Threshold Concentrations of Silver Ions Exist for the Sunlight-Induced

Mar 5, 2018 - Sunlight-induced photoformation of silver nanoparticles (nAg), mediated by natural organic matter (NOM), is significantly affected by th...
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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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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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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

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

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

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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

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

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

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

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

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

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

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

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both important.

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Introduction

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

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

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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.

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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

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

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

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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-

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16

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

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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

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

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

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

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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

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

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

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

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

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

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

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

19

However, little is known

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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)

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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.

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Materials and Methods

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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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

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

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

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

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

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

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

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

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

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

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

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

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the Xenon lamp off.

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

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

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

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

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

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

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

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

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

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

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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

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

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

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

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

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

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

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

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

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

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

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

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

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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

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

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

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

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

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

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

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

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

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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

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nAg Diameter (nm)

nAg Peak SPR Wavelength (nm) 140

.45

.15

.2

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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)

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

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

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

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

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

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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

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

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

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

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

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

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

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

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

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

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

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

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The wavelength of the SPR peak

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(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)

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2

3

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5

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Irradiation Time (h)

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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

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1

2

3

4

5

6

Irradiation Time (h)

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

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

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

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

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concentration was 0.5 mg/L.

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

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

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

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

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

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

(1)

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

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

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

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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

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

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

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

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

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

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

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

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

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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

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40

50

60

70

30

2θ θ (degrees)

40

50

60

70

2θ θ (degrees)

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(d )

(c )

Ag (111) AgCl (200) 30

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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)

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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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

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completely eliminated under CIC.

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(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)

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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)

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

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

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

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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.

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

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

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

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

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

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

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

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

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The interplay of reduction and oxidation reactions affected the CIC

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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

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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

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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

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solutions.

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

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

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

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Ag+ + e-  Ag0

(2)

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Ag+ + O2−  Ag0 + O2

(3)

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Ag0 + Ag0  Ag02

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

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

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(a )

(b ) FFA Concentration (mM)

XTT Formazan (mM)

.004 SRNOM SRHA Control

.003

.002

.001

0.000

.05 SRNOM SRHA Control

.04 .03 .02 .01 0.00

0.0

.5

1.0

1.5

2.0

2.5

3.0

0

Irradiation Time (h)

243

1

2

3

4

5

6

Irradiation Time (h)

(c )

(d )

4

3

2

1 SRNOM SRHA

0

.50

.48

.46

.44 SRNOM SRHA Control

.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

Irradiation Time (h)

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

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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

Initial Formation Rate r (min-1)

Initial Formation Rate r (min-1)

(a )

.14 Control 10% D2O

.12

40% D2O

.10 .08 .06 .04 .02 0.00

Ag(I) Concentration (mg/L)

5

10

15

20

25

30

Ag(I) Concentration (mg/L)

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

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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

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We

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(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

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.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

Ag(I) Concentration (mg/L)

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.

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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

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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

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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

REFERENCES

410

(1)

411

08/20/2015.

412

(2)

413

nanoparticles: Impact on stability and toxicity. Environ. Sci. Technol. 2012, 46, 6900-6914.

414

(3)

415

specific antibacterial activity of silver nanoparticles. Nano Lett. 2012, 12, 4271-4275.

416

(4)

417

oxygen on antimicrobial activity of silver nanoparticles versus silver ions. Environ. Sci. Technol. 2011,

418

45, 9003-9008.

419

(5)

420

growth: Effect of pH, concentration, and organic matter. Environ. Sci. Technol. 2009, 43, 7285-7290.

421

(6)

422

Bernhardt, E. S., More than the ions: The effects of silver nanoparticles on Lolium multiflorum.

423

Environ. Sci. Technol. 2011, 45, 2360-2367.

424

(7)

425

available sock fabrics. Environ. Sci. Technol. 2008, 42, 4133-4139.

426

(8)

427

from consumer products used in the home. J. Environ. Qual. 2010, 39, 1875-1882.

428

(9)

429

nanometer particle within pores of silica host. J. Appl. Phys. 1998, 83, 1705-1710.

430

(10)

431

Environ. Sci. Technol. 2010, 44, 2169-2175.

The project on emerging nanotechnologies. http://www.nanotechproject.org/. Accessed on

Levard, C.; Hotze, E. M.; Lowry, G. V.; Brown, G. E., Environmental transformations of silver

Xiu, Z. M.; Zhang, Q. B.; Puppala, H. L.; Colvin, V. L.; Alvarez, P. J. J., Negligible particle-

Xiu, Z. M.; Ma, J.; Alvarez, P. J. J., Differential effect of common ligands and molecular

Fabrega, J.; Fawcett, S. R.; Renshaw, J. C.; Lead, J. R., Silver nanoparticle impact on bacterial

Yin, L.; Cheng, Y.; Espinasse, B.; Colman, B. P.; Auffan, M.; Wiesner, M.; Rose, J.; Liu, J.;

Benn, T. M.; Westerhoff, P., Nanoparticle silver released into water from commercially

Benn, T.; Cavanagh, B.; Hristovski, K.; Posner, J. D.; Westerhoff, P., The release of nanosilver

Cai, W.; Zhong, H.; Zhang, L., Optical measurements of oxidation behavior of silver

Liu, J.; Hurt, R. H., Ion release kinetics and particle persistence in aqueous nano-silver colloids.

ACS Paragon Plus Environment

22

Page 23 of 28

Environmental Science & Technology

432

(11) Lowry, G. V.; Espinasse, B. P.; Badireddy, A. R.; Richardson, C. J.; Reinsch, B. C.; Bryant, L. D.;

433

Bone, A. J.; Deonarine, A.; Chae, S.; Therezien, M.; Colman, B. P.; Hsu-Kim, H.; Bernhardt, E. S.;

434

Matson, C. W.; Wiesner, M. R., Long-term transformation and fate of manufactured Ag nanoparticles in

435

a simulated large scale freshwater emergent wetland. Environ. Sci. Technol. 2012, 46, 7027-7036.

436

(12)

437

Photoreduction and stabilization capability of molecular weight fractionated natural organic matter in

438

transformation of silver ion to metallic nanoparticle. Environ. Sci. Technol. 2014, 48, 9366-9373.

439

(13)

440

nanoparticles in aquatic environments: Chemical and morphology change induced by oxidation of Ag0

441

and reduction of Ag+. Environ. Sci. Technol. 2014, 48, 403-411.

442

(14)

443

natural organic matter: Formation and transformation of silver nanoparticles. Environ. Sci. Technol.

444

2013, 47, 7713-7721.

445

(15)

446

nanoparticles by dissolved organic matter. ACS Nano 2012, 6, 7910-7919.

447

(16)

448

S., Interactions of aqueous Ag+ with fulvic acids: Mechanisms of silver nanoparticle formation and

449

investigation of stability. Environ. Sci. Technol. 2013, 47, 757-764.

450

(17)

451

toxicity reduction of silver nanoparticles in the presence of perfluorocarboxylic acids under UV

452

irradiation. Environ. Sci. Technol. 2014, 48, 4946-4953.

453

(18)

454

acid, and light exposure on the long-term fate of silver nanoparticles. Environ. Sci. Technol. 2016, 50,

455

12214-12224.

456

(19)

457

Pellarin, M., Photo-oxidation of individual silver nanoparticles: A real-time tracking of optical and

458

morphological changes. J. Phys. Chem. C 2013, 117, 2274-2282.

459

(20)

460

matter optical properties and quantum yields of singlet oxygen and hydrogen peroxide. Environ. Sci.

461

Technol. 2010, 44, 5824-5829.

462

(21)

463

production of reactive oxygen species by single-walled carbon nanotubes in water. Carbon 2011, 49,

464

5099-5106.

465

(22)

Yin, Y. G.; Shen, M. H.; Zhou, X. X.; Yu, S. J.; Chao, J. B.; Liu, J. F.; Jiang, G. B.,

Yu, S. J.; Yin, Y. G.; Chao, J. B.; Shen, M. H.; Liu, J. F., Highly dynamic PVP-coated silver

Hou, W. C.; Stuart, B.; Howes, R.; Zepp, R. G., Sunlight-driven reduction of silver ions by

Yin, Y. G.; Liu, J. F.; Jiang, G. B., Sunlight-induced reduction of ionic Ag and Au to metallic

Adegboyega, N. F.; Sharma, V. K.; Siskova, K.; Zboril, R.; Sohn, M.; Schultz, B. J.; Banerjee,

Li, Y.; Niu, J.; Shang, E.; Crittenden, J., Photochemical transformation and photoinduced

Zhou, W.; Liu, Y. L.; Stallworth, A. M.; Ye, C.; Lenhart, J. J., Effects of pH, electrolyte, humic

Grillet, N.; Manchon, D.; Cottancin, E.; Bertorelle, F.; Bonnet, C.; Broyer, M.; Lermé, J.;

Dalrymple, R. M.; Carfagno, A. K.; Sharpless, C. M., Correlations between dissolved organic

Chen, C. Y.; Jafvert, C. T., The role of surface functionalization in the solar light-induced

Sutherland, M. W.; Learmonth, B. A., The tetrazolium dyes MTS and XTT provide new ACS Paragon Plus Environment

23

Environmental Science & Technology

Page 24 of 28

466

quantitative assays for superoxide and superoxide dismutase. Free Radic. Res. 1997, 27, 283-289.

467

(23) Fu, H. Y.; Liu, H. T.; Mao, J. D.; Chu, W. Y.; Li, Q. L.; Alvarez, P. J. J.; Qu, X. L.; Zhu, D. Q.,

468

Photochemistry of dissolved black carbon released from biochar: Reactive oxygen species generation

469

and phototransformation. Environ. Sci. Technol. 2016, 50, 1218-1226.

470

(24)

471

formation through electron transfer from carboxylated single-walled carbon nanotubes in water.

472

Environ. Sci. Technol. 2014, 48, 11330-11336.

473

(25)

474

organic matter-sensitized photohydroxylation reactions. Environ. Sci. Technol. 2011, 45, 2818-2825.

475

(26) Haag, W. R.; Hoigne, J.; Gassman, E.; Braun, A. M., Singlet oxygen in surface waters. 1. Furfuryl

476

alcohol as a trapping agent. Chemosphere 1984, 13, 631-640.

477

(27)

478

multiwalled carbon nanotubes: Role of reactive oxygen species. Environ. Sci. Technol. 2013, 47, 14080-

479

14088.

480

(28)

481

generated hydroxyl radical. J. Environ. Monitor. 2010, 12, 1658-1665.

482

(29) Mousavi, M. P. S.; Gunsolus, I. L.; Pérez De Jesús, C. E.; Lancaster, M.; Hussein, K.; Haynes, C.

483

L.; Bühlmann, P., Dynamic silver speciation as studied with fluorous-phase ion-selective electrodes:

484

Effect of natural organic matter on the toxicity and speciation of silver. Sci. Total Environ. 2015, 537,

485

453-461.

486

(30) Liu, J.; Sonshine, D. A.; Shervani, S.; Hurt, R. H., Controlled release of biologically active silver

487

from nanosilver surfaces. ACS Nano 2010, 4, 6903-6913.

488

(31)

489

aqueous suspensions via hydrogen reduction. J. Phys. Chem. B 2004, 108, 13948-13956.

490

(32)

491

dependence of surface plasmon resonances. Proc. Natl. Acad. Sci. U. S. A. 2010, 107, 14530-14534.

492

(33)

493

nanoparticles:  The influence of size, shape, and dielectric environment. J. Phys. Chem. B 2003, 107,

494

668-677.

495

(34)

496

water. Environ. Sci. Technol. 2012, 46, 5378-5386.

497

(35) Sun, Y.; Xia, Y., Shape-controlled synthesis of gold and silver nanoparticles. Science 2002, 298,

498

2176-2179.

499

(36)

Hsieh, H. S.; Wu, R. R.; Jafvert, C. T., Light-independent reactive oxygen species (ROS)

Page, S. E.; Arnold, W. A.; McNeill, K., Assessing the contribution of free hydroxyl radical in

Qu, X. L.; Alvarez, P. J. J.; Li, Q. L., Photochemical transformation of carboxylated

Page, S. E.; Arnold, W. A.; McNeill, K., Terephthalate as a probe for photochemically

Evanoff, D. D.; Chumanov, G., Size-controlled synthesis of nanoparticles. 1. "Silver-only"

Peng, S.; McMahon, J. M.; Schatz, G. C.; Gray, S. K.; Sun, Y. G., Reversing the size-

Kelly, K. L.; Coronado, E.; Zhao, L. L.; Schatz, G. C., The optical properties of metal

Li, X.; Lenhart, J. J., Aggregation and dissolution of silver nanoparticles in natural surface

Henglein, A.; Meisel, D., Radiolytic control of the size of colloidal gold nanoparticles. ACS Paragon Plus Environment

24

Page 25 of 28

Environmental Science & Technology

500

Langmuir 1998, 14, 7392-7396.

501

(37)

502

Optical spectrum, controlled growth, and some chemical reactions. Langmuir 1999, 15, 6738-6744.

503

(38)

504

J. Phys. Chem. B 2002, 106, 7729-7744.

505

(39)

506

charging of silver nanoparticles. Environ. Sci. Technol. 2011, 45, 1428-1434.

507

(40) Ershov, B. G.; Janata, E.; Henglein, A.; Fojtik, A., Silver atoms and clusters in aqueous solution:

508

Absorption spectra and the particle growth in the absence of stabilizing Ag+ ions. J. Phys. Chem. 1993,

509

97, 4589-4594.

510

(41) Belloni, J., Nucleation, growth and properties of nanoclusters studied by radiation chemistry:

511

Application to catalysis. Catal. Today 2006, 113, 141-156.

512

(42)

513

chromophoric dissolved organic matter (CDOM) optical and photochemical properties. Environ. Sci.-

514

Process Impacts 2014, 16, 654-671.

515

(43) McNeill, K.; Canonica, S., Triplet state dissolved organic matter in aquatic photochemistry:

516

Reaction mechanisms, substrate scope, and photophysical properties. Environ. Sci.-Process Impacts

517

2016, 18, 1381-1399.

518

(44)

519

tetrazolium salts and its impact on superoxide assaying. Chem. Commun. 2016, 52, 11595-11598.

520

(45)

521

species (ROS) from effluent organic matter. Environ. Sci. Technol. 2014, 48, 12645-12653.

522

(46)

523

intermediates from effluent organic matter: The role of chemical constituents. Water Res. 2017, 112,

524

120-128.

525

(47) Mostafa, S.; Rosario-Ortiz, F. L., Singlet oxygen formation from wastewater organic matter.

526

Environ. Sci. Technol. 2013, 47, 8179-8186.

527

(48)

528

Soc. 1992, 139, 1963-1970.

529

(49)

530

Environ. Sci. Technol. 2009, 43, 8113-8118.

531

(50)

532

interactions among silver nanoparticles, silver ions, and reactive oxygen species. Langmuir 2012, 28,

533

10266-10275.

Henglein, A., Radiolytic preparation of ultrafine colloidal gold particles in aqueous solution: 

Kamat, P. V., Photophysical, photochemical and photocatalytic aspects of metal nanoparticles.

Jones, A. M.; Garg, S.; He, D.; Pham, A. N.; Waite, T. D., Superoxide-mediated formation and

Sharpless, C. M.; Blough, N. V., The importance of charge-transfer interactions in determining

Zhao, J. F.; Zhang, B. W.; Li, J. Y.; Liu, Y. C.; Wang, W. F., Photo-enhanced oxidizability of

Zhang, D. N.; Yan, S. W.; Song, W. H., Photochemically induced formation of reactive oxygen

Zhou, H.; Lian, L.; Yan, S.; Song, W., Insights into the photo-induced formation of reactive

Graedel, T. E., Corrosion mechanisms for silver exposed to the atmosphere. J. Electrochem.

Geranio, L.; Heuberger, M.; Nowack, B., The behavior of silver nanotextiles during washing.

He, D.; Garg, S.; Waite, T. D., H2O2-mediated oxidation of zero-valent silver and resultant

ACS Paragon Plus Environment

25

Environmental Science & Technology

Page 26 of 28

534

(51)

He, D.; Jones, A. M.; Garg, S.; Pham, A. N.; Waite, T. D., Silver nanoparticle−reactive oxygen

535

species interactions: Application of a charging−discharging model. J. Phys. Chem. C 2011, 115, 5461-

536

5468.

537

(52) Latch, D. E.; McNeill, K., Microheterogeneity of singlet oxygen distributions in irradiated humic

538

acid solutions. Science 2006, 311, 1743-1747.

539

(53)

540

uncoated silver, gold, nickel, and silicon nanoparticles and their antibacterial effects. Langmuir 2013,

541

29, 4647-4651.

542

(54)

543

propanol by OH radical. J. Phys. Chem. A 1997, 101, 926-936.

544

(55) Ryan, C. C.; Tan, D. T.; Arnold, W. A., Direct and indirect photolysis of sulfamethoxazole and

545

trimethoprim in wastewater treatment plant effluent. Water Res. 2011, 45, 1280-1286.

546

(56) Wilkinson, F.; Helman, W. P.; Ross, A. B., Rate constants for the decay and reactions of the

547

lowest electronically excited singlet-state of molecular-oxygen in solution: An expanded and revised

548

compilation. J. Phys. Chem. Ref. Data 1995, 24, 663-1021.

549

(57)

550

peroxide from natural organic matter. Geochim. Cosmochim. Acta 2011, 75, 4310-4320.

551

(58)

552

and discharging the colloidal silver microelectrode. J. Am. Chem. Soc. 1981, 103, 1059-1066.

553

(59) Linnert, T.; Mulvaney, P.; Henglein, A.; Weller, H., Long-lived nonmetallic silver clusters in

554

aqueous solution: Preparation and photolysis. J. Am. Chem. Soc. 1990, 112, 4657-4664.

555

(60)

556

the use of hydroquinone as a selective reducing agent. Langmuir 2009, 25, 2613-2621.

557

(61)

558

metal and semiconductor particles. Chem. Rev. 1989, 89, 1861-1873.

559

(62)

560

nanoparticles. J. Am. Chem. Soc. 2010, 132, 70-72.

561

(63) Henglein, A., Physicochemical properties of small metal particles in solution: "Microelectrode"

562

reactions, chemisorption, composite metal particles, and the atom-to-metal transition. J. Phys. Chem.

563

1993, 97, 5457-5471.

564

(64)

565

Ag@AgCl: A highly efficient and stable photocatalyst active under visible light. Angew. Chem.-Int.

566

Edit. 2008, 47, 7931-7933.

567

(65)

Zhang, W.; Li, Y.; Niu, J. F.; Chen, Y. S., Photogeneration of reactive oxygen species on

Luo, N.; Kombo, D. C.; Osman, R., Theoretical studies of hydrogen abstraction from 2-

Garg, S.; Rose, A. L.; Waite, T. D., Photochemical production of superoxide and hydrogen

Henglein, A.; Lilie, J., Storage of electrons in aqueous-solution: The rates of chemical charging

Gentry, S. T.; Fredericks, S. J.; Krchnavek, R., Controlled particle growth of silver sols through

Henglein, A., Small-particle research: Physicochemical properties of extremely small colloidal

Ivanova, O. S.; Zamborini, F. P., Size-dependent electrochemical oxidation of silver

Wang, P.; Huang, B. B.; Qin, X. Y.; Zhang, X. Y.; Dai, Y.; Wei, J. Y.; Whangbo, M. H.,

Garg, S.; Ma, T.; Miller, C. J.; Waite, T. D., Mechanistic insights into free chlorine and reactive ACS Paragon Plus Environment

26

Page 27 of 28

Environmental Science & Technology

568

oxygen species production on irradiation of semiconducting silver chloride particles. J. Phys. Chem. C

569

2014, 118, 26659-26670.

570

(66)

571

thermo-chemical transformation of AgCl and Ag2S in environmental matrices and its implication.

572

Environ. Pollut. 2017, 220, Part B, 955-962.

573

(67)

574

B., Electron transfer in peptides: On the formation of silver nanoparticles. Angew. Chem.-Int. Edit.

575

2015, 54, 2912-2916.

576

(68)

577

semiconducting silver chloride particles: Kinetics and modelling. J. Colloid Interface Sci. 2015, 446,

578

366-372.

579

(69)

580

natural waters by inductively-coupled plasma mass spectrometry: Influence of organic matter. Mar.

581

Chem. 2006, 98, 109-120.

582

(70)

583

from contaminated sediments of San Francisco Bay, California, U.S.A. Mar. Chem. 1997, 56, 15-26.

584

(71)

585

nanoparticles in natural waters by photoinduced Fe(II, III) redox cycling. Environ. Sci. Technol. 2016,

586

50, 13342-13350.

587

(72)

588

K., Humic acid-induced silver nanoparticle formation under environmentally relevant conditions.

589

Environ. Sci. Technol. 2011, 45, 3895-3901.

590

(73)

591

Gardea-Torresdey, J. L., Enhanced formation of silver nanoparticles in Ag+-NOM-Iron(II, III) systems

592

and antibacterial activity studies. Environ. Sci. Technol. 2014, 48, 3228-3235.

593

(74)

594

+

Ag to silver nanoparticles and antagonize bactericidal activity. Environ. Sci. Technol. 2014, 48, 316-

595

322.

Yin, Y.; Xu, W.; Tan, Z.; Li, Y.; Wang, W.; Guo, X.; Yu, S.; Liu, J.; Jiang, G., Photo- and

Kracht, S.; Messerer, M.; Lang, M.; Eckhardt, S.; Lauz, M.; Grobety, B.; Fromm, K. M.; Giese,

Ma, T.; Garg, S.; Miller, C. J.; Waite, T. D., Contaminant degradation by irradiated

Ndung'u, K.; Ranville, M. A.; Franks, R. P.; Flegal, A. R., On-line determination of silver in

Rivera-Duarte, I.; Flegal, A. R., Pore-water silver concentration gradients and benthic fluxes

Li, L.; Zhou, Q.; Geng, F.; Wang, Y.; Jiang, G., Formation of nanosilver from silver sulfide

Akaighe, N.; MacCuspie, R. I.; Navarro, D. A.; Aga, D. S.; Banerjee, S.; Sohn, M.; Sharma, V.

Adegboyega, N. F.; Sharma, V. K.; Siskova, K. M.; Vecerova, R.; Kolar, M.; Zboril, R.;

Kang, F. X.; Alvarez, P. J. J.; Zhu, D. Q., Microbial extracellular polymeric substances reduce

596

ACS Paragon Plus Environment

27

Environmental Science & Technology

598 599

TOC Initial Formation Rate r (min-1)

597

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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

Ag(I) Concentration (mg/L)

Sunlight Irradiation Time

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