Dechlorination-Hydroxylation of Atrazine to Hydroxyatrazine with

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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: [email protected]; [email protected]; [email protected]

ACS Paragon Plus Environment


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Abstract: Hydroxylation of atrazine to nontoxic hydroxyatrazine is generally considered as an

15

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

34

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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36

degradation,1 atrazine is frequently detected in the natural water.2 The pollution of atrazine has raised

37

growing concerns because of its endocrine disrupting effect3,

38

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

53

that of atrazine.20 Among these intermediates, hydroxyatrazine was regarded as a totally nontoxic

54

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),

56

Hydroxyl radical (•OH) was a strong oxidant (E0(•OH/OH-) = 2.8 V vs SHE) and highly reactive

57

with most organic compounds. The reaction constant of •OH and atrazine was as high as 2.4 -



58

3.0×109 M-1•s-1.22,

59

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

61

•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

67

could also be found, the dealkylation pathway is usually predominant. This was because attacking

68

side chains of atrazine by •OH has priority, generating these chlorinated products.30 Further

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dechlorination-hydroxylation of these chlorinated products under attack of •OH was more difficult to

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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

75

effective dechlorination strategy.31-33 Among various nucleophiles, thiosulfate is the most widely

76

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

78

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

83

might largely eliminate the acute toxicity of atrazine.

84

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),

86

ion chromatograph (IC), liquid chromatography mass spectrometry (LC-MS), and liquid

87

chromatography-high resolution mass spectrometry (LC-HRMS). According to the results of

88

LC-HRMS and isotope experiments, the possible reaction mechanism is proposed. Meanwhile, the

89

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

93

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),

96

isopropanol (CH3CH2(OH)CH3), ethanol (CH3CH2OH), H2SO4 and NaOH were purchased from

97

National Medicines Corporation Ltd., China. Superoxide dismutase (SOD) and catalase (CAT) were

98

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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102

flask, followed by transferred the flask into a shaker (100 r•min-1) to initiate the atrazine degradation

103

immediately. 500 μL of solution was withdrawn at regular intervals (5 s, 15 s, 25 s, 40 s, and 60 s).

104

Subsequently, 500 μL of NaOH (pH = 12) was added instantly to stop the reaction as atrazine

105

degradation was completely inhibited at alkaline condition (Figure S2). After that, the mixture was

106

filtered through 0.22 μm filter membranes for the subsequent measurement. During a typical

107

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

109

(10 mg•L-1) was added into the three-neck flask. The subsequent experimental procedures were the

110

same as those of aerobic atrazine degradation. After the reaction, some precipitate was generated in

111

the solution. The reaction solution was then centrifuged to separate the precipitate from solution. The

112

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).

114

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 ×

116

4.6 mm, 5 µm, Agilent) and a ultraviolet detector (wavelength: 220 nm). The concentrations of Cl-

117

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

120

(SI). To detect the shortly lived and highly reactive intermediates during atrazine degradation with

121

sodium thiosulfate, a Dionex Ultimate 3000 series liquid chromatography combined with Q Exactive

122

hybrid Quadrupole-Orbitrap mass spectrometer (Thermo Scientific, Bremen, Germany) was

123

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

125

was not needed and the solution could be fed into the MS directly. The system was worked with a

126

heated electrospray ionization (HESI) source in negative mode with a spray voltage of -3.2 kV. The

127

capillary temperature was set as 300 °C, the S-lens rf level was set as 50% and a mass resolution of

128

70 000 was used. Other parameters were set as default values.40 The mass and charge ratio (m/z)

129

value of target ions was restricted between 100.00 and 500.00. The mass spectra were analyzed by

130

Xcalibur 2.1 software (Thermo Scientific). The possible molecular formulas of the selected peaks in

131

the full mass scan were calculated within 5 ppm mass tolerance allowing the elements of C, H, N, O

132

and S.

133

Theory Calculations. Density functional theory (DFT) calculations were conducted with Gaussian

134

09 software package (Gaussian 09, Revision D.01, Gaussian, Inc., Wallingford CT, 2013). Structures

135

of reactants (HS2O3-, Atrazine), transition states, intermediates (C8H14N5S2O3H) and products were

136

optimized at the B3LYP/6-311G (d, p) level. Solvent effects were included by using self-consistent

137

reaction field (SCRF) technique. The obtained structures of the transition states were confirmed by

138

vibrational analysis and the transition states have one imaginary frequency. Intrinsic reaction

139

coordinate (IRC) calculations were carried out to verify the connectivity between the calculated

140

transition states to the corresponding reactants and products. The Gibbs free energies of activation

141

(∆G⧺) and the reaction (∆Gr) for the nucleophilic substitution (reaction between HS2O3- and atrazine)

142

and hydrolysis reaction (reaction between C8H14N5S- and H2O) were calculated.41

143 144

Results and discussion

145

Figure 1a displays the time profiles of atrazine reacting with sodium thiosulfate under different initial



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146

pH values. It was found that pH 2 favored the reaction of atrazine and sodium thiosulfate, achieving

147

nearly 100% of atrazine degradation within 120 s. However, the atrazine degradation efficiency

148

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.

151

We did not observe the degradation of atrazine at pH ˃ 4 without HS2O3-. So the pH dependent

152

reactivity of thiosulfate to atrazine might be related to the speciation of thiosulfate generated at

153

different pH values, and the gradual decrease of atrazine degradation efficiency along with the

154

increase of pH values might be arisen from the thiosulfate species change from HS2O3- to S2O32-.

155

Meanwhile, we also monitored the temporal pH change of the systems versus reaction time, and

156

observed that pH values did not change obviously throughout the entire degradation process (SI

157

Figure S3), ruling out the contribution of pH change to the atrazine degradation with thiosulfate. It is

158

worth noting that some precipitate was generated during the atrazine degradation, and the precipitate

159

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

161

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

163

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

166

benign strategy to remediate atrazine contamination in acid condition.

167

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,

170

0.020, 0.014 and 0.008 s-1 at the thiosulfate to atrazine molar ratios of 180, 90, 45, 20, 10, 2.5,

171

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

173

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

175

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

178

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



190

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dealkylation and alkylic-oxidation intermediates.

191

To understand how HS2O3- could induce the hydroxylation of atrazine, we first check the possible

192

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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212

imaginary frequency corresponding to vibration of peripheral S atom moving between HS2O3- anion

213

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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234

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

276

a non-oxidative strategy for atrazine detoxification in seconds.

277



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ASSOCIATED CONTENT

279

Supporting Information

280

Additional descriptions, Figures, and tables as mentioned in the text. This material is available free

281

of charge via the Internet at http://pubs.acs.org.

282 283

AUTHOR INFORMATION

284

Corresponding Author

285

*Phone/Fax: +86-27-6786 7535; e-mail: [email protected]; [email protected];

286

[email protected].

287

Notes

288

The authors declare no competing financial interest.

289

Acknowledgements. This work was supported by Natural Science Funds for Distinguished Young

290

Scholars (Grant 21425728), National Key Research and Development Program of China (Grant

291

2016YFA0203002), the 111 Project (Grant B17019), Self-Determined Research Funds of CCNU

292

from the Colleges’ Basic Research and Operation of MOE (Grant CCNU14Z01001 and

293

CCNU16A02029), Excellent Doctorial Dissertation Cultivation Grant from Central China Normal

294

University (2015YBZD024 and 2016YBZZ031), and the CAS Interdisciplinary Innovation Team of

295

the Chinese Academy of Sciences. Specially, we gratefully acknowledge the help of Yue’e Peng

296

and Menglan Zhang in China University of Geosciences, Wuhan for their mass spectrometer

297

analysis.

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425 426

Figure Captions

427 428

Figure 1. (a) Time profiles of atrazine reacting with NaS2O3 under different initial pH values. The

429

initial concentration of atrazine and NaS2O3 were 10 mg·L-1 (0.0464 mmol·L-1) and 0.66 g·L-1 (4.18

430

mmol·L-1), respectively. (b) Thiosulfate speciation as a function of pH values. The thiosulfate

431

species distribution was calculated by the speciation program Visual MINTEQ 3.0. (c) Degradation

432

of atrazine and (d) plot of ln(C0/C) versus time in different thiosulfate/atrazine molar ratio systems.

433

The initial pH values were 2. The initial concentration of atrazine was 10 mg·L-1 (0.0464 mmol·L-1).

434



Page 20 of 24

435 436

Figure 2. (a) Time profiles of atrazine (black line) and dechlorination efficiency (green line) in

437

NaS2O3 system. (b) The HPLC chromatogram of the samples at different reaction time. The peaks at

438

retention time 6.48 min and 10.78 min were belonged to hydroxyatrazine and atrazine, respectively.

439

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

443

for •O2-, CAT for H2O2, iso-propanol for •OH). (b) The corresponding atrazine degradation rate

444

constant.

445


Page 21 of 24


446 447

Figure 4. (a) Basic geometrical parameters of the transition state structures for nucleophilic

448

substitution between atrzaine and thiosulfate (HS2O3-). (b) Profile of the potential energy surface for

449

the nucleophilic substitution process. Mass spectra of atrazine degradation intermediates ((c)



Page 22 of 24

450

C8H14N5S2O3-, m/z 292.05569; (d) C8H14N5S-, m/z 212.09760) detected by nanospray liquid

451

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

454

198.10301) detected in isotope experiments.

455

456 457

Scheme 1. The possible pathway for the transformation of atrazine to hydroxyatrazine with HS2O3-.


Page 23 of 24


458 459

Figure 5. (a) Time profiles of the atrazine, simazine, 2-chloro-4-amino-6- isopropylamino-1, 3,

460

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

462

10 mg·L-1. The initial concentration of thiosulfate was 0.66 g·L-1. The HPLC chromatograms of (b)

463

SIM, (c) CIAT, and (d) CEAT degradation product sampled at different reaction time.

464 465 466 467 468 469 470



471

TOC Art Figure

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Dechlorination-Hydroxylation of Atrazine to Hydroxyatrazine with

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