Reaction pathways for the photodegradation of the organophosphorus cyanophos in aqueous solutions

Article abstract of DOI:10.1111/j.1751-1097.2009.00654.x

Photodegradation in aqueous solutions is an important pathway for many agrochemicals such as pesticides. In the present work, the photochemical transformation of cyanophos (CYA) was investigated in aqueous solutions using UV light within the 254-313 nm range as well as solar light. The study was performed in order to have a deep insight into the mechanistic pathways for the photochemical disappearance of CYA. Upon UV irradiation of an aerated solution of CYA, the degradation quantum yield was found equal to 1.8 × 10 ^-2. It is independent of the excitation wavelength but varies with oxygen concentration. It increased by a factor of 2 from oxygen-saturated to oxygen-free solution. Photosensitized experiments were performed using acrylamide and hydroquinone as energy acceptor and energy donor substrates, respectively. They show that both singlet and triplet excited states were involved in the photochemical behavior of CYA. The laser flash photolysis experiments clearly showed the involvement of the triplet excited state which was efficiently quenched by molecular oxygen and acrylamide with the rate constants 1.97 × 10⁹ and 2.71 × 10⁹ mol ^-1 L s^-1, respectively. The photoproducts structures were proposed according to the mass spectral data using the LCMS technique. The analytical study shows that various processes such as hydrolysis, homolytic bond dissociations and Photo-Fries process occur.

Full text of DOI:10.1111/j.1751-1097.2009.00654.x

Photochemistry and Photobiology, 2010, 86: 247–254
Reaction Pathways for the Photodegradation of the Organophosphorus
Cyanophos in Aqueous Solutions
Matthieu Menager1, Jean Franc¸ois Pilichowski1,2 and Mohamed Sarakha*1
1
´
Clermont Universite, Universite Blaise Pascal, Laboratoire de Photochimie Moleculaire et
Macromoleculaire, UMR CNRS 6505, Clermont Ferrand, France
`
CNRS, UMR 6505, Aubiere, France
´
´
´
2
Received 12 June 2009, accepted 24 September 2009, DOI: 10.1111 ⁄ j.1751-1097.2009.00654.x
mation with solar light via direct and ⁄ or indirect photoreac-
ABSTRACT
tions (9–13). It is then of great interest to know to what extent
they can be degraded by such unavoidable processes and to
elucidate the by-products that are photogenerated and which
can be in some cases more harmful than the parent substrate.
Several pesticides absorb solar light (k > 295 nm) and can
therefore undergo direct photochemical dissociation leading to
the formation of various by-products. When the pollutant does
not absorb solar light, it may still efficiently undergo photo-
chemical transformations via indirect photoprocesses.
Cyanophos (O,O-dimethyl O-4-cyanophenyl phosphoro-
thioate; CYA) is an insecticide from the organophosphorus
family with a commercial name of Cyanox (14). Owing to its
toxic level, its use is avoided in developing countries. It is
already dropped from the list of some European countries.
Several studies have reported that such compounds are not
easily hydrolyzed and thus they are highly persistent and
accumulate in various aquatic compartments such as rivers
and lakes (15,16). As shown in Scheme 1, the aerobic
metabolism of CYA in water-sediment systems undergoes
Photodegradation in aqueous solutions is an important pathway
for many agrochemicals such as pesticides. In the present work,
the photochemical transformation of cyanophos (CYA) was
investigated in aqueous solutions using UV light within the 254–
313 nm range as well as solar light. The study was performed in
order to have a deep insight into the mechanistic pathways for
the photochemical disappearance of CYA. Upon UV irradiation
of an aerated solution of CYA, the degradation quantum yield
was found equal to 1.8 · 10)2. It is independent of the excitation
wavelength but varies with oxygen concentration. It increased by
a factor of 2 from oxygen-saturated to oxygen-free solution.
Photosensitized experiments were performed using acrylamide
and hydroquinone as energy acceptor and energy donor sub-
strates, respectively. They show that both singlet and triplet
excited states were involved in the photochemical behavior of
CYA. The laser flash photolysis experiments clearly showed the
involvement of the triplet excited state which was efficiently
quenched by molecular oxygen and acrylamide with the rate
constants 1.97 · 109 and 2.71 · 109 mol)1 L s)1, respectively.
The photoproducts structures were proposed according to the
mass spectral data using the LC ⁄ MS technique. The analytical
study shows that various processes such as hydrolysis, homolytic
bond dissociations and Photo-Fries process occur.
Cynophos
INTRODUCTION
The increasing use of pesticides and other man-made chemicals
represents the main cause of contamination of soils, ground
and surface waters. As these compounds show a relatively high
persistence, they may cause serious health and environmental
problems that must be prevented and controlled (1–3). An
important effort with great success has been made for the
detection of these pollutants in the environment even at trace
concentrations (4–8). However, the study of their fate and even
their removal from environmental compartments appear to be
urgent ecological problems which have to be seriously under-
taken. Many of these pollutants, and more particularly
pesticides which are present in aqueous solutions as well as
at the surface of soils, can undergo photochemical transfor-
*Corresponding author email: mohamed.sarakha@univ-bpclermont.fr
(Mohamed Sarakha)
ꢀ 2009 The Authors. JournalCompilation. The AmericanSocietyofPhotobiology 0031-8655/10
Scheme 1. Proposed degradation pathways in water sediment systems
(17).
247

248 Matthieu Menager et al.
Scheme 2. Photodecomposition pathways for Cyanox in silica and soil (18).
transient absorbance at preselected wavelength was monitored by a
detection system consisting of a pulsed xenon lamp (150 W), mono-
chromator and a 1P28 photomultiplier. A spectrometer control unit
was used for synchronizing the pulsed light source and programmable
shutters with the laser output. This also housed the high-voltage power
supply for the photomultiplier. The signal from the photomultiplier
was digitized by a programmable digital oscilloscope (HP54522A). A
32-bit RISC-processor kinetic spectrometer workstation was used to
analyze the digitized signal.
Analyses. UV–visible spectra were recorded on a Cary 300 scan
(Varian) spectrophotometer. LC ⁄ MS studies were carried out with Q-
TOF-Micro ⁄ water 2699 from the CRMP center at University Blaise
Pascal. It is equipped with an electrospray ionization source (ESI) and
a Waters photodiode array detector. Each single experiment permitted
the simultaneous recording of both UV chromatogram at a preselected
wavelength and an ESI-MS full scan. Data acquisition and processing
were performed with the MassLynx NT 3.5 system. Chromatography
was run using a Nucleosil column 100-5 C18 ec (250 · 4.6 mm, 5 lm).
Samples (5–10 lL) were injected either directly or after evaporation of
an appropriate volume of the solvent for better detection. The flow rate
was 0.3 mL min)1 and the injected volume was 50 lL. The elution was
accomplished with acidified water (acetic acid 0.4%) and acetonitrile
using the following gradient mode: % acetonitrile (time): 2 (initial); 10
(10 min); 70 (35 min); 2 (42 min).
the cleavage of the P–O-methyl and P–O-aryl bonds together
with the oxidation of the P=S to the oxon group and the
hydrolysis of the cyano moiety (17).
From the photochemical point of view, some studies were
performed on solid support such as silica and soil and in
acetone solution (18). Under illumination, the half-life was
estimated to be 2 and 4 days on soil and silica, respectively,
while it was evaluated to be 120 min in acetone solution.
Photo-oxidation reaction leading to the formation of the
oxon derivative and the scission of the P–O bond generating
4-cyanophenol, as a major product, are reported to be
the main observed processes (Scheme 2). To our knowl-
edge, no detailed studies in aqueous solutions have been
undertaken.
The present work was conducted in order to get a better
insight into the photochemical behavior of CYA in aqueous
solutions. The main goal was the study of the photochemical
kinetic aspect by elucidating the nature of the by-products. The
use of both conventional and time-resolving techniques will
permit us to propose a detailed mechanism for the disappear-
ance of CYA and the photogeneration of the products.
The consumption of CYA and the formation of the by-products were
monitored by an analytical HPLC using an HP1050 apparatus equipped
with a photodiode array detector. The experiments were performed by
UV detection at either 230 or 250 nm and by using a reverse-phase
MachereyNagelcolumn(NucleodurC8, 250 mm · 4.6 mm, 5 lm). The
following gradient program was used by employing acidified water
MATERIALS AND METHODS
Materials. CYA or (O-(4-cyanophenyl) O,O-dimethyl phosphoto-thio-
ate) (98%) was purchased from Aldrich. Cyanophenol was from
Jansen Chimica. Acrylamide and hydroquinone (>99%) were pro-
vided by Aldrich. They were all used as received without further
purification. All other reactants were of the highest grade available.
The solutions were prepared with deionized ultra pure water which was
purified with the Milli-Q device (Millipore) and its purity was
controlled by its resistivity.
(acetic acid 0.4%) (1) and acetonitrile (2) at 1 mL min)1
.
Time (min)
Initial
3
13
20
30
% A
% B
95
5
80
20
80
20
5
95
95
5
Steady-state irradiations. For kinetic as well as analytical purposes,
aqueous solutions were irradiated with a parallel beam using a xenon
arc lamp (1600 W) equipped with a Schoeffel monochromator. The
bandwidth was 10 nm. Solution in a quartz cell (1 cm optical path
length) was deoxygenated by argon or nitrogen bubbling or oxygen-
ated by oxygen bubbling for 20 min prior to irradiation. The cell was
then closed using a septum. The initial concentration of the solution
was checked by HPLC analysis after bubbling. The irradiations at
254 nm were obtained with PHILIPS TUV 6 W lamp delivering a
pH measurements were carried out with a JENWAY 3310 pH-
meter equipped with an Ag ⁄ AgCl glass combination electrode 9102
Orion. The pH of the solutions was adjusted using dilute solutions of
HCl or NaOH. The accuracy achieved was within 0.01 pH units.
RESULTS
parallel beam. Potassium ferrioxalate was used as
a chemical
actinometer as reported in the literature (19). The pH of the
solutions was adjusted using dilute solutions of HClO4 or NaOH.
For analytical purposes, irradiations were performed in a device
equipped with germicide lamps (up to 6) emitting at 254 nm and a
100 mL cylindrical quartz reactor. A similar setup was used for the
irradiation at 313 nm. Solar irradiation was performed in a quartz
reactor (d = 1.5 cm) in Clermont-Ferrand (France, latitude 420ꢁ
above sea level).
Steady-state irradiation studies
The absorption spectrum of CYA in aqueous solution shows
two well-defined bands (Fig. 1). The short wavelength absorp-
tion band, attributed to a p–p* transition, presents an
absorption maximum at 232 nm with a molar absorption
coefficient of 14 000 mol)1 L cm)1
.
The second band
Laser flash photolysis. Transient absorption experiments in the
20 ns to 400 ls time scale were carried out on a nanosecond laser flash
photolysis spectrometer from Applied Photophysics (LKS.60). Exci-
tation (k = 266 nm) was from the fourth harmonic of a Quanta Ray
GCR 130-01 Nd:YAG laser (pulse width ꢀ5 ns), and was used in a
right-angle geometry with respect to the monitoring light beam. A
3 cm3 volume of an argon-saturated solution was used in a quartz cell,
and was stirred after each flash irradiation. Individual cell samples
were used for a maximum of five consecutive experiments. The
(kmax = 266 nm and ꢀ = 1200 mol)1 L cm)1) is owing to an
n-* transition and clearly extends to 320–330 nm permitting a
non-negligible overlap with the solar emission spectrum. Thus,
CYA may be degraded to a certain extent by direct excitation
from solar light. It is worth noting that no obvious degrada-
tion of aqueous solution of CYA in the dark and at room
temperature was observed within the 0.1–2.0 mmol L)1 range.

Photochemistry and Photobiology, 2010, 86 249
Table 1. Disappearance quantum yield of CYA as
a function
of oxygen concentration ([CYA] = 8.0 · 10)5 mol L)1; pH = 5.4;
kexcitation = 254 nm). The concentrations of oxygen were taken from
Murov et al. (35).
Disappearance
quantum yield
[O2] (mol L)1
)
1.27 · 10)3
2.7 · 10)4
<10)5
1.4 · 10)2
1.8 · 10)2
2.7 · 10)2
clearly demonstrates that the excitation within both bands,
namely n–p* and p–p*, leads more likely to the same
photochemical behavior.
The effect of oxygen concentration was also studied by
bubbling either argon or oxygen. The determined quantum
yields, gathered in Table 1, show that the photochemical
reaction was partially inhibited by molecular oxygen. The
quantum yield was lower by a factor of 2 from oxygen-free
aqueous solution to oxygen-saturated solution. This clearly
indicates that the two excited states, namely singlet and triplet
excited states, are involved in the CYA photochemical process.
The irradiation of CYA was also realized using solar light
and we found an efficient degradation. The complete disap-
pearance occurred after 500 min in a sunny day.
Figure 1. UV absorption spectrum of cyanophos in aqueous solution
at pH = 5.4.
This suggests that dark processes such as hydrolysis are
negligible in agreement with the literature (15,16).
Figure 2 gives the typical evolution of the absorption
spectrum as a function of irradiation time when an aerated
aqueous solution of CYA was exposed to light within the 254–
313 nm range. It clearly shows a rapid decrease in the 232 nm
band from the early stages of irradiation, indicating the
disappearance of the pesticide. Moreover, owing to the
formation of by-products, an increase in absorbance is also
seen at k > 250 nm.
Laser flash photolysis studies
The disappearance kinetics of CYA was further studied
using HPLC. In all cases, the degradation process appeared to
fit well the first-order kinetic model. As an example, under
excitation at 254 nm (Fig. 2), the total disappearance was
observed after roughly 6 h irradiation time with a half-life of
110 min. In air-saturated solutions, the disappearance quan-
tum yield at 254 nm was evaluated at 1.8 · 10)2. It was found
to be independent of the initial concentration of the pesticide,
of the pH of the solution and also of the excitation wavelength
within the studied range (254–313 nm). Contrary to several
other organophosphorus compounds (20,21), the latter result
In order to get a better insight into the transformation
mechanism, we undertook nanosecond laser flash photolysis
experiments. The laser excitation from the fourth harmonic
(266 nm) of Nd:YAG laser of an argon-saturated aqueous
solution of CYA (1.1 · 10)4 mol L)1) at pH = 5.4 led to the
formation of at least two short-lived transients (Fig. 3a).
The spectral features of solvated electrons were clearly
observed at the end of the laser pulse, namely an absorption
maximum at 720 nm (22,23). The absorption band was not
observed when the solution was bubbled with N2O, used as
electron trap. As clearly shown in Fig. 3b, the amount of
solvated electrons did not vary linearly with pulse energy
showing that their formation, under our experimental condi-
tions, is mainly through a biphotonic process. However, at the
end of laser pulse in the presence of N2O, a second short-lived
transient was observed. It presents a band with a maximum
located at 280 nm and two shoulders at 370 and 510 nm. Its
absorbance increased linearly with laser energy, indicating its
formation via a monophotonic process (Fig. 3b). The disap-
pearance of this latter species followed a pseudo first-order
kinetics with a rate that depended on oxygen concentration. In
N2O-saturated solution, the decay rate constant (kobs) was
found equal to 1.7 · 105 s)1 while it was evaluated at 7.6 · 105
and 2.7 · 106 s)1 in aerated and oxygen saturated solution,
respectively. Such a transient reacts with oxygen as energy
acceptor and it can more likely be attributed to the triplet
excited state.
In order to obtain evidence for the formation of the triplet
state in the photoreactivity of CYA, we used another energy
acceptor substrate instead of oxygen. Acrylamide has largely
been employed for this purpose in the literature (24,25). The
laser excitation at 266 nm of an argon-saturated aqueous
Figure 2. UV–Vis absorption changes observed for the irradiation of
aerated solution of cyanophos (7.8 · 10)5 mol L)1) upon irradiation
with light in the 254–313 nm range at pH = 5.5. Inset shows the
disappearance of cyanophos obtained by HPLC measurements as a
function of irradiation time.

250 Matthieu Menager et al.
Figure 3. (a) Transient absorption spectra from an argon-saturated aqueous solution (D) and N2O-saturated aqueous solution (s) of CYA under
excitation at 266 nm (1.1 · 10)4 mol)1 L, pH = 5.4). (b) Evolution of the absorbance at 275 and 720 nm as a function of laser energy.
solution of the mixture acrylamide (2.0 · 10)3 mol L)1) and
CYA (1.1 · 10)4 mol L)1) at pH = 5.4 led to a rapid decay of
the absorbance at 275 nm, indicating that an energy transfer
process is efficiently operating and that the triplet excited state
was formed upon excitation of CYA.
The second-order rate constant for the reaction between
molecular oxygen and the triplet excited state may be
determined from the following kinetic plot:
given concentration of X, kq is the second-orderrate constant for
the energy transfer process and k0 is the first-order rate for the
decay of the triplet excited state in the absence of X.
As shown in Fig. 4, the plots kobs as a function of [X] are
linear and present a similar value of ko = 1.7 · 105 s)1. The
second-order rate constant for the energy transfer process was
found equal to 1.9 · 109 and 2.9 · 109 mol)1 L s)1 for molec-
ular oxygen and acrylamide, respectively.
It is worth noting that in deaerated solution, the complete
disappearance of the triplet excited state gave rise to a long-
lived transient T2. It presents an absorption band with a
maximum at 400 nm (Fig. 5a) and disappears within 200 ls.
Its concentration decreased by increasing the concentration of
oxygen (Fig. 5b). This is in complete agreement with the fact
that the triplet excited state is the precursor of T2.
kobs ¼ kq½Xꢁ þ ko
where [X] represents the concentration of either oxygen or
acrylamide, kobs is the observed rate constant in the presence of a
Photosensitized process
The photosensitized process through an energy transfer
mechanism was further shown to occur using CYA as an
energy acceptor and hydroquinone as a donor substrate
(26,27). The latter compound was used as sensitizer as
benzophenone and acetophenone were not sufficiently water
soluble under our experimental conditions. Under excitation,
hydroquinone is known to generate its triplet excited state with
a quantum yield of 0.39 (28). Steady-state irradiation of
hydroquinone (9.0 · 10)4 mol L)1) using a monochromatic
light at 300 nm in the presence of CYA (2.3 · 10)4 mol L)1
)
was performed in aqueous solution. Under these experimental
conditions, the light absorption is mainly owing to hydroqui-
none (kmax = 286 nm). We observed an efficient disappear-
ance of CYA indicating that the energy transfer from the
triplet state of hydroquinone to that of CYA occurred. The
Figure 4. Evolution of the observed rate constant, kobs, a function of
either oxygen or acrylamide concentration (excitation at 266 nm;
[CYA] = 1.1 · 10)4 mol L)1, pH = 5.4).
Figure 5. (a) Absorption spectra of T2 recorded 10 ls after laser pulse. [CYA] = 1.1 · 10)4 mol L)1, pH = 5.4, kexcitation = 266 nm.
(b) Evolution of absorbance at k = 440 nm on 10 ls for various concentrations of oxygen. [CYA] = 1.1 · 10)4 mol L)1, pH = 5.4,
kexcitation = 266 nm.

Photochemistry and Photobiology, 2010, 86 251
quantum yield of such energy transfer process was evaluated at
0.05.
the concentration of oxygen the higher the percentage of
4-cyanophenol obtained. The conversion was estimated to
be 42% in oxygen-saturated solution. The irradiation of
CYA in the presence of acrylamide gave a negligible effect
on the amount of CYA. It is worth noting that the formation
of 4-cyanophenol was negligible when the irradiation was
performed in acetonitrile as a solvent instead of water. Thus,
its formation mainly involves the singlet excited state more
likely through a photohydrolysis process as shown with
several types of organophosphorous pesticides (15,29,30).
Such reaction may more likely occur through a heterolytic
mechanism.
Analytical studies
Several techniques were tested to track and to identify the
stable products formed during the steady-state photolysis
of CYA aqueous solutions, e.g. HPLC, HPLC ⁄ ESI ⁄ MS
and NMR. Due to the nature of the pesticide used in this
study, the latter technique was tested using 31P-NMR but
because of the limit of sensitivity at the employed con-
centrations, no significant results were obtained. Thus, the
analytical studies were mainly performed by HPLC and
HPLC ⁄ ESI ⁄ MS.
Aerated aqueous solutions of 80 lmol L)1 CYA were
irradiated at 254 nm and analyzed by HPLC. A typical
chromatogram obtained for about 30% conversion of CYA
(Fig. 6) shows various stable by-products.
The same products were observed when the excitation was
performed at 300 nm and also by using solar light. Their
concentration increased as a function of irradiation time. The
elucidation was realized by undertaking HPLC ⁄ ESI ⁄ MS
experiments (with both ESI-negative and -positive modes).
The system was equipped with a diode array system that
permits to obtain the mass spectrum together with the
absorption spectrum. It should be noted that as products P2,
P3 and P7 were observed in the initial solution and as their
concentrations were unchanged or decreased during irradia-
tion, they will not be discussed. Table 2 gathers the main
fragments observed in the mass spectrum and the UV features
of tentative degradation products.
The by-product P5 presents an absorption spectrum similar
to that of CYA. The molecular fragment, as well as some main
fragments, is similar to those obtained in the case of CYA but
with a difference of 16 units. This is in complete agreement
with the substitution of the sulfur atom by an oxygen atom
giving rise to the formation of the oxon derivative as largely
observed with organophosphorus compounds (18,31). Under
our experimental conditions, its formation is highly inhibited
in the presence of oxygen and also in the presence of
acrylamide showing that the triplet excited state is more likely
involved. Such oxidative reaction occurs in competition with
the deactivation of the triplet state by oxygen as demonstrated
by laser flash photolysis studies.
P4 was identified as 4-cyanophenol. Its presence was further
confirmed by using authentic samples. It roughly represents
15% of the CYA conversion. The formation of 4-cyanophenol
appeared to depend on the concentration of oxygen: the higher
The product P6 presents a molecular mass similar to that of
the initial compound CYA. It shows a different UV spectrum
but a similar fragmentation in the ESI+ mode leading us to the
conclusion that an isomer of CYA was formed. It is more likely
generated via the singlet excited state as no effect of oxygen was
observed. The experimental data clearly show that its formation
involves a pathway different from that leading to the formation
of 4-cyanophenol: (1) the rate of its formation does not depend
Figure 6. HPLC chromatogram recorded after 90 min irradiation of
cyanophos (80 lmol L)1
) in air-saturated solution. kexcitation =
254 nm; pH = 5.4. The observed drift of the baseline is due to the
used gradient program. Its shape was unchanged when water alone
was injected.

252 Matthieu Menager et al.
Table 2. Mass spectrum features and UV absorption maxima of tentative by-products obtained by UV irradiation of cyanophos in aqueous
solution. The features of cyanophos are also given for comparison.
Chemical structure
CYA
Rt (min)
kmax (nm)
[M + H+] with ESI+
[M ) H+] with ESI)
Main fragments
206; 175; 182; 162;154
228; 182; 166; 141
23
233; 266
244
242
P1
P4
Not identified
5.1
232; 274
246
N.S.
N.S.
182
188
118
10.6
No fragmentation
P5
13.8
232; 270
228
244
N.S.
N.S.
190; 175; 166;154; 146
P6
16
236; 255 (sh)
182; 162; 154
P8
18.5
236
N.S.
261
217; 141
Rt = retention time; sh = shoulder. N.S. = no signal. The bold values refer to the molecular ion.
on acrylamide concentration and (2) its formation was not
completely inhibited as acetonitrile was used as a solvent instead
of water. In agreement with the results obtained with various
other pesticides such as carbamates (32) and phenylureas (33) a
photo-Fries type reaction through the homolytic dissociation of
the P–O bond can be proposed.
lived zwitterionic intermediate (T2) arising from the triplet
excited state, as observed from laser flash photolysis studies,
followed by its hydrolysis may be the precursor of the
carbamoyle derivative. The latter products may be further
hydrolyzed or photolyzed as reported in various studies
(17,34) to yield P8. It is worth noting that in our study no
photoisomerization into the benzoisonitrile derivative was
observed, showing that it is not stable under our experi-
mental conditions.
The formation of P8 involves the reactivity on the cyano
group of the pesticide. The presence of the fragment ion at 217
is in agreement with a decarboxylation process as largely
observed with carboxylic compounds. The formation of P8 is
arising from the triplet excited state pathway as it was totally
absent when the irradiation was performed in the presence of
oxygen as well as in the presence of acrylamide. It has to be
pointed out that it represented the main by-product when the
degradation was sensitized by excitation of hydroquinone at
300 nm. Moreover, while performing the sensitized experi-
ments, a new product appeared to be present. It was identified
on the basis of its mass spectrum. Its molecular fragment was
found at 262 together with a unique fragment at 245. It was
identified as O-(4-carbamoylphenyl) O,O dimethyl thiophos-
phate (Table 3).
Table 3. Chemical structure and the elucidation of the fragment at
245.
The mechanism for the formation of these by-products
may be drawn using the mechanistic study reported in the
literature on 4-cyanophenol (34). The formation of a long-

Photochemistry and Photobiology, 2010, 86 253
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CONCLUSION
The irradiation of the organophosphorus CYA with UV light
k > 254 nm as well as solar light led to an efficient disap-
pearance of the pesticide. The process was shown to be
partially dependent on oxygen concentration. Both singlet and
triplet excited states were involved. The degradation of CYA
was further demonstrated by using hydroquinone as photo-
sensitizer. The light excitation of CYA permitted the forma-
tion of various by-products via oxidation, hydrolysis,
homolytic dissociations and photo-Fries processes. The oxi-
dation reaction leads to the formation of the highly toxic oxon
derivative.
14. Mullie, W. C., A. O. Diallo, B. Gadji and M. D. Ndiaye (1999).
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