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Environmental Processes

Excited Triplet State Interactions of Fluoroquinolone Norfloxacin with Natural Organic Matter: A Laser Spectroscopy Study Xi-Zhi Niu, Evan G. Moore, and Jean-Philippe Croue Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b02835 • Publication Date (Web): 23 Aug 2018 Downloaded from http://pubs.acs.org on August 24, 2018

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Environmental Science & Technology

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Excited Triplet State Interactions of Fluoroquinolone Norfloxacin

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with Natural Organic Matter: A Laser Spectroscopy Study

6

Xi-Zhi Niu1*§, Evan G. Moore2*, Jean-Philippe Croué1*

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8 9

1

Curtin Water Quality Research Centre, Department of Chemistry, Curtin University, GPO Box U1987, Perth, WA 6845, Australia

10 11

2

School of Chemistry and Molecular Biosciences, University of Queensland, St Lucia, Brisbane, QLD 4072, Australia

12 13 14

*corresponding authors:

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X.Z.N: [email protected]; +1 5204259716

16

E.G.M: [email protected]

17

J.P.C: [email protected]

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§: author present address: Department of Chemical and Environmental Engineering,

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University of Arizona, Tucson, AZ85721, USA

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Revised for submission to: Environmental Science & Technology

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1 ACS Paragon Plus Environment


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Abstract

25

In sunlit waters, the fate of fluoroquinolone antibiotics is significantly impacted by

26

photodegradation. The mechanism of how natural organic matter (NOM) participates in the

27

reaction has been frequently studied, but still remains unclear. In this work, the interactions

28

between the excited triplet state of the fluoroquinolone antibiotic norfloxacin (3NOR*) and a

29

variety of NOM extracts were investigated using time-resolved laser spectroscopy. The

30

observed transient absorption spectrum of 3NOR* showed a maximum at ca. 600 nm, and

31

global fitting gave a lifetime of 1.0 µs for 3NOR* in phosphate buffer at pH=7.5. Quenching

32

of 3NOR* by Suwannee River hydrophobic acids (HPO), Beaufort River HPO, and Gartempe

33

River HPO yielded rate constants of 1.8, 2.6, and 4.5 (×107 molC-1s-1) respectively, whereas

34

HPO from South Platte River unexpectedly increased the lifetime of 3NOR* with an as yet

35

unknown mechanism. Concurrent photodegradation experiments of NOR (5 µM) in the

36

presence of these NOM were also performed using a sunlight simulator. In general, the

37

effects of NOM on the photodegradation rate of NOR were in agreement with observations

38

from transient absorption studies. We suggest that adsorption of NOR to NOM is one of the

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major factors contributing to the observed quenching. These results yield a new insight into

40

the likely role of NOM in sunlight-induced degradation of micropollutants.

41


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Introduction

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The ubiquitous occurrence of antibiotics in various watersheds continues to receive

44

increasing attention as an emerging water management issue.1, 2 One of the most commonly

45

prescribed antibiotics in many countries are fluoroquinolones (FLQs), and their

46

concentrations in different waterways at µg ·L-1 levels have been widely recorded in Europe,

47

China, Australia, and USA.3-6 Since hydrolysis7 and microbial degradation8 have been

48

reported to be relatively inefficient at removing FLQs, their fate in waters is mainly governed

49

by adsorption9, 10 and photolysis.7, 11-15 Indeed, the photolytic rate of FLQs was found to be

50

relatively high in buffered water under simulated sunlight conditions (e.g., 2.45 hr-1 at pH 8.0

51

for norfloxacin, NOR).11 However, in real waterways, the rate of photodegradation may be

52

inhibited by the presence of dissolved natural organic matter (NOM).11, 12

53

Over the past two decades, the photochemical transformation of FLQs has been widely

54

studied, and the photolytic degradation of FLQs including norfloxacin (NOR) is believed to

55

occur from its excited triplet state (3NOR*).16, 17 The excited state behaviour of NOR has

56

been examined spectroscopically, and is relatively well-known. As shown in equations 1-3

57

below, 3NOR* is generated by intersystem crossing (IC, eq-2) from the initially populated

58

excited singlet state,17, 18 and has a characteristic excited state absorption (ESA) feature in its

59

transient absorption spectrum, with a maxima at ca. 620 nm.17 Subsequent to excited triplet

60

state formation, several photochemical reaction mechanisms have been reported for NOR,

61

including C-F bond cleavage, piperazine chain oxidation, and/or fluorine atom substitution by

62

an -OH group, yielding a variety of photoproducts.11, 17, 19

63

∗

64

65

ℎ

eq-1

∗→ ∗

eq-2

∗ ℎ


eq-3


Page 4 of 25

66

Given the importance of 3NOR* in these pathways, the population and lifetime of this excited

67

state will play a central role in determining the rate of photodegradation. Accordingly, the

68

presence of aquatic NOM may impact on the photodegradation rate of NOR in two ways: 1.)

69

by inhibiting photolytic rates (kNOR) through competitive light screening, or 2.) by interacting

70

with 3NOR*, and thus significantly changing its excited state lifetime. While the former

71

mechanism has been described in several previous studies,11,

72

information available on potential interactions between 3NOR* and NOM. Hence, in the

73

present work, we have aimed to fill this gap by investigating the excited state lifetime of

74

3

75

laser spectroscopy. Concurrent photodegradation experiments were also carried out using a

76

sunlight solar simulator, and the results we have obtained have important implications for the

77

photodegradation mechanism of antibiotic micropollutants such as NOR in NOM-enriched

78

waters.

79

Experimental

80

Chemicals and materials. Norfloxacin (Fluka analytical), furfuryl alcohol (Acros Organics),

81

sodium phosphate monobasic/sodium phosphate dibasic, and phosphoric acid (Ajax

82

Finechem) were purchased commercially, and all reagents were of analytical grade. Four

83

purified natural organic matter (NOM) isolates were selected for this study, which were

84

Suwannee River hydrophobic acids (SWR-HPO), Beaufort River hydrophobic acids (BF-

85

HPO), Gartempe River hydrophobic acids (GR-HPO), and South Platte River hydrophobic

86

acids (SPR-HPO). These extracts were isolated according to the method previously reported

87

by Leenheer et al.20 All stock solutions were prepared in 1×PBS (phosphate-buffered saline,

88

pH=7.5) using ultrapure water (18.2 MΩ cm, Milli-Q, Purelab Classic) for buffer preparation.

12, 14

there is very little

NOR* in the presence of different NOM fractions using pump-probe transient absorption


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89

Analytical. Total organic carbon (TOC) and pH were measured with a Shimadzu TOC

90

analyser (TOC-L) and a Thermo Scientific pH meter. The UV-Vis absorbance of solutions

91

was measured using an Agilent spectrophotometer (Cary 60). The concentration of

92

norfloxacin was analysed by High Performance Liquid Chromatography (HPLC) using an

93

Agilent 1100 system coupled to a Diode Array Detector (DAD), with full details for HPLC

94

methods as described elsewhere.11

95

Transient absorption spectroscopy. The excitation source for transient absorption

96

measurements was an amplified laser system (Spitfire ACE, Spectra Physics), delivering ca.

97

100 fs 800 nm laser pulses at a 1 kHz repetition rate, which were coupled to an OPA system

98

(Topas Prime, Light Conversion), yielding 0.2 mJ excitation pulses at 335 nm. Ground and

99

excited state difference spectra (∆OD) at various delay times were measured using a sub-

100

nanosecond spectrometer (EOS, Ultrafast Systems), incorporating two 512 pixel CCD

101

cameras as the sample and reference channel. A white light continuum probe between ca. 380

102

and 800 nm was generated using a Nd:YAG based supercontinuum white light source (STM-

103

2-UV, Leukos Systems). The instrument response function (IRF) of this setup had a full

104

width at half maximum (FWHM) of ca. 100 ps. Sample absorbances were ca. 0.3-0.4 over

105

the 2 mm path length cell used, and samples were stirring magnetically during measurements.

106

No detectable changes were observed in the UV-Vis absorption spectrum at the completion

107

of transient absorption studies, indicating a lack of any significant photodecomposition. All

108

spectra were analysed using commercially available software (Igor, Version 6.1.2.1,

109

Wavemetrics).

110

Photodegradation experiment. Steady-state photodegradation experiments were performed

111

employing a sunlight simulator (SUNTEST XLS +, ATLAS, USA), using an experimental

112

setup which has been previously described.11 Briefly, solutions of 5 µM NOR (pH 7.5)



113

containing various concentrations of NOM were irradiated, and all photodegradation

114

experiments were conducted in duplicate.

115

Results and Discussion

116

Transient absorption spectra of norfloxacin. The absorption spectrum of the ground state

117

NOR is given in the supporting information (Figure S1). Using pulsed 335 nm excitation, the

118

time-resolved transient absorption (TA) spectrum of NOR in 1×PBS was recorded (Figure 1),

119

yielding spectral features which agree well with previous observations.17, 21-23

120

As shown in Figure 1, the observed TA spectra at different time delays display several

121

features including a strong positive band at ca. 600 nm and a negative band at ca. 430 nm

122

which can be readily assigned to the known transient species of NOR. Specifically, the latter

123

can be attributed to stimulated emission from the initially populated singlet state of NOR

124

(1NOR*), which is relatively short lived and matches well with the fluorescence maximum

125

previously reported in the literature.16 Instead, the longer lived signal centered at ca. 600 nm

126

is assigned to excited state absorption by the triplet state (3NOR*), with a peak position that

127

also agrees with literature.17,21-23 Interestingly, another transient feature, which appears as a

128

shoulder at ca. 700 nm is also apparent, but has a much weaker signal. This peak has been

129

reported in several previous studies,21,

130

originate from a triplet excimer of NOR (3NOR*-NOR), which is relatively long lived but

131

does not participate in subsequent photodegradation of NOR.23 In the current work, this

132

excimer was not considered in the analysis, and the weak signal associated with this transient

133

species was instead included into the horizontal offset (y0) of the global fitting procedure..

134

In order to understand the fate of NOR in irradiated solutions, the lifetime of the longer-lived

135

3

23, 24

and most recently has been demonstrated to

NOR* is of particular importance, and the observed transient absorbance features for the


Page 6 of 25

Page 7 of 25


136

singlet and triplet excited states at all wavelengths were globally well represented by a bi-

137

exponential decay function of the form: #$

() =

138

% !" $

()

#$

+

% ' !" (

()

+ *

eq-4

139

where I(t) is the observed differential absorption (∆OD) intensity, A1 is a pre-exponential

140

scaling factor for 1NOR*, τ1 is the observed singlet lifetime; likewise A2 and τ2 are the pre-

141

exponential scaling factor and lifetime for 3NOR* and y0 is a horizontal offset.

142

As shown in Figure 2-a, the resulting fit for experimental data was excellent at all

143

wavelengths, and the corresponding fit residual can also be found in Figure S2. In particular,

144

the global fit results for TA spectra at 450 nm and 600 nm are also shown in Figure 2-b,

145

while Figure 2-c and Figure S3 depict the Decay Associated Difference Spectra (DADS) for

146

the 1NOR* and 3NOR*excited states. Because global analysis simultaneously fits multiple

147

data points on the TA curve (Figure 2-c), interference from other transient species in the

148

solution could be reduced, i.e., the TA curves of concurrent transient species may intersect

149

that of 3NOR* but unlikely replicate. From duplicate experiments, we obtain an evaluated τ2

150

lifetime of 1.0 µs for 3NOR* in 1×PBS (Table 1 & Figure 2-c), which is comparable to other

151

literature results. For example, previous reports gave a lifetime 1.3 µs for 3NOR* at pH 7.4

152

using sodium hydrogen carbonate as buffer.17 Similarly, the τ1 lifetime we evaluate for

153

1

154

0.01 M phosphate buffer.17

155

Although the TA features of 3NOR* have been well characterised in previous studies, we also

156

confirmed its nature as an excited triplet state by measuring its lifetime in de-oxygenated

157

solutions, which yielded a significant increase in τ2 (Table 1). In the current study, the excited

158

state lifetime of the long-lived 3NOR* species upon interactions with natural organic matter

NOR* was 1.0 ns, which is similar to the previously published value of 1.5 ns reported in



Page 8 of 25

159

was further investigated, since 3NOR* is believed to be the key excited state species involved

160

in determining the photochemical fate of NOR, due to its considerably longer lifetime (Table

161

1 & Figure 2-c).

162

3

163

used in our previous study involving sunlight-induced photodegradation of NOR.11

164

Specifically, these freshwater NOM isolates are well-characterised hydrophobic (isolated

165

from XAD-8 resin) fractions from four different water sources. SUVA254 values are provided

166

in Table S1. Prior to adding NOM into NOR solution, the transient spectra of the four NOM

167

isolates were recorded as controls. In each case, the observed TA spectra all appeared very

168

similar, with a small positive signal observed at between ca. 400-500 nm. The spectrum for a

169

concentrated sample of BF-HPO (150 mg.L-1) is presented in Figure S4 as a representative

170

example. Importantly, the ∆OD of these NOMs were broad, featureless, and presented much

171

weaker signals even at much higher concentrations (Figure S4) compared to that of

172

35.1 mg.L-1 for 0.11 mM NOR.

173

The quenching of excited triplet states by NOM for a variety of model sensitizers such as

174

2-acetonaphthone and

175

Surprisingly, in the current work, low concentrations of GR-HPO, SWR-HPO, and BF-HPO

176

from ca. 5-10 mg.L-1 (including the data point at 50 mg.L-1 for SWR-HPO) instead led to an

177

enhancement in the lifetime of 3NOR* from 1.0 µs to ca. 1.20-1.27 µs (corresponding to the

178

green zone indicated in Figure 3). Subsequently higher concentrations of NOM then resulted

179

in the expected decrease in τ2, which was observed for these three isolates. While

180

unexpected, the initial increase in τ2 may be rationalised by a decrease in 3NOR*self-

181

quenching

182

previously reported.23 In the presence of NOM, the otherwise bimolecular self-quenching

183

process which has a near-diffusion controlled rate23 may be inhibited, and hence the addition

NOR* interactions with NOM. The four NOMs investigated in the current work were also

3-methoxyacetophenone

has

been previously

documented.25

(eq-5), and evidence of highly efficient 3NOR* self-quenching has been


Page 9 of 25


184

of low concentrations of NOM would lead to an increase in the observed τ2 if the rate of

185

quenching by NOM (ca. 5 mg.L-1) was lower than the self-quenching rate of 3NOR* (vide

186

infra).

187

∗+ → [ − ] ∗

188

189

/01

. ∗+ . + ∗ 234

∗

eq-5 eq-6 eq-7

190

To evaluate the bimolecular quenching rate constants of 3NOR* by NOM (56234 , eq-7), a

191

Stern-Volmer lifetime analysis with the following form was undertaken for SWR-HPO,

192

BF-HPO, and GR-HPO (Figure 4):

193

7

= 7 + 56234 [.]

eq-8

8

194

Where τ is the lifetime of the excited state to be fitted (herein τ2), τ0 is the initial lifetime of

195

the corresponding excited state. Also, to exclude potential interference by the self-quenching

196

of 3NOR*, the initial increase in τ2 from 1.0 µs to ca. 1.20-1.27 µs was considered as a

197

threshold, and hence the second data point in Figure 3 was taken as τ0 (instead of 1.0 µs) with

198

only subsequent data points utilised in this analysis, and the resulting bimolecular quenching

199

rate constant we obtain are summarised in Figure 4. It is worth noting that the linearity of the

200

plots was relatively low with R2 values in the range of 0.58-0.72. One of the contributing

201

factors to these low R2 values is the possible variation of dissolved oxygen (DO) in solutions,

202

which is a potential quencher of 3NOR* (eq-9).

203

3(

∗

eq-9

204

Although this is the first study reporting the quenching rate constants of 3NOR* by NOM, the

205

values we obtain here for 59:;< are comparable to other transient studies performed using

206

human albumins.26 For example, the bimolecular quenching rate constant of 3NOR* by 9 ACS Paragon Plus Environment


207

tyrosine (C9H11NO3) was reported26 to be 0.7×109 M−1s−1, corresponding to 7.78×107 L

208

molC-1s-1. Importantly, the 59:;< values obtained also support our hypothesis that low

209

concentrations of NOM inhibit the potential self-quenching of 3NOR* by NOR (Figure 3)

210

and resultantly increased τ2, since self-quenching of 3NOR* was reported to be nearly

211

diffusion-controlled 23 with a rate constant of 0.9×109 M−1 s−1, which is significantly higher

212

than 59:;< . For example, the self-quenching exerted by 0.11 mM NOR is estimated to be

213

9.9×104 s−1 (quenching rate constant × concentration), while quenching by 5 mg/L SWR-HPO

214

is ca. 4.2×103 s−1. In other words, by inhibiting the bimolecular quenching of 3NOR*, low

215

concentrations of NOM appear to increase the lifetime of 3NOR* since quenching by NOM at

216

these low concentrations was not significant. However, at higher concentration, the

217

quenching by NOM has a more significant effect on τ2 with increasing TOC content (Figure

218

3 & Figure 4).

219

Interestingly, for SPR-HPO, a larger increase in τ2 from 1.0 µs to 1.57 µs was observed in the

220

presence of a relatively low NOM concentration of 5 mg.L-1 (Figure 3). Although increasing

221

SPR-HPO concentrations were then also followed by a decrease in τ2, it was remarkable that

222

most data points for SPR-HPO remained above the other three NOM isolates and τ2 only

223

dropped to ca. 1.20-1.27 µs when the concentration was as high as 225 mg.L-1 (Figure 3).

224

Although the exact reason for this difference remains unknown, a likely explanation may be

225

due to the potential for energy transfer from the excited state of SPR-HPO to the ground state

226

of NOR (eq-6). In the current setting, NOM could function as either a quencher or an energy

227

source for the production of 3NOR*. Owing to the complex nature of NOM, direct evidence

228

for this process is not available on the ns-µs time scale. However, it is worth noting that a

229

previous steady-state photochemical study27 of ciprofloxacin (CIP) which has a similar

230

structure and chemical properties compared with NOR raised the hypothesis that the excited


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231

triplet state of humic substances could facilitate the formation of 3CIP*. SPR-HPO from

232

South Platte River (Colorado, U.S.A.) has been used in our lab for several previous

233

photochemical studies,11, 28, 29 and it was demonstrated to be less aromatic with a SUVA254 of

234

2.9.28 Moreover, the van Krevelen diagram of SPR-HPO obtained based on FTICR-MS

235

analysis has showed that it is rich in aliphatic compounds,30 and when used at the same TOC

236

content, it showed a higher yield of photochemically produced reactive intermediates (PPRIs)

237

compared to BF-HPO and SWR-HPO.28 In the present case, the overall effect of SPR-HPO is

238

further complicated considering the apparent self-quenching of 3NOR* by NOR, energy

239

transfer between different moieties, and eventually quenching of 3NOR* by moieties of SPR-

240

HPO. Due to these uncertainties, and the apparent complexity of the 3NOR* behaviour in the

241

presence of SPR-HPO, it was not included in the Stern-Volmer analysis.

242

Implications for the photodegradation of norfloxacin. As mentioned, the photochemical

243

transformation of NOR is believe to initiate from its triplet state. Although self-sensitized

244

singlet oxygen (1O2) or radicals may also participate in the photodegradation of NOR,11, 12 the

245

production of these reactive species similarly originates from

246

photochemical degradation rate of NOR (kobs) in sunlit solutions will be determined by two

247

parameters of 3NOR*, namely the population and lifetime. The population or concentration of

248

3

249

3

NOR*. Hence, the

NOR* in our system is dependent on its formation rate ɑ described as: ɑ = 2.303B 23C∗ ∑(*,F GF HF )

eq-10

250

where ϕ3NOR* is the quantum yield of 3NOR*, and Sλ is the light-screening coefficient. The

251

light attenuation due to competitive absorbance by NOM can be compensated by a light

252

screening correction factor (CF) in the wavelength range of 300-360 nm, which is the inverse

253

of Sλ, and our calculations for CF and Sλ are available in the SI. After light attenuation

254

correction, ɑ in the presence of NOMs should be the same with that in buffered water.



Page 12 of 25

255

Ultimately, the lifetime of 3NOR* was the only central factor influencing kobs in the solar

256

simulator, and the impact of the four NOM isolates on kobs was analysed. NOR

257

photodegradation data was fit using a first order kinetics model, [23C]

I [23C] = −5JKL

258

eq-11

8

259

The photochemical degradation rate of NOR after light screening correction (kobs_CR) was

260

expressed in the form: * 5JKL_C = 5JKL + 5234

261

eq-12

262

* where 5JKL was the kobs obtained without the presence of NOM, and kNOM is the contribution

263

of NOM to kobs_CR. A negative value of kNOM indicated that the presence of NOM inhibited

264

the photodegradation of NOR.

265

Considering the much lower concentration of NOR utilised (5µM) and the significantly lower

266

irradiance in the solar simulator experimental conditions, self-quenching by NOR as noted

267

and discussed using the amplified laser system for transient absorption measurements was not

268

expected. In accordance with the TA results, with increasing TOC content, SWR-HPO,

269

GR-HPO, and BF-HPO showed an inhibitory effect on kobs_CR (b, c, and d in Figure 5 &

270

Table S2), since these three isolates were capable of quenching the excited state lifetime of

271

3

272

increase was observed with increasing TOC content (Figure 5-a & Table S2), which was also

273

in agreement with the TA data (Figure 3). Although several previous studies have

274

investigated the role of NOM on the photodegradation rate of FLQs (inhibition vs.

275

enhancement),11,

276

differences in experimental conditions and the unknown mechanism governing this process.

277

In the current study, several different NOM have been tested with different TOC contents,

NOR* (Figure 4). On the other hand, SPR-HPO did not decrease kobs_CR and indeed a slight

12, 14, 27

there have previously been discrepancies in conclusions due to


Page 13 of 25


278

and the mechanism can be successfully explained by transient interactions between 3NOR*

279

and NOM.

280

Implication from NOR adsorption to NOM. Intermolecular interactions between NOR and

281

NOM are likely essential for the observed quenching effect. At circumneutral or slightly

282

alkaline pH, NOR is present predominantly in its zwitterionic form,14 and NOM

283

(deprotonated) is known to exhibit considerable adsorption with FLQs.31-33 In this work, both

284

the laser spectroscopy studies and the steady-state irradiation experiments were conducted at

285

pH 7.5, and NOR is believed to associate with NOM to different degrees, depending on the

286

properties of NOM. It has been reported that zwiterrionic CIP (which has a similar structure

287

with NOR) adsorption onto NOM is mainly governed by several processes31-34, including; 1.)

288

electrostatic interactions between the secondary and tertiary amines of the FLQ and the

289

carboxylate groups of NOM; 2) van der Waals and multiple H-bonding interactions with

290

NOM. In agreement with the

291

we have previously reported

292

carboxyl-rich moieties than SPR-HPO, facilitating the former electrostatic interaction

293

between amine groups of NOR with the carboxylates of NOM. The higher total dissolved

294

amino acids (Table 2) 35 in SWR-HPO may also induce higher affinity with NOR than SPR-

295

HPO through interactions with the carboxylate group of NOR. SWR-HPO also showed

296

higher contents of C=O, C-O, and O-C-O groups than SPR-HPO (Table 2), and these

297

functional groups were previously proposed to facilitate H-binding with NOR.31 By

298

comparing SWR-HPO and SPR-HPO in Table 2, it is reasonable to suggest that NOR should

299

possess a higher affinity with the higher SUVA isolates (i.e. SWR-HPO, GR-HPO, and BF-

300

HPO). Although the specific moieties in NOM contributing to 3NOR* scavenging were not

301

identified, higher intermolecular affinity likely promotes the observed quenching effect by

302

these three NOM isolates, which was not observed in SPR-HPO.

13

C-NMR results reported in Table 2,35 the FTICR-MS results

30

indicate that SWR-HPO incorporated significantly more



Page 14 of 25

303

Environmental implications. Given the current state of mechanistic knowledge, scientists

304

have been investigating two major effects of NOM on the sunlight-induced photodegration of

305

micropollutants. The first is that NOM can function as a photosensitizer, and enhance the

306

photodegradation of micropollutants. This is particularly important for compounds which

307

show low direct photolytic rates, and examples in this category include ibuprofen,28,

308

cimetidine,28, 37 and amoxicillin.38 The second is the role of NOM as a potential inhibitor in

309

the photo-oxidation of certain classes of compounds, with sulfonamide antibiotics as

310

previously discussed by Wenk et al being a primary example.39 Additionally, if degradation

311

of the target compound has a high photolytic quantum yield, the role of NOM-

312

photosensitized PPRIs becomes trivial, and NOM becomes simply a sunlight competitor. For

313

example, the degradation of sulfathiazole has been reported with a relatively high quantum

314

yield of 0.079 at pH 8.0, and in this case NOM acts only by affecting the photodegradation

315

rate as a light screener.29 The results reported here suggest a new mechanism by which NOM

316

may impact the photochemical fate of micropollutants, especially broad-spectrum

317

micropollutants with relatively easily accessible transient excited state characteristics, such as

318

FLQs. The ns-µs events described in the current work typically lie unrevealed since the fast

319

photochemical/photophysical processes occurring in sunlit waters are usually ‘hidden’ when

320

studying the photochemical fate of micropollutants using continuous irradiation methods.

321

Due to the similarity in photochemical and photophysical properties among different FLQs,

322

other widely detected FLQs such as ciprofloxacin and enrofloxacin are likely subjected to the

323

same mechanisms presented in the current study.

324

Associated content

325

Supporting Information

326

Additional tables on SUVA254 of NOM isolates and the photochemical production of 1O2 by

327

these NOMs, raw data for kobs, CF300-360, and kobs_CR; and figures on UV-Vis absorbance of 14 ACS Paragon Plus Environment

36

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328

NOR (0.11mM), 150 mg/L SR-HPO, and NOR (0.11mM) plus 150 mg/L SR-HPO, the

329

global fitting processing result and fitting residual for Figure 2-a, Decay Associated

330

Difference Spectra of 1NOR*, representative transient absorption spectrum of NOM, 335 nm

331

laser source, and light screening correction factor calculation models.

332

Acknowledgment

333

Water Research Australia (WaterRA Scholarship 4513-15), Chemcentre, and Curtin

334

University (Curtin International Postgraduate Research Scholarship) are acknowledged for

335

providing financial support to X.Z.N. E.G.M. gratefully acknowledges financial support by

336

the Australian Research Council (ARC). We also thank the anonymous reviewers for the

337

helpful comments on the paper. This study forms part of the Ph.D. thesis of X.Z.N.

338



339

Reference

340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388

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Figure 1. Observed μs transient absorption spectra of NOR (0.11 mM) in 1 × PBS aqueous buffer (λex = 335 nm) from t = 0.015 (red) to 5 μs (purple).



447 448 449 450 451

Figure 2. a) Corresponding decay kinetics (hollow circles) in 10 nm data increments from 400 (red) to 900 nm (purple) together with results from global fitting (black); b) global fitting results at 450 and 600 nm, respectively; c) Decay Associated Difference Spectra (DADS) of excited triplet state (green).


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452 453

Figure 3. τ2 (3NOR*) observed in the presence of different NOMs.

454



455 456 457

Figure 4. Inverse of τ for 3NOR* (τ3NOR*) (μs-1) plotted against the TOC contents of added NOMs; NPQR unit: L molC-1s-1. R2 of the plots were 0.72, 0.62, and 0.62, respectively. O

458


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Figure 5. NOR photodegradation rate constants in the presence of different NOMs, before (∆) and after (○) light screening correc@on; and kNOM (●).

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Page 24 of 25

Table 1. Lifetimes and transient absorption maxima (λmax) of transient species of NOR. 1

τ (µs)

3

NOR* 450 1.1×10-3 1.0×10-3

λmax (nm) 0.11 mM NOR in 1×PBS De-oxygenated‡

NOR* 600 1.0 4.7

465 466

Table 2. Integrated Areas of 13C-NMR Spectra and total dissolved amino acids (TDAA).

13

C-NMR chemical shift (ppm)

TDAA

SWR-HPO

SPR-HPO

0-60

C-C/C-H

27.9

52.4

60-90

C-O

16.8

15.4

90-110

O-C-O

7.8

3.4

110-160

AromC

26.1

11.6

160-190

COOH

16.7

15.2

4.8 6.3 15

2 4 10.2

190-220 C=O µg N/mgC µg C/mgC

467


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