Evaluation of heterogeneous photo-Fenton oxidation of Orange II ...

Q IWA Publishing 2010 Water Science & Technology—WST | 62.6 | 2010

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Evaluation of heterogeneous photo-Fenton oxidation of Orange II using response surface methodology Y. H. Gong, H. Zhang, Y. L. Li, L. J. Xiang, S. Royer, S. Valange and J. Barrault

ABSTRACT A mesoporous SBA-15 doped iron oxide (Fe2O3/SBA-15) was synthesized by co-codensation, characterized and used as heterogeneous catalysts for the photo-Fenton decolorization of azo dye Orange II under UV irradiation. Response surface methodology (RSM) was used to investigate operating condition effects, such as hydrogen peroxide concentration, initial pH and catalyst loadings, on the decolorization rate. UV irradiation is found to enhance the activity of the catalyst in the process. RSM analysis evidenced the influence of the initial pH value and H2O2 concentration on the dye degradation rate. The coupled UV/Fe2O3/SBA-15/H2O2 process at room temperature is revealed as a promising friendly process for wastewater treatment. Indeed, the use of a heterogeneous catalyst allows an easy active phase recycling without multi-step recovering while the heterogeneous catalyst used here exhibits high catalytic activity for the reaction considered. Key words

Y. H. Gong H. Zhang (corresponding author) Y. L. Li L. J. Xiang Department of Environmental Engineering, Wuhan University, P.O. Box C319 Luoyu Road 129#, Wuhan 430079, China E-mail: [email protected] L. J. Xiang S. Royer S. Valange J. Barrault LACCO, Universite´ de Poitiers-CNRS, ESIP, 40 avenue du Recteur Pineau, F-86022, Poitiers Cedex, France

| Fe2O3/SBA-15, heterogeneous catalysis, Orange II, photo Fenton, RSM

INTRODUCTION Synthetic dyes are manufactured and used for numerous

Among advanced oxidation processes, Fenton and

industrial applications such as the textile, leather goods,

photo-Fenton technologies are gaining growing acceptance

food manufacture and other chemical usages. It is estimated

as effective dyes wastewater treatment methods. However,

that among the total amount of dyes, 1 – 2% in manu-

such technologies imply some major drawbacks: (i) the

facturing and 1 – 10% in use are released into water,

narrow range of pH in which the reaction proceeds; (ii) the

air and soil (Forgacs et al. 2004). Consequently, a further

difficulties for catalyst recovering after wastewater treat-

treatment is needed to limit their impact on the environ-

ment; (iii) the resulting sludge can contain heavy metals and

ment. Unfortunately, traditional wastewater treatment

thus requires further treatment stages. Consequently, the

processes are inefficient in handling these dye pollutants,

use of heterogeneous solid Fenton catalysts could be an

mainly because of their biological resistance and chemical

alternative method of these problems. Among the possible

stability (Ganesh et al. 1994; Razo-Flores et al. 1997;

Fenton proposed catalysts, mesoporous SBA-15 silica

Konovalova et al. 2000; Lucas & Peres 2006). Therefore,

supported iron oxide (Fe2O3/SBA-15) was reported as a

decolorization and mineralization of dyes waste is a

very promising Fenton catalyst, and it has been successfully

challenge

tested in other Fenton-like processes handling industrial

for

Consequently,

scientists, various

but

also are

the

industry. and

organic pollutants (Molina et al. 2006; Segura et al. 2009;

proposed as solutions, to treat dye pollutants. Advanced

Xiang et al. 2009). Indeed, authors reported active and

oxidation processes (AOPs) have emerged as interesting

stable catalysts at low reaction temperature (, 508C) and

and effective alternative technologies.

limited pressure in the phenol oxidation reaction using

doi: 10.2166/wst.2010.432

methods

for

developed,

Y. H. Gong et al. | Heterogeneous photo-Fenton oxidation of Orange II

1321

Table 1

|

Water Science & Technology—WST | 62.6 | 2010

Properties of the as-synthesized support and the Fe-doped sample

Sample

Fe2O3 content (wt.%)

Isotherm-Hysteresis

SBET (m2 g21)

DBJH (nm)

Vmeso (cm3 g21)

d100 (nm)

a0 (nm)

Fe2O3/SBA-15

8.5

IV– H1

647

6.9

1.01

9.60

11.1

H2O2 as oxidant. As for the heterogeneous photo-Fenton

Melero et al. 2007). In this procedure, iron precursor (Iron

reactions, hydrogen peroxide concentration, pH and cata-

chloride hydrate) is added to the synthesis media, and

lyst loading are key parameters to control the kinetics of

precipitated by ammonia adding after silica condensation.

pollutant degradation. The traditional one-factor-at-a-time

A full description of the synthesis procedure can be found

approach has been widely used for process optimization.

elsewhere (Xiang et al. 2009). The sample physical proper-

In this case, only one parameter is varied at a time.

ties are gathered in Table 1.

Consequently, to study the influence of one parameter, the

N2 adsorption-desorption experiments were performed

others theoretically remain constant. However, this may

at 21968C on a Tristar 2010 instrument from Micromeritics.

only be a partial approach of exploring these parameter

Iron content in the calcined sample was measured by means

effects experimentally since it does not take into account

of Atomic Emission Spectroscopy with Induced Coupled

possible interactions among the parameters (Ramirez et al.

Plasma (ICP-AES) analysis collected in a Varian Vista AX

2005; Zhang et al. 2009). An alternative method, namely

system. X-ray diffraction experiments were performed on a

experimental statistical design, can overcome this short-

Bruker AXS D5000 instrument equipped with a monochro-

coming. Recently, the multivariable analysis method has

matized CuKa radiation. Signal recorded for 2u between

been applied to investigate the operational parameter effects

0.758 and 5.008 allows the calculation of the d(100) distances,

for dyes and phenolic aqueous solutions during photo-

as well as the a0 cell parameters (assuming an hexagonal

Fenton processes (Martinez et al. 2005; Kasiri et al. 2008). In this work, iron-based mesoporous SBA-15 type silica

structure). Signal was also recorded for 2u ranging from 158 and 758 to detect any iron crystallized phase.

material (labeled Fe2O3/SBA-15) was synthesized by using

The physical properties of the catalysts were presented

a single step procedure, characterized and tested as

and discussed in a previous work (Sample SBA-Ref; Xiang

heterogeneous catalysts for the photo-Fenton decoloriza-

et al. 2009), and are only summarized here. The catalyst

tion of azo dye Orange II. The influence of different

presents high surface area, large pore size and high pore

variables, namely initial pH (X1), catalyst loading (X2) and

volume (Table 1). The isotherm is of type IV according with

hydrogen peroxide concentration (X3), was studied using a

the IUPAC classification, with sharp and parallel adsorp-

response surface methodology (RSM). The response factor

tion and desorption branches (hysteresis H1). This is clearly

considered was the decolorization rate constant.

characteristic of hexagonal mesostructured solid. The small angle X-ray diffraction experiments shows the presence of the (100), (110) and (200) reflections characteristic of the

METHODS

hexagonal cell, showing that the catalyst present a wellorganized pore network. While no reflection of any

Catalyst preparation and characterization

crystallized iron phase can be detected by XRD, TEM

The Fe2O3/SBA-15 catalyst was prepared according to the

evidenced the presence of small crystallized particles

procedure reported by Molina et al. (Martinez et al. 2002;

dispersed in the silica porosity. It can be noted that TEM

Table 2

|

Experimental range and levels of the independent variables

Variables

Initial pH: 21

Fe2O3/SBA-15 (g L

):

H2O2 concentration (mmol L21):

Symbol

21.414

21

0

X1

2.3

3.0

4.5

X2

0.28

0.40

0.70

1.00

1.12

X3

0.34

2.00

6.00

10.00

11.60

11

6.0

11.414

6.6


1322

Table 3

|


around the silica porous matrix and is reported to be

Design matrix in coded units and the experimental responses

efficient for the phenol catalytic wet peroxide oxidation Fe2O3/SBA-15 loading 21

H2O2 concentration 21

(mmol L

k 21

reaction (Xiang et al. 2009).

Run No.

pH

(g L

1

21

21

2

1

21

21

0.0178

3

21

1

21

0.0316

)

)

(min

21

)

0.0313

4

1

1

21

0.0177

5

21

21

1

0.0993

6

1

21

1

0.0810

7

21

1

1

0.105

8

1

1

1

0.0866

9

21.41

0

0

0.0810

10

1.41

0

0

0.0600

11

0

2 1.41

0

0.0707

12

0

1.41

0

0.0639

13

0

0

2 1.41

0.00545

14

0

0

1.41

0.0815

15

0

0

0

0.0572

16

0

0

0

0.0561

17

0

0

0

0.0603

18

0

0

0

0.0602

19

0

0

0

0.0553

20

0

0

0

0.0590

Heterogeneous photo-Fenton experiments In a typical experiment, a solution of 200 mL containing 100 mg L21 Orange II is mixed with the Fe2O3/SBA-15 catalyst in a Pyrex glass vessel. The pH of the Orange II solution was adjusted to the desired value (with HCl or NaOH solution). Then the Fe2O3/SBA-15 suspended solutions containing Orange II were equilibrated in the dark for 30 minutes before being submitted to irradiation. After the equilibration period, 1 mL aliquot was withdrawn from the reaction medium to determine the initial Orange II concentration (C0). Hydrogen peroxide was added to the reaction solution at the beginning of the irradiation. The irradiated source was a 15 Watt UV disinfection lamp from Xinghui Lamp Co. Ltd (Hunan, China) emitting a 254 nm light at an intensity of 2.6 £ 1025 Ein L21 s21, as established by ferrioxalate actinometry (Montalti et al. 2006). Throughout the experiment, the suspension was air bubbled to ensure a good dispersion of the catalyst in the reaction medium. At regular irradiation time intervals (e.g. 0, 10, 15,

also allows the detection of large aggregates of small iron

30 min,…), 1 mL aliquots were sampled, filtered through a

particles outside the silica porosity.

membrane of pore size 0.22 mm, and then analyzed to

In conclusion, the Fe2O3/SBA-15 catalyst can be

determine the Orange II concentration (Ct). Decolorization

described as small crystallized iron clusters dispersed in or

experiments were monitored by a DR 2800 UV –Vis

(a)

(b)

1.0

1.2 1.0 Adsorption Fe2O3/SBA-15/H2O2 UV/H2O2 UV/Fe2O3/SBA-15/H2O2

0.6 0.4

Absorbance

C /C0

0.8

0.8

0 min 10 min 20 min 30 min 40 min 50 min 60 min

0.6 0.4

0.2 0.2 0.0 0

10

20

30 Time (min)

Figure 1

|

40

50

60

0.0 200

300

400

500

600

700

Wavelength (nm)

Control experiment (a) under different conditions and (b) UV-vis spectra change (10 times diluted) of Orange II aqueous solution under heterogeneous photo-Fenton conditions. Orange II ¼ 100 mg L21, Fe2O3/SBA-15 ¼ 0.7 g L21, H2O2 ¼ 0.7 mmol L21, pH ¼ 3.0.

1323



spectrophotometer (HACH, United States) at a maximum

1.0

absorption wavelength of 485 nm. First cycle

C/C0

0.8

Response surface method analysis

Second cycle

Third cycle

0.6 0.4

The RSM analysis is performed using a central composite design (CCD) method. Analysis of the experimental data

0.2

was supported by the statistical graphics Design-Expert (Version 7.1) software system (Stat-Ease, Inc., USA).

0.0 0

Analysis of variance (ANOVA) was used for graphical

20

40

analyses of the data to identify interaction between the different process variables and responses. The quality of the

Figure 2

|

fit quadratic model was expressed by the coefficient of

60

80 100 120 140 160 180 Time (min)

Cyclic degradation of 100 mg/L Orange II aqueous solution during the photocatalytic degradation. Orange II ¼ 100 mg L21, Fe2O3/SBA-15 ¼ 0.7 g L21, H2O2 ¼ 0.7 mmol L21, pH ¼ 3.0 each run.

determination, R 2, and its statistical significance was checked by the Fisher’s F-test using the same program. Model terms were evaluated by the P-value (probability) with 95% confidence level. Three-dimensional plots and their

respective

contour plots were obtained

using

MATLAB 7.0 software, taking into account the effects of the three factors at five levels. The levels of the three major factors identified are summarized in Table 2. The notations (2 1) and (þ1) refer to the low level and the high level of the two-level-factorial-design experiment, respectively. The notations of (21.414) and (þ1.414), and (0) are those levels of start points and center point used. At first, 8 ( ¼ 23) runs of two-level-factorial-design experiment for three parameters were performed randomly (Runs 1-8 in Table 3). This design is considered to be the most suitable to obtain knowledge of the influence of the variables on the process (Ormad et al. 2006). In order to check the assumption of linearity in the factor effects,

in order to estimate the extent of adsorption and the benefits of UV irradiation (Figure 1). Negligible adsorption of Orange II (9%) onto the Fe2O3/SBA-15 surface was found after 60 min of equilibration in the dark. A decrease of 29% in Orange II concentration was achieved by the heterogeneous Fenton reaction (no UV irradiation), which was much smaller than that obtained under the heterogeneous photo-Fenton conditions. Indeed, 95% of Orange II was decomposed after only 60 min in reaction. It can be noted that the UV/H2O2 conditions led to lower rate of Orange II degradation than that obtained by the heterogeneous photo-Fenton process, even if the two processes lead to similar value of Orange II degradation after 60 min. The high rate of degradation observed for the heterogeneous photo-Fenton process can be attributed to the contribution

further experiments to the star points and center point were performed randomly based on the conditions illustrated in

0.10

Table 3 (Runs 9-20). k (min–1)

0.08 0.06 0.04

RESULTS AND DISCUSSION 0.02

Preliminary and sequential study of Orange II photodegradation

0.00

pH

Experiments were first carried out under different conditions (Fenton, UV/H2O2, heterogeneous photo-Fenton)

+1

–1

Figure 3

|

–1

+1

Fe2O3/SBA-15

Average effect of the studied factors on the activity.

+1

–1 H2O2


1324

Table 4

|


ANOVA for response surface quadratic model

Source of variation

Sum of squares

Degree of freedom

Model

0.014

Residual

2.184 £ 1024

10

2.184 £ 1025

Lack of fit

1.953 £ 1024

5

3.906 £ 1025

Pure error

2.308 £ 10

25

5

4.616 £ 1026

Cor. Totalp

0.014

9

Mean square

F-value

p-value

0.014

69.81

,0.0001

8.46

0.0175

19

p

Totals of all information corrected for the mean

of the UV radiation on the reduction of surface ferric ions,

(Figure 2). From the gathered data, it seems that the

which can result in an increase in the overall active sites

Fe2O3/SBA-15 catalyst would be a quite effective and

(Litter & Blesa 1988). Furthermore, irradiation could also

stable catalyst under the heterogeneous photo-Fenton

accelerate the decomposition of H2O2 resulting in the

conditions.

formation of active hydroxyl radicals, that would result in an increase in the overall catalytic activity. It is important to note that some eluted iron (from the catalyst) can obviously

Factorial design of experiments for photo-Fenton

results in an increase in homogenous catalytic activity, and

heterogeneous system

consequently in the global rate of Orange II degradation. This is only possible under acidic reaction conditions, conditions leading to partial iron dissolution. Nevertheless, the fraction of iron that dissolves at ambient temperature remains low (0.4% after 4 h of reaction at 258C for the CWPO of phenol at pH ¼ 3.7, Xiang et al. 2009). The Orange II degradation is clearly visible on the UV spectra presented in Figure 1b. Indeed, intensity of Orange II at 485 nm rapidly decreased with the increase of irradiation time without any appearance of new peaks,

The results of the average effect of each experimental factor are presented in Figure 3. An increase in pH results in negative effects on the catalytic activity. In contrary, an increase in H2O2 concentration results in an increase in activity. Surprizingly, catalyst loading was less important for Orange II decolorization rate, and only a small increase in activity is observed. In conclusion, a low pH and a high H2O2 concentration are the most favorable conditions for the catalytic Photo-Fenton reaction.

while the optical spectra of Orange II only slightly decrease during the Fenton reaction (data not shown). Note that the 0.10

decay of the absorbance at 229 nm and at 310 nm is in the dye molecule. The analysis of normalized concentration-time data illustrated that the pseudo-first order kinetics fit the data well in the heterogeneous photo-Fenton process, and then pseudo-first order rate constant k was considered as response factor in the following RSM analysis. Successive reactions were also performed to test the

Predicted value (min–1)

considered as evidence of aromatic fragment degradation 0.08

0.06

0.04

0.02

stability of the catalyst under reaction. At the end of the first cycle of reaction, the catalyst was recovered by filtration

0.00 0.00

and washing, and used for a second, and after for a third reaction cycle. The decrease in final degradation efficiency was less than 3% after three repetitive experiments

0.02

0.04

0.06

Actual value Figure 4

|

0.08

0.10

(min–1)

Regression plot of measured data against predicted values from the quadratic response surface model describing Orange II decolorization rate (k).


1325

(a)

(b)


(c)

0.1

0.1

0.08

0.06

k (min–1)

0.07

k (min–1)

k (min–1)

0.08 0.06 0.04

1

0.5

0

X2 Figure 5

|

–0.5

–1

–0.5

0

0.5

1

X1

0

0.06 0.04

0.02 0.05

0.08

0.02 1

1

0.5 1 0 –0.5 –1 –0.5 0 X1 X3

0.5

0 –0.5 0 0.5 –1 –0.5 X2 X3

0.5

1

Response surface plot of the effect of reaction pH and Fe2O3/SBA-15 dosage (a), the effect of reaction pH and H2O2 concentration (b), the effect of H2O2 concentration and Fe2O3/SBA-15 dosage (c) on Orange II degradation.

Based on the results shown in Table 2, RSM model of second order polynomial equations were determined as follows,

predicting k, revealing a reasonably good adequation between predictions and experiments. The corresponding response surface plots obtained from the quadratic RSM equation are presented in Figure 5.

k ¼ 0:058 2 7:809 £ 1023 X1 þ 1:571 £ 1024 X2 þ 0:032X3

The P values for X1 (0.0002) and X3 (,0.0001) were quite smaller than 0.05, which mean that pH and H2O2

2 2:625 £ 1025 X1 X2 2 1:149 £ 1023 X1 X3 þ 1:366

concentration were extremely important in affecting Orange

£ 1023 X2 X3 þ 5:349 £ 1023 X21 þ 3:749 £ 1023 X22

II decolorization rate k. The k response surface in Figure 5a

2 8:155 £ 1023 X23

decolorization of Orange II reaction. Indeed, catalyst

clearly showed that acidic condition is beneficial for the loading

of 21

The coefficients of determination R 2 gave the porportion of the total variation in the response variable explained. The R 2 obtained in this work (0.9843) ensured a satisfactory adjusment of the quadratic model to the experimental data. The analysis of variance results were listed in Table 4. The model is significant at the 5% confidence level and the model explained the data variability adequately since the p-value of the model is much less than 0.05 (Benatti et al. 2006; Ghafari et al. 2009). Adequate Precision value (AP) was used to assess the discrimination between the range of the predicted values at the design points to the average prediction error (Beg et al.

2.0 mmol L

0.4 g L21

and

H2O2

concentration

lead to k values of 0.0313 min

21

of and

0.0178 min21 at pH ¼ 3.0 and 6.0, respectively. It is accepted that the pH changed the amount of the photoactive

species,

Fe(III),

dissolved

from

the

catalyst.

Moreover, the variation of initial pH value of the suspension may also change the catalyst surface properties, such as surface charge and surface hydroxyl concentration, which thereafter can affect the decolorization of Orange II rate. The k response surface in Figure 5b and 5c clearly showed that Orange II decolorization was increased with the increase of H2O2 concentration. Finally, it was worth nothing that the catalyst concentration (X2) does not strongly affect the response factor.

2003). The model is considered to give accurate prediction if the AP value is higher than four (Ghafari et al. 2009). The AP value obtained for our analysis is 31.589, which confirms that the model can be used in predicting Orange II decolorization constant rate (k). The predicted values

CONCLUSIONS We have shown that mesoporous silica supported iron

obtained by the model equation were presented in Figure 4,

oxide (Fe2O3/SBA-15) could behave as efficient and stable

and compared to the experiment points. It is evident from

catalyst for dye decolorization in heterogeneous photo-

this figure that the proposed empirical model is suitable for

Fenton system. Dye degradation carried out over this

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heterogeneous catalytic system has been found sensitive to the reaction pH and H2O2 concentration. The coupled UV/Fe2O3/SBA-15/H2O2 system at room temperature appeared as a promising process for dye wastewater treatment.

ACKNOWLEDGEMENTS This study was supported by National Natural Science Foundation of China (Grant No. 20977069) and Natural Science Foundation of Hubei Province (China) through “The Outstanding Youth Scholars Program” (Grant No. 2007ABB028). The Chinese Science Council is warmly acknowledged by Mr. Luojing Xiang for the one year PhD. grant in France (No. 2007101600).

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