Inhibitory effects of natural organic matter on methyltriclosan photolysis kinetics

Article abstract of DOI:10.1039/c8ra03512a

This study evaluated the effects and related mechanisms of natural organic matter (NOM) on the photolysis of methyltriclosan (MTCS), a metabolite of triclosan. Addition of two representative NOM isolates, Pony Lake fulvic acid (PLFA-microbial origin) and Suwannee River fulvic acid (SRFA-terrestrial origin), significantly inhibited the direct photolytic rate of MTCS by ～70%. The MTCS photolytic rate in the presence of PLFA was greater than for SRFA. NOM not only suppressed photolysis by light-shielding, but also produced ROS to oxidatively degrade MTCS and/or triplet NOM (³NOM?) to sensitize degradation. The dual effects of light-screening and photo-sensitization led to an overall decrease in photolysis of MTCS with a positive concentration-dependence. Upon addition of NOM, EPR documented the occurrence of ¹O₂ and OH in the photolytic process, and the bimolecular k value for the reaction of ¹O₂ with MTCS was 1.86 × 10⁶ M^-1 s^-1. ROS-quenching experiments indicated that the contribution of OH (19.1-29.5%) to indirect photolysis of MTCS was lower than for ¹O₂ (38.3-58.7%). Experiments with D₂O further demonstrated that ¹O₂ participated in MTCS photodegradation. Moreover, the addition of sorbic acid and O₂ gas to the reaction confirmed the participation of ³NOM? as a key reactant in the photochemical transformation of MTCS. This is the first comprehensive analysis of NOM effects on the indirect photolysis of MTCS, which provides new insights for understanding the environmental fate of MTCS in natural environments.

Full text of DOI:10.1039/c8ra03512a

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Inhibitory effects of natural organic matter on
methyltriclosan photolysis kinetics†
Cite this: RSC Adv., 2018, 8, 21265
Wei Liu, ‡a Lide Jin,‡a Kai Chen,a Yanyan Li,a Randy A. Dahlgren,ab Meiping Maa
a
*
and Xuedong Wang
This study evaluated the effects and related mechanisms of natural organic matter (NOM) on the photolysis
of methyltriclosan (MTCS), a metabolite of triclosan. Addition of two representative NOM isolates, Pony Lake
fulvic acid (PLFA-microbial origin) and Suwannee River fulvic acid (SRFA-terrestrial origin), significantly
inhibited the direct photolytic rate of MTCS by ꢀ70%. The MTCS photolytic rate in the presence of PLFA
was greater than for SRFA. NOM not only suppressed photolysis by light-shielding, but also produced
ROS to oxidatively degrade MTCS and/or triplet NOM (3NOM*) to sensitize degradation. The dual effects
of light-screening and photo-sensitization led to an overall decrease in photolysis of MTCS with
a positive concentration-dependence. Upon addition of NOM, EPR documented the occurrence of 1O2
and cOH in the photolytic process, and the bimolecular k value for the reaction of 1O2 with MTCS was
1.86 ꢁ 106 Mꢂ1
s
ꢂ1. ROS-quenching experiments indicated that the contribution of cOH (19.1–29.5%) to
indirect photolysis of MTCS was lower than for 1O2 (38.3–58.7%). Experiments with D2O further
demonstrated that 1O2 participated in MTCS photodegradation. Moreover, the addition of sorbic acid
and O2 gas to the reaction confirmed the participation of 3NOM* as a key reactant in the photochemical
transformation of MTCS. This is the first comprehensive analysis of NOM effects on the indirect
photolysis of MTCS, which provides new insights for understanding the environmental fate of MTCS in
natural environments.
Received 24th April 2018
Accepted 5th June 2018
DOI: 10.1039/c8ra03512a
rsc.li/rsc-advances
process.5 Due to their low volatility and water solubility, the
photolysis of TCS and MTCS is one of the most efficient
1. Introduction
degradation pathways in aquatic environments. In recent years,
considerable research has focused on mechanistic studies of
TCS photolysis and demonstrated that TCS can be transformed
into highly carcinogenic chemicals, such as 2,8-dichlor-
odibenzo-p-dioxin.6 Moreover, the transformation of phenolic
compounds into methlyation derivatives may render its lipo-
philic and more bioaccumulative than the parent compound
itself.7,8 For example, it was reported that MTCS is more
persistent, lipophilic, bio-accumulative and less sensitive
towards photo-degradation in the environment than MTCS.9–11
In spite of the recent research on TCS degradation, there is
a paucity of research concerning mechanistic pathways for
MTCS photolysis. Therefore, additional studies are necessary to
elucidate MTCS photolysis pathways and the associated envi-
ronmental risks.
This study investigated the fate of MTCS in response to
irradiation with simulated sunlight and focused on the indirect
photolysis induced by reactive oxygen species (ROS). Due to the
ubiquitous nature of dissolved natural organic matter (NOM) in
the environment, it is widely used as a photosensitizer to absorb
light and subsequently generate reactive triplet state NOM
(3NOM*) and/or induce production of ROS, such as singlet
oxygen (1O2), superoxide anions (cO2ꢂ), hydrogen peroxide
Triclosan (5-chloro-2-(2,4-dichlorophenoxy)-phenol, TCS) is
a common antimicrobial agent widely used in household-
cleaning goods and personal care products such as shampoo,
liquid soaps and toothpaste.1 Methyltriclosan (2,4-dichloro-1-(4-
chloro-2-methoxyphenoxy) benzene; MTCS) is a methylated
product of TCS produced by several decomposition pathways.2
Due to wide-spread use, TCS and MTCS have become ubiqui-
tous contaminants in numerous environmental and animal
matrices including human body uids.3,4
The degradation pathway of TCS and MTCS in natural
environments includes biodegradation and photodegradation.
Anaerobic and aerobic microbes can degrade TCS and MTCS,
however, microbial degradation is a slow and inefficient
aKey Laboratory of Watershed Sciences and Health of Zhejiang Province, Southern
Zhejiang Water Research Institute, Wenzhou Medical University, Wenzhou 325035,
People's Republic of China. E-mail: zjuwxd@163.com; Fax: +86-577-8669 9122; Tel:
+86-577-8668 9733
bDepartment of Land, Air and Water Resources, University of California, Davis,
California 95616, USA
† Electronic supplementary information (ESI) available: Total ion chromatogram
and mass spectrum of MTCS, DLLME procedure, apparent-rst-order kinetic rate
constants. See DOI: 10.1039/c8ra03512a
‡ These authors should be considered co-rst authors.
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(H2O2), and hydroxyl radicals (cOH).12 NOM can also transfer
absorbed energy to other suitable receptors thereby inducing
a corresponding degradation of the receptor.
2.2 Analytical methods
FFA was analyzed by high performance liquid chromatography
(HPLC, 1260, Agilent, Wilmington, DE, USA) with a photodiode
array detector and XDB-C18 column. MTCS was quantied by
gas chromatography-mass spectrometry (Agilent GS-MS 7890-
5977) using a HP-5MS capillary column (30 m ꢁ 0.32 mm I.D.,
0.25 mm lm thickness, Agilent). The detailed operational
parameters are available in the ESI† section. The total ion
chromatogram (TIC) and mass spectrum (MS) prole of MTCS
(40 mg Lꢂ1) was shown in Fig S1.†
An electron paramagnetic resonance instrument (Bruker
A300 spectrometer, Bruker, Germany) was used to record elec-
tron paramagnetic resonance (EPR) signals for 1O2 and cOH
with trapping reagents of TEMP and DMPO, respectively.
Previous studies revealed that NOM interacts with hydro-
phobic organic contaminants (HOCs) via a variety of mecha-
nisms.13–15 The effect of NOM on HOC photolysis is a dual action
mechanism, suppressing photolysis by light-shielding and
producing ROS to oxidatively degrade HOCs or produce triplet
NOM (3NOM*) to sensitize degradation of HOCs.11 Therefore, it
is of great practical signicance to study the effects of NOM on
photochemical degradation of HOCs. Xu et al. (2011) reported
that the photolytic rate of amoxicillin in the presence of fulvic
acid was signicantly increased and the photolytic contribution
ratios of singlet and triplet excited state NOM was 48–74%.14
However, the contribution of NOM on photolysis of HOCs is
highly dependent on the pollutant structure and concentra-
tions.16 MTCS is a typical HOC with low water solubility, but so
far the effects of NOM on the photochemical behavior of MTCS
have not been rigorously examined.
Pony Lake fulvic acid (PLFA) and Suwannee River fulvic acid
(SRFA) are derived from microbial and terrestrial NOM sour-
ces, respectively, and are two of the most representative NOM
isolates utilized in previous studies.17,18 SRFA has lower
nitrogen and sulfur functional groups and more aromatic
moieties than PLFA, and thus their effects on photolysis of
HOCs are likely to differ.19,20 Herein, we investigated the effects
of SRFA and PLFA on the photodegradation of MTCS. A series
of experiments were conducted to elucidate the ROS genera-
tion mechanism and indirect photolysis of MTCS. To the best
of our knowledge, this is the rst comprehensive investigation
on the indirect aqueous photolysis of MTCS in the presence of
PLFA and SRFA. These results enhance our understanding of
the photochemical fate of MTCS and provide a theoretical
basis to predict the environmental fate of other similar
emerging pollutants.
2.3 Photolytic experiments
A photochemical reactor (BL-GHX-V, Shanghai, China) was used
for the photolytic experiments. A 300 W mercury lamp with
a 290 nm cutoff lter was used as the sunlight simulation
source. The irradiance spectrum of light source was measured
by a SP-300 spectroradiometer (Acton Research Corporation,
USA). Fig. S2† conrmed that the 290 nm cutoff lters provide
the transmission of wavelengths above 290 nm. An air-cooled
system was used to refrigerate the photochemical reactor.
To avoid the quenching effect of methanol on hydroxyl
radicals, MTCS standard solution (50 mg Lꢂ1) was rst evapo-
rated using a gentle nitrogen ow, and then redissolved in
acetonitrile to form MTCS stock solution (40 mg Lꢂ1). The
MTCS stock solution was stored in the dark to avoid photo-
chemical degradation. An appropriate volume of deionized
water was added to prepare the initial concentration of 40 mg
Lꢂ1 MTCS (40 mL), which was then transferred into a quartz
tube for photolytic experiments. Prior to photolysis, the quartz
tube was wrapped with aluminum foil to shield the solution
containing analytes from light. At given time intervals, an
aliquot (5 mL) was transferred to a centrifuge tube for dispersive
liquid–liquid microextraction (DLLME) pretreatment (Scheme
S1†). In the direct photolytic experiment, NOM was not added to
the photolytic solution, but it was fortied into the photolytic
solution for indirect photolysis. NOM was fortied at concen-
trations of 5–25 mg Lꢂ1. All the experiments were carried out at
least in triplicate.
2. Materials and methods
2.1 Chemicals
Methyltriclosan (MTCS) (50 mg Lꢂ1 in methanol) was purchased
from AccuStandard (New Haven, CT, USA). Sorbic acid (SA) and
isopropyl alcohol (IPA) were obtained from Aladdin Industrial
(Shanghai, China). Sodium azide (NaN3, 99.5%), furfuryl
alcohol (FFA, 98%), rose Bengal (RB, 93%), 2,2,6,6-tetrame-
thylpiperidine (TEMP, 99%) and 5,5-dimethyl-1-pyrroline-N-
oxide (DMPO, 97%) were purchased from Sigma-Aldrich
2.4 Reactive species analysis
(1) Role of 1O2 and cOH. The indirect photolytic mechanism
(Shanghai, China). Pony Lake fulvic acid (PLFA-microbial was studied following addition of NaN3 (10 mmol Lꢂ1, quencher
origin) and Suwannee River fulvic acid (SRFA-terrestrial of 1O2) and IPA (100 mmol Lꢂ1, quencher of cOH) to the MTCS
origin) were acquired from the International Humic photolytic solution.21
1
Substance Society (Denver, CO, USA) and dissolved in deionized
(2) Rate constant (k value) of O2 and MTCS. Under 300 W
water to form NOM isolate stock solution (100 mg Lꢂ1). mercury lamp irradiation, rose bengal (RB; 2 mmol Lꢂ1
,
1
Chromatographic-grade acetonitrile (CH3CN) and methanol a photosensitizer for O2 production) was photolyzed synchro-
(CH3OH) were obtained from Merck (Darmstadt, Germany). All nously in solutions of MTCS and FFA (200 mM). To limit direct
other chemicals were of analytical grade and used as received photolysis, the MTCS solution was irradiated in a photochem-
without further purication. Deionized water (>18 MU cm) was ical reactor equipped with a series of 420 nm cutoff lters.21 An
prepared using a Millipore Milli-Q system (Bedford, MA, USA). aliquot (0.5 mL) was extracted at a regular interval to determine
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FFA concentrations by HPLC. Aer liquid–liquid extraction clarify the inhibitory mechanism of PLFA and SRFA on MTCS
using n-hexane as the extraction solvent, the residual MTCS photodegradation.
concentration was determined by GC-MS.
Subsequently, it further revealed that MTCS photolysis rates
3
3
(3) Role of NOM*. The role of NOM* was investigated by in the presence of SRFA ((1.62 ꢃ 0.02) ꢁ 10ꢂ3 minꢂ1) was lower
addition of sorbic acid (1 mmol Lꢂ1) under oxygenated or than PLFA ((2.71 ꢃ 0.12) ꢁ 10ꢂ3 minꢂ1) (Fig. 1a and Table S1†).
deoxygenated conditions created by purging air or N2 into the This difference originates from the contrasting chemical and
solution, respectively.
optical properties of SRFA and PLFA.25 For example, Guerard
et al. (2009) demonstrated that a large photo-enhancement of
sulfadimethoxine (SDM) and triclocarban (TCC) occurred in the
presence of PLFA, while little enhanced photolysis in the
SRFA.17 The higher aromatic content in terrestrially derived
NOM (SRFA) allows it to absorb more light per unit carbon
resulting in a larger molar absorptivity or specic UV absor-
bance compared to autochthonous NOM (PLFA).26 Moreover,
Felcyn et al. (2012) found that NOM from microbial origin
(PLAF) was an excellent photosensitizer compared to NOM from
terrestrial sources (SRFA).18
3. Results and discussion
3.1 Effects of NOM on the photolytic kinetics of MTCS
The maximum absorption spectrum of MTCS contains two
bands in the ultraviolet region with absorption at 207 nm and
283 nm (Fig. S3†), respectively. The slight absorption above
290 nm suggested the direct absorption of simulating sunlight.
Thus, the direct photolysis of MTCS using simulating sunlight
(l > 290 nm) without NOM t an apparent-rst-order kinetic
model (Fig. 1a). The apparent MTCS direct photolysis rate
constant (k) and correlation coefficient (R2) were (6.81 ꢃ 0.20) ꢁ
10ꢂ3 minꢂ1 and 0.996 (Table S1†), respectively. Subsequently,
the direct MTCS photolysis transformation efficiency in pure
water was quantied as quantum yields (F) measured relative to
a p-nitroanisole/pyridine actinometer.22 The calculated F was
0.011, revealing the nonnegligible effect of direct MTCS
photolysis under simulating light. Moreover, the k values for
indirect photolysis were signicantly decreased to (2.71 ꢃ 0.12)
ꢁ 10ꢂ3 and (1.62 ꢃ 0.02) ꢁ 10ꢂ3 minꢂ1 in the presence of PLFA
and SRFA (20 mg Lꢂ1), respectively. These results showed that
PLFA or SRFA prominently inhibited the photolysis of MTCS.
Prior studies revealed that sunlight mediated organic
compounds photodegradation in the presence of NOM involved
direct pollutants photolysis and/or indirect contribution
induced by reactive species such as hydroxyl radicals, single
oxygen, triple state NOM and so on.23,24 Thus, it is necessary to
Photodegradation rates for MTCS were signicantly
decreased from (6.81 ꢃ 0.20) ꢁ 10ꢂ3 minꢂ1 to (1.40 ꢃ 0.09) ꢁ
10ꢂ3 or (1.00 ꢃ 0.02) ꢁ 10ꢂ3 minꢂ1 (Table S2†) with increasing
NOM concentrations from 0 to 25 mg Lꢂ1 (Fig. 1b and c).
However, MTCS photodegradation efficiency was slightly higher
in the presence of PLFA compared to SRFA (Fig. 1d). Because the
photolysis of organic contaminants in sunlight-induced natural
waters is caused by NOM-derived active substances (e.g., 1O2,
3
cOH and NOM*), NOM with differing chemical compositions
can lead to different indirect reactions and rates.
3.2 Effects of ROS on MTCS indirect photodegradation
A series of quencher experiments were then carried out to
qualitatively describe the effects of 1O2 and cOH in the indirect
photolysis process. MTCS solutions containing 20 mg Lꢂ1
NOM were investigated in pure water under irradiation (l >
290 nm), with the addition of IPA as a cOH scavenger and NaN3
1
as a O2 scavenger (Fig. 2).21 The MTCS photolytic rate in the
presence of PLFA decreased from (2.71 ꢃ 0.12) ꢁ 10ꢂ3 to (1.91
ꢃ 0.06) ꢁ 10ꢂ3 minꢂ1 with the addition of IPA revealing the
specic contribution of cOH to MTCS photodegradation in the
presence of PLFA. Further, the k value prominently decreased
from (2.71 ꢃ 0.12) ꢁ 10ꢂ3 to (1.12 ꢃ 0.05) ꢁ 10ꢂ3 minꢂ1 in the
presence of NaN3, which demonstrated that 1O2 played
a crucial role in the indirect photolysis of MTCS. Similarly,
MTCS photodegradation constants in the presence of SRFA
Fig. 1 (a) Photodegradation kinetics of MTCS in the absence and
presence of NOM (PLFA – Pony Lake fulvic acid and SRFA is Suwannee
River fulvic acid). (b) Effects of PLFA concentrations on the photo- Fig.
2
Effects of NaN3 and isopropyl alcohol (IPA) on photo-
degradation kinetics of MTCS. (c) Effects of SRFA concentrations on degradation kinetics of MTCS in the presence of (a) PLFA (Pony Lake
the photodegradation kinetics of MTCS. (d) Kinetics comparison of fulvic acid, 20 mg Lꢂ1) and (b) SRFA (Suwannee River fulvic acid,
MTCS photodegradation in the presence of PLFA and SRFA.
20 mg Lꢂ1).
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decreased from (1.62 ꢃ 0.02) ꢁ 10ꢂ3 to (1.31 ꢃ 0.04) ꢁ 10ꢂ3 conrmed the important of reactive oxygen species on the
and (1.00 ꢃ 0.01) ꢁ 10ꢂ3 minꢂ1 with the addition of IPA and MTCS indirect photolysis.
NaN3 (Table 1), respectively. The contribution of cOH and 1O2
1
to the MTCS photolysis process (RcOH and R O ) were calculated
as follows:
2
1
3.3 EPR detection of cOH and O2
EPR was used to identify ROS formation during the MTCS
ꢄ
k OHðPWÞ
kPW
kPW ꢂ kPWþIPA
photodegradation process.31–34 TEMP is a typical capture agent
for 1O2, which reacts with 1O2 to produce a stable para-
magnetic adduct TEMP-1O2 with a characteristic line of
1 : 1 : 1.31 Typical 1 : 1 : 1 triplet signals were obtained aer
irradiation of MTCS, TEMP and NOM (Fig. 3a and c) revealing
generation of 1O2 during the photolysis process. Moreover, the
signal intensity increased signicantly with prolonged irradi-
ꢄ
R OH
¼
¼
(1)
(2)
kPW
1O2ðPWÞ
kPW
k
kPW ꢂ k
ðPWþNaN3Þ
¼
¼
1O2
R
kPW
where kPW, kPW+IPA and kPW+NaN are the photodegradation rate
constants of MTCS without quencher and in the presence of IPA
3
1
1
and NaN3, respectively. The k O (PW) and kcOH(PW) terms are the
ation time. To conrm the contribution of O2 on the forma-
2
MTCS photodegradation rate constants induce by 1O2 and cOH,
tion of TEMP-1O2, NaN3 was added to the MTCS, NOM and
TEMP mixture to inhibit 1O2. The NaN3 strongly decreased the
triplet signal for TEMP-1O2 (Fig. 3), which veried that
formation of the TEMP-1O2 spin adduct was inhibited. For
comparison, the EPR signal of TEMP-1O2 was also collected
during the photolysis process without NOM. Fig. S5a† revealed
that limited TEMP-1O2 signal was obtained in the absence of
NOM. In total, these results conrmed that 1O2 was generated
during MTCS photolysis in the presence of PLFA and SRFA.
DMPO is a typical capture agent for cOH, which reacts with
cOH to produce DMPO-cOH, a stable paramagnetic adduct
with a characteristic line of 1 : 2 : 2 : 1.35 Four characteristic
peaks with a 1 : 2 : 2 : 1 quarter pattern were obtained aer
irradiation of MTCS and NOM, indicating generation of cOH
(Fig. 4). The signal intensity for DMPO-cOH increased signi-
cantly with increased irradiation time (Fig. 4a and c). The
signal sharply decreased with IPA addition under identical
conditions (Fig. 4b and d), conrming that the EPR signal was
formed by reaction of DMPO and cOH. Moreover, no DMPO-
cOH signal was obtained in the absence of NOM during the
respectively.27,28 Table 1 summarizes the photodegradation rate
constants (k) for MTCS under different conditions. The contri-
bution ratios of 1O2 and cOH to MTCS photodegradation in the
presence of PLFA were 58.7% and 29.5%, respectively. The
corresponding contribution ratios for 1O2 and cOH in the
presence of SRFA were 38.3% and 19.1%, respectively. These
results conrmed that both 1O2 and cOH play crucial roles in the
indirect photodegradation of MTCS in the presence of PLFA and
SRFA. The contribution ratio from cOH (19.1–29.5%) in the
indirect photolysis of MTCS was smaller than that of 1O2 (38.3–
58.7%).
To clarify the dual contribution of light-shielding and ROS
on the MTCS photolysis, light screening factors (S) was calcu-
lated during the photolysis process in the presence of PLFA and
MTCS.29 The results revealed that S of SRFA and PLFA for the
MTCS photolysis were 0.101 and 0.047, respectively, suggesting
the higher contribution of light-shielding on MTCS photolysis.
Thus, we concluded that the overall decrease in MTCS photol-
ysis was caused by the inhibitive effect of light-shielding and
promoted effect of ROS in the presence of PLFA and SRFA.
Subsequently, the steady-state concentration of cOH was
determined by monitoring the generation of 2-hydroxytereph-
thalic acid (hTPA) using terephthalate (TPA) as a probe.30
Fig. S4† shown that the hTPA concentrations increased by
prolonging the photolysis time in the presence of SRFA and
PLFA. The calculated steady state concentrations of cOH were
1.73 ꢁ 10ꢂ16 and 1.33 ꢁ 10ꢂ16 M for the SRFA and PLFA,
respectively. However, limited hTPA were detected in the
photolysis systems with the addition of IPA. All of these results
Table 1 Photolytic rate constants for MTCS under UV-Vis irradiation (l
> 290 nm)a
k
SRFA*(ꢁ 10ꢂ3 minꢂ1
)
PLFA*(ꢁ 10ꢂ3 minꢂ1
)
kPW
1.62 ꢃ 0.02
1.31 ꢃ 0.04
0.31
2.71 ꢃ 0.12
1.91 ꢃ 0.06
0.80
kPW+IPA
kcOH(PW)
kPW+NaN
Fig. 3 EPR spectra of 1O2 spin-trapping with TEMP under different
conditions. (a) PLFA (20 mg Lꢂ1), (b) PLFA (20 mg Lꢂ1)/NaN3, (c) SRFA
(20 mg Lꢂ1), (d) SRFA (20 mg Lꢂ1)/NaN3. PLFA is Pony Lake fulvic acid
and SRFA is Suwannee River fulvic acid.
1.00 ꢃ 0.01
1.12 ꢃ 0.05
3
1
k O (PW)
0.62
1.59
2
a
*PLFA is Pony Lake fulvic acid (microbial origin, 20 mg Lꢂ1) and SRFA
is Suwannee River fulvic acid (terrestrial origin, 20 mg Lꢂ1).
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Fig. 5 (a) Loss of MTCS versus loss of furfuryl alcohol (FFA) in the
presence of rose bengal. (b) Effect of D2O on the photodegradation of
MTCS in aqueous solution. SRFA (20 mg Lꢂ1) is Suwannee River fulvic
acid.
3.5 Photolysis of MTCS in D2O solution
1
Due to the longer lifetime of O2 in deuterated solvents, D2O
Fig. 4 EPR spectra of cOH spin-trapping with DMPO under different
conditions. (a) PLFA (20 mg Lꢂ1), (b) PLFA (20 mg Lꢂ1)/IPA, (c) SRFA
(20 mg Lꢂ1), (d) SRFA (20 mg Lꢂ1)/IPA. PLFA is Pony Lake fulvic acid and
SRFA is Suwannee River fulvic acid.
was used as the MTCS solvent to further examine the role of 1O2
in the photolytic process. Literature sources report that the
reaction rate in D2O should be about 14 times higher than in
H2O, if the degradation was fully controlled by 1O2.42 Thus, the
rate constant in D2O should be signicantly increased
compared to H2O under the premise that the reaction with 1O2
was the primary transformation pathway for MTCS degrada-
tion.43 However, the measured degradation rates for MTCS in
D2O ((2.13 ꢃ 0.08) ꢁ 10ꢂ3 minꢂ1) did not show a signicant
increase over that in H2O ((1.62 ꢃ 0.02) ꢁ 10ꢂ3 minꢂ1) (Fig. 5b).
This indicates that indirect photolysis related to 1O2 was not the
exclusive mechanism responsible for photodegradation of
MTCS. Therefore, we concluded that MTCS photodegradation
in the presence of NOM could be ascribed to synergistic effects
MTCS photolysis process (Fig S5b†). In sum, the EPR results
demonstrated the generation of cOH and 1O2 during MTCS
photolysis in the presence of NOM.
1
3.4 Kinetic studies for O2 reaction with MTCS
1
To evaluate the bimolecular reaction rate constant of O2 with
MTCS at pH 7.0, rose bengal (RB) and FFA were used as
photosensitizer and reference compounds in the MTCS photo-
degradation system, respectively.36 From the steady-state 1O2
concentration and reaction rate constant, it is possible to
determine the 1O2 contribution to the photolytic process.
Concentrations of the target (MTCS) and reference (FFA)
compounds were simultaneously monitored to evaluate their
reaction kinetics with 1O2. The bimolecular reaction rate
constant of FFA with 1O2 (kFFA) was determined to be 1.20 ꢁ 108
1
among O2, cOH and direct photolysis.
3
3.6 Role of NOM*
As an effective photosensitizer, NOM can absorb sunlight to
form excited states (e.g., 3NOM*). The 3NOM* can then react
1
with dissolved oxygen to generate a series of ROS, such as O2,
cO2ꢂ, and cOH. Organic molecules can subsequently be
degraded by these ROS or react by transferring electrons or
energy with 3NOM*. Thus, it is necessary to distinguish the
effect of 3NOM* on MTCS photodegradation in the presence of
NOM. Oxygen can be used as a triplet-excited-state quencher to
Mꢂ1 sꢂ1 and was used in calculating the bimolecular reaction
1
1
rate constant for MTCS with O2 (k ) via eqn (3):
O2
k
1O2;S
½Sꢅ
½FFAꢅ
½Sꢅt
k
½FFAꢅt
ln
¼
ln
(3)
1O2;FFA
0
0
3
inhibit the contribution of NOM*.15,44 Hence, MTCS photo-
1
1
1
where k O ,FFA is the rate constant of FFA with O2, and k O ,S is degradation was conducted with O2 (air-saturated) and without
2
2
the rate constant of MTCS with O2.10,37–39 The bimolecular O2 (N2-saturated) to evaluate the contribution of 3NOM* (Fig. 6a
1
1
reaction rate constant for the reaction of O2 with MTCS was and Table S3†). Photodegradation rates for MTCS in N2-satu-
1.86 ꢁ 106 Mꢂ1 sꢂ1 (Fig. 5a), which was slightly lower than those rated conditions ((3.44 ꢃ 0.03) ꢁ 10ꢂ3 minꢂ1 and (4.12 ꢃ 0.11)
for other organic contaminants. For example, Barbieri et al. ꢁ 10ꢂ3 minꢂ1) were signicantly higher than for air-saturated
(2008) reported that the k value for the reaction between conditions ((1.62 ꢃ 0.02) ꢁ 10ꢂ3 minꢂ1 and (2.71 ꢃ 0.12) ꢁ
bisphenol A and 1O2 was 1.01 ꢁ 108 Mꢂ1 sꢂ1 in pure water (pH ¼ 10ꢂ3 minꢂ1) in the presence of SRFA and PLFA, respectively.
10),40 which was nearly two orders of magnitude higher than for These results indicate that the direct reaction of MTCS with
1O2 with MTCS. Xie et al. (2013) conrmed that the direct 3NOM* was important in the absence of oxygen.
1
photolytic rate and secondary reaction rate of anions with O2
To conrm the involvement of 3NOM* in aerobic MTCS
was much higher than those of neutral molecules.41 Thus, we photolysis, the effect of a triplet quencher (sorbic acid) on MTCS
concluded that O2 involved into the MTCS photodegradation indirect photolytic rates was evaluated.45 It revealed that MTCS
1
and its rate constant dependent on the organic pollution photodegradation rates in the presence of SRFA and PLFA were
molecular structure.
signicantly decreased from (1.62 ꢃ 0.02) ꢁ 10ꢂ3 and (2.71 ꢃ
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of Wenzhou Medical University (Grant QTJ16013), and Zhejiang
Provincial Xinmiao Talent Project (Grant 2016R413021).
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