Application of photoactive electrospun nanofiber materials with immobilized meso-tetraphenylporphyrin for parabens photodegradation

Article abstract of DOI:10.1016/j.cattod.2014.06.015

The pollution of aqueous environment by trace amounts of anthropogenic chemical substances has a hazardous impact on regular development of plants and animals as well as on human health. The paper presents the results of studies on the heterogeneous degradation of butyl- and benzylparaben in aqueous solutions using photochemically catalyzed processes. Meso-tetraphenylporphyrin (TPP) was immobilized in the polyurethane nanofiber material by electro-spinning method. The xenon lamp was used as a simulated sunlight source. The influence of various process parameters on reaction rate was investigated. The reuse of the carrier with the immobilized photosensitizer was examined. The major role of the singlet oxygen (¹Δ_g) during the photodegradation was proved by using sodium azide and radical scavengers. The adsorption isotherms of parabens onto nanofiber material were determined using BET model. The kinetic study showed that the heterogeneous photodegradation of parabens could be modeled using Langmuir-Hinshelwood model and rate constants have been reported. The reaction pathway for the photodegradation of parabens via ¹Δ_g was proposed.

Full text of DOI:10.1016/j.cattod.2014.06.015

Catalysis Today 240 (2015) 160–167
Contents lists available at ScienceDirect
Catalysis Today
journal homepage: www.elsevier.com/locate/cattod
Application of photoactive electrospun nanofiber materials with
immobilized meso-tetraphenylporphyrin for parabens
photodegradation
M. Gmureka,∗, M. Bizukojc´ a, J. Mosingerb,c, S. Ledakowicza
a Faculty of Process and Environmental Engineering, Lodz University of Technology, Wolczanska 213, 90-924 Lodz, Poland
b Faculty of Science, Charles University, Hlavova 2030, 128 43 Praha 2, Czech Republic
Institute of Inorganic Chemistry, v.v.i., Academy of Sciences of the Czech Republic, 250 68 Rezˇ, Czech Republic
c
ˇ
a r t i c l e i n f o
a b s t r a c t
Article history:
The pollution of aqueous environment by trace amounts of anthropogenic chemical substances has a
hazardous impact on regular development of plants and animals as well as on human health. The paper
presents the results of studies on the heterogeneous degradation of butyl- and benzylparaben in aqueous
solutions using photochemically catalyzed processes. Meso-tetraphenylporphyrin (TPP) was immobilized
in the polyurethane nanofiber material by electro-spinning method. The xenon lamp was used as a simu-
lated sunlight source. The influence of various process parameters on reaction rate was investigated. The
reuse of the carrier with the immobilized photosensitizer was examined. The major role of the singlet
oxygen (1ꢀg) during the photodegradation was proved by using sodium azide and radical scavengers.
The adsorption isotherms of parabens onto nanofiber material were determined using BET model. The
kinetic study showed that the heterogeneous photodegradation of parabens could be modeled using
Langmuir–Hinshelwood model and rate constants have been reported. The reaction pathway for the
photodegradation of parabens via 1ꢀg was proposed.
Received 18 January 2014
Received in revised form 8 April 2014
Accepted 17 June 2014
Available online 11 July 2014
Keywords:
Esters of p-hydroxybenzoic acid
Heterogeneous photosensitized oxidation
Molecular singlet oxygen
Langmuir–Hinshelwood model
Photoactive materials
© 2014 Elsevier B.V. All rights reserved.
1. Introduction
Immobilization of chromophores also leads to the maintenance of
their molecular dispersion [9]. However, the process of chemical
It has been known for over fifty years that the combination of
molecular oxygen, visible light and some dyes leads to the oxidation
of organic substances. The use of dye-sensitized photooxidation
reaction in the field of medicine (photodynamic therapy [1,2]),
chemistry (fine chemical synthesis [3,4]), environmental engineer-
ing (wastewater treatment [5,6]), etc. as a tool for the applications
have stimulated research in this area. Two general photosensitized
oxidation mechanisms are well known as: Type I also called radical-
involving mechanism and Type II-singlet oxygen (1ꢀg) mechanism.
It has been known that 1ꢀg can be used for water purification [7].
The immobilization of photosensitizers onto various materials is a
desirable approach due to their easy separation from the reaction
mixture and reuse. The immobilization of dyes could be associated
with a decrease of 1ꢀg quantum yield. This may be due to the lim-
itations in oxygen diffusion into and from the carrier material [8].
or physical bonding of photosensitizer with the carrier causes par-
tial loss of activity. The first commercially available immobilized
photosensitizer, Rose Bengal immobilized on Merrifield polymer,
showed a 100-fold decrease in the efficiency of singlet oxygen gen-
eration [10]. Whereas, the works on tetra-phenylporphyrin (TPP)
embedded in polyurethane (PUR) nanofiber showed that the pho-
toactivity of the photosensitizer was probably dependent on the
photosensitizer–carrier configuration. In a homogeneous aqueous
solution of dissolved TPP the lifetime of 1ꢀg generated by the
photosensitizer (3.5 ␮s) is almost twice shorter than that in the
heterogeneous system (6.3 ␮s), aqueous solution in which TPP is
incorporated into nanofibers [11]. Thus, the generation of 1ꢀg is
possible by the use immobilized photosensitizer without the essen-
tial loss of its properties. Therefore, the immobilization of TPP
onto polyurethane nanofibers is an original and innovative way to
remove impurities from water environment.
Esters of p-hydroxybenzoic acid, commonly known as parabens
are preservatives used in a variety of personal care products,
cosmetics, pharmaceutical and food products. The presence of
parabens was detected in wastewater, rivers, soil and house dust
∗
Corresponding author. Tel.: +48 42 631 37 93.
E-mail addresses: marta.gmurek@p.lodz.pl,
marta.gmurek@gazeta.pl (M. Gmurek).
http://dx.doi.org/10.1016/j.cattod.2014.06.015
0920-5861/© 2014 Elsevier B.V. All rights reserved.

M. Gmurek et al. / Catalysis Today 240 (2015) 160–167
161
Table 1
Structure of n-butylparaben and benzylpraben and their toxicity [13].
Substances
Toxicity (␮g dm−3
D. magna 48 h
)
P. subcapitata 72 h
O. latipes 96 h
p-Hydroxybenzoic acid n-butyl ester
Acute toxicity EC50
1900
9500
3100
Chronic toxicity–NOEC
21 d
800
72 h
800
14 d VTG
30
p-Hydroxybenzoic acid benzyl ester
Acute toxicity
2100
1200
730
Chronic toxicity NOEC
21 d
840
72 h
520
14 d VTG
20
[12,13]. The potential hazard of parabens has been studied. The
results obtained with Daphnia magna indicate on the toxicity of
parabens towards the aquatic environment [14,15]. The toxic-
ity and estrogenicity tests have shown that p-hydroxybenzoic
acid benzyl ester (benzylparaben) is the most toxic and estro-
genic compounds out of five studied parabens [16]. Butyl and
benzyl esters of p-hydroxybenzoic acid exhibit both acute and
chronic toxicity to algae (Pseudokirchneriella subcapitata), and
fish (Oryzias latipes) (Table 1) [13] as well as to bacteria–Vibrio
fisheri (LC50 = 2.5 mg dm−3 → butylparaben (LOEC = 0.7 mg dm−3);
photosensitizer. Reactors were symmetrically positioned around
light source—a 100 W xenon lamp, simulating solar radiation.
During the experiments the reaction mixture was agitated by
a gentle air or oxygen stream. The BuP and BeP reaction decay
was monitored by HPLC (Waters) with a UV diode array detec-
tor. A detailed detection procedure and experiment setup were
previously published in [20]. The investigations were conducted
at ambient temperature and the sample temperature was kept
constant within 1 ◦C before and after the irradiation. The initial
reaction rates were calculated by differentiating exponential curve
that fitted experimental points for the first 30 min of reaction at
the correlation factor higher than 0.97.
LC50 = 0.11 mg dm−3 → benzylparaben
(LOEC = 0.02 mg dm−3),
15 min) [17].
Parabens can be considered as “pseudo-persistent” pollutants
as a result of their continuous discharges into the environment
throughout wastewater treatment and the issue of concern related
to their eco-toxicological potencies [14–16].
Herein we present the results of photooxidation of butyl- and
benzylparaben (Table 1) by visible light via 1ꢀg generated by
immobilized TPP onto electrospun nanofibers material. The objec-
tive was to define the conditions that maximize the reaction rate
of photosensitized degradation of parabens in the aquatic solution
and to determine the kinetic model that describes the process in a
heterogeneous system. Identification and analysis of toxicity of the
products formed during the process were carried out as well.
Identification of BuP and BeP photoproducts was done via
ultra-high performance liquid chromatography with a diode array
detector (UPLC® Acquity, Waters, USA) coupled in-line with a
quadrupole–TOF hybrid mass spectrometer (Synapt G2, Waters,
USA). Reversed phase chromatography was performed on an
Acquity UPLC® BEH C18 column (2 mm × 150 mm × 1.7 ␮m) at
40 ◦C (Waters) utilizing a gradient elution profile and mobile phase
consisting of 0.1% formic acid in water (A) and 0.1% formic acid
solution in acetonitrile (B) with the flow rate at 0.35 ml min−1
.
The linear gradient elution profile composed of 5% solvent B for
2 min, from 5% to 15% B in 1 min, from 15% to 30% B in 2 min, from
30% to 50% B in 2 min, from 50% to 60% B in 3 min. and 60% B for
4 min. Analyzes were conducted in the negative (ESI-Q1) electro-
spray ionization. The capillary needle voltage was 3 kV as well as
the source temperature was maintained at 120 ◦C (ESI-Q1). Other
volateges were as follows: sampling cone voltage, 40 V; extrac-
tion cone voltage, 4 V Desolvation gas (nitrogen) flow rate was
1000 l h−1 at 400 ◦C. Cone gas flow was 100 l h−1. Nitrogen was used
as both the ESI nebulizer gas. Data acquisition was performed with
a Waters MassLynx data system.
2. Methods
p-Hydroxybenzoic acid n-butyl ester (>99%) (Butylparaben,
BuP) and p-hydroxybenzoic acid benzyl ester (>99%) (Benzyl-
paraben, BeP) were obtained from Fluka and Sigma-Aldrich,
respectively. Meso-tetraphenylporphyrin (TPP) and zinc phthalo-
cyanine (Zn-Pc) were photosensitizer were prepared by the
electrospinning technique [18] and characterized in [11,19]. The
study was carried out in a semi-continuous system in flat reactors
(6 × 10 cm, 0.01 dm3). The inner wall of the reactors was covered
by nanofiber material in form of thin fabrics with the immobilized
Toxicity of the BuP and BeP reaction solution before and
after irradiation was analyzed using the commercially available
ToxTrak test system (Hach, Loveland, CO, USA) and spectropho-
tometer Odyssey Hach®—model DR/2500 (Method 10017). For the

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M. Gmurek et al. / Catalysis Today 240 (2015) 160–167
Fig. 1. Absorption spectra of parabens and TPP immobilized onto PUR nanofabric on the background of emission spectrum of xenon lamp.
Fig. 2. Decay of BuP and BeP concentration at alkaline (A), (B) and neutral (C), (D) solution during photosensitized oxidation, adsorption parabens on nanofabrics and reaction
in the presence of 0.02 M sodium azide, 0.1 M t-BuOH and 100 IU/ml SOD, CBuP/BeP = 8 × 10−5 M.
◦

M. Gmurek et al. / Catalysis Today 240 (2015) 160–167
163
toxicological test Gram-negative bacteria (Escherichia coli, ATCC
8739) was used. The ToxTrak test is a colorimetric test based on
the reduction of resazurin, a redox-reactive dye by bacterial res-
piration. Upon microbial respiration, blue resazurin is reduced to
resorufin with a change of color into pink. The change in absorbance
at 603 nm was measured spectrophotometrically and provided for
a relative determination of percentage respiration inhibition.
3. Results
3.1. Preliminary study
Comparing the emission spectrum of the used Xe lamp and
absorption spectra of parabens at neutral and alkaline solution
(Fig. 1), the occurrence of partial photolysis can be suspected.
The performed experiments in neutral solution in the absence
of the photosensitizer indicated on about 4% and 10% degradation
of BuP and BeP due to photolysis, respectively. In alkaline solutions
the irradiation caused to decreasing of BuP and BeP concentrations
by 0.5% and 7%, respectively. The absorption spectrum of TPP immo-
bilized onto PUR nanofiber material showed that it can efficiently
absorb radiation emitted by xenon lamp. The calculated photon flu-
ence of polychromatic light entering the reaction space was equal
to E07cm = 554 ␮E s−1 dm−3 and E011cm = 324 ␮E s−1 dm−3. The TPP
incorporated in PUR nanofibers at the concentration of 1% wt.
TPP used in experiments absorbed Ea7cm = 87.5 ␮E s−1 dm−3 and
Ea11cm = 51.1 ␮E s−1 dm−3
.
3.2. Influence of process parameters
The series of experiments were conducted to examine the differ-
ences in the kinetic behavior of the heterogeneous photosensitized
oxidation in alkaline and neutral solutions. In the first step of
study the effect of absence or presence of radiation on the reac-
tion were carried out. The study of photodegradation pathways
was performed in the presence of reactive oxygen trappers. Sodium
azide was used as a water-soluble physical quencher of 1ꢀg [21],
t-BuOH as hydroxyl radicals scavenger [22] and superoxide dismu-
tase (SOD) as a scavenger of superoxide radical (O2•−) [23].
Addition of NaN3 to the reaction solution led to total inhibition
of parabens degradation (Fig. 2) in the alkaline solution however
in the neutral environmental solution there was no impact on the
extent of parabens concentration reductions. The experiments pre-
formed in the presence of t-BuOH as well as SOD did not cause
any changes in the reaction run in both neutral and alkaline solu-
tions. The observed influence of the additives on parabens decay
rate demonstrated a dominant role of the singlet oxygen reaction
with BuP and BeP in the alkaline solution but in the neutral solution
the presence of 1ꢀg as well as hydroxyl radicals were not observed
(no reaction with NaN3 and t-BuOH). As it can be seen in Fig. 2, the
photodegradation occurred only in the alkaline solution while in
the neutral solution parabens adsorbed on the carrier.
The important parameter influencing the reaction rate of the
photosensitzed oxidation process is the concentration of pho-
tosensitizer immobilized into polyurethane nanofibers. In order
to investigate this relationship, a series of experiments with
nanofibers material incorporated with various TPP weight concen-
trations: 0.1%, 0.5% and 1% was carried out. The influence of the
presence of 0.3 wt% zinc phthalocyanine (Zn-Pc) immobilized onto
the same carrier was also examined. The use of different concentra-
tions of the immobilized TPP into nanofiber material showed that
with the increasing amount of photosensitizer the reaction rate
increased (Fig. 3A). The higher concentration of TPP in nanofibers
material caused the higher degradation of parabens. The important
finding of these studies is that the use of even a small addition of
Fig. 3. Dependence of the initial BuP reaction rate (r0) upon (a) different % wt. of pho-
tosensitizer concentration: (the area weight for 0.1%wt.–1%wt. TPP was 1.04 g/m2,
for mixture of photosensitizer nanofabrics—2 g/m2); (b) different area weight for
nanofabric 1%wt.TPP.
Zn-Pc resulted in the significant acceleration of the reaction rate.
It should be noted, that nanofibers material with a mix of photo-
sensitizers had area weight significantly higher (2 g/m2) than the
carrier with the same amount of TPP—1.04 g/m2. It was therefore
necessary to check the effect of the nanofibers material area weight
on the photosensitized reaction rate (Fig. 3B).
As it can be seen in Fig. 3, the use of different area weights
of nanofibers accelerates the reaction rate. The application of the
nanofibers material with the highest area weight (2.3 g/m2) under
the same reaction conditions led to the 24% increase of BuP degra-
dation efficiency and 85% increase of BuP reaction rate, compared to
the nanofibers material with lowest area weight (1.00 g/m2). The
use of a larger area weight did not affect the mechanism of the
process (which was confirmed by experiments with sodium azide).
The great advantage of the immobilized photosensitizers is the
possibility of their reuse. A series of experiments was performed
with the same nanofibers material several times. The possibility
of the manyfold application was examined by comparing the rel-
ative decrease in BuP concentration after each use of nanofibers
material using different light intensity. The changes of BeP concen-
trations after irradiation of nanofibers material containing 1%wt.
TPP and 0.3%wt. Zn-PC were followed. After each use the nanofibers
material were rinsed with distilled water and dried under ambient
conditions. The solutions of the same concentration have been oxy-
genated by air and exposed to radiation. As it is shown in Fig. 4, the
nanofibers material can be used several times but each subsequent
use results in lower efficiency of photodegradation. The increase
of the photon flux entering the reaction space led to the higher
parabens decay rate.

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M. Gmurek et al. / Catalysis Today 240 (2015) 160–167
Fig. 4. Cycling runs of photosensitized oxidation (A) BuP (E07cm = 554 ␮E s−1 dm−3, 1%wt. TPP, 2.1 g/m2), (B) BuP (E011cm = 324 ␮E s−1 dm−3, 1%wt. TPP, 2.1 g/m2), (C) BeP
(E07cm = 554 ␮E s−1 dm−3, 1%wt. TPP + 0.3% wt. Zn-Pc, 2.0 g/m2).
After the first irradiation (E07cm = 554 ␮E s−1 dm−3
)
of
nanofibers material with 1% wt. TPP approximately 50% loss
of initial BuP concentrations was achieved, for lower photon flux
(E011cm = 324 ␮E s−1 dm−3) 35% of BuP concentration decay was
observed. After a four-fold use of nanofibers material the efficiency
of photodegradation was about twice lower. The decrease of
BuP concentrations was equal 26% and 14% for E07cm and E011cm
,
respectively. The application of the mixture of photosensitizers
into nanofibers material (1%wt. TPP + 0.3% wt. Zn-Pc) gave higher
degrees of reduction. At the first exposure about 70% decrease
of BeP concentration was achieved. The second time about 51%
degradation of BeP was reached, for the third—about 37%, and for
the fourth approximately 30% of BeP concentration was removed.
The observed decreases in the operating efficiency of photo-
sensitizers immobilized into nanofibers material was probably
the result of their autobleaching initiated by light. After 8 h of
nanofibers material irradiation apparent decrease in luminescence
dyes was observed, which could be due to their bleaching and
decreasing the activity in the generation of 1ꢀg. The reuse of
nanofibers material also resulted in changes in its structure, as well
as decreased mechanical resistance to damage.
3.3. Kinetic studies
In order to determinate the kinetics of photosensitized oxidation
process in the heterogeneous condition, the amount of adsorbed
pollutant was examined. The adsorption isotherm of BuP and BeP
onto nanofibers material at alkaline solution were determined
using Brunauer-Emmet-Teller (BET) equation in the liberalized
form:
ꢀ
ꢁ
1
1
qmax
1 − X
1
=
+
(1)
qe(1 − X)
X
qmaxKBET
where X = Ce/Cs, qmax is the amount of adsorbed paraben which
takes place in a complete monolayer [mol g−1], qe is amount of
appropriative paraben adsorbed onto nanofibers material at equi-
librium (mol g−1). Cs is the saturated solubility of appropriative
paraben in water, Ce is appropriative paraben equilibrium liquid-
phase concentration. KBET is the equilibrium constant characterizes
the interaction energy of adsorbate with the surface of adsorbent.
In the heterogeneous system the most frequently used
kinetic model to formulate photodegradation rate equations is
Fig. 5. (A) Linearization of Langmuir–Hinshelwood’s equation for BuP and BeP,
1%wt. TPP, 2.1 g/m2, E07cm = 554 ␮E s−1 dm−3, CO = 0.27 mM, pH = 10.8; (B) Lin-
2
earized BET isotherm for BuP and BeP adsorption onto nanofabrics 1 wt% TPP
2.1 g/m2. Inset: The isotherm of adsorption.

M. Gmurek et al. / Catalysis Today 240 (2015) 160–167
165
Fig. 6. (A) TOC and initial concentration decreases during the photosensitized oxidation, (B) Changes in toxicity (as measured by inhibition E. coli) before treatment and
after 180 min of radiation in the presence of TPP incorporated into PUR nanofabrics, CBuP = 4 × 10−4 M, CBeP = 4 × 10−4 M, TOCBuP = 56.93 ppm, TOCBeP = 69.15 ppm,
◦
◦
◦
◦
E07cm = 554 ␮E s−1 dm−3, CO = 0.27 mM, pH = 10.8.
2
Langmuir-Hinshelwood model [24–26]. This model assumes that
the reactions take place at the surface of the carrier, and the
reaction rate is proportional to the fraction of the surface covered
by the reactant. L-H model describes the competitive adsorption of
substrate, reaction intermediates and photodegradation products,
moreover assumes that all intermediate photoproducts have the
same binding characteristics to the immobilized photosensitizers
as the main substrate [27,28]. The simplify L-H model can be used to
explain the relation between the initial reaction rates and the con-
taminant concentration after adsorption equilibrium was achieved:
of experimental data, allowing the use of these models to predict
process performance.
3.4. Toxicity results
Analysis of the content of total organic carbon (TOC) for both
parabens and its decrease during the photosensitized oxidation
process was performed. The samples were analyzed during the pro-
cess after 60 min, 120 min and 180 min in the alkaline environment.
As it can be seen from Fig. 6A, despite a significant decrease in the
initial concentration of parabens their mineralization is minimal,
implying the necessity to examine intermediates and their toxicity.
This part of experiments was focused on the comparison of
the toxicities of pure paraben reaction solution before irradia-
tion and reaction solution containing photoproducts, which were
formed as a result of 180 min treatment. Toxicity tests were based
on a colorimetric method (ToxTrak). Escherichia coli was used as
the test bacterial strain. Each sample containing appropriative
paraben before or after the process was prepared in four series
and compared with a control sample. Such comparison allows for
the determination of the background inhibition under the specific
ꢀ
ꢁ
1
r0
1
kLH
1
1
=
+
(3)
KkLH C0
where kLH is the apparent reaction rate constant, K is the dynamic
equilibrium constant of parabens adsorption onto nanofibers
material.
The results were described successfully by Langmuir-
Hinshelwood (L-H) model (Fig. 5A). A BET isotherm was also
used to fit the adsorption data according to the linear form of
isotherm model (Eq. 2) (Fig. 5B). The parameters of BET as well as
kinetic L-H parameters are presented in Table 2. The high values of
determination coefficients (above 0.99) indicated the good fitting
Table 2
Values of parameters of BET isotherms and L-H kinetic parameters.
BET isotherm
Langmuir-Hinselwood model
qmax (mol g−1
)
KBET (–)
R2 (–)
kLH (M s−1
)
K (M−1
)
R2 (–)
BuP
BeP
1.00 × 10−5
2.23 × 10−5
2.55
2.32
0.994
0.993
(1.217 0.024) × 10−8
(1.077 0.043) × 10−8
9109 182
14552 582
0.995
0.993

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M. Gmurek et al. / Catalysis Today 240 (2015) 160–167
Table 3
Photoproducts of the photosensitized oxidation of parabens identified by UPLC–qTOF-MS.
No. Molecular formula Compounds
Retention
time, min
Analyzed mass
of M-H ion m/z
Real monoisotopic
mass of M-H ion
m/z
Absolute error of
determination of mass
of M-H ion
Main fragments
Butylparaben
1
2
3
4
5
6
7
8
9
C11H14O3
C7H6O3
C7H6O5
C7H6O7
C6H6O
C6H6O2
C6H4O2
C11H14O4
C7H6O4
Butylparabena
9.88
5.62
5.30
2.36
5.58
1.98
1.78
8.21
193.0883
137.0251
168.9900
200.9858
93.0367
109.0438
108.0218
209.0923
153.0295
193.0943
137.0317
169.0215
201.0113
93.0419
109.0368
108.0211
209.0892
153.0266
−0.0060
−0.0066
−0.0315
−0.0255
−0.0052
+0.0070
−0.0007
+0.0034
+0.0029
→137.0251, 93.0325
→93.0325
p-Hydroxybenzoic acida
Phenola
→76.0546, 66.0321
→81.0427
Hydroquinonea
1,4-Benzoquinone
Dihydroxybenzoic butyl ester
2,4-Dihydroxy benzoic acid;
3,4-Dihydroxy benzoic acid
2-Hydroxy (1,4)
→95.0121
→153.0295
3.06; 3.12
→109.0283, 78.9601
10
C6H4O3
3.20
123.0076
123.016
−0.0084
→108.0218, 95.0121
benzoquinonea
Benzylparaben
1
2
3
4
5
6
7
8
9
C
14H12O3
Benzylparabena
9.95
5.62
5.30
2.36
5.58
1.98
1.78
8.83
227.0687
137.0251
168.9900
200.9858
93.0367
109.0438
108.0912
243.0661
153.0295
227.0787
137.0317
169.0215
201.0113
93.0419
109.0368
108.0211
243.0736
153.0266
−0.0100
−0.0066
−0.0315
−0.0255
−0.0052
+0.0070
−0.006
→137.0251, 93.0325
→93.0325
C7H6O3
C7H6O5
C7H6O7
C6H6O
C6H6O2
C6H4O2
p-Hydroxybenzoic acida
Phenola
Hydroquinone
→76.0546, 66.0321
→81.0427
1,4-Benzoquinone
Dihydroxybenzoic benzyl ester
2,4-Dihydroxy benzoic acida;
3,4-dihydroxy benzoic acida
2-Hydroxy (1,4)
→95.0121
C14H12O4
−0.0075
+0.0029
→153.0295
C7H6O4
3.06; 3.12
→109.0283, 78.9601
10
C6H4O3
3.20
123.0076
123.016
−0.0084
→108.0218, 95.0121
benzoquinonea
a
Analyzed by the standard.
conditions, in which the sample was prepared, and the correct cal-
culation of the inhibition of E. coli activity. The respiration inhibition
was calculated with the following equation:
After 180 min of the photosensitized oxidation process of BuP
and BeP, the toxicity of the reaction solution were significantly
decreased for both cases (Fig. 6B). On the basis of the proposed
methodology (ToxTrak) it can be stated that the solutions contained
non-toxic reaction products. It is known from literature that ben-
zylparaben is more toxic than butylparaben (Table 1), our results
confirmed this knowledge and indicated that BeP photoproducts
occurred to be more toxic.
ꢂ
ꢃ
ꢀAsample
ꢀAcontrol
% Inhibition = 1 −
× 100
(4)
Fig. 7. Proposed reaction pathway for parabens photodegradation via singlet oxygen; BuP, R CH CH2CH3; BeP, R CH2C6H5.

M. Gmurek et al. / Catalysis Today 240 (2015) 160–167
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3.5. Identification of photoproducts
initial degradation rates and initial concentration satisfies
Langmuir–Hinshelwood kinetics.
The UHPLC-qTOF-MS analysis allowed for the identification of
the main products formed during parabens degradation. In order
to determine the intermediates formed during the photosensitized
oxidation of parabens the reaction solution was irradiated for 30,
60, 120, 180 and 240 min in the presence of nanofibers mate-
rial containing 1% wt. TPP and 0.3% wt. Zn-Pc. It is known that
parabens undergo hydrolysis to p-hydroxybenzoic acid (4-HBA).
In order to test this hypothesis, also the photodegradation of 4-
HBA by 1ꢀg was examined. In addition, phenol, hydroquinone,
2-hydroxy (1,4) benzoquinone, 2,4-dihydroxybenzoic acid, 3,4-
dihydroxybenzoic acid as the intermediates were also identified by
UHPLC-MS analysis. All studies were carried out at pH 10.8 using
a light intensity equal to 554 ␮E s−1 dm−3. The main photoprod-
uct of BuP and BeP photodegradation was 4-hydroxybenzoic acid
formed by the hydrolysis of parabens. In the subsequent stages the
4-HBA was degraded to a compound of the formula C7H6O5. Due
to the mechanism of the 1ꢀg reaction with organic compounds it
may be a compound formed by the attachment of singlet oxygen or
hydroxyl groups. The second main degradation product was dihy-
droxybenzoic butyl ester (for BuP) and dihydroxybenzoic benzyl
ester (for BeP), which resulted from hydroxylation of parabens. The
analysis indicates that 2,4-dihydroxybenzoic acid as well as 3,4-
dihydroxybenzoic acid are the photo-products of photosensitized
oxidation of BuP and BeP.
The photosensitized oxidation in a heterogeneous system per-
mits the reuse of the carrier with immobilized photosensitizer,
however the multiple usage of them needs still improvements. as
the reusability of the photosensitizer by using it in an immobilized
form onto insoluble nanofibers carrier is essential from economic
and application point of view.
Hydroxylation of parabens is the major reaction that occurrs
during the photosensitized oxidation. The main photoproducts
of the photodegradation of parabens via singlet oxygen are 4-
hydroxybenzoic acid and monohydroxy parabens.
The toxicity test shows a significant decrease of toxicity after
photosensitized oxidation, indicating that reduction of parabens
concentration corresponds to an actual detoxification of the treated
reaction solution.
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