Photochemical and photocatalytic degradation of trans-resveratrol

Article abstract of DOI:10.1039/c2pp25239b

Photochemical and photocatalytic degradation of the emerging pollutant trans-resveratrol has been studied under different irradiation wavelengths and using different TiO₂ catalysts. trans-Resveratrol was more easily degraded when irradiated usi

Full text of DOI:10.1039/c2pp25239b

Photochemical &
Photobiological Sciences
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Photochemical and photocatalytic degradation of
trans-resveratrol†‡
Cite this: Photochem. Photobiol. Sci., 2013,
12, 638
Cláudia Gomes Silva,*a Judith Monteiro,a Rita R. N. Marques,a Adrián M. T. Silva,a
Cristina Martínez,b Moisés Canle L.b and Joaquim Luís Fariaa
Photochemical and photocatalytic degradation of the emerging pollutant trans-resveratrol has been
studied under different irradiation wavelengths and using different TiO2 catalysts. trans-Resveratrol was
more easily degraded when irradiated using the whole spectral range (UV-Vis) rather than with UV and
near-UV to visible irradiation. The main intermediate of trans-resveratrol phototransformation was
identified as its isomer cis-resveratrol. Different TiO2 catalysts were used to carry out the photocatalytic
degradation of trans-resveratrol. Catalysts properties such as crystallite dimensions, surface area and
presence of hydroxy surface groups are shown to be crucial to the photocatalytic efficiency of the
materials tested. From the point of view of trans-resveratrol abatement, the photocatalytic process was
more efficient than the pure photochemical one resulting in higher degradation rates and higher
organic content removal. Six photoproducts of trans-resveratrol phototransformation were identified
mainly resulting from the attack of the hydroxyl radical to the organic molecule.
Received 2nd July 2012,
Accepted 19th September 2012
DOI: 10.1039/c2pp25239b
www.rsc.org/pps
antibiotics and steroids, residues from pharmaceutical manu-
facturing and also from hospitals.3,4 The concentration of
1. Introduction
Worldwide concerns on water pollution have resulted in PPCPs can be particularly high in pharmaceutical industry
increasingly intensive research activity on new treatment tech- effluents, reaching several hundreds of mg L−1 5
nologies over the last few decades. On the other hand, contem- Heterogeneous photocatalysis has gained relevance among
.
porary social habits have led to the occurrence of new types of the existing advanced oxidation processes (AOPs) as a low-cost
pollutants in natural watercourses. Biological treatments are competitive process, mainly because it can be operated at
not always sufficient to treat well waters polluted with many ambient temperature and pressure.6,7
recalcitrant or persistent compounds.
In particular, titanium dioxide (TiO2) has been widely used
Pharmaceuticals and personal care products (PPCPs) are a as a photocatalytic material for solving environmental pro-
class of water contaminants that have emerged in the last few blems, especially for removing chemical pollutants from waste-
decades due to their massive use for personal health or cos- water.8,9 The widespread use of TiO2 in photocatalytic
metic reasons.1,2 PPCPs comprise a diverse collection of applications is due to its strong oxidizing power, high chemi-
chemical substances, including drugs, fragrances, lotions, and cal stability and relative inexpensiveness.8
cosmetics. The introduction of these compounds in the
In this work we have studied the photochemical and photo-
environment includes human activity (e.g., household and catalytic degradation of trans-resveratrol (3,4′,5-trihydroxy-
hygienic uses, swimming), veterinary drug use, especially trans-stilbene), an antioxidant stilbenoid compound used in
medical applications for lowering the risk of coronary
heart disease, bearing also important anticarcinogenic
aLaboratory of Catalysis an Materials, Associate Laboratory, Faculdade de
properties.10–12 trans-Resveratrol can be found in grape skin,
Engenharia, Universidade do Porto, Rua Dr. Roberto Frias s/n, 4200-465 Porto,
peanuts, soy, tea and other plants.13 It is introduced in the
Portugal. E-mail: cgsilva@fe.up.pt; Fax: +351 225 081 449; Tel: +351 225 081 779
human diet mainly through the ingestion of wine in which
both trans- and cis-isomers are found.14 trans-Resveratrol has
emerged as a pollutant in wastewater since it has been syn-
thesized in large scale to be included in the formulation of
several personal care products with anti-aging effects.15,16
To the best of our knowledge this is the first study dedi-
cated to the photocatalytic degradation of trans-resveratrol.
bChemical Reactivity and Photoreactivity Group, Dept. of Physical Chemistry &
Chemical Engineering, University of A Coruña, Rúa da Fraga, 10, E-15008 A Coruña,
Spain. E-mail: mcanle@udc.es; Fax: +34 981 167065; Tel: +34 981 167000
†This paper is published as part of the themed issue of contributions from the
7th European Meeting on Solar Chemistry and Photocatalysis: Environmental
Applications held in Porto, Portugal, June 2012.
‡Electronic supplementary information (ESI)
available. See DOI:
10.1039/c2pp25239b
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There are only a few references in the literature reporting the by the instrument software (OMINC) to equivalent absorption
interaction of light (mostly in the UV range) with the trans- units in the Kubelka–Munk scale. The specific surface area
resveratrol molecule.17,18
(SBET) of TiO2 materials was determined by multipoint analysis
In this paper we have studied the photodegradation of of N2 adsorption isotherms at 77 K using the BET method,
trans-resveratrol under different irradiation wavelengths and with a Quantachrome NOVA 4200e multi-station apparatus.
irradiation intensities. The photocatalytic degradation of trans-
During the photodegradation experiments, samples were
resveratrol using TiO2 as a photocatalyst is also described regularly withdrawn from the reactor and centrifuged prior to
using different types of TiO2 and different irradiation wave- analysis for separation of any suspended solid. The solutions
length ranges.
were then analyzed by HPLC using a Hitachi Elite LaChrom
liquid chromatograph equipped with an L-2450 diode array
detector (DAD). The stationary phase consisted of a Purospher
Star RP-18 endcapped column (250 mm × 4.6 mm, 5 μm par-
ticles) working at room temperature. The mobile phase was a
mixture of water (A) and methanol with a gradient concen-
tration at a flow rate of 1 mL min−1. The gradient was as
follows: 0 min, 70% A; 22 min, 37% A; 27 min, 37% A; 30 min,
70% A; 36 min, 70% A.
Selected samples were analyzed using a Thermo Accela
HPLC equipped with a PDA detector, coupled to a Thermo
LTQ Orbitrap Discovery mass spectrometer using ESI− ioni-
zation, operating in full-scan mode (R > 30 000), between 50
and 400 uma. Separation was carried out using a Phenomenex
Luna C18 column (150 mm × 4.6 mm, 5 μm particles) at a flow
rate of 0.5 mL min−1. The mobile phase gradient was similar
to that used for HPLC-DAD analysis.
2. Experimental
2.1 Reagents and materials
Titanium(IV) oxide powder anatase 99.8% metal basis (TiO2-SA)
and trans-resveratrol (≥99%) were purchased from Sigma-
Aldrich. TiO2 AEROXIDE® P25 (TiO2-EP) was supplied by
Degussa AG, now Evonik. TiO2-SG was prepared by using a
sol–gel procedure as described elsewhere.19
2.2 Photodegradation experiments
The photodegradation of trans-resveratrol was carried out in a
300 mL glass immersion reactor with a gas inlet, a gas outlet
and a sampling port. The photoreactor was equipped with dis-
tinct types of light sources located axially and held in a quartz
immersion tube: a UV Heraeus TNN 15/32 low pressure
mercury vapour lamp (λexc = 254 nm) and a UV-Vis Heraeus TQ
150 medium pressure mercury vapour lamp (λexc = 254, 313,
365, 405, 436, 546 and 578 nm). The light intensity of both
lamps was approximately 50 mW cm−2, as determined by
using a UV-Vis spectroradiometer (USB2000+, OceanOptics,
USA). A quartz jacket with water recirculation was employed to
cool the irradiation source and cancel the infrared radiation,
thus preventing any heating of the solution/suspension. In
order to cut-off the UV-B and UV-C emission lines, a DURAN
50® glass cooling jacket was placed around the UV-Vis lamp
(main resulting emission lines at λexc = 365, 405, 436, 546 and
578 nm, see ESI, Fig. SI1‡). The different irradiation wave-
length ranges were designated as UV (TNN 15/32 lamp), UV-Vis
(TQ 150 lamp) and Vis (TQ 150 lamp with DURAN 50® cut-off
filter). In a typical reaction, the reactor was charged with
250 mL of a 20 mg L−1 trans-resveratrol solution. A 20% vol.
oxygen stream was continuously bubbled through the reaction
system. Reactions were performed under natural pH con-
ditions (pH = 5.6). In the case of photocatalytic reactions, TiO2
was used as a catalyst with a load of 1 g L−1. Before turning the
light on, the suspensions were magnetically stirred for 15 min,
to establish the adsorption–desorption equilibrium. The first
sample was taken out at the end of the dark adsorption
period, just before the light was turned on, in order to set the
initial trans-resveratrol concentration in solution (C0).
Total organic carbon (TOC) measurements were performed
in a Shimadzu TOC-5000 analyzer.
3. Results and discussion
3.1 Photochemical degradation of trans-resveratrol under
different irradiation wavelengths
Photochemical degradation of trans-resveratrol was carried out
under different irradiation conditions: (i) monochromatic
ultraviolet light with an emission line at 254 nm (UV); (ii) poly-
chromatic light in the range 200–600 nm (UV-Vis); (iii) poly-
chromatic light at λ ≥ 365 nm (mostly visible light irradiation,
Vis).
The UV-Vis spectrum of trans-resveratrol shows a maximum
at 310 nm with a molar absorption coefficient at this wave-
length of 26 669 M−1 cm−1 (Fig. 1a). HPLC analysis of the
samples taken during photochemical experiments revealed
that the main intermediate of trans-resveratrol photochemical
degradation is its geometrical isomer cis-resveratrol, identified
by its UV-Vis absorption spectra, with a maximum at 286 nm
(Fig. 1a).
Fig. 1b shows the decay of trans-resveratrol concentration
following UV, Vis and UV-Vis irradiation. trans-Resveratrol is
more easily converted when irradiated over the whole spectral
range (UV-Vis), which is attributed to better overlap of the exci-
tation light with its absorption spectrum.
2.3 Analytical techniques
In opposition, the slowest reaction rate was observed when
Diffuse reflection infrared Fourier transformed (DRIFT) spec- UV light (λexc = 254 nm) was used. At this wavelength trans-
troscopic analysis of TiO2 samples was performed on a Nicolet resveratrol shows a minimum of absorbance, which accounts
510P FTIR Spectrometer. The interferograms were converted for the lower photodegradation efficiency observed. As shown
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in Fig. SI1,‡ the cut-off filter transmission uptake is at ca. degradation process (k), determined by non-linear curve
350 nm. trans-Resveratrol shows residual absorption at λ ≥ fitting to the experimental data, are shown in Table 1, as well
350 nm, which can explain the slow rate observed for the as the percentage of trans-resveratrol conversion and TOC
photochemical degradation reaction when radiation in the Vis removal after, respectively, 5 min (X5 min) and 3 h of irradiation
range is used.
(XTOC, 3 h).
As shown in Table 1, k increases with the irradiation wave-
Independently of the irradiation wavelength used, the kin-
etics of the photodegradation reactions followed a first order length following the order UV < Vis < UV-Vis. The same ten-
rate law. Kinetic rate constants of the photochemical dency was observed for trans-resveratrol conversion at 5 min of
the reaction, with almost complete elimination when irradiat-
ing in the whole UV-Vis spectrum. Nevertheless, although total
conversion of both trans- and cis-resveratrol was achieved at
the end of the reactions using both UV and Vis irradiation, no
TOC removal was observed, which indicates that both isomers
are converted into other organic by-products but not minera-
lized to CO2 and H2O. On the other hand, UV-Vis irradiation
leads to 87% TOC removal, indicating that both trans-resvera-
trol and organic intermediates are almost completely
mineralized.
In order to study the influence of different light intensities
in the photodegradation of trans-resveratrol under UV-Vis
irradiation, a dye (C.I. Direct Green 26) solution was circulated
through the quartz jacket placed around the irradiation
source, acting as a light filter. This dye shows absorption in
the range 200–750 nm (Fig. SI2‡), with maxima at 254, 382 and
610 nm and molar absorption coefficients at these wavelengths
of 32 346, 28 931 and 25 745 M−1 cm−1, respectively. Solutions
of the dye with different concentrations were used to filter
different light fractions. Radiation intensities were determined
by ferrioxalate actinometry20 and treated as fractions of the
initial flow (φ/φ0), i.e., the photonic flow obtained when water
is passing through the quartz jacket. Results show that the rate
of transformation of trans-resveratrol increases with UV-Vis
light intensity (Fig. 2a). It was also found that at an early stage
of the reaction (5 min), trans-resveratrol conversion is linearly
proportional to light intensity (Fig. 2a inset), which indicates
that the number of photons absorbed by the system is pro-
portional to the number of trans-resveratrol molecules being
transformed. Fig. 2b shows the time evolution of the area of
Fig. 1 UV-Vis spectra of trans- and cis-resveratol and respective molecular
the HPLC peak corresponding to cis-resveratrol. Kinetics of for-
mation and disappearance of cis-resveratrol are also light
intensity dependent. For high intensities the low amounts of
structure (a). Evolution of the normalized concentration (C/C0) of trans-resvera-
trol (C0 = 20 mg L−1) during the photodegradation reactions under UV ( ), Vis
■
●
▲
(
) and UV-Vis ( ) irradiation – inset: area of the HPLC peak corresponding to
cis-resveratrol formed are rapidly degraded under those
cis-resveratrol (b).
Table 1 Rate constants (kapp and k), trans-resveratrol conversion after 5 min of irradiation (X5 min) and TOC removal at 3 h of reaction (XTOC, 3 h), obtained for photo-
catalytic experiments using different reaction conditions
Reaction conditions
k (×10−1 min−1
)
kapp (×10−1 min−1
)
kapp/k
X5 min (%)
XTOC,3 h (%)
Photo-products
UV
Vis
UV-Vis
Vis/TiO2-SA
Vis/TiO2-EP
Vis/TiO2-SG
UV/TiO2-SG
UV-Vis/TiO2-SG
0.49 0.06
3.8 0.6
6.63 0.07
—
—
—
—
—
—
—
—
—
—
—
0.58
0.79
1.55
1.92
1.34
39
86
96
66
79
94
44
98
0
0
2, 3, 5, 6
1, 2, 3
1, 3, 4
n.a.
87
74
72
75
65
90
2.21 0.03
3.0 0.1
5.9 0.2
0.94 0.07
8.9 0.3
n.a.
1, 2, 3, 4
1, 2, 3, 4
n.d.
n.a.: not analyzed; n.d.: not detected.
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Fig. 3 Evolution of the normalized concentration (C/C0) of trans-resveratrol
■
during the photocatalytic experiments under Vis irradiation using TiO2-SG ( ),
●
▲
TiO2-EP ( ) and TiO2-SA ( ) catalysts.
absence of catalyst, and it may serve to simulate conditions of
solar irradiation. TiO2 P25 (EP) has been widely used in
numerous photocatalytic studies being generally accepted as
the benchmark catalyst for this type of reaction. It is consti-
tuted by 80% of anatase and 20% of rutile allotropes. Both
TiO2-SA and TiO2-SG are constituted by pure anatase nanopar-
ticles. Results show that for all cases total removal of trans-
resveratrol was achieved after 30 min of irradiation (Fig. 3).
Kinetics of photocatalytic degradation of trans-resveratrol
follow a pseudo-first order rate model (as predicted by the
Langmuir–Hinshelwood model21), the corresponding apparent
kinetic rate constants (kapp) being summarized in Table 1.
TiO2-SG showed the best photocatalytic performance with the
highest kapp, followed by TiO2-EP and TiO2-SA. Using this cata-
lyst, a practically complete degradation was achieved after just
5 min of visible-light irradiation. The rate constants obtained
for reactions using TiO2-EP and TiO2-SA were one half and one
third lower than those obtained with TiO2-SG, respectively.
Noteworthy, regardless of the catalyst being used, high
levels of TOC removal were attained at the end of 3 h of
irradiation. However, kapp for the reactions using TiO2-EP and
TiO2-SA at the studied conditions were lower than the rate con-
stant (k) of the corresponding photochemical degradation
reaction (Table 1). As shown before, in the pure photochemical
regime using the same irradiation conditions, no TOC conver-
sion was observed, which means that organic molecules are
only being photo-transformed and not actually eliminated. In
the case of the photocatalytic process, not only trans-resvera-
trol is being removed from the solution but also most of the
intermediate compounds are being degraded up to complete
mineralization.
Fig. 2 Evolution of the normalized concentration (C/C0) of trans-resveratrol (a)
and area of the HPLC peak corresponding to cis-resveratrol (b) during photoche-
mical degradation reactions under different UV-Vis irradiation intensities: φ/φ0
=
■
□
●
○
▲
1 ( ); φ/φ0 = 0.78 ( ); φ/φ0 = 0.44 ( ); φ/φ0 = 0.16 ( ); φ/φ0 = 0.05 ( ). Inset
(a): correlation between light intensity and trans-resveratrol conversion after
5 min of irradiation (X5 min).
conditions. As light intensity decreases, there is a progressive
increase in the amount of cis-resveratrol being formed (and
not being degraded) with a gradual decrease in its disappear-
ance rate.
The slower rates observed for trans- and cis-resveratrol
depletion when low intensities are used can be attributed to
the lower ratio between the amount of photons available and
the number of molecules in solution.
3.2 Photocatalytic degradation of trans-resveratrol
The different results obtained with the various TiO2 cata-
lysts can be ascribed to the distinct physico-chemical charac-
teristics of the materials. It has been recognized that TiO2
photocatalytic efficiency is related to surface and structural
properties of the semiconductor such as crystal structure,
3.2.1 Effect of TiO2 nature. Photocatalytic degradation of
trans-resveratrol was performed under irradiation at λ ≥
365 nm using different TiO2 catalysts. This wavelength interval
was selected because no mineralization was observed in the
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surface area, particle size distribution, porosity, band gap and other hand, the pKa values of trans-resveratrol corresponding
surface hydroxyl density.8 In this case, all TiO2 samples are to the mono-, di- and tri-protonation of the molecule are 9.3,
mainly constituted by anatase crystallites, but with different 10.0 and 10.6, respectively.27 At the working pH conditions
particle sizes. TEM images of the different TiO2 materials (not trans-resveratrol is, therefore, in its non-ionized form. Thus, it
shown) confirm the presence of anatase crystallites of sizes is not expected that electronic interactions between the cata-
around 10 nm, 30 nm and 50 nm for TiO2-SG, TiO2-EP and lyst’s surface and the organic molecule play a major role in the
TiO2-SA, respectively. Particle size and, ultimately, crystallite photocatalytic process.
dimensions play an important role in the photoactivity of a
Overall, the smaller crystallite size, the higher content on
semiconductor catalyst, since the electron/hole recombination hydroxy surface groups and the higher surface area are
process has been shown to be particle size dependent.22 Gen- believed to be the main features that make TiO2-SG the most
erally it is assumed that the smaller the particle (in a nano- efficient catalyst for trans-resveratrol photodegradation.
metric range), the higher the efficiency of the photocatalytic
3.2.2 Effect of irradiation wavelength. Similarly to the
process. Nevertheless, some studies revealed that the photoca- photochemical degradation study, the photocatalytic degradation
talytic efficiency does not monotonically increase with decreas- of trans-resveratrol was performed using TiO2-SG under different
ing particle size, and there exists an optimal particle size of irradiation wavelengths: UV, Vis and UV-Vis (Fig. 4). As in the
about 10 nm for the pure nanocrystalline TiO2 photocata- case of pure photochemical reactions, trans-resveratrol was more
lyst.22,23 Among the materials tested, TiO2-SG is the one consti- easily removed when UV-Vis light was used and the lower photo-
tuted by smaller crystal particles, which could be one of the catalytic efficiency was obtained under UV irradiation.
reasons for the higher efficiency observed with this material.
Pseudo-first order apparent rate constants (kapp) for the
Another important characteristic that contributes to the photocatalytic reactions performed under different irradiation
performance of TiO2 photocatalysts is the presence of surface sources are listed in Table 1. The highest value of kapp was
hydroxy groups. The presence of OH surface groups was con- obtained for the UV-Vis system and the lowest for the reaction
firmed by DRIFT analysis of the TiO2 materials (Fig. SI3‡). under UV light. Since trans-resveratrol absorbs at the different
DRIFT spectra of the different materials show the presence of wavelength ranges of irradiation, one cannot exclude the con-
a weak band at around 1640 cm−1 caused by the bending tribution of direct photodegradation when studying the photo-
vibration of coordinated water as well as from Ti–OH groups.24 catalytic process. In order to quantify the effect of the
The presence of a broad band located between 2600 and photocatalytic contribution, we calculated the ratio between
3800 cm−1 attributed to the stretching vibration of hydrogen- the kinetic rate constants obtained for photocatalytic (kapp
)
bonded surface water molecules and hydroxy groups can also and photochemical (k) reactions. It was observed for all cases
be observed.19,25 This vibration is much more pronounced for that the utilization of TiO2-SG photocatalyst produced an
TiO2-SG, which suggests the existence of a higher availability increase in the kinetics of trans-resveratrol degradation. This
of OH groups at the surface of this material. In photocatalytic enhancement of the kinetics of the process was more evident
reactions, the surface OH groups and adsorbed water react when UV irradiation was used, with an increase of almost
with photogenerated holes leading to the formation of 100% on the value for the kinetic rate constant. For Vis and
hydroxyl radicals (HO˙) with strong oxidation power that are UV-Vis photocatalytic systems the increase was 55% and 34%,
able to degrade organic compounds until their mineralization. respectively.
The surface area of the photocatalysts is another aspect to
be taken into account since it is directly related to the concen-
tration of active sites for adsorption and reaction. In this case,
the measured BET surface areas for the TiO2 materials used
were 99, 56 and 14 m2 g−1 for TiO2-SG, TiO2-EP and TiO2-SA.
After the dark adsorption period a decrease of 31% on the con-
centration of trans-resveratrol was observed when TiO2-SG was
used and of 22% and 23%, respectively, for the photocatalytic
reactions using TiO2-EP and TiO2-SA. This indicates that
adsorption of trans-resveratrol on the surface of the TiO2-SG
catalyst may play an important role in the efficiency of the
photocatalytic degradation process. It is known that the
adsorption process on the photocatalyst surface depends also
on the electrical charge of both the organic substrate and the
catalyst. Due to the amphoteric character of TiO2, either a posi-
tive or negative charge can develop on its surface depending
on the pH range. The point of zero charge of TiO2, i.e., the pH
at which the surface charge is neutral, is around pH = 6.26
Fig. 4 Evolution of the normalized concentration (C/C0) of trans-resveratrol
Since reactions were performed at pH = 5.6, it is expected that
during the photocatalytic experiments using TiO2-SG and different irradiation
■
●
▲
the catalyst’s surface may be slightly positively charged. On the wavelengths: UV-Vis ( ), Vis ( ) and UV ( ).
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It is also relevant to notice that in the pure photochemical takes place through “a)” (see Scheme 1), one of the fragments
regime, when UV and Vis irradiation was used, no TOC may undergo oxidation with HO˙ to photoproduct 3 (tR
=
removal was observed after 3 h of irradiation. In the case of 10.9 min, 4-hydroxybenzaldehyde, m/z = 121 g mol−1), while
the photocatalytic process, 65% and 75% of TOC reduction the other fragment may also add HO˙ to yield 4 (tR = 9.0 min,
was achieved using UV and Vis irradiation, respectively. These 3,5-dihydroxybenzaldehyde, m/z = 137 g mol−1). If, in turn, the
results indicate that in the presence of a photocatalyst (TiO2- bond cleavage takes place through “b)”, addition of HO˙ leads,
SG in this case), trans-resveratrol is not only more rapidly con- in various steps, to photoproduct 5 (tR = 7.6 min, 2-(3,5-dihy-
verted to its intermediates but there is also mineralization of droxyphenyl)-2-hydroxyacetic acid, m/z = 183 g mol−1). Finally,
other organic compounds present in the reaction media to intermediate
3 may undergo further oxidation by HO˙
CO2 and H2O.
addition to yield 6 (tR = 8.8 min, 4-hydroxybenzoic acid, m/z =
137 g mol−1).
From the results above it is clear that HO˙ is the main reac-
tive species in the photodegradation of trans-resveratrol.
Similar observations were reported by Bader et al., who found
also intermediates 3, 4 and 5 as products of γ-ray radiolysis of
aerated trans-resveratrol solutions.28
3.3 Reaction mechanism
Samples taken at 15 min of irradiation were analyzed to deter-
mine the reaction photoproducts by high-resolution mass spec-
troscopy, as described above. The chromatograms showed a
number of peaks, of which only the main ones were identified as
a function of the working conditions (Table 1). Considering the
observed photoproducts, we propose the mechanism described
in Scheme 1 for both the direct and photocatalyzed degradation
of trans-resveratrol (tR = 17.1 min, m/z = 227 g mol−1). cis-Resvera-
trol was also identified (tR = 20.7 min) as an intermediate, prob-
ably generated following a photoisomerization.
4. Conclusions
Photochemical and photocatalytic degradation of trans-resvera-
trol can be achieved under different irradiation wavelengths,
ranging from the UV to the Visible. In both cases, the main
degradation intermediate is its geometrical isomer, cis-resvera-
trol. In the absence of a catalyst, irradiation in the whole spec-
tral range (UV-Vis) leads to trans-resveratrol conversion faster
than when only UV or Vis light is used. The degree of trans-
resveratrol conversion has been shown to be proportional to
the UV-Vis irradiation intensity. Additionally, under UV-Vis
irradiation a decrease in the organic content is observed, as a
result of the partial mineralization of trans-resveratrol and of
its photodegradation intermediates.
Two different pathways are proposed: (i) a multi-step dehy-
dration process to yield intermediate 1 (tR = 17.7 min, 3,4′,5-tri-
hydroxydiphenylacetylene, m/z = 225 g mol−1 18
which may
)
derive from hydrogen abstraction from the double bond of
trans-resveratrol (tR = 17.1 min) by two hydroxyl radicals (HO˙)
with loss of two water molecules or from addition of two
hydroxyl radicals to both of the carbon atoms of the double
bond followed by subsequent dehydration of the so-formed
diol; (ii) addition of HO˙ to the double bond followed by
capture of a hydrogen (H˙) to yield photoproduct 2 (tR
=
TiO2 can be successfully used as a catalyst for the photo-
catalytic degradation of trans-resveratrol. The efficiency of this
material depends on its intrinsic physical–chemical properties.
Among the materials tested, the best performance was
obtained for TiO2 produced by a sol–gel method, which is con-
stituted by anatase crystallites of very small dimensions and
shows higher surface area and a higher amount of hydroxy
surface groups. The photocatalytic process shows better
efficiency for trans-resveratrol mineralization than the photo-
chemical one. The best photocatalytic system is the one using
TiO2-SG as a catalyst and UV-Vis irradiation, producing the
fastest trans-resveratrol degradation and the highest organic
content elimination.
8.4 min, 5-(1-hydroxy-2-(4-hydroxyphenyl)ethyl)benzene-1,3-
diol, m/z = 245 g mol−1). Two different types of bond cleavage
in 2 may lead to intermediates 3, 4 and 5. If the bond cleavage
The identification of six photoproducts allowed us to
propose a reaction mechanism for trans-resveratrol photo-
degradation, mainly based on successive reactions with HO˙
leading to oxidation and fragmentation of the molecule.
Acknowledgements
This work was partially supported by Pest-C/EQB/LA0020/2011,
financed by FEDER through COMPETE and by FCT – Funda-
ção para a Ciência e a Tecnologia. Additional funding by
Scheme 1 Proposed mechanism for direct and photocatalyzed degradation of
trans-resveratrol.
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Photochemical & Photobiological Sciences
national research grant REEQ/1106/EQU/2005 is acknowl- 13 J. Burns, T. Yokota, H. Ashihara, M. E. J. Lean and
edged. R.R.N.M. and C.G.S. gratefully acknowledge FCT for
their PhD (SFRH/BD/65425/2009) and Post-Doctoral (SFRH/
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644 | Photochem. Photobiol. Sci., 2013, 12, 638–644
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