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Remediation and Control Technologies
Dechlorination-Hydroxylation of Atrazine to Hydroxyatrazine with Thiosulfate: A Detoxification Strategy in Seconds Yi Mu, Guangming Zhan, Cuimei Huang, Xiaobing Wang, Zhihui Ai, Jian-Ping Zou, Shenglian Luo, and Lizhi Zhang Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b06351 • Publication Date (Web): 22 Feb 2019 Downloaded from http://pubs.acs.org on February 23, 2019
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Environmental Science & Technology
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Dechlorination-Hydroxylation of Atrazine to Hydroxyatrazine with
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Thiosulfate: A Detoxification Strategy in Seconds
3
Yi Mua, b, Guangming Zhana, Cuimei Huanga, Xiaobing Wanga, Zhihui Aia,*, Jianping Zoub,*,
4
Shenglian Luob, Lizhi Zhanga,*
5 a
6
Key Laboratory of Pesticide & Chemical Biology of Ministry of Education, Institute of
7
Environmental Chemistry, Central China Normal University, Wuhan 430079, People’s Republic of
8
China
9 10 11 12
b
Key Laboratory of Jiangxi Province for Persistent Pollutants Control and Resources Recycle, Nanchang Hangkong University, Nanchang 330063, People’s Republic of China *Corresponding
author. Phone/fax: +86-27-6786 7535.
Email address: jennifer.ai@mail.ccnu.edu.cn; zjp_112@126.com; zhanglz@mail.ccnu.edu.cn
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Abstract: Hydroxylation of atrazine to nontoxic hydroxyatrazine is generally considered as an
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efficient detoxification method to remediate atrazine-contaminated soil and water. However,
16
previous studies suggested that hydroxylation was not the dominant pathway for atrazine degradation
17
in the hydroxyl radical-generating systems such as Fenton reaction, ozonation and UV/H2O2. Herein
18
we report that the addition of sodium thiosulfate can realize rapid hydroxylation of atrazine to
19
hydroxyatrazine at pH ≤ 4 under room temperature. High resolution mass spectra and isotope
20
experiments results revealed that the hydroxylation of atrazine was involved with nucleophilic
21
substitution and subsequent hydrolysis reaction as follows. HS2O3-, as a species of thiosulfate only at
22
pH ≤ 4, first attacked C atom connecting to chlorine of atrazine to dechlorinate atrazine and produce
23
C8H14N5S2O3-. Subsequently, the S−S bond of C8H14N5S2O3- was cleaved easily to form SO3 and
24
C8H14N5S-. Next, C8H14N5S- was hydrolyzed to generate hydroxyatrazine and H2S. Finally, the
25
comproportionation of SO3 and H2S in-situ produced S0 during hydroxylation of atrazine with
26
thiosulfate. This study clarifies the importance of degradation pathway on the removal of pollutants,
27
and also provides a non-oxidative strategy for atrazine detoxification in seconds.
28 29
Keywords: Atrazine; Thiosulfate; Detoxification; Nucleophilic substitution; Hydroxylation;
30
Hydroxyatrazine
31 32
Introduction
33
Atrazine (2-chloro-4-(ethylamino)-6-isopropylamino-s-triazine) has been introduced to control the
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agricultural weeds since 1950s and it has become one of the most widely used herbicides with 70000
35
- 90000 tons per year in the world. Because of its large scale application and resistance to microbial
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degradation,1 atrazine is frequently detected in the natural water.2 The pollution of atrazine has raised
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growing concerns because of its endocrine disrupting effect3,
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Recently, atrazine has been regarded as a class C carcinogen by World Health Organization (W. H.
39
O).7 Thus, it has been forbidden in the EU. However, atrazine is still widely used in many countries.
40
So it is imperative to advance a facile and effective strategy to remove atrazine.
41
4
and the risk to human health.5,
6
Various strategies have been developed to remove atrazine from water, such as biological
42
methods,8,
9
photolytic destruction,10 advanced oxidation processes,11 advanced reduction
43
processes,12 electrochemical oxidation,13 molecular oxygen activation,14 microwave induced
44
degradation,15 and ozonation. Generally, the pathways of atrazine degradation with these strategies
45
usually involved the dealkylation, alkylic-oxidation, dechlorination, and hydroxylation processes
46
simultaneously.16 Thus, different intermediates could be detected during the degradation process
47
such as desisopropylatrazine (CEAT), desethylatrazine (CIAT), desethyldesisopropylatrazine
48
(CAAT),
49
1-(4-chloro-6-(isopropylamino)-
50
4-ethylamino-6-isopropylamino-s-triazine (EIT), and hydroxyatrazine. Although these intermediates
51
are less toxic than atrazine,17-19 their further degradation was substantially slower than atrazine.
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Therefore, the cumulative toxicity of these intermediates to aquatic creatures might be greater than
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that of atrazine.20 Among these intermediates, hydroxyatrazine was regarded as a totally nontoxic
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compound.21 Thus, direct transformation of atrazine to hydroxyatrazine is considered as one of most
55
efficient methods to remove atrazine contamination.
2-acetamido-4-chloro-6-(isopropylamino)-s-triazine 1,3,5-triazin-2-ylamino)
-ethanol
(CDIT), (CNIT),
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Hydroxyl radical (•OH) was a strong oxidant (E0(•OH/OH-) = 2.8 V vs SHE) and highly reactive
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with most organic compounds. The reaction constant of •OH and atrazine was as high as 2.4 -
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3.0×109 M-1•s-1.22,
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atrazine might be proceeded via •OH attack of the s-triazine ring at the position of the chlorine group
60
(Eq. 1). In recent years, scientists intensively investigated the mechanism of atrazine degradation in
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•OH generating systems including Fenton reaction,24,
62
H2O2/UV,26,
63
chlorine on s-triazine ring by •OH was rather difficult, because the dechlorination-hydroxylation of
64
atraznie is not the dominant degradation pathway in these •OH generating systems (Figure S1).29 For
65
example, in the Fenton reaction system, atrazine was proposed to form chlorinated products (e.g.
66
CIAT, CEAT, CAAT, CDAT, and CAAT).23 Although trace amount of hydroxylated by-products
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could also be found, the dealkylation pathway is usually predominant. This was because attacking
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side chains of atrazine by •OH has priority, generating these chlorinated products.30 Further
69
dechlorination-hydroxylation of these chlorinated products under attack of •OH was more difficult to
70
take place owing to the low reactivity of •OH toward these chlorinated products. Thus, it still
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challenges us to propose a facile and efficient dechlorination-hydroxylation strategy to complete
72
transformation of atrazine to nontoxic hydroxyatrazine.
27
23
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It was thus thought that the dechlorination and subsequent hydroxylation of
25
ozone oxidation,23 TiO2 photocatalysis,17
and electrochemical oxidation.28 Unfortunately, they found that displacement of
(1)
73 74
Rather than the oxidation by hydroxyl radical, SN2 nucleophilic substitution was found to be an
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effective dechlorination strategy.31-33 Among various nucleophiles, thiosulfate is the most widely
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used one for its low cost, nontoxic, and high reactivity.34 For example, Gan et al. reported that
77
thiosulfate could facilitate the removal of halogen content from the halogenated aliphatic
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hydrocarbons, such as 1,3-dichloropropene, chloropicrin, methyl bromide, propargyl bromide, and
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methyl iodide, via nucleophilic substitution.35, 36 They further discovered that the chloroacetanilide
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herbicides, including propachlor, alachlor, metolachlor, and acetochlor, could be dechlorinated by
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thiosulfate.37 Given that atrazine has a special structure of Cl substitution on s-triazine ring, it is still
82
unknown whether atrazine could be dechlorinated by thiosulfate via nucleophilic substitution, which
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might largely eliminate the acute toxicity of atrazine.
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In this study, we systematically investigate the reaction of atrazine and thiosulfate at different pH
85
values, and check the degradation products with high performance liquid chromatography (HPLC),
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ion chromatograph (IC), liquid chromatography mass spectrometry (LC-MS), and liquid
87
chromatography-high resolution mass spectrometry (LC-HRMS). According to the results of
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LC-HRMS and isotope experiments, the possible reaction mechanism is proposed. Meanwhile, the
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reactions of thiosulfate and other typical s-triazine pollutants are also checked, aiming to elucidate
90
the intrinsic reactivity of thiosulfate to remove chlorinated s-triazine compounds.
91 92
Experiment Section
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Chemicals. Atrazine (C8H14N5Cl), simazine (C7H12N5Cl), desethylatrazine (CIAT; C6H10N5Cl),
94
desisopropylatrazine (CEAT; C5H8N5Cl), hydroxyatrazine (OIET; C5H9N5O) and sodium thiosulfate
95
(Na2S2O3) were bought from Alfa Aesar. Sodium bisulfite (NaHSO3), sodium sulfide (Na2S),
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isopropanol (CH3CH2(OH)CH3), ethanol (CH3CH2OH), H2SO4 and NaOH were purchased from
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National Medicines Corporation Ltd., China. Superoxide dismutase (SOD) and catalase (CAT) were
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bought from Sigma-Aldrich.
99
Atrazine Degradation Experiments. Atrazine degradation experiments (10 mg•L-1, 100 mL) were
100
conducted in conical flasks at 25 oC. The initial pH was adjusted to predetermined pH values (2, 3, 4,
101
5, and 6) with 1 mol•L-1 NaOH and 1 mol•L-1 HCl solutions. 0.066 g of Na2S2O3 was adding into the
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flask, followed by transferred the flask into a shaker (100 r•min-1) to initiate the atrazine degradation
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immediately. 500 μL of solution was withdrawn at regular intervals (5 s, 15 s, 25 s, 40 s, and 60 s).
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Subsequently, 500 μL of NaOH (pH = 12) was added instantly to stop the reaction as atrazine
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degradation was completely inhibited at alkaline condition (Figure S2). After that, the mixture was
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filtered through 0.22 μm filter membranes for the subsequent measurement. During a typical
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anaerobic atrazine degradation with thiosulfate process, a 100 mL three-neck flask containing 0.066
108
g of Na2S2O3 was vacuumed and filled with nitrogen gas. Then 100 mL of anoxic atrazine solution
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(10 mg•L-1) was added into the three-neck flask. The subsequent experimental procedures were the
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same as those of aerobic atrazine degradation. After the reaction, some precipitate was generated in
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the solution. The reaction solution was then centrifuged to separate the precipitate from solution. The
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obtained precipitate was dried under anaerobic condition and then characterized with X-ray
113
diffraction (XRD, D8 Advance, Cu Kα radiation, λ = 0.15418 nm, Bruker, Germany).
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Analysis Methods. The atrazine concentrations were measured by high pressure liquid
115
chromatograph (HPLC, Ultimate 3000, Thermo) with a TC-C18 reverse phase column (150 mm ×
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4.6 mm, 5 µm, Agilent) and a ultraviolet detector (wavelength: 220 nm). The concentrations of Cl-
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were determined by ion chromatograph (IC, Dionex ICS-900, Thermo). Atrazine degradation
118
products were detected by liquid chromatography–mass spectrometry (LC-MS, TSQ Quantum
119
Access MAX, Thermo). The detailed analysis methods were provided in the Supporting Information
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(SI). To detect the shortly lived and highly reactive intermediates during atrazine degradation with
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sodium thiosulfate, a Dionex Ultimate 3000 series liquid chromatography combined with Q Exactive
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hybrid Quadrupole-Orbitrap mass spectrometer (Thermo Scientific, Bremen, Germany) was
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introduced to acquire accurate masses, possible elemental compositions and structure of degradation
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intermediates.38, 39 Thanks to the exceptional selectivity of high resolution MS, sample pretreatment
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was not needed and the solution could be fed into the MS directly. The system was worked with a
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heated electrospray ionization (HESI) source in negative mode with a spray voltage of -3.2 kV. The
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capillary temperature was set as 300 °C, the S-lens rf level was set as 50% and a mass resolution of
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70 000 was used. Other parameters were set as default values.40 The mass and charge ratio (m/z)
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value of target ions was restricted between 100.00 and 500.00. The mass spectra were analyzed by
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Xcalibur 2.1 software (Thermo Scientific). The possible molecular formulas of the selected peaks in
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the full mass scan were calculated within 5 ppm mass tolerance allowing the elements of C, H, N, O
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and S.
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Theory Calculations. Density functional theory (DFT) calculations were conducted with Gaussian
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09 software package (Gaussian 09, Revision D.01, Gaussian, Inc., Wallingford CT, 2013). Structures
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of reactants (HS2O3-, Atrazine), transition states, intermediates (C8H14N5S2O3H) and products were
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optimized at the B3LYP/6-311G (d, p) level. Solvent effects were included by using self-consistent
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reaction field (SCRF) technique. The obtained structures of the transition states were confirmed by
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vibrational analysis and the transition states have one imaginary frequency. Intrinsic reaction
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coordinate (IRC) calculations were carried out to verify the connectivity between the calculated
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transition states to the corresponding reactants and products. The Gibbs free energies of activation
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(∆G⧺) and the reaction (∆Gr) for the nucleophilic substitution (reaction between HS2O3- and atrazine)
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and hydrolysis reaction (reaction between C8H14N5S- and H2O) were calculated.41
143 144
Results and discussion
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Figure 1a displays the time profiles of atrazine reacting with sodium thiosulfate under different initial
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pH values. It was found that pH 2 favored the reaction of atrazine and sodium thiosulfate, achieving
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nearly 100% of atrazine degradation within 120 s. However, the atrazine degradation efficiency
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gradually decreased with increasing the pH value from 2.0 to 4.0, and the atrazine degradation was
149
completely inhibited at pH ˃ 4. As shown in Figure 1b, S2O32- was the major thiosulfate species at
150
pH ≥ 2, while 30%, 6% and 1.2% of thiosulfate existed as HS2O3- at pH = 2, 3, and 4, respectively.
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We did not observe the degradation of atrazine at pH ˃ 4 without HS2O3-. So the pH dependent
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reactivity of thiosulfate to atrazine might be related to the speciation of thiosulfate generated at
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different pH values, and the gradual decrease of atrazine degradation efficiency along with the
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increase of pH values might be arisen from the thiosulfate species change from HS2O3- to S2O32-.
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Meanwhile, we also monitored the temporal pH change of the systems versus reaction time, and
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observed that pH values did not change obviously throughout the entire degradation process (SI
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Figure S3), ruling out the contribution of pH change to the atrazine degradation with thiosulfate. It is
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worth noting that some precipitate was generated during the atrazine degradation, and the precipitate
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mainly consisted of S0, as revealed by XRD patterns (SI Figure S4). To check the effect of in-situ
160
generated S0 on degradation of atrazine, the precipitate was collected and then used to degrade
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atrazine. However, atrazine could not be removed by the precipitate (SI Figure S5), ruling out the
162
contribution of in situ generated S0 to the atrazine degradation. For comparison, thiosulfate was
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replaced with other sulfate species (such as S2- and SO32-) for the atrazine degradation reaction at pH
164
2.0. As shown in Figure S5, neither S2- nor SO32- could remove atrazine even prolonging the reaction
165
time to 30 min. We therefore concluded that thiosulfate offered an efficient and environmentally
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benign strategy to remediate atrazine contamination in acid condition.
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The amount of thiosulfate on the degradation of atrazine was then investigated at pH = 2 and the
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initial atrazine concentrations of 10 mg•L-1 (Figure 1c). All the atrazine degradation curves obeyed
169
pseudo first-order reaction kinetic equations, and the rate constants (k) were 0.035, 0.034, 0.024,
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0.020, 0.014 and 0.008 s-1 at the thiosulfate to atrazine molar ratios of 180, 90, 45, 20, 10, 2.5,
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respectively (Figure 1d). Although the atrazine degradation rate gradually decreased with the
172
decrease of thiosulfate amount, more than 35% of atrazine could still be degraded within 120 s at the
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thiosulfate to atrazine molar ratio of 2.5. As the atrazine degradation rate did not increase when
174
increasing the thiosulfate to atrazine molar ratio from 90 to 180, we chose 90 as the optimal initial
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thiosulfate to atrazine molar ratio in this study.
176
Subsequently, we identified the intermediates and products of atrazine degradation at pH = 2. As
177
shown in Figure 2a, the Cl- concentration rapidly increased along with the decrease of atrazine
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concentration and then approached the theoretical value (0.046 mmol•L-1) within 120 s, indicating
179
that the dechlorination process took place along with the atrazine degradation. We thus checked the
180
degradation products with HPLC and LC-MS. During the atrazine degradation, only one product
181
peak (retention time 6.48 min) appeared after 15 s. The peak intensity of this product quickly
182
increased along with the decrease of atrazine concentration (Figure 2b). This product was identified
183
as hydroxyatrazine by LC-MS analysis (SI Figure S6). The concentration of hydroxyatrazine was
184
0.046 mmol·L-1 after 5 min of treatment and the corresponding selectivity reached 99.2% (SI Figure
185
S7). Subsequently, we also investigated the reaction hydroxyatrazine and thiosulfate with the same
186
experimental procedure as that of atrazine degradation, and found that hydroxyatrazine could not be
187
removed by HS2O3- even extending the reaction time to 30 min (SI Figure S8). This result suggested
188
that the cleavage of the C−Cl bond of atrazine and subsequent formation of hydroxyatrazine was the
189
sole pathway for the atrazine degradation with HS2O3-, which could avoid the generation of toxic
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dealkylation and alkylic-oxidation intermediates.
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To understand how HS2O3- could induce the hydroxylation of atrazine, we first check the possible
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generation of reactive oxygen species (ROS, i.e., •O2-, H2O2, and •OH) and their contribution to
193
hydroxylation of atrazine with thiosulfate by adding excess scavengers (SOD for •O2-, iso-propanol
194
for •OH and catalase for H2O2) (Figure 3a).42-43 We found that the atrazine degradation rates did not
195
change after addition of the scavengers, suggesting that •O2-, H2O2 or •OH were not involved in the
196
atrazine degradation in this study. We also explored the effect of molecular oxygen on atrazine
197
degradation with HS2O3- by comparing the aerobic atrazine degradation rate with anaerobic one at an
198
initial pH of 2 (SI Figure S9a). Both aerobic and anaerobic atrazine degradation curves followed a
199
pseudo first-order kinetics model (SI Figure S9b). The anaerobic atrazine degradation rate constant
200
(0.032 s-1) was almost the same as the aerobic one (0.033 s-1), ruling out the participation of
201
molecular oxygen in atrazine degradation with thiosulfate at pH = 2.
202
Previous study suggested that the peripheral S atom of thiosulfate was more nucleophilic than its
203
ligand O,44 and could attack an electron deficient site. Chlorine substituent on the s-triazine ring of
204
atrazine would decrease the electron density of neighboring C atom because chlorine was an electron
205
withdrawing group. Therefore, HS2O3- could facilitate dechlorination-hydroxylation of atrazine into
206
hydroxyatrazine in the manner of nucleophilic substitution reaction. To gain mechanic insight into
207
the nucleophilic substitution of atrazine with HS2O3-, density functional theory (DFT) calculations
208
were first introduced to simulate the possible pathway. Figure 4a presents the geometrical
209
information of transition state corresponding to the nucleophilic substitution of atrazine with HS2O3-.
210
The transition state was obtained using the B3LYP method with the 6-311G (d, p) basis set.
211
Frequency calculation was conducted to confirm that the transition state possessed only one
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imaginary frequency corresponding to vibration of peripheral S atom moving between HS2O3- anion
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and atrazine. IRC analysis was used to further validate that obtained transition state corresponded to
214
the mechanism of nucleophilic substitution reaction between HS2O3- and atrazine (SI Figure S10).
215
We found that two bonds were undergoing substantial changes during the nucleophilic substitution,
216
which were the breaking C-Cl bond of atrazine and forming S−C bond (dash line in Figure 4a),
217
resulting in formation of product C8H14N5S2O3H (SI Figure S11). B3LYP/6-311G (d, p) energy
218
profiles for the nucleophilic substitution in Figure 4b revealed that the Gibbs free energy of this
219
reaction was -20 kJ•mol-1, suggesting that nucleophilic substitution of atrazine with HS2O3- was
220
energetically favorable.
221
The rapid transformation of atrazine to hydroxyatrazine with HS2O3- suggested that this
222
nucleophilic substitution intermediate (C8H14N5S2O3H) might be shortly lived and easily dissociated
223
in the reaction solution. We thus employed nanospray liquid chromatography-high resolution mass
224
spectrometry (LC-HRMS) to check the generation of C8H14N5S2O3H during hydroxylation of
225
atrazine with HS2O3-. In the negative ion mode mass spectra of reaction solution, the deprotonated
226
nucleophilic substitution product C8H14N5S2O3- (m/z 292.05569) was detected and its mass tolerance
227
was -1.574 ppm (Figure 4c and S12). Besides C8H14N5S2O3-, another organic sulfide peak of m/z
228
212.09787 was also found, corresponding to C8H14N5S- with the mass tolerance of -4.217 ppm
229
(Figure 4d and S13). The appearance of C8H14N5S- suggested that the S−S bond of C8H14N5S2O3-
230
was cleaved easily to form C8H14N5S- and SO3 (Eq 3). Therefore, hydroxyatrazine might be derived
231
from C8H14N5S-. As stated above, neither molecular oxygen nor reactive oxygen species (i.e., •O2-,
232
•OH, H2O2) were involved in the transformation of atrazine into hydroxyatrazine in the presence of
233
thiosulfate, we thus proposed that the formation of hydroxyatrazine was proceed via hydrolysis of
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C8H14N5S- and oxygen atoms in hydroxyatrazine might be originated from the solvent H2O.45 To
235
validate this point, we employed DFT calculations to investigate the reaction between C8H14N5SH
236
and H2O (Figure 4e), and found the Gibbs free energies of activation (ΔG⧺) and reaction (ΔGr) were
237
35.4 kJ•mol-1 and -19.6 kJ•mol-1 respectively, suggesting that hydrolysis of C8H14N5SH is virtually
238
irreversible (Figure 4f). To further check the origin of O atom in hydroxyatrazine, we performed the
239
atrazine degradation with thiosulfate in solvent H2O18 under aerobic condition. As shown in the
240
Figure 4g, only the deprotonated hydroxyatrazine C8H14N518O- of m/z 198.10301 was detected,
241
confirming that solvent H2O supplied O atom for hydroxyatrazine via hydrolysis reaction.
242
According to the above results, we proposed a possible pathway for the rapid hydroxylation of
243
atrazine to hydroxyatrazine with HS2O3- (Scheme 1). First, HS2O3-, as a species of thiosulfate only at
244
pH ≤ 4, was functioned as a nucleophile to attack C atom connecting to chlorine substituent of
245
atrazine, resulting in the dechlorination of atrazine and the formation of C8H14N5S2O3- (Eq. 2).
246
Subsequently, the S−S bond of C8H14N5S2O3- was cleaved easily to form SO3 and C8H14N5S- (Eq. 3).
247
Next, C8H14N5S- was hydrolyzed to generate hydroxyatrazine and H2S (Eq. 4), as confirmed by the
248
results of isotope experiments. Finally, the comproportionation of SO3 and H2S in-situ produced S0
249
during the hydroxylation of atrazine with HS2O3- (Eq. 5).
250
C8H14N5Cl + HS2O3- → C8H14N5S2O3- + HCl
(2)
251
C8H14N5S2O3- + H2O → C8H14N5SH + SO3 + OH-
(3)
252
C8H14N5SH + H2O → C8H14N5OH +H2S
(4)
253
3H2S + SO3 → 4S0↓ + 3H2O
(5)
254
The generality of thiosulfate in the hydroxylation of s-triazine compounds was also investigated
255
via degradation experiments of other three typical s-triazine pollutants such as simazine (SIM),
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desethylatrazine (CIAT), and desisopropylatrazine (CEAT). As shown in Figure 5a, thiosulfate
257
exhibited extremely high reactivity for the degradation of simazine, CIAT, and CEAT, and only one
258
hydroxylated product was generated during the reaction procedure (Figure 5b - 5d), suggesting that
259
thiosulfate can be used to remove different s-triazine pollutants generally. Moreover, we found that
260
SO42-, NO3-, PO43-, Cl-, and NH4+, the typical coexisted ions in the pesticide wastewater,46 did not
261
affect the atrazine degradation with thiosulfate (SI Figure S14 - S15). Therefore, this thiosulfate
262
induced detoxification method was applicable to the pesticide wastewater treatment.
263
Environmental implications. Hydroxylation of atrazine to nontoxic hydroxyatrazine is considered
264
as one of the most efficient methods to remediate atrazine contamination. Scientists mainly focus on
265
the hydroxylation of atrazine with •OH, although the hydroxylation of atrazine with •OH is not the
266
dominate degradation pathway, and thus the dealkylation and alkylic-oxidation intermediates are
267
often generated during the traditional •OH-based advanced oxidation processes, while the cumulative
268
toxicity of dealkylation and alkylic-oxidation intermediates might be greater than that of atrazine. In
269
this study, we have proposed a non-oxidative strategy to hydroxylate atrazine into nontoxic
270
hydroxyatrazine with thiosulfate. Different from the •OH-based oxidative strategy, thiosulfate could
271
induce complete transformation of atrazine to nontoxic hydroxyatrazine at pH ≤ 4, and thus avoid the
272
toxic dealkylation and alkylic-oxidation intermediates generation. Although thiosulfate could not
273
mineralize atrazine into CO2 and NH3+, its hydroxylation of atrazine into nontoxic hydroxyatrazine
274
could greatly alleviate the biological toxicity and improve the biodegradability of atrazine. This
275
study clarifies the importance of degradation pathway on the removal of pollutants, and also provides
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a non-oxidative strategy for atrazine detoxification in seconds.
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ASSOCIATED CONTENT
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Supporting Information
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Additional descriptions, Figures, and tables as mentioned in the text. This material is available free
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of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
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Corresponding Author
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*Phone/Fax: +86-27-6786 7535; e-mail: jennifer.ai@mail.ccnu.edu.cn; zjp_112@126.com;
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zhanglz@mail.ccnu.edu.cn.
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Notes
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The authors declare no competing financial interest.
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Acknowledgements. This work was supported by Natural Science Funds for Distinguished Young
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Scholars (Grant 21425728), National Key Research and Development Program of China (Grant
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2016YFA0203002), the 111 Project (Grant B17019), Self-Determined Research Funds of CCNU
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from the Colleges’ Basic Research and Operation of MOE (Grant CCNU14Z01001 and
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CCNU16A02029), Excellent Doctorial Dissertation Cultivation Grant from Central China Normal
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University (2015YBZD024 and 2016YBZZ031), and the CAS Interdisciplinary Innovation Team of
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the Chinese Academy of Sciences. Specially, we gratefully acknowledge the help of Yue’e Peng
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and Menglan Zhang in China University of Geosciences, Wuhan for their mass spectrometer
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analysis.
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Reference
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Figure Captions
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Figure 1. (a) Time profiles of atrazine reacting with NaS2O3 under different initial pH values. The
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initial concentration of atrazine and NaS2O3 were 10 mg·L-1 (0.0464 mmol·L-1) and 0.66 g·L-1 (4.18
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mmol·L-1), respectively. (b) Thiosulfate speciation as a function of pH values. The thiosulfate
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species distribution was calculated by the speciation program Visual MINTEQ 3.0. (c) Degradation
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of atrazine and (d) plot of ln(C0/C) versus time in different thiosulfate/atrazine molar ratio systems.
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The initial pH values were 2. The initial concentration of atrazine was 10 mg·L-1 (0.0464 mmol·L-1).
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Figure 2. (a) Time profiles of atrazine (black line) and dechlorination efficiency (green line) in
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NaS2O3 system. (b) The HPLC chromatogram of the samples at different reaction time. The peaks at
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retention time 6.48 min and 10.78 min were belonged to hydroxyatrazine and atrazine, respectively.
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The initial concentration of atrazine and NaS2O3 were 10 mg·L-1 (0.0464 mmol·L-1) and 0.66 g·L-1
440
(4.18 mmol·L-1). The initial pH of the system was 2.0.
441 442
Figure 3. (a) Time profiles of atrazine in NaS2O3 system with addition of different scavengers (SOD
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for •O2-, CAT for H2O2, iso-propanol for •OH). (b) The corresponding atrazine degradation rate
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constant.
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Figure 4. (a) Basic geometrical parameters of the transition state structures for nucleophilic
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substitution between atrzaine and thiosulfate (HS2O3-). (b) Profile of the potential energy surface for
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the nucleophilic substitution process. Mass spectra of atrazine degradation intermediates ((c)
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C8H14N5S2O3-, m/z 292.05569; (d) C8H14N5S-, m/z 212.09760) detected by nanospray liquid
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chromatography-high resolution mass spectrometry. (e) Basic geometrical parameters of the
452
transition state structures for hydrolysis reaction. (f) Profile of the potential energy surface for the
453
nucleophilic hydrolysis reaction. (g) The mass spectra of hydroxyatrazine (C8H14N518O-, m/z
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198.10301) detected in isotope experiments.
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456 457
Scheme 1. The possible pathway for the transformation of atrazine to hydroxyatrazine with HS2O3-.
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Figure 5. (a) Time profiles of the atrazine, simazine, 2-chloro-4-amino-6- isopropylamino-1, 3,
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5-triazine (CAIT), and 2-chloro-4-ethylamino-6-amino-1, 3, 5-triazine (CEAT) degradation in
461
thiosulfate systems. The initial pH values were 2. The initial concentration of target pollutants was
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10 mg·L-1. The initial concentration of thiosulfate was 0.66 g·L-1. The HPLC chromatograms of (b)
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SIM, (c) CIAT, and (d) CEAT degradation product sampled at different reaction time.
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TOC Art Figure
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