Oxidation of p-hydroxybenzoic acid by Fenton's reagent

Article abstract of DOI:10.1016/S0043-1354(00)00285-2

Fenton's reagent has been shown to be a feasible technique to treat phenolic-type compounds present in a variety of food processing industry wastewaters. A model compound, p-hydroxybenzoic acid was oxidised by continuously pumping two solutions of ferrous iron and hydrogen peroxide. Typical operating variables like reagent feeding concentrations and flowrate, temperature and pH were studied. A mechanism of reactions based on the classical Fenton's chemistry was assumed, and computed concentration profiles of the parent compound, ferrous ion and dihydroxybenzene were compared to experimental results. The model qualitatively predicted the influence of several operating variables, however, calculated results suggested the presence of parallel routes of substrate elimination and/or a initiating rate constant with a higher value. The low efficiency of a well-known hydroxyl radical scavenger (tert-butyl alcohol) also supports the contribution of oxidising species different from the hydroxyl radical to substrate removal. Further evidence of the presence of reactions different from the hydroxyl radical oxidation was observed from comparison of the simultaneous Fenton's or UV/H₂O₂ oxidations of p-hydroxybenzoic acid, tyrosol and p-coumaric acid. (C) 2000 Elsevier Science Ltd. Fenton's reagent has been shown to be a feasible technique to treat phenolic-type compounds present in a variety of food processing industry wastewaters. A model compound, p-hydroxybenzoic acid was oxidized by continuously pumping two solutions of ferrous iron and hydrogen peroxide. Typical operating variables like reagent feeding concentrations and flowrate, temperature and pH were studied. A mechanism of reactions based on the classical Fenton's chemistry was assumed, and computed concentration profiles of the parent compound, ferrous ion and dihydroxybenzene were compared to experimental results. The model qualitatively predicted the influence of several operating variables, however, calculated results suggested the presence of parallel routes of substrate elimination and/or a initiating rate constant with a higher value. The low efficiency of a well-known hydroxyl radical scavenger (tert-butyl alcohol) also supports the contribution of oxidizing species different from the hydroxyl radical to substrate removal. Further evidence of the presence of reactions different from the hydroxyl radical oxidation was observed from comparison of the simultaneous Fenton's or UV/H₂O₂ oxidations of p-hydroxybenzoic acid, tyrosol and p-coumaric acid.

Full text of DOI:10.1016/S0043-1354(00)00285-2

Wat. Res. Vol. 35, No. 2, pp. 387–396, 2001
2000 Elsevier Science Ltd. All rights reserved
Printed in Great Britain
#
PII: S0043-1354(00)00285-2
0043-1354/00/$ - see front matter
OXIDATION OF P-HYDROXYBENZOIC ACID BY FENTON’S
REAGENT
1
FRANCISCO J. RIVAS *, FERNANDO J. BELTRAN , JESUS FRADES and PACO
1M
2
´
´
1
BUXEDA
1
Departamento de Ingenieria Quimica y Energe
´
tica, Universidad de Extremadura, Avenida de Elvas S/N,
6071 Badajoz, Spain and Departamento de Ingenieria Quimica Universidad de Castilla la Mancha,
Plaza de Manuel Meca s/n 13400 Almaden, Ciudad Real, Spain
2
0
´
(
First received 15 November 1999; accepted in revised form 17 May 2000)
Abstract}Fenton’s reagent has been shown to be a feasible technique to treat phenolic-type compounds
present in a variety of food processing industry wastewaters. A model compound, p-hydroxybenzoic
acid was oxidised by continuously pumping two solutions of ferrous iron and hydrogen peroxide.
Typical operating variables like reagent feeding concentrations and flowrate, temperature and pH were
studied.
A mechanism of reactions based on the classical Fenton’s chemistry was assumed, and computed
concentration profiles of the parent compound, ferrous ion and dihydroxybenzene were compared to
experimental results. The model qualitatively predicted the influence of several operating variables,
however, calculated results suggested the presence of parallel routes of substrate elimination and/or a
initiating rate constant with a higher value. The low efficiency of a well-known hydroxyl radical scavenger
(tert-butyl alcohol) also supports the contribution of oxidising species different from the hydroxyl radical
to substrate removal.
Further evidence of the presence of reactions different from the hydroxyl radical oxidation was observed
2 2
from comparison of the simultaneous Fenton’s or UV/H O oxidations of p-hydroxybenzoic acid, tyrosol
and p-coumaric acid. # 2000 Elsevier Science Ltd. All rights reserved
Key words}Fenton, hydroxybenzoic acid, phenolic compounds, wastewater, oxidation
NOMENCLATURE
mation, distilleries, etc.). Thus, substances like gallic
acid, tannic acid, tyrosol, hydroxybenzoic acid, etc.,
can be found at concentrations from 400 to 900 ppm
of gallic acid (wine distilleries) to values as high as
2500–6000 ppm of tannic acid (table olive elabora-
´
tion) (Lopez and Ovelleiro, 1978; Kopsidas, 1992).
Given the bactericide nature of these chemical
structures when found at high concentrations the
classic biological treatment cannot be applied and
alternative wastewater treatments have to be taken
into consideration. Chemical oxidation technologies
based on the presence of UV radiation or/and ozone,
sonolysis, etc. have been successfully used in con-
taminated water remediation of a broad spectrum of
substances. However, as a rule of thumb, these
systems involve prohibitive costs associated to
process scaling-up, specially when dealing with highly
Ar
aromatic ring
C
C
i
i
concentration of species i
concentration of reagent i in the feeding solution
dihydroxybenzene
dihydroxycyclohexadienyl radical
semiquinone radical
rate constant
chelator of iron
phenol
p-hydroxybenzoic acid
flowrate of reagent i feeding solution
*
HQ
HQ
HQ
k
L
Ph
ꢀ
0
ꢀ
pHB
Q
i
t-BuOH tert-butyl alcohol
reaction volume
V
INTRODUCTION
Phenolic-type compounds are widely occurring spe- polluted wastewaters. The application of Fenton’s
cies in wastewater generated in food-processing reagent (hydrogen peroxide in combination with a
industries (i.e. olive oil production, tomato transfor- ferrous salt) as a possible way for wastewater
purification was not practised until the late 1960s.
This oxidation system has been used to treat both
Tel.: +34-924-289385; fax: +34-924-271304; e-mail: individual organic and inorganic substances under
fjrivas@unex.es laboratory conditions and also real effluents from
*
Author to whom all correspondence should be addressed.
387

3
88
Francisco J. Rivas et al.
different sources like chemical manufacturers, refin- influence of typical operating variables, the kinetic of
ery and fuel terminals, engine and metal cleaning, etc. the process and intermediates formed during the
(Bidga, 1996). The process is based on the formation reaction.
of reactive oxidising species able to efficiently degrade
the organic content of the wastewater stream.
Nevertheless, the nature of these species is still under
discussion and its formulation is a subject of
controversy in the past and recent Fenton’s-related
literature (Bossmann et al., 1998; Pignatello et al.,
METHODS
Experiments were carried out in the dark, batchwise by
continuously feeding two solutions of hydrogen peroxide
and ferrous chloride to a solution of p-hydroxybenzoic acid
1
et al., 1996).
999; Walling, 1998; MacFaul et al., 1998; Sawyer (pHB) in the presence of oxygen. The pumping time was in
�
most of the cases 30 min at a flowrate of 0.31 mL min , so
1
the reaction volume (1 L) could be considered constant
throughout the length of the experiment. The aqueous
solution was homogenised by magnetic agitation to avoid
concentration gradients. Mass transfer tests were carried out
In the past decades many authors have widely
investigated the kinetics and reactions involved in
Fenton’s chemistry. In spite of the large number of
works published on this subject, the influence of by varying the speed of the magnetic stirrer. Results
indicated a perfect mix of the solution under the conditions
crucial parameters (such as pH), role of added
used. Systematically, samples were withdrawn and analysed
substrates and nature of the oxidising species present
in the system, are still unclear (Walling, 1998;
to follow different parameters like pHB, intermediates and
reagent concentrations (iron and total peroxides). Prelimin-
MacFaul et al., 1998; Sawyer et al., 1996; Yamazaki ary experiments on pHB Fenton degradation were carried
and Piette, 1991).
out by quenching the samples using both sodium thiosul-
phate and/or tert-butyl alcohol. Results were compared to
runs completed at the same conditions without the use of a
reducing agent or radical scavenger. Since no differences
were obtained, the rest of the experiments were conducted
In the literature, three main species have been
contemplated in Fenton’s chemistry. Two of them
involve the presence of hydroxyl radicals (classic
Fenton’s chemistry) either ‘‘free’’ or ‘‘caged’’ radicals without the addition of these substances
All chemicals (p-hydroxybenzoic acid, tyrosol, coumaric
acid, catechol, hydroquinone, maleic and oxalic acids, tert-
butyl alcohol, hydrogen peroxide and ferrous chloride) were
purchased from Aldrich and used without further purifica-
(
Walling and Amarnath, 1982) and, hence, all the
reactions associated to this species. The third oxidant
has been postulated to be aquo or organocomplexes
of high-valence iron, the ferryl ion (Sawyer et al., tion.
1
996).
Proofs based on product distribution, EPR-spin
Analysis of the parent compound and main intermediates
formed was carried out by HPLC in isocratic mode
immediately after sampling. The system was equipped with
a 1046 Hewlett-Packard UV/VIS detector set at 210 nm. A
Waters Nova-Pack C18 column was used utilising a 20 : 80
trapping techniques, effect of added substrates, etc.,
have been used to demonstrate the existence of one or
another oxidising intermediate. However, as stated (v/v) methanol/water mixture (0.01 M in phosphoric acid) as
�
1
mobile phase with a flowrate of 1 mL min . One mL of
sample was injected by means of an automatic 1010
Hewlett-Packard injector. Analysis error was calculated to
be in the range Æ 5%. Total peroxides concentration was
by Yamazaki and Piette (1991), it is quite likely that
both hydroxy radicals and ferryl complexes coexist in
Fenton’s mechanism and, depending on operating
conditions (substrate nature, metal–peroxide ratio, determined iodometrically. Ferrous ion in the solution was
scavengers addition, etc.) one of them will pre- analysed by the 2,4,6, tripyridyl-s-triazine (TPTZ) method
(
3
Collins et al., 1959). Thus, 1 mL sample was buffered with
mL of a solution containing 2 M acetic acid–2 M sodium
dominate.
In any case, it has been demonstrated that
Fenton’s and Fenton’s-like processes are suitable
� 3
acetate, afterwards, 1 mL of TPTZ reagent (10 M) was
added and the final volume brought to 10 mL with ultrapure
methods to destroy toxic phenolic wastes and non- water. Absorbance of the solution was measured at 593 nm
�
1
extinction coefficient 22,600 M cm ).
� 1
(
biodegradable effluents to render them more suitable
for a biological secondary treatment (Chen and
Pignatello, 1997).
The pH of the reaction media was followed by means of a
Radiometer Copenhagen pH-meter (HPM82). This para-
meter remained practically unchanged throughout the
6 4
In this work, p-hydroxybenzoic acid (C H OH- experiments showing variations of less than 13% in the
COOH), a phenolic model compound found in worst of the cases. Therefore, experiments were conducted
in the absence of any buffer solution.
Intermediate identification was carried out as follows: 1 L
industrial synthetic processes (Nagata et al., 1996)
wastewater from food processing factories and
of the treated aqueous solution (90% conversion of the
has been oxidised in aqueous solutions by means of
parent compound) was first acidified with hydrochloric acid
Experiments were (pH52) and extracted two times with 100 mL of methylene
chloride. The organic phase was then dried with anhydrous
2
+
3+
2 2
the system Fe /Fe /H O .
completed in the dark at relatively high concentra-
tion of the parent compound (0.01 M) to simulate
more realistically the amounts of phenolic com-
sodium sulphate and concentrated in a rotary evaporator
to less than 2 mL. In each analysis 1 mL of sample was
injected into a Hewlett-Packard HP 5890 gas chromato-
pounds present in the aforementioned type of graph coupled to a 5972 Mass Spectrometer operated
effluents. The potential synergistic or inhibiting effect in Scan mode. A 30 m–0.25 mm–0.25 mm film thickness
HP-5MS cross-linked 5% phenyl methyl silicone column
was used. The oven temperature programme was set
at 323 K for 2 min ramped up to 583 K (8 K min ) and
kept for 15 min at this temperature. The carrier gas was
of other phenolic-type compounds like tyrosol
(4-hydroxyphenetyl alcohol) and p-coumaric acid
(4-hydroxycinnamic acid) has also been tested. An
� 1
attempt has been made to gain an insight into the helium.

Oxidation of p-hydroxybenzoic acid
389
RESULTS AND DISCUSSION
From these results, it can be inferred that under the
conditions used in this work, the limiting reactant
seems to be in most of the cases hydrogen peroxide,
Influence of operating conditions
2
+
Two series of experiments were completed by since Fe
varying the concentration of both ferrous cation process, and thereby, playing a catalytic role in the
0–0.01 M) and hydrogen peroxide (0–4 M) feeding mechanism. This catalytic effect was analytically
is likely regenerated throughout the
(
solutions to ascertain the effect of these parameters confirmed since, on the one hand, ferrous ion
on pHB conversion. As expected, an increase in concentration found in the bulk of the reaction at
concentration of any Fenton’s reagent constituents any time was practically coincident to the theoretical
led to higher conversions in pHB removal (see Figs 1 amount pumped into the reactor and, on the other-
and 2). However, an optimum value (around hand, hydrogen peroxide was immediately consumed
�
3
2+
5
Â10 M) for Fe
concentration was noticed, after being injected, experiencing none or negligible
experiencing no appreciable enhancement of the regeneration. This could also be corroborated since
process rate with a further increase in the ferrous no further conversion in pHB was observed when
ion amount fed to the pHB solution (see Fig. 1).
2 2
the H O feeding pump was stopped, in spite of
2
+
the presence of significant amounts of Fe
solution.
in
Some experiments were also conducted to deter-
mine the influence of different temperatures and pH
on the effectiveness of the process. Thus, an increase
of the starting pH from 3 to 7 made the process to
come to a halt. These results are in agreement with
the majority of studies reported on Fenton’s chem-
istry. The usual optimum working pH when using
this system is in the range 3–5. However, neutral pH
has also been found to be the most appropriate to
oxidise some chemical species like polynuclear
aromatic hydrocarbons, nitrobenzene, amines, etc.,
(
Beltra
Broadly speaking, it is assumed that the poor
conversions achieved at neutral pH are related to
´
n et al., 1998).
2
+
Fig. 1. Degradation of p-hydroxybenzoic acid in aqueous reduction in stability of both Fe
2 2
and H O when
solution by Fenton’s reagent. Evolution of the normalised
remaining concentration of pHB with time. Influence of
increasing pH from acidic conditions. Moreover, at
neutral pH ferrous iron is readily oxidised by
atmospheric oxygen and ferric iron precipitates
Ã
ferrous iron concentration in the feeding solution (C 2þ ).
Fe
Conditions:
Ã
T ¼ 293 K,
pH=3.2,
CpHBo=0.01 M,
2+
C
=2.65 M, pumping time=30 min, flowrate= preventing the regeneration of Fe . However, other
H2O2
.31 mL min . Dotted lines: model calculations assuming
�
1
0
factors may affect the influence of pH, specially the
presence of organic and/or inorganic species able to
stabilise or promote by complexation the auto-
oxidation of ferrous ion (Yamazaki and Piette, 1991).
The range of temperatures from 284 to 314 K was
also investigated, although no influence of this
parameter was noticed under the operating condi-
tions used in this work. Few studies in the literature
have dealt with the effect of temperature in Fenton’s
systems, although values of activation energy for the
classic initial step to generate the hydroxyl radical
�
1
� 1
s .
ki2=76 M
(
probably the limiting step) have been reported to be
just a few kilocalories (Hardwick, 1956). Also, Lin
and Lo (1997) showed low activation energies for the
chemical oxygen demand (COD) depletion when
treating a desizing wastewater. In this case, however,
they experienced an optimum value of temperature
around 303 K.
Fig. 2. Degradation of p-hydroxybenzoic acid in aqueous
solution by Fenton’s reagent. Evolution of the normalised
Some intermediates formed in the process were
remaining concentration of pHB with time. Influence of identified by HPLC and GC/MS. Among them
hydrogen peroxide concentration in the feeding solution
Ã
maleic and oxalic acids, catechol and hydroquinone
(
C
). Conditions: T=293 K, pH=3.2, CpHBo=0.01 M,
H2O2
Ã
were analysed throughout the experiments using
high-performance liquid chromatography. Corre-
sponding peaks were identified by comparison of
C
0
2þ =0.01 M,
pumping
time=30 min,
flowrate=
.31 mL min . Dotted lines: model calculations assuming
Fe
�
1
�
1
� 1
s .
ki2=76 M

3
90
Francisco J. Rivas et al.
retention time of standards prepared in the labora-
tory.
Although the primary step in the mechanism
corresponds to the classic initiating reaction (genera-
Generation of dihydroxybenzenes (catechol and tion of hydroxyl radicals) proposed by Haber and
hydroquinone mainly) is supposed to occur through Weiss (1934), formation of ferryl species should not
decarboxylation of the parent compound (p-hydro- be discarded as discussed later in this section. The
xybenzoic acid) or/and decarboxylation of the values of the rate constants have been taken from
hydroxylated species of pHB (dihydroxybenzoic Chen and Pignatello (1997) unless the opposite is
�
1
� 1 � 1
acid). This mechanism has already been proposed stated. (Units are M
s
and s for second-order
for the case of similar structures like phenoxyacetic reaction rates and first-order reaction rates, respec-
acid (Pignatello, 1992) or phenylacetic acid (Barbeni tively.)
et al., 1987). The oxidising species involved in the
decarboxylation process is a matter of controversy
and will be treated in the kinetic section of this study.
In Fig. 3 the evolution with time of by-product
concentrations analysed by HPLC is presented. From
this figure it is seen that a maximum in catechol and
hydroquinone concentrations was noticed when
roughly a 70% pHB removal was achieved. Maleic
and oxalic acids accumulated in the reaction media
due to their refractory nature to Fenton’s reagent
The following set of reactions has been considered:
Fe þþH2O2 ! Fe3þþOH þHO
2
�
ꢀ
ki2 ¼ 76 ð1Þ
Fe þþH2O2! Fe2þþH þHO ki3 ¼ 0:02 ð2Þ
3
þ
ꢀ
2
ꢀ
7
H2O2þHO
Fe2þþHO
ꢀ
! HO þH2O kp1¼ 2:7 Â 10 ð3Þ
2
! Fe3þþOH� kp2¼ 4:3 Â 10 ð4Þ
8
ꢀ
Fe þHO ! Fe2þþO2þHþ kr1 ¼ 10
3þ
ꢀ
4
ð5Þ
2
(
Bidga, 1996) or the generation of more reactive
intermediates that consume the oxidising species
formed in the system.
Fe þHO þH ! Fe3þþH2O2 k01¼ 1:2 Â 10
2þ
ꢀ
þ
6
2
ð6Þ
ð7Þ
ð8Þ
ð9Þ
Mechanism of pHB acid elimination by Fenton’s
reagent
Fe þ O ! Fe2þþO2 kr2¼ 1:5 Â 10
3þ
ꢀ
�
8
2
Fe þ O 2H ! Fe3þþH2O2 k02 ¼ 10
2
þ
ꢀ
� þ
þ
7
In this work a simplistic mechanism based on the
work of Chen and Pignatello (1997) on the iron-
catalysed oxidation of phenol by hydrogen peroxide
has been assumed; nevertheless, to avoid mathema-
tical complexity some reactions have been discarded.
Among them radical–radical recombination has been
neglected given the low probability of these reactions
to occur. Also, the reaction of free hydroxyl radicals
with phenol has been assumed to proceed by aryl
addition generating the corresponding dihydroxy-
benzene, hence, parallel routes leading to different
products were not considered. Furthermore, given
the low working pH in this study (pH54.8) reactions
of the peroxyl radical have also been ruled out.
2
ꢀ
ꢀ
�
þ
5
HO2! O2 þH
kd¼ 8 Â 10
ꢀ
�
þ
ꢀ
10
O2 þH ! HO2 kdr¼ 5 Â 10
ð10Þ
þ H2O þ CO2
OH2Ar2COOH þ HO ! OH2Ar
ꢀ
ꢀ
1
0
kh¼ 1:14 Â 10
ð11Þ
þ Fe3þ ! OH2Ar þ Fe
2þ
OH2Ar
ꢀ
4
kr3¼ 4:4 Â 10
ð12Þ
9
OH2Ar þ HO
ꢀ
! ðOHÞ � Ar
ꢀ
kPh¼ 7:3 Â 10
2
ð13Þ
3
þ
2þ
ðOHÞ � Ar
ꢀ
þ Fe ! ðOHÞ 2Ar þ Fe
2
2
4
kr4¼ 4:4 Â 10
ð14Þ
ð15Þ
3þ
2þ
ðOHÞ � Ar þ Fe ! OH2Ar2O
ꢀ
þ Fe
2
2
kr5¼ 4:4 Â 10
3þ
2þ
OH2Ar2O
ꢀ
þ Fe ! benzoquinone þ Fe
4
kr6¼ 4:4 Â 10
ð16Þ
ð17Þ
OH2Ar2OH þ HO
ꢀ
! ðOHÞ 2Ar þ products
3
Fig. 3. Degradation of p-hydroxybenzoic acid in aqueous
solution by Fenton’s reagent. Evolution of intermediate
9
kHQ¼ 1:0 Â 10
concentration. Conditions: T=293 K, pH=3.2, CpHBo
=
Ã
Ã
0
.01 M,
C
=2.0 M,
0 min, flowrate=0.31 mL min
C
2þ =0.01 M, pumping time=
H2O2
Fe
�
1
3
.
where Ar stands for the aromatic ring.

Oxidation of p-hydroxybenzoic acid
391
The value of kh was determined in experiments of adienyl radical and HQ semiquinone radical in
0ꢀ
UV/H
where (Beltra
2
O
2
according to the method described else- equation (16).
n et al., 1995). Accordingly, given the If the steady-state hypothesis for radical species is
´
similarities of reactions (12), (14) and (16) the values assumed, the set of first-order differential equations
of kr3 and kr4 were assumed to be in the same order of (18)–(23) may be solved numerically by means of the
magnitude as kr6. The value of kHQ was taken from fourth-order Runge–Kutta method.
Farhataziz and Ross (1977).
From the mechanism of reactions (1)–(17) the
corresponding mass balances of molecular species in
a perfectly mixed reactor yield the following system
of differential equations:
Application of the mechanism: kinetic considerations
Figures 1 and 2 depict the theoretical (dotted lines)
and experimental (symbols+solid lines) profiles of
the remaining pHB acid dimensionless concentration
with time for the series of experiments carried out at
different concentrations of the reagents fed to the
reactor. Qualitatively, the applied mechanism is able
to predict the influence of these operating parameters
(i.e. concentration of feeding solutions); however, the
model overestimates the experimental results, spe-
cially for the first 15–20 min of reaction.
*
Ferrous iron:
Ã
dCFe2þ
QFe2þ
C
2þ
Fe
¼
� ðki2CH O þ kO1CHO2ꢀ
2
2
dt
V
Á
þkO2CO� ꢀ CFe2þ þ ðki3CH O
2
2
2
þkr1CHO
ꢀ
þ kr2CO� ꢀ
2
2
þkr3CPhꢀ þ kr4CHQꢀ
þkr5CHQ þ kr6CHQꢀ CFe3þ
Á
At the sight of the S-shaped theoretical profile
concentrations, it is reasonable to assume that either
the value of the rate constant for the initiating step
ð18Þ
(ki2) possesses a higher value or the mechanism
*
Ferric iron:
involves a parallel reaction which generates an
additional oxidant different from the hydroxyl
radical (for instance a high-valence iron complex).
A sensitivity analysis of the rate constants used in the
mechanism demonstrated that the value of ki2 is the
most influencing parameter in the shape of calculated
profiles.
The low value of ki2 was already questioned by
Bossmann et al. (1998) when compared to the rate
constant of the ligand exchange between aquocom-
dCFe3þ
¼
ðki2CH O þ kO1CHO2ꢀ
:
2
2
dt
þkO2CO� ꢀÞCFe2þ þðki3CH O
2
2
2
þkr1CHO2ꢀ þ kr2CO� ꢀ
2
þkr3CPhꢀ þ kr4CHQꢀ þ kr5CHQ
þkr6CHQ0ꢀÞCFe3þ
ð19Þ
2
+
plexes of Fe
(
and hydrogen peroxide (equation
*
Hydrogen peroxide:
24)):
h
Ã
dCH O
QH2O2
C
i2þ
2
2
H2O2
¼
k ¼ 2Â106 M� 1 s� 1
FeðLÞ ðH OÞ
þH2O2
dt
V
x
2
y
�
Á
�
ki2CFe2þ þ ki3CFe3þ CH O
þ kO1CHO2ꢀ þ kO2CO� ꢀ CFe2þ
h
i2þ
2
2
�
Á
 FeðH O2ÞðLÞ ðH OÞ
þH2O
ð24Þ
2
x
2
y� 1
ð20Þ
ð21Þ
2
2
+
where L is any inorganic or organic ligand of Fe
Subsequently, the complex containing the H O as
.
2
2
*
*
Phenol:
a chelator undergoes an inner-sphere two-electron
transfer reaction to form the ferryl ion:
dCPh
dt
¼
kr3CPhꢀCFe3þ � kPhCPhCOH
h
i
2þ
ki1
FeðH O2ÞðLÞ ðH OÞ
!
2
x
2
y� 1
Hydroquinone:
h
i
2
þ
 FeðOHÞ ðLÞ ðH OÞ
ð25Þ
2
x
2
y� 1
dCHQ
¼
kr4CHQꢀCFe3þ
2
4
� 1
dt
where ki1 presents a value in the range 10 –10 s
depending on the nature of chelators of the ferrous
ion (Yamazaki and Piette, 1991; Bossmann et al.,
�
p-Hydroxybenzoic acid:
Á
�
kr5CFe3þ þ kHQCHOꢀ CHQ
ð22Þ
ð23Þ
4
+
1
998). The intermediate Fe
25) may generate free hydroxyl radicals through the
complex in equation
*
(
following reaction:
dCpHB
¼
� khCHOꢀCpHB
dt
h
i2þ
þHO
FeðOHÞ ðLÞ ðH OÞ
þH2O
2
x
2
y� 1
i3þ
where Qi stands for the flowrate of the reagent i
Ã
h
feeding solution with C concentration, Ph is phenol,
HQ dihydroxybenzene, HQꢀ the dihydroxycyclohex-
i
þHO�
!
FeðLÞ ðH OÞ
ꢀ
ð26Þ
x
2
y

3
92
Francisco J. Rivas et al.
In contrast to the hypothesis of the ferryl ion
formation, other authors claim that the variation in
the value of ki2 to generate the hydroxyl radical is due
to the presence of organic ligands. Thus, according to
Sedlak and Hoigne
4
´
(1993), k would have a value of
i2
� 1 � 1
3
reaction media. Other substances able to increase the
.1 Æ 0.6Â10 M
s when oxalate ions are in the
value of k would be gallic acid, catechol, dihydroxy-
i2
benzoic acid, etc. (Sun and Pignatello, 1993). Given
that the structure of these compounds is able to
increase the value of k , it is likely that the parent
i2
compound (pHB) itself acts as a chelator of ferrous
iron accelerating the rate of the initial step (see
equation (27)). In this case a complex similar to that
Fig. 4. Degradation of p-hydroxybenzoic acid in aqueous
proposed by Walling and Amarnath (1982) using solution by Fenton’s reagent. Evolution of the normalised
remaining concentration of pHB with time. Influence of
hydrogen peroxide concentration in the feeding solution
mandelic acid may be formed:
Ã
(
C
0
C
). Conditions: T=293 K, pH=3.2, CpHBo=0.01 M,
H2O2
Ã
2þ =0.01 M,
pumping
time=30 min,
flowrate=
.31 mL min . Dotted lines: model calculations assuming
Fe
�
1
3
� 1 � 1
s .
k
i2=10 M
ð27Þ
The stoichiometric coefficient for the complex
2
+
would likely be Fe(pHB)
2
in the same manner as
found for a similar structure like picolinic acid
(
Sawyer et al., 1996).
The mechanism of reactions was tested by varying
the value of k from 76 to an average value between
i2
2
4
� 1 � 1
1
0
and 10 M
assumption of a higher value of ki2 or taking into
s . In this case, either the
4
+
consideration the formation of Fe (ki1) as an
additional oxidant (reactions (24) and (25)), would
likely lead to similar results, since it has been
reported previously that both the hydroxyl radical Fig. 5. Degradation of p-hydroxybenzoic acid in aqueous
solution by Fenton’s reagent. Evolution of the normalised
remaining concentration of pHB with time. General
stoichiometry (Yamazaki et al., 1998; Walling, 1998)
and the ferryl ion present a similar reactivity and
conditions:
Ã
T=293 K,
Ã
pH=3.2,
CpHBo=0.01 M,
=2.65 M, pumping time=30 min,
although they differ in selectivity.
C
2þ =0.01 M,
C
Fe
H2O2
�
1
Figure 4 shows the theoretical (dotted lines) and flowrate=0.31 mL min . Dotted lines: model calculations
3
� 1 � 1
s . Specific conditions:
assuming ki2=10 M
pumping time=10 min, flowrate=0.92 mL min
m ,
. ,
experimental (symbols) concentration profiles of
pHB depletion with time for the experiments
presented in Fig. 2. It can be observed that the
model is significantly improved if compared to results
�
1
;
Ã
CpHBo=0.0033 M; * , C
time=60 min, flowrate=0.15 mL min
¼2.65 M;
& , pumping
H2O2
� 1
.
�
1
� 1
obtained when utilising a value of ki2 of 76 M
It is also noticeable that the higher the value of the
hydrogen peroxide concentration fed, the higher the ybenzene and peroxides concentration with time for
effect of the ki2 value on theoretical profiles. both experimental and model calculations. It should
s
.
2
+
Figure 6 shows the evolution of Fe , dihydrox-
To validate the kinetic model Fig. 5 also presents be pointed out that calculated results in the figure
the computed values obtained when varying several representing peroxide concentration just correspond
operating variables other than hydrogen peroxide to hydrogen peroxide. From Fig. 6 it can be inferred
concentration fed to the reactor. Thus, as observed that the value of ki2 does not significantly affect the
2
+
from this figure the kinetic model does a good job concentration profiles of Fe , dihydroxybenzene
when changing the flowrate of reagents injected, the and phenol (results not shown in the latter case).
initial concentration of the parent compound and the However, the theoretical accumulation of hydrogen
concentration of ferrous iron fed. Moreover, radical peroxide in the bulk of reaction is reduced when ki2 is
3
concentrations simulated by the mechanism are in the increased to a value of 10 (see Fig. 6B). Discrepan-
�
13
order of magnitude expected, for instance ꢁ 10
� 7
–10
M
M
cies between experimental peroxide concentration
and theoretical evolution of H concentration
suggest the generation of organic peroxides
�
11
for free hydroxyl radicals and ꢁ 10
depending on nature) for organic radicals.
2 2
O
(

Oxidation of p-hydroxybenzoic acid
393
Fig. 6. Degradation of p-hydroxybenzoic acid in aqueous solution by Fenton’s reagent. Evolution of
dihydroxybenzene (HQ), hydrogen peroxide and total peroxides (ROOH) and ferrous iron concentration
Ã
� 1 � 1
s .
with time. Conditions: as in Fig. 1 (C 2þ =0.01 M). Dotted lines: model calculations. (A) ki2=76 M
Fe
3
� 1 � 1
s .
(B) ki2=10 M
according to the following mechanism (MacFaul 2-furancarboxaldehyde, also identified by GC/MS,
et al., 1998):
were likely generated through more complicated
mechanisms involving organic radical–radical
termination, internal rearrangement, etc. Table 1
shows the intermediates detected and identified by
GC/MS.
>
CH
ꢀ
þO2!> CHOO
ꢀ
ð28Þ
ð29Þ
>
CHOO
ꢀ
þ > CH2! CHOOHþ > CH
ꢀ
It is noteworthy that reaction (29) constitutes a In this work hydroxylated species from pHB
different route of substrate elimination and should (dihydroxy and trihydroxybenzoic acids) were de-
not be ruled out in certain cases as for example tected only at trace levels. Contrarily, Nagata et al.
cyclohexene elimination (MacFaul et al., 1998).
(1996) found the hydroxylated species of hydroxy-
Dihydroxybenzoic acid, phenol and polyphenols benzoic acid as the principal intermediates detected
like catechol, hydroquinone, resorcinol and 1,2,3 when a sonochemical treatment was applied. These
benzenetriol were detected by GC/MS and their results may be explained if: (a) under the conditions
presence was attributed to simple decarboxylation/ used in this work the main oxidising species is not
ꢀ
hydroxylation reactions of either pHB or intermedi- HO or (b) dihydroxybenzoic acid undergoes dec-
ates formed from the former. Other species like arboxylation so rapidly that its concentration does
2
,2-dimethyl 2,3-dihydro 7-benzofuranol or 5-methyl not reach the levels of other intermediates.

3
94
Francisco J. Rivas et al.
Table 1. Intermediates identified in the Fenton’s oxidation of
phenolic-type compounds by GC/MS
a
Identified compound
Phenol
Substrate
1
1
1
1
1
1
1
1
1
1
1
2
3
2
2
2
2
1
1
-Methyl-4-(1-methylethyliden) cyclohexene
,2 Benzenediol
Hydroquinone
Catechol
1,2,3 Benzenetriol
Dihydroxybenzoic acid
3
2
5
,4 Dihydroxybenzaldehyde
,3, Dihydro 2,2, dimethyl 7-benzofuranol
-Methyl 2-furancarboxaldehyde
Resorcinol
2
3
2
4
2
4
-Furancarboxylic acid
,4 Furandicarboxylic acid
,3, Dihydrobenzofurane
-Hydroxybenzaldehyde
,4-bis (1,1 dimethylethyl) phenol
Methylphenol
Fig. 7. Degradation of p-hydroxybenzoic acid in aqueous
solution by Fenton’s reagent. Evolution of the normalised
remaining concentration of pHB with time. Influence of
added tert-butyl alcohol (t-BuOH). Conditions: as in Fig. 1
a
Ã
1
}p-hydroxybenzoic acid;
(C 2þ =0.01 M). Dotted 3 lines: model calculations
Fe
� 1
� 1
).
2
3
}p-hydroxybenzoic acid+tyrosol+p-coumaric acid;
}p-coumaric acid.
(ki2=10 M
s
Although the decarboxylation process does not and Amarnath, 1982). Once more, these contra-
preclude the involvement of free hydroxyl radicals, dictory statements seem to corroborate the hypoth-
Siegel and Lanphear (1979) concluded that benzoyl- esis of the existence of different oxidising
formic acid (with a structure similar to pHB) species, namely HO
decarboxylated to benzoic acid through oxidation prevails depending on external operating conditions
with a highly reactive iron species. and specially the nature of the substrate to be
ꢀ, Fe IV, ROOꢀ, etc.; one of those
To cast light on the nature of the oxidising species oxidised. The shape of the curves obtained for the
involved, some experiments were conducted in the experiments completed in the presence of t-BuOH
presence of an apparent hydroxyl radical scavenger, indicates that at the first stages of the process an
tert-butyl alcohol (t-BuOH). Two values of t-BuOH oxidising species that is not scavenged by t-BuOH is
concentration were examined, 0.01 and 0.05 M. An being formed, although the primary step of the
induction period at the beginning of the process process is likely the attack of the hydroxyl radical to
was experienced (see Fig. 7), increasing the length pHB.
of this period as the t-BuOH amount in the media
was increased. Again theoretical calculations pathways of organic substrate removal, a series of
just by considering HO as the unique reactive experiments was carried out by utilising several
species) overestimated the experimental results, substrates of polyphenolic nature similar to that of
Finally, to confirm the existence of additional
(
ꢀ
3
� 1 � 1
even when utilising a value of ki2 of 10 M
From these results some considerations can be order kinetic constants of pHB, tyrosol and p-
s .
p-hydroxybenzoic acid. As a first step, the second-
postulated. On the one hand, some authors coumaric acid in their reaction with the hydroxyl
(
Rahhal and Richter, 1988) have claimed that the radical were calculated utilising the system UV/H O2
2
inefficiency of t-BuOH to prevent substrate oxidation in excess of H
in Fenton’s systems is attributable to the absence of 11.4, 6.7 and 5.7Â10 M
HO . However, in this study some inhibition was the aforementioned rate constants corresponding to
2 2
O (Beltran et al., 1995). Values of
´
9
� 1 � 1
s were determined for
ꢀ
noticed, indicating that t-BuOH and the generated pHB, tyrosol and p-coumaric acid–hydroxyl radical
oxidising species reacted to some extent. On the reactions, respectively.
other hand, other works published in the literature
(
Figure 8 shows the evolution of the three
Yamazaki and Piette, 1991) have pointed out that compounds studied when treated separately by
the Fenton reaction is scavenged by t-BuOH no Fenton’s reagent (solid symbols). As can be observed
matter the nature of the oxidising agent. In addition, from this figure, the reactivity follows the trend
it could be suggested that radicals formed from pHB>Tyrosol ꢁ p-coumaric acid. Therefore, accord-
3
+
2+
the oxidation of t-BuOH could reduce Fe to Fe
ing to the rate constant values, the higher the
promoting the system. However, since primary reactivity with the hydroxyl radical the faster
radicals are not oxidized by ferric iron, Fenton’s Fenton’s reaction. However, if pHB acid, tyrosol and
reagent oxidation of t-BuOH gives a dimer as p-coumaric acid are oxidised simultaneously (same
essentially the sole product, and therefore regenera- initial concentration) different results are obtained.
tion of ferrous iron is not possible (Walling From Fig. 8 (open symbols) a shift in reactivity can

Oxidation of p-hydroxybenzoic acid
395
pounds from aqueous solutions. p-Hydroxybenzoic
acid removal has been investigated by using the
iron II catalysed decomposition of hydrogen
peroxide. The study of typical operating variables
of this system resulted in the following state-
ments:
2
+
*
Fe seems to play a catalytic role in the process,
being regenerated throughout the experiments. As
a consequence, hydrogen peroxide is the limiting
2
+
reactant and an optimum value of the Fe /H
ratio may, thereby, be found.
2 2
O
*
Activation energy of the process must be low,
since an increase in reaction temperature from 284
to 314 K did not result in appreciable changes in
the pHB acid conversion rate.
Fig. 8. Degradation of p-hydroxybenzoic acid, tyrosol and
p-coumaric acid in aqueous solution by Fenton’s reagent.
Evolution of the normalised remaining concentration with
time. Conditions: T=293 K, pH=3.2,
Ã
C
=2.65 M, pumping time=30 min,
0
=0.0033 M,
*
*
Ã
A remarkable inhibition of the reaction was
noticed when pH was increased from acidic
(pH=3.2) to neutral conditions (pH=7).
Among the intermediates identified, the presence
of phenol, catechol, hydroquinone and trihydroxy-
benzene suggests a decarboxylation step in the
mechanism.
C
flowrate=0.31 mL min . Experiments carried out sepa-
2þ =0.01 M,
C
Fe
H2O2
�
1
rately (solid symbols) and simultaneously (open symbols).
be observed as follows: p-coumaric acid>pHB>
tyrosol.
A plausible explanation to these ‘‘anomalous’’
results is, as stated previously, the presence of
oxidising agents other than the hydroxyl radical that
present a different selectivity towards the model
compounds studied. Similarly, Pignatello and co-
workers (Pignatello et al., 1999) gave evidence of an
additional pathway of cyclohexane loss when com-
paring experiments carried out by the photo-
Fenton system and other reactions that only
Although the chemistry of Fenton’s systems
involves a rather complex mechanism, an attempt
has been made to fit experimental results to a
simplistic kinetic model. In the model the most
important reactions of the classic Fenton’s chemistry
generation of hydroxyl radicals) have been consid-
ered. From the application of the kinetic model some
inferences were deduced:
(
2 2
produce hydroxyl radicals, for instance UV/H O .
*
Although the model rightly predicts the quantita-
The change in the kinetic deuterium isotope
effect (KDIE) that they experienced, was attributed
to high-valent oxoiron complexes. However,
the classic Fenton’s reaction in the dark showed
tive effect of the operating parameters checked, it
overestimates the experimental results, specially at
the beginning of the process. The S-shaped
theoretical concentration profiles obtained suggest
a higher value of the reaction rate that generates
the oxidising species involved in pHB removal. An
increase in one order of magnitude in this kinetic
constant resulted in the improvement of model
predictions.
no differences when compared to the UV/H
system.
2 2
O
A simultaneous oxidation experiment of pHB,
tyrosol and p-coumaric acid using the system UV/
2 2
H O (hydrogen peroxide in excess to discard the
photolysis contribution) showed a similar order of
reactivity to that found for the simultaneous Fen-
ton’s treatment (not shown). These results confirm
the existence of reactive species formed from the
original substrates (organic radicals) which would
react with a different selectivity depending on the
nature of the substance to be oxidised. This
synergising effect has already been observed in
previous works when using similar radical generating
systems like wet air oxidation (Kolaczkowski et al.,
*
The anomalous low efficiency of tert-butyl alcohol
(
hydroxyl radical scavenger) to prevent pHB acid
oxidation prompts the thought that other oxidis-
ing species are implicated in the system.
*
Oxidation of pHB, tyrosol and p-coumaric acid
conducted by using the Fenton’s reagent and the
2 2
UV/H O system gives a different order of
reactivity whether experiments are carried out
with individual compounds or simultaneously.
This result indicates the formation of organic
radicals involved in the mechanism.
1
997).
CONCLUSIONS
Acknowledgements}This work has been supported by the
junta de Extremadura and Fondo Social Europeo (Project
Fenton’s reagent seems to be an appropriate
method to efficiently remove phenolic-type com- IPR98EC001).

3
96
Francisco J. Rivas et al.
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