Life Cycle Payback Estimates of Nanosilver Enabled Textiles under

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Critical Review

Life cycle payback estimates of nano-silver enabled textiles under different silver loading, release, and laundering scenarios informed by literature review Andrea L Hicks, Leanne M. Gilbertson, Jamila S. Yamani, Thomas L. Theis, and Julie B. Zimmerman Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.5b01176 • Publication Date (Web): 02 Jun 2015 Downloaded from http://pubs.acs.org on June 9, 2015

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Environmental Science & Technology

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Title: Life cycle payback estimates of nano-silver enabled textiles under different silver loading,

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release, and laundering scenarios informed by literature review

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Andrea L. Hicks 1,+,*, Leanne M. Gilbertson 2,+, Jamila S. Yamani 2, Thomas L. Theis 1, Julie B.

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Zimmerman2,3

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+

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University of Illinois at Chicago, Institute for Environmental Science and Policy, 2121 West Taylor (MC 673), Chicago, IL 60612

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Co-first authors

Department of Chemical and Environmental Engineering, Yale University, New Haven, CT 065208286

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School of Forestry and Environmental Studies, Yale University, New Haven, CT 06520

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* Author to whom correspondence should be addressed; E-Mail: [email protected]; Tel.: +1-312-996-5236; Fax: +1-312-355-0760.

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Abstract: Silver was utilized throughout history to prevent the growth of bacteria in food and wounds. Recently, nano-scale silver has been applied to consumer textiles (nAg-textiles) to eliminate the prevalence of odor-causing bacteria. In turn, it is proposed that consumers will launder these items less frequently thus, reducing the life cycle impacts. While previous studies report that laundering processes are associated with the greatest environmental impacts of these textiles, there is no data available to support the proposed shift in consumer laundering behavior. Here, the results from a comprehensive literature review of nAg-textile life cycle studies are used to inform a cradle-to-grave life cycle impact assessment. Rather than assuming shifts in consumer behavior, the impact assessment is conducted in such a way that considers all laundering scenarios to elucidate the potential for reduced laundering to enable realization of a net life cycle benefit. In addition to identifying the most impactful stages of the life cycle across nine-midpoint categories, a payback period and uncertainty analysis quantifies the reduction in lifetime launderings required to recover the impacts associated with nano-enabling the textile. Reduction of nAg-textile life cycle impacts is not straightforward and depends on the impact category considered.

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Introduction

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Although recently popularized, the use of silver for its antimicrobial properties dates back to

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antiquity, when Hippocrates used it to treat ulcers and prevent the infection of wounds1. Prior to the

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widespread use of refrigeration in North America, silver coins were used to prevent the spoiling of

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water, milk, wine, and vinegar2. While these are just a few examples, the versatility of silver in

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antimicrobial applications is readily apparent. Alexander and Klasen present a comprehensive review

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of the historical uses of silver.2-4

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With the advent of nanotechnology, manufactured nano-silver (nAg) has become popular for

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use in novel applications, primarily due to its enhanced antimicrobial properties.5 Innovation in nAg-

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enabled products has increased significantly since 2000, demonstrated by the increased market value,

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patent applications, and nAg products available to consumers in USA (Figure 1a) and worldwide. The

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global market value for nAg coatings and products is expected to grow from $290 million in 2011 to

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$1.2 billion by 2016.6 Annual United States nAg production is estimated between 2.8 – 20 tonnes per

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year7. Figure 1a presents the results of a nAg patent application search of the United States Patent

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Office database by application year. Patent application data are commonly used as an indicator of

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innovation and technological change particularly with respect to nanotechnology.8-18

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Figure 1 The Project on Emerging Nanomaterials (PEN) maintains an inventory of all nano-enabled

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products currently on the market, 25% of which are enabled with nAg (462 total products).19 These are

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subdivided into several product categories: appliances, automotive, cross cutting, electronics and

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computers, food and beverage, goods for children, health and fitness, and home and garden as

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presented in Figure 1b. The category with the greatest number of nAg products is health and fitness,

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which includes nAg-enabled textiles (nAg-textiles). These textiles include, but are not limited to,

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shirts, pants, socks, underwear, and linens. The primary function of the embedded silver is to prevent

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odors in the products through its antimicrobial properties. In the case of medicinal textiles, the function

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extends beyond the prevention of odors to that of wound infection. ACS Paragon Plus Environment

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Figure 2

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It is expected, and further supported by the patent search and inventory data, that nAg-textiles

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will continue to represent a significant demand of nAg in the consumer market. These products have

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unique characteristics that differentiate them from their conventional counterparts. Yet these novel

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products present tradeoffs in terms of environmental and human health impact costs across the life

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cycle, including mining of silver ore, nAg manufacture, and release from the textiles during the use

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phase (i.e., during wear and laundering).

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This paper presents a current and comprehensive literature review of nano-enabled textiles life

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cycle assessment (LCA) studies. Critical research gaps are identified and addressed by conducting an

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impact assessment that takes into account laundering habits, silver loading (µg silver/g textile), silver

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release, and product lifetimes over multiple environmental and human health midpoint impact

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categories. Finally, suggestions for future research directions are provided and emphasize the need for

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additional insight on proper linking technology to prevent nAg release during wear and laundering as

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well as how the adoption and use of nAg-textiles may change consumer laundering behavior.

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Review of Life cycle Assessment Literature

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LCA is a systematic tool whereby the environmental impacts of a product or process are

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analyzed across all life cycle stages, including raw materials acquisition, manufacture, use, end of life,

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and transportation. The current research findings will be discussed within each phase of the nAg-

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textile.

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Raw Materials

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The principal intermediate products considered include nano-scale silver and the textile itself,

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which is typically composed of either cotton or polyester (for the most common commercially

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available nAg textiles). There is the potential for the environmental impact of the raw materials to shift

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among life cycle stages as a function of the amount of silver embedded in the textile, the synthesis

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method, silver attachment method, and the type of textile material considered.

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Silver

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Silver exists primarily in the environment as an ore with other metals such as lead, zinc,

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copper, and gold.20 In 2013, 819 million ounces of silver was mined worldwide, with the United States

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being the ninth largest producer.8,21 A typical silver-bearing ore contains 0.085% silver, 0.5% lead,

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0.5% copper, and 0.3% antimony.22 Silver mining occurs in either an open pit or underground mine

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and begins with crushing and grinding the source rock. In addition to extremely low initial silver

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concentrations in the ore, losses associated with the various processing stages contribute to the

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environmental impacts associated with silver mining. In general, losses from tailings comprise 20% of

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the ore in mixed mining operations and 12% of the ore in primary silver mines.20 Additional silver

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losses at each step include 10% from tank leaching, 63% from heap leaching, 0.12% from smelting,

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and 0.02% to slag.23 Floatation separation is used to create a concentrate, which typically contains

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1.7% silver, 10-15% lead, 10-15% copper, and 6% antimony.22

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The variety of ore and mining activity from which silver is obtained leads to a significant

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variation in the environmental impact of the raw materials acquisition phase for silver. As such, the

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life cycle inventory information for silver found in the Ecoinvent database includes a mix of ores for

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calculating the environmental impact of silver mining.24 Swart and Dewulf used multiple

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environmental impact methods to compare the mining and refining impact of copper with respect to

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other metals, such as silver.25 They found the mining and refining of silver to have a greater impact

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than copper, on the order of approximately 7 to 850 times, expressed in copper mining equivalents.

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Synthesis of Nano-Silver

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Synthesis of silver nanoparticles has received significant attention over the past decade. Since

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the primary mechanism of silver antimicrobial activity involves the formation and release of

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monovalent silver ion, the increased surface areas achieved by producing silver at the nano-scale is an

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advantage.26 A variety of physical and chemical methods are used to synthesize silver nanoparticles

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ranging from vapor deposition to chemical reduction.27 More recently, the principles of green

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chemistry have been applied to the synthesis of silver nanoparticles. These methods aim to reduce the ACS Paragon Plus Environment

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environmental and human health impacts associated with the synthesis methods by using benign

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reagents (i.e., solvent, reducing agent, and stabilizing agent), which include plant extracts, enzymes,

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biodegradable polymers, and agricultural waste.28-30 While these methods reduce the net life cycle

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impacts associated with the use of toxic reagents, there are tradeoffs in terms of control over purity,

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particle size, morphology, and performance.29,31 Still, a review of the current literature reveals that the

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majority of silver nanoparticles are synthesized in water using nitrate as the precursor salt, sodium

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borohydride (NaBH4) or sodium citrate as the reducing agent, and citrate as the stabilizing agent.27 As

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such, these reagents serve as a representative synthesis method in the LCA section discussed below.

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Fabric Production

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Cotton and polyester are commonly used as the base materials in nAg textile products. While

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these materials serve a similar functional role, they are manufactured using significantly different raw

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materials and processes, which consequently lead to different environmental impacts.

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The collective environmental impacts of cotton vary significantly depending on where it is

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grown, irrigation and pesticide requirements, and fluctuations in yield.32 The United States grows

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approximately 16% of the world’s cotton, which requires between 3.27 and 8.91 cubic meters (m3) of

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water per kilogram (kg) of cotton produced.32 In addition to water consumption, there are significant

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impacts associated with insecticide and pesticide use.33 Cotton is estimated to be grown on 3% of the

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world’s farmland, yet requires 25% of global pesticide use.34 Post-harvest, cotton is typically

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processed using a scouring method in which the waxy outer layer is dissolved using sodium hydroxide

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to allow for better color penetration when dyed.34 Subsequent processing steps, including baling cotton

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lint, preparation and blending, spinning, knitting, dyeing and finishing, and making up (transforming

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the fabric into a garment), are completed to fully transform the cotton into a garment. The total energy

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consumption is 66.6 megawatt hours per metric tonnes (MWh/MT) of cotton garment.33,35

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Polyester is a petroleum-based product. Polyester textiles are typically manufactured from

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polyethylene terephthalate (PET), a synthetic polymer, and comprise 40% of the world textile

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market.36-37 PET consumption in the United States and Canada was estimated at 5.5 million metric ACS Paragon Plus Environment


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tonnes (MMT) annually.36 The PET (or polyester chips) are produced through polycondensation of

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terephthalic acid (TPA) with ethylene glycol.36,38 TPA is a xylene derivative, which is produced from

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petroleum naptha, while ethylene glycol is made from the oxidation of ethylene, which is a petroleum

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feedstock. In general, the yarn is produced by melting the PET chips, which are then drawn through

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spinneret holes to produced filaments to be collected into thread.33,36 The threads are made into yarn,

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which is woven into fabric, and then made into garments. The estimated energy consumption for the

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manufacture of 1 MT of polyester garment is 91.5 MWh.35

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Manufacture of Silver-Enabled Textiles

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Nano-silver incorporation into textile fabric

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Multiple methods exist for the incorporation of nano-scale silver into textiles. In general, nAg

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textiles may be categorized one of three ways: coated, woven, and embedded (this is also true of the

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attachment of silver at greater than the nano-scale, which has the potential to release nAg through use).

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When textiles are coated, the completed garment or fabric is dipped in the silver, whereas in the woven

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category single fibers are dipped in silver. In the case of embedded textiles, the silver is incorporated

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into the fibers themselves. Methods of attachment may include surface coating, covalent bonding (with

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a linking agent), and interwoven silver fibers.39-43 Although the majority of manufactured silver-enabled

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textiles are synthetic, these techniques have also been used to enable cotton and silk with nAg.40-41

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Use

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Current LCA studies

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To date, two LCA studies have explored the impacts associated with the life cycle use stage of

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nAg textiles. Walser et al. completed a prospective LCA comparing nAg t-shirts, conventional t-shirts,

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and t-shirts treated with tricolsan (an alternate antibacterial agent).44 Two production technologies for

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the synthesis of nanoparticles were also compared, commercialized flame spray pyrolysis with melt-

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spun incorporation of silver nanoparticles, and plasma polymerization with silver co-sputtering on a

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laboratory, pilot, and estimated commercial scale. Shirts were treated with either silver or tricolosan.

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Two nAg loadings were applied, 31 mg of pure nAg or 47 mg of nAg-tricalcium phosphate aggregates ACS Paragon Plus Environment

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(TCP) (which is equivalent to 0.93 mg of nAg). Shirts treated with the alternative antibacterial

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contained 22 mg of tricolsan.

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In the second study, Meyer, et al. performed a screening-level LCA on nAg-enabled socks.45

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For this study it was assumed that each pair of socks contained 60.8 g of cotton fiber and 10.2 mg of

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nAg. The production of nAg was modeled to include silver extraction and refining ore into a silver

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feedstock for nAg fabrication, but the fabrication process itself was not included. The authors noted

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that there are multiple methods for nAg synthesis, including: grinding, flame pyrolysis, laser ablation,

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vapor deposition, and liquid-phase reduction. As such, modules were created to model impacts

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associated with some of the nAg fabrication processes: yet, since this was not the intended focus of

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their study, they were not included in the overall results. A summary of silver loadings compiled from

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literature can be found in Table 1.

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Nano-silver release from textiles

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A significant amount of research has been conducted to characterize the amount of silver in

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these textiles and the extent to which it is released (Table 1). There is a wide range in silver content

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(0.9-21,600 micrograms (µg) per gram (g) of textile) and silver release rates during laundering (0.5-

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100% of the initial silver content over various laundering lifetimes) reported in the literature. The same

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antimicrobial properties that warrant use of nAg in these textiles is also a cause for concern given the

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potential environmental impact associated with its release during laundering. The United States

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Environmental Protection Agency estimates that in 2009, 6.8 MT of silver was used in the United

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States for nAg-textiles with a range of 0.1-0.2 mg silver/ g textile (100-200 µg silver/g textile).45 Based

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on the compiled literature values (Table 1), this is likely a very conservative estimate.

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Table 1 Consumer Laundering Behavior

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Results from life cycle assessments of textiles suggest that the laundering phase is associated

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with the greatest environmental impact.52-53 Thus, manufacturers of silver-enabled textiles claim that

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due to the reduction of odor, their products will require less laundering than conventional alternatives, ACS Paragon Plus Environment


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which will lead to a reduction in use-phase environmental impacts.54 However, to date, there is no

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published research confirming changes in consumer laundering behavior. Nonetheless, this is a key

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assumption in current LCA studies. Differences also exist in the assumptions and laundering protocols

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modeled in the literature. While Walser et al. simulated the environmental impact of three different

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laundering scenarios (varying the amount of water and electricity used per cycle, frequency of shirt

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laundering, and duration of shirt lifetime), it remains unclear which of these assumed behaviors is the

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most accurate representation of consumer behavior. 44 Thus, it is impossible to evaluate the

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environmental costs and benefits associated with adoption of silver- textiles. The assessment

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completed by Meyer et al. includes use phase assumptions such as an equivalent number of lifetime

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launderings and no difference in laundering habits between the conventional and nano-enabled

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textile.45 Additionally, it was assumed that by the end of its lifetime, 100% of the silver was released

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through laundering, yet they did not account for the associated environmental impacts.

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Inhibition of Bacterial Growth

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While nAg is incorporated into fabrics to prevent growth of odor causing bacteria, further

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resolution is necessary to fully evaluate the efficacy of the technology. Laboratory scale studies have

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shown significant inhibition of bacterial growth on textiles55-57 and other substrates (e.g., stainless steel

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and glass58) that are treated with nanosilver. Additionally, laboratory studies have found it is possible

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for the nano-scale silver to migrate from the textile when exposed to synthetic sweat.61-62 However, in

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situ studies found that silver chloride treated fabric did not significantly decrease the amount of

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bacterial colonies on the skin and found no significant odor reduction or antimicrobial activity on the

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fabric.59-60

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End of Life

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In general, options for textile disposal include municipal solid waste (in the United States, this

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typically occurs in a landfill or through incineration), or through down cycling (e.g., denim turned into

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insulation).63-64 LCA studies on nAg textiles have largely neglected analysis of the textile itself, and

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have instead focused on the impact of the silver. As shown in Table 1, silver will be released from the ACS Paragon Plus Environment

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textile in varying degrees during the wash cycle where it is assumed to enter the wastewater system,

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and be treated at a wastewater treatment plant (WWTP). In the United States, only 20% of households

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are connected to septic systems, with the remainder having their wastewater treated at a WWTP.65

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Between 90-95% of the silver entering the WWTP is removed from the water during treatment66-67, but

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remains in the biomass created in the activated sludge system.66-70 As such, consideration of

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environmental and human health impacts of silver is shifted to the landfill or land application of

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biosolids scenarios.

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Nitrification is a process by which ammonia is oxidized through bioconversion to nitrite and

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nitrate. nAg particles in wastewater have been found to inhibit nitrification66,71-72, with a threshold

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level of 0.1 mg/L causing inhibition. The silver-containing biomass generated in the activated sludge

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system may be anaerobically digested or composted in order to reduce volume, inactivate pathogens,

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and produce biogas. No significant difference in biogas production as a function of silver levels has

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been observed from digestion68 or composting73, however some reduction in landfill gas (methane)

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generation was observed at high silver levels.69

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Questions remain as to the chemical form of the nAg in the treatment system. The speciation of

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nAg in the WWTP is relevant to a life cycle impact assessment, particularly in regards to toxicity in

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the environment. Most studies have concluded that as the silver enters the treatment plant it will be

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primarily in the form of Ag2S66,73-74 and that secondary forms, such as AgCl75, are possible. Ag2S has

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not been found to be bioavailable in freshwater conditions, and therefore is not considered to be as

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toxic as ionic silver76, however, to the extent that ionic silver is present, there is the potential to

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negatively impact the environment.77-78

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Unintended Consequences of Silver Usage

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While the antimicrobial properties of nAg offer a potential benefit upon incorporation into

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various textiles, there is also potential for significant unintended environmental and human health

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consequences resulting from the widespread adoption and use of nAg products. In addition to being

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inherently cytotoxic, Wakshlak et al. proposed a so-called “zombie effect” in which bacterial cells are ACS Paragon Plus Environment


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rendered nonviable due to silver exposure, and then become repositories of silver that subsequently kill

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other viable bacteria79. Furthermore, the potential for microbes to develop a resistance to silver has

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been demonstrated and suggests that eventually, silver may become ineffective as an antimicrobial

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agent.80-85 Beyond bacteria, nAg has been shown to have a negative impact on aquatic organisms,

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including Daphnia magna86 and Drosophila87. Potential adverse human health effects resulting from

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exposure to soluble silver include eye, skin, respiratory, and intestinal irritation, changes in blood cells,

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and liver and kidney damage88. Chronic exposure to silver88 can result in skin discoloration (gray or

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blue-black), called Argyria, due to do dermal deposits of silver. This is a permanent condition,

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however, it is not considered physically harmful from a health perspective89. Currently the United

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States Environmental Protection Agency (US EPA) categorizes nAg and nAg enabled products as

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pesticides, which are subject to approval and regulation.90

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Overall LCA

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To date, there are three nAg textile LCA studies each using a different combination of impact

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categories to represent their results. Walser et al.44 report greenhouse gas emissions (in units of carbon

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dioxide equivalents) and ecotoxicity (in units of 1,4-dichlorobenzene equivalents), while Meyer et al.45

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include nine environmental and human health categories available through the TRACI impact

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assessment method. Polygiene54 computed Comparative Environmental Impact Factors (CEIFs), based

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on environmental impact categories of climate change, ecotoxicity, human toxicity, eutrophication,

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ozone depletion, photochemical smog, waste, and acidification. Meyer et al. concluded that the

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impacts from laundering (e.g., water and energy demand) are significantly greater than those of the

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added silver.45 Polygiene presented similar conclusions adding that reductions in laundering frequency

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would significantly decrease the life cycle impact of the textile.54 Furthermore, Walser et al. concluded

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that the potential for reductions in launderings could compensate for the increased burden of adding

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the silver, and that the toxicity impacts associated with the release of silver during laundering are

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minor when compared to the rest of the life cycle.44 These results are consistent with studies on

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conventional textiles, which also found the use phase to have the greatest environmental impact.52-53 ACS Paragon Plus Environment

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While these results are consistent for nAg-enabled textiles for clothing, the most impactful life cycle

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stage can shift depending on the application. For example, Pourzahedi and Eckelman modeled the

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environmental impacts of nAg-enabled single use bandages using the environmental and human health

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impact categories available through the TRACI assessment method.91 In their study, the silver content

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was found to be the most influential factor when considering nanoparticle synthesis and bandage

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manufacturing.

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Summary of Current Research Gaps

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Several research gaps exist in the life cycle considerations for nAg-textiles and are discussed in

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order of life cycle stage. First, there is limited information on the industrial scale manufacturing

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impacts, as opposed to laboratory, scale for nAg synthesis. Multiple processes exist for the fabrication

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of nanosilver particles, each with varying degrees of energy and resource intensity. The silver

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manufacturing process used in the forthcoming analysis represents the most commonly used synthesis,

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is not considered the most intensive process, and is modeled on the laboratory scale. Second, the

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relationship between nAg concentration (or loading) in textiles and the functional efficacy is not yet

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fully understood. As a result, there is significant variability in the initial silver concentrations of

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commercially available nAg textiles, as presented in Table 1. Third, the use phase has received the

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most significant attention, focusing on silver release during laundering conditions and the influence

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that detergent, bleach, water temperature, water chemistry, and laundering methodology have on its

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release. However, the greatest variability in silver release is most likely associated with the silver

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attachment method. Furthermore, laundering behavior modifications (i.e., reduced laundering of nAg-

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textiles) has not yet been studied and is vitally important to resolve the life cycle impacts of nAg-

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textiles given the critical role of consumer behavior on the realization of the aforementioned life cycle

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benefits. Finally, the ultimate form and fate of the released silver remains unresolved. Laundering

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studies conducted in the absence of bleach suggest that Ag2S66,70,73-74 will be the dominant form of

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silver entering the WWTP. However when bleach is present (as is often the case in a consumer

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setting), AgCl75 has been found. These two forms of silver are highly insoluble, suggesting that neither ACS Paragon Plus Environment


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would have a significant impact on microbial activity in WWTP and landfill scenarios. However, if the

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silver is in another form, such as ionic silver, there is significant potential for disruption of current

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system balances to occur in not only in the landfill setting, but also in the WWTP and ultimately the

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water body where the effluent is discharged.

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Life Cycle Assessment to Address Research Gaps

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Building upon the current silver textiles LCA literature, the following impact assessment is

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intended to address some of the identified research gaps discussed above, particularly as it relates to

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the uncertainties with consumer laundering behavior, initial nAg concentration, and the release rate of

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the nAg during laundering. Nine environmental and human health midpoint impact categories are used

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to identify those life cycle stages of a nAg-textile that contribute most significantly to the cumulative

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impacts as well as compare the impacts to those of a conventional textile made from either cotton or

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polyester. Finally, pay back periods, in terms of number of launderings, are evaluated for a subset of

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representative midpoint impact categories to determine whether changes in consumer habits (i.e.,

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reduced number of lifetime launderings) are sufficient to overcome the impacts imposed by enabling a

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textile with nAg.

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Methods

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The life cycle environmental and human health impacts are modeled using SimaPro (version

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8.0.1 software), Ecoinvent, USLCI, and ELCD inventories, and the TRACI (2.1) assessment

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method.92-94 The midpoint categories considered include: ozone depletion (kg CFC-11 eq), global

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warming potential (kg CO2 eq), smog (kg O3 eq), acidification (mol H+ eq), eutrophication (kg N eq),

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carcinogenics (CTUh), non carcinogenics (CTUh), respiratory effects (kg PM10 eq), and ecotoxicity

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(CTUe).

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Goal and Scope

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Since the use phase has been identified to have both the greatest life cycle impact and the

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greatest uncertainty based on a review of current literature, it serves as the focus of the LCA presented

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here. Impacts from the use phase are compared with those of silver acquisition, silver nanoparticle ACS Paragon Plus Environment

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synthesis, textile raw materials acquisition and manufacture, transportation of, and end of life disposal,

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as presented in Figure 2.

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Figure 2 Reference Flow and System Boundary

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The functional unit for the midpoint impact assessment is considered as the impact per shirt

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lifetime. The shirt lifetime is defined here as the number of launderings (assumed to be 100 unless

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otherwise noted) rather than the number of wears. The number of launderings was chosen because this

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is the unit around which use-phase environmental impacts manifest. Inputs and outputs associated with

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all stages of the life cycle are considered in this assessment, including potential benefits and

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implications associated with incorporating silver into textiles (e.g., decreased laundering, silver ion

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release). The manufacturing and maintenance of the washers, dryers, and clotheslines is not considered

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in this study. To compare key aspects of the life cycle, the stages are represented in this study as

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follows: silver mining and refining (Silver M/R), nanoparticle fabrication (NP Fab), textile fabrication

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(textile), transportation (transport), laundering, disposal, and release of silver ions (Ag+). It should also

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be noted that washing machines do exist that release silver during the wash cycle as a method of

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microbial control, however, these novel washing machines are not considered in this study.

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Assumptions

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Silver (Mining and Refining)

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The silver mining and refining stage includes raw materials and processes associated with the

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mining and refining of silver ore into silver nitrate.

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Nanoparticle Fabrication

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The nanoparticle fabrication stage encompasses the synthesis of silver nanoparticles from silver

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nitrate using NaBH4, the most commonly used reducing agent27, using the methodology of Solomon et

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al.95

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Textile Fabrication



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14 346

The textile fabrication stage includes raw material acquisition, refining, and production of the

347

cotton (CO) or polyester (PL) shirts. Both shirts are assumed to have a total mass of 130 g.44 The raw

348

materials and manufacturing impacts of the polyester shirt are created using the same protocol as

349

Walser et al.44

350

Silver Loading

351

Based on the compiled literature data in Table 1, the average silver loading of 10,800 µg silver/

352

g textile is modeled as the base case scenario while 0.9 and 21,600 µg silver/g textile represent the

353

lowest and highest silver loadings, respectively.

354

Transportation

355

The transportation stage accounts for the impacts associated with transportation the shirt after

356

fabrication and is scaled per shirt. This is modeled based on a shirt manufacturing location of Lebanon,

357

Kansas (geodetic center of the continental United States) and a use stage destination of Chicago,

358

Illinois.96 This results in an allocation of 939 kilometers (km) to rail and 160 km to trucking.

359

Transportation among all other stages of the life cycle is included with their respective inventory

360

inputs.

361

Use Phase (i.e., Laundering)

362

The use phase includes both high efficiency (HE) and regular efficiency (RE) washing machine

363

scenarios to account for the range of efficiencies currently on the market. The HE washer is assumed

364

to use the following resources per load: 0.228 kilowatt hour (kWh) of electricity, 56.8 liters (L) of

365

water, 29.6 milliliters (mL) of detergent, and launder 32 shirts.97-99 The RE washer is assumed to use

366

the following resources per load: 1.173 kilowatt hour (kWh) of electricity, 87.1 L of water, 29.6 mL of

367

detergent, and launder 27 shirts.99-101 Estimates of washer capacity and laundry soap consumption are

368

based on a field experiment utilizing size large men’s t-shirts completed by the authors.

369

Three drying scenarios are posited, a HE dryer, a RE dryer, and a clothesline (LN). The

370

electricity per load of laundry is 2.31 kWh and 3.87 kWh for the HE and RE dyers, respectively; while

371

no electricity is consumed under the LN scenario.102-103 Both dryers modeled are assumed to be ACS Paragon Plus Environment

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electric. Each washer-dryer combination is assumed to have the same load capacity: 32 shirts for the

373

HE appliances and 27 shirts for the RE appliances.

374

The lifetime of both the silver-enabled and conventional shirts is assumed to have a base

375

lifetime of 100 launderings, as presented in the LCA results. During the laundering phase, silver is

376

released from the shirts and enters the wash water.46 The silver released has the potential to contribute

377

to the environmental and human health impacts, and is thus considered in this study. Silver release is

378

modeled as first order decay with a release constant of 2%, in the LCA results. This results in a

379

release, over the assumed lifetime of 100 launderings of approximately 86% of the initial silver

380

content of the textile. The silver released is assumed to enter a WWTP, and undergoes standard

381

treatment conditions, where 95% will be removed into the biomass.65-66 The silver contained in the

382

biosolids is expected to be in the form of Ag2S and AgCl, two relatively stable forms of silver in the

383

environment.66,70,73-75 The 5% of silver discharged from the WWTP is modeled in the form of Ag+

384

silver ions released to freshwater.

385

Disposal

386

The shirt and remaining silver content are disposed of in a landfill setting, characterized as inert

387

material. Silver release in and from the landfill is not considered, as modern landfills are designed with

388

linings and leachate collection systems.

389

Payback period and uncertainty

390

The concept of environmental payback is well established and has been used to compare the

391

relative environmental impact of numerous conventional and emerging technologies, particularly in

392

energy applications such as fuels and photovoltaics.104-106 The environmental payback concept is

393

utilized here to determine the reduction in launderings required to recover the impacts associated with

394

nAg-enabling a shirt. The following relationship was used to model this relationship for each midpoint

395

category:

396 397 ACS Paragon Plus Environment


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16 398

Impact of conventional t-shirt, laundered 100 times

399

=

Impact of Ag-enabled t-shirt, laundered z times

(1)

400 401 402

Using this relationship, the breakeven point occurs when the life cycle environmental and human

403

health impacts associated with the nAg-enabled shirt equal that of the conventional shirt laundered 100

404

times. This is possible with a reduction in launderings (z < 100 launderings) of the Ag-enabled shirt.

405

The degree of reduction necessary depends on a number of factors, not limited to the amount of silver

406

embedded within the shirt, the release rate of silver ions during laundering, and the impact of each unit

407

process for the relevant midpoint category.

408

The impacts of the conventional and nAg shirt were extracted from the previously described

409

TRACI models. The impact of nAg shirt was then subdivided into impacts of the life cycle that will

410

remain the constant (e.g., textile, transport, and disposal), and those that will change as a function of

411

silver content (silver mining and refining and nanoparticle fabrication) and those that will change as a

412

function of the number of launderings (laundering frequency). − ℎ = + ∗ + ∗ + !"# + !$ + %$ 2,

413 414

where the MR (impact of mining and refining silver) and NP (impact of nanoparticle synthesis)

415

(impact /(µg silver /g textile)) are described as a function of the silver content (Co, µg silver /g textile)

416

of the shirt. The impact of laundering (Laund) is described as a function of the impact per laundering

417

(impact/z) and number of launderings of the shirt during its lifetime (z). The three impacts that remain

418

constant under both scenarios are Text (textile, including cotton or polyester raw materials and

419

manufacturing), Trans (transportation of the textile), and Disp (disposal of the textile at the end of its

420

life).

421 422

A linear relationship between initial silver content (Co) and the number of launderings during the lifetime (z) is assumed, where the x-intercept represents Co,max, the maximum amount of silver that


Page 17 of 42


17 423

can be incorporated into the shirt that allows for potential realization of a net benefit by reducing the

424

number of lifetime launderings to zero.

425 426

Since silver release, as Ag+, contributes significantly to the ecotoxicity impact category, an additional term was added to the above equation. − ℎ = + ∗ + ∗ + !"# + !$ + %$ + − " ()* 0.05 3

427

This additional term reflects the ecotoxicity impact due to the release of silver during laundering. The

428

first order release (decay) constant, /, represents the percent of silver lost to the wash water during

429

laundering (1/laundering), and 0.05 represents the fraction of that silver that is released to the

430

environment in ionic form. I is the ecotoxicity impact of the released ionic silver (impact/µg Ag).

431

Due to the non-linear nature of this expression and variability of both and /, a Monte Carlo

432

simulation was performed using MatLab (version R2014b) to consider the number of launderings

433

across all possible scenarios of initial silver concentration (Co, 0.9-21,600 µg silver/g textile) and

434

release rate constant (k, 0.01-0.99). In this analysis, and / are varied across their relevant range

435

using a linearly spaced mesh grid, resulting in 4x104 ( , / points at which the expression was

436

evaluated. The impacts associated with the textile (raw materials and manufacturing), transport, and

437

disposal stages remain constant and are equivalent for the conventional and nAg shirt.

438

Results and Discussion

439

The LCA conducted here evaluates the environmental and human health impacts of cotton and

440

polyester shirts (both with and without silver) under four laundering scenarios (washing

441

machine/drying scenario: HE/LN, HE/HE, RE/LN, RE/RE). The impacts associated with the various

442

washing and drying scenarios are presented in Figure 3 on a per shirt per laundering basis in the units

443

of greenhouse gas equivalents (kg CO2e).

444

Figure 3



Page 18 of 42

18 445

Unsurprisingly, the life cycle greenhouse gas emissions (GHGs) due to laundering vary based

446

on the washing and drying methods used. In all four laundering scenarios considered, electricity is the

447

most significant contributor to the overall GHGs. As expected, use of HE appliances significantly

448

reduces the environmental impact compared to RE appliances. The HE/LN and RE/RE include the

449

minimum and maximum impact values, respectively, and are thus, used henceforth in the analysis.

450

The disaggregated cumulative life cycle environmental and human health impacts of these

451

laundering scenarios for conventional and nAg-enabled (Ag) cotton (CO) and polyester shirts (PL) are

452

compiled in Figure 4. Since several of the impact categories exhibit similar trends in results, four

453

representative categories are presented and include a distribution of environmental and human health

454

impacts: ozone depletion, global warming potential, carcinogenicity, and ecotoxicity. The results for

455

the remaining five impact categories are available in Supplemental Information (Figure S1).

456

Figure 4

457

The results indicate that the life cycle phase (i.e., silver mining and refining, nanoparticle

458

synthesis, laundering, textile, disposal, silver release or transportation) associated with greatest

459

environmental impact is dependent on the midpoint impact category considered. For example, the

460

production and manufacture of the textile (cotton or polyester) contributes most significantly to ozone

461

depletion (Figure 4a) while laundering contributes most significantly to global warming potential.

462

Mining and refining the silver is a significant contributor to the carcinogenicity impact category, and

463

the release of silver as Ag+ contributes most significantly to ecotoxicity.

464

Further evaluation of the underlying resource inputs and processes was carried out for each

465

category. For ozone depletion and the textile stage (Figure 4a), the majority of the impacts are

466

associated with knitting and spinning the fibers of the polyester shirt. However, for the cotton shirt,

467

the greatest portion of the textile impact is due to the farming and processing of the cotton. When

468

considering global warming potential (Figure 4b), the laundering phase is associated with the greatest

469

environmental impact when a dryer is used due to the electricity consumption (previously described in

470

Figure 3). When a dryer is not used, the environmental impact of laundering is significantly reduced ACS Paragon Plus Environment

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19 471

and impacts associated with the textile fabrication become the most significant contributor to GHGs.

472

These results are representative of not only the global warming potential impact category, but similarly

473

for smog formation, acidification, and respiratory effects (Figure S1).

474

For the carcinogenicity impact category (Figure 4c), the textile fabrication is the most

475

significant contributor for the conventional shirt and the two nAg-cotton shirt scenarios. For the nAg-

476

enabled polyester shirts, the impacts from silver mining and refining are most significant, due to the

477

impact of the polyester shirt being less than that of the cotton shirt. These impacts will shift as a

478

function of the initial concentration, Co, of silver incorporated into the shirt. A similar trend is

479

observed for the eutrophication and non-carcinogenicity midpoint impact categories (Figure S1).

480

A novel trend is observed for the ecotoxicity impact category (Figure 4d). Here, the importance

481

of the silver discharged from the WWTP to surface water, as Ag+, is observed for the nAg shirts. This

482

is due to the potential of silver ions to disrupt healthy function of a variety of organisms and thus, to

483

impart negative impacts on the ecosystem.107-108 Based on the literature review (Table 1) the release of

484

silver ions during laundering remains unresolved and warrants further exploration to determine those

485

factors that contribute most significantly to the release of silver from the textile and ways to minimize

486

the associated environmental impacts.

487

It is important to note that regardless of the impact category considered, nanoparticle synthesis,

488

transportation, and end of life disposal are not significant contributors to the environmental and human

489

health impacts. The conventional washing and drying laundering scenario (RE/RE) utilizes

490

significantly more energy than the high efficiency washer and line drying (HE/LN) laundering

491

scenario. This suggests that shifts in consumer laundering behavior, including replacement of RE with

492

HE appliances and line drying or reduced laundering as a function of the nano-enabled properties of

493

the textile, has the potential to significantly reduce laundering impacts, and subsequently GHGs. While

494

the median concentration of silver (10,800 µg silver/g textile) (Table 1) was used for these impact

495

assessments, there is potential for the impacts to shift as technology improves (e.g., attachment

496

method, functional efficacy). These combined results suggest that the overall environmental and ACS Paragon Plus Environment


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20 497

human health impacts of next generation nAg-textiles is highly dependent on multiple life cycle stages

498

and that approaches to reduce such impacts are not straightforward.

499

Determining Environmental Payback

500

Considering the utilization of nAg-textiles may shift consumer-laundering behavior, it is

501

important to determine the potential reduction in environmental impacts associated with increased

502

wears per wash. This is pursued by determining the reduction of launderings required for the overall

503

life cycle impacts of a nAg-enabled shirt to be equivalent to those of a conventional shirt (equations 1

504

and 2), establishing the payback point for the different midpoint impact categories. The results are

505

presented as either a relationship between the initial silver concentration (Co) and the number of

506

launderings (z) (Figure 5), or as a heat map representing the number of launderings (0-100) across all

507

possible Co and k combinations (Figure 6). While the analysis was carried out for all nine-midpoint

508

impact categories, the same four representative categories are shown here and the remaining five can

509

be found in the Supplementary Information (Figure S2).

510

Figure 5

511

The compiled results in Figure 5 illustrate the potential opportunity to realize a net

512

environmental and human health benefit from enabling shirts with nAg in terms of the number of

513

lifetime launderings. The conventional shirt is assumed to have a lifetime of 100 launderings and thus

514

serves as the upper limit (y-intercept). As is indicated in Figures 5 a-d, the impacts of the nAg-shirt are

515

equivalent to that of the conventional shirt when the initial silver concentration (Co) is zero, and thus,

516

the number of lifetime launderings is 100. For all other values of Co, the solid line (green for HE/LN

517

and blue for RE/RE) bounds the shaded region, which represents the number of lifetime launderings

518

permitted to still achieve a net life cycle benefit. The x-intercept represents the maximum

519

concentration of silver (Co,max) that can be incorporated into the shirt and have the potential to recover

520

the life cycle impacts. In other words, the consumer would never launder the shirt (z = 0) during its

521

lifetime. The scales of the x-axes, Co, differ by orders of magnitude and the Co,max decreases

522

accordingly for different midpoint categories with the trend being global warming potential > ozone ACS Paragon Plus Environment

Page 21 of 42


21 523

depletion, ecotoxicity > carcinogenics. This follows the relative contribution of silver mining and

524

refining to the overall impact category, which contributes significantly to the carcinogenicity and only

525

marginally to the global warming potential. Since the ecotoxicity impact includes Ag+ release, the

526

inclusion of the release factor (e-kz) results in a non-linear relationship with the number of launderings.

527

Finally, because the HE/LN laundering scenario is associated with comparatively lower environmental

528

and human health impacts, there is a smaller gain in terms of allowable silver concentration for every

529

unit reduction in laundering (i.e., a steeper negative slope) compared to the RE/RE scenario. In the

530

calculations above, a single release constant was assumed (k = 0.02). Since there is significant

531

uncertainty in both the initial silver concentration, Co, and the release constant, k, and because both

532

variables significantly influence the resulting ecotoxicity impact, an additional sensitivity analysis was

533

carried out for the ecotoxicity payback period using Monte Carlo simulation. The evaluated range for

534

each variable is based on reported values found in the literature (Table 1). Thus, the number of

535

launderings was calculated across all possible scenarios within the ranges of Co (0.9-21,600 µg/g) and

536

k (0.01-0.99) using Equation 2. The results are compiled as a heat map (the color gradient

537

representative of the number of lifetime launderings) for the HE/LN and RE/RE scenarios (Figure 6).

538 539

Figure 6 Based on the lifetime laundering assumptions outlined previously, 100 launderings suggests

540

that no reduction in lifetime launderings is necessary to recover impacts associated with nAg-enabling

541

the shirt (this is the case for Co = 0 µg/g). On the other hand, zero launderings suggests that in order to

542

recover the impacts associated with incorporating nAg, the shirt can never be washed during its

543

lifetime. The maximum allowable silver content (Co,max) associated with each laundering scenario is

544

15,000 and 1200 µg Ag/g textile for RE/RE and HE/LN, respectively. It is interesting to note that k is

545

significant only under certain conditions, particularly when k is small, ~0.01- 0.05, and when Co is

546

below 6,000 and 600 µg Ag/g textile for RE/RE and HE/LN, respectively. Otherwise, the resulting

547

number of permitted lifetime launderings is driven by Co. Additionally, since the HE/LN laundering

548

scenario is associated with comparatively lower environmental and human health impacts, there is a ACS Paragon Plus Environment


Page 22 of 42

22 549

smaller gain in terms of allowable silver concentration for every unit reduction in laundering compared

550

to the RE/RE scenario. Finally, for most values of Co and k, a large reduction in washes (90-100%) is

551

required to realize a net ecotoxicity benefit indicating that a reduction in life cycle environmental

552

impact via reduction of life time launderings may not be attainable.

553

Shifting Consumer Behavior

554

As mentioned previously, shifting consumer laundering behavior in terms of frequency has the

555

potential to significantly reduce the lifetime environmental and human health impacts of nAg-enabled

556

shirts with respect to the impact categories of global warming potential, smog formation, acidification,

557

and respiratory effects. If the added silver reduces the lifetime number of launderings, there will be an

558

added environmental benefit with respect to the use phase. On the other hand, carcinogenic potential

559

(and other impact categories that follow the same trend) are largely independent of the number of

560

reduced launderings. Making the necessitated reduction in the number of launderings unrealistically

561

high. At the same time, it is unknown whether the adoption of a nAg shirt would actually cause the

562

consumer to launder the textile less frequently between wears.

563

Added Functionality

564

While these nAg textiles are primarily marketed to consumers for their odor reduction

565

capabilities, there are additional potential benefits resulting from silver’s antimicrobial properties. One

566

such benefit is reduced transmission of infection or transfer of contamination, which has a clear

567

applicability in hospitals or other environment where sterility is imperative. Hospital acquired

568

infections are the fourth highest cause of death in the United States (behind heart disease, cancer, and

569

stroke).109 Contaminated hospital objects, such as the floor, bed linens, gowns, over-bed tables, and

570

blood pressure cuffs serve as potential sources of contamination.110 The Center for Disease Control

571

(CDC) has reported that Methicillin-resistant Staphylococcus aureus infections (better known as

572

MRSA) can spread through indirect contact, by touching contaminated objects (such as towels, sheets,

573

wound dressings, and clothes) that have come into direct contact with the infected wound.111 Hospital

574

linens have been evaluated for their role in microbial transfer; in one study, clean linen (before patient ACS Paragon Plus Environment

Page 23 of 42


23 575

contact), dirty linen, and staff uniforms were all found to be contaminated with microbes, with 57% of

576

the species found identified as pathogenic.112 Biocidal textiles (including nAg) have been proposed as

577

an effective method for reducing hospital acquired infections.113,114 A 2012 study estimated that

578

hospital acquired infections in adults cost approximately $10 billion in the United States annually.115

579

Thus, there are significant human health and economic benefits that can be realized through the

580

adoption of antimicrobial textiles (such as those enabled with nAg).

581

Conclusions

582

Upon comprehensive review of the nAg-textile LCA literature, an impact assessment under a variety

583

of scenarios (i.e., silver loading, silver release, laundering) was completed to address critical research

584

gaps. This enabled identification of the most important aspects of the nAg-enabled shirt life cycle,

585

based on the assumptions used in the model, across nine environmental and human health midpoint

586

impact categories. For those impact categories where the use-phase dominates, such as global warming

587

potential, shifts in laundering behavior have significant influence on the net impact. As such, there is

588

potential to realize an environmental benefit by incorporating nAg in textiles intended to reduce the

589

frequency of laundering. However, when considering other environmental impact categories where the

590

silver mining and refining contribute significantly, such as carcinogenics, there is reduced likelihood

591

of achieving impact payback through reduction in the number of launderings (i.e., when Co,max is order

592

of magnitude lower). The release of silver, as Ag+, in the wash water is an important influence on the

593

ecotoxicity impacts. One way to reduce the release of silver from textiles is to advance research aimed

594

at improving methods for incorporating silver into the textile fabric. This will not only prevent

595

unnecessary silver release during laundering (and reduce k), but also has the potential to extend the

596

functional performance lifetime of the silver-enabled t-shirt. In addition, reducing in the amount of

597

incorporated silver (Co) will subsequently reduce the cumulative life cycle impacts associated with this

598

next generation textile technology. Further research is required to optimize the minimum amount of

599

silver required to achieve the desired functional efficacy (i.e., prevention of odor causing bacterial

600

growth). ACS Paragon Plus Environment


Page 24 of 42

24 601

While the results presented herein address several existing gaps, additional research is required

602

to fully resolve the outstanding issues (e.g., consumer behavior, attachment technology, functional

603

efficacy, impacts and form of nAg released) to ensure sustainable implementation of nano-silver

604

textile applications. Still, much can be gleaned from a regulatory standpoint. First, it is important to

605

consider the final product and its entire life cycle when evaluating the safety and environmental impact

606

of a nAg enabled textile. Second, because the different environmental and human health impact

607

categories present strikingly different profiles, generalized conclusions cannot be drawn and thus, must

608

be treated case-by-case. Finally, shifts in consumer behavior cannot be assumed. Critical and

609

comprehensive evaluation, like that presented herein, will elucidate the contribution of consumer

610

behavior to the overall potential realization of a net life cycle benefit.

611

Acknowledgments

612

ALH, LMG, JBZ, and TLT acknowledge the generous support of the U.S. Environmental Protection

613

Agency Assistance Agreement No. RD83558001-0 that funded this research. JSY recognizes the

614

support from the U.S. Environmental Protection Agency STAR Fellowship Assistance Agreement No.

615

FP-917515010. This work has not been formally reviewed by EPA. The views expressed in this

616

document are solely those of the authors and do not necessarily reflect those of the Agency. EPA does

617

not endorse any products or commercial services mentioned in this publication.

618

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619

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620

2 Alexander, J.W. History of the medical use of silver. Surg. Infect. 2009, 10, 289-292.

621

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862 863 864 865 866 867 868 869 870 871 872


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Reduction in Laundering Required per Unit nAg Loading to Achieve Parity ( )

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Page 35 of 42 Environmental Science & Technology

Tables and Figures

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Page 36 of 42

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Number of Silver Products

Textile Suspended

120

19

Number of Annual Nano Silver Patent Applications

Nanosilver

2 3 4 5 6 7 8 9 10 11 12

Product Category

Figure 1. a) United States nAg patent applications by year b) PEN nAg search by product category (Retrieved July, 25, 2014) nAg patent applications per year from 1990-2013. Overlap in the different product categories is possible since multiple product matrices were investigated in the patent application search. Specific search terms included textile, fabric, shirt, pants, and clothing. Suspended nAg products include personal care products and coatings Embedded products are those where the product encases the silver, such as in composites. The medical category includes applications such as silver bandages to promote wound healing. Textile applications refer to clothing items such as shirts and socks, and are the focus of this work. Overall, there has been a significant increase in nAg applications over the past decade.


Page 37 of 42


Table 1. Compiled research to date on silver content and release from commercially available textiles arranged by product type Silver Released During Silver Content Product Type Laundering Conditions (µg silver/g textile) (% of total silver content) Sock 36 1111 Benn and Westerhoff used a laundering protocol where socks were placed 1 in 1 liter glass bottles with ultrapure water and agitated for either 1 or 24 Sock 58 117 hours. In their paper the authors acknowledge that although detergent was Sock 2.1 ND not used in the study, most consumers do use detergent when laundering textiles. Socks in this study had between 0.9 – 1,358.3 µg of silver/g of Sock 1,358

Life Cycle Payback Estimates of Nanosilver Enabled Textiles under

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