Direct photolysis of chlorophenols in aqueous solutions by UV radiation from excilamps

Article abstract of DOI:10.1134/S1070427211030128

Direct photolysis of 2- and 4-chlorophenol and 2,4-dichlorophenol in aqueous solutions by UV radiation from XeBr (282 nm) and KrCl (222 nm) excilamps at various pH values was subjected to a comparative study.

Full text of DOI:10.1134/S1070427211030128

ISSN 1070-4272, Russian Journal of Applied Chemistry, 2011, Vol. 84, No. 3, pp. 407–411. © Pleiades Publishing, Ltd., 2011.
Original Russian Text © V.B. Batoev, G.G. Matafonova, N.I. Filippova, 2011, published in Zhurnal Prikladnoi Khimii, 2011, Vol. 84, No. 3, pp. 415–419.
ORGANIC SYNTHESIS AND INDUSTRIAL
ORGANIC CHEMISTRY
Direct Photolysis of Chlorophenols in Aqueous Solutions
by UV Radiation from Excilamps
a
a
b
V. B. Batoev , G. G. Matafonova , and N. I. Filippova
a
Baikal Institute of Nature Management, Siberian Branch,
Russian Academy of Sciences, Ulan-Ude, Buryatia, Russia
b
Regional Ecological-Informational Center, Limited Liability Company, Ulan-Ude, Buryatia, Russia
Received July 5, 2010
Abstract—Direct photolysis of 2- and 4-chlorophenol and 2,4-dichlorophenol in aqueous solutions by UV
radiation from XeBr (282 nm) and KrCl (222 nm) excilamps at various pH values was subjected to a
comparative study.
DOI: 10.1134/S1070427211030128
It is known that chlorophenols (CPs) are widely
used in syntheses of herbicides, dyes, medicinal agents
and can serve as antiseptics and preservatives for
wood, leather, and fabrics. These compounds are formed
as by-products in chlorine bleaching of cellulose and
chlorination of residual phenols in disinfection of
potable water. Chlorophenols are highly toxic for the
aqueous biota and belong to priority aquatic toxicants
chlorophenol, occurs more effectively with KrCl and
XeBr exilamps, compared with photolysis with a
mercury or XeCl excilamp [9]. For the initial pH
values of a 4-chlorophenol solution, the kinetic of
photolysis with a KrCl excilamp has been calculated
[10] and a higher efficiency of the photolytic de-
composition of 4-chlorophenol, compared with
enzymatic destruction, has been revealed [11].
[
1, 2]. They decompose in aqueous solutions both
The goal of our study was to make a comparative
analysis of the direct photolysis of chlorophenols in
aqueous solutions under exposure to light of KrCl and
XeBr excilamps at varied pH of the medium.
under treatment with UV radiation (direct photolysis)
and in combined treatment, e.g., with UV radiation in
the presence of oxidizing agents or photocatalysts [3].
As UV radiation sources commonly serve low- and
medium-pressure mercury lamps having a broad
emission spectrum. In this case, the kinetics of CP
photolysis is, as a rule, characterized by comparatively
low decomposition rate constants and long half-
decomposition periods. A promising way to intensify
destruction processes of ecotoxicants of this kind is to
use modern UV radiation sources, excilamps emitting
light in a narrow spectral range [4, 5].
EXPERIMENTAL
Our experiments were performed with 2-chloro-
phenol (2-CP), 4-chlorophenol (4-CP), and 2,4-dichloro-
phenol (2,4-DCP) (Merck, >98% purity). As a source
of UV radiation served a XeBr or KrCl excilamp with
barrier-discharge excitation (Institute of High-Current
Electronics, Siberian Branch, Russian Academy of
Sciences, Tomsk). Aqueous solutions of the CP objects
Previously, studies of spectral-luminescent proper-
ties have shown the efficiency of photolysis of me-
thylated phenols in water with KrCl (222 nm), XeBr
3
–1
of study (volume 20 cm , initial concentration 20 mg l )
were irradiated in a reactor (d = 5 cm, h = 3.2 cm)
mounted under the exit window of an excilamp at
room temperature (23 ± 2°C) under permanent
agitation with a magnetic rabble. The energy absorbed
by a solution upon irradiation for 1 to 20 min was 0.1–
(
283 nm) and XeCl (308 nm) excilamps [6] and of
phenol, 4-chlorophenol, and 4-bromophenol with KrCl
and XeBr exilamps [7, 8]. It has been found that direct
photolysis of a herbicide, 2,4-dichlorophenoxyacetic
acid, whose main decomposition product is 2,4-di-
–
3
7.9 J cm .
4
07

4
08
BATOEV et al.
(
a)
(b)
2
-CP
2-CP
4
-CP
4
-CP
2
,4-DCP
2
,4-DCP
τ, min
τ, min
–
1
0
Fig. 1. c/c vs. the duration τ of photolysis of chlorophenols (c = 20 mg l ) by light from (a) XeBr and (b) KrCl excilamps. pH
value: (1) 2, (2) 5.4–5.7, and (3) 11.
The residual concentration of 2- and 4-CP in the
depends on the solution exposure duration at pH 5.4–
5.7 (initial solution pH) and 2 and 11.
course of irradiation was determined colorimetrically
by using the reaction with 4-aminoantipyrine [12]. The
concentration of of 2,4-DCP was determined by HPLC
on a Milikhrom A-02 liquid chromatograph with a UV
detector. We used a ProntoSIL-120-5-C18 column
It is known that CPs in an aqueous solution are
mostly in the molecular form at pH < pK and in the
a
dissociated form at pH > pK , with the dissociated
a
forms (anions) considered to be more reactive than
those in the molecular state [13]. It can be seen in
Table 1 that, at pH 11, UV absorption spectra of CPs
show a bathochromic shift of band peaks in the
medium- and long-wavelength regions, which is
characteristic of the formation of anionic forms. The
maximum photolysis rates and minimum half-
decomposition periods were observed in the case of
irradiation with a XeBr excilamp (282 nm) for 2-CP
and 2,4-DCP at pH 11, and for 4-CP at pH 2 and at the
initial pH value (5.7) (Fig. 1, Table 2). A possible
reason is that light with a wavelength λ = 282 nm is
(
2 × 75 mm), 1 : 1 acetonitrile–water mixture as
–
1
eluent, and elution rate of 150 μl min . The initial
solution pH was brought to 2 or 11 by introduction of a
solution of H SO (4.6 × 10 M) or NaOH (2.5 × 10 M)
and was monitored with an I-16 ion meter (Gomel
plant of measuring instruments, Belarus). Electronic
absorption spectra of CP solutions were recorded with
an Agilent 3843 UV-VIS spectrophotometer (United
States).
–3
–4
2
4
Figure 1 shows how the ratio between the residual
concentration c and the initial concentration c of CP
0
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 84 No. 3 2011

DIRECT PHOTOLYSIS OF CHLOROPHENOLS IN AQUEOUS SOLUTIONS
Table 1. Spectral characteristics of chlorophenols in aqueous solutions
409
–1
–1
Chlorophenol
λmax, nm
рНin
Form
ε
λmax, M cm
2
4
2
-CP
223
2 and 5.6
2 and 5.6
11
Molecular
The same
Anionic
3788
1958
8242
3733
2
2
2
74
37
93
11
The same
-CP
225
2 and 5.7
2 and 5.7
11
Molecular
The same
Anionic
8599
1609
12056
2599
2
2
2
80
44
98
11
The same
,4-DCP
225
2 and 5.4
2 and 5.4
11
Molecular
The same
Anionic
6099
2062
9726
3654
2
2
3
84
45
05
11
The same
Table 2. Pseudofirst-order rate constants k, half-decomposition periods t1/2, and quantum efficiencies φ of photolysis in
irradiation of chlorophenol solutions at various pH of the medium with XeBr (282 nm) and KrCl (222 nm) excilamps
–1
–1
–1
Chlorophenol
рНin
λ, nm
ε
λ
, M cm
k, min
t1/2, min
φ
–
2
2
4
2
-CP
2
222
222
222
282
282
282
4288
4253
4860
1519
1498
2440
8.5×10–
8.5×10
8.1
8.2
7.9
6.1
4.5
0.8
0.034
0.034
0.031
0.036
0.049
0.174
2
2
1
1
1
5
1
2
5
1
.6
.6
–
1
1
8.7×10–
1.1×10
–
1.5×10–
9.0×10
–
1
1
2
1
1
2
-CP
2
5
1
2
5
1
222
222
222
282
282
282
7662
8185
4739
1436
1545
1458
1.5×10–
4.6
3.6
17.0
0.9
0.9
16.1
0.034
0.041
0.015
0.264
0.241
0.014
.7
.7
1.9×10
–
1
1
4.0×10–
8.0×10
–
7.9×10–
4.3×10
–
2
2
2
2
2
1
,4-DCP
2
5
1
2
5
1
222
222
222
282
282
282
6575
6293
4677
2091
1979
1114
6.6×10–
10.4
10.2
10.0
10.4
7.6
0.017
0.019
0.026
0.015
0.022
0.077
.4
.4
6.8×10
–
1
1
6.9×10–
6.7×10
–
9.1×10–
1.8×10
3.8
effectively absorbed by low-energy electronically
excited states of 4-CP molecules and 2-CP and 2,4-
DCP anions. This leads to rupture of C–Cl, O–H, and
C–C bonds. To compare our results with published
data on the CP photolysis by UV light at other
wavelengths and intensities, we estimated the quantum
efficiencies of photolysis. The first-order quantum
efficiency of photolysis of a substance, φ, can be
calculated by the formula [14, 15]:
k
,
ϕ =
2.303I ε l
λ λ
–
1
where k is the pseudofirst-order rate constant (min );
I , light intensity at a wavelength λ (einstein l min );
ε , molar absorption coefficient at a wavelength
λ (M cm ); and l, optical path length in the cuvette
–
1
–1
λ
λ
–1
–1
(cm).
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 84 No. 3 2011

4
10
BATOEV et al.
anions also exceeded those for molecules. This is in
X
2
-CP
agreement with published data on photolysis at 296 nm
for 2-CP (φ = 0.03–0.04 for molecules and 0.20–0.30
for anions) [16–18], and also for 2,4-DCP (φ = 0.01–
0
.02 for molecules and 0.10 for anions) [19, 20].
The average quantum efficiency of CP-4 photolysis
is 0.25 in the pH range 1–13 under irradiation at λ =
54 or 296 nm (medium- and high-pressure mercury
2
lamp) [16]. As follows from the data in Table 2, the
value of φ282 for 4-CP molecules is comparable with
the above value, whereas φ222 is an order of magnitude
smaller and is comparable with the value of φ (0.017),
previously found in photolysis of 4-CP molecules with
a high-pressure mercury lamp, which emits light in a
broad spectral range 238–579 nm with peaks at 254,
–2
D, J cm
X
4-CP
3
13, and 366 nm [21]. At the same time, the photolysis
of 4-CP anions is characterized by the smallest values
of φ, with φ282 ≈ φ , which is indicated by the
222
corresponding values of the rate constants. The
wavelength of 282 nm nearly corresponds to the long-
wavelength absorption peak of 4-CP molecules
(
280 nm), which is due to the electron transition from
the ground state S to S (πσ*) partly localized on the
0
2
C–Cl bond [22]. Therefore, we assume that, upon
irradiation of a 4-CP molecule at λ = 282 nm, a
chloride atom is easily abstracted to give the main
intermediates, hydroquinone and para-benzoquinone,
which is indicated by the yellow coloration of a
solution and has been additionally confirmed pre-
viously by a chromato-mass-spectrometric analysis
–2
D, J cm
X
2
,4-DCP
[23]. 2-CP and 2,4-DCP molecules more difficultly
undergo photolysis, compared with 4-CP molecules,
because of the formation of intermolecular hydrogen
bonds between a Cl atom in an ortho position and a
hydrogen atom of the OH group of another molecule.
Also possible is formation of intramolecular hydrogen
bonds and bonds with water molecules [18].
–2
D, J cm
Fig. 2. Photolysis efficiency X of chlorophenols vs. the UV
irradiation dose D from (1–3) XeBr and (4–6) KrCl
excilamps. pH value (1, 4) 2, (2, 5) 5.4–5.7, and (3, 6) 11.
X, % relative to the initial concentration.
The photolysis of 2-CP anions and 4-CP molecules
by light at λ = 222 nm was characterized by a
substantially lower quantum efficiency, compared with
the respective values of φ . Despite that the light
2
82
from KrCl excilamp falls within the range in which
short-wavelength absorption peaks lie, transitions from
high-energy excited electronic states probably do not
lead to bond rupture, being involved in other effects,
e.g., nonradiative processes [24].
This equation is applicable to the excilamps we
used because the halfwidth of the emission band does
not exceed 1 nm.
It can be seen in Table 2 that the values of φ for 2-
CP anions and 4-CP molecules at λ = 282 nm
substantially exceed (φ ) those for 2-CP molecules
A comparison of the CP photolysis efficiencies at
various UV irradiation doses demonstrated that in the
strongly acidic medium and at initial pH values of
2
82
and 4-CP anions. The values of φ282 for 2,4-DCP
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 84 No. 3 2011

DIRECT PHOTOLYSIS OF CHLOROPHENOLS IN AQUEOUS SOLUTIONS
411
solution the maximum decomposition efficiency of
-CP and 2,4-DCP molecules at the lowest expended
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2
dose was achieved with the KrCl excilamp (Fig. 2).
This agrees with the results of photolysis of 4-CP
solutions [25]. Nevertheless, full decomposition of 2-
CP and 2,4-CP anions at pH 11 required smaller UV
radiation doses with the XeBr exсilamp, which was
also observed in photolysis of 4-CP molecules at pH 2
and 5.7.
7. Sokolova, I.V., Chaikovskaya, O.N., Svetlichnyi, V.A.,
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(
1) Under the experimental conditions, the rate of
1
1
1
0. Gomez, M., Murcia, M.D., Gomez, J.L., et al., Chem.
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direct photolysis of chlorophenols by light of XeBr and
KrCl excilamps increased in the order 2,4-di-
chlorophenol < 2-chlorophenol < 4-chlorophenol for
molecules (pH 2 and 5.4–5.7) and 4-chlorophenol <
,4-dichlorophenol < 2-chlorophenol for anions (pH
1). The maximum photolysis rates were observed
with the XeBr excilamp (282 nm) for 4-chlorophenol
molecules and 2-chlorophenol and 2,4-dichlorophenol
anions.
1. Gomez, M., Matafonova, G., Gomez, J.L., et al.,
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(
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1
1
1
1
1
1
1
1
2
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(
2) The maximum photolysis efficiency of 2-
chlorophenol and 2,4-dichlorophenol molecules with
the smallest absorbed energy dose was achieved with
the KrCl excilamp (222 nm), whereas photolysis of 2-
chlorophenol and 2,4-dichlorophenol required smaller
irradiation doses for the XeBr excilamp (282 nm). The
opposite dependence was observed for 4-chlorophenol
molecules and anions.
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The authors are grateful to E.T. Pavlova and
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