Iron(III) photo-induced degradation of isoproturon: correlation between degradation and toxicity.

Article abstract of DOI:10.1002/ps.505

The degradation of isoproturon photoinduced by Fe(III) was investigated under both artificial and solar light. The monomeric species Fe(OH)2+ present under the experimental conditions ([Fe(III)] = 3 x 10(-4) M) is the main Fe(III) species responsible for

Full text of DOI:10.1002/ps.505

Pest Management Science
Pest Manag Sci 58:707±712 (online: 2002)
DOI: 10.1002/ps.505
Iron(III) photo-induced degradation of
isoproturon: correlation between degradation
and toxicity
1
1
2
2
Fr e´ d e´ ric Galichet, Gilles Mailhot, Fr e´ d e´ rique Bonnemoy, Jacques Bohatier and
1
Mich e` le Bolte *
1
Laboratoire de Photochimie Mol e´ culaire et Macromol e´ culaire UMR 6505 CNRS, Universit e´ Blaise Pascal, F-63177 Aubi e` re cedex France
Laboratoire de Biologie Compar e´ e des Protistes UMR 6023 CNRS, Universit e´ Blaise Pascal, F-63177 Aubi e` re cedex France
2
Abstract: The degradation of isoproturon photoinduced by Fe(III) was investigated under both
2‡
arti®cial and solar light. The monomeric species Fe(OH) present under the experimental conditions
À4
(
[Fe(III)]=3Â10
M) is the main Fe(III) species responsible for the degradation of isoproturon. The
.
2‡
process involves the attack on the pollutant by OHradicals generated by irradiation of Fe(OH) . The
major primary photoproducts were identi®ed; they accumulate in the solution medium before being
degraded. The toxicity of the solution to marine bacterium Vibrio ®sheri (Beijerinck) Lehmann &
Neumann was monitored during the degradation process. It increased in the early stages of the
reaction and, among the photoproducts, the N-formyl derivative appeared to be the major product
responsible for the increase in toxicity.
#
2002 Society of Chemical Industry
Keywords: isoproturon; iron(III); hydroxyl radicals; photodegradation; toxicity
1
INTRODUCTION
pollutant, Fe(III) aquacomplexes can absorb the solar
light and undergo a photoredox process giving rise to
Isoproturon, a herbicide of the family of phenylurea
derivatives, inhibits photo-synthetic electron transfer
and is used for pre- and post-emergence control of
annual grasses and many annual broad-leaved weeds
5
,6
Fe(II) and hydroxyl radicals.
2
Among the Fe(III)
(which refers to
‡
aquacomplexes, Fe(OH)
2
‡
Fe(OH)ꢀH O† ) is photolysed with the highest
2
5
1
6
quantum yield, according to the reaction:
in spring and winter wheat. Because it must show
herbicidal activity over a long period of time, iso-
proturon degradation has to be slow; it undergoes
enzymatic and microbial degradation with DT50 in soil
of 6±28 days. Accordingly, the non-degraded isopro-
turon and the degradation products may be a source of
contamination of aquatic bodies and have to be
eliminated from run-off and ground water. It has been
reported that photodegradation in sunlight can pro-
vide an alternative route for pollutant degradation.
However, due to the very low, if not negligible,
absorption of solar radiation by isoproturon, its
photodegradation has to be induced by absorbing
species such Fe(III) aqua-complexes. In the presence
of Fe(III), two different photoprocesses can be
observed, depending on the nature of the pollutant.
hn
.
2
‡
2‡
Fe(OH) À! Fe ‡ OH
.
The pollutant is attacked by OH radicals, which are
strong oxidizing agents, leading to its degradation. We
have previously investigated the degradation of various
7±13
pollutants photo-induced by Fe(III).
the complete mineralization of the pollutant is
In most cases
2‡
.
observed due to the reoxidation of Fe
by OH
radicals which confers a catalytic aspect to the process.
In this work we investigated the Fe(III) photo-
induced degradation of isoproturon in aqueous solu-
tion, and monitored the evolution of toxicity during
the process.
(
nitrilotriacetic acid, ethylenediaminetetraacetic acid
1) If the pollutant is able to complex Fe(III), eg
3
2
2 EXPERIMENTAL
or ethylene diaminetetra(methylene phosphonic
4
acid), a complex between the pollutant and Fe(III)
2.1 Materials
Isoproturon
(3-(4-isopropylphenyl)-1,1-dimethyl
which absorbs in the near UV-visible spectral region is
formed. In this case, an intramolecular photoredox
process leading to the degradation of the pollutants is
observed. (2) If Fe(III) is not complexed by the
urea) of `technical grade' quality was obtained from
Aventis Agrochimie.
Ferric perchlorate nonahydrate (Fe(ClO ) Á 9H O;
4
3
2
97%) and barium hydroxide octahydrate (Ba(OH) Á
2
*
Correspondence to: Mich e` le Bolte, Laboratoire de Photochimie Mol e´ culaire et Macromol e´ culaire UMR 6505 CNRS, Universit e´ Blaise
Pascal, F-63177 Aubi e` re cedex France
(
Received 2 January 2001; revised version received 25 June 2001; accepted 21 February 2002)
#
2002 Society of Chemical Industry. Pest Manag Sci 1526±498X/2002/$30.00
707

F Galichet et al
8H O; puriss) were Fluka products and were kept in a
desiccator. The Fe(III) solutions were prepared by
lowed by HPLC (UV detection at 240nm) by
measuring the area of the corresponding peaks.
2
À3
diluting stock solutions (2.0Â10
M
Fe(ClO ) Á
Carbon dioxide produced on irradiation was deter-
mined as barium carbonate. It was removed from the
solution by a ¯ow of oxygen (scrubbed free from
carbon dioxide traces by passing through a concen-
trated barium hydroxide solution) and trapped in two
4
H O) to the appropriate Fe(III) concentration.
3
9
2
2
-Propanol (HPLC grade Chromanorm >99.7%)
was a Prolabo product. 8-Hydroxyquinoline-5-sul-
fonic acid (HQSA; 98%) and 5,5-dimethyl-1-pyr-
roline N-oxide (DMPO; 97%) were purchased from
Aldrich. All solutions were prepared with deionized
À3
consecutive 6.0Â10 M barium hydroxide solutions.
The solutions were collected, barium carbonate
allowed to precipitate and the excess of barium
4
ultra-pure water (resistivity r=18.2Â10 ꢀm). pH
À3
measurements were carried out with an ORION pH
meter to 0.01 unit. The ionic strength was not con-
trolled.
hydroxide titrated with 6.0Â10 M hydrochloric acid.
Electron spin resonance (ESR) spectra were ob-
tained using a Brucker ER-200D spectrometer at
9
.30GHzwith a modulation ®eld of 100KH.z A
Xe±Hg Hanovia Lamp was used for irradiation in the
ESR spectrometer cavity (l>350nm).
The toxicity of photoproducts and the in¯uence of
2.2 Apparatus
UV-visible spectra were recorded on a CARY 3 double
beam spectrophotometer.
irradiation on the toxicity of solutions was determined
1
HPLC analyses were carried out using a Waters 540
chromatograph equipped with a Waters 996 photo-
diode array detector giving the UV-visible spectrum
corresponding to each peak. The ¯ow rate was 1ml
with a Microtox test. This test consists in determin-
ing the concentration (EC ) of toxic compound that
50
inhibits 50% of the natural luminescence of a marine
bacterium, Vibrio ®scheri (Beijerinck) Lehmann &
Neumann. The emission is measured after various
exposure times, usually 5, 15 or 30min. A decrease in
EC50 corresponds to an increase of toxicity. Assays
were carried out with a Microbics M500 analyser. All
materials for analyses (test reagent, diluent, osmotic
adjusting solution, reconstitution solutions) were
supplied by Azur Environmental (Carlsbad, CA,
USA). Due to the presence of Fe(III), the solution
À1
min . Liquid chromatography/positive electrospray/
mass spectra (LC/ES/MS) were obtained from Service
Central d'Analyse, CNRS, Vernaison, France. The
eluent was methanol‡water and the column was a
Touzard and Matignon Kromasil C18 (250mmÂ
2
mm ID, pore diameter 5mm).
To measure the quantum yields, monochromatic
irradiations at 296, 313 and 365nm were carried out
with a high-pressure mercury lamp (Osram HBO
1
was acidic (pHꢁ3.50) and, for the Microtox test, it
2
00W) equipped with a grating monochromator
Bausch and Lomb). The beam was parallel and the
was neutralised prior to measurement and the Fe(III)
hydroxide ®ltered off.
(
reactor was a quartzcell of 1-cm path length. The light
intensity was measured by ferrioxalate actinometry
1
5
À1
À2
(
I0(365nm) ꢁ3.7Â10
photons s cm ; I0(313nm)
À2
ꢁ
3 RESULTS
3.1 Isoproturon and iron(III) in aqueous solution
Isoproturon, 3-(4-isopropylphenyl)-1,1-dimethyl urea
1
5
À1
14
2
photons s cm ).
Â10
photons
s
cm
;
I0(296nm) ꢁ9.8Â10
À1
À2 14
À1
A second irradiation set-up used for kinetic experi-
ments was an elliptical stainless steel chamber. A high-
pressure mercury lamp (Philips HPW type 125W), the
emission of which at 365nm was selected by an inner
is only slightly soluble in water (65mg litre , 3.1Â
À4
10 M at 22°C). Unless otherwise noted, the con-
centration used in the work was 10 M prepared from
À4
À4
a stock solution at 2Â10 M. Isoproturon is stable in
®
lter, was located at a focal axis of the elliptical
chamber. The reactor, a water-jacketed Pyrex tube
2.8cm diameter), was centred at the other focal axis.
The reaction medium was well stirred. The unit
the dark in aqueous solution in the presence or in the
absence of Fe(III) at room temperature.
The absorption spectrum of isoproturon in aqueous
(
solution presents a maximum at l=240nm (e
16000 litre mol cm ) and a shoulder at 280nm
=
40
2
1
5
À1
À3
À1 À1
delivered an intensity I ꢁ4.6Â10 photons s cm
a
over a large volume (60ml).
À1
À1
(e280 =1000 litre mol cm ). Isoproturon does not
absorb at wavelengths longer than 300nm.
Under our experimental conditions ([Fe(III)]=
À4
2‡
2
.3 Analysis
3.0Â10 M and pH 3.50), Fe(OH) (subsequently
2
5
‡
The Fe(II) concentration was determined by com-
plexometry with o-phenanthroline, using a molar
absorption coef®cient at the maximum (510nm)
used to refer to Fe(OH)ꢀH2O† ) is the predominant
6
monomeric Fe(III) hydroxy complex. However, the
concentration in monomeric species rapidly decreased
after the dissolution of ferric perchlorate in water. The
disappearance was attributed to the possible formation
of soluble aggregates, the ®rst step toward the
formation of polymeric species and the precipitation
4
À1
À1
e510 =1.118Â10 litre mol cm
for the Fe(II)±
1
4
phenanthroline complex.
The method for measuring the monomeric con-
centration of Fe(III) was modi®ed from K uÈ enzi's
1
0
15
procedure as described elsewhere. Isoproturon
degradation and photoproducts formation were fol-
of amorphous Fe(OH)3. Since there was a con-
2
tinuous evolution, the concentration of Fe(OH) was
‡
7
08
Pest Manag Sci 58:707±712 (online: 2002)

Iron(III) photo-induced degradation of isoproturon
systematically measured before irradiation and the
2
percentage of Fe(OH) was calculated as follows:
‡
‰
Fe(OH)2‡Š
Fe(OH)2
‡
ˆ
 100
%
‰
Fe(III)Štot
Fe(III)]tot being the total concentration of Fe(III) in
[
the starting solution.
À4
A mixture of Fe(III)‡isoproturon (3Â10 M ‡
À4
1
0
M) is stable for days in the dark and the UV-
visible spectrum is the sum of the UV-visible spectra of
the components; there was no detectable complexa-
tion in the ground state.
Figure 2. Formation of Fe(II) as a function of irradiation time at 365nm:
[isoproturon]=10À4M, [Fe(III)]=3Â10À4M, 72% Fe(OH)2‡
.
3.2 Photochemical behaviour
The monochromatic excitation wavelengths were
chosen in the UV part of the solar emission, at 296,
3
13, 334 and 365nm. They are all energetic enough to
(
Fig 1(B)). The rate constants were calculated to be
À1
.
cause the photoredox process giving rise to OH
radicals from Fe(III) aquacomplexes. The majority of
the experiments were performed at 365nm.
À3
À3
7
Fe(OH) , respectively.
.2Â10 and 5.4Â10 min for 72 and 48% of
2‡
Together with the degradation of isoproturon, the
formation of Fe(II) was observed. The concentration
in Fe(II) increased quickly then leveled off (Fig 2)
re¯ecting the reoxidation of Fe(II) into Fe(III).
The quantum yield, Φ, of isoproturon degradation
that measures the ef®ciency of the photochemical
reaction is given by the ratio of the number of
degraded molecules to the number of photons
absorbed for the same period of time (Table 1). It
was calculated as a function of irradiation wavelength
and of percentage of monomeric species. As usually
observed, the quantum yield decreases when the
irradiation wavelength increases (the energy of the
photons decreases) and increases with the percentage
Isoproturon does not absorb at those wavelengths.
À4
Mixtures of isoproturon (10 M) and Fe(III)
À4
2‡
(
(
3Â10 M) at two different percentages of Fe(OH)
48 and 72%) were irradiated at 365nm. The decrease
in isoproturon is shown in Fig 1A. The data are in
agreement with a ®rst-order reaction, as evidenced by
[
[
isoproturon]0
isoproturon]t
plotting log
against irradiation time (t)
10
of monomeric species.
The formation of
.OH radicals upon irradiation
(
spin-trapping experiments DMPO (1mg ml ) was
lꢂ305nm) was con®rmed by ESR spectroscopy. For
À1
À4
added to an aqueous solution of Fe(III) (3Â10
M
with 72% of monomeric species) and isoproturon
(10 M). A 1:2:2:1 quartet (with a coupling constant
a=14.6 Gauss) was observed, characteristic of the
À4
.
stable adduct with OH radicals (Fig 3). The existence
of a quartet instead of a triplet of doublet is due to the
identical value of the coupling constants of the
electron a with nitrogen and a with hydrogen.
N
H
Table 1. Quantum yields (Φ) of isoproturon degradation in
a
aqueous solution
2
Wavelength (nm) Fe(OH) (%) Quantum yield
‡
2
3
3
3
3
3
3
96
13
34
65
65
65
65
72
72
72
72
48
74
92
0.15
0.046
0.033
0.010
0.0029
0.0097
0.015
Figure 1. (A) Isoproturon degradation upon irradiation at 365nm:
À4
À4
2‡
[
isoproturon]=10 M; [Fe(III)]=3.0Â10 M, 48 and 72% Fe(OH)
;
a
[isoproturon]=10À4M [Fe(III)]=3.0Â10À4M.
(
B) Log [isoproturon] /[isoproturon] as a function of irradiation time, t.
0 t
Pest Manag Sci 58:707±712 (online: 2002)
709

F Galichet et al
These four appear to be primary photoproducts. The
chemical structures are given in Table 2.
Photoproducts 1, 5 and 6 were identi®ed by co-
injection of authentic samples obtained either com-
mercially (1) or synthesized by organic chemists at
SEESIB (Laboratoire de Chimie de 1'UniversiteÂ
Blaise Pascal; 5 and 6). Compound 5 was a very
minor photoproduct whereas compound 1 appeared to
result from subsequent reactions. The structures of
photoproducts 2, 3, 4 were proposed from the positive
electrospray mass spectra (HPLC/ES/MS) (Table 2).
The structures were deduced mainly from the mol-
ecular mass and the major fragments obtained from
the spectra, together with the UV-visible spectra and
the retention times of the photoproducts. The com-
parison with the mass spectrum of isoproturon was
also very useful.
3
.4 Toxicity
The toxicity of the isoproturon solution was tested on
V ®scheri (Microtox) throughout the process of
degradation photoinduced by Fe(III). It appears that
a minimum in EC , corresponding to a maximum of
À1
Figure 3. Spin trapping experiments with DMPO (1mg ml ) by ESR
À4
spectroscopy on irradiation (l>305nm): [isoproturon]=10 M,
[
Fe(III)]=3.0Â10À4M, 72% Fe(OH)2‡
.
50
toxicity occurs (Fig 6). The results were similar for 15
and 30min exposure. During the process of miner-
alization of isoproturon, a strong increase in toxicity is
observed, followed by a decrease to complete inno-
cuity of the solution. This evolution was compared to
the formation of the three major photoproducts (2, 3
and 6). Their kinetics of formation nicely ®t the
evolution of the toxicity of the irradiated solution.
Previous work on the biodegradation of isoproturon
established that, among these three photoproducts,
the N-formyl derivative (photoproduct 6) is the most
In the presence of 1% of isopropanol, no signal was
detected by ESR spectroscopy and the degradation of
isoproturon was totally inhibited.
These two results show that isoproturon degrada-
.
tion is only due to OH radicals produced upon
irradiation of Fe(III) aquacomplexes.
3
.3 Identification of the photoproducts
Only a few photoproducts were observed even at a
high percentage conversion. Figure 4 gives the HPLC
chromatogram of a solution irradiated for 60min,
corresponding to 70% of isoproturon degradation. We
noted the appearance of six main new peaks and the
decrease of peak of isoproturon in comparison with the
chromatogram at t=0. The kinetics of formation of
photoproducts 2, 3, 4 and 6 are represented in Fig 5.
16
toxic.
The mineralization process was monitored by
carbon dioxide evolution produced from the mixture
À4
À4
of isoproturon (10 M) and Fe(III) (3.0Â10 M),
60% of monomeric species; after 24h of irradiation at
365nm, 53% of the carbon of the starting isoproturon
was recovered as carbon dioxide. This result shows the
ef®ciency of isoproturon removal from water, photo-
induced by Fe(III).
Figure 4. HPLC chromatogram of an aqueous mixture of isoproturon
À4
À4
2‡
(
(
10 M) and Fe(III) (3Â10 M, 72% Fe(OH) ), irradiated 60min at 365nm
70% of isoproturon has been degraded). Detection: l=240nm; eluent
Figure 5. Kinetics of formation of photoproducts (&) 2, (*) 3, (~) 4 and
(!) 6 (conditions as in Fig (4)).
À1
water ‡methanol (45‡55 by volume), flow rate 1ml min
.
7
10
Pest Manag Sci 58:707±712 (online: 2002)

Iron(III) photo-induced degradation of isoproturon
Table 2. Mass spectra of photoproducts
Peak
Retention time (min) Ionic fragments Molar mass/ions (M‡H)‡
Structure
1
2.2
72
88/89
2
3
4.25
4.8
191; 165; 120; 72
208/209
222/223
205; 134; 72
4
5.7
165; 136; 94; 72
206/207
5
14.9
151; 136; 94
165; 136; 94; 72
136; 94
192/193
206/207
220/221
Isoproturon
17.1
27.9
6
4
DISCUSSION
Isoproturon degradation is achieved upon irradiation
in the initiation of the process. Accordingly, the ®rst
step of the degradation is the photochemical genera-
2
‡
.
of Fe(III) aquacomplexes, more precisely Fe(OH)
.
,
tion of OH radicals, followed by the attack on the
.
through the formation of OH radicals. These very
powerful oxidative agents are the only species involved
pollutant. However, OH radicals are known to also
react with high rate-constant with Fe(II), regenerating
the absorbing Fe(III) species, so the complete mech-
anism can be summarized in Scheme 1.
The competition between isoproturon and Fe(II)
.
for reaction with OH radicals accounts for the
evolution of Fe(II) during the process: in the early
stages of the reaction, the concentration of isoproturon
is far higher than the concentration of Fe(II). As a
result, its reaction with isoproturon is favoured. An
equilibrium is then reached and the Fe(II) concentra-
tion levels off (Fig 2). Such an evolution has been
observed in all our investigated systems.
The transitory accumulation of the aromatic photo-
products in the irradiated solution makes possible their
identi®cation. The nature of the photoproducts shows
Figure 6. Comparison of (~) EC50 evolution and formation of
photoproducts (*) 2, (*) 3 and (&) 6.
that three sites on the molecules are primarily attacked
.
by OH radicals: CH of the isopropyl group (1), CH
3
Pest Manag Sci 58:707±712 (online: 2002)
711

F Galichet et al
Scheme 1. Photocatalytic mechanism of
.OH radical generation from Fe(III) aqua-
complexes.
triacetic acid transformation photoinduced by complexation
with iron (III) in aqueous solution, Trans Met Chem 18:37
(1993).
3
G uÈ nter FK, Hilger S and Canonica S, Determination of the
reaction quantum yield for the photochemical degradation of
Fe(III)±EDTA: implications for the environmental fate of
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(1995).
Figure 7. Sites of attack of isoproturon during irradiation.
4
5
Matthijs E, De Oude NT, Bolte M and Lemaire J, Photo-
degradation of ferric ethylenediaminetetra (methylene phos-
phonic acid) (EDTMP) in aqueous solution, Water Res
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Benkelberg HJ and Warneck P, Photodecomposition of iron(III)
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.
À
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6 Faust BC and Hoign e J, Photolysis of Fe(III)-hydroxycomplexes
the formation of the adduct aromatic ring OH radical
(
site 3) leads to the formation of the hydroxylated
derivative 3. The formation of photoproducts 2, 4, 5
and 6 involves a classical mechanism ®rst investigated
as sources of OH radicals in clouds, fog and rain, Atmos Environ
24A:79±89 (1990).
7
Mazellier P, Mailhot G and Bolte M, Photochemical behavior of
1
7
by Von Sonntag and Schumann. However, iron
through its redox properties interferes all through the
mechanism as already described in our previous
the iron(III)/2,6-dimethylphenol system, New J Chem 21:389±
397 (1997).
8 Brand N, Mailhot G and Bolte M, Degradation and photodegra-
dation of tetraacetylethylenediamine (TAED) in the presence
of iron(III) in aqueous solution, Chemosphere 34:2637±2648
9
±11
work.
The evolution of the toxicity during the early stages
of the degradation (Fig 6) shows a strong increase from
the starting toxicity of isoproturon, but the degrada-
tion process is very ef®cient and after 6h the level of
toxicity returned to the initial level, the mineralization
process leading it to a decrease afterwards. These
results are evidence for that the degradation of the
pollutant has to be monitored all through the process.
Some photoproducts can be more toxic than the
starting compound and this may cause a serious
problem for the aquatic environment.
(
1997).
9
Mailhot G, Astruc M and Bolte M, Degradation of tributyltin
chloride in water photoinduced by iron(III), Appl Organomet
Chem 13:53±61 (1999).
10 Brand N, Mailhot G and Bolte M, Degradation photoinduced by
Fe(III). method of alkyl phenolethoxylates removal in water,
Environ Sci Technol 32:2715±2730 (1998).
1
1 Brand N, Mailhot G, Sarakha M and Bolte M, Primary
mechanism in the degradation of 4-octylphenol photoinduced
by Fe(III) in water±acetonitrile solution, J Photochem Photobiol
A: Chem 135:221±228 (2000).
1
1
2 Mazellier P, Sarakha M and Bolte M, Primary mechanism for the
iron(III) photoinduced degradation of 4-chlorophenol in
aqueous solution, New J Chem 23:133±135 (1999).
3 Mazellier P, Jirkovsky J and Bolte M, Degradation of diuron
photoinduced by iron (III) in aqueous solution, Pestic Sci
ACKNOWLEDGEMENTS
The authors thank C Tixier, A Cuer and H Vescham-
bre (SEESIB Laboratory UMR 6504, CNRS Uni-
versit e Blaise Pascal) for the synthesis of authentic
compounds 5 and 6. The authors are indebted to the
anonymous reviewers for their helpful comments and
suggestions.
4
9:259±267 (1997).
14 Calvert JG and Pitts JM, Photochemistry, Wiley, New York. p 783
(1966).
1
1
5 Flynn CM, Hydrolysis of inorganic iron (III) salts, Chem Rev
4:31±41 (1984).
8
6 Tixier C, Meunier L, Bonnemoy F and Boule P, Phototrans-
formation of three herbicides, chlorbufam, isoproturon and
chlorotoluron. In¯uence of irradiation on toxicity, Internet J
Photoenergy 2:1±8 (2000).
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Pest Manag Sci 58:707±712 (online: 2002)