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Removal of silver nanoparticles by coagulation processes
Accepted Manuscript Title: Removal of Silver Nanoparticles by Coagulation Processes Author: Qian Sun Yan Li Ting Tang Zhihua Yuan Chang-Ping YuQian Sun and Yan Li contributed equally to this work.
S0304-3894(13)00545-1 http://dx.doi.org/doi:10.1016/j.jhazmat.2013.07.066 HAZMAT 15291
To appear in:
Journal of Hazardous Materials
Received date: Revised date: Accepted date:
19-2-2013 22-7-2013 27-7-2013
Please cite this article as: Q. Sun, Y. Li, T. Tang, Z. Yuan, C.-P. Yu, Removal of Silver Nanoparticles by Coagulation Processes, Journal of Hazardous Materials (2013), http://dx.doi.org/10.1016/j.jhazmat.2013.07.066 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Removal of Silver Nanoparticles by Coagulation Processes
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Qian Sun1+, Yan Li1+, Ting Tang1,2, Zhihua Yuan1, Chang-Ping Yu1,*
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Chinese Academy of Sciences, Xiamen 361021, China
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Huainan 232001, China
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College of Earth and Environment, Anhui University of Science & Technology,
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Key Laboratory of Urban Environment and Health, Institute of Urban Environment,
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Qian Sun and Yan Li contributed equally to this work.
* Corresponding Author: Tel: (86)-592-6190768, Email: [email protected]
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ABSTRACT
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Commercial use of silver nanoparticles (AgNPs) will lead to a potential route for
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human exposure via potable water. Coagulation followed by sedimentation, as a
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conventional technique in the drinking water treatment facilities, may become an
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important barrier to prevent human from AgNP exposures. This study investigated the
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removal of AgNP suspensions by four regular coagulants. In the aluminum sulfate and
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ferric chloride coagulation systems, the water parameters slightly affected the AgNP
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removal. However, in the poly aluminum chloride and polyferric sulfate coagulation
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systems, the optimal removal efficiencies were achieved at pH 7.5, while higher or
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lower of pH could reduce the AgNP removal. Besides, the increasing natural organic
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matter (NOM) would reduce the AgNP removal, while Ca2+ and suspended solids
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concentrations would also affect the AgNP removal. In addition, results from the
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transmission electron microscopy and X-ray diffraction showed AgNPs or
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silver-containing nanoparticles were adsorbed onto the flocs. Finally, natural water
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samples were used to validate AgNP removal by coagulation. This study suggests that in the case of release of AgNPs into the source water, the traditional water treatment process, coagulation/ sedimentation, can remove AgNPs and minimize the silver ion concentration under the well optimized conditions.
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Keyword: Silver nanoparticles; Coagulation; Removal efficiency; Flocs; Silver ion
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release
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1. Introduction Nanoparticles (NPs) are defined as materials with at least one dimension less than
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100 nm. Because of the unique antimicrobial properties, silver NPs (AgNPs) are one
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of the most widely used NPs in a range of consumer goods such as food packaging,
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clothing, medical devices, and cleaning agents. The worldwide production of AgNPs
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was approximately 500 tons/year (2008), [1], and the widespread use inevitably leads
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to AgNP release into the environment [2]. Concerns have been raised about potential
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adverse effects of AgNPs on human health and the environment [3] Evidences have
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shown that AgNPs are toxic to cells [4], bacteria [5,6], algae [7], and other aquatic
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organisms [8]. Furthermore, there is also a risk that the use of AgNPs will lead to the
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development of antibiotic resistance among harmful bacteria [9], and may also
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damage or alter beneficial microbial communities [10].
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AgNPs can be diminished by aggregation in the aquatic environment. These
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processes are governed by the physicochemical properties of AgNPs, including size and surface charge, and are directly influenced by the solution condition, such as pH, natural organic matter (NOM), and ionic strength [11]. AgNPs appeared to agglomerate under acidic conditions or higher ionic strength, especially higher concentrations of divalent electrolytes [11-14]. For example, Ca2+ concentrations
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above 2 mM would enhance AgNP agglomeration [11]. However, the interactions of
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AgNPs with NOM are still not clear. Greater stability of citrate and PVP-coated
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AgNPs were observed in the presence of humic substances [11], while a negligible
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effect of aquatic fulvic acid was found on the agglomeration state of uncoated AgNPs 3
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[13]. Besides, the capping agents, which are used in the synthesis of NPs to prevent
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aggregation, may affect the stability of AgNPs [12]. In addition, AgNPs can be
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reduced by silver ion releasing into the aquatic environment. Previous study showed
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that more than 10% of silver ions were released from citrate coated AgNPs (2 mg/L)
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in the air-saturated water at pH 5.6 [15]. The silver ion releasing rates increased with
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temperature, and decreased with increasing pH or addition of NOM [15].
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AgNPs present in aquatic systems may represent a possible route for human
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exposure, especially if the AgNPs-contaminated aquatic system is used as the source
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of potable water. Coagulation, which is one of the most common processes for the
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water treatment to removal particles and NOM, provides a potential way to remove
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AgNPs or other engineered NPs. However, the related study received few attentions
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to date. Previous studies demonstrated the removal of CdTe NPs and C60 NPs by alum
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[16,17], and reported that multiwall carbon nanotubes (MWCNT) could be removed
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from aquatic system via coagulation [18]. Metal oxide NPs, including Fe2O3, ZnO,
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However, the performance of AgNP removal by coagulation has not been fully
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investigated. Moreover, the removal mechanism would be more complicated, since
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AgNPs could keep releasing silver ion in the aquatic system, which would partly
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cause the toxicity of AgNPs [10].
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TiO2, NiO and silica, were also reported to be partially removed by coagulants [19]. These studies have shown that NPs could be removed through coagulation but the removal efficiency varied significantly and was dependent on the NP types, coagulant types, and other aqueous conditions, such as pH, NOM and suspended solids.
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The present study attempts to fill this knowledge gap by investigating the
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removal efficiencies of AgNPs by coagulation and elucidating the mechanism.
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Considering AgNPs could keep releasing silver ion, the removal efficiencies were
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evaluated by determining both silver ion concentrations and total silver concentrations
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in the aquatic phase. Four regular coagulants, including aluminum sulfate, ferric
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chloride, poly aluminum chloride (PACl) and polyferric sulfate (PFS), were applied to
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investigate the effect of coagulant types on AgNP removal. Besides, different aqueous
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conditions, including pH, NOM, cation concentration, and suspended solids
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concentration, were investigated. Furthermore, the coagulation flocs were
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characterized for better understanding of removal mechanism. Finally, natural water
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samples were carried out for the validation of AgNP removal by coagulation.
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2. Material and methods
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2.1. AgNP synthesis
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AgNPs were synthesized from AgNO3 by adding NaBH4 with polyvinyl alcohol
(PVA) as a capping agent [6]. 5 mL of 14 mM NaBH4 was added into 90 mL 0.06% (wt) PVA solution. Afterwards, 5 mL of 14 mM AgNO3 was injected at the rate of one drop per sec. Following 10 min of 700 rpm stirring at room temperature, AgNPs were
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concentrated and purified by centrifugal ultrafiltration (Millipore, Amicon Ultra-15 3
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K, USA), rinsed twice with Milli-Q water (Millipore, 18.2 MΩ·cm, USA). Initial
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physic-chemical characterization was performed after the synthesis. AgNP stock
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suspensions were stored at 4 °C before use. No aggregation during storage was
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observed.
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2.2. AgNP characterization The particle size and morphology of AgNPs were determined by transmission
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electron microscopy (TEM) at 100 kV (Hitachi, H7650, Japan). Samples were
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prepared by placing a drop of fresh suspension on the TEM copper grids, followed by
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solvent evaporation at room temperature overnight. Fig. S1 shows the morphology,
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size distribution and stability of the AgNPs used in this study. The TEM micrograph
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(SI Fig. S1a) shows that AgNPs are spherical in shape and well-dispersed. The size
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distribution was evaluated by software Image J. Fig. S1a shows the size distribution
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of AgNPs, with an average diameter of 12.1 nm.
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The UV-Vis spectra from 300-700 nm were obtained using an UV-Vis
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spectrophotometer (Shanghai Metash, UV-5200, China). Fig. S1b shows the silver
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surface plasmon absorption band with a maximum at 393.5 nm. The crystallography
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of AgNPs was determined by X-ray diffraction (XRD) (PANalytical, X’ Pert Pro, Netherlands) operated at a voltage of 40 kV and a current of 30 mA with Cu Kα radiation. Fig. S1c show Bragg reflections with 2θ values of 38.0, 44.2, 64.3 and 77.3°, indicating the (111), (200), (220) and (311) crystal planes of the face-centered-cubic
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phase of synthesized AgNPs, respectively.
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2.3. Jar test
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Coagulation experiments were performed in 500 mL glass beakers on a
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conventional jar test apparatus using a six-paddle stirrer (Jiamei Instruments, Jintan, 6
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China). Before dosing, AgNP stock suspensions were diluted to 1 mg/L with borate
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buffer (5 mM, pH 8.4) to reduce the releasing of silver ion [15]. pH, NOM
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concentration, calcium concentration, or kaolin concentration were adjusted to a
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certain concentration. Four coagulants, including aluminum sulfate, ferric chloride,
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PACl and PFS, were individually added into each 500 mL AgNP suspension (1.0 mg
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Ag/L) to test the efficiency of AgNP removal. The dosages of aluminum sulfate, PACl,
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ferric chloride and PFS were predetermined based on the minimum quantity of
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coagulants to produce flocs, as described in detail in Supplementary Information (SI).
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The applied dosages for aluminum sulfate, ferric chloride, PACl, and PFS were 495,
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400, 30, and 47 mg/L, respectively. The jar test procedures consisted of a 2.0 min
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rapid mix (300 rpm), a 10 min slow mix (60 rpm), and a 20 min settling period. A
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small amount of samples was taken immediately after 1.0 min rapid mix to measure
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the zeta potential by Zetasizer (Malvern Instruments, ZetaPALS, U.K.). After settling,
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the supernatants were carefully collected to analyze the concentrations of AgNPs and
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Milli-Q water, where the solution pH was adjusted to alkaline condition to enhance
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the dissolution. The stock solutions were then filtered (Millipore, 0.45 µm, USA), and
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the total organic carbon (TOC) of the stock solution was determined by TOC analyzer
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(TOC-V CPH, Shimadzu, Japan). The kaolin stock suspension was prepared to study
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silver ion.
The suspension pH was adjusted by adding 1 M HNO3 or NaOH. Suwannee
River humic acid (HA) (Sigma-Aldrich) solution was prepared to represent NOM in the water. HA stock solution of 1 g/L were prepared by dissolving the HA powder in
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the effect of suspended solids on the removal of AgNPs. All the coagulation
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experiments were performed in triplicate.
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2.4. Floc characterization
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After finishing the jar test for determining the effect of pH on AgNP removal,
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flocs were immediately collected from beakers after the sedimentation. Floc samples
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were vacuum-dried at 40 oC and gently grounded with mortar and pestle. The powders
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were resuspended in Milli-Q water. The morphology of the flocs was analyzed by
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TEM analysis, and the crystal structure of the flocs was identified by XRD analysis.
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2.5. Silver speciation and determination
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The quantification of AgNP stock suspensions was analyzed by inductively
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coupled plasma optical emission spectrometry (PerkinElmer Optima 7000 DV, USA)
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after nitric acid digestion. The AgNP concentrations in the stock suspension were in
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the range of 100-120 mg/L. Silver concentrations in the coagulation solution were
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analyzed by inductively coupled plasma mass spectrometry (Agilent 7500cx, USA). The dissolved silver ion was isolated by removing AgNPs using centrifugal ultrafilter devices (Millipore Amicon Ultra-4 3 K, USA), subjected to centrifugation for 30 min at 4000 rpm [15], while the total silver concentration was analyzed after nitric acid
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digestion. The AgNP concentration was calculated by deducting dissolved silver ion
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from total silver.
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2.6. Water sample collections
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Ten L of grab samples were collected in Oct, 2012 from a local source water 8
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reservoir (Fujian Province, China). Samples were transported in ice-packed coolers to
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the laboratory and stored at 4 oC. Sample was characterized and the physicochemical
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parameters are described in SI Table S1.
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3. Results and discussion
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3.1. Effect of pH on AgNP removal
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In this study, zeta potentials of 1.0 mg/L PVA coated AgNP suspension at pH 6.5,
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7.5 and 8.5 were -7.3, -9.4 and -18.2 mV, indicating a destabilization of AgNPs at
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lower pH. The measurement agreed with a previous study, which observed that the
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agglomeration was enhanced with lower pH due to the less negative zeta potential
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[11,12]. Coagulation at different pH values was carried out in the jar test to investigate
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the effect of pH on AgNP removal (Fig. 1a). AgNP removal efficiencies increased as
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pH increased from 6.5 to 7.5, and decreased as pH increased to 8.5. Especially using
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PFS and PACl as coagulants, the removal efficiencies by PFS were 16, 76 and 15% at
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pH value 6.5, 7.5, 8.5, while the removal efficiencies by PACl were about 4, 90 and
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pH values for all the four coagulants. In aluminum sulfate system, the zeta potentials
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were 8.5, 5.4 and -1.1 mV at pH 6.5, 7.5 and 8.5. Compared with AgNP suspensions
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without coagulants, zeta potentials of AgNP suspensions became less negative (pH 8.5)
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or positive (pH 6.5 and 7.5) in the presence of coagulants, indicating the AgNP
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73% respectively. The optimized pH at 7.5 was selected for all the four coagulants in the following study.
Zeta potential was measured immediately after 1 min rapid mixing. As shown in
Fig. 1b, an increase in the measured zeta potentials was observed due to a decrease in
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removal was partly attributed to the charge neutralization, where the cationic
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aluminum or iron hydrolysis products would be strongly adsorbed on the negative
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AgNPs and effective destabilization was achieved [20]. Similar removal mechanism
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was also observed for ferric chloride (at pH 7.5 and 8.5), PACl (at pH 7.5 and 8.5),
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and PFS (at pH 7.5). However, more positive zeta potentials were measured for ferric
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chloride and PACl at pH 6.5, while a more negative zeta potential was measured for
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PFS at pH 8.5. Generally, a suspension that exhibits an absolute zeta potential less
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than 20 mV is considered unstable and will result in precipitation of particles in
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solution [12], while the absolute zeta potential higher than 20 mV is stable. The higher
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absolute zeta potential for PACl at pH 6.5 or PFS at 8.5 would explain the insufficient
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removal of AgNPs, since the particles were stable due to the surface charge. However,
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the good removal of AgNPs by ferric chloride at pH 6.5 suggested that other
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mechanism of coagulation might be involved.
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3.2. Effect of NOM on AgNP removal
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In this study, different concentrations of NOM were chosen according to reported
surface water TOC [21]. Zeta potential of AgNPs with concentration of 1.0 mg/L at pH 7.5 was -9.4 mV, and it decreased to -32.7 mV while 20 mg TOC/L NOM was added, indicating greater stability of AgNPs in the presence of humic substances,
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likely because NOM imparted negative charge to AgNP surface and increased the
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absolute surface potential [22], or the addition of NOM in the suspension decreased
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the zeta potential.
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The effect of NOM on the AgNP removal is shown in Fig. 2. The removal
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efficiencies using aluminum sulfate and ferric chloride reduced slightly as the NOM
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concentration increased. However, the removal rates by PACl were 90, 57 and 19%
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with the NOM concentrations of 0, 5 and 20 mg/L, respectively, while the removal
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rates by PFS were 76, 37 and 19%. The lower AgNP removal efficiencies were
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probably because NOM may consume part of the coagulant due to the preferential
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interaction between NOM and the coagulants [17,19,23]. The AgNP removal
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efficiency increased to 92.9% and 95.6% as the dosages of PFS and PACl increased to
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five times, indicating the increase in the coagulant dosages would obtained the higher
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removal efficiency.
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3.3. Effect of cation ion on AgNP removal
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The zeta potentials of AgNP suspensions at pH 7.5 with Ca2+ concentrations of 0,
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1, and 10 mM were -9.4, -5.4, and -1.9 mV, respectively. The negative surface charge
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of AgNPs approached zero with increasing Ca2+ concentrations, indicating the
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destabilization of AgNPs would be enhanced under higher Ca2+ ion strength. The
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electrical double-layer on the AgNP surface is strongly compressed by Ca2+ and zeta potential approaches zero [11], and agglomeration of AgNPs was reported to be enhanced by the Ca2+ [11-13]. In addition, the previous study has also demonstrated the effect of Ca2+ on the coagulation performance [24]. The AgNP removal
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efficiencies at various Ca2+ concentrations during the coagulation process are shown
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in Fig. 3a. In the aluminum sulfate and PACl coagulation systems, AgNP removal
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efficiencies decreased as Ca2+ concentrations increased. This is probably due to the
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increase of zeta potential to enhance AgNP stability (Fig. 3b) but will need further
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experiments for confirmation. In PFS coagulation system, AgNP removal efficiency
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slightly increased as Ca2+ concentrations increased. The increase of zeta potential
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from negative to around zero as Ca2+ concentrations increased from 0 to 1 and 10 mM,
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could consequently reduce the AgNP stability.
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3.4. Effect of suspended solids on AgNP removal
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AgNP removal efficiencies slightly changed with the addition of kaolin in the
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aluminum sulfate and ferric chloride system. However, the removal efficiencies
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improved with higher kaolin concentrations in PACl and PFS coagulation system (Fig.
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4). Purposeful addition of supplementary particles is often used to improve the
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coagulation efficiency since the opportunity for particle-particle contact was increased
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[18,25]. In the PACl and PFS coagulation systems, the addition of kaolin could
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increase the opportunities for kaolin-AgNPs contact, and consequently, AgNP
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removal was likely improved by the direct AgNP surface adsorption on the kaolin
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particle followed by removal via particle destabilization or sweep flocculation.
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Similar improved removal efficiency of NPs was reported, where MWCNT removal was enhanced by kaolin in the coagulation process [18]. 3.5. Silver ion releasing
Considering that AgNPs could keep releasing silver ions in the solutions [15,26],
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which would attribute to AgNP removal, the silver ion release of AgNPs in the
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coagulation processes was investigated. The silver ion concentrations after
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coagulation with different pH values are shown in Fig. 5. Higher silver ion
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concentrations were observed under lower pH value in ferric chloride and PFS
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systems. Especially for PFS at pH 6.5, the silver ion concentrations were 105 µg/L,
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which counted for 65% of the AgNP removal. This suggested that the silver ion
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release was strongly pH dependent, and ion release rates decreased with increasing
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pH, which was in accordance with the previous study [15].
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The silver ion concentrations decreased as NOM concentrations increased (Fig.
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S2) in the aluminum sulfate and ferric chloride systems. The surface adsorption of
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NOM onto AgNPs might reduce the silver ion releasing [27]. Besides, NOM in the
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solution might work as a competitive sink for oxidation compounds which could
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inhibit the oxidation of AgNPs [28]. The silver ion releasing decreased with addition
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of humic acids, which was in accordance with previous study [15]. However, in the
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PACl and PFS system, the highest silver ion concentrations were observed at NOM
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concentration of 5 mg/L. This might be due to the release of silver ion from the
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insufficient removal of AgNPs in the solution (Fig. 2). While for 20 mg/L NOM, the
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silver ion release could be reduced with the addition of NOM [15, 27, 28]. The effects
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of cationic strength and suspended solids concentration on the silver ion concentrations are shown in Fig. S2b and S2c. The silver ion concentrations had slight variation with Ca2+ concentrations or kaolin concentrations, with an exception of an increasing silver ion concentration observed in PACl and PFS systems with higher concentrations of kaolin.
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It is worth noting that silver ion concentrations were lower in ferric chloride and
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PACl systems compared to aluminum sulfate and PFS systems. It was likely due to
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the formation of AgCl, since the solubility product constant (ksp) of AgCl is quite low
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(1.56x10-10). Therefore, the ferric chloride and PACl, which could release chloride
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ions in the solution, were recommended to be the coagulants for the AgNP removal,
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since they could not only remove AgNPs but also control the silver ion concentrations
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to reduce the toxicity.
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3.6. Confirmation of AgNP removal by coagulation
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TEM analysis was applied to investigate the microscopic configuration of AgNPs
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distributed in the flocs (Fig. 6). Black spherical shape spots were absorbed on the light
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color flocs. The particle sizes were 5-25 nm, which were within the same range of the
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pristine AgNPs. In contrast, as shown in the TEM images in SI Fig. S3, no such spot
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was observed in samples without AgNPs (i.e., flocs only). Therefore, the black spots
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were considered to be the AgNPs or silver-containing NPs.
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In addition, XRD analysis was applied to further study the chemical form of
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silver in the coagulation flocs. XRD spectra, with (111), (200), (220) and (311) facets,
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validate the existence of AgNPs with the crystal structure in the coagulation flocs of
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aluminum sulfate, PACl and PFS, as shown in Fig. 7a, 7c, 7d. Therefore, in the aluminum sulfate, PACl, and PFS coagulation systems, the AgNP removal was mainly achieved by adsorption onto the coagulants and sedimentation with the flocs in the form of AgNPs.
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In the ferric chloride coagulation system, characteristic peaks of the flocs were
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observed at 27.7, 32.2, and 46.2° (Fig. 7b), indicating the existence of AgCl. Besides,
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additional peaks at 35.7 and 43.6°, also indicated the existence of Fe2O3, which is
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likely formed due to the hydrolysis of FeCl3 followed by two-step phase
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transformation [29]. However, peaks for AgNPs, with (111), (200), (220) and (311)
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facets, were not observed. This implied AgNPs was mainly removed by release of
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silver ions in the ferric chloride coagulation system, followed by the formation of
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AgCl.
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3.7. AgNP removal by coagulation in the raw water
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Raw water from a local reservoir was collected to investigate the removal
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efficiencies of AgNPs by the coagulation process. The water pH was 7.43, and TOC
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was 1.53 mg/L. Detail characterization parameters are shown in SI Table S1. Jar tests
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were carried out by the four coagulants with the solution pH at 7.5. The removal
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efficiencies are shown in Table 1. The highest removal was achieved by PACl with the
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removal efficiency close to 100%, followed by the aluminum sulfate with the removal
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efficiency of 99%. The removal efficiencies by ferric chloride and PFS were 92 and
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91%, respectively. The results demonstrated that the traditional water treatment
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process, coagulation and sedimentation could effectively remove AgNPs from the raw
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water.
3.8. Environmental implication Incorporation of AgNPs into a wide variety of consumer products has led to
concern due to their unintended release to the aquatic environment. The aggregation,
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dissolution, chemical transformation, precipitation of AgNPs in the environmental
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water could limit the transport of AgNPs and lead to the removal of AgNPs before
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entering the drinking water treatment facility. Even if AgNPs can be transported to the
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source water and enter the drinking water treatment processes, our results have
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demonstrated that the traditional treatment process, coagulation/sedimentation, could
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not only remove AgNPs, but also limit the silver ion to a low level. Therefore, human
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exposure to AgNPs through potable water could be reduced by the well optimized
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coagulation process.
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4. Conclusions
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The removal of AgNPs depends on the coagulant types, dosage and the
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suspension pH, and the optimum removal efficiencies were achieved at pH 7.5. All
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the four coagulants could effectively remove AgNPs. However, the ferric chloride and
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PACl could reduce the silver ion concentrations to a lower level, and these two
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chloride coagulants were recommended for the AgNP removal. Results from TEM
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and XRD showed AgNPs or silver-containing nanoparticles were adsorbed onto the
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flocs. Moreover, the removal efficiencies of AgNPs would be affected by the natural
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water characteristics, for instance, NOM, cationic strength, and suspended solids
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concentration. Therefore, the coagulation process should be optimized accordingly to provide an effective way for AgNP or other engineered NP removal. Acknowledgements
We appreciate Dr. Xin Yu, and Ms. Zihong Fan for the raw water sample collections.
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This work was supported by the Natural Science Foundation of Fujian Province,
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China (2011J05035), the Special Program for Key Basic Research of the Ministry of
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Science and Technology, China (2010CB434802), the Science and Technology
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Planning Project of Xiamen, China (3502Z20120012), the Science and Technology
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Innovation and Collaboration Team Project of the Chinese Academy of Sciences,
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Technology Foundation for Selected Overseas Chinese Scholar of MOHRSS, China,
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and the Hundred Talents Program of the Chinese Academy of Sciences.
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Reference
347
[1] N.C. Mueller, B. Nowack, Exposure modeling of engineered nanoparticles in the
348
environment, Environ. Sci. Technol. 42 (2008) 4447-4453.
349
[2] B. Nowack, Nanosilver revisited downstream, Science 330 (2010) 1054-1055.
350
[3] S.W.P. Wijnhoven, W.J.G.M. Peijnenburg, C.A. Herberts, W.I. Hagens, A.G.
351
Oomen, E.H.W. Heugens, B. Roszek, J. Bisschops, I. Gosens, D. van de Meent, S.
352
Dekkers, W.H. de Jong, M. van Zijverden, A.J.A.M. Sips, R.E. Geertsma,
353
Nano-silver- a review of available data and knowledge gaps in human and
354
environmental risk assessment, Nanotoxicology 3 (2009) 109-U78.
355
[4] C. Carlson, S.M. Hussain, A.M. Schrand, L.K. Braydich-Stolle, K.L. Hess, R.L.
356
Jones, J.J. Schlager, Unique cellular interaction of silver nanoparticles: Size-
358 359 360
cr
us
an
M
d
te
Ac ce p
357
ip t
343
dependent generation of reactive oxygen species, J. Phys. Chem. B 112 (2008) 13608-13619.
[5] Z.M. Xiu, J. Ma, P.J.J. Alvarez, Differential effect of common ligands and molecular oxygen on antimicrobial activity of silver nanoparticles versus silver ions,
361
Environ. Sci. Technol. 45 (2011) 9003-9008.
362
[6] O. Choi, C.-P. Yu, G.E. Fernandez, Z. Hu, Interactions of nanosilver with
363
Escherichia coli cells in planktonic and biofilm cultures, Water Res. 44 (2010)
364
6095-6103.
17
Page 17 of 31
[7] A.J. Miao, Z.P. Luo, C.S. Chen, W.C. Chin, P.H. Santschi, A. Quigg, Intracellular
366
uptake: A possible mechanism for silver engineered nanoparticle toxicity to a
367
freshwater alga Ochromonas danica, PLoS One 5 (2010) e15196.
368
[8] H.J. Jo, J.W. Choi, S.H. Lee, S.W. Hong, Acute toxicity of Ag and CuO
369
nanoparticle suspension against Daphnia magna: The importance of their dissolved
370
fraction varying with preparation methods, J. Hazard. Mater. 227 (2012) 301-308.
371
[9] S.S. Khan, E.B. Kumar, A. Mukherjee, N. Chandrasekaran, Bacterial tolerance to
372
silver nanoparticles (SNPs): Aeromonas punctata isolated from sewage environment,
373
J. Basic Microb. 51 (2011) 183-190.
374
[10] Z. Yuan, J. Li, L. Cui, B. Xu, H. Zhang, C.-P. Yu, Interaction of silver
375
nanoparticles with pure nitrifying bacteria, Chemosphere 90 (2013) 1404-1411.
376
[11] F. Piccapietra, L. Sigg, R. Behra, Colloidal stability of carbonate-coated silver
377
nanoparticles in synthetic and natural freshwater, Environ. Sci. Technol. 46 (2012)
378
818-825.
380 381 382
cr
us
an
M
d
te
Ac ce p
379
ip t
365
[12] A.M. El Badawy, T.P. Luxton, R.G. Silva, K.G. Scheckel, M.T. Suidan, T.M. Tolaymat, Impact of environmental conditions (pH, ionic strength, and electrolyte type) on the surface charge and aggregation of silver nanoparticles suspensions, Environ. Sci. Technol. 44 (2010) 1260-1266.
383
[13] X.A. Li, J.J. Lenhart, H.W. Walker, Dissolution-accompanied aggregation
384
kinetics of silver nanoparticles, Langmuir 26 (2010) 16690-16698.
385
[14] K.A. Huynh, K.L. Chen, Aggregation kinetics of citrate and polyvinylpyrrolidone
386
coated silver nanoparticles in monovalent and divalent electrolyte solutions, Environ.
18
Page 18 of 31
Sci. Technol. 45 (2011) 5564-5571.
388
[15] J. Liu, R.H. Hurt, Ion release kinetics and particle persistence in aqueous
389
nano-silver colloids, Environ. Sci. Technol. 44 (2010) 2169-2175.
390
[16] Y. Zhang, Y.S. Chen, P. Westerhoff, J.C. Crittenden, Stability and removal of
391
water soluble CdTe quantum dots in water, Environ. Sci. Technol. 42 (2008) 321-325.
392
[17] H. Hyung, J.H. Kim, Dispersion of C-60 in natural water and removal by
393
conventional drinking water treatment processes. Water Res. 43 (2009) 2463-2470.
394
[18] R.D. Holbrook, C.N. Kline, J.J. Filliben, Impact of source water quality on
395
multiwall carbon nanotube coagulation, Environ. Sci. Technol. 44 (2010) 1386-1391.
396
[19] Y. Zhang, Y.S. Chen, P. Westerhoff, K. Hristovski, J.C. Crittenden, Stability of
397
commercial metal oxide nanoparticles in water, Water Res. 42 (2008) 2204-2212.
398
[20] J.M. Duan, J. Gregory, Coagulation by hydrolysing metal salts, Adv. Colloid
399
Interface Sci. 100 (2003) 475-502.
400
[21] T. Ouyang, Z. Zhu, Y. Kuang, River water quaility and pollution sources in the
402 403 404
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an
M
d
te
Ac ce p
401
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387
Pearl River Delta, China, J. Environ. Monit. 7 (2005) 664-669. [22] Y. Zhang, Y.S. Chen, P. Westerhoff, J. Crittenden, Impact of natural organic matter and divalent cations on the stability of aqueous nanoparticles, Water Res. 43 (2009) 4249-4257.
405
[23] C.R. O'Melia, W.C. Becker, K.K. Au, Removal of humic substances by
406
coagulation, Water Sci. Technol. 40 (1999) 47-54.
407
[24] Y. Wang, B.Y. Gao, X.M. Xu, W.Y. Xu, The effect of total hardness and ionic
408
strength on the coagulation performance and kinetics of aluminum salts to remove
19
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humic acid, Chem. Eng. J. 160 (2010) 150-156.
410
[25] W. Stumm, C. R. O'Melia, Stoichiometry of coagulation. J. Am. Water Works Ass.
411
(1968) 60514-60539.
412
[26] W. Zhang, Y. Yao, N. Sullivan, Y.S. Chen, Modeling the primary size effects of
413
citrate-coated silver nanoparticles on their ion release kinetics, Environ. Sci. Technol.
414
45 (2011) 4422-4428.
415
[27] S.T. Dubas, V. Pimpan, Humic acid assisted synthesis of silver nanoparticles and
416
its application to herbicide detection, Mater. Lett. 63 (2008) 2661-2663.
417
[28] G.S. Wan, C.H. Liao, F.J. Wu, Photodegradation of humic acid in the presence of
418
hydrogen peroxide, Chemosphere 42 (2001) 379-387.
419
[29] W.T. Dong, C.S. Zhu, Use of ethylene oxide in the sol-gel synthesis of
420
alpha-Fe2O3 nanoparticles from Fe(III) salts, J. Mater. Chem. 12 (2002) 1676-1683.
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421
Coagulants
Al2(SO4)3
FeCl3
PACl
PFS
Removal Rates (%)
99±1
91±9
100a
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Table 1 Removal efficiencies of AgNPs from the raw water by different coagulants
a
Raw water is just enough for one sample.
92±7
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Figure Captions
425
Fig. 1. Effect of pH on the AgNP removal efficiencies (a) and zeta potential (b).
427
Fig. 2. Effect of NOM on the removal efficiencies of AgNPs.
428
Fig. 3. Effect of Ca2+ on the removal efficiencies of AgNPs (a) and zeta potential (b).
429
Fig. 4. Effect of the suspended solids on the AgNP removal.
430
Fig. 5. Effect of pH on the silver ion releasing.
431
Fig. 6. TEM images of flocs by different coagulants: (a) aluminum sulfate; (b) ferric
432
chloride; (c) PACl; (d) PFS.
433
Fig. 7. XRD patterns of flocs by different coagulants: (a) aluminum sulfate; (b) ferric
434
chloride; (c) PACl; and (d) PFS.
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a
100 80 60
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6.5 7.5
40
8.5
0
435
Al2(SO4)3
FeCl3
PACl
30
b
an
10
M
0 -10
-30
437 438
7.5 8.5
Al2(SO4)3
FeCl3
PACl
PFS
Ac ce p
436
6.5
d
-20
te
zeta potential (mV)
20
-40
PFS
cr
20
us
AgNPs removal efficiencies (%)
120
Fig. 1.
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Page 23 of 31
438 439
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100 80
cr
NOM 0 mg/L
60
NOM 5 mg/L
NOM 20 mg/L
us
40 20 0 Al2(SO4)3
440 Fig. 2.
PACl
PFS
M
441
FeCl3
an
AgNPs removal efficiencies (%)
120
Ac ce p
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442
a
100
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80
Ca2+ 0 mM
60
cr
Ca2+ 1 mM
40
Ca2+ 10 mM
20 0 FeCl3
PACl
25
M
15
5 0
445 446
Ac ce p
-5
-10
Ca2+ 0 mM Ca2+ 1 mM Ca2+ 10 mM
d
10
b
te
Zeta potential (mV)
20
444
PFS
an
Al2(SO4)3
443
us
AgNPs removal efficiencies (%)
120
Al2(SO4)3
FeCl3
PACl
PFS
Fig. 3.
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446 447
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100 80
cr
Kaolin 0 mg/L
60
Kaolin 10 mg/L Kaolin 30 mg/L
us
40 20 0 Al2(SO4)3
448 Fig. 4.
PACl
PFS
M
449
FeCl3
an
AgNPs removal efficiencies (%)
120
Ac ce p
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450
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450
120
80
pH 6.5 60
452
cr
pH 7.5
40
Al2 (SO4 )3
FeCl3
PACl
us
pH 8.5
20 0
451
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100
PFS
an
Silver ion concentraion (µg/L)
140
Fig. 5.
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454 455 456
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Fig. 6.
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60
70
(111)
8000
20
80
c
6000
2000 50
60
70
5500 20
80
60
70
80
d
(200)
30
40
an
40
50
(220)
60
(311)
70
80
o
Position ( 2θ)
Position (o 2θ)
M
456 457
te
d
Fig. 7.
Ac ce p
460
8500
6500
(311)
50
(111)
9000
7000 (220)
30
40
7500
(200)
4000
459
30
8000
6000
3000
○
8500
7000 5000
b
cr
50
Fe2O3
us
Counts
(311) 40
AgCl
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(220)
30
○
○ ▲
2000
458
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(200)
3000
1000 20
○○
9500
4000
1000 20
10000
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Counts
a
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Counts
Counts
6000
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GRAPHICAL ABSTRACT
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Highlights
465
● This study investigated the removal of AgNP suspensions by four regular
467
coagulants.
468
● The optimal removal efficiencies for the four coagulants were achieved at pH 7.5.
469
● The removal efficiency of AgNPs was affected by the natural water characteristics.
470
● TEM and XRD showed that AgNPs or silver-containing NPs were adsorbed onto the
471
flocs.
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S0304-3894(13)00545-1 http://dx.doi.org/doi:10.1016/j.jhazmat.2013.07.066 HAZMAT 15291
To appear in:
Journal of Hazardous Materials
Received date: Revised date: Accepted date:
19-2-2013 22-7-2013 27-7-2013
Please cite this article as: Q. Sun, Y. Li, T. Tang, Z. Yuan, C.-P. Yu, Removal of Silver Nanoparticles by Coagulation Processes, Journal of Hazardous Materials (2013), http://dx.doi.org/10.1016/j.jhazmat.2013.07.066 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Removal of Silver Nanoparticles by Coagulation Processes
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Qian Sun1+, Yan Li1+, Ting Tang1,2, Zhihua Yuan1, Chang-Ping Yu1,*
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Chinese Academy of Sciences, Xiamen 361021, China
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2
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Huainan 232001, China
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College of Earth and Environment, Anhui University of Science & Technology,
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Key Laboratory of Urban Environment and Health, Institute of Urban Environment,
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Qian Sun and Yan Li contributed equally to this work.
* Corresponding Author: Tel: (86)-592-6190768, Email: [email protected]
1
Page 1 of 31
ABSTRACT
17
Commercial use of silver nanoparticles (AgNPs) will lead to a potential route for
18
human exposure via potable water. Coagulation followed by sedimentation, as a
19
conventional technique in the drinking water treatment facilities, may become an
20
important barrier to prevent human from AgNP exposures. This study investigated the
21
removal of AgNP suspensions by four regular coagulants. In the aluminum sulfate and
22
ferric chloride coagulation systems, the water parameters slightly affected the AgNP
23
removal. However, in the poly aluminum chloride and polyferric sulfate coagulation
24
systems, the optimal removal efficiencies were achieved at pH 7.5, while higher or
25
lower of pH could reduce the AgNP removal. Besides, the increasing natural organic
26
matter (NOM) would reduce the AgNP removal, while Ca2+ and suspended solids
27
concentrations would also affect the AgNP removal. In addition, results from the
28
transmission electron microscopy and X-ray diffraction showed AgNPs or
29
silver-containing nanoparticles were adsorbed onto the flocs. Finally, natural water
31 32 33
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samples were used to validate AgNP removal by coagulation. This study suggests that in the case of release of AgNPs into the source water, the traditional water treatment process, coagulation/ sedimentation, can remove AgNPs and minimize the silver ion concentration under the well optimized conditions.
34 35
Keyword: Silver nanoparticles; Coagulation; Removal efficiency; Flocs; Silver ion
36
release
37
2
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38
1. Introduction Nanoparticles (NPs) are defined as materials with at least one dimension less than
40
100 nm. Because of the unique antimicrobial properties, silver NPs (AgNPs) are one
41
of the most widely used NPs in a range of consumer goods such as food packaging,
42
clothing, medical devices, and cleaning agents. The worldwide production of AgNPs
43
was approximately 500 tons/year (2008), [1], and the widespread use inevitably leads
44
to AgNP release into the environment [2]. Concerns have been raised about potential
45
adverse effects of AgNPs on human health and the environment [3] Evidences have
46
shown that AgNPs are toxic to cells [4], bacteria [5,6], algae [7], and other aquatic
47
organisms [8]. Furthermore, there is also a risk that the use of AgNPs will lead to the
48
development of antibiotic resistance among harmful bacteria [9], and may also
49
damage or alter beneficial microbial communities [10].
51 52 53 54 55
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AgNPs can be diminished by aggregation in the aquatic environment. These
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processes are governed by the physicochemical properties of AgNPs, including size and surface charge, and are directly influenced by the solution condition, such as pH, natural organic matter (NOM), and ionic strength [11]. AgNPs appeared to agglomerate under acidic conditions or higher ionic strength, especially higher concentrations of divalent electrolytes [11-14]. For example, Ca2+ concentrations
56
above 2 mM would enhance AgNP agglomeration [11]. However, the interactions of
57
AgNPs with NOM are still not clear. Greater stability of citrate and PVP-coated
58
AgNPs were observed in the presence of humic substances [11], while a negligible
59
effect of aquatic fulvic acid was found on the agglomeration state of uncoated AgNPs 3
Page 3 of 31
[13]. Besides, the capping agents, which are used in the synthesis of NPs to prevent
61
aggregation, may affect the stability of AgNPs [12]. In addition, AgNPs can be
62
reduced by silver ion releasing into the aquatic environment. Previous study showed
63
that more than 10% of silver ions were released from citrate coated AgNPs (2 mg/L)
64
in the air-saturated water at pH 5.6 [15]. The silver ion releasing rates increased with
65
temperature, and decreased with increasing pH or addition of NOM [15].
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AgNPs present in aquatic systems may represent a possible route for human
67
exposure, especially if the AgNPs-contaminated aquatic system is used as the source
68
of potable water. Coagulation, which is one of the most common processes for the
69
water treatment to removal particles and NOM, provides a potential way to remove
70
AgNPs or other engineered NPs. However, the related study received few attentions
71
to date. Previous studies demonstrated the removal of CdTe NPs and C60 NPs by alum
72
[16,17], and reported that multiwall carbon nanotubes (MWCNT) could be removed
73
from aquatic system via coagulation [18]. Metal oxide NPs, including Fe2O3, ZnO,
77
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However, the performance of AgNP removal by coagulation has not been fully
79
investigated. Moreover, the removal mechanism would be more complicated, since
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AgNPs could keep releasing silver ion in the aquatic system, which would partly
81
cause the toxicity of AgNPs [10].
74 75 76
TiO2, NiO and silica, were also reported to be partially removed by coagulants [19]. These studies have shown that NPs could be removed through coagulation but the removal efficiency varied significantly and was dependent on the NP types, coagulant types, and other aqueous conditions, such as pH, NOM and suspended solids.
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The present study attempts to fill this knowledge gap by investigating the
83
removal efficiencies of AgNPs by coagulation and elucidating the mechanism.
84
Considering AgNPs could keep releasing silver ion, the removal efficiencies were
85
evaluated by determining both silver ion concentrations and total silver concentrations
86
in the aquatic phase. Four regular coagulants, including aluminum sulfate, ferric
87
chloride, poly aluminum chloride (PACl) and polyferric sulfate (PFS), were applied to
88
investigate the effect of coagulant types on AgNP removal. Besides, different aqueous
89
conditions, including pH, NOM, cation concentration, and suspended solids
90
concentration, were investigated. Furthermore, the coagulation flocs were
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characterized for better understanding of removal mechanism. Finally, natural water
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samples were carried out for the validation of AgNP removal by coagulation.
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2. Material and methods
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2.1. AgNP synthesis
96 97 98
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AgNPs were synthesized from AgNO3 by adding NaBH4 with polyvinyl alcohol
(PVA) as a capping agent [6]. 5 mL of 14 mM NaBH4 was added into 90 mL 0.06% (wt) PVA solution. Afterwards, 5 mL of 14 mM AgNO3 was injected at the rate of one drop per sec. Following 10 min of 700 rpm stirring at room temperature, AgNPs were
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concentrated and purified by centrifugal ultrafiltration (Millipore, Amicon Ultra-15 3
100
K, USA), rinsed twice with Milli-Q water (Millipore, 18.2 MΩ·cm, USA). Initial
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physic-chemical characterization was performed after the synthesis. AgNP stock
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suspensions were stored at 4 °C before use. No aggregation during storage was
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observed.
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2.2. AgNP characterization The particle size and morphology of AgNPs were determined by transmission
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electron microscopy (TEM) at 100 kV (Hitachi, H7650, Japan). Samples were
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prepared by placing a drop of fresh suspension on the TEM copper grids, followed by
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solvent evaporation at room temperature overnight. Fig. S1 shows the morphology,
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size distribution and stability of the AgNPs used in this study. The TEM micrograph
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(SI Fig. S1a) shows that AgNPs are spherical in shape and well-dispersed. The size
111
distribution was evaluated by software Image J. Fig. S1a shows the size distribution
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of AgNPs, with an average diameter of 12.1 nm.
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The UV-Vis spectra from 300-700 nm were obtained using an UV-Vis
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spectrophotometer (Shanghai Metash, UV-5200, China). Fig. S1b shows the silver
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surface plasmon absorption band with a maximum at 393.5 nm. The crystallography
117 118 119
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of AgNPs was determined by X-ray diffraction (XRD) (PANalytical, X’ Pert Pro, Netherlands) operated at a voltage of 40 kV and a current of 30 mA with Cu Kα radiation. Fig. S1c show Bragg reflections with 2θ values of 38.0, 44.2, 64.3 and 77.3°, indicating the (111), (200), (220) and (311) crystal planes of the face-centered-cubic
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phase of synthesized AgNPs, respectively.
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2.3. Jar test
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Coagulation experiments were performed in 500 mL glass beakers on a
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conventional jar test apparatus using a six-paddle stirrer (Jiamei Instruments, Jintan, 6
Page 6 of 31
China). Before dosing, AgNP stock suspensions were diluted to 1 mg/L with borate
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buffer (5 mM, pH 8.4) to reduce the releasing of silver ion [15]. pH, NOM
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concentration, calcium concentration, or kaolin concentration were adjusted to a
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certain concentration. Four coagulants, including aluminum sulfate, ferric chloride,
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PACl and PFS, were individually added into each 500 mL AgNP suspension (1.0 mg
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Ag/L) to test the efficiency of AgNP removal. The dosages of aluminum sulfate, PACl,
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ferric chloride and PFS were predetermined based on the minimum quantity of
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coagulants to produce flocs, as described in detail in Supplementary Information (SI).
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The applied dosages for aluminum sulfate, ferric chloride, PACl, and PFS were 495,
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400, 30, and 47 mg/L, respectively. The jar test procedures consisted of a 2.0 min
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rapid mix (300 rpm), a 10 min slow mix (60 rpm), and a 20 min settling period. A
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small amount of samples was taken immediately after 1.0 min rapid mix to measure
136
the zeta potential by Zetasizer (Malvern Instruments, ZetaPALS, U.K.). After settling,
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the supernatants were carefully collected to analyze the concentrations of AgNPs and
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Milli-Q water, where the solution pH was adjusted to alkaline condition to enhance
143
the dissolution. The stock solutions were then filtered (Millipore, 0.45 µm, USA), and
144
the total organic carbon (TOC) of the stock solution was determined by TOC analyzer
145
(TOC-V CPH, Shimadzu, Japan). The kaolin stock suspension was prepared to study
138 139 140
silver ion.
The suspension pH was adjusted by adding 1 M HNO3 or NaOH. Suwannee
River humic acid (HA) (Sigma-Aldrich) solution was prepared to represent NOM in the water. HA stock solution of 1 g/L were prepared by dissolving the HA powder in
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the effect of suspended solids on the removal of AgNPs. All the coagulation
147
experiments were performed in triplicate.
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2.4. Floc characterization
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After finishing the jar test for determining the effect of pH on AgNP removal,
150
flocs were immediately collected from beakers after the sedimentation. Floc samples
151
were vacuum-dried at 40 oC and gently grounded with mortar and pestle. The powders
152
were resuspended in Milli-Q water. The morphology of the flocs was analyzed by
153
TEM analysis, and the crystal structure of the flocs was identified by XRD analysis.
154
2.5. Silver speciation and determination
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The quantification of AgNP stock suspensions was analyzed by inductively
156
coupled plasma optical emission spectrometry (PerkinElmer Optima 7000 DV, USA)
157
after nitric acid digestion. The AgNP concentrations in the stock suspension were in
158
the range of 100-120 mg/L. Silver concentrations in the coagulation solution were
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analyzed by inductively coupled plasma mass spectrometry (Agilent 7500cx, USA). The dissolved silver ion was isolated by removing AgNPs using centrifugal ultrafilter devices (Millipore Amicon Ultra-4 3 K, USA), subjected to centrifugation for 30 min at 4000 rpm [15], while the total silver concentration was analyzed after nitric acid
163
digestion. The AgNP concentration was calculated by deducting dissolved silver ion
164
from total silver.
165
2.6. Water sample collections
166
Ten L of grab samples were collected in Oct, 2012 from a local source water 8
Page 8 of 31
reservoir (Fujian Province, China). Samples were transported in ice-packed coolers to
168
the laboratory and stored at 4 oC. Sample was characterized and the physicochemical
169
parameters are described in SI Table S1.
170
3. Results and discussion
171
3.1. Effect of pH on AgNP removal
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In this study, zeta potentials of 1.0 mg/L PVA coated AgNP suspension at pH 6.5,
173
7.5 and 8.5 were -7.3, -9.4 and -18.2 mV, indicating a destabilization of AgNPs at
174
lower pH. The measurement agreed with a previous study, which observed that the
175
agglomeration was enhanced with lower pH due to the less negative zeta potential
176
[11,12]. Coagulation at different pH values was carried out in the jar test to investigate
177
the effect of pH on AgNP removal (Fig. 1a). AgNP removal efficiencies increased as
178
pH increased from 6.5 to 7.5, and decreased as pH increased to 8.5. Especially using
179
PFS and PACl as coagulants, the removal efficiencies by PFS were 16, 76 and 15% at
180
pH value 6.5, 7.5, 8.5, while the removal efficiencies by PACl were about 4, 90 and
184
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pH values for all the four coagulants. In aluminum sulfate system, the zeta potentials
186
were 8.5, 5.4 and -1.1 mV at pH 6.5, 7.5 and 8.5. Compared with AgNP suspensions
187
without coagulants, zeta potentials of AgNP suspensions became less negative (pH 8.5)
188
or positive (pH 6.5 and 7.5) in the presence of coagulants, indicating the AgNP
181 182 183
73% respectively. The optimized pH at 7.5 was selected for all the four coagulants in the following study.
Zeta potential was measured immediately after 1 min rapid mixing. As shown in
Fig. 1b, an increase in the measured zeta potentials was observed due to a decrease in
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Page 9 of 31
removal was partly attributed to the charge neutralization, where the cationic
190
aluminum or iron hydrolysis products would be strongly adsorbed on the negative
191
AgNPs and effective destabilization was achieved [20]. Similar removal mechanism
192
was also observed for ferric chloride (at pH 7.5 and 8.5), PACl (at pH 7.5 and 8.5),
193
and PFS (at pH 7.5). However, more positive zeta potentials were measured for ferric
194
chloride and PACl at pH 6.5, while a more negative zeta potential was measured for
195
PFS at pH 8.5. Generally, a suspension that exhibits an absolute zeta potential less
196
than 20 mV is considered unstable and will result in precipitation of particles in
197
solution [12], while the absolute zeta potential higher than 20 mV is stable. The higher
198
absolute zeta potential for PACl at pH 6.5 or PFS at 8.5 would explain the insufficient
199
removal of AgNPs, since the particles were stable due to the surface charge. However,
200
the good removal of AgNPs by ferric chloride at pH 6.5 suggested that other
201
mechanism of coagulation might be involved.
202
3.2. Effect of NOM on AgNP removal
204 205 206
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ip t
189
In this study, different concentrations of NOM were chosen according to reported
surface water TOC [21]. Zeta potential of AgNPs with concentration of 1.0 mg/L at pH 7.5 was -9.4 mV, and it decreased to -32.7 mV while 20 mg TOC/L NOM was added, indicating greater stability of AgNPs in the presence of humic substances,
207
likely because NOM imparted negative charge to AgNP surface and increased the
208
absolute surface potential [22], or the addition of NOM in the suspension decreased
209
the zeta potential.
210
The effect of NOM on the AgNP removal is shown in Fig. 2. The removal
10
Page 10 of 31
efficiencies using aluminum sulfate and ferric chloride reduced slightly as the NOM
212
concentration increased. However, the removal rates by PACl were 90, 57 and 19%
213
with the NOM concentrations of 0, 5 and 20 mg/L, respectively, while the removal
214
rates by PFS were 76, 37 and 19%. The lower AgNP removal efficiencies were
215
probably because NOM may consume part of the coagulant due to the preferential
216
interaction between NOM and the coagulants [17,19,23]. The AgNP removal
217
efficiency increased to 92.9% and 95.6% as the dosages of PFS and PACl increased to
218
five times, indicating the increase in the coagulant dosages would obtained the higher
219
removal efficiency.
220
3.3. Effect of cation ion on AgNP removal
M
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211
The zeta potentials of AgNP suspensions at pH 7.5 with Ca2+ concentrations of 0,
222
1, and 10 mM were -9.4, -5.4, and -1.9 mV, respectively. The negative surface charge
223
of AgNPs approached zero with increasing Ca2+ concentrations, indicating the
224
destabilization of AgNPs would be enhanced under higher Ca2+ ion strength. The
226 227 228
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225
d
221
electrical double-layer on the AgNP surface is strongly compressed by Ca2+ and zeta potential approaches zero [11], and agglomeration of AgNPs was reported to be enhanced by the Ca2+ [11-13]. In addition, the previous study has also demonstrated the effect of Ca2+ on the coagulation performance [24]. The AgNP removal
229
efficiencies at various Ca2+ concentrations during the coagulation process are shown
230
in Fig. 3a. In the aluminum sulfate and PACl coagulation systems, AgNP removal
231
efficiencies decreased as Ca2+ concentrations increased. This is probably due to the
232
increase of zeta potential to enhance AgNP stability (Fig. 3b) but will need further
11
Page 11 of 31
experiments for confirmation. In PFS coagulation system, AgNP removal efficiency
234
slightly increased as Ca2+ concentrations increased. The increase of zeta potential
235
from negative to around zero as Ca2+ concentrations increased from 0 to 1 and 10 mM,
236
could consequently reduce the AgNP stability.
237
3.4. Effect of suspended solids on AgNP removal
cr
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233
AgNP removal efficiencies slightly changed with the addition of kaolin in the
239
aluminum sulfate and ferric chloride system. However, the removal efficiencies
240
improved with higher kaolin concentrations in PACl and PFS coagulation system (Fig.
241
4). Purposeful addition of supplementary particles is often used to improve the
242
coagulation efficiency since the opportunity for particle-particle contact was increased
243
[18,25]. In the PACl and PFS coagulation systems, the addition of kaolin could
244
increase the opportunities for kaolin-AgNPs contact, and consequently, AgNP
245
removal was likely improved by the direct AgNP surface adsorption on the kaolin
246
particle followed by removal via particle destabilization or sweep flocculation.
248 249 250
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us
238
Similar improved removal efficiency of NPs was reported, where MWCNT removal was enhanced by kaolin in the coagulation process [18]. 3.5. Silver ion releasing
Considering that AgNPs could keep releasing silver ions in the solutions [15,26],
251
which would attribute to AgNP removal, the silver ion release of AgNPs in the
252
coagulation processes was investigated. The silver ion concentrations after
253
coagulation with different pH values are shown in Fig. 5. Higher silver ion
254
concentrations were observed under lower pH value in ferric chloride and PFS
12
Page 12 of 31
systems. Especially for PFS at pH 6.5, the silver ion concentrations were 105 µg/L,
256
which counted for 65% of the AgNP removal. This suggested that the silver ion
257
release was strongly pH dependent, and ion release rates decreased with increasing
258
pH, which was in accordance with the previous study [15].
ip t
255
The silver ion concentrations decreased as NOM concentrations increased (Fig.
260
S2) in the aluminum sulfate and ferric chloride systems. The surface adsorption of
261
NOM onto AgNPs might reduce the silver ion releasing [27]. Besides, NOM in the
262
solution might work as a competitive sink for oxidation compounds which could
263
inhibit the oxidation of AgNPs [28]. The silver ion releasing decreased with addition
264
of humic acids, which was in accordance with previous study [15]. However, in the
265
PACl and PFS system, the highest silver ion concentrations were observed at NOM
266
concentration of 5 mg/L. This might be due to the release of silver ion from the
267
insufficient removal of AgNPs in the solution (Fig. 2). While for 20 mg/L NOM, the
268
silver ion release could be reduced with the addition of NOM [15, 27, 28]. The effects
270 271 272 273
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269
cr
259
of cationic strength and suspended solids concentration on the silver ion concentrations are shown in Fig. S2b and S2c. The silver ion concentrations had slight variation with Ca2+ concentrations or kaolin concentrations, with an exception of an increasing silver ion concentration observed in PACl and PFS systems with higher concentrations of kaolin.
274
It is worth noting that silver ion concentrations were lower in ferric chloride and
275
PACl systems compared to aluminum sulfate and PFS systems. It was likely due to
276
the formation of AgCl, since the solubility product constant (ksp) of AgCl is quite low
13
Page 13 of 31
(1.56x10-10). Therefore, the ferric chloride and PACl, which could release chloride
278
ions in the solution, were recommended to be the coagulants for the AgNP removal,
279
since they could not only remove AgNPs but also control the silver ion concentrations
280
to reduce the toxicity.
281
3.6. Confirmation of AgNP removal by coagulation
cr
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277
TEM analysis was applied to investigate the microscopic configuration of AgNPs
283
distributed in the flocs (Fig. 6). Black spherical shape spots were absorbed on the light
284
color flocs. The particle sizes were 5-25 nm, which were within the same range of the
285
pristine AgNPs. In contrast, as shown in the TEM images in SI Fig. S3, no such spot
286
was observed in samples without AgNPs (i.e., flocs only). Therefore, the black spots
287
were considered to be the AgNPs or silver-containing NPs.
M
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282
In addition, XRD analysis was applied to further study the chemical form of
289
silver in the coagulation flocs. XRD spectra, with (111), (200), (220) and (311) facets,
290
validate the existence of AgNPs with the crystal structure in the coagulation flocs of
292 293 294
te
Ac ce p
291
d
288
aluminum sulfate, PACl and PFS, as shown in Fig. 7a, 7c, 7d. Therefore, in the aluminum sulfate, PACl, and PFS coagulation systems, the AgNP removal was mainly achieved by adsorption onto the coagulants and sedimentation with the flocs in the form of AgNPs.
295
In the ferric chloride coagulation system, characteristic peaks of the flocs were
296
observed at 27.7, 32.2, and 46.2° (Fig. 7b), indicating the existence of AgCl. Besides,
297
additional peaks at 35.7 and 43.6°, also indicated the existence of Fe2O3, which is
298
likely formed due to the hydrolysis of FeCl3 followed by two-step phase
14
Page 14 of 31
transformation [29]. However, peaks for AgNPs, with (111), (200), (220) and (311)
300
facets, were not observed. This implied AgNPs was mainly removed by release of
301
silver ions in the ferric chloride coagulation system, followed by the formation of
302
AgCl.
303
3.7. AgNP removal by coagulation in the raw water
cr
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299
Raw water from a local reservoir was collected to investigate the removal
305
efficiencies of AgNPs by the coagulation process. The water pH was 7.43, and TOC
306
was 1.53 mg/L. Detail characterization parameters are shown in SI Table S1. Jar tests
307
were carried out by the four coagulants with the solution pH at 7.5. The removal
308
efficiencies are shown in Table 1. The highest removal was achieved by PACl with the
309
removal efficiency close to 100%, followed by the aluminum sulfate with the removal
310
efficiency of 99%. The removal efficiencies by ferric chloride and PFS were 92 and
311
91%, respectively. The results demonstrated that the traditional water treatment
312
process, coagulation and sedimentation could effectively remove AgNPs from the raw
314 315 316
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304
water.
3.8. Environmental implication Incorporation of AgNPs into a wide variety of consumer products has led to
concern due to their unintended release to the aquatic environment. The aggregation,
317
dissolution, chemical transformation, precipitation of AgNPs in the environmental
318
water could limit the transport of AgNPs and lead to the removal of AgNPs before
319
entering the drinking water treatment facility. Even if AgNPs can be transported to the
320
source water and enter the drinking water treatment processes, our results have
15
Page 15 of 31
demonstrated that the traditional treatment process, coagulation/sedimentation, could
322
not only remove AgNPs, but also limit the silver ion to a low level. Therefore, human
323
exposure to AgNPs through potable water could be reduced by the well optimized
324
coagulation process.
ip t
321
4. Conclusions
us
326
cr
325
The removal of AgNPs depends on the coagulant types, dosage and the
328
suspension pH, and the optimum removal efficiencies were achieved at pH 7.5. All
329
the four coagulants could effectively remove AgNPs. However, the ferric chloride and
330
PACl could reduce the silver ion concentrations to a lower level, and these two
331
chloride coagulants were recommended for the AgNP removal. Results from TEM
332
and XRD showed AgNPs or silver-containing nanoparticles were adsorbed onto the
333
flocs. Moreover, the removal efficiencies of AgNPs would be affected by the natural
334
water characteristics, for instance, NOM, cationic strength, and suspended solids
336 337 338
M
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an
327
concentration. Therefore, the coagulation process should be optimized accordingly to provide an effective way for AgNP or other engineered NP removal. Acknowledgements
We appreciate Dr. Xin Yu, and Ms. Zihong Fan for the raw water sample collections.
339
This work was supported by the Natural Science Foundation of Fujian Province,
340
China (2011J05035), the Special Program for Key Basic Research of the Ministry of
341
Science and Technology, China (2010CB434802), the Science and Technology
342
Planning Project of Xiamen, China (3502Z20120012), the Science and Technology
16
Page 16 of 31
Innovation and Collaboration Team Project of the Chinese Academy of Sciences,
344
Technology Foundation for Selected Overseas Chinese Scholar of MOHRSS, China,
345
and the Hundred Talents Program of the Chinese Academy of Sciences.
346
Reference
347
[1] N.C. Mueller, B. Nowack, Exposure modeling of engineered nanoparticles in the
348
environment, Environ. Sci. Technol. 42 (2008) 4447-4453.
349
[2] B. Nowack, Nanosilver revisited downstream, Science 330 (2010) 1054-1055.
350
[3] S.W.P. Wijnhoven, W.J.G.M. Peijnenburg, C.A. Herberts, W.I. Hagens, A.G.
351
Oomen, E.H.W. Heugens, B. Roszek, J. Bisschops, I. Gosens, D. van de Meent, S.
352
Dekkers, W.H. de Jong, M. van Zijverden, A.J.A.M. Sips, R.E. Geertsma,
353
Nano-silver- a review of available data and knowledge gaps in human and
354
environmental risk assessment, Nanotoxicology 3 (2009) 109-U78.
355
[4] C. Carlson, S.M. Hussain, A.M. Schrand, L.K. Braydich-Stolle, K.L. Hess, R.L.
356
Jones, J.J. Schlager, Unique cellular interaction of silver nanoparticles: Size-
358 359 360
cr
us
an
M
d
te
Ac ce p
357
ip t
343
dependent generation of reactive oxygen species, J. Phys. Chem. B 112 (2008) 13608-13619.
[5] Z.M. Xiu, J. Ma, P.J.J. Alvarez, Differential effect of common ligands and molecular oxygen on antimicrobial activity of silver nanoparticles versus silver ions,
361
Environ. Sci. Technol. 45 (2011) 9003-9008.
362
[6] O. Choi, C.-P. Yu, G.E. Fernandez, Z. Hu, Interactions of nanosilver with
363
Escherichia coli cells in planktonic and biofilm cultures, Water Res. 44 (2010)
364
6095-6103.
17
Page 17 of 31
[7] A.J. Miao, Z.P. Luo, C.S. Chen, W.C. Chin, P.H. Santschi, A. Quigg, Intracellular
366
uptake: A possible mechanism for silver engineered nanoparticle toxicity to a
367
freshwater alga Ochromonas danica, PLoS One 5 (2010) e15196.
368
[8] H.J. Jo, J.W. Choi, S.H. Lee, S.W. Hong, Acute toxicity of Ag and CuO
369
nanoparticle suspension against Daphnia magna: The importance of their dissolved
370
fraction varying with preparation methods, J. Hazard. Mater. 227 (2012) 301-308.
371
[9] S.S. Khan, E.B. Kumar, A. Mukherjee, N. Chandrasekaran, Bacterial tolerance to
372
silver nanoparticles (SNPs): Aeromonas punctata isolated from sewage environment,
373
J. Basic Microb. 51 (2011) 183-190.
374
[10] Z. Yuan, J. Li, L. Cui, B. Xu, H. Zhang, C.-P. Yu, Interaction of silver
375
nanoparticles with pure nitrifying bacteria, Chemosphere 90 (2013) 1404-1411.
376
[11] F. Piccapietra, L. Sigg, R. Behra, Colloidal stability of carbonate-coated silver
377
nanoparticles in synthetic and natural freshwater, Environ. Sci. Technol. 46 (2012)
378
818-825.
380 381 382
cr
us
an
M
d
te
Ac ce p
379
ip t
365
[12] A.M. El Badawy, T.P. Luxton, R.G. Silva, K.G. Scheckel, M.T. Suidan, T.M. Tolaymat, Impact of environmental conditions (pH, ionic strength, and electrolyte type) on the surface charge and aggregation of silver nanoparticles suspensions, Environ. Sci. Technol. 44 (2010) 1260-1266.
383
[13] X.A. Li, J.J. Lenhart, H.W. Walker, Dissolution-accompanied aggregation
384
kinetics of silver nanoparticles, Langmuir 26 (2010) 16690-16698.
385
[14] K.A. Huynh, K.L. Chen, Aggregation kinetics of citrate and polyvinylpyrrolidone
386
coated silver nanoparticles in monovalent and divalent electrolyte solutions, Environ.
18
Page 18 of 31
Sci. Technol. 45 (2011) 5564-5571.
388
[15] J. Liu, R.H. Hurt, Ion release kinetics and particle persistence in aqueous
389
nano-silver colloids, Environ. Sci. Technol. 44 (2010) 2169-2175.
390
[16] Y. Zhang, Y.S. Chen, P. Westerhoff, J.C. Crittenden, Stability and removal of
391
water soluble CdTe quantum dots in water, Environ. Sci. Technol. 42 (2008) 321-325.
392
[17] H. Hyung, J.H. Kim, Dispersion of C-60 in natural water and removal by
393
conventional drinking water treatment processes. Water Res. 43 (2009) 2463-2470.
394
[18] R.D. Holbrook, C.N. Kline, J.J. Filliben, Impact of source water quality on
395
multiwall carbon nanotube coagulation, Environ. Sci. Technol. 44 (2010) 1386-1391.
396
[19] Y. Zhang, Y.S. Chen, P. Westerhoff, K. Hristovski, J.C. Crittenden, Stability of
397
commercial metal oxide nanoparticles in water, Water Res. 42 (2008) 2204-2212.
398
[20] J.M. Duan, J. Gregory, Coagulation by hydrolysing metal salts, Adv. Colloid
399
Interface Sci. 100 (2003) 475-502.
400
[21] T. Ouyang, Z. Zhu, Y. Kuang, River water quaility and pollution sources in the
402 403 404
cr
us
an
M
d
te
Ac ce p
401
ip t
387
Pearl River Delta, China, J. Environ. Monit. 7 (2005) 664-669. [22] Y. Zhang, Y.S. Chen, P. Westerhoff, J. Crittenden, Impact of natural organic matter and divalent cations on the stability of aqueous nanoparticles, Water Res. 43 (2009) 4249-4257.
405
[23] C.R. O'Melia, W.C. Becker, K.K. Au, Removal of humic substances by
406
coagulation, Water Sci. Technol. 40 (1999) 47-54.
407
[24] Y. Wang, B.Y. Gao, X.M. Xu, W.Y. Xu, The effect of total hardness and ionic
408
strength on the coagulation performance and kinetics of aluminum salts to remove
19
Page 19 of 31
humic acid, Chem. Eng. J. 160 (2010) 150-156.
410
[25] W. Stumm, C. R. O'Melia, Stoichiometry of coagulation. J. Am. Water Works Ass.
411
(1968) 60514-60539.
412
[26] W. Zhang, Y. Yao, N. Sullivan, Y.S. Chen, Modeling the primary size effects of
413
citrate-coated silver nanoparticles on their ion release kinetics, Environ. Sci. Technol.
414
45 (2011) 4422-4428.
415
[27] S.T. Dubas, V. Pimpan, Humic acid assisted synthesis of silver nanoparticles and
416
its application to herbicide detection, Mater. Lett. 63 (2008) 2661-2663.
417
[28] G.S. Wan, C.H. Liao, F.J. Wu, Photodegradation of humic acid in the presence of
418
hydrogen peroxide, Chemosphere 42 (2001) 379-387.
419
[29] W.T. Dong, C.S. Zhu, Use of ethylene oxide in the sol-gel synthesis of
420
alpha-Fe2O3 nanoparticles from Fe(III) salts, J. Mater. Chem. 12 (2002) 1676-1683.
cr us
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421
Coagulants
Al2(SO4)3
FeCl3
PACl
PFS
Removal Rates (%)
99±1
91±9
100a
ip t
423
Table 1 Removal efficiencies of AgNPs from the raw water by different coagulants
a
Raw water is just enough for one sample.
92±7
cr
422
Ac ce p
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M
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424
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Page 21 of 31
424
Figure Captions
425
Fig. 1. Effect of pH on the AgNP removal efficiencies (a) and zeta potential (b).
427
Fig. 2. Effect of NOM on the removal efficiencies of AgNPs.
428
Fig. 3. Effect of Ca2+ on the removal efficiencies of AgNPs (a) and zeta potential (b).
429
Fig. 4. Effect of the suspended solids on the AgNP removal.
430
Fig. 5. Effect of pH on the silver ion releasing.
431
Fig. 6. TEM images of flocs by different coagulants: (a) aluminum sulfate; (b) ferric
432
chloride; (c) PACl; (d) PFS.
433
Fig. 7. XRD patterns of flocs by different coagulants: (a) aluminum sulfate; (b) ferric
434
chloride; (c) PACl; and (d) PFS.
cr
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d te Ac ce p
435
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426
22
Page 22 of 31
a
100 80 60
ip t
6.5 7.5
40
8.5
0
435
Al2(SO4)3
FeCl3
PACl
30
b
an
10
M
0 -10
-30
437 438
7.5 8.5
Al2(SO4)3
FeCl3
PACl
PFS
Ac ce p
436
6.5
d
-20
te
zeta potential (mV)
20
-40
PFS
cr
20
us
AgNPs removal efficiencies (%)
120
Fig. 1.
23
Page 23 of 31
438 439
ip t
100 80
cr
NOM 0 mg/L
60
NOM 5 mg/L
NOM 20 mg/L
us
40 20 0 Al2(SO4)3
440 Fig. 2.
PACl
PFS
M
441
FeCl3
an
AgNPs removal efficiencies (%)
120
Ac ce p
te
d
442
24
Page 24 of 31
442
a
100
ip t
80
Ca2+ 0 mM
60
cr
Ca2+ 1 mM
40
Ca2+ 10 mM
20 0 FeCl3
PACl
25
M
15
5 0
445 446
Ac ce p
-5
-10
Ca2+ 0 mM Ca2+ 1 mM Ca2+ 10 mM
d
10
b
te
Zeta potential (mV)
20
444
PFS
an
Al2(SO4)3
443
us
AgNPs removal efficiencies (%)
120
Al2(SO4)3
FeCl3
PACl
PFS
Fig. 3.
25
Page 25 of 31
446 447
ip t
100 80
cr
Kaolin 0 mg/L
60
Kaolin 10 mg/L Kaolin 30 mg/L
us
40 20 0 Al2(SO4)3
448 Fig. 4.
PACl
PFS
M
449
FeCl3
an
AgNPs removal efficiencies (%)
120
Ac ce p
te
d
450
26
Page 26 of 31
450
120
80
pH 6.5 60
452
cr
pH 7.5
40
Al2 (SO4 )3
FeCl3
PACl
us
pH 8.5
20 0
451
ip t
100
PFS
an
Silver ion concentraion (µg/L)
140
Fig. 5.
Ac ce p
te
d
M
453
27
Page 27 of 31
454 455 456
Ac ce p
te
d
M
an
us
cr
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453
Fig. 6.
28
Page 28 of 31
60
70
(111)
8000
20
80
c
6000
2000 50
60
70
5500 20
80
60
70
80
d
(200)
30
40
an
40
50
(220)
60
(311)
70
80
o
Position ( 2θ)
Position (o 2θ)
M
456 457
te
d
Fig. 7.
Ac ce p
460
8500
6500
(311)
50
(111)
9000
7000 (220)
30
40
7500
(200)
4000
459
30
8000
6000
3000
○
8500
7000 5000
b
cr
50
Fe2O3
us
Counts
(311) 40
AgCl
▲
9000
(220)
30
○
○ ▲
2000
458
▲
(200)
3000
1000 20
○○
9500
4000
1000 20
10000
ip t
5000
Counts
a
(111)
Counts
Counts
6000
29
Page 29 of 31
GRAPHICAL ABSTRACT
an
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460
M
461
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464
Ac ce p
463
d
462
30
Page 30 of 31
464
Highlights
465
● This study investigated the removal of AgNP suspensions by four regular
467
coagulants.
468
● The optimal removal efficiencies for the four coagulants were achieved at pH 7.5.
469
● The removal efficiency of AgNPs was affected by the natural water characteristics.
470
● TEM and XRD showed that AgNPs or silver-containing NPs were adsorbed onto the
471
flocs.
an
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cr
ip t
466
472
M
473
Ac ce p
te
d
474
31
Page 31 of 31