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New Insights into the Stability of Silver Sulfide Nanoparticles in Surface Water: Dissolution through Hypochlorite Oxidation Lingxiangyu Li, Zhenlan Xu, Andreas Wimmer, Qinghua Tian, and Xin Ping Wang Environ. Sci. Technol., Just Accepted Manuscript • Publication Date (Web): 13 Jun 2017 Downloaded from http://pubs.acs.org on June 14, 2017

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Environmental Science & Technology

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New Insights into the Stability of Silver Sulfide Nanoparticles in Surface Water: Dissolution through

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Hypochlorite Oxidation

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Lingxiangyu Li,*,† Zhenlan Xu,§ Andreas Wimmer,‡ Qinghua Tian† and Xinping Wang†

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†

School of Sciences, Zhejiang Sci-Tech University, Hangzhou 310018, China

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§

Institute of Quality and Standard of Agro-Products, Zhejiang Academy of Agricultural Sciences, Hangzhou

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310021, China

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‡

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Division of Analytical Chemistry, Department of Chemistry, Technical University of Munich, Garching

85748, Germany

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Corresponding Author:

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∗

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E-mail: [email protected]

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Tel: +86 571 86843228

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Fax: +86 571 86843600

Dr. Lingxiangyu Li

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ACS Paragon Plus Environment


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TOC

25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 2


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ABSTRACT

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Silver sulfide nanoparticles (Ag2SNPs) are considered to be stable in the environment due to the extreme low

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solubility of Ag2S (Ksp: 6.3×10-50). Little is known about the stability of Ag2SNPs in surface water disinfected

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with aqueous chlorine, one of the globally most used disinfectants. Our results suggested that both uncoated

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and polyvinylpyrrolidone (PVP)-coated Ag2SNPs (100 µg/L) underwent dissolution in surface water

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disinfected with aqueous chlorine at a dose of 4 mg/L, showing the highest dissolved silver ion concentrations

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of 22.3 and 10.5 µg/L within 45 min, respectively. The natural organic matter (NOM) and dissolved oxygen

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(DO) posed effects on the Ag2SNPs dissolution by chlorine; NOM accelerated Ag2SNPs dissolution while DO

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reduced the rate and extent of Ag2SNPs dissolution. We further demonstrated that Ag2SNPs dissolution was

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primarily attributed to active oxidative substances including hydroxyl radical and H2O2 originating from the

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hypochlorite oxidation. Additionally, water containing Ag2SNPs disinfected with hypochlorite showed stronger

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interference on the zebra fish (Danio rerio) embryo hatching than Ag2SNPs and hypochlorite on their own.

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This work documented that Ag2SNPs could undergo dissolution in surface water through hypochlorite

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oxidation, posing potential risks to aquatic organisms, and therefore showed new insights into the stability of

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Ag2SNPs in natural environment.

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INTRODUCTION

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The occurrence of silver sulfide nanoparticles (Ag2SNPs) in natural environment was primarily attributed to

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sulfidation of silver nanoparticles (AgNPs), being one of the most promising engineered nanomaterials used in

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products all over the world.1,2 Laboratory and field studies have shown that AgNPs could be converted almost

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entirely to Ag2SNPs in the wastewater treatment system, along with a buildup of Ag2SNPs in sewage sludge.3,4

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Thus, measurable levels of Ag2SNPs have been observed in sewage sludge and even surface water.5-7

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Previous studies showed good stability of Ag2SNPs in soil and compost even over a six months period.8,9

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Furthermore, sulfidation has been considered as natural antidote for AgNPs toxicity because several reports

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documented that Ag2SNPs posed substantially reduced toxicity to aquatic organisms and microorganisms

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compared to that of AgNPs due to the negligible solubility of Ag2S.10,11 In a recent study, nevertheless, we

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observed that Ag2SNPs could undergo dissolution in the aquatic environment containing ferric ions through the

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hydroxyl radical formed during reduction of Fe(III) to Fe(II) in the light ,12 indicating that the stability of

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Ag2SNPs might be overestimated in past studies. More recently, Kaegi et al.13 reported that oxidation of

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Ag2SNPs in wastewater effluent by ozone resulted in a substantial increase in acute toxicity to green algae.

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Therefore, it is essential to investigate the stability of Ag2SNPs under various environmental conditions to gain

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comprehensive understanding of its risks to human and environmental health.

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Chlorination has been widely applied for drinking water disinfection, which was found to degrade organic

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pollutants rapidly, along with the formation of disinfection by-products (DBPs).14,15 To date, however, little is

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known about the stability of Ag2SNPs during water disinfection with free chlorine such as aqueous chlorine,

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one of the globally most used disinfectants. Potential dissolution of Ag2SNPs would pose risks, in particular

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when the levels of silver-based nanoparticles (e.g., Ag2SNPs) in surface water gradually increase.6,7,16

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Moreover, high chlorine doses and long contact times are required in some cases.17 In recent, it has been

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reported that exposure to aqueous chlorine can affect the oxidation of AgNPs in water.18 Accordingly, can Ag+ 4


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be remobilized from Ag2SNPs by aqueous chlorine? How can Ag2SNPs dissolution occur in the surface water

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disinfected with free chlorine? More importantly, what is the impact that Ag2SNPs dissolution has on aquatic

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organisms?

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Herein, the primary aim of this work was to investigate the stability of Ag2SNP in surface water disinfected

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with aqueous chlorine at safe levels recommended by the United States Environmental Protection Agency (U.S.

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EPA).19 Two types of Ag2SNP, namely uncoated Ag2SNPs (U-Ag2SNPs) and polyvinylpyrrolidone

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(PVP)-coated Ag2SNPs (P-Ag2SNPs), were comprehensively examined. We investigated the Ag2SNPs

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dissolution in the presence of hypochlorite under different conditions, and proposed the potential pathway of

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Ag2SNPs dissolution by hypochlorite on the basis of experimental data. We further explored the toxicity of

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Ag2SNPs dissolution to zebra fish (Danio rerio) embryo development to evaluate the impact of aqueous

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chlorine disinfection on the risk of Ag2SNPs in the aquatic environment.

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MATERIALS AND METHODS

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Materials. In this study the reagents except for the natural organic matter (NOM) were purchased from

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Sigma-Aldrich (St. Luis, USA). The NOM (2R101N) from the Suwannee River was acquired from the

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International Humic Substance Society (Denver, USA). Ultrapure water from a Direct-Q-system (Millipore,

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Billerica, USA) with a resistivity of 18.2 MΩ/cm was used for preparation of all solutions. The P-Ag2SNPs

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with an average size of 57.2 ± 5.3 nm were prepared as described in our previous study.20 The U-Ag2SNPs

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with an average size of 107.8 ± 19.5 nm (on the basis of size measurement through TEM images, n = 156)

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were prepared by reaction of AgNO3 with NaSH in aqueous solution.21 Here the hydrodynamic size of

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U-Ag2SNPs was 201.2 ± 17.3 nm by using dynamic light scattering (DLS, Malvern Zetasizer Nano-ZS90),

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with a polydispersity index of 0.609. In brief, an aqueous solution of NaSH (50 mL, 1 mM) was added to

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AgNO3 solution (50 mL, 2 mM) under vigorous stirring. The transparent AgNO3 solution turned bright yellow

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immediately. After 3 min, the reaction mixture gradually turned tawny. After 30 min of mixing, precipitate was 5



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observed at the bottom of the vessel. The precipitate was concentrated by using centrifugation (9384 g, 30 min)

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and purified by using ultrapure water for three cycles. Following 30 min of ultrasonication (KQ-600E, 40 kHz,

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600 W) at about 0oC (ice-water mixture), the U-Ag2SNPs stock solution was stored at 4oC for later use. The

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U-Ag2SNPs and P-Ag2SNPs should undergo ultrasonication (KQ-600E, 40 kHz, 600 W) for 30 min at 0oC to

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disperse NPs before use.

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Effect of Chlorine Disinfection on the Stability of Ag2SNPs in Surface Water. River, lake, and landscape

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water samples were collected manually in Hangzhou city, Zhejiang, China. Briefly, river water was taken from

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the Qiantang River. Lake water was collected from the West Lake, and landscape water was from a fountain in

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front of the labrary on the campus of Zhejiang Sci-Tech University. The landscape water originated from an

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effluent of a wastewater treatment plant to save water resource. All samples were collected in 500 mL

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polyvinyl chloride (PVC) containers, which were rinsed threefold with the sample before collection. Samples

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were filtered through a 0.45 µm glass fiber filter (Pall Corporation, Michigan, USA) and stored at 4oC until use.

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The basic characterization of surface water is shown in the Table S1 (Supporting Information). The total silver

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concentrations of surface water were all below 0.1 µg/L (limit of detection) based on inductively coupled

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plasma mass spectrometry (ICP-MS, Agilent 8800, USA) measurement. The dissolved organic carbon (DOC)

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concentration of samples was determined with a TOC-L total organic carbon analyzer (Shimadzu, Germany),

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showing the DOC concentrations of river, lake and landscape water were 3.6 ± 0.1, 3.0 ± 0.1 and 2.7 ± 0.2

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mg/L (Table S1), respectively. The samples were spiked with stock Ag2SNPs and NaClO, leading to

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concentrations of 100 µg/L Ag2SNPs and 4 mg/L free chlorine respectively, followed by stirring (100 rpm) at

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room temperature. A chlorine dose of 4 mg/L was used due to the recommendation for drinking water

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chlorination by the U.S. EPA.19 Afterward, a sample of 4 mL was taken from each bottle at certain intervals

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(e.g., 0, 0.5, 8, 20, 30, 45, 60 and 120 min). Centrifugal filtration (Amicon Ultra-4, 3kD, Millipore) at 9384 g

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for 8 min was used to collect the dissolved silver ions released from Ag2SNPs followed by ICP-MS 6


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measurement to observe the dissolution of Ag2SNPs in surface water disinfected by hypochlorite. To validate

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the centrifugal filtration at 9384 g for 8 min, the recovery efficiency of 100 µg/L AgNO3 solution is 95.7 ±

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1.9%, suggesting negligible loss of the analyte during the centrifugal filtration (9384 g, 8 min). All

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experiments were performed in triplicate.

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Effect of NOM and DO on the Ag2SNPs Dissolution by Aqueous Chlorine. The P-Ag2SNP and U-Ag2SNP

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stock solutions were diluted with ultrapure water in glass bottles (100 mL) followed by adding NaClO stock

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solution (40 mg/L) prepared by sodium hypochlorite solution with an available chlorine content of 4.00-4.99%.

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Finally, different volumes of NaClO stock solution were added to Ag2SNPs solution, yielding 0.1, 1 and 4

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mg/L NaClO with 100 µg/L Ag2SNPs respectively. The homogenized mixtures (pH: ～8) were stirred (100

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rpm) at room temperature. At each time interval (0, 0.5, 8, 20, 30, 45, 60 and 120 min), a 4-mL aliquot of the

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aqueous sample was taken from each bottle. The dissolved silver ions collected with centrifugal filtration at

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9384 g for 8 min were quantified using ICP-MS. All experiments were performed in triplicate.

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The effect of NOM on the dissolution of Ag2SNPs by chlorine was investigated. A NOM stock solution with

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the determined content of 100 mg/L DOC was prepared dissolving the dry powder in ultrapure water, followed

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by filtration using a 0.45 µm membrane syringe. ICP-MS measurement confirmed no silver impurities (< 0.1

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µg/L) in the NOM. Different volumes of the so prepared NOM stock solution were added to P-Ag2SNP and

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U-Ag2SNP solutions based on the desired DOC concentrations (2 and 10 mg/L). Afterward, NaClO stock

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solution was added to the mixture of Ag2SNPs and NOM, followed by stirring, sampling and silver ions

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measurement as mentioned above.

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To examine the role of dissolved oxygen (DO) in the dissolution of Ag2SNPs by hypochlorite, experiments

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using (99.999%) N2-purged (4 h) deoxygenated ultrapure water for preparing mixtures of Ag2SNPs and

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hypochlorite were performed in a glove box with N2 headspace. Then, a 4-mL aliquot of an aqueous sample

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was taken from each bottle at intervals (0, 0.5, 8, 20, 30, 45, 60 and 120 min), followed by centrifugal filtration 7



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and silver ions measurement.

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Chlorine Measurement. In the present study, concentrations of ClO- were measured applying the DPD

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(N,N-diethyl-p-phenylenediamine) method as developed by Garg et al.18 In brief, 50 mM phosphate buffer was

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prepared by mixing 75 mM NaH2PO4 and 25 mM Na2HPO4, and 6 mM DPD stock solution was prepared by

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dissolving 30 mg DPD in 30 mL of 50 mM H2SO4 solution. Then, 300 µL of the phosphate buffer and 100 µL

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of the DPD stock solution were added to 2.6 mL of each sample, followed by vortexing for 5 seconds. The

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mixture was measured at 551 nm using a UV-visible spectrophotometer (UV-1800, Shimadzu). Considering

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potential DPD oxidation caused by other oxidants in the sample, 100 µL of a 1.5 mM glycine was firstly added

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to 2.5 mL of each sample to remove ClO-,18,22 followed by 300 µL of phosphate buffer and 100 µL of DPD

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stock solution. The slight changes in the absorbance of samples with glycine were observed during the

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Ag2SNPs dissolution (Figure S1). The concentrations of ClO- in samples were calculated according to the

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absorbance of a prepared NaClO calibration.

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Effect of Radical Scavengers on the Dissolution of Ag2SNPs by Chlorine. To examine the contribution of

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active oxidative substances to the dissolution of Ag2SNPs by chlorine, effects of radical scavengers including

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tertiary butyl alcohol (TBA), catalase, and superoxide dismutase (SOD) on the Ag2SNPs dissolution were

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investigated. Here TBA (20 mM), catalase (1000 U/mL) and SOD (1000 U/mL) stock solutions were added to

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the mixture of Ag2SNPs (100 µg/L) and hypochlorite (4 mg/L) to yield 2 mM TBA, 300 U/mL catalase and

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300 U/mL SOD, respectively. These mixtures were stirred (100 rpm) at room temperature, and samples were

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collected at intervals (0, 0.5, 8, 20, 30, 45, 60 and 120 min), followed by determination of silver ions using

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centrifugal filtration and ICP-MS.

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Toxicity of Ag2SNPs Dissolution to Zebra Fish (Danio rerio) Embryos. Zebra fish (Danio rerio) were

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cultured at 25ºC and a photoperiod of 14 h light and 10 h dark. The embryos were collected, counted, and

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rinsed several times in ultrapure water to remove any residue on the embryo surface. Twenty healthy embryos 8


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[～4 hour postfertilization (hpf)] were incubated in each well of 12-well transparent plates (NEST Biotech,

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USA). 3 mL of Ag2SNPs (100 µg/L), dispersion, NaClO (4 mg/L) solution, or mixture of Ag2SNPs and NaClO

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were added to each well. Throughout the whole exposure period (3 d), solutions were changed every 24 h and

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the development status of embryos was observed to assess the embryo mortality and hatching rate. Three

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replicate trials were conducted in this study.

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RESULTS AND DISCUSSION

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Effect of Chlorine Disinfection on the Stability of Ag2SNPs in Surface Water. The effect of chlorine

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disinfection on the stability of Ag2SNPs in environmental water was investigated by spiking hypochlorite into

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surface water with 100 µg/L Ag2SNPs. Dissolution of Ag2SNPs was observed (Figure 1); both P-Ag2SNPs and

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U-Ag2SNPs rapidly released silver ions within the first 45 min, showing the highest silver ion concentrations

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of 22.3 ± 2.1 and 10.5 ± 1.3 µg/L, respectively. This suggests that hypochlorite for surface water disinfection at

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the dose recommended by the U.S. EPA could result in Ag2SNPs dissolution. Moreover, compared to

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U-Ag2SNPs, P-Ag2SNPs showed a higher rate and extent of dissolution (Table S2 and Figure 1), implying that

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P-Ag2SNPs likely underwent dissolution by hypochlorite in surface water. For example, the highest rate

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constant of P-Ag2SNPs dissolution was 1.02 min-1, which is much higher than that of U-Ag2SNPs (0.60 min-1)

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(Table S2). Also, the highest rate constants of P-Ag2SNPs and U-Ag2SNPs were observed in the river sample

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(Table S2), which might be related to the NOM, since the river water showed the highest DOC level(Table S1).

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To comprehensively understand the Ag2SNPs dissolution by hypochlorite, the kinetics, potential pathways and

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impact were further investigated.

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Dissolution Kinetics of Ag2SNPs by Chlorine. Both P-Ag2SNPs and U-Ag2SNPs showed negligible

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dissolution in the absence of hypochlorite (Figure 1 and Figure 2A,B), while a measurable amount of silver

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ions was observed in the presence of aqueous chlorine (Figure 2), even at hypochlorite concentrations of 0.1

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mg/L, far lower than the chlorine disinfection dose recommended by the U.S. EPA. This suggests that 9



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hypochlorite is a key factor controlling the dissolution of Ag2SNPs. The time-resolved content of silver ions

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was highly dependent on the concentration of hypochlorite; the higher the concentration of hypochlorite, the

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larger the extent of Ag2SNPs dissolution (Figure 2A,B). Besides this extent, the dissolution rate of Ag2SNPs

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also increased with the increase in the initial hypochlorite concentration (Figure 2A,B). Moreover, the amount

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of dissolved silver rapidly increased again when a second equivalent of hypochlorite was added to the solution

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(Figure S2), demonstrating the key role of hypochlorite in the Ag2SNPs dissolution. At a dose of 4 mg/L

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hypochlorite 15.3 ± 2.5 and 17.1 ± 1.1 µg/L silver ions (120 min) were measured for P-Ag2SNPs and

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U-Ag2SNPs, respectively, being an order of magnitude higher than that at dose of 0.1 mg/L hypochlorite.

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As shown in Figure 2, P-Ag2SNPs show faster dissolution rates compared to U-Ag2SNPs, which may be

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attributed to the hydrophilic characterization of PVP. A previous study has already documented that

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hydrophilic coating could affect the dissolution dynamics and particularly accelerate the dissolution rate of

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nanoparticles.23 Also, the fast dissolution rates of P-Ag2SNPs may be related to their small size, since generally

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small size of NPs would undergo dissolution fast compared to big NPs.24 Accordingly, the time scale to reach

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the dissolution equilibrium was shorter for P-Ag2SNPs compared to U-Ag2SNPs; the amount of released silver

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ions from P-Ag2SNPs remained constant after a rapid increase within the first 45 min, while U-Ag2SNPs

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dissolution reached equilibrium at 60 min. Also, the molar ratio of ClO- to Ag2S was 30 to reach equilibrium of

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P-Ag2SNPs dissolution, while great excess of ClO- was needed for U-Ag2SNPs (Figure 2C). However, the

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amount of silver ions released from P-Ag2SNPs was comparable to that of U-Ag2SNPs at the same

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concentration of hypochlorite (Figure 2).

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Similarly, NOM posed effects to the dissolution of Ag2SNP by hypochlorite (Figure S3); the dissolution rate

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increased with increasing concentration of NOM, being likely attributed to the electron transfer-mediating role

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of NOM, which has been proven to be able to accelerate electron transfer as electron shuttle mediators.25

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Moreover, the increasing rate of Ag2SNP dissolution in the presence of NOM may be related to the stability of 10


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Ag+ by NOM due to formation of Ag+-NOM complex. A recent study on dissolution of sulfidized AgNPs

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showed that fulvic acid from the Pony lake accelerated the dissolution of partly-sulfidized AgNPs through

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formation of very stable complexs with Ag+.26 On the other hand, the extent of U-Ag2SNPs dissolution was

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greatly reduced by the presence of NOM, which is consistent with previous studies on that adsorption of NOM

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reduces dissolution of nanoparticles due to the increased colloidal stability.27

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Pathway of Ag2SNPs dissolution by ClO-. Here we demonstrated that both P-Ag2SNPs and U-Ag2SNPs

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could undergo dissolution in aqueous environment even with a lower dose of chlorine than recommended by

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the U.S. EPA. The chlorine-dependent dissolution of Ag2SNPs was influenced by DO; both extent and rate of

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dissolution dramatically increased with the decreasing concentration of DO (Figure 3), which is in contrast to

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the general consensus presenting that dissolution of metal-based NPs is accelerated by DO.28,29 Here, DO

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suppressed the dissolution of Ag2SNPs, which might be attributed to the production of O2 during Ag2SNPs

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dissolution by chlorine. As shown in Figure 4A, the concentration of DO gradually increases in the solution. In

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a recent study on AgNPs dissolution by chlorine, effect of O2 was dependent on the molar ratio of AgNP to

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OCl-; at [AgNP]/[ClO-] > 4, the dissolution of AgNPs was completely inhibited under deoxygenated condition,

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while little effect of O2 removal was observed on the AgNPs dissolution at [AgNP]/[ClO-] ≈ 1.18 Also, the pH

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value gradually decreased during Ag2SNPs dissolution (Figure 4B), indicating that OH- is consumed during the

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Ag2SNPs dissolution by ClO-. In general, hypochlorite oxidation is initiated by a single-electron oxidation,30

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resulting in the occurrence of ClO• radical which was observed applying spin-trap EPR using the stable spin

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adduct formed with 5,5-dimethyl-1-pyrroline N-oxide (DMPO) in this study (Figure S4). The ClO• radical can

247

react with OH- to form H+, Cl- and superoxide radical anions,31,32 resulting in the decrease of the solution’s pH

248

value. Accordingly, the free Cl- concentration increased within the first 10 min, followed by the gradual

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decrease (Figure 4C), which might be attributed to the formation of [AgClx]-(x-1) complexes due to the

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occurrence of silver ions in the solution with sufficient Cl-.33 11



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Sulfate was detected (ICS-1000, DIONEX) when Ag2SNPs underwent dissolution through hypochlorite

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(Figure 4D), suggesting the sulfur element of Ag2SNPs underwent oxidation to form the SO42- during Ag2SNPs

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dissolution by hypochlorite . A recent study reported that O3-induced Ag2SNPs oxidation occurred, along with

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the release of silver ions and sulfate formation.13 As shown in Figure 4E, the concentration of ClO- rapidly

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decreases from 4.7 to 4.0 mg/L in the initial time, and then gradually increases to 4.3 mg/L, keeping constant

256

along with the time. This suggests a ClO- depletion at first, followed by formation during the Ag2SNPs

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dissolution process. Hydroxyl radical and Cl- could form ClO- in the presence of DO through several reaction

258

steps as described in previous studies.34

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Additionally, the hydroxyl radical can oxidize most redox-sensitive elements in the solution because of its

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extremely high oxidation ability (standard reduction potential: 2.8 V).35 Here, we postulated that highly active

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oxidative substances like the hydroxyl radical and H2O2 may induce the dissolution of Ag2SNPs. To investigate

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whether the dissolution of Ag2SNPs by hypochlorite is primarily attributed to these active substances, the

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effect of radical scavengers such as TBA, SOD and catalase on the Ag2SNPs dissolution was examined.

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As shown in Figure 4F, the extent of Ag2SNPs dissolution is significantly influenced by 2 mM TBA; the

265

amount of released silver ions is reduced from 15.3 ± 2.5 to 10.8 ± 0.1 µg/L, demonstrating that the hydroxyl

266

radical contributes to the Ag2SNPs dissolution. Furthermore, we observed that the amount of silver ions was

267

negligible after addition of SOD or catalase (Figure 4F), which suggests that the Ag2SNPs dissolution was

268

completely inhibited by SOD and catalase. Taken together, this indicates that active oxidative substances, in

269

particular the hydroxyl radical and H2O2, might primarily contribute to the Ag2SNPs dissolution. The

270

generation of superoxide radical and H2O2 during the hypochlorite oxidation has been demonstrated in

271

previous studies.36,37 Here, the occurrence of H2O2 in the Ag2SNPs solution with ClO- was confirmed by using

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visual observation and instrumental analysis (Figures S5 and S6).

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On the basis of these findings, we proposed that the Ag2SNPs dissolution process was mainly attributed to 12


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highly active oxidative substances formed during the hypochlorite oxidation (Figure 5). We thus postulated

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that the dissolution of Ag2SNPs through hypochlorite oxidation may primarily proceed through the following

276

equations.

277

ClO-

278

ClO• + OH-

279

O2•- + H2O + ClO-

280

Ag2S H2O2/•OH

281

ClO• + e

(1)

H+ + Cl- + O2•-

(2)

H2O2 + •OH + Cl- + O2

(3)

Ag+ + SO42-

(4)

According to previous studies, hypochlorite oxidation was initiated by a single-electron oxidation, forming

282

ClO• radical (eq 1),30 followed by generation of superoxide radical anion through reactions between water and

283

ClO• radical, along with the occurrence of H+ and Cl- (eq 2).31,32 In general, superoxide radical anion could

284

further undergo reactions to form hydroxyl radical, O2, H2O2 and water. Here, we proposed that the superoxide

285

radical anion underwent interactions with hypochlorite in water to yield H2O2, O2 and •OH in the solution (eq

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3).38,39 And then the H2O2 together with hydroxyl radical primarily contributed to the Ag2SNPs dissolution

287

with a simultaneous oxidation of the sulfide to sulfate (eq 4). Evidently, our data supported the proposed

288

mechanism that Ag2SNPs dissolution was primarily attributed to the hypochlorite oxidation.

289

Environmental Implications. Sulfidation has been widely considered as natural antidote for metallic NPs; the

290

toxicity of AgNPs to aquatic organisms and microbes could be dramatically reduced by sulfidation due to the

291

negligible dissolution of Ag2SNPs in previous studies.10,11 In general, ionic silver primarily controls the

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toxicity of particles containing silver.40 The dissolution, therefore, potentially affects the hazard of Ag2SNPs,

293

which should be taken into account when assessing their risks to aquatic ecosystems. In the present study,

294

Ag2SNPs underwent dissolution by hypochlorite at a dose recommended by the U.S. EPA, indicating potential

295

risks to aquatic organisms. Indeed, Ag2SNPs dissolution completely inhibited zebra fish embryos from

296

hatching (Figure 6); higher than 90% embryos were dead in the first 24 h when exposed to Ag2SNPs in the 13



297

presence of hypochlorite, which interferes embryos hatching by directly affecting their viabilities, showing a

298

large reduction in the survival rate. In contrast, all embryos hatched when exposed to either P-Ag2SNPs or

299

U-Ag2SNPs alone (Figure 6), suggesting that the safety of Ag2SNPs is dependent on their stability.

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The rapid increase of silver ions released from Ag2SNPs by hypochlorite indicates that acute toxicity may

301

occur in the effluent of water disinfection by aqueous chlorine, in particular when the level of Ag2SNPs in

302

surface water is rapidly increasing due to the erosion of soils receiving a large amount of sewage sludge with

303

high concentrations of Ag2SNPs.5,41 Our findings provided robust evidence that stability and safety of

304

Ag2SNPs was likely overestimated in previous studies. Clearly, researchers should raise attention to the risk of

305

potential transformations, especially the dissolution of Ag2SNPs in surface water disinfected by aqueous

306

chlorine.

307 308

ASSOCIATED CONTENT

309

Supporting Information

310

Additional details of results are included. It includes the basic characterization of surface water (Table S1), the

311

rate constant (k) of Ag2SNPs (100 µg/L) dissolution in surface water disinfected with hypochlorite (Table S2),

312

the changes in the absorbance of samples with glycine through the Ag2SNPs dissolution (Figure S1), the effect

313

of ClO- on the P-Ag2SNPs dissolution, and a second equivalent of ClO- was added after 120 h (Figure S2),

314

effect of NOM on the dissolution of Ag2SNPs by hypochlorite (Figure S3), ClO• radical EPR signal observed

315

from NaClO solution (Figrue S4), and the visual observation and instrumental analysis confirmed the

316

occurrence of H2O2 through the Ag2SNPs dissolution by hypochlorite (Figures S5 and S6). This material is

317

available free of charge via the internet at http://pubs.acs.org.

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320

AUTHOR INFORMATION

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Corresponding Author

322

∗

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Notes

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The authors declare no competing financial interest.

E-mail: [email protected]

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ACKNOWLEDGEMENTS

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We thank the Zhejiang Province of Natural Science Foundation (2015C32039, LQ16B070003) and Science

328

Foundation of Zhejiang Sci-Tech University (17062003-Y) for financial support. The authors thank Miss Xiaoxi

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Yang (Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences) for providing the

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zebra fish embryo. The authors also thank the anonymous reviewers for their valuable comments and

331

suggestions on this work.

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8274-8281.

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Figure 1. Dissolution kinetics of Ag2SNPs (100 µg/L) in surface waters by aqueous chlorine (4 mg/L). (A)

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P-Ag2SNPs. (B) U-Ag2SNPs. Statistic analysis for dissolution rates of Ag2SNPs in the presence of

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hypochlorite at the initial 8 min showed that the highest rate was observed in the river water among the

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different surface water samples.

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Figure 2. Dissolution of Ag2SNPs by aqueous chlorine. (A) Effect of hypochlorite concentrations on the

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dissolution kinetics of P-Ag2SNPs. (B) Effect of hypochlorite concentrations on the dissolution kinetics of

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U-Ag2SNPs. (C) Dissolution of Ag2SNPs by different molar ratios of ClO- to Ag2S. It should be noted that the

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amount of silver ion was measured after Ag2SNPs (100 µg/L) mixed with different amount of NaClO for 120

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min, since the dissolution has been equilibrium after 120 min on the basis of our data.

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Figure 3. Effect of DO on the dissolution of Ag2SNPs by aqueous chlorine. Here the concentrations of

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Ag2SNPs and NaClO were 100 µg/L and 4 mg/L, respectively. (A) P-Ag2SNPs. (B) U-Ag2SNPs.

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Figure 4. Experimental data to support the proposed mechanism of Ag2SNPs dissolution by chlorine. (A) DO.

516

(B) pH. (C) Free Cl- concentration. (D) SO42- concentration. (E) ClO- concentration. (F) Effect of radical

517

scavengers on the Ag2SNPs dissolution by chlorine. The concentrations of P-Ag2SNPs and NaClO were

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simultaneously amplified by 8 folds for the cases A and B (namely concentrations of P-Ag2SNPs and NaClO

519

were 800 µg/L and 32 mg/L, respectively) to measure the changes of DO and pH. Similarly, for the case of D,

520

the concentrations of P-Ag2SNPs and NaClO were simultaneously amplified by 24 folds (namely 2.4 and 96

521

mg/L for P-Ag2SNPs and NaClO, respectively) to measure the SO42- changes. The other cases of C, E and F,

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the concentrations of P-Ag2SNPs and NaClO were 100 µg/L and 4 mg/L, respectively.

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Figure 5. Pictorial summary of the proposed pathway of Ag2SNPs dissolution through hypochlorite oxidation.

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Figure 6. Toxicity of NaClO (4 mg/L), Ag2SNPs (100 µg/L), and Ag2SNPs with NaClO to zebra fish (Danio

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rerio) embryos.

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New Insights into the Stability of Silver Sulfide ... - ACS Publications

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