Life Cycle Assessment and Release Studies for 15 Nanosilver

Article

Life Cycle Assessment and Release Studies for 15 Nanosilver-enabled Consumer Products: Investigating Hotspots and Patterns of Contribution Leila Pourzahedi, Marina Eller Vance, and Matthew J. Eckelman Environ. Sci. Technol., Just Accepted Manuscript • Publication Date (Web): 24 May 2017 Downloaded from http://pubs.acs.org on May 28, 2017

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Environmental Science & Technology

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Life Cycle Assessment and Release Studies for 15 Nanosilver-enabled Consumer Products: Investigating Hotspots and Patterns of Contribution

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Leila Pourzahedi1, Marina Vance2,3, Matthew J. Eckelman1,*

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* Corresponding author: [email protected]

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Department of Civil and Environmental Engineering, Northeastern University, Boston, MA, USA Institute for Critical Technology and Applied Science, Virginia Tech, Blacksburg, VA, USA 3 Department of Mechanical Engineering, University of Colorado Boulder, Boulder, CO, USA 2

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ABSTRACT

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Increasing use of silver nanoparticles (AgNPs) in consumer products as antimicrobial agents has

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prompted extensive research towards evaluation of their potential release to the environment and

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subsequent ecotoxicity to aquatic organisms. It has also been shown that AgNPs can pose significant

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burdens to the environment from life cycle emissions associated with their production, but these

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impacts must be considered in the context of actual products that contain nanosilver. Here a cradle-to-

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gate life cycle assessment for production of 15 different AgNP-enabled consumer products was

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performed, coupled with release studies of those same products, thus providing a consistent analytical

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platform for investigation of nanosilver impacts across a range of product types and concentrations.

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Environmental burdens were assessed over multiple impact categories defined by USEPA’s TRACI

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method. Depending on the product composition and silver loading, the contribution of AgNP synthesis

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to the overall impacts was seen to vary over a wide range, from 1% to 99%. Release studies found that

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solid polymeric samples lost more silver during wash compared to fibrous materials. Estimates of direct

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ecotoxicity impacts of AgNP releases from those products with the highest leaching rates resulted in

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lower impact levels compared to cradle-to-gate ecotoxicity from production for those products.

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Considering both cradle-to-gate production impacts and nanoparticle release studies, in conjunction

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with estimates of life cycle environmental and health benefits of nanoparticle incorporation, can inform

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sustainable nano-enabled product design.

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KEYWORDS

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nanosilver, nanoparticle leaching, life cycle assessment, ecotoxicity, nanofabrics, nanocomposites

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ACS Paragon Plus Environment


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INTRODUCTION

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A global material flow analysis for silver nanoparticles (AgNPs) was carried out based on the world

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economy in 2010, showing a production rate of nearly 500 tons/year.1 The main market applications of

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these AgNPs were found to be in medical devices, coatings, textiles, and cosmetics.1 AgNPs have been in

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use for over 100 years in colloidal form for biocidal purposes, wound treatment, as pigments, and in the

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photography industry.2 According to the Project of Emerging Nanotechnologies’ Consumer Products

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Inventory,3 as of April 2016, nearly 2,000 products were listed as containing nanomaterials, roughly 25%

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of which claim to contain AgNPs. It was also found that antimicrobial protection was the most desired

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function of the products utilizing nanomaterials, especially those containing silver.4 Silver nanoparticles

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exhibit high surface activity compared to the bulk metal,5 and are known for their broad spectrum

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antimicrobial function against gram-negative and gram-positive bacteria.5–7 Their antibacterial mode of

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action is believed to be through the generation of reactive oxygen species during the release of silver

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ions after nanoparticles cross the bacterial cell membrane.5,7

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The use of AgNPs as an antimicrobial agent has multiple potential benefits. A comparison between a

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nanosilver-coated wound dressing and a conventional antibiotic cream for the treatment of burn

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wounds showed that lower levels of infections, overall costs of treatment, and length of stay in the

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hospital was achieved by using the AgNP-based treatment.8 Use of nanosilver rather than other broad-

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spectrum antibiotics in consumer products is also thought to be promising in terms of slowing antibiotic

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resistance.9 Product manufacturers have made use of AgNPs’ antibacterial properties in order to deliver

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odor neutralizing textiles and containers that can keep food items fresh for longer.3 Such products also

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have potential environmental co-benefits, saving energy and water by reduced washing of textiles

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(which also helps to extend product lifetimes), or saving resources in the food system (including organics

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waste treatment) by reducing food spoilage.

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AgNPs have also been receiving significant attention from toxicologists concerned with environmental

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and human health risks caused by their exposure to unintended releases.10,11 In addition to the

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established bactericidal properties,7,12–14 AgNPs have displayed cytotoxic effects towards higher

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organisms.7 Researchers have already gathered evidence of this toxicity against higher cell lines such as

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zebrafish, rats, and humans,7,15–17 in aquatic environments or as aerosolized particles in the

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atmosphere.10 The toxicity of AgNPs has been shown to depend primarily upon particle characteristics

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such as size, shape, choice of capping agent, and environmental conditions.7,18,19 Particles of smaller size


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exhibit higher levels of toxicity towards organisms.19,20 It has also been shown that AgNPs’ interaction

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with gram-negative organisms is shape-dependent,21 while surface charge can greatly affect the toxicity

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of these particles towards gram-positive bacteria.22 It has been speculated that the contributing factor

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to this process is the dissolution of AgNPs and generation of silver ions.5,7,13,16 When tested under strictly

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anaerobic conditions, AgNPs showed no particle-specific toxicity and ion release was shown to be the

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toxic mode of action.23

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Incorporation and Releases of Nanosilver from Consumer Products

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To control the release and antimicrobial action of AgNPs, manufacturers have started incorporating

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AgNPs particles within the matrix of their products or applying them in novel coatings. Subsequently the

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rate of silver ion release is a function of the method of incorporation and the concentration of AgNPs in

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the matrix or coating.24 For glass substrates, spin coating at different speeds,25 and dip coating by simply

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immersing the substrate in solutions have been reported.26 Various methods have been developed for

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embedding AgNPs in polymer matrices such as polymerization of polyethylene samples in the presence

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of AgNPs,27 vapor deposition of particles on nylon-11 matrix followed by heat treatment,28 simultaneous

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silver sputtering and plasma polymerization of an organosilicon matrix,29 layer-by-layer assembly of

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AgNP containing films on polyethylene terephthalate subtrates,30 melt blending of silver and

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polyethylene composites and laminating by extrusion,24 homogenous casting of an AgNP-polyethylene

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suspension,24 and deposition of an AgNP layer on polyethylene using a metallic sprayer.24

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To apply AgNPs to textiles, a variety of processes exist that either impregnate the particles within the

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fibers or distributes them as a coating. The application method is an important determinant of the final

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concentration of silver on fabrics after washing processes. Prior literature have assessed the relation

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between the incorporation method and amount of particle release by synthesizing AgNP-enabled

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textiles and analyzing the wash water. Perelshtein et al. used ultrasound irradiation to deposit AgNPs on

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nylon, polyester and cotton fabrics, and reported no loss of silver after 20 wash cycles as a result of the

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high temperature and speed of AgNP dispersion.31 Another method reported by Radetic et al. used

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corona treatment, a pulsed high voltage electric discharge, to aid with silver adhesion onto fabric

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surface, and saw higher antimicrobial performance of the fabric before washing the samples.32

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Maneerung et al. developed a method to impregnate AgNPs in bacterial cellulose fibers using chemical

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reduction, and reported accumulation of particles at the fabric surface, but signs of deeper particle

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penetration and slower silver ion release with higher concentration of the reducing agent.33 Other



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reported methods are electrospinning AgNP-containing gelatin fibers,34 thermal reduction during melt

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processing of polyamide-6 fibers,35 and pad-dry-cure techniques.36,37 A recent study by Reed et al.

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looked into the post-laundering antibacterial activity of four fabric samples with different silver

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integration methods, namely, covalently tethered, electrostatically attached, silver salt coated, and

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metallic silver coated fibers.38 The authors again concluded that the silver incorporation method and

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initial loading are the primary drivers of silver release levels during use.38

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The current experimental literature on AgNP-enabled products has predominantly focused on their

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antimicrobial efficacy towards bacteria or higher organisms, or the release of particles under various

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conditions. Table S1 in the Supporting Information (SI) shows a list of some of the prior experimental

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studies on AgNP release from commercially available products. Experiments were performed so as to

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simulate particle leaching during use conditions. Quadros et al. studied the release of silver from

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products for children to various liquid media, and found higher silver dissolution levels in media with

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high salt concentrations.39 Benn et al. also experimented on several AgNP-containing household

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products and reported a wide range of release rates. They observed no correlation between the amount

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of silver on the product and the released amount, for fabric samples.40 In a study on AgNP-enabled

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socks, Benn and Westerhoff detected colloidal and ionic silver releases from the samples, and suggested

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that the manufacturing process could be a cause of the different observed release rates from the

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socks.41 There is also an extensive body of knowledge developed by Mitrano and colleagues on release

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of AgNPs from textiles into artificial sweat,42 and in wash-water after standard laundering.43 In addition

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to monitoring the quantity of released particles, they have also studied the transformation of the forms

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of silver after release and the effects of aging on silver speciation.43,44 The textiles studied by Mitrano et

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al. were primarily lab-prepared and not commercial; therefore they have not been included in Table S1.

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Life Cycle Assessment of Nano-Enabled Products

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The environmental implications of AgNP are not just confined to their potential releases and ecotoxicity.

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Life cycle assessment (LCA) models have been developed to quantify the overall environmental burdens

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of AgNPs (including non-nano emissions) from material extraction to end-of-life, as well as to

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contextualize nanomaterial impacts relative to entire AgNP-enabled products. Several studies have

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looked into quantifying the life cycle environmental impacts of AgNP-enabled textiles. A screening level

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LCA on an AgNP-enabled sock was performed, suggesting use phase as the main driver of environmental

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impacts. This study has also considered various AgNP synthesis techniques, showing the dependence of


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the impacts to the manufacturing method.45 This finding was confirmed by cradle-to-gate LCA of

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different physical, chemical, and bio-based AgNP synthesis methods, providing results based on particle

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size as well as mass, to reflect the antimicrobial activity of AgNPs in a comparative analysis.46 To assess

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the change in the impact profile of products due to the addition of AgNPs, a comparative cradle-to-grave

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LCA of a conventional and AgNP-enabled T-shirts was carried out by Walser et al., demonstrating higher

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impacts at the production level, mainly driven by silver mining.47 To build on the release study of Reed et

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al.,38 Hicks and Theis performed a comparative LCA on the polyester fabrics to evaluate the effects of

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laundering behaviors, concluding that for some impact categories such as non-carcinogenic toxicity, the

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life cycle impacts of nano-enabling the textiles are in fact so large that they cannot be offset by any

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change in laundering habits.48 In previous work, we conducted a cradle-to-grave LCA study for a

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commercial AgNP-enabled wound dressing,49 we found AgNPs to be the main contributor to the

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environmental impacts of the product (again largely due to silver mining and electricity use), despite

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their low concentration (only 6% by weight). Additionally, we found that manufacturing of the bandage

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resulted in higher environmental impact levels compared to the waste incineration at its end-of-life.

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Therefore, for this AgNP-enabled product, indirect non-nano emissions upstream were found to be the

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predominant driver of impacts, compared to direct AgNP releases during disposal.

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Of course, every material in a product, nanomaterials included, is added in order to provide

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performance benefits during use, and any full life cycle assessment that specifies a product-based

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functional unit must account for these benefits in order to provide a complete picture of product

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impacts relative to conventional alternatives. There have been several LCA studies that attempt to

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account for the benefits of nano-enabling products;47,50–56 these have been either strictly net-benefit-

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focused or comparative (nano versus non-nano) in nature. Our own work on comparing life cycle

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impacts and benefits has focused on carbon nanotubes, including cumulative energy benefits of reduced

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product weight,51,52 or improved electrical performance,52,56 as well as prospective benefits to health

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from improved pollution sensing.53

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One of the challenges of applying LCA to nano-enabled products is that the use of nanomaterials

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provides an unprecedented or added technological advantage, not typically achievable by the

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conventional materials to which they are being compared. In some cases (as with the carbon nanotube

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studies discussed above), the improved performance reported through laboratory testing can be

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extrapolated to commercial scales, while in others, actual testing of commercial products is required.



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For example, nanosilver-enabled bandages may reduce infection rates, reduce hospital antimicrobial

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resistance, and shorten recovery times, all of which require clinical studies for verification.8 Where direct

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substitution can be assumed based on equivalent functional unit, one approach for including the

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benefits of using nano-enabled products when assessing their overall life cycle environmental burdens is

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the use of system expansion methods to account for the avoided burdens of the alternative product it

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replaces; however, often these benefits are estimated in an anticipatory fashion rather than empirically

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determined.49

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The present study expands on previous work by comparing the effects of AgNP incorporation on

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environmental impacts for a wide range of products, bringing together new and previously reported

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results for product manufacturing and AgNP releases in a consistent framework. LCA was used to

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estimate potential environmental impacts from upstream at producing and incorporating AgNPs relative

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to other product materials, while use-phase AgNP releases from products were estimated under

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simulated use conditions, determined experimentally and from literature. This work does not examine

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potential benefits of AgNP incorporation for each product, which have in certain cases (specifically t-

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shirts) been examined previously and depend on multiple contextual and behavioral factors,47,48 but

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remain unreported in most cases, nor does the work examine product end-of-life considerations. It is

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also not the intention to compare consumer products as in a comparative LCA, as the actual benefits

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imparted by AgNPs are not captured in an appropriate functional unit. Rather, the goal of the study is to

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identify material- or function-specific patterns in both product cradle-to-gate impacts and nanoparticle

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releases that could in turn inform AgNP specification, incorporation, or concentration guidelines for new

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products.

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METHODS

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Scope and System Boundary

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A cradle-to-gate LCA was performed for 15 commercially available products to quantify the impacts

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associated with their production and determine the relative contribution of AgNPs to the overall impacts

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of the product. Products were chosen based on their availability in the market. Out of 15 products, six

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were purchased for the purpose of this study, while models for the remaining nine products were based

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on available data in literature. The material composition and manufacturing steps for each of the

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products are described in the SI. Acid digestion and release experiments were performed on product

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samples to determine the silver concentration and use-phase leaching, respectively. Complete life cycle

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inventory (LCI) data for each product can be found in Tables S2-S16 of the SI.

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Product Descriptions

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Purchased Products. Products obtained were readily available in the market. Samples from products

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were imaged using Scanning Electron Microscopy (SEM) to confirm the presence of AgNPs. Energy

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Dispersive X-ray Spectroscopy (EDX) analysis was also performed to determine the elemental

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composition of the samples. For EDX, samples were mounted on an aluminum pin using carbon tape and

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sputtered with a layer of platinum prior to imaging. These images can be seen in Figure S1 in the SI.

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Product compositions were gathered from labels and manufacturer specifications. Four varying silver

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containing wound dressings were chosen for comparison, namely Acticoat 7 (by Smith & Nephew),

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Silvercel (by Systagenix), Aquacel Ag (by ConvaTech), and Polymem (by Ferris Mfg. Corp.). These wound

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dressings have different material compositions, silver forms and concentrations. A nanosilver-enabled

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food storage, GoGreen Food Container (by Kinetic), was chosen to look at the effects of incorporating

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AgNPs in non-fibrous materials. A silver containing sock, named here as Silver Containing Socks #1 (by

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Fox River Mills Inc.), was also considered here as part of the analysis to account for another application

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of AgNPs to textiles. Detailed description of the manufacturing steps considered for these products can

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be found in the SI. Silver concentrations for these purchased products were determined by the

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experimental procedure described below.

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Products Based on Literature. From a study by Quadros et al. on leaching from AgNP-containing

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products, three products were chosen.39 Silver concentrations reported for all products was derived

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using thermally assisted nitric acid digestion. The weight of all the products were based on

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measurements. The chosen products were as follows: Baby blanket (by Baby Pink or Blue Ltd.) with



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AgNPs embedded in polyethylene fleece; Children’s cup (by Baby Dream Co. Ltd.) made from injection

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molded polypropylene; and Plush toy (by Pure Plushy Inc.) with an interior polystyrene foam filling and

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exterior fur, with AgNPs concentrated in the filling material.

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Four products were chosen from another study by Benn et al., which looked into AgNP release from

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consumer products used at home.40 Silver concentrations were found using nitric acid digestion of

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samples and product weights were reported in the study. Product descriptions are as follows: Medical

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mask (by Nanbabies) composed of polypropylene fibers containing silver to work against bacteria and

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fungi; Medical cloth (by Nanbabies) with silver, assumed to be made from cotton; Towel (by Good4U)

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with AgNPs was assumed to be composed of a fiber blend of 80% polyester and 20% nylon; and Athletic

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T-shirt (by Puckskin) made from polyester.

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One product was chosen from a study on release from silver containing food containers by Echegoyen et

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al.57 The concentration of silver was determined through calcination in a muffled oven followed by nitric

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acid digestion of the remaining ashes. Product description is as follows: FreshLonger plastic bag (by

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Sharper Image®) is an AgNP infused re-sealable bag for food storage to prevent food spoilage. It was

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assumed that the bag is produced from LDPE using plastic film extrusion. LCI data were compiled for a

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20 × 20 cm2 sample. The thickness of each sheet of the bag was assumed to be 2 mils (0.051 mm).

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Finally, AgNP releases from sock fabrics were studied by Benn and Westerhoff, for several commercially

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available socks.41 Silver concentration was determined through a heated nitric acid digestion process.

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For comparison with the purchased sock, one was chosen from this study with a different fiber

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composition and silver concentration. Silver containing socks #2 (by Arctic Shield) uses X-Systems AgNP

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fiber technology for odor protection. These socks are composed of 50% cotton, 39% polyester, 6%

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nylon, and 5% spandex.

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Experimental Procedures

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To determine the total silver content of products, where not reported, digested triplicate samples from

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each product were ashed by adapting methods used by Echegoyen and Nerín and Tulve et al.57,58 Pieces

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weighing 3 – 10 mg from each product were cut and placed into 2-ml glass vials. The vials were then

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heated in a tube furnace (Thermo Scientific Lindberg Blue M Mini-Mite) at 600°C until samples were

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completely combusted (3 – 10 min). Then ash-containing vials were filled with nitric acid (HNO3, 70%


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ACS certified, Spectrum) and heated to ~100°C for 4 hours, with additional nitric acid added over time to

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keep vials full. After this period, samples were removed from heat and left to cool. An amount of 1 ml

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hydrogen peroxide (30% ACS certified, Fisher) was added and samples were diluted to a volume of 250

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ml with ultrapure water. Samples were analyzed for silver concentration by inductively-coupled plasma

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mass spectroscopy (ICP-MS, X-Series, Thermo Electron), which had a calibration curve for Ag ranging

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from 1 – 5000 ppb. To estimate the amount of silver that could potentially be released during product

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use, a method used by Wu et al. was adapted.59 Circles with a diameter of 10 mm were cut from each

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product and immersed into 9 ml of phosphate buffered saline (PBS 1X, Ricca). Samples were placed in a

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water bath at 37°C for 3 days, then filtered (450-nm, Teflon), acidified with the addition of 1 ml HNO3

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(70%), and analyzed by ICP-MS. For products with reported loading levels of AgNPs, if the concentration

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of AgNPs derived by acid digestion was higher than reported amounts, literature values were preferred

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for LCA modeling to project a more conservative scenario.

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Life Cycle Assessment Modeling

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Life cycle impacts were modeled using the SimaPro 8.1 software package (PRé Consultants). Material

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and energy inputs were associated with their respective unit process in the US-EI database (Earthshift),

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as this study was done for the case of U.S. The US-EI dataset was created to represent the North

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American region by applying U.S. electrical grid conditions to the European ecoinvent life cycle inventory

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database. Environmental impacts were assessed using the U.S. Environmental Protection Agency’s (EPA)

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Tool for the Reduction and Assessment of Chemical and Other Environmental Impacts (TRACI) 2.1

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method.60 This method allows for quantification of environmental stressors recognized by the EPA’s

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programs and regulations and are deemed to be the minimum set of categories for assessing

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environmental impacts.61 This method considers the environmental impact categories of ozone

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depletion (OD) in kg CFC-11 equivalents (eq.), global warming potential (GW) in kg CO2 eq.,

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photochemical smog formation (PS) kg O3 eq., acidification (AC) in kg SO2 eq., eutrophication (EU) in kg

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N eq., human health impacts from carcinogenic and non-carcinogenic substances (HHC and HHNC) in

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health comparative toxic units or CTUh, human health impacts from criteria air pollutants (HHCR) in kg

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PM2.5 eq., and ecotoxicity (EC) in CTUe, and fossil fuel depletion (FF) in MJ surplus. The cradle-to-gate

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LCAs were performed on a product basis, considering impacts from raw material extraction to product

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manufacture. The use phase and end-of-life stages were not considered in analysis.

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Due to uncertainty in the method of AgNP production for each commercial product, a single synthesis

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route was assumed for all products for consistent comparison, namely chemical reduction of silver

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nitrate (AgNO3) with trisodium citrate. According to Tolaymat et al.,62 AgNO3 is the most popular silver

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salt precursor reported in studies of nanoparticle synthesis, with citrate being the most used reducing

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and stabilizing agent in synthesis for special applications. Effects of using synthesis routes other than

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citrate reduction is illustrated as a sensitivity analysis. Detailed LCI data for multiple AgNP synthesis

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routes can be found in our earlier work.46 To assess the uncertainty associated with model input

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parameters, a Monte Carlo simulation was performed using SimaPro 8.1, for 10,000 iterations. For all

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background unit processes, the default log-normal distributions of the US-EI database were used, while

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for all foreground processes the probability distributions were defined utilizing the pedigree matrix

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approach,63 with details reported in Table S19 of the SI.

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RESULTS AND DISCUSSION

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Analysis of Process Contributions to Cradle-to-Gate Life Cycle Environmental Impacts

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Life cycle impact assessment was performed on a product basis with relative results illustrated in Figure

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1. The contribution of AgNPs to cradle-to-gate product impacts covered a wide range, from ~1% up to

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99%. In general, the environmental impacts of AgNPs were found to be due primarily to the electricity

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use and emissions from the silver mining processes, with variations across environmental impact

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categories, as discussed in our earlier work,46 and consistent with previous literature.47

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For products including the baby blanket, T-shirt, and towel, due to the low concentration of silver

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particles, AgNP impacts were overshadowed by those from producing the polyester or polyethylene

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terephthalate (PET) fabric. Electricity use for spinning and knitting processes, with associated emissions

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from electric power plants, were the contributing factors to the global warming potential, ozone

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depletion, smog, and acidification impact categories. Coal mining and combustion was the main driver

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for eutrophication and carcinogenic impacts, while natural gas consumption for electricity generation

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and the emissions of combustion were found to be driving the non-carcinogenic and respiratory effects.

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Ecotoxicity impacts of these products was dominated by the use of xylene as a precursor to terephthalic

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and the disposal of incineration residues of PET. Fossil fuel depletion was due to natural gas extraction

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for electricity generation and use of xylene for producing terephthalic acid and ethylene for producing

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ethylene glycol, both used in manufacturing PET granulates.

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For sock #2, the contribution of AgNPs was also negligible compared to the other components, but for

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this product, electricity consumed during cotton yarn and fabric manufacturing dominated the overall

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burdens. In ozone depletion, global warming, smog and acidification categories, electricity consumption

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during cotton weaving processes and yarn production, and emissions associated with electricity

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generation were found to be the source of impacts. Another contributing factor to ozone depletion was

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the use of trichloromethane as a pesticide during cotton cultivation. Eutrophication and ecotoxicity

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impacts were driven by fertilizer and pesticide runoff. Human health impacts, carcinogenic, non-

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carcinogenic and respiratory, were caused by coal-fired electricity generation, and pesticide emissions.

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Sock #1 had higher silver concentration compared to sock #2 and utilized less resource-intensive

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polymeric textiles, so the relative contribution of AgNPs was discernable.

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321 322 323 324 325 326 327 328 329

Figure 1. Process contribution to environmental impacts of different products. Components were categorized generally as paper packaging, nanosilver, extrusion and molding processes for plastics, cellulosic and cotton fibers, and polymers. Concentration of silver is shown in parentheses in percentage to total product. Products are listed in ascending silver concentration from the top left corner to the bottom right. Abbreviations for TRACI impact categories: ozone depletion (OD), global warming (GW), photochemical smog (PS), acidification (AC), eutrophication (EU), human health: carcinogens (HHC), human health: non-carcinogens (HHNC), human health: criteria air pollutants (HHCR), ecotoxicity (EC), fossil fuel depletion (FF).


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In the case of the plush toy, the process of styrene production and the emissions from incineration of

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solid and hazardous waste from this process contributed significantly to the overall impacts. Non-

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carcinogenic health impacts of this product were affected mainly by the release of arsenic ions to

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groundwater from emissions of sulfidic tailings of silver mining. In the ozone depletion category, the

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contribution of AgNPs are a result of bromochlorodifluoromethane (BCF) emissions from heating

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processes during manufacture. The low concentration of AgNPs in the children’s drinking cup resulted in

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negligible impacts compared to the electricity used during the injection molding process, except for the

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ecotoxicity impact category which was dominated by the emissions from disposal and incineration of

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hazardous and chemical waste associated with the production of the polypropylene, and the fossil fuel

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depletion impact category as a result of crude oil consumption. The process of injecting molding plastics

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requires more energy than transforming them into films, as done to manufacture the plastic bag.

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Therefore, higher relative impacts can be seen due to shaping in the food container than the plastic bag.

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The fossil fuel depletion categories of these three products are dominated by the polymer production.

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The medical cloth and mask were the products with the highest concentration of nanosilver. As a result

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all the impact categories are dominated by the impacts of AgNPs. Ozone depletion, global warming, and

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fossil fuel depletion were dominated by emissions and resource use associated with the heating step of

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particle synthesis (BCF release, CO2 release, and natural gas consumption respectively). Contributing

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factors to smog and acidification impacts were emissions from blasting processes during silver

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extraction and refining from combined metal mining. Sulfidic tailing disposal was found to be the key

350

factor affecting carcinogenic, and ecotoxic impacts (from chromium releases and antimony emissions

351

respectively). Emissions from refining copper ores and desilverizing lead were the main causes of non-

352

carcinogenic human health impacts. The same processes contributed to respiratory effects in addition to

353

emissions from natural gas production.

354 355

Between the wound dressings, the contribution of AgNP to impacts was highest in Acticoat 7. AgNP

356

loading on the Silvercel dressing is in a similar range as Acticoat 7 (7.8% vs. 10.1%), but due to the use of

357

calcium alginate and CMC fibers, with precursors including chlorinated compounds and formaldehyde,

358

the contribution of silver was from 10% (in eutrophication or non-carcinogenic impact categories) to

359

60% (in ozone depletion) lower, showing the environmental burdens of the product are not only driven

360

by the concentration of silver, but by other product components as well. For Aquacel Ag and Polymem

361

with lower AgNP-to-product weight ratios, impacts were driven by the production of CMC fibers and



Page 14 of 27

14 362

polyurethane foam, respectively. Polyurethane impacts are driven by upstream impacts of consuming

363

toluene diisocyanate and polyols.

364 365

In general, it can be concluded from the cradle-to-gate LCA results that products with nanosilver

366

concentrations 10%, it was seen that AgNP impacts dominated all environmental categories.

370

Figure 2 examines the correlations between the concentration of silver and the percentage contribution

371

of AgNPs to the overall environmental impacts of the product, with R2 values ranging from 0.36-0.87.

372

The strongest positive correlations exist for fossil fuel depletion and global warming potential impact

373

categories. This is due to the fact that chemical reduction of AgNPs with trisodium citrate requires

374

external heating. Additionally, AgNPs contributed to the smog impact category as a result of nitrogen

375

oxide emissions from the blasting processes during silver extraction, thus, the higher the silver

376

concentration, the higher the contribution to this impact category.

377 378 379 380 381 382 383 384

Figure 2. Evaluation of relationship between silver loading and contribution of silver to impact category. The x-axis shows the loading percentage, the y-axis shows the percentage of impact by silver. Abbreviations for TRACI impact categories: ozone depletion (OD), global warming (GW), photochemical smog (PS), acidification (AC), eutrophication (EU), human health: carcinogens (HHC), human health: noncarcinogens (HHNC), human health: criteria air pollutants (HHCR), ecotoxicity (EC), fossil fuel depletion (FF).


Page 15 of 27


15 385

These results are dependent on the method of particle synthesis and can change by considering other

386

energy inputs or precursors. Figure 3 demonstrates a sensitivity analysis performed to quantify the

387

range of potential environmental impacts of 1 kg AgNPs, using methods described in our earlier work.46

388

Seven methods have been included by this study comprising a mix of current AgNP synthesis trends in

389

physical, chemical, and bio-based synthesis categories. Methods such as flame spray pyrolysis were

390

shown to have the highest levels of impacts while a chemical reduction method with sodium

391

borohydride appeared to be more favorable overall. Comparative results on a mass basis showed an

392

approximate 80% difference in impact levels over all impact categories, between the most and least

393

impactful synthesis methods. Results of the sensitivity analysis performed here demonstrated that for

394

categories such as ozone depletion, global warming and fossil fuel depletion, the contribution of AgNPs

395

could drop as much as 80% or increase by 20% by using other synthesis routes. In eutrophication,

396

carcinogenic, non-carcinogenic, and ecotoxicity impact categories, the chemical reduction with

397

trisodium citrate represents the lower bound of potential impacts, but using other methods could result

398

in a 300% increase of AgNP specific impacts. Impacts of acidification and respiratory effects can

399

decrease by 40% or potentially increase by almost 100%. Smog impacts can range from 20% lower or

400

230% higher levels as a result of change in the production method. These results depend on the

401

precursors used during AgNP synthesis, the heating requirements and electricity consumption. A

402

contribution analysis for each synthesis method and each impact category is provided in Tables S21-27

403

of the SI, summarizing previous work.46 The diverse results indicate the need for using process-specific

404

nanomaterial synthesis data in LCA wherever possible to assess the environmental impacts of nano-

405

enabled products.

406 407

Table S20 summarizes the product-based coefficients of variation (CVs) for all impact category results,

408

derived from the Monte Carlo simulation. The CVs for carcinogenics and ecotoxicity impact categories

409

were found to be among the highest, as is commonly reported. This is due in part to the underlying

410

uncertainties of the characterization factors for metals (importantly here silver) as well as general

411

incompleteness in fate and toxicity data and the current methodological constraints for their

412

development for LCA purposes.64



Page 16 of 27

16

413 414 415 416 417 418 419 420

Figure 3. High and low boundaries for environmental impacts of AgNPs with different synthesis methods (black lines). Blue columns indicate the impacts of producing 1 kg of AgNPs via chemical reduction with trisodium citrate, which was assumed in the present study. Abbreviations for TRACI impact categories: ozone depletion (OD), global warming (GW), photochemical smog (PS), acidification (AC), eutrophication (EU), human health: carcinogens (HHC), human health: non-carcinogens (HHNC), human health: criteria air pollutants (HHCR), ecotoxicity (EC), fossil fuel depletion (FF).

421

Analysis of AgNP Releases

422

Figure 4 demonstrates the percentage of released particles relative to the initial AgNP concentration for

423

each product. Silver release rates from fabric samples reported previously are also shown in Figure 4 for

424

comparison. The concentrations are depicted as those derived by the acid digestion and the axes are

425

shown in log-log format. Varying colors of the data points are an illustration of the different techniques

426

used to determine silver release rates from samples. Our experimental release studies were performed


Page 17 of 27


17 427

using a phosphate buffered saline solution to maintain a constant pH and simulate contact with skin,

428

and thus our release results differed from those already existing on the purchased materials. Quadros et

429

al. reported microwave heating tap water samples to 38 ◦C, refrigerating them for 24 hours, and

430

reheating to 38 ◦C, while refrigerating orange juice samples at 5 ◦C for 3 days, both in 1:50 mass ratios of

431

product to liquid media.39 Benn et al. washed product samples in tap water for one hour at room

432

temperature.40 Echegoyen and Nerin followed EU regulations for migration tests in 3% v/v acetic acid in

433

40 ◦C for 10 days.57 Within the tested samples of this study, releases from the Aquacel Ag samples were

434

the greatest due to the characteristics of the Hydrofiber technology, which dissolves in contact with

435

solutions, enabling higher release rates. Non-textile polymeric samples showed higher amounts of AgNP

436

release, suggesting more surface bound particles compared to fibrous material. This trend was also

437

detected among polymeric and textile samples tested by Quadros et al. (2013). Below detection limit

438

AgNP release level was reported for Sock #2 due to the entanglement of particles within the fabric

439

matrix, so the result for this sample is not shown in Figure 4. Release rates are also dependent upon the

440

frequency and duration of wash, and the release media. Fabric samples of Reed et al. were laundered

441

four times, each for 30 minutes.38 Electrostatic and tethered samples with surface-bound particles

442

showed higher levels of silver release rates. The AgNP leaching percentage for the X-static sample,

443

however, was shown to be within the same range and order of magnitude as that of Silvercel and Sock

444

#1 which used the same silver-coated fiber technology. This result corroborates the finding that the

445

method of AgNP incorporation is a critical determinant of the amount of AgNPs released from products

446

during regular use.38

447



Page 18 of 27

18

448 449 450 451 452

Figure 4. Results of different release studies on the products considered in this study. Due to the differences in the experimental methods, they are shown in varying colors. Axes are in log-log format. Sock #2 from Benn and Westerhoff is not shown due to undetectable amounts of AgNP release.

453

Comparison to Previous Studies

454

Larger-scale mathematical models have been previously developed to determine the probability of

455

AgNP release from products, or their concentration in different compartments of the environment.

456

Using particle flow analysis concepts, Arvidsson et al. developed estimations for AgNP emissions during

457

use of wound dressings, textiles and electronics, resulting in 10% release from wound dressings and up

458

to 100% release from textiles and electronics.65 Keller et al. utilized global market information in

459

conjunction with material flow analysis tools to determine the likelihood of nanomaterial emissions to

460

the environment and landfills.1 This study by Keller et al.,1 compiled estimates for release of particles

461

during use from available studies of different nanomaterials.41,66–70 Based on the aforementioned study,

462

for AgNPs a high estimate of 25% release from medical products, and almost 100% release from textiles

463

to waste water treatment plants were used. Our analysis shows up to 5% AgNP release from AgNP-

464

enabled wound dressings, agreeing with the results of Arvidsson et al.,65 but showing significantly lower

465

release rates from AgNP-enabled textiles. This is due to the assumption made by several prior studies of

466

100% release of particles from textiles during use. Our results show this percentage is highly dependent

467

on the fabrication method of the textiles, and the silver incorporation techniques. This is supported by a

468

recent work by Reed et al., showing a 3-80% AgNP release range, after four wash cycles, for four


Page 19 of 27


19 469

different textiles.38 Keller et al. also estimated nanoparticle emissions to the environment from food

470

packaging products containing AgNPs to be as low as 5% and reaching a high of 25%, with 5% release

471

rate to wastewater treatment plants.1 Results of our experiments show almost 2% release of silver from

472

the food container, which are within the ranges defined by their mathematical models.

473 474

Ecotoxicity of Nanosilver Releases from Products

475

Nanosilver released to aquatic environments can be a source of ecotoxicity impacts due to the release of

476

free silver ions. The highest silver release amount is from the food container, releasing 7.92 mg silver. As

477

an additional comparison between production-phase and use-phase ecotoxicity, impacts from these

478

direct AgNP releases were estimated using existing characterization factors from literature. Several

479

characterization factors have been developed for AgNPs using a variety of methods, summarized here.

480

A worst-case scenario is direct release of AgNPs to water bodies, as might occur for outdoor product use

481

and washing. Most of the products considered, however, would likely have wash water delivered to

482

wastewater treatment plants or septic systems. Both Walser et al. and Manda et al. assumed 90.7%

483

elimination of AgNPs by WWTPs. Those authors assumed that 75% of the remaining fraction would

484

dissolve and applied a characterization factor for dissolved silver.47,71 Another study found that silver ion

485

release from silver nanoparticle dissociation in freshwaters has been found to be approximately 50% of

486

AgNP mass.72

487

characterization factor of 1.94×105 CTUe/kg,73 gives a range of 0.12-0.77 CTUe for AgNP releases from

488

the food container. (Modeling of nanoparticle fate and exposure based on colloid theory rather than

489

partition coefficients,74 such as with the SimpleBox4Nano model,75 or the work of Pu et al.,76 or Deng et

490

al.,77 may yield more accurate nano-specific characterization factors. Inclusion of chemical speciation

491

and complexation models will also improve accuracy, particularly consideration of sulfidation reactions

492

that have been shown to reduce AgNP toxicity.78) These nanomaterial release-based ecotoxicity impacts

493

are 2-12 times less than the cradle-to-gate ecotoxicity of the same food container product of 1.5 CTUe

494

(see Table S17). While both direct and indirect ecotoxicity results are uncertain, this comparison

495

underscores that the inclusion of upstream environmental impacts from nanomaterial production is

496

critical when assessing the potential environmental and health impacts of AgNP-enabled products.

497

Conversely, from Figures 1 and 2, several products with silver concentrations less than 0.03% showed

498

relatively low nanomaterial-related impacts during production, while at the same time having high AgNP

499

release rates compared to other products under study, ranging from 0.5-2.5%. Overall, these results

Combining these values for untreated/treated releases with the USEtox Ag+



Page 20 of 27

20 500

illustrate the complementary nature of LCA models and release studies and the importance of including

501

both in the assessment of environmental impacts of products.

502 503

Recommendations for Life Cycle Modeling

504

AgNP-enabled products comprise the majority of nano-enabled consumer product market. The results

505

of this study quantify the cradle-to-gate environmental impacts associated with the production of these

506

products and their potential silver emissions during use and cleaning. As shown in this study, as with our

507

previous work,49 impacts associated with indirect and non-nano emissions upstream can dominate the

508

overall environmental burdens of manufacturing nano-enabled products. Importantly, for nearly half of

509

the products considered, cradle-to-gate environmental impacts were dominated by the conventional

510

material constituents, such as cotton fiber or plastic resins. LCA and other assessment and design

511

guidance efforts should therefore be careful to consider environmental burdens and risks associated

512

with products overall and not focus solely on the marginal impacts introduced by incorporation of

513

nanomaterials.

514 515

Both the quantities of AgNPs used in nano-enabled products as well as their release, fate, and potential

516

toxicity of AgNPs from those products are important considerations when assessing environmental

517

impacts. An exclusive focus on negative impacts is insufficient for providing actual design guidance for

518

individual products, however, as the potential environmental benefits of including AgNPs must be

519

included in order to provide a genuinely life cycle perspective. In addition, while this manuscript focuses

520

on AgNP releases during product use, other work has modeled worker exposure to indoor nanoparticle

521

releases during product manufacturing,79 as well as linked predicted releases to age-specific exposures

522

by incorporating geospatial and demographic information.80 Release experiments can help determine

523

the concentration of AgNPs left on the product at end-of-life, thus enabling more accurate modeling of

524

silver concentrations reaching landfills or waste incineration plants. Information on the contribution of

525

silver to environmental impacts and releases from the product matrix can assist in identifying the

526

minimum AgNP concentrations for adequate product performance as well as the desired method of

527

impregnation (surface coating or subsurface embedding) for a more controlled (or mitigated) release,

528

therefore minimizing impacts from both AgNP production and releases.

529 530

Finally, it is worth noting that while LCAs can provide insight to potential areas of improvements in

531

product manufacturing, there has been a call towards more prospective LCA for the case of emerging


Page 21 of 27


21 532

technologies. The level of uncertainty and inherent lack of data across all stages of LCA, from the

533

inventory analysis of these immature technologies to robust measurement and modeling of particle-

534

specific fate and transport tied to nano-specific characterization factors, implies that retrospective LCA

535

modeling approaches are inappropriate for environmental guidance of decision makers in research and

536

development (R&D), as alluded to by Wender et al.81 Therefore, use of anticipatory LCAs have been

537

suggested as an all-inclusive alternative framework that incorporates societal values, technological

538

forecasting, and risk research into environmental impact assessment, to better inform and guide R&D of

539

these technologies prior to their commercialization.81–85

540 541

ACKNOWLEDGMENTS

542

We acknowledge NSF award SNM-1120329, as well as the George J. Kostas Nanoscale Technology and

543

Manufacturing Research Center at Northeastern University and the Virginia Tech Center for Sustainable

544

Nanotechnology. We thank the student S. Guldin for his assistance with sample preparation.

545 546

SUPPORTING INFORMATION AVAILABLE

547

Additional tables and figures can be found in the Supporting Information, including list of prior release

548

studies (Table S1), SEM images (Figure S1) and life cycle inventory data (Tables S2-S16) for nanosilver-

549

enabled products, absolute LCA results (Table S17), full AgNP leaching results with standard error values

550

(Table S18), Pedigree matrix assumptions (Table S19), CV results (Table S20), and contributing elements

551

to various AgNP synthesis methods (Tables S21-S27). This material is available free of charge via the

552

Internet at http://pubs.acs.org.



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