Sensitized photooxidation of triclosan pesticide. A kinetic study in presence of vitamin B2

Article abstract of DOI:10.1016/j.jphotochem.2021.113213

Kinetic and mechanistic aspects of Riboflavin (Rf, vitamin B2)-sensitized photochemical degradation of Triclosan (TCS) have been studied by time-resolved and stationary techniques. TCS is a broadly-used biocide, also employed in a series of industrial products as a multifunctional additive. Rf, in the presence of light and oxygen, generates singlet molecular oxygen (O₂(¹Δ_g)) and superoxide radical anion (O₂^[rad]–). Results indicate that TCS quenches the triplet excited state of Rf (³Rf*), O₂(¹Δ_g), and O₂^[rad]–. The reactive rate constant for the interaction TCS-O₂(¹Δ_g) is 62-faster in alkaline medium with respect to pH 7. Photosensitized degradation of TCS by Rf was much faster than for phenol, a model pollutant, in similar conditions of pH. Kinetic analysis indicated that the reaction of TCS with ³Rf* and/or O₂^[rad]– is the prevailing oxidative route. Based on the environmental importance of the TCS, the products were determined by UHPLC-MS / MS analysis.

Full text of DOI:10.1016/j.jphotochem.2021.113213

Journal of Photochemistry & Photobiology, A: Chemistry 412 (2021) 113213
Contents lists available at ScienceDirect
Journal of Photochemistry & Photobiology, A: Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Sensitized photooxidation of triclosan pesticide. A kinetic study in presence
of vitamin B2
a
b
c
c
´
Agustina Reynoso , David Possetto , Eduardo De Geronimo , Virginia C. Aparicio ,
a,
a,
´
Jose Natera *, Walter Massad *
a Instituto para el Desarrollo Agroindustrial y de la Salud (IDAS), CONICET – UNRC, Depto. de Química – FCEF-QyN - Universidad Nacional de Río Cuarto, Argentina
b
´
Instituto de Investigaciones en Tecnologías Energeticas y Materiales Avanzados (IITEMA) - CONICET – UNRC, Depto. de Química – FCEF-QyN - Universidad Nacional
de Río Cuarto, Argentina
c
´
Instituto Nacional de Tecnología Agropecuaria - Centro Regional Buenos Aires Sur. Estacion Experimental Agropecuaria Balcarce, Argentina
A R T I C L E I N F O
A B S T R A C T
Keywords:
Kinetic and mechanistic aspects of Riboflavin (Rf, vitamin B2)-sensitized photochemical degradation of Triclosan
(TCS) have been studied by time-resolved and stationary techniques. TCS is a broadly-used biocide, also
employed in a series of industrial products as a multifunctional additive. Rf, in the presence of light and oxygen,
Triclosan
Photodegradation
Pesticide
•
generates singlet molecular oxygen (O2(1Δg)) and superoxide radical anion (O2–).
Reactive oxygen species
Vitamin B2
•
Results indicate that TCS quenches the triplet excited state of Rf (3Rf*), O2(1Δg), and O2–. The reactive rate
constant for the interaction TCS-O2(1Δg) is 62-faster in alkaline medium with respect to pH 7. Photosensitized
degradation of TCS by Rf was much faster than for phenol, a model pollutant, in similar conditions of pH. Kinetic
•
analysis indicated that the reaction of TCS with 3Rf* and/or O2– is the prevailing oxidative route. Based on the
environmental importance of the TCS, the products were determined by UHPLC-MS / MS analysis.
1. Introduction
resistance from microorganisms, as is the case with antibiotics, and is
highly toxic to aquatic organisms [11,12]. The high toxicity of TCS is
Triclosan (TCS) is a fungicide and broad-spectrum antimicrobial
used as a preservative and disinfectant in personal care products, such as
soaps, deodorants, antiperspirants, detergents, cosmetics, and anti-
microbial creams. Moreover, it is used as an additive in plastics, poly-
mers, and textiles to give these materials antibacterial properties [1–4].
The extensive use of this antimicrobial agent has caused its incorpora-
tion into the environment through sewage, thus resulting in a frequent
pollutant of terrestrial environments [5].
due to the by-products that are generated during its photodegradation,
within which the most important are dioxins [13,14].
TCS is a chlorinated phenoxyphenol with a pKa ~ 7.9 [15].
Photolysis experiments have shown that it is photodegradable in its
phenolate form (TCSꢀ ), while it is photostable in its phenolic form [16].
The chemical structures for neutral and ionized species of TCS are shown
in Fig. 1.Therefore, it is important to know what happens with TCS in
different aquatic environments., where TCS could be degraded by pho-
tosensitized processes. It is well documented that different natural
photosensitizers are present in aquatic environments [17–19]. One of
the main photosensitizers are humic acids (HA), which are characterized
by generating singlet oxygen (O2(1△g)) with a quantum yield of 0.04
[20]. Another photosensitizer present in rivers, lakes, and seawater is
the vitamin B2 (Riboflavin, Rf) in concentrations 0.1–2 nM [21], which
generates O2(1△g) with a quantum yield = 0.47 [22], and the super-
It is detected in concentrations of parts per trillion in surface waters
and parts per million in biosolids. The high concentration of TCS in
biosolids and aquatic sediments should not only be attributed to its
extensive use, but also to the strong adsorption on organic matter and
environmental persistence of this compound [6], thus generating a
negative impact on aquatic ecosystems. Other authors have shown that
TCS is a pollutant with a long lifetime (t1/2 ~ 8 days) [7], that it depends
on the environmental compartment and the prevailing conditions [8],
which can cause potential long-term risks to human health [9,10,11].
The accumulation of the compound in the ecosystem can lead to
•
oxide anion (O2-) with a quantum yield of 0.009 [23]. Both photosen-
sitizers have been reported as capable of degrading different phenolic
pollutants [22,24–29].
* Corresponding authors.
E-mail addresses: jnatera@exa.unrc.edu.ar (J. Natera), wmassad@exa.unrc.edu.ar (W. Massad).
https://doi.org/10.1016/j.jphotochem.2021.113213
Received 8 August 2020; Received in revised form 28 January 2021; Accepted 14 February 2021
Available online 20 February 2021
1010-6030/© 2021 Elsevier B.V. All rights reserved.

A. Reynoso et al.
Journal of Photochemistry & Photobiology, A: Chemistry 412 (2021) 113213
2.2. Apparatus and methods
The solvent in all the experiments in neutral media was a mixture of
H2O-MeOH in equal amounts, to ensure complete solubilization of TCS.
Whereas a 0.01 M NaOH aqueous solution was used as basic media.
Absorption spectra were recorded using a Hewlett Packard 8452A
diode array spectrophotometer.
2.2.1. Continuous photolysis
Stationary aerobic photolysis of solutions containing TCS and the
sensitizer (Rf, A445nm = 0.5) were carried out using a home-made pho-
tolyzer [30] with two blue light emitting diodes as light sources (LEDs,
λmax 467 nm, polarization current 30 mA, and flux of photons of
40 W/m2 for each one). The photon flux of the blue LED was measured
with a Luxon SPM-1116SD solar energy meter. The solution to be pho-
tolysed was placed in a typical fluorescence cell with an optical path
length of 1 cm. The sample was magnetically stirred throughout the
experiment guaranteeing homogeneity in the irradiation.
2.2.2. Laser flash photolysis experiments
Fig. 1. Structures and absorption spectra of acid-base equilibrium of TCS
(0,2 mM) species. (a): TCS in H2O-MeOH 1:1v/v; (b): TCS in H2O-MeOH 1:1v/v
plus 10 mM NaOH.
Laser flash photolysis H2O-MeOH solutions of Rf 0.05 mM saturated
with argon was carried out with equipment previously described [31,
32]. To perform the transient absorption spectra, the signals were
adjusted at each wavelength employing a bi-exponential decay function.
Lifetime values were shared in the global fit using origin 8.0 software
(Origin lab Corp. Northampton, MA, USA). The spectra were obtained by
plotting the pre-exponential factors (ai) for each lifetime using ΔA(t) =
All this background shows the importance of investigating the pho-
tosensitized degradation of TCS by Rf. For this reason, we perform a
kinetic and mechanistic study of this system in different aqueous media.
Also, the photoproducts were identified.
∑
τi
a0 aieꢀ t/ with i = 2 [29–35].
2. Experimental
i
2.1. Materials
2.2.3. O2-uptake experiments
For the O2-uptake experiments, the photolysis measurements were
carried out in a home-made photolyzer [33] which is provided with a
quartz halogen lamp (OSRAM XENOPHOT HPLX 64640, 150w-24 V
G35, OSRAM Augsburg, Germany). The light source was equipped with
an appropriate cut-off filter according to sensitizer used, 420 nm for Rf
experiments, and 320 nm for PN experiments. These cut-off filters were
used to remove radiation were TCS absorbs. The light was passed
through a water filter and focused into a hermetically sealed reaction
cell with a specific oxygen electrode (Orion 97ꢀ 08). The solutions were
continuously stirred.The reactive rate constant of deactivation of
Triclosan (TCS) 5-Chloro-2-(2,4-dichlorophenoxy)phenol, superox-
ide dismutase from bovine erythrocytes (SOD), deuterium oxide 99.9 %
(D2O), riboflavin (Rf), ammonium acetate, and formic acid were pur-
chased to Sigma Chem. Co. Perinaphthenone (PN), furfuryl alcohol
(FFA), monodeuterated methanol (CH3OD), and sodium hydroxide were
from Aldrich (Milwaukee, WI, USA). Methanol (HPLC quality) was ob-
tained from Sintorgan. All the experiments were carried out at room
temperature, using freshly prepared solutions.
Scheme 1. Possible pathways for the Rf-sensitized process in the presence of a hypothetical electron donor (TCS), which does not absorb at the irradiation
wavelength. The overall rate constant involving both processes will be named kt, being kt = kr + kq, which accounts for the physical, kq, and reactive, kr, contribution
to the overall deactivation, respectively.
2

A. Reynoso et al.
Journal of Photochemistry & Photobiology, A: Chemistry 412 (2021) 113213
O2(1Δg) by TCS, kr (see process (11) presented in Scheme 1), was
determined by the method introduced by Foote and Ching [34]. The PN
synthetic sensitizer was used as O2(1Δg) generator. Assuming the reac-
tion of O2(1Δg) with the substrate is the only form of oxygen con-
sumption, the ratio of the slope of pseudo first-order plot for oxygen
consumption by TCS and by a reference compound (with a known krRef
value), under identical conditions, is equal to krTCS / krRef ratio. The
reference (Ref) used was FFA, with a reported krRef value in H2O-MeOH
of 6.2 × 107 Mꢀ 1 s-1 [28,29].
In all cases, conversions less than 10 % were used to avoid possible
interference from the photoproducts.
2.2.4. Time resolved phosphorescence detection of O2(1Δg) (TRPD)
The overall rate constant for deactivation of O2(1Δg) by TCS (kt = kq
+ kr, processes (12) and (11), in Scheme 1) was determined using a
previously described method [36]. Briefly, a Nd:YAG laser (Spectron
Laser System, SL400) was used as excitation source. The output at
355 nm was used to excite PN, and the O2(1Δg) emission at 1270 nm was
detected at right angles using an amplified Judson J16 / 8Sp germanium
detector, after passing through the appropriate filters. The detector
output was coupled to an Agilent Technologies DSO 6012A digital
oscilloscope and a personal computer for signal processing. In general,
6–8 shots were averaged, in order to achieve a good signal / noise ratio,
from which the decay curve was obtained [37]. D2O-MeOD, instead of
H2O-MeOH, was used as a solvent to extend the lifetime of O2(1Δg) [29].
From a first order fitting of the decay the lifetime of O2(1Δg) was eval-
Fig. 2. Changes in the UV–vis difference absorption spectra in air equilibrated
H2O-MeOH 1:1 v/v solution of 0.035 mM Rf plus 0.1 mM TCS upon irradiation
at 470 nm, taken vs. 0.035 mM Rf in the same solvent. Inset A: Changes in the
UV–vis difference absorption spectra of 0.035 mM Rf plus 0.1 mM TCS vs
0.035 mM Rf in Ar-saturated H2O-MeOH 1:1 v/v solutions. Irradiation times for
both figures: 0, 60, 120, 480, 2040s. Inset B: absorbance decrease of TCS at
280 nm in air equilibrated (-◼-) in Ar-saturated (-▸-).
was irradiated at λ> 450 nm in the same device used in the steady-state
photolysis experiments previously described in section 2.3. Aliquots to
different irradiation time were analyzed by UHPLC-MS / MS to photo-
products determination. The identification of the degradation products
formed was carried out from the mass-to-charge ratios, isotopic relation
of chlorine atoms, and spectra of the [Mꢀ H]ꢀ ions produced in the
negative electron spray ionization- (ESIꢀ ) mode. Also, the MS spectra
obtained were compared with reference spectra reported by other au-
thors [38,39]. This procedure allows us to guarantee the proposed
products structure with 99 % confidence.
uated in the presence (τ) and absence (τ0) of TCS, and the data were
plotted as a function of TCS concentration, according to a simple
Stern-Volmer treatment.
2.3. TCS degradation products
To obtain the photolysis products of TCS in the presence of Rf, a
home-made photoreactor was used, equipped with a merry-go-round
equipped with nine LEDs that emitting at 470 nm. In this region, the
absorption was due only to Rf. The irradiation was carried out for
300 min, taking aliquots every 30 min, which were kept in the dark until
further analysis.
MZmine 2.13.1 was used for data extraction and analysis of the
transformation product [40].
The degradation products of TCS were identified by an ultra-
performance liquid chromatography-tandem mass spectrometer
(UHPLC-MS/MS) analysis, using an ACQUITY UPLCTM system (Waters
Corp., Milford, MA, USA) coupled to a Quattro Premier TM XE tandem
quadrupole mass spectrometer (Waters, Manchester, UK). Mass Lynx 4.1
software equipped with Quan Lynx software (Waters) was used to con-
trol the instruments and to process the data. ACQUITY UPLC equipment
consisting of a binary pump, an auto-sampler, and a column heater was
used. Chromatographic separation was carried out on a UPLC BEH C18
3. Results and discussion
The results are interpreted and discussed by the mechanism shown in
Scheme 1, where Rf is the photosensitizer and TCS is the substrate that
can react with Rf electronic excited states (Rf*) or with the reactive
oxygen species (ROS) generated from Rf*. This scheme represents a
generic photosensitized process in which the absorption of visible light
promotes the sensitizer to electronically excited states singlet (1Rf*) and
triplet (3Rf*) (processes (1) and (3)). 3Rf* can transfer an electron to
oxygen in its ground state generating the superoxide radical anion
species (O●2 ꢀ ) (process (7)) or transfer energy to it generating the singlet
oxygen species (O2(1Δg)) (process (8)). The latter can decompose by
collision with solvent molecules (process (10)) or it can interact chem-
ically (process (11)) or physically (process (12)) with the photo-
oxidizable substrate.
(1.7 μm, 2.1 × 100 mm; Waters). Solvent A was water (0.1 mM NH4Ac/
0.01 % formic acid), and solvent B was methanol (0.1 mM NH4Ac/0.01
% formic acid). The flow rate was set at 0.3 mL minꢀ 1 and the column
temperature was 35 ◦C.The chromatography was performed as follows:
B was 10 % in 0ꢀ 0.5 min, linearly increased to 90 % in 0.5ꢀ 9 min; held
at 90 % for 9 ꢀ 12 min and returning to the initial condition in 1 min. An
auto-sampler was used to inject 20 μL of the samples. The Quattro Pre-
mier TM XE tandem quadrupole mass spectrometer was operated in a
negative mode with the electrospray-ionization (ESI) source. The oper-
ating parameters were optimized under the following conditions:
capillary voltage, 3 kV, ion source temperature 120 ◦C, desolvation
temperature 450 ◦C, cone gas flow 80 L hꢀ 1, desolvation gas flow 800 L
hꢀ 1(both gases were nitrogen) and collision gas flow 0.3 mL min-
ꢀ 1(argon gas). Liquid chromatography–mass spectrometry (LC–MS) full-
scan was used to select a list of molecular ions whose levels are signif-
icantly altered between case and control samples. These molecules are
then subjected to precursor-ion (PI) scans to acquire MS/MS data by
manually setting the m/z values of the PIs.
3.1. Photosensitized degradation
The experiments were performed at pH conditions where the neutral
form of TCS is the main species (Fig. 1). Spectral changes were observed
in a difference spectra of an irradiated (λ > 450 nm) solution of
0.035 mM Rf plus 0.1 mM TCS vs 0.035 mM Rf in H2O–MeOH (Fig. 2), a
region where TCS does not absorb (see figure S1 with the absorption
spectrum of Rf for comparative purposes). The time evolution of the
absorption spectrum was evaluated in air (Fig. 2, main) and inert at-
mosphere (Fig. 2, inset), to discern the oxygen participation in the
photosensitized process. The differences between the spectra shown in
A solution of 0.1 mM TCS plus 0.04 mM Rf in MeOH-H2O mixture
3

A. Reynoso et al.
Journal of Photochemistry & Photobiology, A: Chemistry 412 (2021) 113213
might indicate the interaction between TCS and the electronic excited
states of the sensitizer, quenching of 3Rf* by TCS was investigated.
The quenching of 1Rf* was not taken into account, because at the TCS
concentrations used in this work, (< 1 mM) the excited singlet of Rf
(lifetime 4.5 ns [41]) cannot be intercepted by the quencher.
Therefore, the reaction between the TCS and 3Rf*, (process (9)), is
the most likely explanation for the behavior observed in stationary
photosensitization experiments in the absence of oxygen.
The interaction 3Rf*-TCS was studied through laser flash photolysis
experiments in Ar-saturated H2O–MeOH solutions. The presence of TCS
in the mM concentration range neatly decreases 3Rf* lifetime demon-
strating the interaction between them. A Stern-Volmer treatment was
done at 670 nm, where only 3Rf* absorbs. Eq. 1 was used to determine
the bimolecular rate constant (3kq) for the quenching of 3Rf* by TCS
(process (9)).
3
3τ0
/
3τ = 1+ k3qτ0[TCS]
(1)
where 3
τ
and 3τ0 are the experimentally determined lifetimes of 3Rf* in
Fig. 3. Decay-associated spectra of Rf (0.05 mM) in argon-saturated MeOH-
H2O 1:1 v/v in the absence (-◼-) 3Rf*
τ
= 32
μ
s, (-●-) RfH•
τ
= 490
μs and in the
the presence and the absence of TCS, respectively. From the slope of plot
3τ0
/
3τ vs [TCS] was possible to obtain the value of 3kq (Fig. 3, inset). 3Rf*
presence of TCS (-△-) 3Rf*
τ
= 5.6
μ
s, (-◊-) RfH•
τ
= 280 μs. Inset:
quenching yielded a value of 3kq = (0.58 ± 0.04) x 109 Mꢀ 1sꢀ 1. This
value is similar to the reported bimolecular rate constant for phenol
quenching of 3Rf* (Table 1). TCS has a phenolic moiety in its structure,
which may explain this similarity. Furthermore, this result is significant
Stern–Volmer plot for the quenching of 3Rf* by TCS in MeOH-H2O 1:1 v/v.
τ0 and
τ
refer to 3Rf* lifetimes in the absence and the presence of TCS,
respectively. Error bars indicate one standard deviation.
considering that phenol is considered
contaminant model compound.
a paradigmatic water-
Table 1
Rate constant values for the quenching of triplet excited state of Riboflavin by
TCS (3kq). Reactive (kr) and overall (kt) quenching rate constant of O2(1Δg) by
TCS and ratio kr/kt up on PN sensitized irradiation. Literature values for phenol
were also included in the table for comparative purposes. Solvent: H2O–MeOH,
1:1 v/v. kt values were determined in D2O–MeOD, 1:1 v/v solutions.
In the absence of TCS, the transient absorption spectrum of Rf
measured is similar to that previously reported (see Figure S2) [22,42].
The spectrum at 1 μs after the laser pulse presents two a typical maxima,
at 680 nm and 310 nm, attributed to 3Rf* absorption spectrum [22].
Whereas at 180 μs the spectrum shows a wide band between 500 and
Compound
Solvent
3kq(x109)
kt(x108)
kr(x107)
kr/kt
600 nm, with a maximum of 510 nm which can be assigned to the radical
RfH• absorption. RfH• is formed by the triplet-triplet annihilation given
the cation (Rf•+) and anion (Rf•ꢀ ) radicals of Rf (process (5)), followed
by protonation of Rf•ꢀ to produce the neutral radical of Rf (process (14)
in scheme 1). This acid-base equilibrium has a reported pKa = 8.3 [22].
To achieve a better signal-to-noise ratio of the transient spectra, and
considering that 3Rf* and RfH• are the principal contributing species,
the global fitting method [31] was used for the data analysis. Fig. 3
shows the decay-associated spectra of 3Rf* and RfH• obtained using a
biexponential decay for each decay. The values obtained from the fitting
Mꢀ 1sꢀ 1
Mꢀ 1sꢀ 1
Mꢀ 1sꢀ 1
TCS
MeOH-
H2O1:1
v/v
0.58 ± 0.04
0.13 ± 0.02
0.087
MeOD-
D2O1:1
v/v
0.15 ± 0.06
H2O -
0.01 M
NaOH
D2O -
8.04 ± 0.01
0.18
4.5 ± 0.4
0.01 M
NaOH
MeOD
H2O (b)
H2O, pH
10.3(c)
MeOH
(d)
were τ1 = 32 μs and τ2 = 490 μs. The lifetime measured and
decay-associated spectrum for τ1 agrees with the one attributed to the
3Rf* [29,43] (Fig. 3, inset) as long as the spectrum associated with τ2 is
attributed to the species RfH• [29].
Phenol
0.014 (a)
2.68
0.008
0.09
0.00114(b)
5
Figure S3 shown, the transient absorption spectrum of a solution of
Rf (Abs 0.32 at 355 nm) +0.38 mM TCS (95 % 3Rf* quenched by the
TCS). In the Figure S3 no new bands were observed with respect to
Figure S2. Therefore, a global fit with two exponential decays associated
to 3Rf* and RfH• were used for the data fitting. The associated spectrum
0.48
(a) [29](b)
[58](c)
[46]; (d)
[20][57]
attributed to 3Rf* has a lifetime of τ1 = 5.6
μs and matches with that
obtained for 3Rf* in the absence of TCS. While an increase in the ab-
sorption band attributed to RfH•
(τ2 = 280 μs) was observed. When it is
Fig. 2 and inset A of Fig. 2, might indicate that the photodegradation of
TCS eventually proceeds by different mechanisms. The effect of O2
participation in the mechanism is corroborated in the Inset B of Fig. 2,
where it is shows the differences in the changes observed at 280 nm, at
the maximum of TCS absorption, in the two different conditions.
Therefore, in absence of oxygen, spectral changes may be a consequence
of the interaction of TCS with *Rf, whereas those in air equilibrated
solution should be the result of possible reactions of TCS with ROS
generated from the interaction of 3Rf* and O2.
compared to the spectrum obtained in the absence of TCS. The increase
of the [RfH•] in the presence of TCS may be attributed to an electron
transfer reaction from TCS to 3Rf* (processes (9) and (14) in scheme 1).
•
ꢀ
3.3. The interaction of TCS with O2
To evaluate the possible participation of the superoxide anion in the
photodegradation process (process (19)), rates of oxygen uptake were
evaluated, irradiating H2O–MeOH solutions of 0.03 mM Rf plus 0.1 mM
TCS. These experiments were made in the absence and the presence of
3.2. The interaction of TCS with electronically excited states of Rf
•
the enzyme superoxide dismutase (SOD), an O2ꢀ -scavenger. SOD cata-
Since the results obtained in steady-state photolysis experiments
lyzes the dismutation of O●2 ꢀ (process (21)) and has been utilized as a
4

A. Reynoso et al.
Journal of Photochemistry & Photobiology, A: Chemistry 412 (2021) 113213
carried out in MeOD/D2O and D2O plus 0.01 M NaOH. A lifetime of
37
μ
s was obtained for O2(1Δg) in D2O-MeOD and 53
μs in D2O in the
presence of 0.01 M NaOH in the absence of the quenchers. Rate con-
stants were determined by the TRPD method. The kt (kr + kq) value does
not depend on the type of sensitizer or potential interactions of the
substrates with excited states of the sensitizer.
The kt values showed in Table 1 indicate dependence with the ioni-
zation grade of TCS. For TCSꢀ , kt is 30-fold faster than for neutral TCS.
However, in the case of phenolate, kt is 191-fold faster than phenol.
Therefore, the ionization grade has a minor effect in the reactivity of TCS
when is compared to phenol. OH-ionization in OH-aromatic compounds
produces an enhancement in the electron-donor ability of the compound
and, hence, an increase in its O2(1Δg) quenching ability. The mechanism
involves an intermediate complex possessing charge-transfer character
extensively studied [46,51], which depends on the position of the
substituents.
Another observation about the results obtained is that the values of
the kinetic constant for TCS were higher than for phenol.
However, the values of kr determined were relatively lower that
respect to kt, this would indicate that TCS interact with O2(1Δg) mostly
by a physical quenching process (process (12)). The kr/kt ratios in
Table 1 represent a measure of the efficiency of the degradation pathway
via reaction with O2(1Δg). When the kr/kt is ~ 1 indicates high effec-
tiveness in the O2(1Δg)–mediated degradation. Table 1 shows that kr and
the ratio kr/kt also increases in basic medium.
Fig. 4. Stern–Volmer plots for the quenching of O2(1△g) phosphorescence by
TCS in D2O solution plus 0.01 M NaOH (a) and MeOD-D2O 1:1 v/v solution (b).
Inset: first order plots for oxygen uptake in MeOH–H2O 1:1 v/v solution con-
taining PN + TCS (a), in aqueous solution plus 0.01 M NaOH (b) and
MeOH–H2O 1:1 v/v solution containing PN + FFA (c). [TCS] = [FFA] =0.1 mM
and PN as a sensitizer (A363 = 0.5). Error bars indicate one standard deviations.
According, to the low values obtained from the ratio kr/kt (Table 1),
this would indicate that most of the deactivation mechanisms of O2(1Δg)
proceed by a physical pathway and that even, under competitive con-
ditions favorable to process (11), the process reactive path that involves
O2(1Δg) can constitute only a minor contribution to the whole mecha-
nism of Rf-sensitized photooxidation of TCS.
quencher of O●2 ꢀ -mediated photooxidation in similar concentration to
those employed in other works [44,45]. Figure S4 shows that the
addition of SOD generates an approximately a 40 percent decrease in the
rate of oxygen uptake of TCS, which evidences the participation of O2●ꢀ
in the photodegradation of TCS.
The comparison of the quenching rate constant for the reaction of
TCS with 3Rf* (3kq = 0.58 × 109 Mꢀ 1sꢀ 1) or O2(1Δg) (kr = 0.13 × 107
2O●2 ꢀ + 2H+ + SOD → O2(3Σg-) + H2O2
(21)
M
ꢀ 1sꢀ 1), indicates that the oxidation of TCS mainly implies the partic-
ipation of 3Rf*.
3.4. The interaction of TCS with O2(1Δ )
3.5. Feasibility of TCS photooxidation
g
The formation of RfH● (processes (9) and (14)), detected by laser
flash photolysis (Fig. 3), is an indication that there is an electron transfer
reaction from TCS to 3Rf*. From a thermodynamic point of view, the
possibility of the electron transfer process between TCS and Rf is
evaluable through the Gibbs free energy change for an electron transfer
process [52]:
Numerous authors have reported that the reaction between O2(1△g)
and phenolic compounds depends on the degree of ionization of the
hydroxyl groups in the molecule [30,46]. Thus, the interaction
TCS-O2(1Δg) was evaluated in two different media: MeOH-H2O 1:1 v/v
and 0.01 M NaOH aqueous solution, to ensure the presence of the
neutral and ionized form of TCS, respectively.
Since Rf photolyzes very easily at pH > 9 [47], the well-known
O2(1Δg) sensitizer PN was employed instead of the vitamin [48]. PN
generates O2(1Δg) with a quantum yield of 1 in ethanol [30,49].
The reactive rate constant kr (processes (11)) was determined by
monitoring oxygen uptake upon photo-irradiation of PN-TCS mixtures,
through an already described actinometric method [50] using FFA as a
reference compound. The kr values were graphically determined (Inset
A, Fig. 4) in MeOH–H2O, 1:1 v/v (kr = (0.13 ± 0.02) x 107 Mꢀ 1sꢀ 1) and in
water at pH 12 (kr= (8.04 ± 0.01) x 107 Mꢀ 1sꢀ 1) in the presence of PN as
a photosensitizer. The results are summarized in Table 1. These differ-
ences in kr values may be due to the ionization of TCS phenolic moiety in
water at pH 12. Comparative bibliographic data for phenol is also shown
in Table 1, where is observed that the ionized species are more reactive
against O2(1Δg) than the neutral species.
Δ
ETG0 = E0(TCS/TCS+) – E0(Rf/Rf-) – ERf* + C.
(2)
Where the electrode potential of the electron donor, E0(TCS/TCS+)
=
0.68 V at pH = 7 [53], the Rf data has been previously reported: E0(Rf/Rf-)
= 0.80 V is the standard electrode potential of the acceptor; ERf*
=
(2.17 eV) is the 3Rf* energy, and the coulombic energy term C = 0.06 V
[49]. From eq (2) a value of ΔETG0= ꢀ 0.78 eV results which constitutes
an evidence for the operability of process (8).
As a consequence, the species O•2– could be formed through processes
(13)-(16) provided that process (19) is kinetically competitive with
O2(1Δg) generation step (process (8), kET). Since 3kq = 0.58 × 109
Mꢀ 1sꢀ 1 for the TCS and kET = 9.5 × 108 M–1 s–1 in MeOH-H2O (1:1, v/v)
(i.e., 1/9 of the diffusional value) [54], it can be inferred that, for the
same concentrations of TCS and dissolved O2(3Σgꢀ ), the rate value for the
generation of both RfH• -the initial O•2– precursorꢀ is almost equal than
the corresponding one for O2(1Δg). This finding, in addition to the
relatively low kr value exhibited by TCS, situates the O2(1Δg)-oxidative
pathway in a secondary level with respect to the O•2– mediated oxidation,
under Rf photosensitization.
Fig. 4 shows that the presence of sub-mM TCS quenches the O2(1Δg)
phosphorescence emission. These results unambiguously demonstrate
the existence of the interaction between the specific ROS and TCS, via
reactive (kr, process (11)) and/or physical quenching (kq, process (12)).
These possible O2(1Δg) deactivation processes were studied in detail as
described below.
To investigate the different reactivity towards singlet oxygen of
neutral TCS and its ionized form, O2(1Δg) quenching experiments were
5

A. Reynoso et al.
Journal of Photochemistry & Photobiology, A: Chemistry 412 (2021) 113213
The complexity and variability of natural waters make it difficult to
directly extrapolate our results. However, the presence of the natural
Table 2
Retention time and m/z of the TCS degradation products using Rf as a sensitizer
•
ꢀ
photosensitizer Rf and the O2 generated from 3Rf* (among other
sources) makes the degradation of the TCS a matter of time in surface
waters where solar radiation arrives.
at different irradiation times.
Peak N◦/ Photolysis time
Retention time
m/z
142
Name
(1) /30 min
3.92 min
Author contributions
• Agustina Reynoso and David Possetto conducted the experiments.
• Matías I. Sancho and Virginia C. Aparicio performed the (UHPLC-
MS/MS) analysis.
Chlorohydroquinone
(4) / 30 min
(2) /60 min
4.80 min
4.36 min
303
177
´
• Walter Massad and Jose Natera planned the different experiments
and collaborated with the preparation of the manuscript.
´
• Jose Natera organized the final discussion and, wrote the
Monohydroxy-triclosan
manuscript.
All authors discussed on the results and commented on the manu-
script. All authors have given approval to the final version of the
manuscript
3,5-dichlorocatechol
2,4-dichlorophenol
(3) / 300 min
4.42 min
160
Declaration of Competing Interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
3.6. Photooxidation products
According to our knowledge, we have not found a report on the Rf-
sensitized photooxidation of TCS and their potential photoproducts.
To elucidate the nature of TCS photoproducts, UHPLC-MS / MS exper-
iments were carried out. The obtained chromatograms reveal that TCS is
partially degraded after 30 min of irradiation. Moreover, it was possible
to identify some of the possible products of degradation at different
irradiation times. Figure S5 shows the mass spectra obtained by UHPLC-
MS / MS to the different retention times. The chemical structures
assigned to the products obtained and their different retention times are
shown in Table 2. At the end of the photolysis (300 min), four different
products were found, these products are similar to those originated from
typical oxidation and/or dimerization reactions of substrates with
phenolic groups [29,55,56].
Acknowledgments
Financial support from Consejo Nacional de Investigaciones Cien-
´
´
tíficas y Tecnicas (CONICET), Agencia Nacional de Promocion Científica
´
´
y Tecnologica (ANPCyT), Secretarías de Ciencia y Tecnica of the Uni-
versidad Nacional de Río Cuarto (SECyT UNRC).
Appendix A. Supplementary data
Supplementary material related to this article can be found, in the
online version, at doi:https://doi.org/10.1016/j.jphotochem.2021.
113213.
The exposed results lead to propose that the products come from the
electron transfer reaction between the riboflavin sensitizer and the
contaminant, originating the radical ionic species. Moreover, the radical
anion formed from Rf, in the presence of proton donor solvents and the
presence of oxygen, will form different reactive species, which may be
reacted with molecular TCS to give different oxidation products (see
scheme 1 with the proposed mechanism for TCS degradation).
As can be seen from Table 2, hydroxylation of the aromatic ring, it is
likely to lead to the formation of monohydroxy triclosan. This may be
followed by typical fragmentation of the ether bond [29,56,57] and
other species, such as chlorohydroquinone, dichlorocatechol, and
dichlorophenol are formed.
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