Environ Sci Pollut Res (2011) 18:1497–1507
DOI 10.1007/s11356-011-0514-4
RESEARCH ARTICLE
Kinetic degradation of the pollutant guaiacol by dark Fenton
and solar photo-Fenton processes
Youssef Samet & Ines Wali & Ridha Abdelhédi
Received: 4 March 2011 / Accepted: 14 April 2011 / Published online: 3 May 2011
# Springer-Verlag 2011
Abstract This work is first intended to optimize the
experimental conditions for the maximum degradation of
guaiacol (2-methoxyphenol) by Fenton’s reagent, and
second, to improve the process efficiency through the use
of solar radiation. Guaiacol is considered as a model
compound of pulp and paper mill effluent. The experiments
were carried out in a laboratory-scale reactor subjected or
not to solar radiation. Hydrogen peroxide solution was
continuously introduced into the reactor at a constant flow
rate. The kinetics of organic matter decay was evaluated by
means of the chemical oxygen demand (COD) and the
absorbance measurements. The experimental results
showed that the Fenton and solar photo-Fenton systems
lead successfully to 90% elimination of COD and absorbance at 604 nm from a guaiacol solution under particular
experimental conditions. The COD removal always obeyed
a pseudo-first-order kinetics. The effect of pH, temperature,
H2O2 dosing rate, initial concentration of Fe2+, and initial
COD was investigated using the Fenton process. The solar
photo-Fenton system needed less time and consequently
less quantity of H2O2. Under the optimum experimental
conditions, the solar photo-Fenton process needs a dose of
H2O2 40% lower than that used in the Fenton process to
remove 90% of COD.
Keywords Guaiacol . Degradation kinetics . Fenton .
Solar photo-Fenton . COD removal
Responsible editor: Philippe Garrigues
Y. Samet (*) : I. Wali : R. Abdelhédi
UR Electrochimie et Environnement, Ecole Nationale
d’Ingénieurs de Sfax,
Sokra Street Km 3.5,
BPW 3038, Sfax, Tunisia
e-mail: youssef.samet@fss.rnu.tn
1 Introduction
The pulp and paper industry produces large quantities of
wastewater that contain significant concentrations of contaminants like phenols, guaiacols, catachols, and vanillins.
These wastewaters represent a risk factor for human health
and the environment because they are difficult to degrade
toxic compounds that generally end up in aquatic sources,
where they can persist for long periods. Consequently, the
remediation of these wastewaters has received much
attention in the last decades. Among the several processes
used in the treatment of these wastes, the so-called
advanced oxidation processes (AOPs) appear to be a
promising field of study due to the effective complete
mineralization of organic contaminants under mild conditions (Ruppert and Bauer 1994; Andreozzi et al. 2000).
AOPs are based on the use of a very strong oxidizing agent
such as hydroxyl radical (HO·) with E° (HO·/H2O)=2.8 V/
NHE, which is generated in situ in the reaction medium
(Hirvonen et al. 1996; Flotron 2004).
Fenton (H2O2/Fe2+) and photo-Fenton (UV/H2O2/Fe2+)
processes have proved to be effective and economical
AOPs used for the detoxification and degradation of many
organic compounds (Lin et al. 2000; Sabhi and Kiwi 2001;
Catastini et al. 2002; Samet et al. 2009; Walling 1975).
Oxidation with Fenton’s reagent is based on ferrous ion
and hydrogen peroxide, and exploits the reactivity of the
hydroxyl radical produced in acidic solution by the
catalytic decomposition of H2O2 (Walling 1975; Chen
and Pignatello 1997; Chamarro et al. 2001; Kang et al.
2002):
þ
þ
Fe2 þ H2 O2 ! Fe3 þ OH þ HO
k1 ¼ 63 L mol 1 s
1
ð1Þ
1498
Environ Sci Pollut Res (2011) 18:1497–1507
Ferrous iron is slowly regenerated through the so-called
Fenton-like reaction between ferric iron and H2O2 in acidic
aqueous medium (Walling 1975; Chen and Pignatello 1997;
Kang et al. 2002):
Eq. 11 (Walling 1975; Bergendahl and Thies 2004;
Venkatadri and Peters 1993; Kang and Hwang 2000;
Flotron et al. 2005; Sun et al. 2007), or by hydroxyl
addition, as shown in Eq. 12 (Flotron et al. 2005).
Fe3þ þ H2 O2 ! Fe2þ þ HO2 þ Hþ
k2 ¼ 0:01 L mol 1 s 1
RH þ HO ! R þH2 O ! Products
k11 ¼ 107 L mol 1 s 1
Fe3þ þ HO2 ! Fe2þ þ O2 þ Hþ
k3 ¼ 3:1 105 L mol 1 s 1
ð2Þ
ð3Þ
R þ HO ! HOR ! Products
k12 ¼ 107 L mol 1 s 1
ð11Þ
ð12Þ
Nevertheless, numerous competitive reactions can also
occur, namely the following ones, which negatively affect
the oxidation process (Walling 1975; Kang et al. 2002;
Bielski et al. 1985; Buxton et al. 1988; Bergendahl and
Thies 2004; Burbano et al. 2005).
In photo-Fenton process, in addition to the above
reactions the formation of hydroxyl radical also occurs by
the following reactions (Eqs. 13 and 14) (Muruganandham
and Swaminathan 2004; Will et al. 2004; Tamimi et al.
2008).
HO þ H2 O2 ! HO2 þ H2 O
Fe3þ þ H2 O þ hn ! HO þ Fe2 þ Hþ
ð13Þ
H2 O2 þ hn ! 2HO ðl < 300 nmÞ
ð14Þ
k4 ¼ 1:2 107 L mol 1 s
þ
1
ð4Þ
HO þ Fe2þ ! Fe3þ þ OH
k5 ¼ 4:3 108 L mol 1 s
1
HO2 þ H2 O2 ! O2 þ HO þ OH
k6 ¼ 0:5 L mol 1 s 1
1
1
HO þ HO ! H2 O2
k8 ¼ 5:3 109 L mol 1 s
HO þ HO2 ! O2 þ H2 O
k9 ¼ 5:3 1010 L mol 1 s
1
1
HO2 þ HO2 ! H2 O2 þ O2
k10 ¼ 8:3 105 L mol 1 s
þ
FeðOHÞ2þ þ hn ! Fe2 þ HO ðl < 450 nmÞ
ð15Þ
ð6Þ
þ
FeðRCO2 Þ2þ þ hn ! Fe2 þ R þ CO2 ðl < 500 nmÞ
ð16Þ
HO2 þ Fe2þ ! Fe3þ þ HO2
k7 ¼ 1:2 106 L mol 1 s
ð5Þ
At acidic pH (2.5–5), the main compounds absorbing
light in the photo-Fenton system are ferric ion complexes,
e.g., Fe(OH)2+ and Fe(RCO2)2+, which produce additional
Fe2+ (Eqs. 15 and 16) (Sagawe et al. 2001).
ð7Þ
ð8Þ
ð9Þ
ð10Þ
In the presence of substrate, as a target contaminant, the
generated hydroxyl radicals are able to attack most of
contaminants either by hydrogen abstraction, as shown in
The rate of organic pollutant degradation could be
increased by irradiation of Fenton with UV or visible light
(photo-Fenton process). This leads not only to the formation of additional hydroxyl radicals but also to recycling of
ferrous catalyst by reduction of Fe(III). In this way, the
concentration of Fe2+ is increased and the overall reaction is
accelerated.
Solar technology can be used as alternative to UV lamps
to reduce the degradation process costs. Thus, photo-Fenton
degradation of contaminants using solar light has been
successfully used being an economically viable process
since solar energy is an abundant natural energy source and
can be used instead of artificial light sources which are
costly and hazardous (Robert et al. 2004; Gumy et al. 2005;
Monteagudo et al. 2009).
The aim of this work is the optimization of Fenton
process for the degradation of guaiacol as model compound
of pulp and paper mill wastewater. In order to improve the
reaction rate, solutions were subjected to solar radiation
Environ Sci Pollut Res (2011) 18:1497–1507
1499
using a laboratory-scale reactor. The influence of different
operational parameters such as H2O2 dosing rate, initial
concentrations of guaiacol and Fe2+, temperature, and pH
was investigated.
2 Materials and methods
The experiments were performed in an experimental
device, whose scheme is shown in Fig. 1. This device
consists of an aluminum frame (0.5 m length, 0.5 m width,
and 0.6 m height), which supports: (1) a platform in
aluminum placed 34° to the horizontal on which was fixed
a solar reactor consisting of a borosilicate glass tube
(4.5 m length, an inner diameter of 6 mm, and outer
diameter of 8 mm) snake-shaped; and (2) an Erlenmeyer
flask (Pyrex 1 L) at which the guaiacol solution was
prepared with bidistilled water. This Erlenmeyer flask was
immersed in water bath to control the working temperature
by a thermostat (Julabo Labortechnik GMBH, Sellback,
Germany). The treated volume was 0.4 L, the solution was
circulated through the reactor using a peristaltic pump
(Cole-Parmer Instrument, Chicago, Illinois 60648 USA)
3
2
1
4
6
5
Fig. 1 Scheme of the experimental installation. (1) Erlenmeyer flask
(guaiacol solution), (2) H2O2 solution, (3) solar reactor, (4) peristaltic
pump, (5) thermostatic bath, (6) magnetic stirrer
with a flow rate of 140 mL min−1. This pump is
simultaneously used for the flow of the H2O2 solution in
the Erlenmeyer flask with a constant rate of 0.4 mL min−1
(more details were given in the top of the Section 3). The
quantity of the ferrous sulfate was introduced into the
solution at startup. The solutions were continuously stirred
using a magnetic stirrer (Tacussel, France). The pH values
were adjusted using a prepared 1 M sulfuric acid solution
and measured using a pH meter (PHM multi-parameter
analyzer, pH/mV Belgium Kingdom). The hydraulic
system used was a closed circuit which prevented the
evaporation of the solution.
The solar photo-Fenton experiments were performed at
the Electrochemical and Environmental Laboratory, National Engineering School of Sfax (approximately 3 m
above sea level, latitude 34°44′ N; longitude 10°45′ E),
Tunisia. All tests were conducted between 11:00 a.m.
and 3:00 p.m. on sunny days from July to October 2009.
The global solar radiation intensity was approximately
850 W/m2.
For tests using only the Fenton reagent, the experimental
device was kept away from solar radiation by covering with
a black plastic film and an aluminum foil.
Samples (0.5 mL) were withdrawn from the reactor at
selected intervals for COD and absorbance analysis. COD
was measured using a spectrophotometer (Shimadzu UVMini 1240 UV/Vis Spectrophotometer) using a dichromate
solution as the oxidant in strong acid media (Kolthof et al.
1969).
The absorbance was measured with the same spectrophotometer. Guaiacol is colorless; however, its oxidation
products (dihydroxylated rings such as catechol, methoxyhydroquinone, methoxycatechol, and their quinonic
forms) have dark color may be at low concentrations. The
absorption maximum was detected at a wavelength of
604 nm. Therefore, the variation of absorbance of the
treated solutions was followed with time at this wavelength.
Samples were previously centrifuged using a Micro-12
Hanil centrifuge.
All samples were tested in duplicate, and the test was
reproduced three times for each sample, so that the relative
errors could be minimized. All the figures show the average
values. Reaction intermediates were detected by the use of
high-performance liquid chromatography (HPLC) analysis
system (model 1100, Hewlett-Packard) equipped with a
Hamilton PRP ×300 column (Metrohm) as described in our
previous paper (Samet et al. 2002).
Guaiacol, catechol, and methoxyhydroquinone (analytical grade) were purchased from Aldrich (Gillingham,
Dorset, UK) and were used as received. Ferrous sulfate
heptahydrate (FeSO4 7H2O) was obtained from Riedel-de
Haën (Seelze-Hannover, Germany) and used as the Fe(II)
catalyst. Hydrogen peroxide (35% v/v) and sulfuric acid
1500
Environ Sci Pollut Res (2011) 18:1497–1507
were provided by Merck (Darmstadt, Germany). All
solutions were prepared with bidistilled water.
Ln (CODO/COD)
800
−1
COD (mg L )
1000
3 Results and discussion
3.1 Oxidation of guaiacol by the Fenton process
3.1.1 H2O2 injection mode
In most studies, hydrogen peroxide is added at once at
startup. If a sufficiently high concentration of reagents is
added ([H2O2]0 >[Fe2+]0>> [substrate]0), it will form a large
amount of hydroxyl radicals in the early response.
Therefore, substrates are rapidly degraded. However, some
of hydroxyl radicals will be consumed by hydrogen
peroxide in excess (Eq. 4), and by the termination reaction
between hydroxyl radicals (Eq. 8), which has a relatively
high constant rate (5.3×109 L mol−1 s−1). Furthermore, the
total ferrous ions will be oxidized in a few minutes, and the
slow regeneration of these ions according to Eq. 2 will limit
the Fenton process.
The several or continuous addition of hydrogen peroxide
can overcome all these problems. Indeed, a quasi-stationary
concentration of radicals HO· can thus be maintained
throughout the reaction. Several authors have shown that
continuous addition of H2O2 in the Fenton or photo-Fenton
processes is more effective than the addition of all the
quantity of H2O2 at the startup (Monteagudo et al. 2009;
Yalfani et al. 2009; Silva et al. 2007). So in the present
work, H2O2 was injected continuously in the solution from
the beginning to the end of reaction at a constant flow rate
of 0.4 mL min−1.
Since the residual H2O2 interferes with the measurement
of COD (Kang et al. 2002), the residual amount of H2O2
was also measured, using the permanganate titration. This
method is suitable for measuring solutions of hydrogen
peroxide in the range 0.25 to 70 wt.% According to Lin and
Lo (1997), 1 mg L−1 of H2O2 contributes 0.27 mg L−1 COD
concentration. Since no H2O2 residual concentration higher
than 0.25 wt.% was measured, no correction was performed
to COD analysis.
3.1.2 Kinetics of COD removal
Figure 2 shows the decrease of COD during the oxidation
of guaiacol (COD0 =1,080 mg L−1) by Fenton’s reagent. It
can be seen that COD values decreased almost exponentially and 90% of COD removal was obtained after
approximately 45 min. If we suppose that the oxidation
of guaiacol and its by-products by the hydroxyl radicals
HO· is of a first order with respect to COD and the
hydroxyl radicals concentration is constant during the
kapp = 0.07 min−1
5
1200
600
4
2
R = 0.983
3
2
1
0
0
20
400
40
60
Time (min)
200
0
0
10
20
30
40
50
Time (min)
Fig. 2 Trend of COD during the oxidation of guaiacol solution by
Fenton process. The inset panel shows its kinetic analysis assuming
that COD follows a pseudo-first-order reaction. Dosing rate of H2O2
60 mg min−1, [Fe2+]0 =8 mM, pH=3, and T=40°C
treatment, the oxidation rate (r) can be given by the
following equation:
r¼
dCOD
¼ k½HO a COD ¼ kapp COD
dt
ð17Þ
where k is the reaction rate constant, α is the reaction
order related to the hydroxyl radicals, and kapp ¼ k ½HO a
the apparent rate constant.
The integration of Eq. 17 subject to the initial condition
COD=COD0 at t=0 leads to the following equation:
COD ¼ COD0 expð kapp tÞ
ð18Þ
kapp could be calculated from the plot of Ln (COD0/COD)
versus t (inset of Fig. 2). As it can be seen, points lie
satisfactory in straight line with correlation coefficient
greater than 0.96. kapp was used to study the effect of
different concentrations of H2O2, Fe2+, guaiacol, and for
different temperatures and pH.
3.1.3 Effect of the dosing rate of hydrogen peroxide
The dosing rate of H2O2 is considered as one of the most
important factors which should be considered in the
Fenton process. The effect of the dosing rate of hydrogen
peroxide on the efficiency of the oxidation process was
investigated under the operating conditions (COD 0 =
1,080 mg L−1, [Fe2+]0 = 8 mM pH = 3 and T = 40°C)
(Fig. 3). It was found that COD removal efficiency
increases with increasing the dosing rate of hydrogen
peroxide from 3 to 60 mg min−1. The higher percent
removal of COD was attained at 45 min when using
60 mg min−1 H2O2 dosing rate, so further addition of
1.6
kapp x 10 (min )
Environ Sci Pollut Res (2011) 18:1497–1507
−1
−1
H2O2 (mg min )
COD/CODO
1.2
1.0
2
3
15
24
30
60
1.4
1501
10
8
6
4
2
0
0
20
0.8
40
60
H 2 O 2 (mg min
80
−1
100
)
0.6
0.4
0.2
0.0
0
10
20
30
40
50
Time (min)
Fig. 3 Effect of the dosing rate of H2O2 on the COD removal by the
Fenton process. The inset panel shows kapp evolution at different
dosing rate of H2O2. COD0 =1,080 mg L−1, [Fe2+]0 =8 mM, pH=3,
and T=40°C
H2O2 is not necessary. Excessive H2O2 reacts with HO·
competing with organic pollutants and consequently
reducing treatment efficiency.
In all tests, the drop in COD was more significant during
the first minutes of reaction where the concentration of
organic matter is high. This observation clearly appeared in
the case of 60 mg min−1 H2O2 dosing rate.
The inset of Fig. 3 shows the variation of the apparent
rate constants (kapp) values, at different H2O2 dosing rate,
calculated from the straight lines considering a pseudo-firstorder reaction. kapp increased when the dosing rate of H2O2
increased. This increase becomes more significant for H2O2
dosing rate higher than 30 mg min−1, due to the effect of
the additional HO· radicals produced. Given that the
concentration of Fe2+ introduced initially in the solution is
sufficient to react with H2O2, the competitive reactions
(Eqs. 4, 6, and 8–10) did not affect significantly the COD
removal rate.
Moreover, in order to follow the change in solution color
during guaiacol oxidation, the absorbance measurements
were carried out at a wavelength of 604 nm. In the first
minutes, the guaiacol solution undergoes a fast color
change from colorless to dark brown, reaching a peak
level. Later, the solution begins to slowly clear up to a light
brown, and even turns a pale yellow residual color in some
experimental conditions.
The kinetic pathway followed by the guaiacol oxidation
reaction and the experimental results reported here show that
color is not a fortuitous result depending on trace components
or parameters with low significance, but depends directly on
the main reaction intermediates. Indeed, in any experiment,
color shows the path of a reaction intermediate which follows
a slow kinetics that can continue for many minutes. So it
might be possible to establish a relationship between the color
level observed and the intermediate compounds generated
during the oxidation.
Based on HPLC analysis results for the determination of
the reaction intermediates, the mechanism that we proposed
for guaiacol oxidation shows that during the first stages of
oxidation, highly colored intermediate compounds such as
methoxy-p-benzoquinone (yellow) and o-benzoquinone
(red) are generated (Fig. 4). Their color comes from their
quinoidal structure, which contains chromophore groups
substituted in benzene rings. Benzoquinones achieve their
peak level during the first minutes, and then disappear
slowly because they are very stable species due to the
conjugated carbonyl groups contained in their internal
structure.
The effect of the dosing rate of hydrogen peroxide on
color evolution was tested in a set of assays with
constant catalyst concentration [Fe2+]0 =8 mM at pH 3.
Results were reported in Fig. 5, which shows the temporal
absorbance at 604 nm using different dosing rate of H2O2.
In all cases, the degradation reactions were much slower
than color generation. The rate of this decolorization stage
increased with the increase of the dosing rate of H2O2 and
the solution being fully decolorized at shorter reaction
times. With 60 mg min−1 H2O2 dosing rate, the absorbance increased rapidly and has a maximum (A604 =0.517)
after about 6 min of treatment and then decreased and
tended to zero beyond 35 min leading to the almost
complete disappearance of color and so of quinine-type
intermediates. The relationship between the dosing rate of
hydrogen peroxide and the final color of the solution is
therefore established, and it is concluded that the color
observed depends on the level of oxidation reached.
Consequently, it can be said that current color is a good
indicator of the degree of oxidation achieved during the
reaction. Using at least 60 mg min−1 H2O2 dosing rate is
required in these conditions to remove completely the
toxicity associated with aromatic intermediates and that of
guaiacol itself, but some biodegradable acids would
remain in the solution.
3.1.4 Effect of the initial concentration of ferrous iron
Ferrous ion acts as a catalyst in Fenton’s reactions. To
choose the optimal amount of Fe2+ added in the reaction
solution, a set of tests was performed. Figure 6 illustrates
the decrease of COD with time, during the oxidation of
guaiacol solution (COD0 =1,080 mg L−1) using different
initial concentrations of Fe2+. As can be seen, Fe2+ dosage
has a significant effect on the degradation of guaiacol. The
COD percent removal increased from 65% to 90% within
45 min reaction when the initial concentration of Fe2+
increased from 2 to 8 mM. As a catalyst, ferrous ion
1502
Environ Sci Pollut Res (2011) 18:1497–1507
Fig. 4 Reaction pathway of the
degradation of guaiacol by Fenton
and solar photo-Fenton processes
OCH3
OCH3
.
.
OCH3
.
O
OH
OH
- H2O
.
Guaiacol
OCH3
O
O
O
.
- CH3OH
OH
OCH3
.
.
.
OH
OH
OCH3
OCH3
OH
Polymeric products
OH
OH
OH
OH
HO
.
- 2 H2O
2 OH
.
O
.
- 2 H2O
2OH
2 OH
- 2 H2O
OCH3
OCH3
O
O
O
O
O
.
OH
HOOC
COOH
COOH
HOOC
.
OH
HOOC
COOH
HCOOH
.
OH
CO 2
0.6
−1
H2O2 (mg min )
60
15
3
0.5
A604
0.4
0.3
0.2
0.1
0.0
0
10
20
30
40
50
Time (min)
Fig. 5 Temporal results of color evolution at 604 nm (A604). Effect of
the dosing rate of H2O2 upon color formation and abatement. COD0 =
1,080 mg L−1, [Fe2+]0 =8 mM, pH=3, and T=40°C
+
H2O
initiates the decomposition of hydrogen peroxide to
generate the very reactive HO· in Fenton’s reactions.
Therefore, higher initial Fe2+ concentration lead to higher
generation of HO· (Banerjee et al. 2008) and better
degradation of guaiacol and its by-products. However, for
Fe2+ doses higher than 8 mM, the COD percent removal
decreased slightly; it passed from 90% to 82% when the
Fe2+ concentration increased from 8 to 40 mM. This
decrease is essentially due to competitive consumption of
HO· and HO2· radicals (Eqs. 5 and 7) (Bielski et al. 1985;
Buxton et al. 1988)
It is worth noting that, in the Fenton process, the
amounts of Fe2+ ions should be as low as possible for
economic and environmental reasons; high amounts of
Fe2+ ions might produce a larger quantity of Fe3+ sludge.
The removal/treatment of the sludge-containing Fe3+ at
the end of the wastewater treatment is expensive and
needs large amount of chemicals and manpower (Ramirez
et al. 2007).
2+
1.0
1503
10
2
[Fe ]0 (mM)
2
4
8
25
40
1.2
COD/CODO
−1
1.4
kapp x 10 (min )
Environ Sci Pollut Res (2011) 18:1497–1507
0.8
8
6
4
2
0
0
10
20
30
40
50
2+
[Fe ]O (mM)
0.6
0.4
modified by iron (III) that appears in the reacting medium
due to the oxidation of Fenton reagent. Quinone-type
compounds are the main contributors to the color observed
during the reaction, although the contribution of iron (III)
and its complexes is by no means negligible because the
residual color of fully oxidized water increases with the
initial concentration of iron. This suggestion is also
supported by other authors (Federico et al. 2006).
3.1.5 Effect of the initial concentration of guaiacol
0.2
0.0
0
10
20
30
40
50
Time (min)
Fig. 6 Effect of the initial concentration of Fe2+ on the COD removal
by Fenton process. The inset panel shows kapp evolution at different
Fe2+ concentration. COD0 =1,080 mg L−1, dosing rate of H2O2
60 mg min−1, pH=3, and T=40°C
On the other hand, the inset of Fig. 6 shows that kapp
values, calculated from the straight lines, considering a
pseudo-first-order reaction, increased as a function of Fe2+
dosage and reached a maximum around 8 mM of Fe2+.
Therefore, 8 mM Fe2+ was selected as the optimum Fe2+
dosage in this work.
Moreover, Fig. 7 shows the change in absorbance of the
solution versus time measured for four initial concentrations of Fe2+ ions. In all cases the curves showed the same
shape. They passed as from the first minutes through a
maximum whose intensity increased proportionally with the
amount of Fe2+ ions. The final color can be deepened or
It is important from an application point of view to study
the dependence of removal efficiency on the initial
concentration of the pollutant. Therefore, the effect of
guaiacol concentration on the degradation efficiency was
investigated at different initial concentrations (COD0, 453,
710, 1,080, and 1,435 mg L−1) and presented in Fig. 8. It
can be observed that the COD removal decreased with the
increase of the initial concentration of the pollutant. Almost
90% of COD removal was achieved after about 15, 25, and
35 min time of reaction for COD 0 453, 710, and
1,080 mg L−1, respectively. However, at high guaiacol
concentration, the removal of COD needs more time and so
more quantity of H2O2 (e.g., the percent removal of COD is
about 75% after 45 min when using COD0 =1,435 mg L−1).
This is because when the concentration of guaiacol
increases, the quantity of hydroxyl radicals produced
continuously with time does not increase accordingly and
hence the removal rate decreases. Also, from the inset of
Fig. 8, it can be seen that kapp decreased linearly with
COD0. This behavior was similar to those reported by many
researchers (Tamimi et al. 2008; Lucas and Peres 2006;
Modirshahla et al. 2007).
−1
0.6
0.8
COD/COD0
A604
0.4
0.2
−1
1.0
2
4
8
25
0.8
COD0 (mg L )
1435
1080
710
453
2
2+
[Fe ]O (mM)
kapp x 10 (min )
1.0
0.6
12
8
4
0
500
1000
1500
−1
COD0 (mg L )
0.4
0.2
0.0
0.0
0
10
20
30
40
50
Time (min)
Fig. 7 Temporal results of color evolution at 604 nm (A604). Effect of
the initial concentration of ferrous ions upon color formation and
abatement. COD0 =1,080 mg L−1, dosing rate of H2O2 60 mg min−1,
pH=3, and T=40°C
0
10
20
30
40
50
Time (min)
Fig. 8 Effect of the initial concentration of guaiacol on the COD
removal by the Fenton process. The inset panel shows kapp evolution
at different initial COD. Dosing rate of H2O2 60 mg min−1, [Fe2+]0 =
8 mM, pH=3, and T=40°C
1504
Environ Sci Pollut Res (2011) 18:1497–1507
3.1.6 Effect of the initial pH value
It has been illustrated that the pH affects significantly the
degradation of organics by the Fenton reaction and acidic
conditions are required to produce the maximum amount
of hydroxyl radicals by the decomposition of hydrogen
peroxide catalyzed by ferrous ions (Lin and Lo 1997;
Tang and Huang 1996). Several investigations have
indicated that the optimum pH for the degradation of
organics by the Fenton process is in the range 2.5–3.5 and
that the extent of degradation decreases with increasing
pH for pH>3.5 (Samet et al. 2009; Kang and Hwang
2000; Dutta et al. 2002; Malik and Saha 2003; Hsueh et al.
2005).
Figure 9 shows the COD decrease with time and the kapp
curve, during the oxidation of guaiacol solution (COD0 =
1,080 mg L−1) as a function of the initial pH. Clearly, the
COD removal is significantly influenced by the pH and the
optimum pH was 3. The values of kapp increase when pH
increases from 2 to 3, then decrease when pH is raised from
3 to 6. The contributing factors for the low kapp in lower pH
range (<3) include the formation of oxonium ion (i.e.,
H3O2+) due to the strong proton solvating ability of H2O2,
complex species [Fe(H2O)6]2+ and [Fe(H2O)6]3+ (Bossmann
et al. 1998) and enhanced HO· scavenging by H+ (Tang and
Huang 1996; Feng et al. 2003). The poor degradation of
guaiacol at a high pH values (>3) was caused by the
formation of ferrous and ferric hydroxide complexes with
much lower catalytic capability than Fe2+ (Kang and
Hwang, 2000).
On the other hand, the influence of the initial pH on the
evolution with time of the absorbance at 604 nm wave-
COD/CODO
The temperature plays an important role in chemical
oxidation, because it represents a determinant parameter
in the kinetics of homogeneous reactions. The influence
of this parameter on the kinetic rate constants, kapp, for
the guaiacol degradation was investigated in the range
between 30°C and 70°C with tests conditions at COD0
1,080 mg L−1, H2O2 60 mg min−1, [Fe2+]0 8 mM, and
pH 3. The obtained results shown in Fig. 11 indicate that
kapp was significantly influenced by the temperature with
an optimal value of 40°C. The values of kapp quickly
increased when the temperature increased from 30°C to
40°C, suddenly decreased when the temperature was
raised from 40°C to 50°C, and then gradually and slightly
drop off with the increase of temperature in the range of
50–70°C. The decrease of kapp at temperature higher than
40°C is due to the accelerated decomposition of H2O2 into
oxygen and water. Similar results were reported by Wang
(2008).
The data for temperatures between 25°C and 40°C
exhibit an Arrhenius-type behavior with apparent activation
energy of 17,543 Jmol−1 (Eq. 19) calculated from the usual
Ln kapp versus 1/T (Fig. 12).
kapp ¼ 58:78exp
10
17; 543
RT
min
1
ð19Þ
0.7
−1
0.8
8
pH
0.6
6
2
1.0
kapp x 10 (min )
2
3
4
5
6
3.1.7 Effect of the temperature
4
2
3
5
0.5
2
0
0
0.6
2
4
6
8
pH
A 604
pH
1.2
length is shown in Fig. 10. As can be seen, initial pH has no
significant effect on the absorbance formation and abatement because the Fenton treatment at pH values between 2
and 5 led to the almost complete disappearance of color in
the end of the treatment.
0.4
0.3
0.4
0.2
0.2
0.1
0.0
0.0
0
10
20
30
40
50
60
Time (min)
Fig. 9 Effect of the initial pH on the COD removal by the Fenton
process. The inset panel shows kapp evolution at different pH. Dosing
rate of H2O2 60 mg min−1, [Fe2+]0 =8 mM, COD0 =1,080 mg L−1, and
T=40°C
0
10
20
30
40
50
Time (min)
Fig. 10 Temporal results of color evolution at 604 nm (A604). Effect
of the initial pH upon color formation and abatement. COD0 =
1,080 mg L−1, dosing rate of H2O2 60 mg min−1, [Fe2+]0 =8 mM,
and T=40°C
Environ Sci Pollut Res (2011) 18:1497–1507
T (°C)
4
6
4
20
3
40
60
80
T (°C)
Fenton
Solar photo-Fenton
1.0
4
Ln (COD0/COD)
Ln (COD0/COD)
2
5
25
30
35
40
50
60
70
0.8
COD/COD0
8
−1
kapp x 10 (min )
6
1505
0.6
0.4
y = 0.21 x
2
R = 0.992
3
y = 0.12 x
2
R = 0.988
2
1
0
0
2
10
20
30
Time (min)
0.2
1
0.0
0
10
20
30
40
Time (min)
0
0
10
20
30
40
50
Time (min)
Fig. 11 Plot of Ln (COD0/COD)-t at different temperatures. COD0 =
1,080 mg L−1, dosing rate of H2O2 60 mg min−1, [Fe2+]0 =8 mM, and
pH=3
where R is the ideal gas constant (8.314 Jmol−1 K−1) and T
is the reaction absolute temperature (K).
3.2 Oxidation of guaiacol by the solar photo-Fenton process
In order to improve the reaction rate and COD abatement
efficiency, solutions were subjected to solar radiation using
a laboratory-scale reactor (Fig. 1). Figure 13 shows the
trend of the COD/COD0 ratio during the treatment of
guaiacol solution by the two processes under the optimum
experimental conditions already found when using Fenton
Fig. 13 Effect of solar radiation on the kinetic abatement of COD
during the treatment of guaiacol solution (COD0 =453 mg L−1). The
inset panel shows the fitting of the experimental data to a first-order
reaction kinetic model. Dosing rate of H2O2 60 mg min−1, [Fe2+]0 =
8 mM, pH=3, and T=40°C
process (COD0 453 mg L−1, H2O2 60 mg min−1, [Fe2+]0
8 mM, pH 3, and T 40°C). It can be seen that the solar
photo-Fenton system needed less time and consequently
less quantity of H2O2 to reach the same COD recent
removal. In fact, under the optimum experimental conditions, the solar photo-Fenton process need a dose of H2O2
40% lower than that used in the Fenton process to remove
90% of COD. On the other hand, the COD removal rate is
higher with the solar photo-Fenton process as shown the
kapp values in the inset of Fig. 13.
4 Conclusion
-2.6
-2.7
Ln kapp(min−1)
In this study, the degradation of guaiacol has been studied
by applying homogeneous Fenton and solar photo-Fenton
processes. The results showed that:
y = −2.1101 x + 4.0739
2
R = 0.9691
–
-2.8
-2.9
-3.0
–
-3.1
–
3.15
3.20
3.25
3.30
3
3.35
3.40
1/Tx 10 (K 1)
−
Fig. 12 Plot of Ln kapp–(1/T) for the degradation of guaiacol by
Fenton oxidation process. Dosing rate of H2O2 60 mg min−1, COD0 =
1,080 mg L−1, [Fe2+]0 =8 mM, and pH=3
–
–
The solar photo-Fenton process was more efficient
than the Fenton process for COD removal. In the
solar photo-Fenton process, the degradation rate
was increased by 40% which reduced the operating
cost.
The COD and color removal increased with the
increase of the dosing rate of hydrogen peroxide.
The ferrous ion as catalyst accelerated the COD
removal. Fe2+ concentration of 8 mM could be used
as an optimum dosage for Fenton process.
The optimum pH for both COD and color removal
was 3.
The degradation rate was significantly influenced by
the temperature with an optimum value of 40°C.
1506
Acknowledgment This research is funded by the Tunisian Higher
Education and Scientific Research Ministry.
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