobile phase was a mixture of methanol (60%) and deionized water (40%). The flow rate of mobile phase was set at 1 mL/min. Under the analytical conditions, the retention time of 4-CP was 12.5 ± 0.1 min.

2. Materials and methods 2.1. Materials 4-CP with a purity of 99% were purchased from Acros Co., USA and used without further purification. Hydrogen

2.3.2. COD and TOC analysis COD is an important parameter that was measured in order to know how the degree of oxidation of 4-CP changes. TOC of solution was measured in order to known the amount of 4-CP that was degraded to CO2 during

W.S. Kuo, L.N. Wu / Solar Energy 84 (2010) 59–65

oxidation. COD was carried out via a DR 4000 photometer (HACH Co., USA) by using a K2Cr2O7 reagent. TOC was measured by using a Shimadzu VCPH analyzer (Shimadzu Co., Japan). 2.3.3. IC analysis The quantitative determination of acetate, oxalate, and chloride (formed as a result of 4-CP degradation) during Fenton and solar photo-Fenton processes were carried out in an ion-chromatographic system (Dionex ICS-1000) equipped with an IonPac AS12A column (L:200 mm, ID: 4.0 mm). Aqueous solutions of 2.7 mM Na2CO3 and 0.3 mM NaHCO3 were used as the eluents, maintained at a flow rate of 1.5 mL/min and operated on an isocratic mode. The retention times for chloride, acetate, and oxalate peaks were detected at 3.1, 2.1, and 12.3 (±0.1) min, respectively. 3. Results and discussion 3.1. Effect of solar irradiation on degradation efficiency of 4-CP Table 1 showed the degradation and dechlorination efficiency of 4-CP in the first 3 min reaction by Fenton and solar assisted photo-Fenton oxidation. As shown in Table 1a, 4-CP was degraded quickly and almost completely by both Fenton and solar photo-Fenton processes. No significant promoted effect was found for 4-CP degradation by solar irradiation even at a lower H2O2 dosage (2.5 mM). It illustrated that the molecular structure of 4-CP was easily attacked and disrupted by OH radicals. One of the initial degradation pathways of 4-CP could be through dechlorination as evidenced by the formation of chloride Table 1 Degradation and dechlorination of 4-chlorophenol under different operation conditions*. Process

H2O2, mM 2.5 (%)

5.0 (%)

7.5 (%)

a. Degradation efficiency S800 98.23 S400 97.22 Fenton 95.85

99.99 99.99 99.99

99.99 99.99 99.99

b. Dechlorination efficiency S800 69.99 S400 68.13 Fenton 57.25

89.77 79.25 78.51

99.99 96.62 95.14

c. Dechlorination ratio (b/a) S800 71.25 S400 70.08 Fenton 59.73

89.78 79.26 78.52

100.00 96.63 95.15

d. Chlorinated-intermediates ratio (a–b) S800 28.24 S400 29.09 Fenton 38.60

10.22 20.74 21.48

0.00 3.37 4.83

*

Operation conditions: [4-CP]0: 1 mM, [Fe2+]0: 0.3 mM, reaction time (RT): 3 min.

61

in solution. However, as shown in Table 1b, solar irradiation showed a positive effect on the dechlorination efficiency of 4-CP. Furthermore, increasing intensity of solar irradiation and H2O2 dosage increased the dechlorination efficiency of 4-CP, leading to a higher dechlorination ratio and less chlorinated-intermediates generated (Table 1c and d). Under a solar intensity of S800 and a H2O2 dosage of 7.5 mM, the degradation of 4-CP was almost through dechlorination and no chlorinated-intermediate was found in solution. In contrast, under conditions of lower H2O2 dosage and solar irradiation, more chlorinated-intermediates were produced in the initial degradation stage of 4-CP solution. This could be due to the fact that both phenolic AOH and ACl groups of 4-CP molecule possess strong electron-attracting character and could be attacked by OH radicals through nucleophilic addition, resulting in the formation of free inorganic chloride ions during the oxidation processes, which is a clear indication for 4-CP decomposition. The presence of chloride ions during the first 3 min of the reaction suggested that OH radical could attack the carbon atom carrying ACl group, thereby eliminated the organically bound chlorine as inorganic chloride (Cl) ion. The concentration of chloride ions increased with increasing solar light intensity and H2O2 dosage, suggesting the presence of minor amounts of other chlorinated compounds. At a lower H2O2 dosage (2.5 mM), about 38.6% and 28.2% of 4-CP was not degraded through dechlorinated pathway for Fenton and solar photo-Fenton process (S800), respectively. However, as increasing the dosage of H2O2 to 7.5 mM, almost all 4-CP in solution was degraded initially through dechlorination pathway for solar photo-Fenton process, while there was still up to 5% of 4-CP degradation was through other pathway for Fenton process. Because the 4-CP molecules were almost disrupted in the first 3 min reaction, the COD and TOC degradation of 4-CP solution were further used to investigate the mechanism of Fenton oxidation promoted by solar irradiation. Moreover, since the degradation efficiency of 4-CP was also influenced by the dosage of Fenton’s reagents, the effect of H2O2 and Fe2+ dosage were investigated as well. As shown in Fig. 1, the degradation efficiency of COD and TOC increased significantly as H2O2 dosage increased from 0 to 12.5 mM, which may be due to the additional OH radicals produced by higher amount of H2O2. However, as H2O2 dosage exceeded 12.5 mM, no significant improvement in degradation efficiency was observed, possibly due to the fact that the excess H2O2 would serve as the scavenger of hydroxyl radicals instead. Considering degradation efficiency and operation cost, an optimum oxidant concentration of 12.5 mM of H2O2 was therefore selected for the rest of experimental runs. The effect of ferrous ion dosage on degradation of 4-CP was illustrated in Fig. 2. Increasing the ferrous ion concentration from 0.2 to 0.3 mM increased the COD and TOC removal efficiency from 82% and 75% to 96% and 94%, respectively. Almost complete COD degradation with 96% of efficiency can be

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W.S. Kuo, L.N. Wu / Solar Energy 84 (2010) 59–65

Degradation Efficiency , %

100

80

60

40

COD TOC

20

0 0.0

2.5

5.0

7.5

10.0 12.5 15.0

17.5 20.0

initial H2O2 dosage , mM

Fig. 1. Effect of initial H2O2 dosage on degradation of 4-chlorophenol solution ([4-CP]0: 1 mM, [Fe2+]0: 0.3 mM, solar light intensity: S800, RT: 30 min).

Degradation Efficiency, %

100

80

60

40

COD TOC

solar light and promoted the recombination of hydroxyl radicals as remarked by Ghaly et al. (2001). Under the optimum dosage of Fenton’s reagents, the COD degradation of 4-CP by different solar light intensity was shown in Fig. 3. In Fenton process, i.e. no solar light irradiation, the OH radical generated according to Eq. (1) was responsible for the oxidation process, resulting in only 30% COD degradation of 4-CP, in contrast to 80% and 95% COD degradation in solar assisted photo-Fenton process with S400 and S800 irradiation, respectively. Increasing solar light intensity could increase degradation efficiency of 4-CP significantly. The increased efficiency promoted by solar irradiation was mainly attributed to the photo-reduction of ferric ions formed during Fenton reaction to ferrous ion as described in Eq. (3). Consequently, the presence of more ferrous ions in solution increased the utilization efficiency of H2O2 (Fig. 4), leading to additional OH radical generation to degrade 4-CP. The utilization of H2O2 was almost completed within 30 min of reaction under solar irradiation, while there was about 35% of H2O2 remained in solution after 60 min reaction in Fenton process. In addition, carboxyl intermediates such as oxalic acid were produced in solution, which may further form ferric carboxylate complexes during the reaction, leading to another pathway of ferric reduction – photodecarboxylation as described in Eq. (4), and drive the carboxyl intermediates to be further mineralized. 3

½FeðC2 O4 Þ3 

20

k<480mm

 þ hm ƒƒƒƒ! Fe2þ þ 2C2 O2 4 þ CO2 þ CO2

ð4Þ 0 0.2

0.4

0.6 2+

initial Fe

0.8

1.0

dosage, mM

Fig. 2. Effect of initial Fe2+ dosage on degradation of 4-chlorophenol solution ([4-CP]0: 1 mM, [H2O2]0: 12.5 mM, solar light intensity: S800, RT: 30 min).

achieved at a Fe2+ ion concentration of 0.3 mM. Further increase in Fe2+ ion concentration did not improve the efficiency further. Such results revealed that a H2O2/Fe2+ molar ratio of approximately 40 was optimal for a required degradation of 4-CP, and were consistent with the findings in the literature, where an optimal H2O2/Fe2+ ratio of 20–40 was reported for 2-CP degradation (Kavitha and Palanivelu, 2003). Furthermore, increasing initial H2O2 dosage was much more beneficial for COD removal and mineralization efficiency than increasing initial Fe2+ dosage. This could be due to the incessant photo-reduction reaction of Fe3+ (Fe(OH)2+) ion under solar irradiation (Eq. (3)), leading to a lower dosage of Fe2+ being sufficient to catalyze H2O2 for generating hydroxyl radicals in solution. Moreover, solar photo-Fenton reaction at higher initial Fe2+ dosage led to the formation of a brown turbidity in the reaction vessel, which could decrease the absorption of

3.2. Effect of solar irradiation on mineralization efficiency and AOS of 4-CP The effect of solar irradiation on mineralization of 4-CP by Fenton process was shown in Fig. 5. Although Fenton process was able to destroy 4-CP effectively, it showed rather difficulty in mineralizing the compound completely. In this study, only 25% of 4-CP was mineralized by Fenton process (Fig. 5a). Balcioglu and Arslan (1999) reported COD Degradation Efficiency , %

0.0

100

80

S800 S400 Fenton

60

40

20

0 0

10

20

30

40

50

60

70

time , min. Fig. 3. Effect of solar light intensity on COD degradation of 4-chlorophenol solution ([4-CP]0: 1 mM, [H2O2]0: 12.5 mM, [Fe2+]0: 0.3 mM).

W.S. Kuo, L.N. Wu / Solar Energy 84 (2010) 59–65

COD degradation H 2 O 2 residual

60

60 40

20

20

2

40

0

0 0

10

20

30

40

50

60

Mineralization Efficiency , %

80

2

80

R,%

100

100

a

H O residual , %

100

63

70

a 80 60

S800 S400 Fenton

40 20 0 0

time , min.

10

20

30

40

50

60

70

50

60

70

time , min. 100

100 4

40

40

20

20

b

2

AOS

60

2

60

2

80

H O residual , %

R,%

b 80

0

-2 0

0 0

10

20

30

40

50

60

70 -4

time , min.

0 100

60

60

40

40

20

20

2

80

0

0 0

10

20

30

40

50

60

20

30

40

Fig. 5. Effect of solar light intensity on mineralization efficiency (a) and AOS (b) of 4-chlorophenol solution ([4-CP]0: 1 mM, [H2O2]0: 12.5 mM, [Fe2+]0: 0.3 mM).

2

c

80

10

time , min.

H O residual , %

R,%

100

70

time , min. Fig. 4. Relationship of H2O2 residual and COD degradation of 4-chlorophenol solution (a: S800, b: S400, c: Fenton; [4-CP]0: 1 mM, [H2O2]0: 12.5 mM, [Fe2+]0: 0.3 mM).

that no more than 50% of organic compounds were mineralized by Fenton process. In a study of Fenton degradation of 2-CP, Kavitha and Palanivelu (2003) also reported that only 39% of 2-CP was mineralized by Fenton’s process. It was believed that the presence of acetate and oxalate intermediates as confirmed by IC analysis hindered the mineralization in Fenton process. However, after 60 min of reaction in solar assisted photo-Fenton process, 78% and 90% mineralization of 4-CP was observed for samples treated with S400 and S800 irradiation, respectively. The positive effect of solar irradiation on mineralization of 4-CP was possibly achieved by photo-reduction of ferric ion (Eq. (1)) or by photodecarboxylation of ferric complexes (Eq. (4)). Although chloride, acetate, and oxalate were identified as intermediates during IC analysis, most of the intermedi-

ates of 4-CP degradation by Fenton and solar photo-Fenton processes were not identified. Hence, the degradation of 4-CP into other by-products was monitored by the change in the degree of oxidation, namely average oxidation state (AOS). The average oxidation state of the organic carbon was calculated by Eq. (5). AOS ¼ 4ðTOC  CODÞ=TOC

ð5Þ

where TOC is in mol C/L and COD in mol O2/L. The AOS value indicates how chemical substances in the effluent become more oxidized. A higher AOS value implies a higher degree of oxidation. Theoretically, the AOS has a value of +4 for CO2, the most oxidized state of C, and 4 for CH4, the most reduced state of carbon. The AOS has a value of 0.33, 0, and +3 for the possible intermediates of 4-CP degradation – hydroquinone, acetic acid, and oxalic acid, respectively. The oxidation state evolution during the reaction of the different processes studied was compared in Fig. 5(b). The AOS value of solution was found to increase significantly from 0.33 to 1.0, and 2.13 for samples treated with S400, S800 irradiation, respectively, while no significant improvement for AOS values was found for Fenton process. Such results implied that some intermediates with higher AOS value such as acetic acid, oxalic acid were possibly formed and more 4-CP molecules were mineralized in solar assisted photo-Fenton process.

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Table 2 Pseudo-first order rate constants and half-lives of 4-CP solution irradiated by different solar light intensity. Process*

kCODa (min1)

t_1/2c (min)

r2d

kTOCb (min1)

t1/2c (min)

r2d

S800 S400 Fenton

0.0093 0.0039 0.0012

75 178 278

0.92 0.97 0.93

0.0079 0.0032 0.0005

88 217 1386

0.99 0.93 0.96

* a b c d

Operation conditions: [4-CP]0: 1 mM, [H2O2]0: 12.5 mM, [Fe2+]0: 0.3 mM. kCOD: pseudo-first order rate constant based on the degradation of COD in solution. kTOC: pseudo-first order rate constant based on the degradation of TOC in solution. t1/2 = ln 2/k. r2: coefficient of correlation.

3.3. Effect of solar irradiation on degradation rate of 4-CP To quantitatively analyze the effect of solar irradiation on Fenton oxidation of 4-CP, a pseudo-first order model expressed by Eq. (6), which was generally used for reaction kinetic analysis for photo-Fenton process under conditions with a low initial concentration of pollutant in solution (Benitez et al., 2005), was applied to obtain the rate constants. Moreover, because the 4-CP molecules were almost disrupted in the first 3 min reaction, it would be appropriate to assess the overall rate constant with respect to COD or TOC rather than to a particular chloro-phenolic compound. 

dC ¼ kC dt

of oxidation of 4-CP in solar photo-Fenton process. Chlorine ion was identified as the major chloride by-product during 4-CP oxidation and accumulated to stoichiometric amount in solar photo-Fenton processes, while short-chain aliphatic acids such as acetic and oxalic acids were identified and left behind in Fenton process. The kinetics of degradation reaction of 4-CP in the treatment processes could be well described by a pseudo-first order reaction kinetics model. The presence of solar irradiation in Fenton process increased the reaction rate by a factor of 6.5 and 15.8 for COD and TOC degradation, respectively as compared to Fenton process. Accordingly, applying natural solar irradiation in Fenton process seemed to be a powerful driving force in treating wastewater containing 4-CP.

ð6Þ

in which C is the concentration of COD or TOC in solution at time t, and k is the pseudo-first order rate constant. It was found that both the COD and TOC degradation of 4-CP solution followed the pseudo-first order reaction kinetics (r2 > 0.93). The resulting rate constants for the COD and TOC degradation of 4-CP solution were given in Table 2. It was obvious that a significant enhancement for the degradation of 4-CP was achieved by solar assisted photo-Fenton oxidation as indicated by a higher rate constant and shorter half-life. The higher the solar irradiation intensity, the faster the degradation rate of 4-CP was. Particularly, the mineralization rate of 4-CP increased more than 10 times as the Fenton oxidation of 4-CP solution was assisted by S800 solar irradiation, accompanied with a much shorter half-life for 4-CP degradation. 4. Conclusions In this study, a great increase in COD degradation efficiency, mineralization efficiency and AOS value could be achieved by the combination of solar irradiation and Fenton reaction with a lower Fe2+ dosage. A higher intensity of solar irradiation was found to be beneficial for Fenton oxidation of 4-CP. The results indicated that solar assisted photo-Fenton process was effective for complete removal of 4-CP in water. More than 90% of 4-CP was mineralized to CO2 in solar assisted process while only 25% carbon mineralization was observed in Fenton process. In addition, the variation of AOS values indicated a higher degree

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