Complete destruction of p-Nitrophenol in aqueous medium by electro-fenton method

Article abstract of DOI:10.1021/es990901b

An indirect electrochemical method, which is very efficient for the degradation of organic pollutants in water, is described. The method, named electro-Fenton, is based on electrocatalytical generation of Fenton's reagent to produce hydroxyl radicals, which are very active toward organic compounds. An industrial pollutant, p-nitrophenol (PNP), was chosen for this study and was eventually mineralized. The major intermediary degradation products such as hydroquinone, benzoquinone, 4-nitrocatechol, 1,2,4-trihydroxybenzene and 3,4,5-trihydroxynitrobenzene were unequivocally identified by HPLC and GC-MS methods. The rate constants of the hydroxylation reactions were determined. The mineralization of the initial pollutant and the intermediates formed during electro-Fenton treatment was followed by total organic carbon (TOC) analyses. Dependence of mineralization on the amount of electrical energy consumed is shown by the relative decrease ofTOC values. A mineralization reaction mechanism is proposed. An indirect electrochemical method, which is very efficient for the degradation of organic pollutants in water, is described. The method, named electro-Fenton, is based on electrocatalytical generation of Fenton's reagent to produce hydroxyl radicals, which are very active toward organic compounds. An industrial pollutant, p-nitrophenol (PNP), was chosen for this study and was eventually mineralized. The major intermediary degradation products such as hydroquinone, benzoquinone, 4-nitrocatechol, 1,2,4-trihydroxybenzene and 3,4,5-trihydroxynitrobenzene were unequivocally identified by HPLC and GC-MS methods. The rate constants of the hydroxylation reactions were determined. The mineralization of the initial pollutant and the intermediates formed during electro-Fenton treatment was followed by total organic carbon (TOC) analyses. Dependence of mineralization on the amount of electrical energy consumed is shown by the relative decrease of TOC values. A mineralization reaction mechanism is proposed. The destruction of p-nitrophenol by the electro-Fenton method was examined. The method is based on the oxidation of the substrate by hydroxyl radicals generated in situ in an electrochemically induced Fenton reaction. The progress of all reactions and product distribution were monitored by high-performance liquid chromatography and gas chromatography-mass spectrometry. Results showed that p-nitrophenol disappeared, as did the degradation products after reaching a maximum concentration. The rapid degradation of p-nitrophenol was accompanied by the appearance of aromatic intermediates, such as p-nitrocatechol and hydroquinone. The subsequent decrease in p-nitrocatechol was accompanied by an increase in 1,2,4-trihydroxybenzene, while the decrease in hydroquinone was accompanied by the production of benzoquinone. The mineralization mechanism is described. The variation in TOC values taken at different times during treatment showed their dependence on the charge passed or electrical energy consumed.

Full text of DOI:10.1021/es990901b

Environ. Sci. Technol. 2000, 34, 3474-3479
has been investigated (13, 14). Efficiency of the electro-
Complete Destruction of
chemical techniques in water treatment was also demon-
strated in heterogeneous electro-oxidation (15, 16) and
indirect electro-reduction (17-19) studies. Since electro-
chemical techniques are more environmentally friendly
than the chemical methods used in polluted water treatment,
there is a need of improvement of such techniques.
Nitrophenols are among the most common organics of
toxic persistent pollutants in industrial and agricultural
wastewater (20-22). They are considered to be hazardous
wastes and priority toxic pollutants by U. S. Environmental
Protection Agency (EPA) (23a). They are present in industrial
effluents of chemical plants producing pesticides, explosives,
dyestuffs, and products for leather treatment and in the
agricultural irrigation effluents. Moreover, they were also
found in rainwater, in Germany, in concentrations up to
170 nM which are formed in the troposphere by the inter-
action of monoaromatic chemicals with nitrogen oxides and
ozone (23b).
p-Nitrophenol in Aqueous Medium
by Electro-Fenton Method
M E H M E T A . O T U R A N , * , †
J O S E P E I R O T E N , ‡
P A S C A L C H A R T R I N , ‡ A N D
A U R E L J . A C H E R §
Laboratoire d'Electrochimie Mole´culaire,
UMR-CNRS no. 7591, Universite´ Paris7-Denis Diderot,
75251 Paris Cedex 05, France, Laboratoire des Ge´omate´riaux,
Universite´ de Marne la Valle´e, Cite´ Descartes,
77454 Marne la Valle´e Cedex 2, France, and
The Volcani Center, Institute of Soils and Water,
P.O. Box 6, Bet Dagan, 50250 Israel
Purification of wastewater polluted with p-nitrophenol
(PNP) is a very difficult task. The presence of a nitro group
in the aromatic ring enhances the stability to chemical and
biological degradation. These pollutants are unaffected by
aerobic biodegradation (24), while the anaerobic degradation
produces nitroso and hydroxylamines compounds which are
known as carcinogenic (25). The kinetics of p-nitrophenol
(PNP) ozonolysis depends on the pH of the medium and
on partial pressure of ozone (26). The direct photolysis is
very slow and inefficient (27), while the photolysis using
UV+H2O2 (28), UV+H2O2+Fe3+ (photo-Fenton) (29, 30) or
UV+H2O2+TiO2 in suspension as catalyst (photo catalysis)
(31, 32) brings satisfactory results, but they necessitate
addition of chemicals.
The destruction of PNP in aqueous medium by electro-
Fenton method, presented in this study, is a scientific effort
to develop an environmentally friendly and efficient method
for water treatment. This method is based on the oxidation
of the substrate by hydroxyl radicals generated in situ in an
electrochemically induced Fenton reaction. The Fenton’s
reagent (mixture of H2O2 + Fe2+) to produce hydroxyl radicals
is formed in a catalytic manner by electrochemistry.
An indirect electrochemical method, which is very
efficient for the degradation of organic pollutants in
water, is described. The method, named electro-Fenton,
is based on electrocatalytical generation of Fenton’s
reagent to produce hydroxyl radicals, which are very
active toward organic compounds. An industrial pollutant,
p-nitrophenol (PNP), was chosen for this study and was
eventually mineralized. The major intermediary degradation
products such as hydroquinone, benzoquinone, 4-nitro-
catechol, 1,2,4-trihydroxybenzene and 3,4,5-trihydroxy-
nitrobenzene were unequivocally identified by HPLC and
GC-MS methods. The rate constants of the hydroxylation
reactions were determined. The mineralization of the initial
pollutant and the intermediates formed during electro-
Fenton treatment was followed by total organic carbon
(TOC) analyses. Dependence of mineralization on the amount
of electrical energy consumed is shown by the relative
decrease of TOC values. A mineralization reaction mechanism
is proposed.
Experimental Section
Introduction
Chem icals and Standard Materials. p-Nitrophenol (Sigma,
reagent grade, CAS: 100-02-7) and 4-nitrocatechol (Acros,
reagent grade, CAS: 3316-09-4) were used without further
purification. Hydroquinone (CAS: 123-31-9), benzoquinone
(CAS: 106-51-4), 1,2,4-trihydroxybenzene (CAS: 533-73-3),
4-nitropyrogallol (3,4,5 trihydroxynitrobenzene, CAS: 87-
66-1), iron(III) chloride hexahydrate (CAS: 10025-77-1) and
salicylic acid (CAS: 69-72-7) were purchased in the highest
purity available from Aldrich. All these chemicals were used
without any further purification. Deionized water used for
the preparation of solutions and HPLC eluents was obtained
from a Milllipore Milli RO6 system, with a conductivity lower
than 4 × 10-6 S.cm-1. Methanol (chromanorm grade, CAS:
67-56-1), acetic acid (chromanorm grade, CAS: 64-19-7), and
ethyl acetate (pestinorm grade, CAS: 141-78-6) were supplied
by Prolabo. The pressurized oxygen gas (99.99%) to saturate
the solutions was obtained from Carboxyque.
Pollution of water and the environment by toxic and
nonbiodegradable organic materials of industrial or agri-
cultural origin, like pesticides, brings about very serious health
hazards for all living species of the nature. The enormous
diversity of pollutants of different chemical composition
excludes the possibility of using an universal treatment
method and led to the development of special treatment
methods for water decontamination. Among the meth-
ods for water treatment are ozonation (1-3), UV-photo-
degradation using UV/ H2O2, UV/ TiO2, and UV/ H2O2/ Fe3+
systems (4-6) and sensitized sunlight (7, 8). These so-called
advanced oxidation processes (AOPs) (9, 10) have attracted
an increasing scientific and technique attention during
the last two decades. Their applicability has been experi-
mentally verified by the oxidation of some model pollutants
(11, 12). In recent years another wastewater treatment
method, using electrochemically generated hydroxyl radicals,
Electrochemistry. A potentiostat-galvanostat EG&G, model
273A, was used for electrolyses and for measurements of
consumed electrical charge. Electrolyses were carried out in
a divided three-electrode electrochemical cell (Figure 1). The
working electrode (WE) was a 15 cm2 carbon felt (Carbon-
Lorraine), and the counter electrode (CE) was a platinum
* Corresponding author phone: 33-(0)1 44275665; fax: 33-(0)1
44277625; e-mail: oturan@paris7.jussieu.fr.
† Laboratoire d'Electrochimie Mole´culaire, UMR-CNRS no. 7591.
‡ Universite´ de Marne la Valle´e.
§
The Volcani Center, Institute of Soils and Water.
9
3 4 7 4 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 34, NO. 16, 2000
10.1021/es990901b CCC: $19.00
2000 Am erican Chem ical Society
Published on Web 07/13/2000

have passed through the electrolysis cell, while for the
experiments aimed to destroy the PNP, the electrolyses were
carried up to 1500 C. The reaction mixtures were sampled
continuously. Volume of the samples injected to HPLC was
20 µL.
Standard curves have been prepared for quantitative
determination of PNP and intermediary products formed
during electro-Fenton process. Standard solutions containing
authentic expected products were used for qualitative
identification of the intermediates in the electrolysis mixture.
Identification was achieved by comparison of retention times
and also by using them as internal reference, directly injected
into samples taken from the electrolysis.
Preparation of samples for qualitative identification of
the reaction products present in the electrolyzed solutions
was carried out as follows: first the reaction time was chosen,
so that the HPLC chromatogram will show an optimum
presence of the relevant peak(s) that should be identified;
then the reaction mixture was extracted 3 times with ethyl
acetate. The extract was dried over anhydrous sodium sulfate,
filtered, and concentrated in a rotavapor to 2 mL, and 1-2
µL was injected in the capillary column. The GC-MS
temperature program was as follows: the temperature was
held at 50 °C for 5 min and then increased at a rate of
5 °C/ min up to a temperature of 250 °C which was maintained
for 15 min.
FIGURE 1. Schematic setup of the electrochemical cell used for
electrolysis. R: reference electrode; WE: working electrode; CE:
counter electrode.
grid (Goodfellow), placed in the anodic compartment
containing a 0.01 M aqueous HCl solution. The anodic
compartment was separated from the cathodic compartment
by a glass frit of porosity 5. The reference electrode (RE) was
a saturated calomel electrode (SCE), from Radiometer. The
RE and WE were placed in the cathodic compartment, which
has a volume of 150 cm3. The electrolyzer is also equipped
with a magnetic stirrer.
High Perform ance Liquid Chrom atography. Progress of
all reactions and product distribution was monitored by high
performance liquid chromatography (HPLC). Analyses was
achieved by a Gilson HPLC system equipped with a model
118 UV/ Vis detector and a reverse phase hypersil C-18 column
(250 mm × 4,6 mm i. d., 5 µm particle size). The column was
eluted with a mixture water-methanol-acetic acid 65:33:2
v/ v with a flow rate of 0.5 mL/ min. The detection was
performed by UV absorption at 280 nm.
Gas Chrom atography-Mass Spectrom etry. The decay
of PNP and evolution of its hydroxylated derivatives were
also monitored by gas chromatography-mass spectrometry
(GC-MS). Analyses were performed on a Carlo Erba Instru-
ment (QMD 1000). The GC column used was a Quadrex fused-
silica capillary column (30 m × 0.53 mm ID × 0.25 µm film)
with a 80% methyl-20% phenyl polysiloxane stationary phase.
The temperature of the injector was kept at 250 °C. Electron
impact (EI) mode at 70 eV was used.
Results and Discussions
Hydroxyl Radical Production. At the cathodic potential of
-0.50 V/ SCE applied to carbon felt WE, reduction of the
molecular oxygen, dissolved in the aqueous solution, takes
place. In the acidified medium, this reduction leads to the
formation of hydrogen peroxide, H2O2, (34):
O2 + 2H+ + 2e- f H2O2 E° ) 0.69 V/ NHE
(1)
Hydrogen peroxide formed electrochemically at the WE
surface diffuses into solution and reacts with ferrous ions
present in solution, according to Fenton’s reaction:
H2O2 + Fe2+ + H+ f Fe3+ + H2O + OH.
(2)
Total Organic Carbon (TOC) Analysis. TOC analysis of
initial and electro-Fenton treated aqueous PNP solutions
were carried out by Analytik Jena Laboratory using an IDC
Micro N/ C TOC analyzer equipped with an automatic sample
injector and a platinum based catalyst. The carrier gas was
pure oxygen at a rate of 12 dm3/ h. The detector was an
infrared NDIR. The samples were acidified to pH < 4.0 before
to be sent to Jena Analytic Laboratory GmbH for analysis.
Procedure. All the solutions were prepared with water at
mM concentrations. The solutions prepared for electrolysis
were adjusted to pH 2.0 by addition of concentrated HCl, a
Ferric ions produced above are also reducible on the WE at
the same applied potential to reduce molecular oxygen:
Fe3+ + e- h Fe2+ E° ) 0.77 V/ NHE
(3)
Ferrous ions consumed by Fenton’s reaction in homogeneous
medium are regenerated in this way electrochemically on
the cathode (eq 3). On the other hand, molecular oxygen
necessary to form H2O2 (eq 1) which participated in the
hydroxyl radical formation (eq 2) is also produced at anodic
compartment by simply oxidation of water:
catalytic amount of ferric chloride (to reach 0.5 mM Fe3+
)
was added, and then they were saturated with oxygen by
bubbling oxygen for 5 min before starting the electrolysis.
The working electrode potential was adjusted at -0.5 V/ ECS;
the electric current (mA) and the amount of electrical charge
(C) were continuously checked and registered during the
electrolysis.
The kinetic study was done by carrying out a competition
reaction between the substrate and salicylic acid as internal
standard, for which the absolute rate constant of reaction
with hydroxyl radicals is well-known (33). Hydroxyl radicals
were formed in situ by electrochemistry in the presence of
substrate (1.0 mM) and salicylic acid (1.0 mM). Concentration
change of substrate and reference (salicylic acid) during the
electrolysis was continuously followed by HPLC analysis. Also
the relative rate constants of hydroxylation of substrates have
determined by comparison with those of salicylic acid. For
the kinetic study, the process was stopped after 5 coulombs
H2O h 1/ 2O2 + 2H+ + 2e- E° ) 1.23 V/ NHE (4)
The sum of the above cathodic and anodic processes (by
elimination of the electrons) gives the overall equation
1/ 2O2 + H2O f 2OH.
(5)
which clearly shows the catalytic character of the electro-
Fenton method (19).
Analyses of Degradation Products. The HPLC chro-
matograms (Figure 2) of the samples, taken from the
electrochemical reactor during the electro-Fenton process,
shows the disappearance of PNP and formation of its
degradation products peaks, which also disappear after
reaching a maximum concentration. By comparison of the
retention times (tR, in min) of the peaks in the above
9
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TABLE 1. HPLC Identification of PNP Degradation Products
Formed by the Reaction with Electrochemically Generated OH‚
Radicals
FIGURE 3. Concentration change of PNP (C ) 4.0 mM, V ) 0.125
0
dm3) and some of its degradation products during the electro-Fenton
treatment as function of charge (higher scale) and electrical energy
consumed (lower scale). 4-NC: p-nitrocatechol, HQ: hydroquinone,
BQ: benzoquinone.
p-nitrocatechol and hydroquinone. Concentrations of these
intermediates reach to a maximum steady state value at
around 500 C, for an aqueous solution containing initially
4 mM PNP, and then decrease until disappearance. The
decrease of p-nitrocatechol is accompanied by the increase
of 1,2,4-trihydroxybenzene. The evolution of benzoquinone
follows that of hydroquinone. At 1200 C the PNP and its
degradation intermediates have completely disappeared.
Determ ination of Rate Constants. Since the solution was
continuously saturated with molecular oxygen, the produc-
tion rate, therefore the concentration, of hydrogen peroxide
is stable. Ferrous ion concentration was also steady due to
its cathodic regeneration during the electro-Fenton process.
The electrolysis current was remained constant accordingly
these suggestions. Hydroxyl radicals are very short-lived
species. Their concentration may be considered in a quasi-
stationary state. In these conditions the hydroxylation of
PNP is a pseudo first-order reaction. So, the rate constant
of hydroxylation reaction
FIGURE 2. Evolution of HPLC chromatograms during the electro-
Fenton treatment of a PNP solution (C ) 4 mM) in water, as a
0
function of electrical charge Q (coulombs). (a) 0 C, (b) 400 C, (c)
800 C and (d) 1500 C.
chromatograms, with retention times of authentic samples
expected to be the PNP degradation products (which were
also used as internal standards), it was possible to identify
five major decomposition products II-VI. Retention times
and chemical structure of these compounds are given in
Table 1.
Their identification was confirmed by GC-MS. Results are
given in Table 2. For unambiguous identification of the
reaction products, their GC-MS spectra were compared with
spectra of standard compounds performed with the same
apparatus in identical experimental conditions.
Change of concentrations of PNP and some of its
hydroxylated derivatives is shown in the Figure 3 as a function
of charge (C) and of electrical energy (kW.h/ dm3). Figures 2
and 3 show that the rapid degradation of PNP is accompanied
by the appearance of aromatic intermediates such as
PNP + OH. f products
(6)
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TABLE 2. GC-MS Data for the Reaction Mixture after Passing 400 C during the Electro-Fenton Treatment of the PNP Solution with
Initial Concentration C ) 4 mM
0
retention time
(min)
molecular weight
(m/z)
mass of predominant ions in fragmentation pattern
product identification
3.84
12.85
14.89
16.55
18.08
108
110
126
139
155
108 (M+), 80, 54, 37
benzoquinone
hydroquinone
1,2,3-trihydroxybenzene
p-nitrophenol
p-nitrocatechol
110 (M+), 81, 63, 53, 39
126 (M+), 108, 97, 80, 69, 63, 52, 38
139 (M+), 123, 109, 93, 81, 65, 53, 39
155 (M+), 137, 125, 109, 81, 53, 36
TABLE 3. Rate Constants for Hydroxylation Reactions of PNP
and Its Hydroxylated Derivatives by OH‚ Radicals
rate constant (L.mol-1.s-1
)
•
substrate (S)
p-nitrophenol
p-nitrocatechol
hydroquinone
for S + OH f products
(1.2 ( 0.2) × 1010
(1.0 ( 0.2) × 1010
(1.6 ( 0.5) × 1010
(8.6 ( 0.3) × 109
1,2,4-trihydroxybenzene
might be comparatively determined taking the salicylic acid
(SA) reaction as standard. In fact, salicylic acid hydroxyl-
ation was chosen because its hydroxylation rate constant
(2.2 × 1010 M-1 s-1) is well-established (33).
SA + OH. f products
(7)
FIGURE 4. Decrease of TOC value in the reaction mixture during
the electro-Fenton treatment of a PNP aqueous solution (with initial
Assuming that there are no side reactions of PNP and our
reference compound (SA), other than their reaction with
hydroxyl radicals, the rates of these processes are given by
concentration C ) 1.0 mM, V ) 0.125 dm3) as function of charge
0
(C) and energy (kW.h/dm3).
mineralization, although their biodegradation should be in
principle possible. The decrease in mineralization rate may
be influenced by the competition with
-d[PNP]/ dt ) kPNP[OH‚][PNP]
-d[SA]/ dt ) kSA[OH‚][SA]
OH• + Fe2+ f Fe3+ + OH-
which can be integrated and combined to give the following
equation
when the concentration of organic matter decrease. In fact,
this reaction has a highly rate constant; k ) 4.3 × 108 M-1
s-1 (38).
ln([PNP]0/ [PNP]t) ) (kPNP/ kSA) × ln([SA]0/ [SA]t) (8)
Mineralization Mechanism . The reaction mechanism of
hydroxyl radicals with the aromatic compounds is well-
known (34-36). It consists of an electrophilic addition to the
aromatic ring with the formation of a hydroxylated radical,
dihydroxycyclohexadienyl (DHCHD) radical in case of PNP:
where the subscripts 0 and t indicate concentration at
the beginning of the experiment and at time t, respectively.
A plot of ln([PNP]0/ [PNP]t) vs ln([SA]0/ [SA]t) thus yields
the rate constant ratio, therefore kPNP. Equation 8 was also
used to determine the rate constants of p-nitrocatechol,
hydroquinone and 1,2,4-trihydroxybenzene with OH‚ radical
(Table 3).
(9)
Total Organic Carbon (TOC) Measurem ent. The variation
of TOC values found in the samples taken at different times
of electro-Fenton process shows their dependence on the
charge (C) passed or electrical energy consumed (kW.h/ dm3).
Figure 4 illustrates these results during the electro-Fenton
treatment of a 1.0 mM PNP solution. It also proves that the
disappearance of peaks on HPLC chromatograms not only
due to the formation of aliphatics, which cannot be detected
by the UV detector of HPLC, but this is a result of the
mineralization process which takes place during the electro-
Fenton treatment. Mineralization rate is higher at the
beginning of the electrolysis, but diminishes with the decrease
of concentration of compounds in the reaction mixture and
with change in their structure, from aromatics to aliphatics
by ring opening reactions, which are more resistant to
mineralization. Under above conditions, the mineralization
process reached a ratio of 95% at 800 C passed, which is
equivalent to 3 × 10-3 kW.h/ dm3. The continuation of the
process beyond this point resulted in a nonsignificant
reduction of the TOC value. At this stage, organics present
in solution are probably carboxylic acids, which resist to
This radical may undergo different reactions according
to the nature of reaction medium. In an anaerobic medium
it may dimerize and/ or disproportionate (37, 38) leading to
formation of a biphenyl compound and a phenol one,
respectively. In the oxidative medium this radical will undergo
hydroxylation reactions (34, 39, 40). Analysis of the inter-
mediates and monitoring of the TOC values during the
electro-Fenton process helped to understand the possible
pathways of reaction of DHCHD. in the presence of O2 and
ferric ions. The mesomeric electron-donor character of
hydroxyl group on the benzene ring favors the electrophilic
attack of hydroxyl radicals on ortho- and para-positions.
Electrophilic attack of hydroxyl radical on ortho-position of
PNP leads to the formation of p-nitrocatechol (Scheme 1,
reaction 10). Formation of hydroquinone (reaction 12) might
be explained by an ipso attack of the hydroxyl radical (at
para-position of OH group) with a simultaneous leave of the
nitro group. The continuous decrease of TOC value since the
9
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SCHEME 1. Evolution of DHCHD Radical in Oxidative Medium
Leading to the Formation of Compounds II and III
SCHEME 3. Hydroxylation Reaction Pathways for p-Nitro-
catechol and Hydroquinone
SCHEME 2. Proposed Mineralization Pathway for PNP
Following the Electrophilic Addition of OH‚ Radical
SCHEME 4. Proposed Mineralization Pathway for Compounds
V and VI
beginning of electro-Fenton process might be explained by
partial mineralization of this radical (reaction 11).
The mineralization pathway includes the opening of the
aromatic ring followed by successive oxidations of the
aliphatic products up to CO2 and H2O (Scheme 2), by hydroxyl
radicals in the presence of molecular oxygen (6, 17, 19, 40).
-
The nitrite ion NO2 liberated by reaction 12 and by
destabilized due to the strong electrophilic effect of hydroxyl
and nitro groups. Consequently these bonds easily undergo
oxidative ring opening reactions leading to aliphatic com-
pounds. Scheme 4 shows a plausible mineralization pathway
of compounds V and VI. It can also be seen that the compound
VI has two C-C bonds predisposed to oxidative rupture while
the compound V has only one. This leads to the rate of
mineralization of VI (Scheme 4, reaction 16) to be faster than
those of V (reaction 17) and explains the fact that the presence
of VI in the reaction mixture is in a lower concentration than
V. The products of the ring opening reaction might be
alcohols, aldehydes or ketones (from quinones) which will
be oxidized to carboxylic acids (Scheme 2 and reaction 18)
and then mineralized to CO2 and H2O (reaction 19).
To identify the major carboxylic acid formed from
oxidative cleavage of hydroxylated aromatic compounds, the
experiments with compounds V and VI were performed
separately. Ion chromatographic analyses revealed the maleic
acid as the major byproduct of these reactions. The presence
of fumaric and oxalic acids in a very small concentration has
also been identified. Maleic acid concentration reaches a
maximum value of 0.2 mM at 200 C, in the conditions of
Figure 3, and then slowly decreases. These observations are
in agreement with previous studies (42-45) in which the
maleic acid has been identified as major oxidation product
by advanced oxidation processes with different organic
substrates. Concerning mineralization of carboxylic acids,
Stephan et al. (46) recently proposed a mineralization reaction
pathways induced by hydroxyl radicals.
mineralization process (Scheme 2) is subsequently oxidized
into nitrate ion NO3-. Ion chromatography (Dionex 100)
analyses show that 85-90% of nitrogen initially containing
in PNP was recovered as nitrate ion at the end of process.
Benzoquinone (compound IV) appears in the reaction
mixture when hydroquinone reaches to a concentration of
0.2-0.3 mM (Figures 2 and 3). It is well-known that hydro-
quinone is very easily oxidized into benzoquinone (41) thus
it is probably formed by reaction of hydroxyl radicals (eq 13)
which are also powerful oxidizing agents (E0
)
•
+
OH ,H / H
O
2
2.72 V/ NHE):
(13)
The values of the kinetic constants (Table 3) show that
the reactivity of the compounds II and III versus hydroxyl
radicals is comparable to that of PNP. However, their
hydroxylation starts to compete with of PNP when their
concentrations reach to a sufficient value. Scheme 3 shows
the possible reactions of these compounds with OH. radicals.
Pelizzetti et al. (32) have already studied the photocatalytic
degradation of PNP on irradiated TiO2. Their degradative
process yields hydroquinone and 4-aminophenol as well as
other dihydroxynitrobenzene derivatives. Our experimental
conditions are oxidative; therefore, we could not detect
4-aminophenol which should have resulted from a reductive
(6e- + 6H+) transformation of nitro group (Ar-NO2) into the
corresponding amine (Ar-NH2) (41).
Formation of compound VI (3,4,5-trihydroxynitrobenzene
or 4-nitropyrogallol) by hydroxylation of PNP during the
photo-Fenton process is already described (30). A peak of tR
) 15.9 min found on HPLC chromatogram was attributed to
compound VI by comparison with an authentic standard.
To confirm reaction 14, an electrolysis of compound II was
performed separately; compound VI (identified by com-
parison with authentic products) appears as the major formed
product. This is not mentioned in Table 2, because its
identification leaves some problems remaining: first it is
not accumulated with a significant concentration due to its
rapid degradation (cf. Figure 2), and second its extraction
from water is difficult because of its rather high polarity.
It is to be mentioned that, in the compound V and VI, the
C-C bonds between the adjacent hydroxyl groups are
Finally the overall mineralization reaction of PNP could
be summarized as follows:
O2N-(C6H4)-OH + 28OH. electrical energy8 6CO2 +
-
16H2O + H+ + NO3
Acknowledgments
The authors gratefully acknowledge helpful discussions
with Professor T. Ozgen. They are grateful to Dr. O.
Kampe (ANALYTIK Jena GmbH), B. Yurdakul and L. Joubert
(PERICHROM sarl) for performing the TOC analysis.
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