Mineralisation of Monuron photoinduced by Fe(III) in aqueous solution

Article abstract of DOI:10.1016/j.chemosphere.2004.08.058

The degradation of Monuron (3-(4-chlorophenyl)-1,1-dimethylurea) photoinduced by Fe(III) in aqueous solution has been investigated. The rate of degradation depends on the concentration of Fe(OH)²⁺, the most photoreactive species in terms of

Full text of DOI:10.1016/j.chemosphere.2004.08.058

Chemosphere 57 (2004) 1307–1315
www.elsevier.com/locate/chemosphere
Mineralisation of Monuron photoinduced by Fe(III)
in aqueous solution
a,b,
a,
a
ˇˇ ´
´
Hana MestÕankova
*, Gilles Mailhot *, Jean-Franc¸ois Pilichowski ,
b
c
Josef Kry´sa , Jaromır Jirkovsky´ , Michele Bolte
a
´
`
a
´ ´ ´ `
Laboratoire de Photochimie Moleculaire et Macromoleculaire, Universite Blaise Pascal, F-63177 Aubiere Cedex, France
b
´
Department of Inorganic Technology, Institute of Chemical Technology, Technicka 5, 166 26 Prague 6, Czech Republic
c
´ ˇ
J.Heyrovsky Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolejskova 3,
128 23 Prague 8, Czech Republic
Received 16 February 2004; received in revised form 1 July 2004; accepted 20 August 2004
Abstract
The degradation of Monuron (3-(4-chlorophenyl)-1,1-dimethylurea) photoinduced by Fe(III) in aqueous solution
has been investigated. The rate of degradation depends on the concentration of Fe(OH)2+, the most photoreactive spe-
cies in terms of ÅOH radical formation. These ÅOH radicals are able to degrade Monuron until total mineralisation. The
primordial role of the speciation of Fe(III)-hydroxy complex in aqueous solution, for the efficiency of the elimination of
pollutant, was shown and explained in detail. The formation of Fe(II) in the irradiated solution was monitored and
correlated with the total organic carbon evolution. Degradation photoproducts were identified and a mechanism of
degradation is proposed.
ꢀ 2004 Elsevier Ltd. All rights reserved.
Keywords: Monuron; Fe(III)-hydroxy complex; Phototransformation; Mineralisation
1. Introduction
persed in the different environmental compartments of
the environment but mainly in aquatic environment
via agricultural runoff or leaching. Thus, pesticides are
among the most frequently occurring organic pollutants
in natural waters. Great care has been developed about
possible processes that pesticides might undergo when
they are dispersed in the environment. Among the differ-
ent abiotic degradation processes, solar irradiation is
one of the main factors responsible for pollutant degra-
dation in aquatic environment (Faust, 1999). When the
pollutant does not absorb solar light, its degradation
can be photoinduced by radicals generated by different
species, like Fe(III) aquocomplexes, presented or added
in the aquatic compartment.
The intensive use of pesticides on large areas of agri-
cultural soil has given rise to concern about their fate in
the environment. Less than 1% of total applied pesti-
cides reach the target pests, the vast majority being dis-
*
Corresponding author. Address: Laboratoire de Photochi-
´
`
63177 Aubiere Cedex, France. Tel.: +33 4 73 40 71 73; fax: +33
´ ´
mie Moleculaire et Macromoleculaire, Universite Blaise Pascal,
4 73 40 77 00.
E-mail address: gilles.mailhot@univ-bpclermont.fr (G.
Mailhot).
0045-6535/$ - see front matter ꢀ 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.chemosphere.2004.08.058

ˇ ˇ ´ ´
H. Mest’ankova et al. / Chemosphere 57 (2004) 1307–1315
1308
Among pesticides, phenylurea derivatives are widely
complete mineralisation of bio-recalcitrant pollutants
(Sarria et al., 2003).
used as herbicides because of their inhibition of photo-
synthesis (Bucha and Todd, 1951), for general weed con-
trol of non-crop area and as pre-emergence on fruit
crops. These herbicides have received a special attention
in recent years because of their toxicity and possible car-
cinogenic properties. Several weeks, even months, are
required for their removal from environment (White-
Stevens, 1971; Mackay et al., 1997). This work is focused
on the particular case of Monuron (3-(4-chlorophenyl)-
1,1-dimethylurea). The life times of Monuron in the dif-
ferent compartments of environment were found to be
relatively long; greater than 8weeks in river waters
(Eichelberger and Lichtenberg, 1971) and the half-life
is equal to 170days in the soil (Augustin-Beckers
et al., 1994).
The present paper deals with Monuron degradation
photoinduced by Fe(III) aquacomplexes until its com-
plete mineralisation. The major photoproducts have
been identified and a mechanism of Monuron degrada-
tion is proposed. A special emphasis is given to the role
of the different Fe(III) species.
2. Experimental
2.1. Reagents and solutions
All reagents were of the purest grade commercially
available and were used without further purification.
Monuron (3-(4-chlorophenyl)-1-1-dimethylurea) (99%)
and 8-hydroxyquinoline-5-sulfonic acid monohydrate
(HQSA; 98%) were purchased from Aldrich and used as
received. Ferric perchlorate nonahydrate (Fe(ClO4)3,
9H2O; >97%) was a Fluka product kept in a dessicator.
Solutions of Fe(III) used for the studies (3 · 10ꢀ4mollꢀ1),
were prepared by diluting stock solutions of Fe(ClO4)3,
9H2O (2.0 · 10ꢀ3mollꢀ1) to the appropriate Fe(III) con-
centration. 2-Propanol and acetonitrile were HPLC grade
products and purchased from Merck and Carlo Erba
respectively. For the synthesis of authentic samples of
photoproducts, 4-chlorophenyl isocyanate (98%), methyl-
amine (2M in THF solution) and N-methylformamide
(99%) were Aldrich products.
Several authors have investigated the degradation of
Monuron. Two main ways of biodegradation are de-
picted in the literature for the N-aryl-N0,N0-dimethyl-
urea derivatives. The first one is the hydrolysis of
amide (Kearney, 1965) whereas the second is the
N-demethylation (Tillmanns et al., 1978). Then, the pho-
totransformation of Monuron has been the subject of
several papers. Direct photolysis of Monuron has been
studied in water under solar and UV irradiations (Hill
et al., 1955; Weldom and Timmons, 1961). Crosby and
Tang (1969) identified different photoproducts and pro-
posed a mechanism of phototransformation. The photo-
catalytic degradation of Monuron with TiO2 was also
investigated in detail (Augugliaro et al., 1993; Pramauro
et al., 1993).
All solutions were prepared with deionized ultra pure
water (q = 18.2MXcm). When necessary, the solutions
were deoxygenated at room temperature either by argon
bubbling for 30min in 5ml quartz cell or by continuous
nitrogen bubbling in the reactor used for irradiation
(100ml). We checked that there was no elimination of
Monuron during the bubbling. The pH was measured
with an Orion pH-meter to 0.1 pH unit (pH = 3.3 for
a solution with a concentration in Fe(III) equal to
3.0 · 10ꢀ4 M). The ionic strength was not controlled.
Fe(III)-aquo complexes are known to undergo a pho-
toredox process through an internal electron transfer
yielding Fe(II) and hydroxyl radical (reaction (1)):
hm
Å
FeðIIIÞ ! FeðIIÞ þ HO þ Hþ
ð1Þ
H2O
Among the Fe(III)-aquo complexes, Fe(OH)2+ is the
most photoreactive species in terms of hydroxyl radical
formation (Benkelberg and Warneck, 1995). The per-
centage of Fe(OH)2+ (Fe(OH)2+ refers to FeðOHÞ
ðH2OÞ2þ) in iron(III) aqueous solution is influenced by
2.2. Apparatus
5
many factors. The hydrolysis of iron(III) is a complex
phenomena and many equilibria lead to numerous spe-
In order to measure the quantum yields, monochro-
matic irradiations at 296, 313, 334 and 365nm were
carried out with a high-pressure mercury lamp (Osram
HBO 200W) equipped with a monochromator (Bausch
and Lomb) providing a parallel beam. The reactor was
a cylindrical quartz cell of 2cm path length. The light
intensity was measured by ferrioxalate actinometry
(Calvert and Pitts, 1966) (Table 1). The quantum
yields were calculated for a reaction extent lower than
10%.
´
cies (Flynn, 1984; Faust and Hoigne, 1990). However,
the hydrolysis mainly depends on pH, initial concentra-
tion of FeIII, temperature, ionic strength and nature of
anions (Sylva, 1972).
Such systems, based on Fe(III) aquacomplexes as a
Å
source of OH radicals were proved to be efficient to in-
duce the complete mineralisation of pollutants (Mailhot
et al., 2002). In addition, due to its redox properties, iron
can interfere with the different intermediates generated
all along the process. In terms of wastewater treatment,
such a process could be very useful in pre-treatment be-
fore a biological treatment with the goal of achieving the
In order to irradiate larger volume (V = 60ml) at
exc. = 365nm, for kinetic and analytical experiments, an
k
elliptical stainless steel reactor was used. A high-pressure

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H. MestÕankova et al. / Chemosphere 57 (2004) 1307–1315
1309
Table 1
Photonic flux and initial quantum yields of Monuron disap-
pearance as a function of irradiation wavelength
Total Organic Carbon (TOC) measurements, based
on the combustion of carbon detected by infrared gas
analysis method, were followed with a TOC analyser
Shimadzu (model TOC-5050A). The calibration curves
within the range 1–20mglꢀ1 were obtained by using
potassium hydrogen phthalate for total organic carbon
and a mixture of sodium hydrogen carbonate and
sodium carbonate for inorganic carbon. The threshold
k (nm)
296
313
334
365
I0
5.6 · 1014 1.00 · 1015 6.4 · 1014 2.12 · 1015
(photonssꢀ1
cmꢀ2
UMonuron
)
0.121
0.109
0.106
0.025
of detection is around 1.5mglꢀ1
.
[Fe(III)]0 = 3.0 · 10ꢀ4 mollꢀ1 (90% of monomeric species) and
[Monuron]0 = 4.0 · 10ꢀ4 mollꢀ1
.
3. Results and discussion
mercury lamp enclosed in a glass filter bulb (Mazda
HPW type 125W), whose emission consists of 93% at
365nm (few percents of the light is emitted at 313, 334
and 405nm), was placed in one of the focal axes, while
the photoreactor, a water-jacketed Pyrex tube (internal
diameter = 2.8cm), was centred at the other one. The
reaction medium was continuously stirred.
3.1. Dark experiments
In our experimental conditions ([Fe(III)] = 3 ·
10ꢀ4 M and pH = 3.4 0.1), Fe(OH)2+ was the predom-
inant monomeric Fe(III)-hydroxy complex (Benkelberg
and Warneck, 1995), but its concentration rapidly de-
creased due to hydrolysis and oligomerisation processes
leading to the formation of soluble aggregates. These
processes control the proportion between the various
Fe(III) species that differ in their photoactivities and
thus influence the rate of hydroxyl radical production
(Helz et al., 1994). The initial concentration of
Fe(OH)2+ and the kinetics of its disappearance is
UV–visible spectra were recorded on a Cary 3 double
beam spectrophotometer.
HPLC experiments were carried out using a Waters
chromatograph equipped with two pumps Waters 510,
an auto-sampler 717 and a Waters 996 photodiode array
detector. The flow rate was 1ml minꢀ1 and the eluent a
mixture of pure water and acetonitrile (v/v 70/30). The
column was a Lichrosphere (Merck) RP18 (reverse
phase) of 125 · 4.6mm with a particle diameter of
´
strongly affected by the age of the stock solution (Kry´sova
et al., 2003). For this reason it is necessary to determine
the percentage of Fe(OH)2+ before each experiment by
the HQSA method (see Section 2.3).
˚
5lm and a pore diameter of 100A.
The purity of synthesised authentic photoproducts
was checked by NMR. 1H and 13C spectra were re-
corded on a Bruker Avance DSX spectrometer operat-
ing at 300.13MHz for the proton. The mass spectra
were obtained using a Hewlett-Packard model 5989B
detector.
½FeðOHÞ2þꢁ
2þ
% FeðOHÞ
¼
ꢂ 100
½FeðIIIÞꢁto
where [Fe(III)]to is the starting concentration of total
Fe(III).
The solubility of Monuron in water at 25ꢁC is equal
to 230mglꢀ1 (1.16 · 10ꢀ3 mollꢀ1) (Mackay et al., 1997).
A stock solution of 10ꢀ3 mollꢀ1 concentration was pre-
pared by stirring during 48h.
2.3. Analysis
The method of measuring the monomeric concentra-
tion of Fe(III) (mainly Fe(OH)2+ in our experimental
conditions) was modified from KuenziÕs procedure
(Kuenzi, 1982) and described in our previous papers
(Brand et al., 1998; Mailhot et al., 1999). The measure-
ment is based on the formation of the complex
Fe(HQS)3 between 8-hydroxyquinoline-5-sulfonic acid
(HQSA) and Fe(III) monomeric species which presents
a maximum absorption at 572nm with a molar absorp-
tion coefficient equal to 5075lmolꢀ1 cmꢀ1 (Mailhot
et al., 1999).
O
CH3
Monuron (MW = 198.65 g mol-1)
Cl
NH C N
CH3
Unless otherwise noted,
a
concentration of
1.0 · 10ꢀ4 mollꢀ1 was used all along this work. Monu-
ron was stable in aqueous solution; no degradation
was observed in the dark and at room temperature after
three months. The UV–visible spectrum of Monuron
comprises one maximum at 244nm (e = 18000lmolꢀ1
cmꢀ1) and no absorbance at wavelength greater than
300nm.
Fe(II) concentration was determined by complexom-
etry with ortho-phenanthroline, using e510 = 1.118 ·
104 lmolꢀ1 cmꢀ1 for the Fe(II)-phenanthroline complex
(Calvert and Pitts, 1966).
The mixture Monuron/Fe(III) in our experimental
conditions was thermally stable (in the dark and at room
temperature) in terms of Monuron concentration. So
The concentrations of Monuron and its photo-
=
products were followed by HPLC analysis (kdetection
244nm).

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H. Mest’ankova et al. / Chemosphere 57 (2004) 1307–1315
1310
Table 2
Influence of the monomeric species concentration and oxygen on the initial quantum yield of Monuron disappearance
[Monuron]
% Fe(OH)2+
20%
40%
60%
90%
60% without O2
0.013
1.0 · 10ꢀ4 mollꢀ1
4.0 · 10ꢀ4 mollꢀ1
UMonuron
UMonuron
0.0023
0.0022
0.0060
0.0052
0.0103
0.0108
0.025
0.025
k
irr. = 365nm and [Fe(III)]0 = 3.0 · 10ꢀ4 mollꢀ1
.
there is no precipitation or redox reaction in this system.
Moreover, no change either in concentration of
Fe(OH)2+ or in its disappearance kinetics was observed
in the presence of Monuron. These results confirm the
absence of interactions between Monuron and Fe(III)
in aqueous solution kept in the dark.
Table 3
Initial quantum yields of Fe(II) formation as a function of the
percentage of monomeric species and oxygen
% Fe(OH)2+ 45%
UFe(II) 0.0054 0.017 0.063 0.014
[Fe(III)]0 = 3.0 · 10ꢀ4 mollꢀ1, [Monuron]0 = 1.0 · 10ꢀ4 mollꢀ1
irr. = 365nm.
60%
90%
60% without O2
;
k
3.2. Photodegradation of Monuron
ance are not affected by the starting Monuron
concentration.
Unless otherwise noted, Monuron (1.0 · 10ꢀ4
mollꢀ1) and Fe(III) (3.0 · 10ꢀ4 mollꢀ1) were the con-
centrations used all along this work. The kinetics of
photodegradation were performed at 365nm. This
wavelength is sufficiently energetic to cause the photo-
redox process of Fe(III) aquacomplexes and it is
present in solar spectrum. During the irradiation of
the mixture, the concentration of Monuron continu-
ously decreased.
(c) Influence of the irradiation wavelength. In this set
of experiments, the solutions of ferric perchlorate
(3.0 · 10ꢀ4 mollꢀ1, 90% of monomeric species) and
Monuron (4.0 · 10ꢀ4 mollꢀ1) were irradiated at 296,
313, 334 and 365nm. The quantum yields of Monuron
disappearance are given in Table 1. The quantum yield
is significantly higher at 334nm than at 365nm and
slightly increases at shorter wavelengths. This effect
has to be related to the increase of the quantum yield
Å
3.2.1. Primary stage of the degradation
of OH radical formation when the excitation wave-
length decreases. It is consistent with the notion that
(a) Influence of the monomeric species concentration.
The initial quantum yields of Monuron disappearance
and Fe(II) formation are gathered in Tables 2 and 3
respectively. The results showed that the quantum yields
of Monuron disappearance and Fe(II) formation were
strongly dependent on the Fe(OH)2+ percentage: higher
is the percentage, higher were the quantum yields of
Monuron disappearance and Fe(II) formation. These re-
sults are directly linked to the nature of Fe(III)-hydroxo
species present in solution and give evidence for the
importance of Fe(III) speciation: Fe(OH)2+ is the most
Å
ejection of OH radical from the solvent cage requires
kinetic energy (Benkelberg and Warneck, 1995).
(d) Oxygen effect. The initial quantum yields of
Monuron disappearance and Fe(II) formation were cal-
culated in the presence and in the absence of oxygen.
The results are collected in Tables 2 and 3. There is no
significant effect of oxygen on the initial quantum yields.
The primary step of the Monuron degradation appears
not to be assisted by the oxygen. But oxygen is necessary
for the rest of the degradation to obtain the complete
mineralisation of the pollutant. As we will see latter,
the formation of photoproducts depends on the oxygen
concentration.
Å
photoactive species in terms of OH radicals production
´
(Faust and Hoigne, 1990; Benkelberg and Warneck,
1995) and consequently for Fe(II) formation and the
degradation of the organic compound present in solu-
tion. The quantum yields of Fe(II) formation are always
higher than the quantum yields of Monuron degrada-
tion. This result may be due to the rapid formation of
photoproducts and after a competition for the reaction
of hydroxyl radical.
3.2.2. Kinetics of Monuron degradation and Fe(II) forma-
tion
The photoinduced degradation of Monuron in the
presence of Fe(III) aquacomplexes was also performed
in a reactor with larger volume (60ml) irradiated at
365nm. First of all, it may be noted that the direct pho-
tolysis of Monuron was negligible: no photodegradation
of Monuron occurred in the absence of Fe(III). The rate
of Monuron disappearance was strongly affected by the
concentration of Fe(OH)2+. It increased with increasing
concentration of Fe(OH)2+ (Fig. 1), that is consistent
(b) Influence of Monuron concentration. The effect of
Monuron concentration on the photochemical reaction
(k = 365nm) was investigated for two concentrations in
Monuron (1.0 · 10ꢀ4 mollꢀ1 and 4.0 · 10ꢀ4 mollꢀ1) with
different percentage of Fe(III) monomeric species (Table
2). The initial quantum yields of Monuron disappear-

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H. MestÕankova et al. / Chemosphere 57 (2004) 1307–1315
1311
III
1.10-4 mol.l-1 Fe ; pH = 3.5
1.0
0.8
0.6
0.4
0.2
0.0
III
3.10-4 mol.l-1 Fe ; pH = 3.4
1.00
III
1.10-3 mol.l-1 Fe ; pH = 3.1
without Fe(III)
-4mol l-1
with [Fe(III)] = 3 x 10
III
1.10-2 mol.l-1 Fe ; pH = 2.7
2+
20 % Fe(OH)
45 % Fe(OH)2+
III
5.10-2 mol.l-1 Fe ; pH = 1.75
90 % Fe(OH)2+
0.75
0.50
0.25
0.00
0
1
2
3
4
5
6
Irradiation time/h
0
1
2
3
4
5
6
Fig. 1. Degradation of Monuron upon irradiation at 365nm of
Fe(III) (3.0 · 10ꢀ4 mollꢀ1)/Monuron (1.0 · 10ꢀ4 mollꢀ1) mix-
tures at various percentage of Fe(III) monomeric species.
Irradiation time /h
Fig. 3. Degradation of Monuron upon irradiation at 365nm of
) mixtures at different
Fe(III)/Monuron (1.0 · 10ꢀ4 mollꢀ1
Fe(III) concentrations.
with the values of quantum yields. In this set of experi-
ments, the concentration of Fe(II) was also measured all
along the transformation (Fig. 2). In all cases Fe(II) was
formed, its concentration quickly rose, then reached a
plateau value upon continued irradiation. The [Fe(II)]
at the plateau is a positive function of the initial concen-
tration of Fe(OH)2+. In solution without pollutant,
Fe(II) is formed too, but the concentration at the pla-
teau is lower than in the presence of Monuron. In this
case, the reaction of oxidation of Fe(II) by ÅOH radicals
becomes important: Fe(II) appears to be the major sink
(1.0 · 10ꢀ4; 3.0 · 10ꢀ4; 1.0 · 10ꢀ3; 1.0 · 10ꢀ2 and 5.0 ·
10ꢀ2 mollꢀ1) (Fig. 3). The kinetics of Monuron degrada-
tion are strongly dependent on Fe(III) concentration.
The disappearance of Monuron became faster when
the concentration of Fe(III) increased from 1.0 · 10ꢀ4
to 1.0 · 10ꢀ3 mollꢀ1. This effect can be correlated with
the increase of the monomeric species present in Fe(III)
solution. However at highest Fe(III) concentrations,
1.0 · 10ꢀ2 and 5.0 · 10ꢀ2 mollꢀ1, a decrease of the rate
of Monuron disappearance was observed when com-
pared to the kinetics of the reaction with Fe(III) concen-
tration equal to 1.0 · 10ꢀ3 mollꢀ1. This negative effect is
attributed to two major factors:
Å
of OH radicals (reaction 2) (Buxton et al., 1988):
ꢀ1
Fe2þ þ OH ! Fe3þ þ OHꢀ k ¼ 3:2 ꢂ 108 lmol sꢀ1
Å
ð2Þ
(i) The presence of Fe(III) dimeric species at high con-
centration of Fe(III) (P1.0 · 10ꢀ2 mollꢀ1). The
concentrations of dimeric species were evaluated
at 4.5 · 10ꢀ4 and 4.2 · 10ꢀ3 mollꢀ1 for Fe(III)
solution of 1.0 · 10ꢀ2 and 5.0 · 10ꢀ2 mollꢀ1 respec-
tively, corresponding to 9% and 17% of the total
iron respectively. The remaining concentrations of
monomeric species were still high (9.1 · 10ꢀ3 and
4.16 · 10ꢀ2 mollꢀ1). However, the dimeric species
are the major light-absorbing species at 365nm
3.3. Influence of Fe(III) concentration
The photodegradation of Monuron was performed at
five different concentrations of Fe(III) aquacomplexes
0.25
0.20
-4mol l-1
[Fe(III)] = 3 x 10
90 % of Fe(OH)2+
Å
45 % of Fe(OH)2+
and the quantum yield of OH radical production
is much lower for the dimer than for the monomer
Fe(OH)2+ (Langford and Carey, 1975; Faust and
0.15
20 % of Fe(OH)2+
90 % of Fe(OH)2+
without Monuron
0.10
´
Hoigne, 1990; Helz et al., 1994) (Table 4). The
spectral features associated to the photochemical
properties of the dimeric species lead to a negative
effect for the elimination of pollutant.
0.05
0.00
0
1
2
3
4
5
6
(ii) The pH of the solution that controls the nature of
the Fe(III) monomeric species in solution. The pH
of Fe(III) solutions were equal to 2.5 and 1.75 for
the concentrations of 1.0 · 10ꢀ2 mollꢀ1 and 5.0 ·
10ꢀ2 mollꢀ1 respectively and the fractions of
Irradiation time/h
Fig. 2. Fe(II) formation upon irradiation at 365nm of Fe(III)
(3.0 · 10ꢀ4 mollꢀ1)/Monuron (1.0 · 10ꢀ4 mollꢀ1) mixtures at
various percentages of Fe(III) monomeric species.

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H. Mest’ankova et al. / Chemosphere 57 (2004) 1307–1315
1312
Table 4
Spectral and photochemical properties of Fe(III)-hydroxy
species (Helz et al., 1994)
Table 5
Structure and mass spectra of the main photoproducts
Number Molar mass Structure
Å
Fe(III)
species
e365 nm
U
(M + H)+
and ionic
fragment
OH
(lmolꢀ1 cmꢀ1
)
Fe3+
No absorbance
at k > 300nm
290
ND
215
185, 128
O
C
I
CH2OH
CH3
Fe(OH)2+
0.017 (k = 360nm)
0.007 (k = 350nm)
Cl
Cl
NH
NH
N
N
4þ
Fe2ðOHÞ2
ꢃ5000
O
C
CH3
CH3
3+
Fe(OH)2+
1.0
0.8
0.6
0.4
0.2
0.0
Fe
II
215
187, 144
OH
OH
O
C
CH3
H
Cl
NH
N
III
201
144
+
Fe(OH)2
O
C
CH3
IV
V
Synthesised
Synthesised
Cl
Cl
NH
NH
N
N
CHO
CH3
1
2
3
4
5
O
C
pH
H
Fig. 4. Equilibrium speciation of monomeric Fe(III) aquacom-
plexes at 293K calculated with equilibrium constants given by
´
Faust and Hoigne (1990).
polar than Monuron. Moreover, this N-formyl deriva-
tive was observed to be unstable in water solution at
room temperature: a moderate reactivity leading to the
monomethylated (V) was recorded. The nature of the
primary photoproducts can be explained by the reaction
Fe(III) monomeric species, calculated with the
equilibrium constants of Fe(III)-hydrolytic equilib-
´
rium (Faust and Hoigne, 1990) (Fig. 4), gives 50%
and 10% of Fe(OH)2+ for pH 2.5 and 1.7 respec-
tively. The remaining of the monomeric species is
present as Fe3+ species, which do not absorb at
wavelength longer than 300nm and lead also to a
negative effect for the elimination of pollutant.
of OH radicals on two moieties of Monuron: the aro-
matic ring and the methyl groups of the urea function
(cf. mechanism in scheme 1). The two important steps
Å
Å
of the mechanism are the formation of the OH adduct
on the aromatic ring and the abstraction of a hydrogen
atom from a methyl group of the dimethylurea function.
The ÅOH adduct on the aromatic ring leads to the forma-
tion of the hydroxylated photoproduct (II) whereas the
radicals formed on the methyl group react with oxygen
to form peroxy radical ROOÅ. This radical evolves,
through the formation of tetroxide derivative, to alkoxy
radical. Different routes (b scissions and redox reaction
with Fe(II)) as described in details by Brand et al.
(1998) allow us to account for all the primary identified
photoproducts. Afterwards, the mechanism becomes
more and more complicated as evidence by the presence
of many peaks on the HPLC chromatogram. A non-
negligible thermal reaction (in the dark and at room
temperature) is also observed for the conversion of pho-
toproduct I in photoproduct V.
These results illustrate the importance of the specia-
tion of iron in aqueous solution.
3.4. Photoproducts identification and mechanism of deg-
radation
Several peaks appeared in the HPLC chromatogram
of an irradiated mixture. The same peaks were observed
whatever the percentage of monomeric species. The five
major photoproducts were identified (Table 5): two of
them by co-injection of the authentic samples synthe-
sised in our laboratory using the procedure proposed
by Crosby and Tang (1969). The three other photoprod-
ucts were identified by mass spectra analysis and by
analogy with the photodegradation studies of dimethyl-
phenylurea derivatives (diuron and chlortoluron)
(Mazellier et al., 1997; Poulain et al., 2003). The N-
formyl derivative (IV) was the only photoproduct less
In contrast of the effect on the quantum yield, the ab-
sence of oxygen affects the formation of photoproducts.
In the absence of oxygen, the carbon centred radical ob-
tained by abstraction of hydrogen by ÅOH radical, reacts

ˇ ˇ
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´
H. MestÕankova et al. / Chemosphere 57 (2004) 1307–1315
1313
12
Kinetics of Photoproduct I formation
4x105
3x105
2x105
10
8
TOC for 60 % of Fe(OH)2+
TOC for 90 % of Fe(OH)2+
in absence of oxygen
in presence of oxygen
6
4
1x105
0
2
0
0
1
2
3
4
5
6
7
8
0
10
20
30
40
50
60
70
Irradiation time /h
Irradiation time /h
Fig. 5. Kinetics of the photoproduct I formation in the absence
and in the presence of oxygen. kirr. = 365nm, [Fe(III)] = 3.0 ·
10ꢀ4 mollꢀ1 90% of monomeric species, [Monuron] = 1.0 ·
Fig. 6. Time evolution of TOC vaꢀlu1es during irradiation
monomeric species)/Monuron (1.0 · 10ꢀ4 mollꢀ1) mixtures.
(365nm) of Fe(III) (3.0 · 10ꢀ4 moll
, 60% and 90% of
10ꢀ4 mollꢀ1
.
observed with 90% of Fe(OH)2+: this can be explained
by the higher rate of the reaction in this case. The evo-
lution of Fe(II) concentration (Fig. 7) is a multistep phe-
nomenon. The very fast formation of Fe(II) at the
beginning of the irradiation mainly results from the pho-
toredox process of Fe(OH)2+ species. Then Fe(II) con-
centration reaches a pseudo-constant value (between 7
and 20h of irradiation) corresponding to a photostation-
ary equilibrium between Fe(III) and Fe(II) in the
presence of organic compounds. Afterwards, the con-
centration in Fe(II) decreases and reaches a second pla-
teau value after 25h of irradiation (ꢃ1.2 · 10ꢀ4 mollꢀ1).
This second plateau value corresponds to the photosta-
tionary equilibrium between Fe(III) and Fe(II) in the
absence of any organic compounds as evidenced by
the irradiation of Fe(III) (3.0 · 10ꢀ4 mollꢀ1, 90% of
Fe(OH)2+) alone in aqueous solution (Fig. 7). This result
mainly with Fe(III) aquacomplexes to formed the
photoproduct I (alcohol on the methyl group). The
formation of alcohol, according to reaction (3), becomes
the main pathway (Clark et al., 1988).
H2O
Å
R þ Fe3þ ! ROH þ Fe2þ þ Hþ
The formation of alcohol is increased by a factor of
fifteen in the absence of oxygen (Fig. 5).
ð3Þ
In terms of photoproducts, the principal difference
between the direct, TiO2 or iron sensitised photodegra-
dation of Monuron is due to the substitution of the chlo-
rine atom by OH group during the direct photolysis.
3.5. Mineralisation of Monuron
One of the aims of the present work was to test the
efficiency of Monuron removal from water solution,
when the degradation was photoinduced by Fe(III).
Total organic carbon experiment were undertaken in or-
der to make evidence for the complete mineralisation of
Monuron. As shown in Fig. 6, the total mineralisation
of Monuron was reached after 25h of irradiation with
90% of monomeric species and after 50h with 60% of
monomeric species. The complete mineralisation is ob-
tained due to the continuous formation of radical spe-
cies (ÅOH). A photocatalytic cycle, in homogeneous
phase, based on the couple Fe(III)/Fe(II) and assisted
by oxygen (Poulain et al., 2003) is involved. This result
also shows that the speciation of Fe(III) (concentration
of monomeric species) at the beginning of the irradiation
is a very important parameter for the reaction efficiency.
A lag period of approximately 10h was observed
with 60% of Fe(OH)2+. This was due to the formation,
accumulation and subsequent degradation of primary
organic photoproducts. The lag period was not clearly
[Fe(II)] for 90 % Fe(OH)2+
3.0x10-4
2.5x10-4
2.0x10-4
1.5x10-4
1.0x10-4
12
2+ without Monuron
[Fe(II)] for 90 % Fe(OH)
TOC for 90 % of Fe(OH)2+
10
8
6
4
5.0x10-5
0.0
2
0
0
10
20
30
40
50
Irradiation time /h
Fig. 7. Fe(II) and TOC evolutions with and without Monuron
for long irradiation times (365nm) ([Fe(III)] = 3.0 ·
10ꢀ4 mollꢀ1 90% of monomeric species, [Monuron] = 1.0 ·
10ꢀ4 mollꢀ1).

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H. Mest’ankova et al. / Chemosphere 57 (2004) 1307–1315
1314
Calvert, J.G., Pitts, J.M., 1966. Photochemistry. Wiley, New
York, pp. 783–786.
was confirmed by the correlation with the total organic
carbon concentration in solution: the second photosta-
tionary equilibrium was reached when the complete min-
eralisation of the solution was obtained.
Clark, E.A., Sterrit, R.M., Lester, J.N., 1988. The fate of
tributyltin in the aquatic environment. Environ. Sci. Tech-
nol. 22 (6), 600–604.
Crosby, D.G., Tang, C.S., 1969. Photodecomposition of 3-(p-
Chlorophenyl)-1,1dimethylurea Monuron. J. Agric. Food
Chem. 17, 1041–1044.
4. Conclusions
Eichelberger, J.W., Lichtenberg, J.J., 1971. Persistence of
pesticide in river water. Environ. Sci. Technol. 5, 541–544.
Faust, B.C., 1999. Aquatic photochemical reactions in atmos-
pheric, surface, and marine waters: influences on oxidant
formation and pollutant degradation. In: Boule, P. (Ed.),
Environmental Photochemistry. Springer-Verlag, Berlin.
Faust, B.C., Hoigne´, J., 1990. Photolysis of Fe(III)-hydroxy
complexes as sources of OH radicals in clouds, for and rain.
Atmos. Environ. 24A, 79–89.
Our results confirm that Fe(III) can act as an efficient
photoinducer of phenylurea herbicides elimination from
water solution. Furthermore, photodegradation leads to
the complete mineralisation. This work gives evidence
for the efficiency of Monuron removal from water by a
process of homogeneous photocatalysis providing the
Å
continuous formation of OH radicals. Moreover, the
importance of the speciation of Fe(III) aquacomplexes
present in solution was demonstrated for the efficiency
of such a process. The use of a high concentration of
Fe(III) (>1.0 · 10ꢀ3 mollꢀ1), with the presence of di-
meric and Fe3+ species, has a negative effect on the de-
Flynn, C.M., 1984. Hydrolysis of inorganic(III) salts. Chem.
Rev. 84, 31–41.
Helz, G.R., Zepp, R.G., Crosby, D.G., 1994. Aquatic and
Surface Photochemistry. Lewis, Boca Raton, USA, p. 13.
Hill, G.D., Mac Gahen, J.W., Baker, H.M., Finnerty, D.W.,
Bingeman, C.W., 1955. The fate of substituted urea herbi-
cides on agricultural soils. Agronom. J. 47, 93–104.
Kearney, P.C., 1965. Enzyme hydrolysis, purification and
properties of enzyme responsible for hydrolyzing phenyl-
carbamates. J. Agric. Food Chem. 13, 561–564.
gradation of pollutants. As
a consequence, the
optimum concentrations in Fe(III) necessary in such a
process are sufficiently low to make the treated waters
compatible with a safe aquatic environment.
´
´
´
Kry´sova, H., Jirkovsky, J., Krysa, J., Mailhot, G., Bolte, M.,
2003. Comparative kinetic study of atrazine photodegrada-
tion in aqueous Fe(ClO4)3 solutions and TiO2 suspensions.
Appl. Catal. B 40, 1–12.
Acknowledgments
ˇ
This work was supported by grants GA CR 104/02/
ˇ
0662, GA CR 203/02/0983 and Barrande 2003/20.
Kuenzi, W.H., 1982. Die hydrolyse von eisen(III). PhD
dissertation, ETH no. 7016, Zurich, Switzerland.
Langford, C.H., Carey, J.H., 1975. The charge transfer
photochemistry of the hexaaquoiron(III) ion, the chloro-
pentaaquoiron(III)ion, and the l-dihydroxo dimer explored
with tert-butyl alcohol scavenging. Can. J. Chem. 53, 2430–
2435.
Financial support from the French Embassy in the
ˇˇ ´
´
Czech Republic for a stay of Hana MestÕankova in Cler-
mont-Ferrand is also gratefully acknowledged.
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