Q IWA Publishing 2010 Water Science & Technology—WST | 62.6 | 2010
1320
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
Y. H. Gong et al. | Heterogeneous photo-Fenton oxidation of Orange II
1322
Table 3
|
Water Science & Technology—WST | 62.6 | 2010
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
Y. H. Gong et al. | Heterogeneous photo-Fenton oxidation of Orange II
Water Science & Technology—WST | 62.6 | 2010
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
Y. H. Gong et al. | Heterogeneous photo-Fenton oxidation of Orange II
1324
Table 4
|
Water Science & Technology—WST | 62.6 | 2010
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).
Y. H. Gong et al. | Heterogeneous photo-Fenton oxidation of Orange II
1325
(a)
(b)
Water Science & Technology—WST | 62.6 | 2010
(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
1326
Y. H. Gong et al. | Heterogeneous photo-Fenton oxidation of Orange II
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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