Monitoring of phenol photodegradation by ultraviolet spectroscopy

Article abstract of DOI:10.1016/S1386-1425(02)00172-5

Advanced oxidation processes (AOPs) have been developed as an emerging technology for hazardous organic treatment in industrial wastewater. In this paper, the contribution of ultraviolet (UV) spectroscopy to follow phenol photodegradation was studied in a laboratory photochemical reactor equipped with a low pressure mercury lamp. It has been observed that a multicomponent approach is efficient for the evolution estimation of the initial product or intermediate compounds formed during the photodegradation.

Full text of DOI:10.1016/S1386-1425(02)00172-5

Spectrochimica Acta Part A 59 (2003) 303Á307
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www.elsevier.com/locate/saa
Monitoring of phenol photodegradation by ultraviolet
spectroscopy
B. Roig ꢀ, C. Gonzalez, O. Thomas
´
Laboratoire Genie de l’Environnement Industriel, Ecole des Mines d’Ale`s, 6 avenue de Clavie`res, 30319 Ale`s Cedex, France
Received 25 March 2002; accepted 8 April 2002
Abstract
Advanced oxidation processes (AOPs) have been developed as an emerging technology for hazardous organic
treatment in industrial wastewater. In this paper, the contribution of ultraviolet (UV) spectroscopy to follow phenol
photodegradation was studied in a laboratory photochemical reactor equipped with a low pressure mercury lamp. It
has been observed that a multicomponent approach is efficient for the evolution estimation of the initial product or
intermediate compounds formed during the photodegradation.
# 2002 Elsevier Science B.V. All rights reserved.
Keywords: Photodegradation; UV spectrophotometry; Phenol; Monitoring
1. Introduction
These techniques are also studied for the degra-
dation of pure organic compound solutions.
Among the considered organic compounds, phe-
nol is one of the more studied because of its high
toxicity and its large presence in industrial waste-
water. H2O2/UV processes [4,5], O3/UV process
[6], heterogeneous catalyst/UV [7,8], or sensitizer
compounds [9] have been used for phenol degra-
dation, as well as more powerful systems using
laser source [10], or flash lamp [11] or far UV
monochromatic lamp [12,13].
However, whatever the degradation technique,
numerous by-products are formed. Chromatogra-
phy analysis is generally used for the study of
reaction evolution and the estimation of the
degradation yield of the studied compound. On
the other hand, UV spectrophotometry, frequently
used as detector in chromatographic systems, has
The pre-treatment of industrial wastewater is of
growing interest because of the high toxicity of
released products and treatment price of available
methods. Among these, advanced oxidation pro-
cesses (AOPs), generally based on the association
of chemical and photochemical reactions, are often
chosen. Several systems have proved to be effective
for the treatment of industrial wastewater. The
process often includes chemical additives like
oxidants (H2O2 or O3) [1] or catalyst (TiO2) [2].
Direct ultraviolet (UV) irradiation, without re-
agent nor catalyst may also be used [3].
ꢀ Corresponding author. Tel.: ꢁ
466-782-701
E-mail address: benoit.roig@ema.fr (B. Roig).
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33-466-782-770; fax: ꢁ33-
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1386-1425/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved.
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been often used for the quality control of water
and wastewater [14Á16]. It is a simple and fast
2.2. Apparatus
/
technique, giving very close results to those from
reference methods, for global and specific para-
meters of water pollution such as total organic
carbon (TOC), dissolved organic carbon (DOC) or
nitrate. The aim of this paper is to show the
interest of UV spectrophotometry for the direct
photodegradation monitoring a model compound.
The phenol is chosen and the reaction is followed
by on line UV spectrophotometry and is validated
by HPLC as reference technique.
Experiments are carried out with a photoche-
mical reactor shown in Fig. 1. It is composed of a
UV light source (low pressure mercury lamp)
surrounded by the synthetic quartz (suprasil)
coil. The sample circulates in a 20 ml closed-circuit
and the UV measurement is carried out with a UV
spectrophotometer.
The UV detector is a UV-visible spectrophot-
ometer Anthelie (Secomam) controlled by a soft-
ware DATHELIE version 4.1f. The pathlength of the
Suprasil quartz cell is 2 mm and the scan speed is
1800 nm minꢂ1
.
2. Materials and methods
2.3. Analytical methods
2.1. Materials
Organic compounds (phenol and photo-pro-
ducts) were analyzed by a Waters 600 HPLC
controller equipped with a quaternary gradient
pump, a 100 ml injection circuit and a Varian 2550
UV detector. Separation was obtained with a C18
Phenol (RDH), catechol, muconic acid (Al-
drich), hydroquinone, benzoquinone (Merck) and
organic acids-oxalic, formic, fumaric, maleic,
malic-(Carlo Erba) were used without further
purification. Solutions were prepared with high
purity water from MilliQ system (conductivity
greater than 18 mV).
column (Supelcosil LC 4.6 IDꢃ250 mm). The
/
wavelength used for the UV detection was 254 nm.
The eluting solvent was water/acetonitrile (80/20)
at a flow rate of 1 ml minꢂ1. Products were
identified by comparison of retention times with
standards.
Raw samples containing phenol have been
obtained from petrochemistry industrial waste-
waters.
Fig. 1. Twenty millileters on-line reactor.

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Organic acids as final photo-products were
analyzed by a Shimadzu LC 6A HPLC system
equipped with a Varian 2050 UV detector. Separa-
tion was obtained by ion exchange chromatogra-
phy using a Supelcogel C610H (7.8 IDꢃ300 mm)
/
column. The UV detector wavelength used was
210 nm. The products were eluted with H3PO4
0.1% as mobile phase at a flow rate of 0.5 ml
minꢂ1 and were identified by comparison of
retention times with standards. The amount of
organic acids after irradiation was quantified by
standard calibration curve using butyric acid as
internal standard.
3. Results and discussion
Numerous works have studied the phenol de-
gradation either by direct UV photolysis or by
AOP techniques [4Á6,8]. The main reactions are
/
aromatic ring oxidation (hydroquinone, catechol),
ring cleavage (muconic acid) and finally carboxylic
acid formation (oxalic acid, formic acid) (Fig. 2).
Fig. 2. Simplified scheme of phenol degradation.
3.1. UV analysis
According to this scheme, UV spectrophotome-
try was used to explain the evolution of phenol
photodegradation. Fig. 3 represents the variation
of the absorbance at 268 nm (chosen as a
characteristic wavelength of phenol at pH 7) and
the UV spectrum shape evolution as a function of
time. The absorption increases during the first
minutes showing the formation of aromatic inter-
mediates whereas the phenol concentration de-
creases. These intermediates are then degradated
by cleavage of the aromatic ring until the decrease
of the absorbance is close to zero. At the end, a
residual absorbance around 200 nm corresponding
to the organic acids formation as final degradation
products can be seen.
The estimation of phenol concentration is not
possible neither by the following of the evolution
at a given wavelength nor by a second derivative
method [17]. But a more quantitative analysis can
be carried out with the UV spectra exploitation by
using a multicomponent approach [18]. The prin-
ciple of this technique is to consider the UV
absorption spectrum as the sum of few reference
spectra composing a reference base. In our case,
UV spectra of phenol, catechol, benzoquinone,
hydroquinone, muconic acid and formic acid form
the reference base for the multicomponent calcula-
tion. The concentrations estimated by UV spectro-
photometry are comparable with those obtained
by HPLC (Fig. 4).
The multicomponent approach also allows the
calculation of the evolution of the degradation by-
products. Fig. 5 shows the comparison between
the estimated UV evolution and the HPLC one for
three main degradation compounds (benzoqui-
none, catechol and formic acid).
Figs. 4 and 5 confirm the evolution previously
observed: phenol is degraded in the same time as
the formation of intermediates. Then, all com-
pounds are degradated and organic acids are
formed at the end. This UV analysis allows to
explain the general evolution during phenol de-
gradation and to quantify the main intermediates.

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Fig. 3. UV representation of phenol degradation.
A good correlation was obtained. But, in this
case, the reference basis for the deconvolution
method has been improved. Indeed, spectra of the
matrix have been added in order to take into
account the interferences.
4. Conclusion
The evolution of the photochemical degradation
of phenol can be studied by UV spectrophotome-
try. It allows to follow not only the degradation of
the precursor compounds but also of the main
degradation intermediates. This approach can be
considered as a simple and fast analysis giving
some relevant informations about degradation
mechanisms. This methodology could be extended
to the study of different aromatic molecules like
benzene, aniline, salicylic acid or benzoic acid.
Fig. 4. UV and HPLC phenol concentration evolution with
time.
But a GC/MS analysis is necessary to confirm the
intermediate structures.
This procedure was applied on raw industrial
wastewater containing various phenol concentra-
tions. Fig. 6 shows the relation between the data
founded with the UV measurement and the HPLC
one.

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307
Fig. 5. Evolution of three main degradation by-products analyzed by UV and HPLC.
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