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Sequential studies of silver released from silver nanoparticles in aqueous media simulating sweat, laundry detergent solutions and surface water Jonas Hedberg, Sara SKOGLUND, Maria-Elisa Karlsson, Susanna Wold, Inger ODNEVALL WALLINDER, and Yolanda Susanne Hedberg Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/es500234y • Publication Date (Web): 03 Jun 2014 Downloaded from http://pubs.acs.org on June 19, 2014
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Environmental Science & Technology
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Sequential studies of silver released from silver nanoparticles in aqueous media simulating sweat, laundry detergent solutions and surface water.
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Jonas Hedberg,a,* Sara Skoglunda, Maria-Elisa Karlssona, Susanna Woldb, Inger Odnevall
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Wallindera, Yolanda Hedberga
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a
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and Corrosion Science, SE-100 44 Stockholm, Sweden.
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b
KTH Royal Institute of Technology, School of Chemical Science and Technology, Surface
KTH Royal Institute of Technology, School of Chemical Science and Technology, Applied
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Physical Chemistry, SE-100 44 Stockholm, Sweden.
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* corresponding author:
[email protected]. FAX: +46 8 208284, Tel: +46 7906642.
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Abstract
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From an increased use of silver nanoparticles (Ag NPs) as an antibacterial in consumer
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products follows a need to assess the environmental interaction and fate of their possible
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dispersion and release of silver. This study aims to elucidate an exposure scenario of the Ag
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NPs potentially released from e.g. impregnated clothing by assessing the release of silver and
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changes in particle properties in sequential contact with synthetic sweat, laundry detergent
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solutions and freshwater, simulating a possible transport path through different aquatic media.
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The release of ionic silver is addressed from a water chemical perspective, compared with
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important particle and surface characteristics. Released amounts of silver in the sequential
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exposures were significantly lower, approximately a factor of two, than the sum of each
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separate exposure. Particle characteristics such as speciation (both of Ag ionic species and at
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the Ag NP surface) influenced the release of soluble silver species present on the surface,
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thereby increasing the total silver release in the separate exposures compared with sequential
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immersions. The particle stability had no drastic impact on the silver release as most of the Ag
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NPs were unstable in solution. The silver release was also influenced by a lower pH
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(increased release of silver), and co-transported zeolites (reduced silver in solution).
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Introduction
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The use of silver nanoparticles (Ag NPs) in consumer products, such as clothing, is expanding
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on the global market, primarily due to their antibacterial properties.1 The estimated production
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rate of Ag NPs in Europe is in the range of 0.6-55 tonnes/year.2 At the moment, the
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environmental concentrations of Ag NPs in general are in the ng/L range,3-4 meaning no
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present acute risk for environmental hazard induced by Ag NPs. However, the large
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uncertainty in production volumes, which are increasing, together with the diversity of Ag
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NPs in terms of e.g. size and coating, call for investigations of Ag NPs. Hence, there is a need
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for accurate assessment of their potential environmental dispersion and fate3-4 as silver is
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potentially toxic, especially towards water organisms.5 For example, a median effect
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concentration value (EC50) of 0.187 mg Ag/L for Daphnia magna,6 and a median lethal
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concentration value (LC50) of 0.696 µg Ag/L for Ceriodaphnia dubia7 have been established.
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Gottschalk et al. have estimated, based on modelling, that increased levels of silver released
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into the environment would result in elevated risks for e.g. surface water.8 The chemical
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speciation of released silver and nanoparticle characteristics, such as size and surface charge,
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largely influence silver toxicity.9-10 Increased release of soluble silver has been seen to
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increase toxicity.9-10 Furthermore, Ag NPs may change speciation in different liquid media,
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which influences the extent and chemical form of released silver.9-11 Parameters such as
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particle size, coatings/capping agents, solution pH and composition have been investigated in
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relation to dissolution of Ag NPs, all essentially underlining the complexity of silver release.9-
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A dependency of solubility on particle size was found, with a higher solubility for smaller 2 ACS Paragon Plus Environment
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Environmental Science & Technology
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particles, related to the fact that small particles have a higher surface area.15 Modelling
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attempts have shown that the extent of silver release does not follow expected values
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predicted from either loadings (mass Ag NP added per volume) or surface area,16 due to
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varying particle properties with particle size. Solubility and release rates of silver from Ag
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NPs are also, as expected, highly dependent on solution composition.9, 11, 14, 17
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Reported findings of the environmental dispersion of silver and Ag NPs from consumer
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products show that significant amounts of silver are released from Ag NP impregnated
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clothing already after a few laundry cycles, both as particles and ionic species.18-21 Released
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silver will inevitably change its properties during transport via different aquatic media. To the
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best of the knowledge of the authors, studies of sequential exposures of Ag NPs to different
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aquatic media of relevance for environmental exposure and fate scenarios are, so far, largely
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missing.
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This study aims on simulating the environmental interaction of Ag NPs released from e.g.
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clothing, by monitoring changes in ionic release and changes in particle properties of
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uncoated Ag NPs in sequential contact with artificial sweat (human contact), model laundry
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detergent solutions (washing cycle), and synthetic freshwater, respectively. Artificial sweat
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(AS) was chosen to mimic transformation of Ag NPs, released from e.g. clothing, in human
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skin contact. Its composition is based on an established protocol.22 To take into account the
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potential release of Ag NPs and silver species from clothing or the skin into the washing
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cycle, a laundry relevant medium with typical detergents was the second step.23 Surface water
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was the final step of the sequence, a standard medium for dissolution/transformation,
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designed to have a general relevance for transformation of materials in aqueous media.24
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Naturally, there are additional aquatic settings of relevance for the environmental fate of Ag
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NPs, including e.g. sewage sludge,25 and media containing natural organic matter14, 26 to
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complete a more comprehensive investigation of the fate of Ag NPs. These media will be 3 ACS Paragon Plus Environment
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targeted in future studies, as this study is a first step towards introducing the approach of
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sequential immersions for investigations of environmental transformation of NPs.
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Material and Methods
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Silver nanoparticles A commercial non-coated Ag NP powder (EV NANO Technology Co., Ltd., China),
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produced by laser ablation, was investigated at a particle loading of 1.5 mg Ag NPs in 15 mL
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solution, i.e. 0.1 g Ag /L. The specific surface area (3.38 m²/g) of the Ag NPs was measured
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by the BET (Brunauer, Emmet, Teller) method, Table 1, which is described elsewhere.27-28
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The powder consisted of small particles sized less than 10 nm, and also larger particles and
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agglomerates of several hundred nm, as illustrated in the TEM image published by Skoglund
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et al.29. Low angle laser light scattering (LALLS) (Table S1) measurements at the onset of
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immersion in solution revealed wide particle size distributions with sizes up to several
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hundred µm with slight difference in observed d(50) values in artificial sweat, detergent
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solutions, and freshwater, respectively (Table 1). The surface composition (x-ray
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photoelectron spectroscopy, XPS, Table 1) showed metallic Ag signal in addition to oxidised
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Ag, indicating that the surface film (adventitious carbon and Ag2CO3) is in the order of 5 nm
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or less (see supporting information on details on XPS).
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Table 1. Ag NP characteristics at dry and wet conditions. Isoelectric point (pH)30 Specific surface area30 Surface compound in air (Raman)30 Surface composition, mass% (XPS) Size and morphology
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~3 3.38 m2/g Ag2CO3
d(50), volume based, Artificial sweat d(50), volume based, Detergent solution, no zeolites
O: 8 %, Ag: 72 % (47 % metallic Ag), C: 19 %. See TEM image, Figure 1 in Skoglund et al29. Size range (after sonication): 2-150 nm. 52 ± 30 µm 65 ± 20 µm 4
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Environmental Science & Technology
d(50), volume based, Freshwater
70 ± 3 µm
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Solutions and exposure
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NaCl, 1.0 g/L urea and 1.0 g/L lactic acid. The pH was adjusted to 6.5 using NaOH.
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Approximately 3.6 mL 5 vol.% NaOH was added to 1 L ultrapure water solution, resulting in
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4.5 mM NaOH. The model laundry detergent solution consisted of 2 g/L Berol (Berol 266,
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Akzo Nobel), an alcohol ethoxylate with chain length C9–11 and 4–7 ethylene oxide units,
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1.8 g/L dodecylbenzenesulfonate (LAS, Sigma Aldrich), and 11 g/L zeolite (ball milled
Artificial sweat (AS) was prepared according to the EN 1811 standard22 and contained 5.0 g/L
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Zeolite A, size
