Modeling and kinetics study of Bisphenol A (BPA) degradation over an FeOCl/SiO2 Fenton-like catalyst

Article abstract of DOI:10.1016/j.cattod.2016.01.002

A heterogeneous Fenton-like catalyst was used for the oxidation of Bisphenol A (BPA), which is a typical endocrine disruptor. A simple semi-empirical kinetic equation was developed to define the effects of experimental conditions (initial concentration of H₂O₂, initial concentration of BPA, temperature and the catalyst loading) on the reaction rate. The plausible pathway of the degradation process was proposed. A pseudo-first-order reaction rate expression with respect to BPA concentration was developed to fit all data for these experiments, and the equation of k_obs (observed rate constant) was obtained with the apparent activation energy of 42.2 kJ/mol. In order to reduce the operational cost, the values of parameters (temperature, initial concentration of H₂O₂ and pH) that optimize the catalytic performance were studied by means of a response surface methodology. Finally, a plausible mechanism was proposed on the basis of kinetics and optimization studies, which illustrates the reaction process and optimization in a microscopic view.

Full text of DOI:10.1016/j.cattod.2016.01.002

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Modeling and kinetics study of Bisphenol A (BPA) degradation over an
FeOCl/SiO2 Fenton-like catalyst
Xue-jing Yanga,b, Xi-meng Xua, Xin-chao Xua, Jing Xua, Hua-lin Wanga,b
,
Raphael Semiatc, Yi-fan Hana,d,∗
a State Key Laboratory of Chemical Engineering, East China University of Science and Technology (ECUST), Shanghai 200237, China
b State Environmental Protection Key Laboratory of Environmental Risk Assessment and Control on Chemical Process, ECUST, Shanghai 200237, China
c The Wolfson Department of Chemical Engineering, Technion, Israel Institute of Technology Technion City, Haifa 32000, Israel
d Research Center of Heterogeneous Catalysis and Engineering Sciences, School of Chemical Engineering and Energy, Zhengzhou University, Zhengzhou
450001, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A heterogeneous Fenton-like catalyst was used for the oxidation of Bisphenol A (BPA), which is a typical
endocrine disruptor. A simple semi-empirical kinetic equation was developed to define the effects of
experimental conditions (initial concentration of H2O2, initial concentration of BPA, temperature and the
catalyst loading) on the reaction rate. The plausible pathway of the degradation process was proposed.
A pseudo-first-order reaction rate expression with respect to BPA concentration was developed to fit
all data for these experiments, and the equation of kobs (observed rate constant) was obtained with the
apparent activation energy of 42.2 kJ/mol. In order to reduce the operational cost, the values of parameters
(temperature, initial concentration of H2O2 and pH) that optimize the catalytic performance were studied
by means of a response surface methodology. Finally, a plausible mechanism was proposed on the basis of
kinetics and optimization studies, which illustrates the reaction process and optimization in a microscopic
Received 27 October 2015
Received in revised form
22 December 2015
Accepted 2 January 2016
Available online xxx
Keywords:
FeOCl/SiO2
Bisphenol A
Semi-empirical kinetic modeling
HO• radical
view.
© 2016 Elsevier B.V. All rights reserved.
1. Introduction
tion of BPA has proved to be feasible with a relative longer time,
at least one month [6,7], is necessary for this process. Therefore, a
Environmental endocrine disruptor has proved to interfere with
endocrine in animals and human beings, resulting in cancerous
tumors, birth defects and other system developmental disorders
[1]. Bisphenol A (BPA), known as a typical endocrine disruptor,
has shown high rates of diabetes, mammary and prostate cancers,
reproductive problems, early puberty, obesity and neurological
problems [2]. BPA is generally used as an intermediate in plas-
tic industry, which products include paper, plastic bottles, powder
paintings, building materials and adhesives [3]. BPA can be released
into the natural environment during the manufacture process or by
leaching from the products, thus engendering the source of contam-
inants in the aquatic environment via accumulation [4]. On basis of
a screening-level review of the hazard and exposure information
about BPA, including the uncertainties about low-dose studies, U.S.
environmental protection agency (EPA) has proposed the potential
danger to people when exposing to this chemical [5]. Biodegrada-
rapid, convenient and effective method for the treatment of BPA is
highly desired.
Advanced oxidation processes (AOPs) refer to a set of chemical
technologies and procedures applied in the water treatments aim-
ing at removing organic and inorganic pollutants including dyes
[8–10], chlorinated hydrocarbons [11] and phenolic compounds
[12,13] by oxidation reactions. These processes can be decided by
the way of the generation of hydroxyl radical (HO•), which is a
powerful oxidant [14].
The application of Fenton reagent is a well-known example of
AOPs (Eq. (1)) [15].
M
n+ + H2O2 → M(n+1)+ + OH− + OH•(M = Fe, Cu, Mn)
(1)
However, several drawbacks of the conventional Fenton
reagent, such as the narrow pH range (3.0–5.0) and the production
of sludge hamper this process from practical application [15–17].
The Fenton process using solid Fenton-like or heterogeneous cata-
lysts is regarded as an alternative to solve those problems. Typical
solid catalysts, such as iron powder [18], ferric ion exchanged
Nafion membrane [19], Fe2O3 and Fe containing clay [20], Au
∗
Corresponding author. Fax: +86 21 64251928.
E-mail address: yifanhan@ecust.edu.cn (Y.-f. Han).
http://dx.doi.org/10.1016/j.cattod.2016.01.002
0920-5861/© 2016 Elsevier B.V. All rights reserved.
Please cite this article in press as: X.-j. Yang, et al., Modeling and kinetics study of Bisphenol A (BPA) degradation over an FeOCl/SiO2
Fenton-like catalyst, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2016.01.002

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the degradation pathway of BPA, which explains the experimental
results well.
Nomenclature
Y
kobs
kHO
kapp
A
Target compound
2. Experimental
Observed rate constant, min−1
Intrinsic rate constant, min−1
Apparent rate constant, min−1
Pre-exponential coefficient, min−1
Apparent activation energy, kJ/mol
Oxidants in the system other than HO•
Concentration of hydroxyl radical, mg/L
Concentration of BPA, mg/L
•
2.1. Catalyst preparation
A melt infiltration method was applied for the preparation
of FeOCl/silica catalyst [37]. FeCl3·6H2O (AR, from Sinoreagent,
China) was selected as iron source and fumed silica AEROSIL 200
from Degussa was applied as the supporting material. The loading
amount of Fe is 10 wt%. Both materials were physically mixed in an
agate mortar for 10 min under ambient conditions till the powder
became uniform lemon yellow. Then, the precursor was transferred
to a sealed glass vessel for 24 h infiltration at 353 K. Finally, the
obtained powder was sintered at 623 K in air with a ramping rate
of 5 K/min for 30 min.
Ea
OXi
[HO•]
[BPA]
[H2O2] Concentration of H2O2, mg/L
[Catal.] Catalyst load amount, mg/L
T
Co
R
Temperature
Standard concentration, 1 mg/L
BPA degradation efficiency
2.2. Catalytic performance and kinetic measurement
[21,22], MnOx [23–25], have exhibited high efficiencies toward the
generation of HO• from H2O2 decay.
With referring to the “microfluid” theory proposed by Leven-
spiel [38] the catalyst was continuously stirring at 700 rpm for
mixing completely. The oxidation of BPA was conducted in a glass
batch reactor with a capacity of 150 mL. The temperature was con-
trolled by circulating water through the jacket of the reactor. The
pH value was adjusted with 0.1 M HNO3. The used catalyst was fil-
tered with a 0.22 ␮m nylon membrane and vacuum dried at 313 K
for the recycling.
Without further emphasis, 10 mg catalyst was put into 100 mL
solution containing BPA (60 mg/L) at pH 5.0, and then the mixture
was stirred for 20 min to reach the adsorption equilibrium and a
constant temperature, following with the addition of H2O2. The
sample was taken at various time intervals, and filtered with a
0.22 ␮m nylon membrane immediately to remove the catalyst. The
concentration of BPA was determined by a high performance liq-
uid chromatography (HPLC, Perking Elmer Flexar), equipped with
Spheri-5C18 column and UV detector (225 nm for BPA) [39]. The pH
of the solution was monitored by a pH meter (Leici PHS-3D, China),
which was kept within 0.1 pH unit.
≡ FeIII + H2O2 →≡ FeIIIH2O2
≡ FeIIIH2O2 →≡ FeII + HO2 + H+
≡ FeII + H2O2 →≡ FeIII + OH− + OH•
(2)
(3)
(4)
For the solid Fenton-like systems, HO• is generated by the
decomposition of H2O2 on the surface of the solid catalysts through
a surface complexion mechanism (Eqs. (2)–(4)) [26]. Up to now, the
roles of surface adsorption/desorption and surface reactions are not
clear yet, nor are the rate constants of elementary steps. The equiv-
ocal mechanisms and complicated kinetics pose challenges for the
design and scale-up of heterogeneous Fenton reactors, especially
when optimal operation conditions should be reached through
minimal experimentation from the point of operational cost [16].
Empirical modeling based on an experimental design methodology
is often applied in such a case. An empirical kinetic model may not
only reflect the reaction behavior, but also illustrate the effects of
operating parameters on the rate constants for further designing
Fenton reactors, which should be suitable for the solid catalysts.
In this study, we present the catalytic performance of a silica
supported iron oxychloride (FeOCl/SiO2) in a heterogeneous Fen-
ton process using BPA as the model compound. It is noted that
unsupported FeOCl with a layer structure has already showed high
reactivity to HO• production by H2O2 decomposition, being com-
parable to the conventional Fenton regent [27,28]. To the best of
our knowledge, it is the first attempt to degrade BPA in such a sys-
tem, and the results show that the application of the system may
act as an ideal and inexpensive alternative for traditional biological
treatment of BPA-containing wastewater. A semi-empirical equa-
tion that describes the kinetics of BPA degradation in a continuous
stirred tank reactor was presented, and the effects of several fac-
tors, such as temperature, initial concentration of H2O2 and initial
pH of solution, on the reaction rate are measured in details. The
studies on empirical kinetic modeling of a heterogeneous Fenton
process are very rare [29–31].
2.3. Catalyst characterization
The crystalline phase of the prepared catalyst was analyzed
by X-ray diffraction (XRD) pattern (Rigaku D/Max 2550), with the
assistance of MDI Jade software (5.0 version). The crystalline size
of the FeOCl (according to JCPDS card No. 24-1005) was calcu-
lated by Sherr equation from the FWHM of FeOCl (0 1 0) peak (2
theta = 11.3◦). The specific surface area of the catalyst was cal-
culated from the N2 adsorption–desorption (collected from ASAP
2010, Micromeritics, USA) isotherms at 77 K using the BET method.
The isoelectronic point of the catalyst was determined by a dynamic
laser scattering instrument (Malvern Zetasizer Nano S). The oxi-
dation state of the iron was detected by X-ray photoelectron
spectroscopy (XPS, thermo ESCALAB 250Xi). The concentration of
metal ion was analyzed by ICP-MS (PerkinElmer NexION 300)
A response surface methodology (RSM) was applied in this study
to optimize the performance of the catalyst in terms of BPA degra-
dation and influences of main operational parameters [32]. RSM
can be used for optimization of the process, which can reduce the
amount of experiments needed [33]. It has already proven to be a
reliable statistical tool in the investigation of chemical treatment
processes in other studies [34–36]. On the basis of kinetics and
optimization study from macroscopic and empirical view, we pro-
pose a plausible reaction mechanism at microscopic scale as well as
3. Results and discussion
3.1. Structure of the catalyst
The physical characteristics of the prepared catalyst were out-
lined in Table 1. As indicated by XRD, FeOCl was exhibited as the
sole crystalline phase in FeOCl/SiO2 (Fig. 1a). In a previous study,
we have already demonstrated the supreme capacity of FeOCl for
HO• generation [27]. With an increase in specific surface area to
Please cite this article in press as: X.-j. Yang, et al., Modeling and kinetics study of Bisphenol A (BPA) degradation over an FeOCl/SiO2
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Table 1
Structure and properties of the catalyst.
Properties
Unit
Content
Color
Purple
Structure
FeOCl crystalline; SiO2 amorphous with non-porous structure
Crystalline size
Iron content
Iron oxidation state
pHpzc
nm
wt%
88
11.7
+3
7.8
133.3
Specific surface area
m2/g
(b)
(a)
Adsorption
Desorption
500
400
300
200
100
0
10
20
30
40
50
60
70
80
0.0
0.2
0.4
0.6
0.8
1.0
2theta (degree)
Relative Pressure
(P/P
)
o
Fig. 1. XRD pattern (a) and N2 adsorption–desorption isothermal curve (b) of the FeOCl/SiO2 catalyst.
133.3 m2/g (Fig. 1b), the dispersion of FeOCl on SiO2 reduced the
dosing of iron and enhanced the efficiency for the degradation of
organic compounds. The isoelectronic point of the FeOCl/SiO2 was
7.8, indicating that the surface of the catalyst was positively charged
by immersing into solution of the acidic or neutral pH.
can not be detected by the present experimental conditions, we
infer the degradation pathway by the interaction of BPA and
hydroxyl radicals, as demonstrated in Fig. 4. The 4-isopropyl group
within BPA could be firstly attacked by HO•. The free electron
of singlet state may transfer to 4-isopropyl group or benzene
ring, leading to the formation of p-hydroquinone (p-HQ) or 4-(1-
hydroxy-1-methyl-ethyl)-phenol (HMEP), respectively. The HMEP
will convert into 4-isopropenylphenol (IPeP) under the proceed-
ing of dehydration reaction. In following with the oxidation of
peroxide, 4-hydroxyacetophenone (HAP) may generate as conse-
quence, meanwhile, the hydroxycyclo-hexadienyl radicals may be
formed under the attacking of HO•. Small aliphatic molecules, such
as formic acid, acetic acid or acetaldehyde will be produced simul-
taneously while the aromatic compounds transfer to p-HQ. p-HQ
will further oxidize to p-quinone and an oxidative ring-opening
reaction will finally happen, which leading to the generation of
complete degradation products, CO2 and H2O. Without the inter-
action between free electron (H•), the generation of high-toxic
intermediates, including phenol and substituted indoles will be
diminished [41]. It indicated the advantages of heterogeneous Fen-
ton process compared with photo degradation or photo-Fenton on
the treatment of BPA containing waste water.
3.2. Adsorption and degradation of BPA on an FeOCl/SiO2 catalyst
The time-dependent BPA con-centration was measured during
the isothermal adsorption process (Fig. 2). The concentration of BPA
was reduced by 5% after 24 h, oscillating within the error range in
the whole process at various pH values. The degradation of BPA
is assumed to proceed through Eqs. (5) and (6) [40]. The posi-
tively charged surface of FeOCl/SiO2 has proved not affinitive to
BPA in this pH range. In contrast, with the addition of H2O2, as indi-
cated by the HPLC spectra of BPA (Fig. 3 curve A), the peak area for
BPA (retention time ∼3.5 min) was remarkably reduced in the first
30 min (Fig. 3, curve C and D). The peaks for other products were
observed and consequently diminished after 180 min (Fig. 3, curve
E). The degradation of BPA was likely completed within 180 min.
However, in the absence of H2O2, the spectrum of liquid sample
remained unchanged after 24 h (Fig. 3, curve B). Therefore, we con-
clude that the effect of adsorption on the conversion of BPA is
negligible. In addition, the concentration of leaching Fe ions was
also detected by ICP–MS, which is listed in Table 1. It is indicated
that the catalyst is relatively stable in aqueous and the influence of
the homogenous Fenton reaction was nearly negligible as well.
3.3. Kinetics study
BPA → BPA− + H+pKa1 = 9.6
BPA− → BPA2− + H + pKa2 = 10.2
(5)
(6)
The general elementary rate law for the degradation of the
model organic compound (Y) can be expressed in Eq. (7) [16]:
The evolution of the degradation by-products is of crucial impor-
tance for the degradation pathway as well as the bio-toxic effect of
the process. Though the specific composition of the by-products
ꢀ
d[Y]
dt
•
•
−
= kHO [OH ][Y] +
kox,i[OXi][Y]
(7)
i
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75
70
65
60
55
2
4
6
8
10
12
14
16
18
20
22
24
Adsorption time (h)
Fig. 2. Time-on-stream of BPA concentration at pH 4.0 (᭿), pH 5.5 (᭹), pH 6.8 (ꢀ) during adsorption process.
(E )
2000
(D)
1500
(C)
1000
500
(B)
0
(A)
0.5
1.0
1.5
2.0
2.5
3.0
3.5
Relative retention time (min)
Fig. 3. HPLC spectra of BPA with (A) or without H2O2(B) and the profiles of the degradation products at reaction time 10 min (C), 30 min (D), 180 min (E).
HO• is usually regarded as the most powerful species for the
oxidation/degradation of organic compounds [26,42,43]. The OXi
represents oxidants in the system other than HO•.
previous studies [44–47]. Therefore, the reaction in Eq. (7) can be
expressed as Eq. (9):
The corresponding reaction rate is simplified as Eq. (9):
rBPA = kHO HO• [Y] = k
BPA
]
(9)
•
[
]
[
obs
HO• + BPA → products
(8)
rBPA represents the intrinsic rate for the degradation, while kobs
is the observed rate constant. The value of kobs (Eq. (10)) depends
on the reaction conditions including initial concentrations of H2O2
and BPA, temperature, initial pH and the loading amount of cata-
This reaction can be further simplified by assuming that the con-
centration of HO• (denoting as [HO•]) is kept constant in the whole
experiment (Eq. (8)). Such an assumption is quite often adopted in
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Fig. 4. Degradation pathway of BPA by FeOCl/SiO2 heterogeneous Fenton process.
2.0
1.5
1.0
0.5
4
8
12
16
20
24
28
Reaction time (min)
Fig. 5. Time-dependent BPA concentration during the reaction. Reaction conditions: [BPA]0 = 80 mg/L, [H2O2]0 = 100 mg/L, [Catal]0 = 100 mg/L, T = 303.15 K and pH 6.8.
lyst. In this study, we kept the pH neutral (∼6.8) initially without
further adjustment. By alternation of the variable one at a time and
linearization of the data, the dependence of kobs on the reaction
conditions in Eq. (10) can be determined.
because of a linear relationship between reaction time t and In
([BPA]/[BPA]0) (Fig. 5).
ꢁ
ꢂ
[BPA]
[BPA]0
ln
= −kobs × t
(11)
The observed rate constant can be predicted by a simple empir-
kobs = f([H2O2]0, [Catal], [BPA]0,T, pH)
(10)
ical equation, Eq. (12):
ꢃ
ꢄ
a
ꢁ
ꢂ
ꢁ
ꢂ
b
c
[H2O2]0
[BPA]0
C0
[Catal.]0
C0
Experimental results and liner regression analysis (Eq. (11)) iden-
tified that the degradation of BPA was a pseudo first-order reaction
kobs = kapp
×
×
×
(12)
C0
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whereas, exponent a, b and c represent the apparent rate orders
of H2O2, BPA and the loading amount of catalyst, respectively. C0
is the standard concentration of 1 mg/L. kapp is the apparent rate
constant, which can be expressed as Eq. (13):
calculated, the equation can be expressed as Eq. (15):
ꢃ
ꢄ
1.25
ꢁ
ꢂ
ꢁ
ꢂ−1.63
42183
RT
[H2O2]0
C0
[BPA]0
C0
kobs = Aexp
−
×
×
ꢁ
ꢂ
Ea
ꢁ
ꢂ
1.64
[Catal.]0
C0
kapp = A × exp
−
(13)
×
(15)
RT
A is the pre-exponential coefficient; Ea is the apparent activation
energy for this reaction.
The resulting parity plot depicted in Fig. 10 shows a good agree-
ment between the experimental and calculated results. During the
reaction, the initial conditions may change with time and it resulted
in the error of Eq. (15). In order to lower the error due the change in
BPA concentration, only the data at initial 10 min was taken (Fig. 5),
because the variation of BPA concentration was less than 20% then.
Therefore, Eq. (15) can be used to predict the rate of BPA degra-
dation in the batch reactor with sufficient precision within first
10 min. Also, it is valuable to predict the behavior of a continuous
stirred tank reactor (CSTR), since the initial concentrations of H2O2
and BPA only vary in such a reactor.
3.4. Effect of initial H2O2 concentration
The effect of initial concentration of H2O2 on kobs was derived
from the data of run 1–5 in Table 2. Clearly, the reaction rate
was enhanced with an increase in H2O2 concentration, while kobs
decreased simultaneously. It is well accepted that when the excess
of H2O2 can scavenge HO• since the radicals are non-selectively
active toward all the scavengers in solutions [48,49]. Therefore,
the decrease in the concentration of HO• leads to a drop in the
BPA degradation efficiency. The apparent rate order with respect
to H2O2 was estimated to be 1.25 resulting from a ln [H2O2]0 vs.
lnkobs plot (Fig. 6).
3.9. Effect of diffusion
The apparent rate of heterogeneous reaction is usually domi-
nated by either the rate of diffusion of the solutes to the surface or
the rate of intrinsic chemical reactions on the surface. In this study,
the Thiele modulus: ꢀ was estimated according to Levenspiel to
elucidate the rate-determining step. The ꢀ is expressed as the ratio
3.5. Effect of BPA concentration
The effect of BPA concentration on the apparent kinetic constant
was also obtained by run 6–10 in Table 2. Obviously, the apparent
rate constant decreased with an increase in BPA concentration. The
apparent reaction order with respect to BPA was then determined
to be −1.63 from a ln [BPA]0 vs. ln kobs plot (Fig. 7), being close
to the data in the literature [47,50,51]. HO• is assumed to be gen-
erated from the interaction between H2O2 and iron oxide surface,
since the number of active sites on the catalyst surface for H2O2
activation remained unchanged during the experiment. However,
BPA molecule might be approached on the active sites in competi-
tion with H2O2, thus reducing the production of HO• and affecting
the generation rate of HO• [52].
of the reaction rate to the diffusion rate:
ꢅ
kint
ϕ = L
(16)
D
where kint is the first-order reaction rate constant (s−1), D is the
diffusion coefficient (cm2/s), and L is the thickness of the stagnant
liquid film or the pore length (cm). If ꢀ is estimated to be <0.5,
the rate of diffusion is concluded to be faster than the reaction rate,
while ꢀ > 5 suggest a strong liquid diffusion resistance which results
in a slower diffusion rate.
Since the catalyst used in this study is a non-porous material,
the pore diffusion (internal mass transfer resistance) was not con-
sidered, only the mass transfer rate of solutes by diffusing through
the liquid film (external mass transfer resistance) was evaluated. In
an vessel reactor with complete mixing, by the Film theory the dif-
fusion coefficient of the solutes in liquids is typically 10−5 cm2/s,
and the stagnant layer thickness of the liquid film is estimated
to 10−3 cm [54]. The maximum kobs fitting by the experimental
result is 0.002 s−1, and the corresponding ꢀ is about 0.4 for exter-
nal mass transfer. The value of ꢀ indicates that the reaction rate on
the catalyst surface is slower than the diffusion rate, thus the liquid
diffusion resistance can be ignored in the process.
3.6. Effect of the loading amount of catalyst
The loading amount of catalyst has a significant impact on the
reaction rate, which is expressed by run 14–18 in Table 2. This trend
reflects that the generation of HO• is proportional to the number
of active sites. The apparent rate order with respect to catalyst
(expressed by concentration) is determined to be 1.64 from a ln
[Catal.] vs. ln kobs plot (Fig. 8).
3.7. Effect of temperature
As listed in Table 2 (runs 20–23), the apparent rate constant is
raised with an increase in the temperature. However, the observed
rate constant decreased rapidly at above 323 K. H2O2 tends to
be decomposed into water and oxygen at elevated temperatures
[53]. The data exhibits an Arrhenius-type behavior with a Ea of
42.2 kJ/mol, calculated from a ln kobs vs. 1/T plot (Fig. 9).
Finally, the equation for apparent rate constant can be obtained
(Eq. (14)):
3.10. Optimization of BPA degradation
For a aiming at treating BPA-containing wastewater in CSTR sys-
tem, the independent variables including the initial concentration
of H2O2, initial pH and reaction temperature are optimized. In a
heterogeneous Fenton reaction, the variables impact on the differ-
ent reaction steps and have strong interactions, which results in
complicated and multiple effects on the whole reaction. In order to
improve the process efficiency, the influences of operation param-
eters on the catalytic performance must be evaluated. Here, a new
methodology was applied for the design of the experiments by
modeling the degradation of BPA as a function of H2O2 concen-
tration, initial pH and temperature. This modeling allows drawing
contour plots or curves of constant response value to predict the
values of the response (BPA degradation) at any point of the exper-
ꢁ
ꢂ
42183
RT
kapp = Aexp
−
(14)
3.8. Rate equation of BPA degradation
As mentioned previously, the observed rate constant can be
expressed by Eq. (12). Given all the parameters and coefficients
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Table 2
Effect of operation conditions on the observed rate constants.
Run
[H2O2]0 (mg/L)
[BPA]0 (mg/L)
90
[Catal.] (mg/L)
37
T (K)
kobs (min-1
)
Correlation coefficient
1
2
3
4
5
6
7
8
100
150
200
250
300
100
313.15
0.054 0.013
0.067 0.020
0.109 0.031
0.136 0.071
0.178 0.049
0.119 0.014
0.095 0.029
0.068 0.037
0.066 0.019
0.057
0.026 0.011
0.032 0.042
0.090 0.066
0.114 0.066
0.116 0.044
0.062 0.032
0.063 0.024
0.079 0.037
0.105 0.031
0.168
0.995
0.995
0.987
0.897
0.952
0.992
0.998
0.984
0.998
0.972
0.992
0.920
0.942
0.962
0.974
0.979
0.990
0.965
0.991
0.965
60
70
80
88
95
90
37
313.15
313.15
9
10
11
12
13
14
15
16
17
18
19
20
100
100
30
50
60
70
77
37
90
303.15
308.15
313.15
318.15
323.15
-1.6
-2.0
-2.4
-2.8
-3.2
4.6
4.8
5.0
5.2
5.4
5.6
5.8
ln [H2O ]0
2
Fig. 6. Effect of initial H2O2 concentration on the rate constant for BPA degradation.
Table 3
Furthermore, the analysis of variance (ANOVA) is used to test
the significance and adequacy of the prosed model. The results are
listed in Table 3. The coefficient of determination of 0.98 can prove
the quality of fitting to the quadratic equation [55]. It implied that
about 98% of the variations for the BPA degradation efficiency were
explained by the experimental factors and their interactions. In
addition, the model F-value (Fisher variation ratio) and the prob-
ability value (Prob > F, p-values) were employed as criterion to
evaluate the significance of each variables. The larger the F as well
as the smaller the p value are, the more significant of the corre-
sponding coefficient is. As indicated in Table 3, the temperature (C)
contributes very little. The significant of the interaction between
coefficient can be implied by the F value as well. Thus, the interac-
tion between the dosing amount of peroxide and the pH value of
the solution is intensive and obvious.
Experimental rang and levels of the process independent variables.
Independent variable
Unit
factor
Range and level
−1
0
+1
[H2O2]0
pH
t
mg/L
–
A
B
C
100
4
30
200
5.5
40
300
7
50
◦C
imental range: [H2O2]0: 100–300 mg/L; pH: 4–6.8; temperature:
303–325 K [55], summarized in Table 3.
The function of BPA degradation efficiency (R) after reaction
10 min is presented in Eq. (17):
R = 46.22 + 31.31 × A + 13.70 × B + 7.64 × C + 16.7 × AB
+ 3.64 × 10 − 3 × AC − 0.08 × BC − 8.55 × A2
− 2.42 × 10 − 2 × B2 + 0.09 × C2
By identifying these variables, we establish R-functions for a pair
of experimental variables (A and B), via contour plots representing
curves of constant response R for the values (A, B). In this way, the
optimum values of A and B that yield a maximum value of R (BPA
(17)
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-2.2
-2.4
-2.6
-2.8
4.1
4.2
4.3
4.4
4.5
ln [BPA]0
Fig. 7. Effect of initial BPA concentration on the rate constant for BPA degradation.
-2.0
-2.4
-2.8
-3.2
-3.6
3.0
3.2
3.4
3.6
3.8
4.0
ln[Catal.]
Fig. 8. Effect of catalyst load on the rate constant for BPA degradation.
Table 4
degradation) can be achieved. Since the impact of temperature is
not sensitive to the reaction rate, the interaction of A and B was
solely evaluated under 325 K. As shown in the 3D response sur-
faces and corresponding contour plots (Fig. 11), BPA degradation
efficiency does not change monotonously with any variable. Prob-
ably, the interaction among all variables exerts a comprehensive
effect on the BPA degradation reaction in a complex way. To opti-
mize such a system, in which parameters have strong interactions,
a response surface method is highly desired (Table 4).
Analysis of variance (ANOVA) from central composite design model.
Source of variable
Sum of square
Df
F
p-value
Value
prob>F
Model
A
B
C
AB
AC
0.15
0.018
0.015
9
1
1
1
1
1
1
1
1
1
7
3
23.32
25.32
21.21
9.10
5.30
0.49
<0.0001
<0.0001
0.0004
0.2281
0.0004
0.0079
<0.0001
<0.0001
<0.0001
6.4 × 10−3
3.7 × 10−3
3.4 × 10−4
4.2 × 10−4
0.034
BC
0.60
3.11. Plausible mechanism
A2
48.59
105.50
10.34
B2
0.074
C2
7.3 × 10−3
3.5 × 10−3
2.3 × 10−3
0.98
On the basis of kinetics and the interaction among parameters,
we propose a plausible mechanism (Fig. 12) for the degradation
of BPA on the solid Fenton-like FeOCl catalyst. The liquid diffusion
resistance was ignored so that only the processes of adsorption and
intrinsic reaction were taken into consideration. BPA was added
Residual
Lack of fit
R2
1.21
0.0009
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Fig. 9. Effect of temperature on the rate constant for BPA degradation.
0.12
0.10
0.08
0.06
0.04
0.02
R2 = 0.98
0.02
0.04
0. 06
0.08
0.10
0.12
k
from empirical equation ( )
min-1
obs
Fig. 10. Plot empirical kinetic model (solid line) and kobs obtained from experimental tests (᭿).
into the solution with solid catalysts, and the adsorption equilib-
rium of BPA molecules on the catalyst surface were obtained after
completely mixing. At steady state, we suppose that BPA molecules
are adsorbed on the catalyst surface. Then, H2O2 is adsorbed on
the active sites and produce HO• during a series of surface reac-
tions. As mentioned above, HO• is non-selectively active toward all
the scavengers. By integrating the parameters that may influence
the BPA degradation rate, we presume the radicals on the catalyst
surfaces can be scavenged immediately by: (i) catalyst surfaces;
(ii) adsorbed H2O2 molecules; (iii) adsorbed BPA molecules. There-
fore, radicals are supposed to be survived only in the environments
near the active sites. The amount of radicals scavenged by BPA can
be increased by reducing the catalyst surface and adsorbed H2O2
molecules, which can be achieved by enhancing the interaction
between H2O2 molecules and catalyst surfaces.
The interaction between pH and H2O2 is indicated by the
response surface graphs at 325 K (Fig. 11). For heterogeneous
Fenton-like processes [56], the effect of pH value mainly relates
to the chemical state of Fe, which influences the production of
HO•. Actually, the rate for BPA degradation increased at first, and
then decreased with the other one increasing, when one of the two
parameters was fixed. We argue that the redox of iron atoms in
the catalyst surface affects the adsorption-reaction process of H2O2
molecules, as well as the radical scavenger density. Likely, relatively
low concentrations of adsorbed H2O2 and catalyst surface favorites
BPA degradation (Table 5).
4. Conclusions
A new solid Fenton-like catalyst, FeOCl/SiO2, has found to be
active for the oxidation/degradation of BPA. Variation of opera-
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Fig. 11. 3-D response surface graphs for BPA degradation at 303 K.
Fig. 12. Scheme of plausible reaction mechanism.
Table 5
derived is valuable for the prediction of BPA concentration in the
batch reactor. In addition, the coefficients in the equation also pro-
vide information to analyze the effects of all the variables during
the reaction.
Leaching amount of iron ions during each reaction recycle.
Recycle times
1
2
3
4
Fe leaching (mg/L)
0.012
0.008
0.007
0.007
A response surface methodology has been developed to opti-
mize values of H2O2 concentration, pH and temperature, which
maximizes BPA degradation. On the basis of kinetics, a possible
reaction mechanism for the production of HO• by H2O2 decom-
tion conditions leads to the changes in reaction kinetics, which is
reflected by the apparent rate constants. The power-law equation
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position was proposed and explains the experimental results very
well.
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Acknowledgment
We gratefully acknowledge the support of the National Science
Foundation (21507030, 21576084 and 91534127), the National Key
Basic Research Program of China (2014CB748500), International
Cooperation Project of Shanghai Ministry of Science and Technol-
ogy (14230710700), the Fund of Chinese Post-doctral Community
(200-5R-1507, 20150074), and the Chinese Education Ministry 111
project (B08021).
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Fenton-like catalyst, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2016.01.002