A Threshold Concentration of Silver Ions Exists for Sunlight-Induced

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Environmental Processes

A Threshold Concentration of Silver Ions Exists for Sunlight-Induced Formation of Silver Nanoparticles in the Presence of Natural Organic Matter Huiting Liu, Xueyuan Gu, Chenhui Wei, Heyun Fu, Pedro J. J. Alvarez, Qilin Li, Shourong Zheng, Xiaolei Qu, and Dongqiang Zhu Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b05645 • Publication Date (Web): 05 Mar 2018 Downloaded from http://pubs.acs.org on March 7, 2018

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Environmental Science & Technology

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A Threshold Concentration of Silver Ions Exists for

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Sunlight-Induced Formation of Silver Nanoparticles in

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the Presence of Natural Organic Matter

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Huiting Liu†, Xueyuan Gu†, Chenhui Wei†, Heyun Fu†, Pedro J.J. Alvarez§, Qilin Li§, Shourong Zheng†,

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Xiaolei Qu†*, Dongqiang Zhu‡

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†

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

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University, Jiangsu 210023, China §

Department of Civil and Environmental Engineering, Rice University, Houston TX 77005, United

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States ‡

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

* Corresponding author Xiaolei Qu, phone: +86-025-8968-0256; email: [email protected]

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ACS Paragon Plus Environment

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Abstract

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Sunlight-induced photo-formation of silver nanoparticles (nAg), mediated by natural organic matter

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(NOM), is significantly affected by the concentration of Ag(I) and chloride. The initial photo-formation

17

rates of nAg in Suwannee River humic acid (SRHA) and Suwannee River natural organic matter

18

(SRNOM) solutions were examined under simulated sunlight irradiation. A critical induction

19

concentration (CIC) of Ag(I) (10 mg/L for SRHA and 5 mg/L for SRNOM, respectively) was observed

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below which the nAg formation was minimal. The threshold is attributed to the interplay of reduction

21

and oxidation reactions mediated by NOM, reflecting the need to achieve sufficiently fast growth of

22

silver clusters to outcompete oxidative dissolution. The CIC can be reduced by scavenging oxidative

23

radicals or be increased by promoting singlet oxygen and hydrogen peroxide generation. The presence

24

of chloride effectively reduced the CIC by forming AgCl which facilitates reduction reactions and

25

provides deposition surfaces. SRNOM was more efficient in mediating photo-formation of nAg than

26

SRHA, owing to their differed phototransient generation. These results highlight prerequisites for the

27

photo-formation of nAg mediated by NOM, in which the photochemistry and solution chemistry are

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both important.

29 30

Introduction

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Silver nanoparticles (nAg) possess unique optical, electronic, and antimicrobial properties, and are the

32

most widely used nanomaterial in consumer products.1 When used as a strong and wide-spectrum

33

antimicrobial agent, nAg can be readily incorporated into the matrix or the surface coating of textiles,

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personal care products, medical devices, household appliances, and water treatment devices.1 On the

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other hand, unintended nAg releases can adversely impact a wide range of organisms including

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microorganisms, algae, fungi, plants, invertebrates, vertebrates, and human cell lines.2 The acute

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toxicity of nAg stems mainly from its ability to release Ag+,3, 4 although nanoparticle-specific toxicity


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has also been reported.5, 6 The release of nAg/Ag+ during the lifetime of these nano-enabled products

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seems inevitable and in many cases significant,7,

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environmental risks.

8

leading to concerns regarding its potential

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The speciation of silver is critical for understanding the fate, bioavailability, and toxicity of nAg. It is

42

well-known that nAg undergoes transformation processes including oxidation, dissolution, and reactions

43

with ligands.2 nAg can be oxidized by oxygen, forming a Ag2O shell which can subsequently release

44

Ag+.9,

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(NOM).2, 11 On the other hand, Ag+ can be readily reduced by NOM to form new nAg under sunlight.12-

46

16

47

reduction mediated by phototransients such as superoxide (O2-).14, 15

10

Ag+ can further complex with natural ligands, such as S2-, Cl-, and natural organic matter

The photo-reduction mechanism was suggested to be ligand-to-metal charge transfer (LMCT) and

48

The impact of water chemistry, including pH and co-existing cations, on the photo-reduction process

49

has been investigated.12, 14, 15 Photo-reduction rates increase with increasing pH,12, 14, 15 owing to the pH-

50

dependent Ag+ sorption and/or reductive potential of NOM. The presence of co-existing cations reduced

51

the formation rate of nAg, likely due to the competing effect for sorption sites on NOM.14 Ca2+ was

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reported to induce the aggregation and formation of larger nAg particles.12 Different NOM or NOM

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fractionations possess varying ability to photo-reduce Ag+, which can be attributed to the inherent

54

reducing potential16 and/or the differential light attenuation ability.12 On the other hand, concurrent

55

oxidation/dissolution and reduction reactions are previously reported to occur during the light-induced

56

size/morphology evolution of nAg suspension.13, 14, 17, 18 In many cases, simulated sunlight was found to

57

accelerate the dissolution of nAg through photo-oxidation processes.13,

58

regarding how interactions between oxidative phototransients and silver species influence the initial

59

photo-formation of nAg in NOM solutions. Furthermore, little is known about the impact of chloride,

60

which significantly influences silver speciation in aquatic environment, on the photo-formation of nAg.

19

However, little is known

61

In the present study, we examine the sunlight-induced formation of nAg mediated by two standard

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humic substances, Suwannee River humic acid (SRHA) and Suwannee River natural organic matter


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(SRNOM), over a wide range of Ag(I) concentrations (0.05 – 20 mg/L). Our objectives were to 1)

64

examine the possibility of forming nAg under more realistic conditions found in sunlit natural aquatic

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systems, and 2) elucidate the role of silver and chloride concentrations in the photo-reduction process.

66 67

Materials and Methods

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Materials. Silver perchlorate (>97%), 2,3-bis (2-methoxy-4-nitro-5-sulfophenyl)-2H-tetrazolium-5-

69

carboxanilide (XTT, >90%), furfuryl alcohol (FFA, 98%), N,N-diethyl-p-phenylenediamine sulfate salt

70

(DPD, ≥ 99 %), horseradish peroxidase (HRP, ≥ 250 units/mg), and terephthalic acid (TPA, 98%) were

71

purchased from Sigma-Aldrich, USA. Deuteroxide (D2O, 99.8 atom % D) was provided by Tokyo

72

Chemical Industry, Japan. Isopropyl alcohol (>99.7%) was purchased from Nanjing Chemical Reagent

73

Co., Ltd, China. All solutions were prepared using deionized water (18.2 MΩ•cm) obtained from an

74

ELGA Labwater system (PURELAB Ultra, ELGA LabWater Global Operations, UK).

75

Preparation of SRNOM and SRHA solutions. NOM samples, including SRNOM and SRHA, were

76

obtained from the International Humic Substances Society (IHSS, St. Paul, USA). The stock solutions

77

of NOM were prepared by dissolving 40 mg sample powder into 100 mL deionized water. It was then

78

sonicated in a bath sonicator (KH-800TDB, Kunshan Hechuang Ultrasonic Instrument, China) at 50 W

79

for 15 min. After adjusted to pH of 7.0 ± 0.2 using 0.1 M NaOH, the NOM solutions were filtered

80

through a 0.45-µm membrane (Pall, USA). The total organic carbon (TOC) of NOM solutions was

81

quantified by a TOC analyzer (TOC-5000A, Shimadzu, Japan). Part of the SRNOM solution was

82

dialyzed using dialysis bags (1000 Da, Union Carbide, USA) to remove the chloride.

83

Photo-reduction of Ag+. The photo-reduction experiments were carried out by irradiating 30 mL of

84

the Ag+ and SRNOM/SRHA mixture stirred at 200 rpm in a 50-mL cylindrical cell equipped with a

85

water-circulating jacket for temperature control. The concentration of Ag+ (as AgClO4) and Cl- (as NaCl)

86

stock solutions are 500 mg/L and 300 mg/L. 1.08 mL of 139 mg C/L SRNOM stock solution or 0.69

87

mL of 217 mg C/L SRHA stock solution was mixed with predetermined amount of Ag+ and Cl- stock


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solutions to yield the experiment solution of 30 mL with total Cl- concentration of 0.5 mg/L. The pH of

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the mixture was 7.0 ± 0.2 without buffer. The temperature of the circulating water was maintained at 20

90

± 0.1 °C by a temperature control system (DC0506, Shanghai FangRui Instrument Co., Ltd., China).

91

The dissolved oxygen concentration in the test solution during the reaction remained constant at around

92

8 mg/L as determined by an oxygen microsensor (PreSens, Precision Sensing GmbH, Germany, Figure

93

S1). The simulated sunlight was provided by a 50 W Xenon lamp (CEL-HXF300, AULTT, China)

94

shining from the top of the cylindrical cell without light filters. The lamp spectrum was similar to that of

95

natural sunlight with the wavelength > 300 nm (Figure S2). The irradiation energy at the water surface

96

was 438 mW/cm2, which was monitored periodically using a radiometer (CEL-NP2000-10, Ceaulight,

97

Beijing, China). Samples were exposed to the same light intensity by adjusting the output energy to

98

offset the decay of the lamp. The detailed experimental setup can be found in Figure S3. A small aliquot

99

of 0.5 mL solution was withdrawn periodically from the cell during the irradiation for analysis. Dark

100

controls were conducted in the same experimental setting with the cell wrapped with aluminum foil and

101

the Xenon lamp off.

102

nAg analysis. The formation of nAg was monitored by the absorbance of its surface plasmon

103

resonance (SPR). UV-vis spectra of the samples were recorded using a double beam spectrophotometer

104

(UV-6100, Mapada, China) in a quartz cell with 1-cm light path length. The detection limit for this

105

method is around 0.01 mg/L nAg with diameter of 10 nm±2 nm (Figure S4). The particle size of nAg

106

was measured by dynamic light scattering (DLS) using a ZEN 3500 Zetasizer Nano ZS (Malvern,

107

Worcestershire, UK) equipped with a 532-nm laser. In some experiments, the samples were

108

concentrated by ultrafiltration membranes (Amicon Ultra-15 3 kD, Millipore, MA, U.S.A.), dried on

109

glass slides, and analyzed by an X-ray diffraction spectrometer (XRD, X-TRA, ALR, Switzerland).

110

ROS determination. The production of 1O2, O2−, H2O2, and •OH/lower-energy hydroxylating species

111

by SRNOM and SRHA was investigated using probe molecules as described previously.20-25 O2−

112

generation was quantified by the formation of XTT formazan from XTT at an initial concentration of


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0.05 mM. XTT formazan was quantified by its absorption at 470 nm. The extinction coefficient of XTT

114

formazan is 23,800 M-1cm-1.22 Singlet oxygen formation was monitored by the loss of FFA with an

115

initial concentration of 0.05 mM.26, 27 The FFA concentration was measured at the detection wavelength

116

of 220 nm using an HPLC with a Zorbax Eclipse XDB-C18 column (Agilent 1100, Agilent

117

Technologies, USA). The mobile phase was 30% acetonitrile and 70% 0.1 wt% phosphoric acid at a

118

flow rate of 1 mL/min. The production of •OH/lower-energy hydroxylating species was quantified by

119

the loss of TPA.23, 25, 28 TPA was added at a concentration of 0.5 mM into the NOM solutions. The

120

residual TPA was quantified by HPLC using a mobile phase of 30% acetonitrile/70% 0.1 wt %

121

phosphoric acid and detection wavelength of 254 nm at a flow rate of 1 mL/min. H2O2 generation was

122

measured by the HRP (5 mg/L) catalyzed oxidation of 1 mM DPD.24 The stable oxidation product,

123

DPD•+, was measured by its absorbance at 551 nm using a UV-vis spectrometer.

124

Batch sorption experiments. Batch sorption experiments were conducted in 40-mL polypropylene

125

centrifuge tubes filled with solutions containing 5 mg C/L SRNOM/SRHA and 0.1 to 20 mg/L Ag+. The

126

tubes were wrapped with aluminum foil and agitated on a reciprocating shaker at room temperature for

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1 h which was sufficient to achieve complexation equilibrium. A previous study suggested that the

128

complexation between Ag+ and NOM was almost instant (< 1s).29 The free Ag+ was detected using a

129

silver ion-selective electrode (9616BNWP, Thermo, USA) with a potentiometer (ORION 5 STAR,

130

Thermo, USA) at room temperature. The amount of AgCl formation was calculated by the loss of Cl- in

131

solution which was determined using an ion chromatography (ICS-1000, Dionex, USA) with a Dionex

132

IonPac AS11-HC analytical column (250 mm × 4 mm). The mobile phase (1.0 mL/min) was 10 mM

133

KOH. The concentrations of AgCl2- and AgCl32- were several orders of magnitude lower than AgCl at

134

experimental conditions and were consequently not considered in the mass balance.30 The NOM-

135

complexed Ag+ at a specific equilibrium concentration was calculated based on the mass balance.

136 137

Results and Discussion


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Threshold concentration of Ag(I) for the NOM-mediated photo-formation of nAg

(a )

(b ) .75

1.0

Absorbance

.8

.6

.4

.60

Abs @ 410nm

0 min 0.5 min 1 min 2 min 4 min 6 min 10 min 20 min 30 min 60 min

0.0 200

.30

0.00 300

400

500

600

700

0

800

Wavelength (nm)

(c )

10

20

30

40

50

60

Time (min)

(d ) 16

435

nAg Diameter (nm)

nAg Peak SPR Wavelength (nm) 140

.45

.15

.2

139

Sunlight Dark Control

430 425 420 415 410

14 12 10 8 6 4 2 0

405 0

10

20

30

40

50

60

0

10

20

30

40

50

60

Time (min)

Time (min)

141

Figure 1. (a) UV-vis spectra of 20 mg/L Ag(I) and 5 mg C/L SRNOM during the simulated sunlight

142

irradiation at pH 7.0±0.2. (b) Formation kinetics of nAg during simulated sunlight irradiation and under

143

dark condition in the presence of 20 mg/L Ag(I) and 5 mg C/L SRNOM, as quantified by the SPR

144

absorbance at 410 nm. (c) Evolution of the SPR wavelength during the photo-formation of nAg. (d)

145

Particle diameter of nAg as a function of irradiation time as measured by dynamic light scattering.

146 147

The UV-vis absorption spectra of the solution containing 20 mg/L Ag(I) and 5 mg C/L SRNOM


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during the simulated sunlight irradiation are shown in Figure 1a. The characteristic peak of the SPR of

149

nAg at 409 – 431 nm was observed during the irradiation, which was consistent with previous photo-

150

reduction studies using humic substances.14, 16 An absorption minimum appeared at 320 – 334 nm; this

151

was caused by the interband transition in nAg, which led to the damping of plasmon oscillations.31

152

Figure 1b presents the formation kinetics of nAg under simulated sunlight exposure. The intensity of the

153

SPR peak increased with longer irradiation time and plateaued after 40 min. Meanwhile, the wavelength

154

of the SPR peak shifted from 409 nm to 431 nm in the initial 10 min, indicating the growth of nAg

155

particle size upon simulated sunlight exposure (Figure 1c).32,

156

remained constant afterwards, suggest the particle size plateaued. The DLS measurements showed a

157

sharp increase of nAg size in 20 min, generally consistent with the initial shift of SPR peaks (Figure 1d).

158

However, the particle size measured by DLS still increased after 20 min irradiation. This could be

159

attributed to morphology changes and the aggregation of nAg.14, 34

33

The wavelength of the SPR peak

160

161

(b )

.14

.35

SRNOM SRHA

.12

Ag(I) 5mg/L Ag(I) 7mg/L Ag(I) 8mg/L

.30

Abs @ 410nm

Initial Formation Rate r (min-1)

(a )

.10 .08 .06 .04

.25 .20 .15 .10

.02

.05

0.00 0

5

10

15

20

Ag(I) Concentration (mg/L)


0

1

2

3

4

5

6

Irradiation Time (h)

8

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(c )

Abs @ 410nm

.5

Ag(I) 10mg/L Ag(I) 11mg/L Ag(I) 12mg/L

.4

.3

.2

.1

0

162

1

2

3

4

5

6


163

Figure 2. (a) The initial formation rate of nAg in 5 mg C/L SRNOM or SRHA solutions as a function of

164

the Ag(I) concentration under simulated sunlight irradiation. Formation kinetics of nAg under simulated

165

sunlight in the presence of 5 mg C/L (b) SRNOM and (c) SRHA at given concentrations of Ag(I) as

166

quantified by the SPR absorbance at 410 nm. The solution pH was 7.0±0.2, and the total chloride

167

concentration was 0.5 mg/L.

168 169

To better understand the role of Ag(I) concentration and NOM properties in the photo-reduction

170

process, nAg formation kinetics were compared in the presence of SRNOM and SRHA. The initial

171

formation rate of nAg, r = (dA(t)/dt)t0, was determined by linear regression of the initial increase of

172

the SPR absorbance at 410 nm. The regression was carried out over 2 min irradiation (see an example in

173

Figure S5). The r has been reported to be proportional to the concentration of Ag(I) and NOM:15

174

r =k[Ag(I)][NOM]

(1)

175

Figure 2a shows the initial formation rate of nAg as a function of Ag(I) concentration in irradiated 5

176

mg C/L NOM solutions. It is worth noting that the Ag(I) concentration here refers to the total silver

177

concentration including all silver species. As the NOM concentration was set to be 5 mg C/L, r was

178

expected to increase with increasing Ag(I) concentration according to equation 1. However, the profiles


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observed were comprised of two distinct regimes: Regime I, at low Ag(I) concentration, no SPR

180

absorbance of nAg was observed; Regime II, once the Ag(I) concentration reached a threshold (referred

181

to as critical induction concentration, CIC), r increased with increasing Ag(I) concentration. The CIC

182

was 5 mg/L Ag(I) for SRNOM and 10 mg/L Ag(I) for SRHA as suggested by the long-term irradiation

183

tests (i.e., 6h) (Figure 2b and 2c). It is worth noting that the CIC could be lower than what it appears to

184

be in Figure 2a due to the extremely low initial formation rate near CIC, which can be determined in the

185

long-term irradiation tests. The CICs are much higher than the detection limit of the SPR method, 0.01

186

mg/L, and consequently can not be attributed to the detection method of nAg. The initial formation rate

187

of nAg in NOM solutions decreased with decreasing light intensity (Figure S6a). Nevertheless, the CIC

188

remained constant under simulated sunlight with different light intensities (Figure S6b).

(a )

(b )

Intensity (a.u.)

Intensity (a.u.)

Ag (111)

AgCl (200)

AgCl (200)

Ag (111) Ag (200) 30

189

40

50

60

70

30

2θ θ (degrees)

40

50

60

70

2θ θ (degrees)


10

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(d )

(c )

Ag (111) AgCl (200) 30

190

Intensity (a.u.)

Intensity (a.u.)

Ag (111)

Ag (200)

40

50

AgCl (200)

Ag (220) 60

70

30

2θ θ (degrees)

Ag (220)

Ag (200) 40

50

60

70

2θ θ (degrees)

191

Figure 3. XRD spectra of samples containing (a) 3 mg/L Ag(I) (below CICSRNOM) and (b) 10 mg/L Ag(I)

192

(above CICSRNOM) in 5 mg C/L SRNOM solutions; (c) 5 mg/L Ag(I) (below CICSRHA) and (d) 15 mg/L

193

Ag(I) (above CICSRHA) in 5 mg C/L SRHA solutions after 30 min simulated sunlight irradiation. The

194

solution pH was 7.0±0.2, and the initial chloride concentration was 0.5 mg/L.

195 196

The XRD pattern of the samples initially containing Ag(I) above CIC showed characteristic peaks for

197

the (111), (200), and (220) planes of metallic silver (Figure 3b and 3d),35 as well as a characteristic peak

198

(2θ=32.24°) for the (200) plane of AgCl. The total Cl- concentration in the SRNOM and SRHA

199

solutions was 0.5 mg/L, leading to the formation of AgCl (see the discussion about silver speciation

200

below). The co-existence of metallic silver and AgCl during the photo-formation of nAg mediated by

201

riverine NOM was also reported in a previous study.15 The characteristic peaks of metallic silver were

202

found in the XRD pattern of the sample initially containing Ag(I) below CIC (i.e., no SPR observed) but

203

with very low intensity (Figure 3a and 3c). Note that the detection limit of XRD is around 5 %. The

204

small amount of metallic silver detected could be attributed to the presence of dispersed Ag0 or small

205

silver clusters which cannot induce SPR. Previous study suggested that noble metal clusters, such as Au

206

clusters, with diameter smaller than 5 nm show little SPR, but that with diameter of 5-50 nm show a


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sharp SPR.36-38 According to the UV-vis and XRD data, the photo-formation of nAg was minimal if not

208

completely eliminated under CIC.

209

(b )

(a ) 100

C Ag species (%)

80

C Ag species (%)

100

Free Ag+ + Complexed Ag AgCl

60

40

20

80

60

40

20

0

0 0

5

10

15

20

0

Total Ag(I) Concentration (mg/L)

210

Free Ag+ Complexed Ag+ AgCl

5

10

15

20

Total Ag(I) Concentration (mg/L)

(c ) 1.2e+6 SRNOM SRHA

qe(mg/kg C)

1.0e+6 8.0e+5 6.0e+5 4.0e+5 2.0e+5 0.0 0

211

3

6

9

12

15

18

Ce (mg/L)

212

Figure 4. Speciation of silver in 5 mg C/L (a) SRNOM and (b) SRHA solutions plotted as percentages

213

of silver species versus the initial total Ag(I) concentration; (c) sorption isotherms plotted as complexed

214

concentration (qe, mg/kg C) versus aqueous-phase concentration (Ce, mg/L) of Ag+ at equilibrium in the

215

presence of 5 mg C/L SRNOM or SRHA at pH 7.0±0.2. The total chloride concentration was 0.5 mg/L.


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Sorption experiments were run in triplicates.

217

The concentrations of silver species including free Ag+, NOM-complexed Ag+, and AgCl in 5 mg C/L

218

NOM solutions were summarized in Figure 4. The silver were mostly in the form of free Ag+ and

219

NOM-complexed Ag+. Over the entire Ag(I) concentration tested, free Ag+ constituted 20~75 % of total

220

silver in SRNOM solution and 21~68 % of total silver in SRHA solution. NOM-complexed Ag+

221

constituted 20~73 % of total silver in SRNOM solution and 26~67 % of total silver in SRHA solution.

222

The AgCl formed at low Ag+ concentrations due to its small solubility product, 1.77×10-10.2 Its

223

concentration stabled after the majority of chloride in the system was scavenged. The speciation

224

diagram suggests that the CIC was not caused by the precipitation of Ag+ as AgCl.

225

The interplay of reduction and oxidation reactions affected the CIC

226

The observation of a minimum threshold concentration, CIC, implies the presence of antagonistic

227

reactions to the photo-reductive formation of nAg. We hypothesize that this can be attributed to the

228

interplay of reduction and oxidation reactions between phototransients and silver species, which control

229

the nucleation and growth of silver clusters. Concurrent oxidation/dissolution and reduction reactions

230

are previously reported to occur during the light-induced size/morphology evolution of nAg

231

suspension.13, 14, 17, 18 In many cases, simulated sunlight was found to accelerate the dissolution of nAg

232

through photo-oxidation processes.13, 19 However, little is known regarding how interactions between

233

oxidative phototransients and silver species influence the initial photo-formation of nAg in NOM

234

solutions.

235

NOM can reduce Ag+ to silver atoms (Ag0) under simulated sunlight irradiation through two possible

236

pathways: (1) direct transfer of electrons to complexed Ag+ through the LMCT pathway, or (2)

237

generation of O2− which in turn reduces Ag+ (equation 2 & 3).14, 15, 39

238

Ag+ + e- Ag0

(2)

239

Ag+ + O2− Ag0 + O2

(3)

240

Ag0 + Ag0 Ag02

(4) (Ag0 forms dimers when collide)40 ACS Paragon Plus Environment

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241

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Agm + Agn Agm+n (5) (Further coalescence results in silver clusters and eventually nAg particles)41

242

(a )

(b ) FFA Concentration (mM)

XTT Formazan (mM)

.004 SRNOM SRHA Control

.003

.002

.001

0.000


.04 .03 .02 .01 0.00

0.0

.5

1.0

1.5

2.0

2.5

3.0

0


243

1

2

3

4

5

6


(c )

(d )

4

3

2

1 SRNOM SRHA

0

.50

.48

.46


.42

.40 0

244

TPA Concentration (mM)

H2O2 Concentration (µM)

5

10

20

30

40

50

60

Irradiation Time (min)

0

4

8

12

16

20

24


245

Figure 5. (a) XTT formazan production, representing O2− generation; (b) FFA degradation,

246

representing 1O2 generation; (c) H2O2 generation; and (d) TPA degradation, representing •OH

247

generation as a function of irradiation time with 26.8 mg C/L NOM under simulated sunlight. Error

248

bars represent ± one standard deviation from the average of triplicate tests.

249 250

Under solar irradiation, SRNOM and SRHA can be excited, generating singlet excited state NOM ACS Paragon Plus Environment

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(1NOM*) and charge-separated species (NOM+/-).42 NOM+/- can react with the complexed Ag+ through

252

the LMCT pathway. 1NOM* further undergo energy transfer or charge transfer reactions to form

253

various phototransients including NOM+/-, O2−, 1O2, H2O2, and •OH/lower-energy hydroxylating species,

254

which are expected to mediate the redox reactions of silver species.42, 43 The O2− generation in SRNOM

255

and SRHA solutions was quantified and compared in Figure 5a. A recent study suggested that the XTT

256

assay was not specific to O2− in some cases.44 Nevertheless, the reduction of XTT was almost

257

completely inhibited in the presence of 15 mg/L superoxide dismutase (Figure S7), confirming the

258

generation of O2−.44 The rate of O2− generation in the SRNOM solution was higher than that in the

259

SRHA solution (Figure 5a). Other than reductive O2−, SRHA and SRNOM are known to generate

260

oxidative ROS including

261

irradiation,45, 46 which are compared in Figure 5b, 5c, and 5d respectively. SRNOM produced similar

262

amount of 1O2 as compared with SRHA (Figure 5b). The apparent singlet oxygen quantum yields of

263

SRNOM and SRHA were calculated to be 1.60% and 1.16% (see details in SI), respectively, generally

264

consistent with previous reported values (i.e., 2.02±0.23% and 1.81% for SRNOM; 1.38±0.08% and

265

1.60±0.08% for SRHA).20,

266

species than SRHA (Figure 5c and 5d). The lower •OH/lower-energy hydroxylating species generation

267

rate of SRNOM agrees with a previous report.25 The oxidizing species, including 1O2, H2O2, and

268

•OH/lower-energy hydroxylating species can readily oxidize Ag0 or nAg at neutral pH.10, 17, 48-51 The

269

steady-state concentration of 1O2 and •OH/lower-energy hydroxylating species in 5 mg C/L SRNOM

270

solution are calculated to be 3.2 × 10-13 and 1.3 × 10-16 M, respectively (see details in SI). If assume a

271

diffusion-limited reaction constant between Ag0 and these phototransients (k = 1×1010 M-1s-1)39, the

272

reaction rate constants are 3.2×10-3 s-1 and 1.6 × 10-6 s-1 for 1O2 and •OH/lower-energy hydroxylating

273

species, respectively. It is worth noting that there is strong microheterogeneity of phototransients in

274

irradiated NOM solutions.52 For example, the 1O2 in NOM micro-environment, [1O2]

275

higher than [1O2] measured by FFA method. The reactions between Ag0 and 1O2 happened mostly in

1

O2, H2O2, and •OH/lower-energy hydroxylating species under solar

45, 47

SRNOM generated less H2O2 and •OH/lower-energy hydroxylating


NOM,

is much

15


Page 16 of 28

276

NOM. If use the [1O2]NOM/[1O2] of 130 from the literature,52 the reaction rate constant between Ag0 and

277

1

278

generated by NOM within a short time scale. Previous studies suggested that the interactions between

279

these oxidizing species and citrate-coated/bare nAg facilitated their photo-induced dissolution process.17,

280

53

281

hydroxylating species than SRHA, consistent with its lower CIC (i.e., NOM with higher O2− generation

282

and lower 1O2/H2O2/•OH/lower-energy hydroxylating species generation possesses lower CIC value).

O2 can be as high as 0.42 s-1. This indicates that Ag0 can potentially be oxidized by phototransients

SRNOM generated more reductive O2−, similar amount of 1O2, but less H2O2 and •OH/lower-energy

283

(b )

.18 Control 20% Isopropyl alcohol 30% Isopropyl alcohol

.16 .14 .12 .10 .08 .06 .04 .02 0.00 0

4

8

12

16



(a )

.14 Control 10% D2O

.12

40% D2O

.10 .08 .06 .04 .02 0.00


5

10

15

20

25

30


284

Figure 6. The initial formation rate of nAg in 5 mg C/L SRNOM solutions as a function of the Ag(I)

285

concentration in the presence of (a) isopropyl alcohol and (b) D2O under simulated sunlight irradiation.

286

The solution pH was 7.0±0.2, and the initial chloride concentration was 0.5 mg/L.

287 288

In order to further probe the interplay of oxidation and reduction reactions in the photo-formation of

289

nAg, we carried out scavenger tests. Isopropyl alcohol was first introduced into the reaction system as a

290

radical scavenger, especially for •OH/lower-energy hydroxylating species.54, 55 The CIC of SRNOM

291

decreased from 5 mg/L to 2 mg/L as the concentration of isopropyl alcohol increased from 0 to 30% as

292

shown in Figure 6a. It suggests that the CICSRNOM can be lowered by scavenging oxidative radicals in


16

Page 17 of 28


293

the system. Another test was carried out in the presence of D2O as shown in Figure 6b. The presence of

294

D2O had no impact on the measurement of nAg SPR (Figure S8). D2O can promote the lifetime of 1O2

295

(16-fold greater than in H2O solution)56 and consequently increase its steady-state concentration. This is

296

caused by the kinetic solvent isotope effect as the 1O2 deactivation rate constant is much lower in D2O

297

than that in H2O.56 Meanwhile, the H2O2 generation rate by SRNOM was also expected to increase in

298

the presence of D2O.57 The CICSRNOM was found to increase from 5 mg/L to 15 mg/L as the

299

concentration of D2O increased from 0 to 10% (Figure 6b). In the presence of 40 % D2O, the nAg

300

formation kinetics was low even at 30 mg/L Ag(I). These results suggest that the CIC can be

301

manipulated by scavenging or promoting the oxidative phototransients, indicating that the CIC can be at

302

least partially attributed to the balance between redox reactions.

303

As the Ag(I) concentration exceeded CIC, nAg started to form. This indicates that the balance

304

between redox reactions can be modulated by the Ag(I) concentration. We speculate that higher Ag(I)

305

concentration facilitates the nucleation and growth of silver clusters. On one hand, the presence and

306

growth of silver clusters can facilitate the photo-reduction process by increasing the redox potential of

307

Ag+ and inducing the catalytic reduction of Ag+.39, 58-60 Free Ag+ in solutions has a redox potential of -

308

1.8 V (Ag+ + e- Ag0; E0 = -1.8 V vs NHE).59 Thus silver atoms are not stable compared to the Ag+/e-

309

couple. In the presence of silver clusters, Ag+ was mainly reduced on the cluster surface. The redox

310

potential of Ag+ increases with increasing co-existing silver cluster size and eventually reaches +0.8 V

311

in the presence of nAg particles.60,

312

electrons to oxygen or Ag+, catalyzing the reduction reactions of Ag+.39, 58 It has been reported that the

313

presence of nAg led to 4 times faster Ag+ reduction by O2−.39 On the other hand, the growth of silver

314

clusters hindered the oxidation process as larger silver clusters are more resistant to oxidation and

315

subsequent dissolution due to its higher redox potentials and lower specific surface area.2,

316

speculate that higher Ag(I) concentration can facilitate the initial nucleation and growth of silver

317

clusters by increasing the kinetics of coalescence among Ag0 (Equation 4) or small silver clusters

61

Silver clusters were suggested to rapidly store and transfer


62, 63

We

17


Page 18 of 28

318

(Equation 5). It may also promote the kinetics of surface Ag+ reduction on silver clusters which leads to

319

their growth. Once the Ag(I) concentration exceeds the CIC, silver clusters undergo fast coalescence

320

and surface Ag+ reduction, which help them grow to a critical size at which reduction outcompetes

321

oxidation and subsequent dissolution.

322

At low silver concentrations, a significant part of silver will be complexed with NOM. An earlier

323

study suggests that the photo-reduction process involves Ag+ binding to NOM.14 Thus, the initial

324

formation of silver clusters can be affected by the complexed Ag+ concentration in NOM molecules.

325

The complexed Ag+ concentration was much higher than that in the bulk solution depending on the

326

complexation affinity of NOM. The carbon-normalized Ag+ complexation affinities of SRNOM and

327

SRHA were compared in Figure 4c. SRHA had stronger complexation affinity than SRNOM at high

328

silver concentrations, consistent with a previous study on the Ag+-NOM interactions.29 Their

329

complexation affinities were similar at low silver concentrations. At the Ag(I) concentrations of 5 mg/L

330

(CICSRNOM) and 10 mg/L (CICSRHA), the complexed Ag+ concentrations in SRNOM and SRHA were

331

similar. Thus, the differed CIC of SRNOM and SRHA was not caused by the different complexed Ag+

332

concentration in NOM. The higher ability of SRNOM in mediating the photo-formation of nAg is most

333

likely due to its higher reductive phototransient generation but lower oxidative phototransient generation.

334

The impact of chloride on the CIC


18



Page 19 of 28

335

.16 .14

0 mg/L Cl0.23mg/L Cl-

.12

0.5mg/L Cl-

.10

1 mg/L Cl-

.08 .06 .04 .02 0.00 0

4

8

12

16

20


336

Figure 7. The initial formation rate of nAg in 5 mg C/L SRNOM solutions as a function of the Ag(I)

337

concentration at various chloride concentrations under simulated sunlight irradiation. The solution pH

338

was 7.0±0.2.

339 340

Chloride is ubiquitous in natural aquatic systems and is known to strongly influence the speciation of

341

silver and the dissolution of nAg.18 In order to test the role of Cl- in the photo-formation of nAg, nAg

342

formation kinetics in SRNOM solution was determined at various Cl- concentrations as shown in Figure

343

7. The CICSRNOM was found to decrease with increasing Cl- concentration. It decreased from 10 mg/L to

344

3 mg/L as the Cl- concentration increased from 0.23 mg/L to 1 mg/L. In another experiment, we used

345

dialysis method to remove the Cl- ion from the SRNOM solution (data labeled as 0 mg/L Cl- in Figure

346

7). There was no nAg formation in the dialyzed SRNOM solution at silver concentration of 20 mg/L.

347

Note that part of the low molecular weight fraction of SRNOM (~15%) was removed during dialysis. A

348

previous study suggested that all size fractions of SRNOM were photoactive and the difference of size

349

fractions on the photo-reduction of silver was caused by the differential light attenuation.12 Thus, the

350

inhibited nAg formation in the dialyzed SRNOM solution was attributed to the removal of Cl-, rather

351

than the removal of low molecular weight fraction of SRNOM.


19


Page 20 of 28

352

AgCl is a semiconductor photocatalyst which generates electron-hole pairs under simulated

353

sunlight.64 The electron transfer can reduce Ag+ at the surface of AgCl which produces Ag0 atoms that

354

potentially combine with adjacent atoms to yield silver clusters or nAg.30, 64 Furthermore, on irradiation,

355

AgCl itself can partially transform into nAg.65,

356

photocatalyst.64 A recent study also suggested that an important role of AgCl in the photo-reduction of

357

Ag+ in the presence of peptides was to provide deposition surfaces.67 On the other hand, The co-existing

358

AgCl is also known to generate various oxidizing species including holes, chlorine atom, •OH,

359

hydrogen peroxide, and carbonate radical.68 The formation of AgCl also decreased the total amount of

360

free and NOM-complexed Ag+. Our results suggest that the overall effect of AgCl in the tested

361

conditions is to facilitate the photo-formation of nAg. Nevertheless, the presence of AgCl does not

362

necessarily lead to the formation of nAg through aforementioned mechanisms in our systems (Figure 4a

363

and 4b). This result indicates that photo-formation of nAg is still controlled by the redox balance after

364

taking the role of AgCl into consideration.

365

Environmental implications

66

This process is often used to prepare Ag@AgCl

366

Our work demonstrates the existence of a threshold concentration of Ag+ for the photo-formation of

367

nAg mediated by NOM. The CIC is defined by the photochemistry of NOM and is influenced by the

368

water chemistry. The CIC of NOM in natural conditions could be lower than the CIC values of SRNOM

369

and SRHA which are both in the milligram per liter range. However, the Ag+ concentration in natural

370

surface waters is in the range of ng – µg/L,69,

371

determined in our experimental settings. The growth of silver clusters is expected to be slow due to the

372

extremely low mass transfer at environmental concentrations, which may not be sufficiently fast to

373

outcompete oxidative dissolution. Thus, the possibility of NOM-mediated photo-formation of nAg in

374

natural aquatic conditions needs to be further scrutinized. The existence of a threshold also indicates that

375

some of the results acquired using high Ag+ concentrations cannot be linearly extrapolated to

376

environmental conditions. The photo-formation of nAg could be potentially relevant for surface waters

70

several orders of magnitude lower than the CIC


20

Page 21 of 28


377

heavily impacted by silver input, such as wastewater effluents or mine drainage with comparatively high

378

silver concentrations. In that case, the most abundant silver species is often silver sulfide, which can be

379

transformed into nAg by photo-induced Fe redox cycling.71 In another possible scenario, the photo-

380

formation of nAg could be triggered by pre-existing silver seeds. Silver clusters can be generated from

381

the reduction of Ag+ in the presence of microbes, extracellular polymeric substances, or humic

382

substances under natural conditions without light exposure,16, 72-74 serving as seeds for faster photo-

383

reduction reactions.

384 385

ASSOCIATED CONTENT

386

Supporting Information

387

Dissolved oxygen concentration in the test solution during the irradiation, the experimental setup for

388

irradiation experiments, the spectra of the Xenon lamp and sunlight, the detection limit of UV-vis

389

method for nAg, the calculation of initial formation rate of nAg, the initial formation rate of nAg in

390

SRNOM solutions as a function of the Ag(I) concentration under simulated sunlight with different light

391

intensities, XTT assay with SOD, UV-vis spectra of nAg in D2O, and the calculation of apparent

392

quantum yield of singlet oxygen can be found in the Supporting Information. This material is available

393

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

394 395

AUTHOR INFORMATION

396

Corresponding Author

397

*Xiaolei Qu; Phone: +86-025-8968-0256; email: [email protected]

398 399

ACKNOWLEDGMENTS

400

We thank Ziyan Du, Zhixian Li, Yutong Xu, and Yufan Li for their assistance in measuring nAg

401

photo-formation kinetics. This work was supported by the National Key Basic Research Program of ACS Paragon Plus Environment

21


Page 22 of 28

402

China (Grant 2014CB441103), the National Natural Science Foundation of China (Grant 21407073,

403

21622703, 21225729, and 21237002), and the Department of Science and Technology of Jiangsu

404

Province (BE2015708). We thank the State Key Laboratory of Environmental Chemistry and

405

Ecotoxicology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences for

406

partial funding (KF2014-11). Partial funding was also provided by the NSF ERC on Nanotechnology-

407

Enabled Water Treatment (EEC-1449500).

408 409

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TOC Initial Formation Rate r (min-1)

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Page 28 of 28

CAg(I) Above NOM’s CIC

.14 SRNOM SRHA

.12 .10 .08

CAg(I) Below NOM’s CIC

.06 .04 .02 0.00 0

5

10

15

20


Sunlight Irradiation Time


28

A Threshold Concentration of Silver Ions Exists for Sunlight-Induced

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