Oxygen dependent oxidation of trimethoprim by sulfate radical: Kinetic and mechanistic investigations

Article abstract of DOI:10.1016/j.cclet.2021.02.062

Trimethoprim (TMP) is a typical antibiotic to treat infectious disease, which is among the most commonly detected antibacterial agents in natural waters and municipal wastewaters. In the present study, the impacts of dissolved oxygen (DO) on the oxidation efficiency and pathways of TMP by reaction with sulfate radicals (SO₄^[rad]?) were investigated. Our results revealed that the presence of DO was favourable for TMP degradation. Specifically, TMP would react initially with SO₄^[rad]? via electron-transfer process to form a carbon-centered radical. In the absence of oxygen, the carbon-centered radical could undergo hydrolysis to produce α-hydroxytrimethoprim (TMP?OH), followed by the further oxidation which generated α-ketotrimethoprim (TMP=O). However, in the presence of oxygen, the carbon-centered radical would alternatively combine with oxygen, leading to a sequential reaction in which peroxyl radical and a tetroxide were formed, and finally generated TMP?OH and TMP=O simultaneously. The proposed pathways were further confirmed by density functional theory (DFT) calculations. The results obtained in this study would emphasize the significance of DO on the oxidation of organic micro-pollutants by SO₄^[rad]?.

Full text of DOI:10.1016/j.cclet.2021.02.062

G Model
CCLET-6191; No. of Pages 5
Chinese Chemical Letters xxx (xxxx) xxx–xxx
Contents lists available at ScienceDirect
Chinese Chemical Letters
journal homepage: www.elsevier.com/locate/cclet
Communication
Oxygen dependent oxidation of trimethoprim by sulfate radical:
Kinetic and mechanistic investigations
d
a,b,c
Xuerui Yanga,b, Xi Dinga,b, Lei Zhoua,b,c, , Yuefei Ji , Guangli Xiu
*
a
Shanghai Environmental Protection Key Laboratory for Environmental Standard and Risk Management of Chemical Pollutants, School of Resources &
Environmental Engineering, East China University of Science and Technology, Shanghai 200237, China
State Environmental Protection Key Lab of Environmental Risk Assessment and Control on Chemical Processes, School of Resources & Environmental
b
Engineering, East China University of Science and Technology, Shanghai 200237, China
c
Shanghai Institute of Pollution Control and Ecological Security, Shanghai 200092, China
College of Resources and Environmental Sciences, Nanjing Agricultural University, Nanjing 210095, China
d
A R T I C L E I N F O
A B S T R A C T
Article history:
Trimethoprim (TMP) is a typical antibiotic to treat infectious disease, which is among the most
commonly detected antibacterial agents in natural waters and municipal wastewaters. In the present
Received 18 December 2020
Received in revised form 19 January 2021
Accepted 25 February 2021
Available online xxx
study, the impacts of dissolved oxygen (DO) on the oxidation efficiency and pathways of TMP by reaction
ꢀ
with sulfate radicals (SO4 ꢁ) were investigated. Our results revealed that the presence of DO was
ꢀ
favourable for TMP degradation. Specifically, TMP would react initially with SO4 ꢁ via electron-transfer
process to form a carbon-centered radical. In the absence of oxygen, the carbon-centered radical could
undergo hydrolysis to produce α-hydroxytrimethoprim (TMPꢁOH), followed by the further oxidation
which generated α-ketotrimethoprim (TMP=O). However, in the presence of oxygen, the carbon-centered
radical would alternatively combine with oxygen, leading to a sequential reaction in which peroxyl
radical and a tetroxide were formed, and finally generated TMPꢁOH and TMP=O simultaneously. The
proposed pathways were further confirmed by density functional theory (DFT) calculations. The results
obtained in this sꢀꢁtudy would emphasize the significance of DO on the oxidation of organic micro-
Keywords:
Antibiotics
Sulfate radicals
Dissolved oxygen
Hydroxylation
DFT calculations
pollutants by SO4
.
© 2021 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences.
Published by Elsevier B.V. All rights reserved.
Advanced oxidation process (AOPs) can be used as tertiary
treatment strategies for the decontamination of organic micro-
pollutants that are widely present in municipal wastewater
applied in the remediation of soil, surface water and groundwater
as a promising in situ chemical oxidation (ISCO) technology [4,7].
As one of the most important components in natural waters or
wastewaters, it was found recently that dissolved oxygen (DO),
might be involved in SR-AOPs and led to unpredicted results on the
degradation kinetic as well as pathways [8,9]. For instance, Zhu
et al. [8] found that the degradation of hexachloroethane by
thermo-activated persulfate (TAP) was faster under anaerobic
condꢀiꢁtion than aerobic condition, mainly due to the formation of
S2O8 in the absence of oxygen (ꢀEꢁq. 1).
treatment plants effluents [1]. Among different AOPs, sulfate
ꢀ
radical (SO4 ꢁ) based AOPs (SR-AOPs) have attracted increasing
scientific attention in recent years, due to their strong oxidizing
ability, high selectivity and wide pH flexibility against numerous
ꢀꢁ
organic micro-pollutants [2,3]. SO4
could be generated by
‒
activated peroxymonosulfate (PMS, HSO5
)
or persulfate
(PS, S2O82‒) via various approaches such as heating, UV irradiation,
transition metals, bases and ultrasound [4–6]. Owing to the
advantages mentioned above, SR-AOPs have been widespread
ꢀꢁ
2‒
2‒
SO4 + S2O8 → SO4 + S2O8 k = 6.3 ꢂ 105 L mol-1 s-1 (1)
ꢁ
The new formed S2O8 radical could continue react efficiently
with halogenated organic compounds such as organochlorine
pesticides by a reductive dechlorination process [8]. However,
another theoretical study revealed a contradictory result that DO
was beneficial to the degradation of pollutants, indeed Zhang et al.
[10] showed that DO could decrease the energy barrier of the
SR-AOPs and shifted the chemical equilibrium of the reaction
towards the side of products. The conflicting literature results
suggested the impact of DO on SR-AOPs might be diverse each
* Corresponding author at: Shanghai Environmental Protection Key Laboratory
for Environmental Standard and Risk Management of Chemical Pollutants, School of
Resources & Environmental Engineering, East China University of Science and
Technology, Shanghai, 200237, China.
E-mail address: zhoulei@ecust.edu.cn (L. Zhou).
https://doi.org/10.1016/j.cclet.2021.02.062
1001-8417/© 2021 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences. Published by Elsevier B.V. All rights reserved.
Please cite this article as: X. Yang, X. Ding, L. Zhou et al., Oxygen dependent oxidation of trimethoprim by sulfate radical: Kinetic and
mechanistic investigations, Chin. Chem. Lett., https://doi.org/10.1016/j.cclet.2021.02.062

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other, depending on the target pollutants and reaction conditions.
In addition, oxygen contents in different bodies are also variable,
unlike surface environments, groundwater is often oxygen-
deficient. Therefore, for realistic applications of SR-AOPs, it is of
great importance to evaluate the impact of DO on the oxidation
efficiency and degradation pathways of target pollutants since only
few studies dealt with this point.
In our previous paper the degradation kinetic and transforma-
tion pathways of trimethoprim (TMP) were investigated systemi-
cally by thermo-activated persulfate (TAP) [11]. TMP is
a
bacteriostatic antibiotic for the treatment of infectious diseases
in humans and have been frequently detected in natural waters
[12,13]. An oxygen involved pathway was temporarily proposed by
which α-hydroxytrimethoprim (TMPꢁOH) and α-ketotrimetho-
prim (TMP=O) were generated simultaneously. However, the
detailed impacts of oxygen on degradation kinetics and mecha-
nisms still remain unclear.
Therefore, the main goal of the present study was to investigate
the impact of oxygen on TMP oxidation more deeply. Our
objectives were: 1) To evaluate the effect of oxygen on TMP
degradation kinetic; 2) to identify the oxidation products and to
record their evolution in the absence and presence of oxygen; 3) to
reveal the detailed degradation pathways and mechanisms of TMP
under different oxygen conditions.
Specifically, TAP oxidation experiments were conducted in
75 mL screw-cap cylindrical glass vials with Teflon septa at
predetermined temperature (60 ꢃC) controlled by a thermal water
bath (Xianou Instrument Manufacture Co., Ltd., Nanjing, China).
Photochemical experiments were performed using a merry-go-
round photo reactor (NaAi Instrument Manufacture Co., Ltd.,
Fig. 1. Degradation of TMP under different oxygen conditions by TAP.
([TMP]0 = 40 m
mol/L, [PS]0 = 2 mmol/L, T = 60 ꢃC, pH 7.0).
anaerobic conditions. Under control group (DO 8.23 ꢄ 0.25 mg/L),
TMPlossratewasdetectedat0.033minꢁ1,whichwasfasterthanthat
of anaerobic condition group (DO 1.52 ꢄ 0.20 mg/ L) at 0.018 min-1
(Fig. S1 in Supporting information). All the results suggested that
TMP degradation under control group was faster than that at
anaerobic condition, indicating the involvement of DO in the overall
interaction between sulfate radical and TMP.
It was widely considered sulfate radical and hydroxyl radical
was responsible for the elimination of contaminants in the TAP
ꢀ
processes [16,17]. The work of Tan et al. has reported O2 ꢁ might be
generated under the basic conditions in the antipyrine removal by
TAP [18]. Thus, spin trapping-electron paramagnetic resonance
(EPR) analysis was employed here to identify the potential
involved active species. As shown in (Fig. S2 in Supporting
information), under both two oxygen conditions, the splitting
signals with relative intensity ratio of 1:2:2:1 and a six-line signal
were observed, whose typical spectra were assigned to DMPO-ꢀOH
Shanghai, China) equipped with
a 500 W Xenon lamp and
conducted in a series of 60 mL uncapped cylindrical quartz tubes.
Appropriate volumes of TMP and PS stock solution were
transferred into the vials to achieve a total of 50 mL reaction
solution with predetermined concentrations. Anaerobic experi-
ments were carried out in an anaerobic incubator (YQI-II, CIMO,
Shanghai, China), prior to activation, the reaction solutions were
purged by N2 for 15 min to achieve a DO concentration of
1.28 ꢄ 0.25 mg/L. Reactions solutions were buffered by using
10 mmol/L phosphate, pH values remained within 7.0 ꢄ 0.1 over
the course of the experiments. Sample aliquots (0.75 mL) were
ꢀ
ꢀꢁ
and DMPO-SO4 ꢁ, respectively [19,20]. However, O2 was hardly
detected directly by EPR, possibly due to its low concentration.
These phenoꢀmꢁ ena confirmed TMP removal was mainly affected by
ꢀOH and SO4
.
To further explore the dominant species on TMP removal by
TAP processes under different oxygen conditions, the radical
scavenger experiments were conducted. The different reactivities
of tertiary butanol (TBA) and ethanol (EtOH) toward hydroxyl
radical (kꢀOH +EtOH = 2.0 ꢂ 109 L molꢁ1 s-1, kꢀOH+TBA = 5.7 ꢂ 108
L
ꢀꢁ
molꢁ1 s-1) and sulfate radical (kSO4 + EtOH = 4.65 ꢂ107 L molꢁ1 s-1,
withdrawn intermittently and quenched by 20 mL Na2S2O3
ꢀꢁ
kSO4 + TBA = 6.55 ꢂ105 L molꢁ1 s-1) could be used to distinguish the
(1 mol/L) solution to stop the reactions. Further analysis details
are illustrated in Text S1 (Supporting information). Density
functional theory (DFT) calculations were conducted in a Gaussian
09 package software [14]. Detailed calculation method was shown
in Text S2 (Supporting information). In this contribution, frontier
electron densities (FEDs) of highest occupied molecular orbital
(HOMO) and lowest unoccupied molecular orbital (LUMO) were
dominant radical species in reaction solution [21,ꢀ2ꢁ2]. In the
meanwhile, chloroform (CF) was applied to trap O2 with rate
constant of 3 ꢂ 1010 L molꢁ1 s-1 [21]. Applied in Fig. S3 (Supporting
information), without the addition of any scavenger, 85.7% TMP
removal could be achieved in the control group after 60 min. After
the addition of EtOH, TMP removal sharply decreased from 85.7%–
26.3%. When TBA replaced EtOH, the decline of TMP was
comparatively slighter. In addition, the presence ofꢀꢁCF showed
less adverse effect on TMP removal, indicating O2 playing a
minor role in the TMP oxidation. In the N2 bubbling group, same
trend was also oꢀbꢁserved similarly. All these results suggested that
overweight SO4 was the predominant oxidizing species in the
TAP process.
also calculated. Values of 2FED2
were estimated to predict
HOMO
reaction sites for electron-transfer by sulfate radicals [15].
Firstly, a comparison study was conducted to evaluate the
impact of DO on TMP oxidation by sulfate radical. The theoretical
DO concentration in an air-opened condition at 60 ꢃC was around
3.18 mg/L and this system was taken as the control group. As seen
in Fig. 1, in the case of control group, TMP underwent a fast
degradation with a rate constant of 0.042 minꢁ1. However, when
oxygen was driven out from the reaction solution (anaerobic
condition group), significant decrease on the degradation efficien-
cy was observed, with a rate constant at 0.021 minꢁ1. Correspond-
ingly, half-life time of TMP was increased from 16.5 min to
33.0 min. TMP degradation under air-saturated condition was
2-fold faster than that at anaerobic condition.
In order to obtain a deeper insight on the involved mechanism
of DO in the reaction process, oxidation products generated under
two different oxygen contents were identified by HR-MS tech-
nique. Surprisingly, no difference on the total ion chromatogram
(TIC) spectrum of TMP degradation under different conditions was
observed (Fig. S4 in Supporting information). Specifically, two
oxidation products with m/z of 307.1396 and 305.1242 (Table S1
and Fig. S5 in Supporting information) were identified as the
primary products, assigning as TMPꢁOH (lmax at 270.6 nm) and
TMP=O (lmax at 226.8, 255.0 and 293.2 nm) (Fig. S6 in Supporting
To further confirm the involvement of oxygen, TMP degradation
was also conducted in UV/PS process at ambient temperature
(20 ꢃC), by continuously bubbling N2 (0.2 L/min) to achieve
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information), respectively, which was consistent with our previous
work [11]. It has also been documented by Zhang et al. [23] that
these two chemicals were the only transformation products in
UV-activated PS induced oxidation of TMP in synthetic human
urine. Therefore, the following discussion would be mainly based
on this conclusion.
The mass results seemed contradictory to kinetic study in Fig. 1,
since oxidation products were nearly the same in the absence and
presence of DO. However, from the quantitative point of view,
some differences were observed. The evolution of TMPꢁOH and
TMP=O at different conditions were analyzed by HPLC-DAD
(Fig. S6). As seen in Fig. 2, TMPꢁOH and TMP=O exhibited different
formation profiles under control group and anaerobic conditions.
Specifically, with the continuous degradation of TMP, TMPꢁOH
reached its maximum concentrations at about 30 min in the
presence of DO, afterwards, it begun to decay. A similar pattern was
observed under anoxic condition with the maximum TMPꢁOH
level occurred at 45 min. In the case of TMP=O, the evolution
patterns were quite different, for control group sample, TMP=O
exhibited a similar profile to TMPꢁOH and reached maximum
around 75 min; while in anaerobic condition, TMP=O showed a
continuous increasing tendency. It is noteworthy that over the
experimental course the concentration of TMP=O in the absence of
oxygen lower than that at the control group open to air. In addition,
after the same conversion rate, the yields of TMPꢁOH and TMP=O
in the absence of oxygen were higher than that under control
group. For instance, after 15 min of reaction under control group,
40% of TMP was removed, which corresponds to the area of
TMPꢁOH and TMP=O were 56163 (270 nm) and 50502 (293 nm),
respectively (Fig. S6A). While in anaerobic condition, 30 min of
reaction is required to obtain the same rate of 40% of conversion
and the peak area of 64782 and 106653 for TMPꢁOH and TMP=O,
respectively. This could be explained by the fact that in the
presence of DO, TMPꢁOH and TMP=O might undergo further
oxidation, leading to relative lower concentrations. In general, the
discrepancies on oxidation products evolution profiles and
distributions reflect the intrinsic differences of TMP reaction with
sulfate radicals under different oxygen conditions.
Fig. 2. Degradation of TMP (A) under control group (⬛) and anaerobic
conditions (⬜). Time-dependent evolution of absorption peaks of TMPꢁOH
(B) and TMP=O (C) in HPLC-DAD, TMPꢁOH (270 nm) under control group (⬥)
and anaerobic conditions (◇), TMP=O (293 nm) under control group (~)
To better undersꢀtꢁand the underlying interaction mechanisms
and anaerobic conditions ( ). ([TMP]0 = 40
mmol/L, [PS]0 = 2 mmol/L, T =
4
60 ꢃC, pH 7.0).
through which SO4 reacts with TMP, the reactive sites were
Scheme 1. Proposed oxidation pathways for TMP degradation by TAP, the red line represents the oxygen dependent pathways.
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bimolecular combination to generate a tetroxide intermediate.
Further unimolecular decomposition of tetroxide gives TMPꢁOH
and TMP=O, which has been well accepted by the literature [25].
On the other way, the carbon-centered radical could also undergo
hydrolysis to produce an OH-adduct radical, which can be further
ꢀꢁ
oxidized by another SO4 to form TMPꢁOH, this process was
considered as an oxygen independent pathway. Furthermore,
ꢀ
TMPꢁOH could be further oxidized by SO4 ꢁ, undergoing a similar
path to that from TMP toTMPꢁOH (without oxygen), as 5C was also
the reactive site of TMPꢁOH (Table S3).
To further support our proposed oxidation pathways, DFT
calculations were performed to elucidate the decisive involvement
of DO in TMP oxidation processes, the calculation results are
Fig. 3. The calculated enthalpy profiles (kcal/mol) for the reaction of TMP with
sulfate radical.
illustrated in Figs. 3 and 4. Initially, we calculated the primary
ꢀ
interaction between TMP and SO4 ꢁ. By considering the onꢀꢁe
electron-transfer process, the first step was the addition of SO4
on C5 via a transition state (TS(I-II)) in Fig. 3), resulting in the
generation of an adduct intermediate II. For such process an energy
barrier of 0.2 kcal/mol was obtained, which could be easily to
overcome in TAP. The intermediate II was unstable and could
undergo spontaneous cleavage to give the carbon centered radical
predicated by calculating FEDs values (Tables S2 and S3 in
Supporting information). The result suggests that 5C (Scheme 1)
ꢀꢁ
of TMP opposes the highest 2FED2HOMO value, indicating that SO4
preferentially attacks this position. Based on kinetic study,
products identification, calculation results, as well as literature
data, we attempted to propose the detailed oxidation pathways of
TMP by TAP in the presence and absence of oxygen, as shown in
‒
(III) and HSO4 (or H+ and SO42‒), this process was expected to be
thermodynamically favorable due to the relatively low energy level
of III. The proposed addition and dissociation mechanisms of sulfate
radical have already been documented in literatures [26,27]. Once
the carbon centered radical was formed, there were two different
pathways for the further transformation. When oxygen was present
in the system, DO could combine with species III to form
corresponding peroxyl radical (species VI), this process was
thermodynamically spontaneous due to the lower energy level of
species VI. For the transformation from VI to VII, a small energy
barrier of 2.1 kcal/mol was calculated, which was easy to overcome
Scheme 1. The initial step was believed to be an electron-transfer
ꢀ
reaction from TMP to SO4 ꢁ, and a pyrimidine radical cation could
be formed, which had been experimentally proved by EPR [24]. The
pyrimidine radical cation is unstable and would undergo a
deprotonation process from the bridging methylene carbon
(ꢁCH2ꢁ) to generate a carbon-centered radical. Afterwards, two
different pathways were proposed in which one is oxygen
dependent while the second one is oxygen independent. The first
path was initialed via a quick combination with DO to form the
corresponding peroxyl radical, followed by a peroxyl radical
in our study. The following reaction (VII
→ VIII) was also
Fig. 4. The calculated enthalpy profiles (kcal/mol) for the transformation pathways of carbon centered radical with sulfate radical in the presence and absence of DO.
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thermodynamically favorable, leading to the formation of TMP=O
and TMPꢁOH, spontaneously. However, in the absence of oxygen
system, the transformation of species III would overcome a higher
energy barrier compared to the system with oxygen. The hydroxyl-
ation could directly take place on 9C, leading to the formation of
TMPꢁOH. Therefore, from a thermodynamical point of view, the
oxygen dependent path was more favorable that the oxygen
independent one, this could explain the promotion effect of DO
on TMP decay in TAP.
Acknowledgements
This work was financially supported by the National Natural
Science Foundation of China (No. 21806037). The authors would
also like to thank Dr. Ting Wu from Research Center of Analysis and
Test of East China University of Science and Technology for the
technical support on HR-MS analysis.
Appendix A. Supplementary data
Basedontheproposedmechanisms,theevolutionprofilesofTMP
and its corresponding products (Fig. 2) could be further explained.
With the oxidation of TMP, under control group, the formation of
both TMPꢁOH and TMP=O at the initial stage (within 15 min) was
faster than that at anaerobic condition. Such result was in line with
the DFT calculation that TMPꢁOH/TMP=O could be formed via two
different paths in the presence of oxygen. After 30 min, in the
presence of DO, TMPꢁOH level begun to decay with a relative fast
decomposition rate (30ꢁ60 min), which could be attributed to the
fact that most TMP in the reaction system was consumed at this
stage, the reaction between sulfate radical and TMPꢁOH would
become crucial, leading toa fastformation of TMP=O. In the absence
of DO, the reactionwouldfollowthe sequenceof TMP→TMPꢁOH →
TMP=O, leading to the continuous generation of TMP=O.
Total organic carbon (TOC) changes were also tracked under
different oxygen environments. Applied in Fig. S7 (Supporting
information), the values of TOC dropped drastically after 60 min
under control group. However, in the anaerobic condition group,
TOC remained quite stable. The presence of oxygen will promote
the reduction of TOC, which was of great environmental
significance for the application of SR-AOPs.
Supplementary material related to this article can be
found, in the online version, at doi:https://doi.org/10.1016/j.
cclet.2021.02.062.
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To conclude, from the experimental data and calculation results,
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ꢀ
pathways of TMP reaction with SO4 ꢁ. The absence of DO would
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pathways, although same oxiꢀdꢁation products were detected. Without
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Declaration of competing interest
The authors report no declarations of interest.
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