Enhanced Phototransformation of Tetracycline at Smectite Clay

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Environmental Processes

Enhanced Phototransformation of Tetracycline at Smectite Clay Surfaces under Simulated Sunlight via a Lewis-Base Catalyzed Alkalization Mechanism Liangpang Xu, Hui Li, William A. Mitch, Shu Tao, and Dongqiang Zhu Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b06068 • Publication Date (Web): 18 Dec 2018 Downloaded from http://pubs.acs.org on December 18, 2018

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Environmental Science & Technology

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Enhanced Phototransformation of Tetracycline at Smectite Clay Surfaces under Simulated

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Sunlight via a Lewis-Base Catalyzed Alkalization Mechanism

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Liangpang Xu,† Hui Li,‡ William A. Mitch,§ Shu Tao,† and Dongqiang Zhu†,*

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†School of Urban and Environmental Sciences, Key Laboratory of the Ministry of Education for Earth

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Surface Processes, Peking University, Beijing 100871, China

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‡Department

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48824, United States

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§Department

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United States

of Plant, Soil and Microbial Sciences, Michigan State University, East Lansing, MI

of Civil and Environmental Engineering, Stanford University, Stanford, CA 94305,

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* Corresponding

author D. Zhu, phone: +86 (010) 62766405; email: [email protected]

1

ACS Paragon Plus Environment


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Abstract

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As an important class of soil minerals and a key constituent of colloidal particles in surface aquifers,

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smectite clays can strongly retain tetracyclines due to their large surface areas and high cation

18

exchange capacities. However, the research on phototransformation of tetracyclines at smectite clay

19

surfaces is rarely studied. Here, the phototransformation kinetics of tetracycline pre-adsorbed on two

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model smectite clays (hectorite and montmorillonite) exchanged with Na+, K+, or Ca2+ suspended in

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aqueous solution under simulated sunlight was compared with that of tetracycline dissolved in water

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using batch experiments. Adsorption on clays accelerated tetracycline phototransformation (half-lives

23

shortened by 1.1−5.3 times), with the most significant effects observed for Na+-exchanged clays.

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Regardless of the presence or absence of clay, the phototransformation of tetracycline was facilitated

25

by increasing pH from 4 to 7. Inhibition or enhancement of photolysis-induced reactive species

26

combined with their measurement using scavenger/probe chemicals indicate that the facilitated

27

production of self-photosensitized singlet oxygen (1O2) was the key factor contributing to the clay-

28

enhanced phototransformation of tetracycline. As evidenced by the red shifts and the increased molar

29

absorptivity in the UV-vis absorption spectra, the complexation of tetracycline with the negatively

30

charged (Lewis base) sites on clay siloxane surfaces led to formation of the alkalized form, which has

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larger light absorption rate and is more readily to be oxidized compared to tetracycline in aqueous

32

solution at equivalent pH. Our findings indicate a previously unrecognized, important

33

phototransformation mechanism of tetracyclines catalyzed by smectite clays.

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Key words: tetracycline, smectite clays, phototransformation, singlet oxygen, triplet-excited state

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INTRODUCTION

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Tetracyclines are an important class of antibiotics that have been widely used as veterinary

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therapeutics and animal growth promoters.1,

39

estimated annual consumption of tetracyclines reached 5, 866 tons in the USA in 2016 and 6, 950

40

tons in China in 2013.3, 4 Owing to the relatively low rates of metabolism and absorption, most of the

41

tetracyclines applied for animal husbandry are excreted unmodified by the treated animals and

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consequently released into the environment. Environmental residues of veterinary antibiotics

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including tetracyclines in the environment have raised serious concerns because of their acute and

44

chronic toxicity and potential to promote microbial antibiotic resistance.5-7

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According to recent government surveys, the

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Soil is the major sink for veterinary antibiotics in agricultural runoff, or leached from sewage

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sludge from municipal wastewater treatment plants that have been applied to fields as fertilizer.

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Tetracyclines are frequently detected in agricultural soils at g/kg levels;8 however, in the soils

48

associated with animal farms, concentrations as high as several thousand g/kg have been reported.9,

49

10

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variety of complex interactions (including sorption, transformation, and biodegradation) involved in

51

heterogeneous soil systems. As the major inorganic components in soils and also the key constituents

52

of natural colloidal particles in surface aquifers,11 clay minerals play a critical role in the fate and

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transport of polar and ionizable compounds which can be sorbed by ion exchange and/or surface

54

complexation.12-14 Due to their high cation exchange capacities (CECs), large specific surface areas,

55

and swelling interlayer structures, smectite clays strongly adsorb cationic and zwitterionic

56

tetracycline molecules (which dominate at acidic and slightly basic pH conditions) mainly via a

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cation-exchange mechanism.14-16 Therefore, smectites could be an effective sorbent and transport

58

vehicle of tetracyclines in contaminated soil and aquatic environments near point pollution sources

Prediction of the fate and biological effects of tetracyclines requires a better understanding of a

3



59

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(e.g., livestock and poultry farms).

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Phototransformation is a key process affecting the transformation of many organic contaminants

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in the environment, especially those compounds containing photoreactive groups (e.g., -conjugated

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choromophores).17, 18 The conjugated benzene-enone structures in tetracycline can effectively absorb

63

photons within the ultraviolet-visible (UV-vis) wavelength range (maximum molecular absorption

64

observed at approximately 360 nm19), making the molecule photoreactive and vulnerable to

65

phototransformation under sunlight irradiation. Tetracyclines solubilized in water were reported to

66

have half-lives between a few minutes and days, depending on the photolytic conditions (e.g.,

67

irradiation wavelength and intensity, solution chemistry, and initial concentration).20-22 Besides direct

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phototransformation, organic compounds in aquatic environments may be subject to indirect

69

phototransformation induced by reactive oxygen species (ROS), including singlet oxygen (1O2),

70

superoxide anion (O2•-), and/or hydroxyl radical (•OH), which are generated by photosensitizer

71

species such as dissolved organic matter (DOM), nitrate, and organoiron complexes.18, 23-25 Nitrate

72

and dissolved humic acid were reported to facilitate tetracycline phototransformation in aqueous

73

solutions. 19, 21, 26

74

Several studies have investigated the effects of clay minerals in aqueous dispersions and solid

75

films on the photolysis of organic compounds.27-29 The observed effects on the photolysis rate of

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organic compounds are mainly ascribed to the clay-induced change of light absorption. Nonetheless,

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a mechanistic understanding of how the molecular interaction of organic contaminants with clay

78

surfaces affects the aqueous phototransformation is scare in the literature. Due to the prevalence of

79

Lewis and/or Brønsted acid and base sites, the aluminosilicate surfaces of clays are known to have

80

significant catalytic activity toward the chemical transformation of sorbed organic compounds.30, 31

81

For example, Fe3+-exchanged montmorillonite is capable of catalyzing the formation of 4


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octachlorodibenzodioxin from pentachlorophenol adsorbed on the clay surface via acidic iron-

83

induced radical cation reactions.32 To date only a few studies have been conducted to investigate the

84

photolysis of veterinary antibiotics adsorbed on clay minerals.33-35 Werner et al.33 studied the

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photolysis of chlortetracycline adsorbed on dry powdered kaolinite at ambient moisture level, and

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proposed that photolysis might be an important loss process for chlortetracycline sorbed to soils

87

exposed to sunlight. Compared with kaolinite, which is a 1:1 phyllosilicate and has a negligible CEC,

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the 2:1 type smectites exhibit much stronger adsorption affinities for cationic/zwitterionic tetracylines

89

due to their much higher surface areas and CECs. However, it remains unknown whether smectite

90

clays have catalytic activities for adsorbed tetracycline molecules under sunlight.

91

This study investigated the phototransformation behavior of tetracycline adsorbed on two model

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smectite clays (montmorillonite and hectorite) suspended in aqueous solutions under simulated

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sunlight. A series of experiments were systematically designed to illustrate the photolysis

94

mechanisms using scavenger and probe chemicals. The main objectives of this study were (1) to

95

examine the possible catalytic effects of smectite clays in the phototransformation of tetracycline, (2)

96

to assess the relative importance of each reactive species, and (3) to illustrate the underlying

97

mechanisms for the photocatalytic activities of smectite clays.

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MATERIALS AND METHODS

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Materials. Tetracycline (99%) was purchased from International Laboratory (San Bruno, CA, USA).

100

The molecular structure of tetracycline and the associated stepwise acid dissociation constants (pKa)

101

are summarized in Figure S1, Supporting Information. Furfuryl alcohol (FFA, 98%), 2,3-bis(2-

102

methoxy-4-nitro-5-sulfophenyl)-2H-tetrazolium-5-carboxanilide (XTT, > 90%), terephthalic acid

103

(TPA, 98%), sodium azide (NaN3) (> 99.5%), superoxide dismutase (SOD, 90%, from bovine

104

erythrocytes), isopropyl alcohol (IPA, > 99%), and 2,4,6-trimethylphenol (TMP, 97%) were 5



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105

purchased from Sigma-Aldrich, USA. Analytical grade NaCl, KCl, and CaCl2 were purchased from

106

Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Deuteroxide (D2O, 99.8 atom % D) was

107

purchased from Tokyo Chemical Industry, Japan. Deionized water (18.2 MΩ·cm resistivity at 25 °C)

108

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

109

UK) was used in all the experiments.

110

A montmorillonite (CEC = 110 cmol/kg) (Fenghong Inc., Zhejiang Province, China) and a

111

hectorite (CEC = 73.4 cmol/kg) (Elementis, UK) were used to prepare sorbent materials. The original

112

clay was mixed with a 0.5 M NaCl aqueous solution (20 g in 1 L) for 24 h, and the < 2 μm size

113

fraction was collected by repeated centrifugation and resuspension in 0.1 M NaCl. The collected Na+-

114

clay suspension was dialyzed against dialysis membrane tubes (3500 Daltons) (Union Carbide) until

115

no chloride was detected using AgNO3. After centrifugation, the Na+-clay was treated by

116

washing/centrifuging four times with a solution of 0.1 M KCl or CaCl2 solution, followed by repeated

117

washing with deionized water until no chloride was detected using AgNO3. The clay suspensions

118

were freeze-dried, and the obtained homoionic-exchanged montmorillonites/hectorites were referred

119

to as Na-, K-, and Ca-Mont/Hect. The elemental compositions of Na-Hect and Na-Mont were

120

characterized using an X-ray fluorescence spectrometer (ARL-9800, ARL) (see results in Table S1).

121

Photolysis experiments. The experiments were conducted using a photolysis installation similar

122

to that described in our previous study.36 A 200-mL solution containing 0.3 g homoionic-clay, 0.015

123

mM tetracycline, and 5 mM phosphate buffer was adjusted to pH 4, 5, 6, or 7. The suspension was

124

magnetically stirred at 200 rpm in a 200-mL cylindrical cell equipped with a water jacket. The

125

temperature of the suspension was maintained at 20 ± 0.1 °C by a recirculating water temperature

126

control system (DC0506, Shanghai FangRui Instrument Co., Ltd., China). After reaching sorption 6


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127

equilibrium in dark at 72 hr, the suspension was sampled to determine the sorbed amount of

128

tetracycline. In all photolysis experiments containing clay, the ratio of clay mass to solution volume

129

was carefully selected based on the predetermined sorption isotherm data collected under the same

130

conditions (results presented in Figure S2) to ensure that > 90% of the added tetracycline was sorbed

131

by the clay. The UV-vis absorption spectra of tetracycline in aqueous solutions and in clay

132

suspensions prior to irradiation were recorded using a Shimadzu UV-2600 spectrometer (Kyoto,

133

Japan). The blank spectra of neat clay suspension were also measured and found to be identical before

134

and after 2.5-hr irradiation (Figure S3). The clay exhibited no characteristic absorption peaks. The

135

spectra of tetracycline in the presence of clay were obtained by subtracting the blank spectra of neat

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clay suspension. During the photolysis experiment, the suspension was irradiated by a 50 W xenon

137

lamp (CEL-HXF300, AULTT, China) at a distance of 0.2 m from the top of the solution. The lamp

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spectrum was recorded using a spectrometer USB2000+ (Ocean Optics, FL, USA). The irradiation

139

energy at the water surface was 27.5 mW/cm2 with the wavelength range between 290−800 nm, which

140

was similar to that of natural sunlight. Two 0.5-mL aliquots of clay suspension were collected at

141

designated time intervals during the experimental periods. The suspension was mixed with 2 mL of

142

10 mM EDTA (pH ~11.5), and shaken for 24 hr to achieve complete extraction.33 After centrifugation

143

at 3295 g for 20 min, the supernatant was analyzed directly by high-performance liquid

144

chromatography (HPLC) equipped with a UV detector and a 4.6mm×150mm SB-C18 column

145

(Agilent). Isocratic mobile mixture contained 80% 10 mM oxalic acid:16% acetonitrile:4%methanol

146

(v:v:v), and the detector wavelength was set at 360 nm. An HPLC-mass spectrometer (HPLC-MS)

147

(Sciex API 4000, Applied Biosystem, Singapore) assay was used to determine tetracycline

148

phototransformation products formed in deionized water or in clay suspension (see more details in

149

Texts S1−S2). 7



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150

To further evaluate the role of self-sensitized reactive species in the phototransformation of

151

tetracycline, ROS scavengers including NaN3, SOD, and IPA and TMP (a triplet-state quencher) were

152

used for the inhibition experiments.36-39 The phototransformation reactions were also compared at

153

two different levels of dissolved oxygen obtained with and without N2 purging prior to the irradiation.

154

Without N2 purging the dissolved oxygen concentration was ~ 8.5 mg/L as measured by an oxygen

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microsensor (Microx 4 PreSens, Precision Sensing GmbH, Germany). After purging for 1 hr prior to

156

irradiation, the concentration of residual dissolved oxygen was reduced to 0.3−0.62 mg/L and

157

gradually increased to 2.54−3.4 mg/L after 2.5 hr at the end of the experiments. The

158

phototransformation kinetics of tetracycline was also measured in mixtures of D2O and H2O (90:10,

159

v:v). Compared with H2O, D2O significantly increases the lifetime of 1O2 due to the kinetic solvent

160

isotope effect.40 Comparison experiments were done for samples receiving the same treatment but

161

without clay. Control experiments were also performed in the dark.

162

Measurement of photolysis active species. Production of the ROS species, 1O2, O2•-, and •OH,

163

during the phototransformation of tetracycline in aqueous solutions or aqueous clay suspensions was

164

quantified using probe molecules FFA, XTT, and TPA, respectively. Aqueous clay suspensions

165

containing ROS probe molecules but without tetracycline were also irradiated under the same

166

conditions as blank controls. The formation of 1O2 was quantified by monitoring the loss of FFA as a

167

function of reaction time.41, 42 FFA was added to an aqueous solution or aqueous clay suspension at

168

an initial concentration of 0.08 mM. During the irradiation, the remaining FFA in the solution or clay

169

suspension was measured using an HPLC equipped with a Zorbax Eclipse SB-C18 column (Agilent)

170

and a UV-Vis detector at a wavelength of 220 nm. The mobile phase consisted of 30% acetonitrile:70%

171

0.1 wt% phosphoric acid (v:v). The formed 1O2 in the presence of TMP, a quencher of triplet-excited

172

tetracycline (3TC*), was also measured for selected samples. 8


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173

The formation of XTT formazan from XTT (initially at 0.05 mM) was used to quantify O2•-.43, 44

174

XTT formazan was measured using a UV-vis spectrophotometer (UV-2600 Shimadzu Co., Japan) at

175

475 nm. The extinction coefficient of XTT formazan was 23800 M-1 cm-1.44 The formation of

176

hydroxyterephthalic acid (HTPA) from TPA was used to quantify the production of •OH.45 The TPA

177

stock solution was prepared in 2 mM NaOH and filtered through a 0.45-µm membrane. The initial

178

concentration of TPA in the aqueous solution or clay suspension was 0.5 mM. HTPA was quantified

179

using fluorescence spectroscopy (F-7000, Hitachi, Japan) with excitation and emission wavelengths

180

of 315 and 425 nm, respectively. The production of 3TC* during irradiation with and without the

181

presence of clay was quantified by measuring the loss of the probe molecule TMP (initially at

182

0.025−1.00 mM). TMP concentration was measured using an HPLC with a Zorbax Eclipse SB-C18

183

column at a wavelength of 220 nm. The quantum yields of 1O2, 3TC*, and tetracycline

184

phototransformation were determined according to previously reported methods38, 46-48 (see details in

185

Texts S3−S5).

186

RESULTS AND DISCUSSION

187

Enhanced phototransformation of tetracycline at clay surfaces. Compared with the aqueous

188

solution, the presence of homoionic-exchanged hectorites (Na-Hect, K-Hect, and Ca-Hect) or

189

montmorillonites (Na-Mont, K-Mont, and Ca-Mont) significantly enhanced the phototransformation

190

of tetracycline at pH 7 (Figure 1). The phototransformation of tetracycline was best described by the

191

pseudo-first-order kinetic model (R2 > 0.97), and the calculated apparent pseudo-first-order rate

192

constants (kTC) are summarized in Table 1. The estimated half-lives (t1/2) decreased from 0.88 hr in

193

aqueous solution to 0.14−0.35 hr and to 0.26−0.42 hr in the presence of hectorites and

194

montmorillonites, respectively (Table 1). As > 90% of tetracycline was adsorbed on clays (estimated

195

by sorption isotherms, Figure S2), the enhanced tetracycline phototransformation in the presence of 9



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196

clay can only be attributed to the specific catalytic effects of the clay surface. The photocatalytic

197

effects of clay on tetracycline phototransformation are dependent highly on the types of clays and

198

exchangeable cations. When exchanged by the same type of cation, hectorite exhibited stronger

199

catalytic effects than montmorillonite, in agreement with their adsorption affinities for tetracycline.

200

For both hectorites and montmorillonites, Na+-exchanged clay had the strongest photocatalytic effects

201

among the three homoionic-exchanged clays. Additionally, the phototransformation kinetics of

202

tetracycline in Na-Hect suspension slightly decreased from (1.1 ± 0.1) × 10-3 s-1 to (9.0 ± 0.6) × 10-4

203

s-1 when the loading concentration of tetracycline decreased from 0.015 mM to 0.0015 mM

204

(corresponding to surface coverage decrease from 0.42% to 4.2%) (Figure S4). The results imply that

205

the clay-enhanced phototransformation of tetracycline would most likely occur at even lower

206

environmentally relevant concentrations. Reactions with self-sensitized ROS (including 1O2, O2•-, and •OH) are proposed to affect the

207 208

aqueous phototransformation of tetracycline under simulated sunlight.26,

49

209

phototransformation of tetracycline suppressed by their respective ROS scavengers (e.g., NaN3 for

210

1O

211

of Na-Hect (Figure 2). The respective kTC and t1/2 values are also summarized in Table 1. In both

212

cases, all ROS scavengers suppressed tetracycline phototransformation (kTC decreased by 32.9−60.7%

213

for Na-Hect and by 13.6−64.1% for aqueous solution), suggesting the importance of ROS. Partial

214

removal of oxygen by N2 purging suppressed the phototransformation of tetracycline moderately in

215

the presence of Na-Hect (kTC decreased by 55.7%), whereas the suppression effect was slight for

216

aqueous solution (kTC decreased by 9.1%). Among the three tested ROS scavengers, NaN3 exhibited

217

the strongest suppression effect on tetracycline phototransformation, suggesting a more important

218

role played by 1O2 than by O2•- and •OH. For both aqueous solution and clay suspension, the

Here, the

•-, and IPA for •OH) in the presence of Na-Hect was compared with that in the absence

2, SOD for O2

10


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219

suppression effect induced by TMP, a triplet-state quencher, was close to or slightly less than that by

220

NaN3, but more than that by SOD and IPA. These results imply that 3TC* is also a key reactive species

221

involved in tetracycline phototransformation. It is noteworthy that in all these suppression

222

experiments the initial concentrations of the scavengers were high enough to completely quench the

223

respective reactive intermediates (see more details in Figure S5).

224

Measurement of reactive photolysis species. Figure 3 presents the formation of 1O2 as reflected

225

by the loss of FFA during tetracycline phototransformation with and without the presence of

226

homoionic-exchanged clays at pH 7. The effect of different clays on the production kinetics of 1O2

227

(measured as the pseudo-first-order rate constant of FFA, kobs,

228

correlates very well with their observed effects on the phototransformation kinetics of tetracycline

229

(see the kTC values in Table 1), which was ranked as Na-Hect > K-Hect > Ca-Hect > No clay and Na-

230

Mont > Ca-Mont > K-Mont > No clay. These results strongly support that the clay-facilitated

231

production of 1O2 plays a key role in the photocatalytic reaction of tetracycline. Similar to FFA, the

232

loss of TMP, a quencher of 3TC*, was much more pronounced in Na-Hect suspension than in aqueous

233

solution (Figure 4). The self-sensitized 3TC* could affect tetracycline phototransformation via two

234

different mechanisms: as an oxidant of other ground-state tetracycline molecules, and as a reactive

235

intermediate to induce the formation of 1O2. Structural transformation by triplet-state chromophores

236

is a key pathway for the photolysis of humic substances and many synthetic organic chemicals,18, 50-

237

52

238

Therefore, the suppressed phototransformation of tetracycline by TMP (Figure 2) confirmed the

239

importance of 1O2. As dissolved oxygen is a very efficient triplet-state quencher,38, 53 the observed

240

decrease in photolysis rate by deoxygenation (see Figure 2) suggested that 3TC* acted more as a

241

reactive inducer of 1O2 than as a reactant in direct tetracycline oxidation

FFA,

data presented in Figure 3)

while triplet-state DOM (3DOM*) in natural systems is considered as a major producer of 1O2.18, 50

11



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242

The production of •OH as monitored by the formation of HTPA was compared between aqueous

243

solution and Na-Hect suspension with and without the presence of tetracycline (Figure S6). Like 1O2

244

and 3TC*, the production of •OH was more effective in Na-Hect suspension than in aqueous solution.

245

Notably, Na-Hect suspension itself could produce •OH under simulated sunlight, consistent with the

246

findings in previous studies.54 Adding IPA, a scavenger of •OH, significantly suppressed the

247

formation of HTPA in all cases. The formation of HTPA in Na-Hect suspension containing

248

tetracycline was reduced by IPA to a level lower than that in water containing tetracycline and similar

249

to that in pure water. Combining these observations with the moderate suppressive effects on

250

tetracycline phototransformation by IPA (Figure 2) revealed that •OH was not a key species

251

responsible for the clay-enhanced tetracycline phototransformation. As monitored by the formation

252

of XTT formazan, the production of O2•- was nearly equal between Na-Hect and aqueous solution

253

(Figure S7), ruling out the significance of O2•- in tetracycline phototransformation at smectite clay

254

surfaces.

255

Mechanisms for photocatalytic activity of smectite clays. The phototransformation of

256

tetracycline in aqueous solution is greatly enhanced with increasing pH.20, 21, 26. Lewis base sites are

257

prevalent on aluminosilicate surfaces, including deprotonated Si/Al hydroxide groups at the edge sites

258

and permanent negative charges arising from isomorphic substitution. Therefore, it is hypothesized

259

that the complexation reaction (Lewis-acid-base interaction) between protonated amine group (Lewis

260

acid) (pKa3 = 9.68, see Figure S1) of tetracycline and the negatively charged sites (Lewis bases) on

261

the clay surface would alkalize tetracycline, leading to enhanced phototransformation relative to the

262

aqueous solution at equivalent pH. In line with this hypothesis, the phototransformation of

263

tetracycline was found to be higher in the presence of Na-Hect than in aqueous solution at equivalent

264

pH conditions, and the photolysis rate increased consistently with increasing pH from 4 to 7 (Figure 12


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265

5). Over this pH range, the tetracycline molecules adsorbed on clay surfaces would be dominated by

266

the zwitterionic form.

267

The proposed mechanism is illustrated by comparing the UV-vis absorption spectra of

268

tetracycline on molar absorptivity at various pH with and without the presence of Na-Hect (Figure

269

6). As the pH of aqueous solution increased from 4 to 7, the absorbance peak at 357 nm slightly

270

shifted to 360 nm; however, the peak was pronouncedly shifted to 366 nm and 372 nm, respectively,

271

when the pH further increased to 8 and 9, at which tetracycline is dominated by the alkalized anionic

272

form (justified by the pKa values shown in Figure S1). Under the examined pH range (4−7), the

273

aqueous suspension of Na-Hect showed a significant red shift (~ 15 nm) relative to the aqueous

274

solution at the same pH. For instance, at pH 7 the peak observed at 360 nm in aqueous solution shifted

275

to 375 nm in the presence of Na-Hect, even slightly greater than that at 372 nm for aqueous solution

276

at pH 9. Additionally, the molar absorptivity of tetracycline over the visible wavelength range

277

(390−760 nm) in clay suspension was much larger than that in aqueous solution at the same pH, and

278

the molar absorptivity of tetracycline in clay suspension increased significantly with pH. These

279

observations offer strong evidence that tetracycline becomes alkalized on clay surfaces, and the

280

adsorbed alkalized tetracycline has enhanced light absorption rate. This is likely due to the charge-

281

transfer complexation between the tetracycline molecules and the negatively charged (Lewis base)

282

sites on clay surfaces.

283

Sorption of cationic/zwitterionic tetracycline species (dominated under the examined pH 4 to 7)

284

by smectite clays is mainly controlled by cation exchange reaction via electrostatic interaction with

285

the sites carrying negative charge (Lewis bases) on the clay surface.14, 16 Thus, the photoactivity of

286

the clay surface in tetracycline phototransformation can be mainly attributed to the Lewis base-

287

catalyzed alkalization. Once deprotonated under alkaline conditions, the amino group and the 13



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288

phenolic group in tetracycline molecules become more electron-rich and thus more susceptible to the

289

electrophilic addition reaction of 1O2.55 The phototransformation product analysis using HPLC-MS

290

verified that under the same reaction conditions Na-Hect suspension produced much more

291

tetracycline amine-1O2 reaction product (with a mass peak at 459.356-58) than in aqueous solution (see

292

more details in Figures S8−S11 and Text S2). The Lewis-base catalyzed alkalization mechanism can

293

well explain why Na+-exchanged clays had stronger photocatalytic effects than K+, Ca2+-exchanged

294

clays. Due to the combined effects of monovalent charge and large hydrated ionic radius, Na+

295

adsorbed on the clay surface was much more exchangeable than K+ and Ca2+ when being replaced by

296

cationic/zwitterionic tetracycline molecules.

297

Figure 7 displays the concentration changes of different probe chemicals (FFA, XTT, and TMP)

298

with and without the presence of Na-Hect after 2.5-hr photolysis of tetracycline at various pH. For

299

both clay suspension and aqueous solution, the concentrations of the three probed reactive species

300

(i.e., 1O2 by FFA, O2•- by XTT formazan, and 3TC* by TMP) generally increased as the pH increased

301

from 4 to 7, with the exception of the plateau of TMP from pH 6 to 7. Given the same pH, Na-Hect

302

generally exhibited much more production of 1O2 and 3TC* than aqueous solution; however, the

303

production of O2•- was relatively close between Na-Hect and aqueous solution. These results

304

demonstrate that the enhanced production of 1O2 and 3TC* by tetracycline alkalization contributes

305

significantly to the clay-photocatalized reaction of tetracycline. To better understand the role of 1O2

306

and 3TC*, their steady-state concentrations ([1O2]ss and [3TC*]ss) during the phototransformation

307

process were compared between Na-Hect suspension and aqueous solution at various pH (see details

308

in Texts S3-S4). In both cases, [1O2]ss and [3TC*]ss significantly increased with pH, while [3TC*]ss and

309

[1O2]ss were consistently higher in clay suspension than in aqueous solution at the same pH (Table

310

S2). However, compared with aqueous solution at the same pH, Na-Hect suspension showed close or 14


Page 15 of 33


311

even lower quantum yields () in 1O2 and 3TC* productions, in spite of the much larger  in

312

tetracycline phototransformation (Table S2). This contradiction can be reconciled by the larger light

313

absorption rate and the higher reactivity of alkalized tetracycline on clay surfaces relative to

314

tetracycline in aqueous solution at equivalent pH. Furthermore, the  in tetracycline

315

phototransformation was positively correlated with the loading concentration of tetracycline,

316

increasing from (7.0 ± 0.5) × 10-5 at 0.0015 mM to (2.2 ± 0.2) × 10-4 at 0.015 mM (Figure S4). These

317

results reaffirmed that the photoreactive species (1O2 and 3TC*) were produced through self-

318

sensitization of tetracycline molecules on clay surfaces.

319

The importance of 1O2 in clay-enhanced tetracycline phototransformation was further assessed

320

by switching the solvent from H2O to D2O (results presented in Figure S16). As D2O significantly

321

increases the lifetime of 1O2,40 the photolysis rate of tetracycline would be facilitated by the solvent

322

switch if 1O2 is the key reactive intermediate. After switching the solvent from H2O to D2O, kTC was

323

increased by 94% from (6.2 ± 0.1) × 10-4 s-1 to (1.2 ± 0.1) × 10-3 s-1 for the N2-purged Na-Hect

324

suspension and by 29% from (1.4 ± 0.1) × 10-3 s-1 to (1.8 ± 0.1) × 10-3 s-1 for the non-purged

325

suspension. The larger increase ratio of reaction kinetics observed for the N2-purged system can be

326

accounted for by the much lower availability of dissolved oxygen when compared with the non-

327

purged system. Considering the fact that the positively charged amino group of tetracycline becomes

328

more difficult to dissociate to the alkalized form when the attached proton changed to deuterium (i.e.,

329

the pKa increases from 9.68 to 9.97),59 the actual contribution of

330

phototransformation would be more significant than that reflected by the observed apparent solvent

331

effect.

1O

2

to tetracycline

332

Environmental implications. Smectite clays occur ubiquitously in soils and surface aquifers in

333

large quantities, and serve as an important immobilization or mobile carrying phase for tetracyclines 15



Page 16 of 33

334

due to the extraordinarily strong adsorption affinity and capacity. Here we found that tetracycline

335

adsorbed on montmorillonite and hectorite, two common smectites, can be effectively photocatalyzed

336

by alkalization through a Lewis-acid-base interaction with the negatively charged sites on clay

337

siloxane surfaces, which facilitates the formation of 1O2 and 3TC* and in turn the phototransformation

338

process. These findings imply that smecite clays can be important natural photocatalysts for the

339

phototransformation of tetracyclines and possibly other cationizable amines such as aromatic and/or

340

heterocyclic amines that can be strongly adsorbed via cation exchange and subsequently alkalized by

341

the Lewis base sites on clay surfaces. In particular, the clay-enhanced phototransformation could be

342

an important fate process of tetracyclines in clay-rich waste ponds which are formed after stormwater

343

runoff or in animal husbandry systems or feeding lots.

344

Supporting Information Available

345

Table S1 presents elemental compositions of clays. Figure S1 displays molecular structure and

346

protonation-deprotonation transition of tetracycline. Table S2 presents steady-state concentrations

347

and quantum yields of different photoreactive species at various pH. Figure S2 presents sorption

348

isotherms of tetracycline. Figure S3 presents UV-vis spectra of aqueous suspension of clay. Figure

349

S4 presents tetracycline phototransformation at various initial concentrations. Figure S5 presents

350

tetracycline phototransformation in the presence of different quenchers in varying concentrations.

351

Figures S6−S7 present concentration changes of HTPA and XTT formazan during tetracycline

352

phototransformation. Figures S8−S11 and Texts S1−S2 present analyses of phototransformation

353

products of tetracycline. Figures S12−S15 and Texts S3−S5 present photolysis experiment setup and

354

determination of the apparent quantum yields of different species. Figure S16 compares tetracycline

355

phototransformation in water and deuterated water. This material is available free of charge via the

356

Internet at http://pubs.acs.org. 16


Page 17 of 33


357

ACKNOWLEDGMENTS

358

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

359

2014CB441103) and the National Natural Science Foundation of China (Grants 21428701 and

360

21777002).

17



361

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362

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365 366

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30. Laszlo, P. Chemical reactions on clays. Science 1987, 235, 1473-1477.

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formation in clays: Density functional theory and fourier transform infrared studies. Environ. Sci.

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35. Wu, Q.; Que, Z.; Li, Z.; Chen, S.; Zhang, W.; Yin, K.; Hong, H. Photodegradation of

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36. Fu, H.; Liu, H.; Mao, J.; Chu, W.; Li, Q.; Alvarez, P. J.; Qu, X.; Zhu, D. Photochemistry of

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38. Halladja, S.; ter Halle, A.; Aguer, J.-P.; Boulkamh, A.; Richard, C. Inhibition of humic

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substances mediated photooxygenation of furfuryl alcohol by 2,4,6-trimethylphenol. Evidence 21



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40. Wilkinson, F.; Helman, W. P.; Ross, A. B. Rate Constants for the Decay and Reactions of the

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Lowest Electronically Excited Singlet State of Molecular Oxygen in Solution. An Expanded and

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Revised Compilation. J. Phys. Chem. Ref. Data 1995, 24, 663-677.

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41. Haag, W. R.; Hoigne, J. R.; Gassman, E.; Braun, A. M. Singlet oxygen in surface waters-Part I: Furfuryl alcohol as a trapping agent. Chemosphere 1984, 13, 631-640.

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49. Khan, M. A.; Musarrat, J. Tetracycline-Cu(II) photo-induced fragmentation of serum albumin. Comp. Biochem. Physiol. Part C 2002, 131, 439-446.

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acetaminophen in natural organic matter solutions: The role of triplet states and oxygen. Water

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Res. 2017, 109, 266-273.

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molecular oxygen-mediated photo-oxidation of tetracyclines: kinetics, mechanism and

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microbiological implications. J. Photochem. Photobiol. B: Biol. 1998, 43, 164-171.

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56. Chen, Y.; Li, H.; Wang, Z.; Tao, T.; Hu, C. Photoproducts of tetracycline and oxytetracycline

499

involving self-sensitized oxidation in aqueous solutions: Effects of Ca2+ and Mg2+. J. Environ.

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Sci. 2011, 23, 1634-1639.

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57. Baciocchi, E.; Del, G. T.; Lanzalunga, O.; Lapi, A.; Raponi, D. The singlet oxygen oxidation of

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chlorpromazine and some phenothiazine derivatives. Products and reaction mechanisms. J. Org.

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Chem. 2007, 72, 5912-5915.

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58. Young, R. H.; Martin, R. L.; Feriozi, D.; Brewer, D.; Kayser, R. On the mechanism of quenching

505

of singlet oxygen by amines-III. Evidence for a charge-transfer-like complex. Photochem.

506

Photobiol. 1973, 17, 233-244.

507 508

59. Krȩżel, A.; Bal, W. A formula for correlating pKa values determined in D2O and H2O. J. Inorg. Biochem.2004, 98, 161-166.

24


Page 25 of 33


510

Table 1. Fitting parameters for tetracycline phototransformation under different reaction conditions

511

by pseudo-first-order model with and without the presence of clay.a

512 513 514 515 516 517 518 519 520 521 522 523 524 525 526 527 528 529 530 531 532 533 534 535 536 537 538

Reaction condition

kTC (s-1)b

R2

t1/2 (h)c

No clay

(2.2 ± 0.1) × 10-4

0.994

0.88

No clay+NaN3

(1.0 ± 0.1) × 10-4

0.994

1.93

No clay+SOD

(1.9 ± 0.2) × 10-4

0.993

1.01

No clay+IPA

(1.7 ± 0.2) × 10-4

0.986

1.13

No clay+TMP

(7.9 ± 1.2) × 10-5

0.996

2.44

No clay+ N2

(2.0 ± 0.2) × 10-4

0.989

0.96

Na-Hect

(1.4 ± 0.1) × 10-3

0.985

0.14

Na-Hect+NaN3

(5.5 ± 0.2) × 10-4

0.995

0.35

Na-Hect+SOD

(9.4 ± 0.3) × 10-4

0.991

0.20

Na-Hect+IPA

(9.4 ± 0.4) × 10-4

0.992

0.20

Na-Hect+TMP

(7.4 ± 0.2) × 10-4

0.994

0.26

Na-Hect+N2

(6.2 ± 0.1) × 10-4

0.998

0.31

K-Hect

(7.5 ± 0.5) × 10-4

0.978

0.26

Ca-Hect

(5.6 ± 0.5) × 10-4

0.979

0.35

Na-Mont

(7.4 ± 0.2) × 10-4

0.996

0.26

K-Mont

(4.6 ± 0.2) × 10-4

0.992

0.42

Ca-Mont

(5.6 ± 0.2) × 10-4

0.993

0.35

aReaction

539

conditions: 0.015 mM tetracycline, 1.5 g/L clay (if present), and 5 mM phosphate buffer (pH 7) at 20 °C. bApparent pseudo-first-order rate constants determined as the slope of linear regression with a 95%

540

confidence interval.

541

cHalf-lives

calculated by the equation: t1/2 = ln 2/kTC.

542

25



1.0

1.0 Dark control (No clay) Dark control (Na-Hect) No clay Na-Hect K-Hect Ca-Hect

.6

.6

.4

.4

.2

.2

(a) 0.0

0.0

.5

1.0

1.5

2.0

Dark control (No clay) Dark control (Na-Mont) No clay Na-Mont K-Mont Ca-Mont

.8

Ct/C0

Ct/C0

.8

0.0

Page 26 of 33

(b) 0.0

2.5

.5

543

1.0

1.5

2.0

2.5

Time (h)

Time (h)

544 545

Figure 1. Phototransformation of tetracycline (0.015 mM) with and without the presence of clay (1.5

546

g/L) at pH 7, plotted as the ratio of tetracycline concentration at given time (Ct) to the initial

547

concentration (C0) vs. time. (a) Homoionic-exchanged hectorites (Na-Hect, K-Hect, and Ca-Hect).

548

(b) Homoionic-exchanged montmorillonites (Na-Mont, K-Mont, and Ca-Mont). Error bars represent

549

standard variations from duplicate measurements. Lines are for visual clarity only.

550

1.0

No clay No clay+NaN3 No clay+SOD No clay+IPA No clay+TMP No clay+N2 Na-Hect Na-Hect+NaN3 Na-Hect+SOD Na-Hect+IPA Na-Hect+TMP Na-Hect+N2

Ct/C0

.8 .6 .4 .2 0.0 0.0

.5

1.0

1.5

2.0

2.5

Time (h)

26


Page 27 of 33


551 552 553 554 555 556 557 558 559 560 561 562

Figure 2. Suppressed phototransformation of tetracycline (0.015 mM) with and without the presence

563

of Na+-exchanged hectorite (Na-Hect, 1.5 g/L) at pH 7 by quenching photolysis-induced reactive

564

species, plotted as the ratio of tetracycline concentration at given time (Ct) to the initial concentration

565

(C0) vs. time. Error bars represent standard variations from duplicate measurements. Lines are for

566

visual clarity only. The initial concentrations of NaN3, SOD, IPA, and TMP were 10 mM, 5 mg/L,

567

55 mM, and 0.5 mM, respectively.

27


.080

.080

.075

.075

FFA conc. (mM)

FFA conc. (mM)


.070 .065 Na-Hect (No TC) No clay Na-Hect K-Hect Ca-Hect

.060 .055 .050 0.0

.5

1.0

.070 .065 .060

Na-Mont (No TC) No clay Na-Mont K-Mont Ca-Mont

.055

(a) 1.5

2.0

(b)

.050

2.5

0.0

Time (h)

569

Page 28 of 33

.5

1.0

1.5

2.0

2.5

Time (h)

570 571

Figure 3. Loss of FFA (initially at 0.08 mM) as a function of time during the phototransformation of

572

tetracycline with and without the presence of clay at pH 7. (a) Homoionic-exchanged hectorites (Na-

573

Hect, K-Hect, and Ca-Hect). (b) Homoionic-exchanged montmorillonites (Na-Mont, K-Mont, and

574

Ca-Mont). Error bars represent standard variations from duplicate measurements. The pseudo-first-

575

order rate constants of FFA (kobs, FFA) were 1.4×10-5 s-1, 3.8×10-5 s-1, 2.7×10-5 s-1, 2.5×10-5 s-1, 3.3×10-5

576

s-1, 1.73×10-5 s-1, 2.43×10-5 s-1 for No clay, Na-Hect, K-Hect, Ca-Hect, Na-Mont, K-Mont, and Ca-

577

Mont, respectively.

28


Page 29 of 33


.55

TMP conc. (mM)

.50 .45 .40 .35

No clay Na-Hect Na-Hect (without TC)

.30 0.0 579

.5

1.0

1.5

2.0

2.5

Time (h)

580 581

Figure 4. Loss of TMP (initially at 0.5 mM) as a function of time during the phototransformation of

582

tetracycline with and without the presence of Na+-exchanged hectorite (Na-Hect) at pH 7. Error bars

583

represent standard variations from duplicate measurements.

29



Page 30 of 33

1.0

No clay (pH=4) No clay (pH=5) No clay (pH=6) No clay (pH=7) Na-Hect (pH=4) Na-Hect (pH=5) Na-Hect (pH=6) Na-Hect (pH=7)

Ct/C0

.8 .6 .4 .2 0.0 0.0 585

.5

1.0

1.5

2.0

2.5

Time (h)

586 587

Figure 5. Phototransformation of tetracycline (0.015 mM) with and without the presence of Na+-

588

exchanged hectorite (Na-Hect, 1.5 g/L) at various pH, plotted as the ratio of tetracycline concentration

589

at given time (Ct) to the initial concentration (C0) vs. time. Error bars represent standard variations

590

from duplicate measurements. Lines are for visual clarity only.

30


Page 31 of 33


592

2.0

593

360 nm

pH 4 pH 5 pH 6 pH 7 pH 8 pH 9

594

1.5

595

-1

598 599 600 601 602 603 604 605

4

597

-1

Molar absorptivity (x10 M cm )

596

1.0

.5

0.0

(a)

375 nm

pH 4 pH 5 pH 6 pH 7

1.5

1.0

606 607

.5

608 609 610 611

(b) 0.0 200

300

400

500

600

700

800

Wavelength (nm)

612 613

Figure 6. UV-vis spectra of tetracycline (0.015 mM) in arbitrary units (AU) with and without the

614

presence of Na+-exchanged hectorite (Na-Hect, 1.5 g/L) at various pH. (a) Aqueous solution. (b)

615

Aqueous suspension of Na-Hect. The dashed vertical lines indicate the maximum absorption

616

wavelengths of tetracycline (360 nm and 375 nm) for aqueous solution and Na-Hect, respectively, at

617

pH 7. The spectra of tetracycline in the presence of Na-Hect were obtained by subtracting the blank

618

spectra of neat clay suspension at the same pH.

31



.025

.006

(a)

No clay Na-Hect

XTT Formazan (mM)

FFA loss (mM)

.015 .010 .005

.004 .003 .002 .001 0.000

0.000 4

5

6

4

7

.14

5

6

7

pH

pH .16

(b)

No clay Na-Hect

.005

.020

Page 32 of 33

(c)

No clay Na-Hect

TMP loss (mM)

.12 .10 .08 .06 .04 .02 0.00 4

620

5

6

7

pH

621 622

Figure 7. Comparison of concentration changes of different probe chemicals with and without the

623

presence of Na+-exchanged hectorite (Na-Hect) after 2.5-hr photolysis of tetracycline at various pH.

624

(a) FFA. (b) XTT. (c) TMP. Error bars represent standard variations from duplicate measurements.

625 626

32


Page 33 of 33



Enhanced Phototransformation of Tetracycline at Smectite Clay

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