Dissolved Black Carbon as an Efficient Sensitizer in the

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Environmental Processes

Dissolved Black Carbon as an Efficient Sensitizer in the Photochemical Transformation of 17#-Estradiol in Aqueous Solution Zhicheng Zhou, Beining Chen, Xiaolei Qu, Heyun Fu, and Dongqiang Zhu Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b01928 • Publication Date (Web): 22 Aug 2018 Downloaded from http://pubs.acs.org on August 24, 2018

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Environmental Science & Technology

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Dissolved Black Carbon as an Efficient Sensitizer in the Photochemical Transformation

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of 17β-Estradiol in Aqueous Solution

3 Zhicheng Zhou,† Beining Chen,† Xiaolei Qu,† Heyun Fu,†* and Dongqiang Zhu‡

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†

State Key Laboratory of Pollution Control and Resource Reuse/School of the Environment,

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Nanjing University, Jiangsu 210046, China ‡

School of Urban and Environmental Sciences, Peking University, Beijing 100871, China

9 10 11 12 13 14 15 16 17

Manuscript prepared for Environmental Science & Technology

18 19 20

August 18th, 2018

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ACS Paragon Plus Environment


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ABSTRACT

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Dissolved black carbon (DBC) is an important component of the dissolved organic matter

24

(DOM) pool. Nonetheless, little is known about its role in the photochemical processes of

25

organic contaminants. This study investigated the effect of DBC on the phototransformation of

26

17β-estradiol in aqueous solutions under simulated sunlight. Four well-studied dissolved

27

humic substances (DHS) were included as comparisons. DBC acted as a very effective

28

sensitizer to facilitate the phototransformation of 17β-estradiol. The apparent quantum yield

29

for 17β-estradiol phototransformation mediated by DBC was approximately six times higher

30

than that by DHS at the same carbon concentration. Quenching experiments suggested that

31

direct reaction with triplet-excited state DBC (3DBC*) was the predominant pathway of

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17β-estradiol phototransformation. The higher mediation efficiency of DBC than DHS is

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likely due to the higher contents of aromatic groups and smaller molecular sizes, which

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facilitated the generation of 3DBC*. The apparent quantum yield of triplet-excited states

35

production for DBC was 4~8 times higher than that for DHS. The results suggest that 3DBC*

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may have a considerable contribution to the overall photoreactivity of triplet-excited state

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DOM in aquatic systems. Our findings also imply that DBC can play an important role in the

38

phototransformation of organic contaminants in the environments.

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INTRODUCTION

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Dissolved organic matter (DOM) is a complex mixture of organic molecules and is

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ubiquitous in natural aquatic environments.1 DOM plays a key role in the fate and transport

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processes of organic contaminants, among which photochemical transformation has gained

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intense attention.2-4 DOM can facilitate the transformation of many organic contaminants

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under sunlight through the production of reactive intermediates such as triplet-excited state

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DOM (3DOM*) and reactive oxygen species (ROS, e.g., singlet oxygen/1O2, superoxide

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anion/O2•-, and hydroxyl radical/•OH).5-11 On the other hand, DOM may inhibit the

48

phototransformation of organic contaminants by light screening and/or by quenching the

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reactive intermediates.4,12,13 It is well-recognized that the photoreactivity of DOM depends

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strongly on its chemical and structural properties, including aromaticity, aromatic ketone and

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aldehyde structures, and molecular size.3,14-18 The ability of DOM to absorb photons of the

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solar spectrum is closely related to its aromaticity.15,16 The photosensitized generation of

53

3

54

generation of 1O2 was found to increase with decreasing DOM molecular size.3,17 The

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molecular size of DOM also influences the phototransformation of organic chemicals such as

56

2,4,6-trimethylphenol and the herbicide fenuron.14

DOM* was reported to depend on the aromatic ketones and aldehydes in DOM.18 The

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Dissolved black carbon (DBC), which is the water soluble fraction of black carbon, has

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been recently identified as an important constituent of the natural DOM pool.19-21 DBC has

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been estimated to account for approximately 10% of the global riverine flux of dissolved

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organic carbon to the oceans,21 and it comprises as much as 4~7% of coastal marine DOM20

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and 4~22% of marine DOM.19 The chemical and structural properties of DBC are significantly

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different from well-studied DOM specimens (e.g., dissolved humic substances, DHS) due to

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different source materials and formation processes. DBC was reported to have a relatively

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homogeneous structure comprising small aromatic clusters extensively substituted with

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oxygen-containing functional groups (mainly carboxylic and phenolic groups).22,23 The unique

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molecular structure of DBC may lead to strong photoreactivity. Our recent investigation

67

demonstrated that DBC could effectively generate 1O2 under simulated sunlight, with an

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apparent quantum yield of 4.07 ± 0.19%, more than two times higher than many well-studied

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DOM types.24 It is reasonable to expect that DBC plays an important role in the photochemical

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processes of organic contaminants in aquatic environments. Nonetheless, to our knowledge, no

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relevant study has been conducted to understand whether and how DBC would affect the

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photochemical transformation of organic contaminants.

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As an emerging class of contaminants, endocrine disrupting compounds (EDCs) have

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raised increasing environmental concern due to their adverse effects on the endocrine systems

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of both human and animals.25 Natural estrogens are considered to have the most potent

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estrogenic activity, being a major source of EDCs contamination in aquatic systems.26 They

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are

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phototransformation of 17β-estradiol, a common natural estrogen with strong endocrine

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disrupting activity, was found to be markedly accelerated by DHS including dissolved

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Suwannee River humic acid (SRHA) and Aldrich humic acid.28,29,31

reported

to

undergo

photochemical

reactions

mediated

4


by

DHS.7,27-31

The

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In this study, we investigated the phototransformation of 17β-estradiol in the presence of

82

DBC in aqueous solutions under simulated sunlight. Four well-studied DHS obtained from

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International Humic Substance Society (IHSS) were included as comparisons to better

84

evaluate the photoreactivity of DBC. A series of experiments were designed to illustrate the

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reaction mechanisms. The main objectives of this study were (1) to examine the potential role

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of DBC in the photochemical transformation of natural estrogens in aquatic systems, and (2)

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to differentiate the contribution of each reactive intermediate and illustrate the controlling

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reaction mechanisms.

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MATERIALS AND METHODS

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Materials. 17β-Estradiol (≥ 99.5%), tert-butyl alcohol (≥ 99.5%), superoxide dismutase

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(SOD, from bovine erythrocytes), sodium azide (NaN3, > 99.5%), Rose Bengal (dye content ≥

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90%), furfuryl alcohol (FFA, 98%), and 2,4,6-trimethylphenol (TMP, 97%) were purchased

94

from Sigma-Aldrich, USA. Four model DHS, including Nordic Lake natural organic matter

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(Nordic NOM), Suwannee River fulvic acid (SRFA), SRHA, and Leonardite humic acid

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(LHA) were obtained from IHSS, USA. Deionized water (18.2 MΩ·cm resistivity at 25 °C)

97

was produced by an ELGA Labwater system (PURELAB Ultra, ELGA LabWater Global

98

Operations, UK) and was used in all the experiments.

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Preparation and characterization of DBC. DBC was prepared from the water

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extraction of biochar derived from bamboo (collected from Lishui, Zhejiang Province, China)

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as described in our previous studies.23,24 The biochar was produced by pyrolyzing the biomass

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102

in a muffle furnace under oxygen-limited conditions at 400 °C for 2 h. To obtain DBC, 30 g of

103

biochar was mixed with 500 mL of deionized water in a 1000-mL glass beaker and sonicated

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in a bath sonicator (KH-800TDB, Kunshan Hechuang Ultrasonic Instrument, China) at 100 W

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for 30 min. The biochar suspension was then filtered through a 0.45-µm membrane (Pall,

106

USA). The residue retained on the membrane was collected and subjected to another round of

107

sonication and extraction. After three cycles of water extraction, the obtained DBC solution

108

(i.e., the filtrate passing through 0.45-µm membranes) was collected and freeze-dried. The

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resulting DBC powder was stored in a desiccator at room temperature with protection from

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light. The stock solution of DBC (130 mgC/L) was prepared in deionized water and stored in

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the dark at 4 °C prior to use.

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The chemical and structural properties of DBC were characterized in our previous studies

113

using elemental analysis, Ultraviolet-visible (UV-vis) absorption, Fourier transform infrared

114

(FTIR), Raman, and solid-state

115

molecular weight distributions of DBC and DHS were analyzed by a gel permeation

116

chromatography (GPC) column (PL aquagel-OH MIXED-M, 300 × 7.5 mm, bead diameter of

117

8 µm) (Agilent Technologies, USA) using polystyrenesulfonate sodium salts as the standards

118

on a high-performance liquid chromatograph (HPLC) equipped with a diode array detector

119

(DAD) (Agilent 1100, Agilent Technologies). The elution was performed using a solution of

120

100 mM NaCl and 2 mM phosphate buffer (pH 6.8) at a flow rate of 0.5 mL/min at 35 °C, and

121

the detection wavelength was set to be 254 nm.32 The characterization results indicated that

122

DBC had a similar elemental composition with that of the tested DHS, but significantly

13

C nuclear magnetic resonance (NMR) spectra.23,24 The

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different molecular structures. DBC primarily consisted of small aromatic moieties substituted

124

with oxygen-containing functional groups (mainly carboxyls and phenols). The aromaticity,

125

which is defined as the aromatic carbon content determined by solid-state 13C NMR, was 66%

126

for DBC. This value was higher than that of model DHS, ranging from 19% to 58%. The

127

weight-averaged molecular weight (Mw) of DBC was determined to be 3600 Da, lower than

128

the tested DHS except Nordic NOM (Figure S1 in supporting information, SI). Comparison of

129

UV-vis spectra between DBC and DHS provides similar results (Figure S2). The absorbance

130

ratio at 465 nm to 665 nm (E4/E6) of DOM is suggested to be inversely related to its

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aromaticity.33,34 The absorbance ratio at 254 nm to 365 nm (E2/E3) and slope ratio (SR, ratio of

132

spectral slope coefficient in the range of 275~295 nm to that in the range of 350~400 nm) are

133

reported to negatively correlate to the molecular weight of DOM.34 DBC has lower E4/E6, but

134

higher E2/E3 and SR than DHS, indicating its higher aromaticity and lower molecular weight.

135

The contents of carbon functional groups, Mw, and UV-vis indices of DBC and the four DHS

136

used in this study can be found in Table S1 and S2 in SI.

137

Photochemical reaction experiments. All the photochemical reaction experiments were

138

performed in a 40-mL glass vial placed in a water-circulating jacket. The reaction temperature

139

was controlled at 20 ± 0.1 °C using a DC0506 constant-temperature circulating water bath

140

(Shanghai FangRui Instrument, China). The sunlight was simulated by a Xenon lamp (50 W,

141

CEL-HXF300, AULTT, China) and the reaction solution was irradiated from the top. The

142

lamp spectrum was recorded by an Ocean Optics USB2000+ spectrometer (USA) (Figure S3),

143

and it was similar to that of natural sunlight. The photon irradiance of the spectrometer was

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144

calibrated using the p-nitroanisole/pyridine actinometer35,36 (see details in Figure S4). The

145

calibrated irradiation energy at the surface of the reaction solution was 25.3 mW cm-2 in the

146

range of 295−400 nm, 4.5 times equivalent to the sun power (5.80 mW cm-2). Details about

147

the experimental setup can be found in Figure S5.

148

For the 17β-estradiol phototransformation experiments, 30 mL of 0.015 mM

149

17β-estradiol solution were irradiated in the presence of 5 mg C/L DBC or DHS. A 0.5 mL

150

aliquot of the reaction solution was sampled at predetermined time intervals. The

151

concentration of 17β-estradiol in the aliquot was measured using HPLC-DAD (see details in

152

below). The direct phototransformation of 17β-estradiol was tested in the same experimental

153

setting in the absence of DBC and DHS. Dark control was conducted in a solution containing

154

5 mg C/L DBC in the dark. All reaction solutions were buffered with 10 mM phosphate buffer

155

to pH 6.9.

156

The role of different reactive intermediates in 17β-estradiol phototransformation was

157

probed using their respective quenchers, tert-butyl alcohol (2 mM) for •OH, SOD (2 mg/L) for

158

O2•-, NaN3 (8 mM) and tetrahydrofuran (10 mM) for 1O2, and TMP (0.2 mM) for

159

triplet-excited state DBC (3DBC*).10,11,37-40 The role of 1O2 and 3DBC* was also evaluated by

160

examining the photoreaction in nitrogen (20 mL min-1) purged oxygen-deficient solution. The

161

concentration of dissolved oxygen during the reaction was 7.1 ± 0.9 mg/L in unpurged

162

solutions and 0.5 ± 0.1 mg/L in nitrogen-purged solutions as measured by an oxygen

163

microsensor (Microx 4, PreSens, Precision Sensing GmbH, Germany). The steady-state

164

concentration of 1O2 ([1O2]ss) was determined using FFA (0.2 mM) as a probe compound.6,11,36

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The second-order reaction rate constant between 1O2 and 17β-estradiol (k1O2,estradiol) was

166

measured using Rose Bengal (0.1 mM) as the 1O2 photosensitizer and FFA (0.2 mM) as the

167

reference compound.38 The steady-state concentrations of 3DOM* ([3DOM*]ss) for DBC and

168

the model DHS were determined by monitoring the phototransformation of TMP at different

169

initial concentrations (0.05~1.00 mM).41-44 The aforementioned determination methods can be

170

found in SI.

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Analytical methods. The concentrations of 17β-estradiol, FFA, and TMP in solution

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were determined by HPLC-DAD using a 4.6 × 150 mm Zorbax Eclipse SB-C18 column

173

(Agilent Technologies). Isocratic elution was performed under the following conditions: 45%

174

acetonitrile: 55% water (v:v) with a detection wavelength of 230 nm for 17β-estradiol; 10%

175

acetonitrile: 90% water (v:v) with a detection wavelength of 220 nm for FFA; 60% acetonitrile:

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40% 0.1 wt.% phosphoric acid (v:v) with a detection wavelength of 220 nm for TMP.

177

The intermediates and products of 17β-estradiol phototransformation in DBC solution

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were analyzed using HPLC coupled to a quadrupole time-of-flight mass spectrometer

179

(Q-TOF-MS) equipped with an electron spray ionization (ESI) probe (Triple TOF 5600, AB

180

SCIEX, USA). The mobile phase consisted of 0.3% formic acid in water (A) and methanol

181

(B), and the flow rate was set at 0.2 mL min-1. The linear gradient was programmed as

182

follows: initially at 90% A and held for 2 min, decreased to 10% A in 1 min and held for 23

183

min, then increased to 90% A in 1 min and held for 8 min. The ions were collected in negative

184

ionization mode, and the mass (i.e., m/z) scan range was set from 70 to 1500. The source

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185

conditions were as follows: temperature of 550°C, ion spray voltage floating of -4500 V,

186

declustering potential of -80 V, and collision energy of -10 V.

187 188

RESULTS AND DISCUSSION

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DBC-mediated phototransformation of 17β-estradiol. The phototransformation

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kinetics of 17β-estradiol in phosphate buffered neutral solutions (pH 6.9) with and without

191

DBC is compared in Figure 1a. The direct phototransformation of 17β-estradiol was very slow

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under simulated sunlight, consistent with previous studies.29,30 This is because at neutral pH

193

conditions 17β-estradiol (pKa = 10.7145) is dominated by the undissociated neutral form,

194

which has very low absorption of solar light (wavelength ≥ 290 nm).29 The photosensitized

195

transformation of 17β-estradiol was significantly enhanced in the presence of 5 mgC/L DBC,

196

with more than 80% of 17β-estradiol removed after a 12-h irradiation time. A negligible

197

decrease of 17β-estradiol (< 2% loss) was observed on the same time scale (i.e., 12 h) in the

198

presence of DBC under dark conditions, suggesting that non-photochemical processes played

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a minimal role in the transformation of 17β-estradiol.

200

The phototransformation kinetics of 17β-estradiol was fitted to a pseudo-first-order

201

kinetic model (ln(Ct/C0) = -kobst), where Ct and C0 are the 17β-estradiol concentrations at time

202

t and time zero, respectively, and kobs is the observed pseudo-first-order rate constant (fitting

203

results are shown in Figure 1b and Table S3). The pseudo-first-order kinetic model fitted the

204

reaction data well (R2 > 0.922). The half-life (t1/2) of 17β-estradiol was then calculated based

205

on the value of kobs (t1/2 = ln2/kobs,27,29 Table S3). The kobs for direct phototransformation of

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17β-estradiol was only (1.3 ± 0.2) × 10-6 s-1. The kobs was remarkably increased by up to one

207

order of magnitude to (4.1 ± 0.1) × 10-5 s-1 in the presence of 5 mgC/L DBC. The t1/2 of

208

17β-estradiol was significantly reduced from 150 h to 4.8 h after the addition of DBC. Thus,

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DBC can act as an efficient sensitizer to facilitate the phototransformation of 17β-estradiol

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under simulated sunlight.

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Role of ROS in the indirect phototransformation of 17β-estradiol. Previous studies

212

have shown that the generation of ROS (e.g., •OH, 1O2, and O2•-) is an important pathway for

213

the DOM-mediated phototransformation of organic contaminants.5-11,46-51 To explore the role

214

of ROS in DBC-mediated phototransformation of 17β-estradiol, ROS quenching experiments

215

were carried out and the results are summarized in Figure 2. The presence of 2 mM tert-butyl

216

alcohol scarcely affected the phototransformation rate of 17β-estradiol, suggesting an

217

insignificant role of •OH. This is consistent with our previous observation that DBC produced

218

little •OH under simulated sunlight conditions.24 Interestingly, the phototransformation of

219

17β-estradiol was markedly enhanced by the presence of 2 mg/L SOD (Figure 2), a quencher

220

of O2•-.11,24,29,43 The inhibition effect of O2•- on 17β-estradiol phototransformation is likely due

221

to the quick reduction of the initial photooxidation product of 17β-estradiol (i.e., phenoxyl

222

radical) by O2•-, leading to the regeneration of 17β-estradiol.11,52 Similar results were reported

223

in a previous study wherein the phototransformation rate of N-acetyl-p-aminophenol in

224

solutions containing Suwannee River natural organic matter was found to increase with

225

increasing SOD concentration.11

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226

The role of 1O2 in 17β-estradiol phototransformation was examined in the presence of its

227

quenchers, NaN3 (8 mM) and tetrahydrofuran (10 mM).7,24,40 NaN3 may also react with •OH.

228

But this would not affect the quenching of NaN3 towards 1O2, because little •OH was

229

produced by DBC under simulated sunlight and •OH played minimal role in the

230

phototransformation of 17β-estradiol (see more above). The decay rate of 17β-estradiol was

231

reduced by approximately 17% by NaN3 and 10% by tetrahydrofuran (as reflected by the kobs,

232

Table S3 in SI), indicating that 1O2 was only slightly involved in the phototransformation

233

process. The maximum quenching effect of NaN3 on the formation of 1O2 was assessed by

234

gradually increasing its spiked concentration. The kobs for 17β-estradiol phototransformation

235

decreased from (4.1 ± 0.1) × 10-5 s-1 to (3.4 ± 0.1) × 10-5 s-1 as the concentration of NaN3

236

increased from 0 to 8 mM, and leveled off when it further increased to 10 mM (Figure S6).

237

Therefore, the maximum quenching effect of NaN3 on reaction was approximately 17%. To

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further quantify the contribution of 1O2 in the photoreaction, the [1O2]ss in DBC solution (5

239

mgC/L) and the k1O2,estradiol were determined. [1O2]ss was measured as 5.2 × 10-13 M,

240

consistently higher than that for the four model DHS (in the range of 1.3 ~ 4.2 × 10-13 M, see

241

details in Text S1 and Figure S7 in SI) determined at the same carbon concentration. This

242

agrees well with the previous findings that DBC was more active in generating 1O2 as

243

compared with DHS.24,36 The k1O2,estradiol was determined to be (1.4 ± 0.2)× 107 M-1 s-1 (see

244

details in SI). The pseudo-first-order rate constant for the reaction between 1O2 and

245

17β-estradiol (i.e., kobs,1O2) was then calculated to be (7.3 ± 1.0) × 10-6 s-1. Therefore, 1O2

246

accounted for approximately (18 ± 3)% of the DBC-mediated phototransformation of

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17β-estradiol, which is consistent with the maximum quenching effect of NaN3 on reaction

248

(i.e., 17%). The results also indicated that NaN3 mainly reacted with 1O2 in the examined

249

photoreaction system.

250

Role of 3DBC* in the indirect phototransformation of 17β-estradiol. In addition to the

251

production of ROS, DOM can also mediate the phototransformation of organic contaminants

252

by direct reactions with 3DOM* and the contaminants themselves.5-11,46-51 Such reactions

253

proceed via energy transfer and/or electron transfer pathways.9,11,48,50 The energy transfer

254

between 3DOM* and 17β-estradiol may not occur because the estimated energy of 3DOM*

255

(170~250 kJ mol-1)53,54 is lower than the triplet energy of 17β-estradiol (~270 kJ mol-1).55

256

Consistently, the one-electron reduction potentials of 3DOM* were estimated to be 1.6~1.8 V

257

(vs. standard hydrogen electrode/SHE, pH 8),56 presumably higher than that reported for

258

17β-estradiol (1.50 V, vs. SHE, pH 6).57 Thus, 3DOM* may serve as acceptors of electrons

259

transferred from 17β-estradiol, leading to enhanced phototransformation. To test this

260

possibility, we examined the DBC-mediated photoreaction in the presence of TMP, a

261

well-known quencher of

262

17β-estradiol phototransformation was significantly suppressed in the presence of 0.2 mM

263

TMP. The kobs decreased by 61%, confirming the important role of 3DBC* in the photoreaction.

264

To further probe the contribution of 3DBC*, nitrogen purging experiments were performed.

265

The removal of oxygen, a triplet quencher, would increase the concentration of 3DBC* but

266

would reduce that of 1O2.11,37,38 The kobs for 17β-estradiol phototransformation increased from

3

DOM*.5,10,17,37,39 As shown in Figure 2, the DBC-mediated

13



267

(4.1 ± 0.1) × 10-5 s-1 to (5.4 ± 0.3) × 10-5 s-1 in DBC solution purged by nitrogen, indicating a

268

larger contribution of 3DBC* than 1O2.

269

The contribution of 3DBC* in the photoreaction was further evaluated quantitatively. As

270

the second-order reaction rate constant for the reaction of 3DOM* with 17β-estradiol is

271

unknown, it is difficult to directly assess the contribution of 3DBC* to the reaction. Thus, we

272

estimated the contribution of 3DBC* indirectly by examining the quenching effect.58 Herein

273

TMP was used to quench 3DBC*.5,10,17,37,39 Because the kobs of TMP with 1O2 was nearly an

274

order of magnitude lower than that with 3DBC* (see the results in Text S2), TMP was not

275

expected to significantly inhibit the photoreaction mediated by 1O2. The kobs for 17β-estradiol

276

phototransformation in DBC solutions pronouncedly decreased from (4.1 ± 0.1) × 10-5 s-1 to

277

(3.5 ± 0.2) × 10-6 s-1 as the TMP concentration increased from 0 to 0.5 mM, and plateaued as

278

the TMP concentration further increased (Figure S8). Based on the maximum decrease of kobs

279

in the presence of TMP, the contribution of 3DBC* was estimated to be as high as (91 ± 6)%,

280

confirming the predominant role of 3DBC* in the photoreaction. Summing up the contribution

281

of 3DBC* and 1O2 yields a total contribution of (109 ± 9)%, indicating the minimal role of

282

direct phototransformation and other reactive intermediates (e.g., •OH and O2•-) in

283

17β-estradiol phototransformation.

284

Pathways of the indirect phototransformation of 17β-estradiol. The products formed

285

during the phototransformation of 17β-estradiol in DBC solution were analyzed using

286

HPLC/Q-TOF-MS (see details in Text S3 and Figure S9~S11 of SI). The pathways of

287

17β-estradiol phototransformation in DBC solution were then proposed based on the identified

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products and the reactive intermediates involved in the photoreaction. As shown in Figure 3,

289

phenoxyl radicals were firstly formed by the electron transfer from 17β-estradiol to 3DBC*.

290

Then, the phenoxyl radicals can be self-coupled at different positions (i.e., C-C or C-O-C

291

bonds) to form 17β-estradiol dimers (products P4, m/z = 542.37), trimers (products P5, m/z =

292

812.37), and oligomers.59,60 These radicals can also react with DBC molecules to form

293

cross-coupling products. Consistently, products formed by phenoxyl radicals coupled with

294

hydroxybenzoic acids (products P3, m/z = 408.18) that were likely released from DBC

295

molecules were identified in the reaction system. The cross-coupling between 17β-estradiol

296

and DOM molecules during oxidation reactions has also been reported in earlier studies.59 In

297

addition to the coupling reactions, 17β-estradiol also underwent oxygen insertion by 1O2 to

298

form quinones and ethers (products P1 and P2, m/z = 286.15 and 302.15).59,61,62

299

Comparison of mediation efficiency between DBC and DHS. The mediation effect on

300

17β-estradiol phototransformation was also tested in the presence of four well-studied DHS,

301

Nordic NOM, SRFA, SRHA, and LHA (Figure 4a). All tested DHS were able to mediate the

302

transformation of 17β-estradiol under simulated sunlight, but with mediation efficiencies

303

markedly lower than that of DBC. At the same DOM concentration (e.g., 5 mg/C), the kobs of

304

17β-estradiol phototransformation was 2.1~4.7 times higher for DBC than for DHS. The

305

stronger mediation efficiency of DBC can be better illustrated by comparing the apparent

306

quantum yield of 17β-estradiol phototransformation (Φestradiol), which was determined by

307

normalizing the 17β-estradiol phototransformation rate by the photon flux absorbed by the

308

DOM (see the calculation details in Text S4 of SI)12,48 The Φestradiol for DBC was determined

15



309

to be 3.4 × 10-4, significantly higher than that for the four tested DHS (in the range of 5.4 ~ 6.2

310

× 10-5, Table S3). These results indicate that DBC is a much more effective component in

311

DOM pool to mediate the phototransformation of 17β-estradiol.

312

As discussed above, the enhanced mediation efficiency of DBC could be attributed to its

313

strong ability to generate triplet-excited states. To verify this point of view, the [3DOM*]ss for

314

DBC and DHS (5 mg/C) was compared (see details in Figure S12, S13 and Text S2, and

315

results in Table S4 of SI). The [3DOM*]ss of the tested DHS was in the range of 7.1 × 10-14 ~

316

1.2 × 10-13 M. The [3DOM*]ss for DBC was determined to be 2.3 × 10-13 M, higher than that of

317

DHS. It is noteworthy that the [3DOM*]ss both for DBC and DHS measured in this study fall

318

in the typical concentration range of 3DOM* in sunlight-irradiated waters (e.g., 10-15 ~ 10-13

319

M).53 The apparent quantum yield of 3DOM* (Φ3DOM*) of DBC was calculated to be 7.1 × 10-2,

320

also much higher than that of the four tested DHS (0.9 ~ 1.7 × 10-2, Table S4) as well as those

321

values previously reported for other DHS (in the range of 0.4 ~ 1.6 × 10-2).53,63 A strong

322

positive linear correlation exists between the kobs for 17β-estradiol phototransformation and the

323

[3DOM*]ss for the different tested DOM (R2 = 0.975, p = 0.001, Figure 4b). These results

324

confirm that the high efficiency of DBC in producing triplet-excited states under simulated

325

sunlight is responsible for the enhanced 17β-estradiol phototransformation.

326

The strong ability of DBC to produce triplet-excited states can be attributed to its unique

327

molecular structures. As shown by the 13C NMR results (Table S1 of SI), DBC had a similar

328

content of oxygen-containing groups as compared to the tested DHS; however, the aromaticity

329

of DBC (66%) was markedly higher than that of DHS (in the range of 19~58%). This suggests

16


Page 16 of 35

Page 17 of 35


330

that

DBC

had

more

oxygen-substituted

aromatic

moieties

(e.g.,

aromatic

331

ketones/aldehydes/quinones), which are the main moieties responsible for triplet-excited states

332

generation.18 Accordingly, a positive correlation was observed between log[3DOM*]ss and the

333

aromaticity of the tested DOM (R2 = 0.814, p = 0.04, Figure 5). The results of molecular

334

weight distribution and UV-vis spectra collectively suggested that DBC had smaller molecular

335

weight than DHS. Several previous studies reported the negative correlation between

336

molecular weight of DOM and its ability to produce triplet-excited states and the associated

337

1

338

singlet-excited state of DOM (1DOM*) decays by two competitive processes, i.e., electron

339

transfer between electron donor/acceptor moieties to form charge-separated species (DOM•+/•–)

340

and energy transfer by intersystem crossing to form 3DOM*. DOM with smaller molecular size

341

is expected to have smaller amounts of electron donor/acceptor moieties and hence less

342

electron donor-acceptor interactions, leading to less DOM•+/•– formation and consequently

343

higher production of 3DOM*. Thus, in addition to higher aromaticity, the smaller molecular

344

size of DBC may also contribute to its strong ability to generate triplet-excited states.

345

Consistently, the [3DOM*]ss was found to be inversely related with the Mw of DOM, except for

346

Nordic NOM that has the lowest aromaticity (see Figure S14a in SI). Moreover, there exists a

347

strong positive correlation (R2 = 0.920, p = 0.01, Figure S14b) between [3DOM*]ss and the SR

348

of DOM, a well-accepted indicator that negatively correlates with DOM molecular size.34 The

349

smaller molecular size of DBC also helps explain the higher photoactivity in generating

350

triplet-excited states compared with LHA, which has a similar aromaticity with DBC.

O2,3,14,36,54,64 which can be explained by the charge-transfer (CT) model.3,64 In this model, the

17



351

Environmental Implications. The present study showed that DBC released from black

352

carbon can effectively mediate the phototransformation of 17β-estradiol under sunlight

353

irradiation. The half-life of 17β-estradiol in solution was significantly reduced from 150 h to

354

4.8 h by the presence of 5 mg C/L DBC. Thus, estrogens are expected to undergo faster

355

phototransformation in aquatic systems receiving high flux of DBC. The apparent quantum

356

yield for DBC-mediated phototransformation of 17β-estradiol was found to be approximately

357

six times higher than that mediated by many well-studied DOM (e.g., DHS). It is worth noting

358

that the contribution of DBC to the overall phototransformation of 17β-estradiol may still not

359

be comparable with that by DHS considering the far exceeding content of the latter in most

360

aquatic environments. For instance, DBC may account for approximately 40% of the

361

phototransformation of estrogens mediated by DOM in freshwaters by assuming it comprises

362

10% of the riverine DOM pool.21 Nevertheless, the role of DBC could be more significant for

363

those environments receiving frequent or local-scale transient high inputs of DBC, such as

364

estuarine regions and waters influenced by agricultural biomass burning or vegetation fires.65

365

Owing to the high content of oxygen-substituted aromatic structures and small molecular

366

weights, DBC has a strong ability to generate triplet-excited states. The triplet-excited states of

367

DOM are proposed to be important reactive intermediates involved not only in

368

photodegradation of many types of organic contaminants (e.g., EDCs, pesticides,

369

pharmaceuticals, and personal care products/PPCPs),2,12-14,28-30 but also in photoinduced

370

speciation reactions of trace metals.66,67 Additionally, both 3DOM* and 3DOM*-sensitized 1O2

371

have been shown to be important to the inactivation of pathogens.68,69 Thus, DBC may play a

18


Page 18 of 35

Page 19 of 35


372

significant and previously unrecognized role in these important processes due to its ubiquity

373

and relatively high abundance in natural aquatic environments. This study also highlights that

374

aromaticity and molecular weight are two key descriptors for estimating the ability of DOM to

375

produce triplet-excited states and the associated photoreactivity. Further study is warranted to

376

investigate the photochemistry of DBC from different sources in larger and more complex

377

sample pools of natural DOM.

378 379

ASSOCIATED CONTENT

380

Supporting Information

381

Table S1 and Figure S1, distribution of carbon functional groups and molecular weights of

382

DBC and DHS. Table S2 and Figure S2, UV-vis absorption spectra and indices of DBC and

383

DHS. Figure S3, spectra of the Xenon lamp and sunlight. Figure S4, photon irradiance

384

calibration using actinometer. Figure S5, experimental setup for the photoreaction experiments.

385

Text S1 and Figure S7, determination of [1O2]ss. Text S2, Table S4, Figure S12 and Figure S13,

386

determination of [3DOM*]ss. Text S3 and Figure S9~S11, identification of phototransformation

387

products of 17β-estradiol. Text S4, determination of the apparent quantum yield. Table S3,

388

fitting parameters for 17β-estradiol phototransformation by pseudo-first-order model. Figure

389

S6 and S8, 17β-estradiol phototransformation in DBC solution in the presence of NaN3 and

390

TMP at different initial concentration. Figure S14, [3DOM*]ss as a function of Mw and SR of

391

DOM. This material is available free of charge via the Internet at http://pubs.acs.org.

392

AUTHOR INFORMATION

19



393

Corresponding Author

394

*Phone: +86-025-8968-0373; e-mail: [email protected]

395

ACKNOWLEDGMENTS

396

This work was supported by the National Key Basic Research Program of China (Grant

397

2014CB441103), Natural Science Foundation of Jiangsu (BK20150568), National Natural

398

Science Foundation of China (Grants 21507056 and 21622703), and the Fundamental

399

Research Funds for the Central Universities (Grant 021114380047).

400 401

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29



Page 30 of 35

1.2 (a) 1.0

Ct/C0

.8 .6 .4 with DBC in dark without DBC in light with DBC in light

.2 0.0 0

2

4

6

8

10

12

14

Time (h) .5 (b)

ln (Ct/C0)

0.0 -.5 -1.0 with DBC in dark without DBC in light with DBC in light

-1.5 -2.0 0

593

2

4

6

8

10

12

14

Time (h)

594

Figure 1. (a) Phototransformation of 17β-estradiol in aqueous solutions under simulated

595

sunlight in the absence and presence of DBC. (b) Pseudo-first-order kinetics plotted as

596

ln(Ct/C0) vs time for 17β-estradiol phototransformation; Ct and C0 are the concentrations of

597

17β-estradiol at time t and zero, respectively. Reaction conditions: 0.015 mM 17β-estradiol

598

and 5 mgC/L DBC in 10 mM phosphate buffer (pH 6.9) at 20 °C. Error bars represent ± one

599

standard deviation from the mean of triplicate samples.

600 30


Page 31 of 35


DBC DBC + tert-butyl alcohol DBC + SOD DBC + NaN3

0.0

ln (Ct/C0)

-1.0

DBC + tetrahydrofuran DBC + TMP DBC + N2

-2.0 -3.0 -4.0 0

601

2

4

6

8

10

12

14

Time (h)

602

Figure 2. Phototransformation of 17β-estradiol in aqueous solutions containing DBC under

603

simulated sunlight in the presence of tert-butyl alcohol (2 mM), superoxide dismutase (SOD, 2

604

mg/L), sodium azide (NaN3, 8 mM), tetrahydrofuran (10 mM), 2,4,6-trimethylphenol (TMP,

605

0.2 mM), or nitrogen (N2, 20 mL min-1). Reaction conditions: 0.015 mM 17β-estradiol and 5

606

mgC/L DBC in 10 mM phosphate buffer (pH 6.9) at 20 °C. Error bars represent ± one

607

standard deviation from the mean of triplicate samples.

608

31



HO

OH

17β-estradiol - 3DBC* m/z: 272.17 -e

+1O2

OH

OH O

Page 32 of 35

OH

OH O

P1, m/z: 286.15

oxygen insertion •O

O

O

self-coupling OH OH

C

P4, m/z: 542.37

P3, m/z: 408.18

P2, m/z: 302.15

OH

HO OH OH

O

O

Further oxidation to organic acids

C

O

O

O

O

cross-coupling OH

C

O

OH C

OH OH OH OH

777 OH

self-coupling

P5, m/z: 812.37

OH

OH OH

OH

HO

HO OH

OH OH

OH

O

OH

777 609 610

OH

777

Further coupling to oligomers

Figure 3. Proposed pathways of DBC-mediated phototransformation of 17β-estradiol.

611

32


Page 33 of 35


.5 (a)

ln (Ct/C0)

0.0 -.5 -1.0

without DOM with Nordic NOM with SRFA with SRHA with LHA with DBC

-1.5 -2.0 0

2

4

6

8

10

12

14

Time (h)

5.0 (b) DBC

kobs (x10-5 s-1)

4.0 3.0

R2= 0.975 LHA

2.0

SRFA SRHA

1.0 Nordic NOM

0.0 0.0

.5

1.0 ¡[[

612

1.5

3

2.0 -13

DOM*]ss (x10

2.5

3.0

M)

613

Figure 4. (a) Phototransformation of 17β-estradiol in aqueous solutions under simulated

614

sunlight in the presence of DOM. (b) Pseudo-first-order rate constant (kobs) for 17β-estradiol

615

phototransformation as a function of the steady-state concentration of triplet-excited state

616

([3DOM*]ss). Reaction conditions: 0.015 mM 17β-estradiol and 5 mgC/L DOM in 10 mM

617

phosphate buffer (pH 6.9) at 20 °C. Error bars represent ± one standard deviation from the

618

mean of triplicate samples.

619 33



Page 34 of 35

-12.4

DBC

-12.8 LHA

3

log [ DOM*]ss

-12.6

-13.0

SRFA

-13.2

SRHA

Nordic NOM

-13.4 0

20

40

60

80

Aromaticity (%)

620 621

Figure 5. Steady-state concentration of triplet-excited states ([3DOM*]ss) as a function of

622

DOM aromaticity.

34


Page 35 of 35

623


TOC art

Black carbon in soils

Dissolved black carbon

OH H 3C H .

--

H

.

--

Dissolved humic substances

H

HO

Black carbon in marine sediments

fast

slow

moderate

Phototransformation products

624

35


Dissolved Black Carbon as an Efficient Sensitizer in the

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