The phototransformation of 4-chloro-2-methylphenoxyacetic acid under KrCl and XeBr excilamps irradiation in water

Article abstract of DOI:10.1016/j.jphotochem.2011.11.004

The effect of UV radiation of a KrCl and a XeBr on the photodegradation of 4-chloro-2-methylphenoxyacetic acid (MCPA) was studied. The main photoproducts were identified by gas chromatograph/mass spectrometry (GC/MS). The variation of chlorine-ion and active chlorine in MCPA aqueous solution exposed to excilamp radiation was also defined by analytical methods. Irradiation of MCPA solution with a KrCl excilamp emitting mainly at 222 nm yield 2-methylhydroquinone and lactone of 2-hydroxy-3-methyl-5-chlorophenoxyacetic acid as the main photoproducts. Irradiation of MCPA solution with a XeBr excilamp emitting mainly at 283 nm yield 2-methylhydroquinone as the main photoproduct. Biological processes are not suitable for MCPA removal due to low or total absence of biodegradability of this class of pollutants. Estimation of biodegradability of phototreated MCPA solution was carried out according to ratios of biological oxygen demand (BOD₅) to chemical oxygen demand (COD). The biodegradability of MCPA solutions increased after irradiation.

Full text of DOI:10.1016/j.jphotochem.2011.11.004

Journal of Photochemistry and Photobiology A: Chemistry 228 (2012) 8–14
Contents lists available at SciVerse ScienceDirect
Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
The phototransformation of 4-chloro-2-methylphenoxyacetic acid under KrCl
and XeBr excilamps irradiation in water
Olga N. Tchaikovskayaa,b,∗, Elena A. Karetnikovac, Irina V. Sokolovaa,b, Georgy V. Mayera,
Dmitry A. Shvornevd
a Laboratory of Photophysics and Photochemistry of Molecules, Department of Physics, Tomsk State University, 36, Lenine Ave, Tomsk 634050, Russia
b Department of Photonics, Siberian Physical Technical Institute at Tomsk State University, 1, Novosobornaya Sq., Tomsk 634050, Russia
c Laboratory of Ecological Biotechnology, Institute of Water and Ecological Problems of the Far Eastern Branch of Russian Academy of Sciences,
65, Kim-Yu-Chen St., Khabarovsk 680000, Russia
d Khabarovsk Referent Centre, Federal Service for Veterinary and Phytosanatory Surveillance, 65, K. Marks St., Khabarovsk 680000, Russia
a r t i c l e i n f o
a b s t r a c t
Article history:
The effect of UV radiation of
a KrCl and a XeBr on the photodegradation of 4-chloro-2-
Received 24 August 2011
Received in revised form
11 November 2011
Accepted 12 November 2011
Available online 16 December 2011
methylphenoxyacetic acid (MCPA) was studied. The main photoproducts were identified by gas
chromatograph/mass spectrometry (GC/MS). The variation of chlorine-ion and active chlorine in MCPA
aqueous solution exposed to excilamp radiation was also defined by analytical methods. Irradiation of
MCPA solution with a KrCl excilamp emitting mainly at 222 nm yield 2-methylhydroquinone and lac-
tone of 2-hydroxy-3-methyl-5-chlorophenoxyacetic acid as the main photoproducts. Irradiation of MCPA
solution with a XeBr excilamp emitting mainly at 283 nm yield 2-methylhydroquinone as the main
photoproduct. Biological processes are not suitable for MCPA removal due to low or total absence of
biodegradability of this class of pollutants. Estimation of biodegradability of phototreated MCPA solution
was carried out according to ratios of biological oxygen demand (BOD5) to chemical oxygen demand
(COD). The biodegradability of MCPA solutions increased after irradiation.
Keywords:
UV irradiation
MCPA
Photolysis
Photoinduced biodegradation
Wavelength effect
Crown Copyright © 2011 Published by Elsevier B.V. All rights reserved.
1. Introduction
and biological factors. Microorganisms play the major role among
these factors as they directly contribute to the degradation of pollu-
Phenoxyacetic acid derivatives are widely used soluble herbi-
cides. MCPA (4-chloro-2-methylphenoxyacetic acid) is commonly
used for soil treatment in agriculture [1]. As the result chlorine-
containing organic products are formed in natural waters and
influence remediate processes. According to world ecological
statistics many pesticides are included in to the so-called “disgust-
ing dozen” group of ecotoxicants. Depending on the total herbicide
concentration, dampness, oxygen regime and organic soil com-
pounds, the time of the semi-decomposition of MCPA may take
from several weeks to a few months and may cause the forma-
tion of pollutants of secondary toxicity in the environment [2]. The
decrease of dampness, oxygen concentration and temperature lead
to the inhibition of MCPA decomposition in soil [3,4]. It is known
that in the ecosystems processes of transformation and utilization
of organic compounds take place under certain physical, chemical
tants of different composition and origin [3]. Microbial degradation
as an important removal force of many organic toxicants is used in
coupling systems for wastewater treatment. Usually these systems
are based on physico-chemical oxidizing processes and biodegra-
dation.
Modern environment protection technologies of sewage water
treatment necessarily include photoreactors with UV sources [4].
According to literature reports on pesticide photodegradation [5]
direct photolysis of organic toxicants was reviewed under typi-
cal high and middle pressure conditions of mercury lamps. The
MCPA transformation as a result of direct photolysis and photocat-
alytic processes under these UV-sources were investigated [6–8].
Although detailed structures of basic MCPA photoproducts were
shown, unfortunately the effect of radiation wavelength on their
formation mechanism was not established [7]. It is known that
phototransformation of some pollutants can lead to the formation
of more soluble and more toxic products than parent compounds
[9,10]. The employment of the artificial UV irradiation may lead
to the secondary pollution with extremely toxic substances. Data
on MCPA photoproduct toxicity and stability to further micro-
bial degradation are not fully reported. The European Scientific
∗
Corresponding author at: Siberian Physical Technical Institute at Tomsk State
University, 1, Novosobornaya Sq., Tomsk 634050, Russia. Tel.: +7 3822 533426;
fax: +7 3822 533034.
E-mail address: tchon@phys.tsu.ru (O.N. Tchaikovskaya).
1010-6030/$ – see front matter. Crown Copyright © 2011 Published by Elsevier B.V. All rights reserved.
doi:10.1016/j.jphotochem.2011.11.004

O.N. Tchaikovskaya et al. / Journal of Photochemistry and Photobiology A: Chemistry 228 (2012) 8–14
9
Committee on Toxicity, Ecotoxicity and the Environment (CSTEE
2001) has proposed a draft on the environmental risk assessment
of medical and chemical products. To need to take into account
not only herbicide, but also their phototransformation products
is recommended. Although the hydrolytic and photolytic cleavage
of chlorine containing pesticides and drugs widely used, the main
degradation products toward aquatic organisms were toxic at the
expense of the formation of a radical intermediates [11,12].
Therefore nowadays studies of the effectiveness of modern UV
sources are still very important. The irradiation of these sources
is absorbed by the high-lying electronically excited states of the
organic molecules and may lead to the influence of radiation
wavelength on optimal canals of the molecule phototransforma-
tion. Such sources are a KrCl and a XeBr excilamps [13]. Besides
the results of MCPA phototransformation in water under the
excilamps it is actually interesting to determine the effect of pho-
toproduct toxicity on sequential biological treatment. Biological
processes do not always appear relevant for the removal of chlo-
rinated photoproducts due to low biodegradability of this class of
pollutants.
The aim of the work was to determine the content and structure
of MCPA phototransformation and chlorine-ion and active chlorine
concentrations in MCPA water solution exposed to a KrCl and a XeBr
excilamps, and also to estimate biodegradability of phototreated
MCPA solutions.
2. Materials and methods
2.1. Phototreatment conditions
4-Chloro-2-methylphenoxyacetic acid (95%) was supplied by
Aldrich. Distilled water was used to prepare MCPA solutions. The
initial concentration of MCPA was 2 × 10−3 M.
As sources of UV radiation for photochemical transformation,
we used a pulsed U-type barrier discharge excilamps with working
KrCl (the wavelength of irradiation: ꢀrad ∼ 222 nm) and XeBr
(ꢀrad ∼ 283 nm) molecules. The parameters were ꢁꢀ = 5–10 nm,
peak output power Wpeak = 18 mW cm−2, f = 200 kHz, and pulse
duration 1 ␮s. The incident light intensity at 283 nm was eval-
uated at 3 × 1015 photons s−1 cm−2 by potassium ferrioxalate
actinometry.
Fig. 1. Kinetics of MCPA disappearance and formation of 4-methylhydroquinone in
an air-saturated solution irradiated a KrCl and a XeBr excilamps: initial concentra-
tion 2 × 10−4 M (a) and 2 × 10−3 M.
Aromatic aldehydes were visualized as a orange spots after being
sprayed and 2,4-dinitrophenylhydrazine (0.1%) in 2 M HCl.
2.2. Chemical analyses
2.3. Assessment of biodegradability
Concentrations of chloride ion, active chlorine and quinones
were maintained with standard procedures [14]. Chloride ion was
determined by titration with Hg(NO3)2 [14] in the presence of
diphenylcarbazon; active chlorine measurements were carried out
in burette with methyl orange; a Thermo Evolution 600 (Var-
ian, United States) spectrophotometer with benzolsulfonic acid at
320 nm was applied to analyze the concentration of quinones.
Irradiated and non-irradiated MCPA solutions were extracted
with diethyl ether after acidification to pH 2. The extraction was
repeated twice. The extracts were mixed and evaporated to volume
1.0 ml. The extracted solutions ware analyzed with an ULTRA TRACE
DSQ gas chromatograph with a MS detector (Thermo Electron
Chromatography and Mass Spectrometry Division, United States).
Determination conditions were as follows: column—Trace TR-5MS;
temperature—100 ◦C (5 min); heating at a rate of 10 ◦C min−1 to
180 ◦C (5 min); heating at a rate of 100 ◦C min−1 to 300 ◦C (1 min);
carrier gas—helium.
The ratio of biological oxygen demand (BOD5) to chemical
oxygen demand (COD) is widely used for assessment of biodegrad-
ability of wastewaters [15,16]. This ratio was also applied to
estimate biodegradability of phototreated MCPA solutions. The
activated sludge (AS) was used as inocula for this procedure. The
sample of AS was collected at the sewage treatment facilities of
the Tomsk Petrochemical Enterprise (Siberia, Russia). After sam-
pling the sludge was being shaken for 8 h and then liquid phase
was separated by filtration (membrane filter with pore size 1.2 nm,
Vladipor, Russia). Separation of bacteria from AS liquid fraction
was carried out by filtration through the membrane (pore size
0.2 nm) and washing with sterile NaCl solution (0.5%) to remove
organic compounds from AS. Microorganisms were washed away
from the membrane filter with a sterile NaCl solution to be further
used in experiments. BOD5 and COD calculations were performed
according to standard methods (dilution method) for water and
wastewater examination [17].
Also the separation of photoproducts in the ether extract was
carried by thin layer chromatography (TLC). The analyses were
performed on a silica gel plates (Sorbifol, Russia). Following sol-
vent system was used—toluene:1,4-dioxane:acetic acid (90:20:4).
For investigation of sequential photo- and biodegradation of
MCPA (2 × 10−3 M) 50 ml phototreated and non-phototreated (as
a control) solutions were placed in conical flasks (250 ml). Liq-
uid fraction of AS (prepared by filtration through 1.2 nm pore

10
O.N. Tchaikovskaya et al. / Journal of Photochemistry and Photobiology A: Chemistry 228 (2012) 8–14
Fig. 2. Mass spectrum of the photoproduct (with MW 182) in MCPA solution after 30 min of KrCl excilamps irradiation (a) and lactone of 2-hydroxy-3-methyl-5-
chlorophenoxyacetic acid obtained by Prof. A. Zertal (b).
The incident light intensity at 222 nm was at 2 × 1015 photons s−1
size membrane) was added (50 ml) to each sample of MCPA
solutions.
cm−2. The intensity values exclude the possibility of multiphoton
processes and photoionization in the studied systems.
3. Results and discussion
3.2. GC/MS analysis of phototreated MCPA solutions with
concentration 2 × 10−4 M
3.1. Efficiency of energy absorption by MCPA solutions treated
with excilamps
Earlier 2-methylhydroquinone, 2-methylcathehol, 2-methyl-
phenol, 2-methylresorcinol, 2,6-dimethylhydroquinone were
The selection of excilamp irradiation wavelength is based on
the fact that ꢀrad ∼ 222 nm is absorbed by the high-lying electron-
ically excited states of MCPA. It is possible to make a transfer from
these states to photodissociative states which take place in the pho-
tocleavage of O–H, O–C and C–Cl bonds. It causes the increase of
photolysis efficiency of organic molecules [18,19]. The effect of UV
radiation of a XeBr excilamp at ꢀrad ∼ 283 nm activates the direct
population of photodissociative states which contribute to the
effectiveness of the reaction of the photobreak of C–Cl bond [18,20].
The operating parameters of lighting sources are listed in Table 1.
From UV–vis spectrum of MCPA in aqueous solution two absorp-
tion maxima are located at 278 nm and 226 nm [18]. The radiation
energy (66%) of a KrCl excilamp was concentrated at 222 nm. The
remaining energy was radiated at 196, 235, 240 and 257.8 nm. The
radiation energy (66%) of a XeBr excilamp was concentrated at
283 nm. The remaining energy was radiated at 224, 315 and 360 nm.
Table 1
The accumulated energy in MCPA solutions during KrCl and XeBr excilamps
treatment.
Solution
Time of
phototreatment (min)
Output energy
(J/cm3)
KrCl
XeBr
15
30
60
15
8
15
30
60
1.30
2.61
5.20
1.44
0.63
1.19
2.38
4.64
0.09
0.18
0.36
0.17
0.63
1.17
2.35
4.57
1 × 10−4 M MCPA
2 × 10−4 M MCPA
2 × 10−3 M MCPA

O.N. Tchaikovskaya et al. / Journal of Photochemistry and Photobiology A: Chemistry 228 (2012) 8–14
11
OH
OH
OH
Cl
CH3
HO
CH3
CH3
2-methyl catechol
OH
2-methyl-4-chlorophenol 2-methyl hydroquinone
OH
OH
O
CH3
CH3
CH3
OH
2-methyl resorcinol
2-methyl phenol
O
OH
2-methyl-1,4-benzoquinone
OH
Cl
CH3
Cl
CH3
H
O
C
Fig. 4. Mass spectrum of the photoproduct (with MW 148) in MCPA solution after
30 min of XeBr excilamp irradiation.
OH
2-methyl 6-chloro phenol
Cl
2-methyl 4,6-dichloro phenol
comparison our mass spectrum shows that photoproduct with
molecular ion 182 presents lactone of 2-hydroxy-3-methyl-5-
chlorophenoxyacetic acid (see Fig. 2). The dehalogenated lactone
structure of 2-hydroxy-3-methyl-5-chlorophenoxyacetic acid was
suggested from mass spectrum of the photoproduct with molecu-
lar ion 148. Loss of a particle with a mass of 28 a.m.u. confirms the
presence of C O group in molecule (see Fig. 4). Group of peaks at
m/z = 148 and 150 allows to exclude the presence of Cl as A + 2 ele-
ment in molecule [21]. This fact may prove that the photobreak of
C–Cl bond during the direct photolysis may originate both in MCPA
and its photoproducts.
HO
O
CH3
CH2
C
2,5-dihydroxy-3-methyl
benzoaldehyde
O
Cl
O
CH2
C
O
CH3
lactone 2-hydroxy-3-methyl-
5-chlorophenoxyacetic acid
OH
CH3
H3C
CH3
According to mass spectrum the compound (m/z = 152) is a sub-
stituted salicylaldehyde. As mentioned above peak at m/z = 152
could be corresponded with molecular ion and groups of peaks at
m/z = 152 and 154 shows the absence of A + 2 element (like Cl) in
molecular structure. The number of C atoms N in this photoproduct
was determined with using the following formula [21]:
lactone 2-hydroxy 3-methyl
phenoxyacetic acid
OH
2,6-dimethylhydroquinone
100 Im+1
1.1 Im
N =
,
Fig. 3. The structure of MCPA transformation products.
where Im is signal intensity of molecular ion m (% relative abun-
dance), Im+1 is signal intensity of ions with mass m + 1 (% relative
abundance).
In our case m/z = 152 and m + 1/z = 153, according this for-
mula N = 160,000 × 100/1.1 × 1,800,000 = 8. The molecular weight
of compound is 152 and the rest of MW (56 a.m.u.) accounted on H
and O atoms. We can propose the presence of three O atoms and as
a result we had obtained the formula of photoproduct is C8O3H8.
Number of double bonds R in molecule was calculated with
following formula [21]:
identified in MCPA solution (2 × 10−4 M) [20]. In present work,
according to HPLC data, during 2–4 min UV radiating of KrCl or
XeBr excilamps MCPA was completely transformed. Kinetics pre-
sented in Fig. 1a show that MCPA is rapidly degraded. After 30 min
of irradiation photoproducts mentioned above were also fixed in
solution. But only 2-methylhydroquinone was one of dominating
products among these compounds. The others were the minor.
Another main product after UV pre-irradiation also registered in
solution and for this photoproduct m/z = 152 may be corresponds
to molecular ion and molecular weight for this compound is
also 152. Additionally three photoproducts at m/z = 182, 148 and
158 also corresponded to molecular ions were detected in lower
concentrations (see Fig. 2a). The Cl atom in the structure of a
photoproduct appears only in molecules with MW = 182 and 158.
The structure of identified photoproducts is shown in Fig. 3.
Previously Zertal et al. [7] reported that after high pressure mer-
cury lamp irradiation of a MCPA molecular form at pH 1.5 both
2-hydroxy-3-methyl-5-chlorophenoxyacetic acid and correspond-
ing lactone (MW = 182) were detected on the HPLC chromatogram
and identified by MS and 1H NMR. The main photoproduct result-
ing from the irradiation of an anionic form of MCPA at pH
5.9 was 4-hydroxy-2-methylphenoxyacetic acid (see Fig. 2b). In
x − y
z
R =
+
,
2
2
where x is number of C atoms, y is number of H atoms, and z is
number of N atoms.
For this compound amount of double bonds is 4. Presence of
[M-29] peak in mass spectrum apparently corresponds to alde-
hyde group. So this photoproduct with molecular formula C8O3H8
may be identified as 2,5-dihydroxy-3-methyl-benzoaldehyde.
After the photochemical treatment the presence of aldehyde
in solution was confirmed by TLC. As a result, the presence
of aldehyde was detected in the chromatogram with Kf = 0.63.
The possible way of obtaining this compound includes the

12
O.N. Tchaikovskaya et al. / Journal of Photochemistry and Photobiology A: Chemistry 228 (2012) 8–14
Table 2
Variation of concentration of chlorine-ion and active chlorine in MCPA solutions
(C = 2 × 10−3 M) after UV treatment of KlCl and XeBr light.
Irradiation
time (min)
Chlorine-ion (mg/l)
Active chlorine
(mg/l)
XeBr
KrCl
XeBr
KrCl
15
30
60
36 3.2
56 5.0
68 6.1
22 2.0
34 3.1
46 4.1
0.4
1.4
2.0
0.6
0.8
1.3
ꢀrad ∼ 283 nm which passes exactly through C–Cl photocleavage.
But after the prolonged exposure the formation of dichlorinated
products can occur and the effectiveness of further biodegradation
of MCPA may reduce. Therefore to combine techniques of UV radia-
tion and biological treatment the time of pre-irradiation treatment
must be optimally chosen. Photometric method data also reveal
the presence of quinonic derivatives which concentration was high
(3 × 10−4 M) after the UV irradiation for 60 min. The experimental
data demonstrate that the total concentration of quinonic com-
pounds does not depend on the wavelength of the UV sources.
However, according to [7], the disappearance of MCPA was more
efficient in the presence of quinonic products formed as interme-
diates.
According to GC/MS data after 15 min MCPA irradiation 2-
methyl-4-chlorophenol, 2-methylhydroquinone, 2-methylphenol,
2-methyl-1,4-benzoquinone, lactone of 2-hydroxy-3-methyl-5-
chlorophenoxyacetic acid were found in the solutions. After 30
or 60 min of irradiation by XeBr and KrCl excilamps, respectively
3-methyl-2,5-dihydroxybenzoaldehyde appeared in the solutions
and its concentration depended on the irradiation wavelength
(Table 3). As the result of MCPA photolysis not only 2-methyl-4-
chlorophenol but also 2-methyl-6-chlorophenol was formed. The
formation mechanism of 2-methyl-4-chlorophenol is connected
with direct photobreak of the ether bond in the process of pho-
tocatalytic reaction [7,8], and the quinone influence on the MCPA
oxidation [7], and also with the formation of unstable MCPA per-
oxide, and its phototransformation into 2-methyl-4-chlorophenol
and 2-methyl-4-chlorophenoxyformaldehyde [8]. The formation
of 2-methyl-6-chlorophenol might be the result of 2-methyl-
4-chlorophenol photoisomerization. After the long irradiation
of 2 × 10−3 M MCPA solution with both excilamps 2,6-dichloro-
3-methylphenol (Rt = 9.02 min) and 4,6-dichloro-2-methylphenol
(Rt = 11.28 min) were found only in traces and these derivatives
were not the main photoproducts.
After 15 min of irradiation the concentration of MCPA was
reduced by 85–90% and during 15–60 min period a significant part
of energy was spent not only on MCPA degradation, but also on
the transformation of photoproducts. The ratio between peak areas
of photoproducts after 15 min treatment with different excilamps
shows that XeBr was more effective in terms of photoproduct for-
mation (Table 3). Analysis of the dynamics of main photoproduct
concentrations (as a ratio between peaks areas at different time)
shows that under KrCl excilamp irradiation the concentration of
products continued to increase with the increase of the irradiation
time, except for 2-methyl-6-chlorophenol. The concentration of 2-
methylhydroquinone increased markedly (Table 3). Dominant pho-
toproducts after 60 min irradiation were 2-methyl-4-chlorophenol,
lactone of 2-hydro-3-methyl-5-chlorophenoxyacetic acid and
2-methylhydroquinone. In comparison under XeBr excilamp irra-
diation between 15 and 60 min the concentration of photoproducts
decreased with the exception of 2-methylhydroquinone which
was the dominant product together with lactone of 2-hydro-
3-methyl-5-chlorophenoxyacetic acid. This fact indicates that
the dechlorination process of initial photoproducts is effective
at ꢀrad ∼ 283 nm. Therefore, the results of MCPA photolysis at
Fig. 5. Proposed pathway for the formation of 2,5-dihydroxy-3-methyl benzalde-
hyde.
2-methyl-1,4-benzoquinone reduction before the formation of
2.5-dihydroxy-3-methylphenoxyacetic acid (see Fig. 5). Then
decarboxylation of the last one exactly forms 2,5-dihydroxy-3-
methylbenzoaldehyde. But because of the lack of the analytical
standard of 2,5-dihydroxy-3-methylbenzoaldehyde it is impossible
to confirm the presence of this compound in the solution by typical
GC/MS parameters (retention time and mass-spectrum). However,
the detailed study of the formation mechanism of the product is
the goal of our further research. But in any case this photoproduct
is dehalogenated.
Also the concentration of 2-methyl-4,6-dichlorophenol was not
depend on the UV sources. Dichlorinated derivative in the photo-
transformation of MCPA is connected with the presence of active
Cl− ion in solutions. This dichlorinated product was indicated early
during the photolysis of 10−4 M MCPA without mixing [20] after
the exposure of a XeBr excilamp. Concentration of quinonic com-
pounds after 30 min of phototreatment was 1.5–1.6 × 10−5 M for
both types of excilamps.
3.3. GC/MS analysis of phototreated MCPA solutions with
concentration 2 × 10−3 M
MCPA also readily photodegraded under a KrCl or a XeBr photol-
ysis in higher concentration. According to HPLC data irradiation for
30 min after excilamps irradiation resulted in 90% MCPA transfor-
mation in stirring. The disappearance of MCPA in irradiated solution
reported in Fig. 1b. 2-Methylhydroquinone is formed when MCPA
is irradiated with a XeCl excilamp.
The composition of the main products was similar and
not dependent on the irradiation wavelength. After the expo-
sure to UV light 2-methylphenol, 2-methyl-4-chlorophenol,
2-methyl-6-chlorophenol and lactone of 2-hydroxy-3-methyl-5-
chlorophenoxyacetic acid were recorded in the solution by GC/MS
analysis. The direct photolysis under KrCl irradiation produces
to 2-methyl-4-chlorophenol as the main photoproduct. According
to GC/MC data after 15 min irradiation at 283 nm 2-methyl-1,4-
benzoquinone was additionally found in MCPA solution.
Moreover the increase of active chlorine and the Cl ion quantity
in the irradiated MCPA solutions show that there is an effective
reaction of C–Cl bond photocleavage which proves the increase
in chloride ion concentration in the solution. The Cl− evolution
is consistent with the MCPA degradation. In comparison with a
KrCl, using a XeBr excilamp radiation may accelerate this pro-
cess (Table 2). After 60 min of a XeBr excilamp irradiation chloride
ion concentration in the solution was maximal and approximately
equal to the total amount of its mass fraction in MCPA. This fact
confirms the tentative mechanism of direct photolysis of MCPA at

O.N. Tchaikovskaya et al. / Journal of Photochemistry and Photobiology A: Chemistry 228 (2012) 8–14
13
Table 3
Dynamic of photoproducts (Ct/C15) in air-saturated solution of MCPA irradiated at 222 and 283 nm.
No.
Photoproduct
Rt, min
C15 XeBr/C15 KrCl
Irradiation time, min
KrCl
XeBr
15
15
30
60
6.7
6.7
0.2
7.7
3.6
1
30
60
1
2
3
4
5
6
7
2-Methyl-1,4-benzoquinone
2-Methylphenol
2-Methyl-6-chlorophenol
2-Methyl-4-chlorophenol
2-Methylhydroquinone
2,5-Dihydroxy-3-methylbenzoaldehyde
Lactone of 2-hydroxy-3-methyl-5-chlorophenoxyacetic acid
4.0
4.4
8.4
11.7
4.4
0.6
1.2
2.5
–
1
1
1
1
1
–
1
2.5
2.3
1.5
1.9
2.1
–
1
1
1
1
1
–
1
1.2
1.7
1.1
1.1
1.9
1
0.6
1.6
0
0.5
2.8
1.6
0.6
9.3
10.6
12.8
13.3
6.8
3.6
10
1.2
ꢀrad ∼ 283 nm are of great interest as they indicate the possibility
to decrease the concentration of chlorinated initial photoproducts
and accumulation of 2-methylhydroquinone and 2,5-dihydroxy-3-
methylbenzoldehyde. The photodecomposition of MCPA in water
under the XeBr excilamp light leads to rapid formation of dechlo-
rinated photoproducts. C–Cl bond cleavage is an enhancement
mechanism for photodegradation.
context of the integration of phototransformation process and a
biological treatment, the evolution of global parameters, like BOD
and COD may be helpful.
The results show that using of excilams provides the effective
phototransformation of MCPA and coupling of photo- and biopro-
cesses allows removing from water residuals of initial pollutant and
photoproducts.
Acknowledgements
3.4. The effect of MCPA phototransformation on the sequential
biological treatment
The authors are very gradeful to Prof. A. Zertal (Universite de
Constantine, Algeria) for mass and 1H NMR spectra of lactone of 2-
hydro-3-methyl-5-chlorophenoxyacetic, Prof. J.L. Gomez Corrasco
and Dr. M.D. Murcia for the HPLC analysis, M.G. Kambalina, Y.G.
Kopylova and A.A. Hvazhevskaya for technical help with chlorine-
ion analysis. The work was supported in part by the Russian Basic
Research Foundation (Project No. 10-08-90706-mob st) and the
Federal Target Program “Scientific and Teaching Staff of Innovative
Russia” for 2009–2013 years (No. P1128). The work was carried out
with the help of the Centers of Cooperation of Tomsk State Univer-
sity: “Physics and chemistry of high energy systems” and “Quantum
chemistry, spectroscopy and photonics of nanomaterials”.
A coupling process based on photochemical and biological treat-
ment of wastewater is one of possibilities to increase the efficiency
of degradation of some organic pollutants. But in this case it is nec-
essary to estimate the biodegradability of UV treatment solutions of
toxicants. The value of COD after the phototreatment during 60 min
did not change significantly and was 515 + 56 and 595 + 65 mgO L−1
for non-phototreated and phototreated solutions, respectively.
From the point of view of coupling wastewater treatment, it is
very important that pre-irradiation increase the biodegradability of
wastewater, i.e. increase the ratio BOD5/COD before the activated
sludge process. According our experimental data the ratio of BOD5
to COD for non-phototreated 2 × 10−3 M MCPA solution was 0.13.
The UV pretreatment increased this value to 0.4. The BOD5/COD
value of 0.4 is considered to be a threshold of biodegradability of
organic compounds, and several authors consider it as the boundary
of biodegradability [15,16].
GC/MS analysis shows that in a batch culture with AS as inocula
traces of MCPA and non-halogenated photoproducts disappeared
from media after 14 days of biodegradation. Sequential photo-
biotreatment with a XeBr excilamp caused complete degradation
not only of MCPA but also of 2-methyl-4-chlorophenol. After the
KrCl light pretreatment traces of 2-methyl-4-chlorophenol in solu-
tions were observed. Biodegradation of non-irradiated solution of
MCPA led to decreasing in its concentration by 80–87%.
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