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7 Treatment of Nanoparticles in Wastewater Kim M. Henry AMEC Earth & Environmental Kathleen Sellers ARCADIS U.S., Inc. CONTENTS 7.1 Mass Balance Considerations .....................................................................156 7.1.1 Case Study: SilverCare™ Washing Machine ....................................157 7.1.2 Case Study: Socks with Nano Silver ................................................159 7.2 Treatment Processes ....................................................................................160 7.2.1 Sedimentation ..................................................................................160 7.2.2 Coagulation and Flocculation .......................................................... 161 7.2.3 Activated Sludge ..............................................................................162 7.2.4 Sand Filters ......................................................................................164 7.2.5 Membrane Separation ......................................................................165 7.2.6 Disinfection ......................................................................................165 7.3 Summary .....................................................................................................165 References ..............................................................................................................166 Commercial products incorporating nanomaterials eventually reach the end of their usable life. Sunbathers wash sunscreen containing titanium dioxide (TiO2) nanopar-ticles from their skin; antimicrobial silver particles drain from washing machines in the rinse cycle; paints and coatings f lake; or materials are landf illed. What happens to those nanoparticles at the end of product life? In short, no one knows. Initial atten-tion has focused on the fate of nanoparticles in wastewater treatment. Nanoparticles can enter a municipal wastewater treatment plant as a result of commercial use and discharge. Wastewater discharges from manufacturing processes also can contain nanoparticles. As illustrated by examples in this chapter, however, the discharge and fate of nanomaterials is diff icult to quantify. 155 © 2009 by Taylor & Francis Group, LLC 156 Nanotechnology and the Environment The same unique properties that make nanomaterials so promising in a wide variety of industrial, medical, and scientif ic applications may pose challenges with respect to wastewater treatment. In 2004, because the toxicity of nanomaterials and their fate and transport in the environment were not well understood at the time, the British Royal Society and the Royal Academy of Engineering recommended that “factories and research laboratories treat manufactured nanoparticles and nano-tubes as if they were hazardous, and seek to reduce or remove them from waste streams” [1]. Although the body of research regarding the toxicity, fate, and trans-port of nanoparticles has grown [2], literature surveys in 2006 and 2007 indicate that the behavior of nanomaterials during wastewater treatment has not been well studied [3, 4]. An abstract for a research project to evaluate the removal of various types of nanoparticles during wastewater treatment, which was funded by the U.S. EPA’s National Center for Environmental Research (NCER) for the period from 2007 to 2010, states: “Today, almost no information is available on the fate of manufactured nanoparticles during biological wastewater treatment” [5]. This chapter discusses the potential for various treatment processes to remove nanoparticles from waste streams. A general description of each process is provided, as well as an evaluation of how particular properties of nanomaterials can reduce or enhance the effectiveness of the process. Research findings are provided where available, or an indication is given as to whether research is ongoing at the time of writing this book. While the primary focus is treatment processes in a typical municipal wastewater treatment plant, many of these processes are used in industrial wastewater treatment. Certain processes also may apply to drinking water treatment and, where relevant, the f indings from water treatment research are also discussed. 7.1 MASS BALANCE CONSIDERATIONS Concerns over the presence of nanoparticles in wastewater streams, which could eventually accumulate in sewage sludge or discharge to the environment in treated wastewater, must be put into context. The concentration of a nanomaterial in waste-water depends primarily on: • The amount of local production or use of commercial products containing nanomaterials • Whether the nanomaterials are f ixed in a matrix (such as the carbon nano-tubes in a tennis racket) or free (such as TiO2 nanoparticles in sunscreen) • The amount of the free nanomaterial in the product • The fraction that is washed down the drain • The degree of agglomeration or adsorption occurring in aqueous solution that changes the form of the nanoparticle or removes it from solution • The extent of dilution No studies have been published of which the authors are aware that attempt to quantify the discharge of nanomaterials into wastewater treatment plants. Given the recent growth of the industry, the wide variety of materials entering the market, and the conf identiality of their formulation, this comes as no surprise.Two case studies © 2009 by Taylor & Francis Group, LLC Treatment of Nanoparticles in Wastewater 157 illustrate both the potential for nanomaterials to enter wastewater streams and the diff iculty in making such an estimate when the details of product manufacture are proprietary. Coincidentally, both examples concern the discharge of silver when washing clothes. 7.1.1 CASE STUDY: SILVERCARE™ WASHING MACHINE Samsung’s SilverCare™ option on several models of washing machine uses silver ions to sanitize laundry. Samsung reportedly spent $10M to develop this technology [6]. The details of the technology are, understandably, proprietary. Company litera-ture describes the technology in several ways. According to one account [6], the sys-tem electrolyzes pure silver into nano-sized silver ions “approximately 75,000 times smaller than a human hair”; assuming that a human hair is approximately 60 to 120 micrometers (μm) wide [4], then the silver nanoparticles would be on the order of 1 nm in diameter. Elsewhere [7], Samsung described their system as follows: “[A] grapefruit-sized device alongside the [washer] tub uses electrical currents to nano-shavetwosilverplatesthesizeoflargechewinggumsticks.Theresultingpositivelycharged silver atoms — silver ions (Ag+) — are injected into the tub during the wash cycle.” Thesetwodescriptionsdifferenough tomakeitunclearwhetherthesilverisreleased as a true nanoparticle (ca. 1 nm diameter) or as ionic silver. (Silver has an atomic diameter of 0.288 nm and an ionic radius of 0.126 nm [8], and thus silver ions are smaller than the nanoparticle size range of 1 to 100 nm.) Based on the electrolysis process, both may be present. Key and Maas [9] indicate that electrolysis of a silver electrode in deionized water produces colloidal silver containing both metallic silver particles (1 to 25 wt%) and silver ions (75 to 99 wt%). The silver particlesobserved in colloidal silver generally range in size from 5 to 200 nm; a particle 1 nm in diameter would consist of 31 silver atoms. This information suggests — but certainly does not conclusively prove — that the SilverCare™ washing machine discharges a mixture of silver ions and silver nanoparticles. Silver ions, rather than nanoparticles, may comprise most of the mass. Samsung has offered several indications of the amount of silver released when washing a load of clothing. Their product literature notes that electrolysis of silver generates up to 400 billion silver ions during each wash cycle [6, 10]. The two chew-ing-gum sized plates of silver reportedly last for 3000 wash cycles [10]. Finally, Samsung reportedly has indicated that using a SilverCare™ washing machine for a year would release 0.05 g silver [11]. With respect to the sanitizing function that this release of silver provides, Sam-sung has indicated that the silver ions “eradicate bacteria and mold from inside the washer” and “stick to the fabric” of clothes being washed to provide antibacterial function for up to 30 days [10]. A Samsung representative stated that “silver nano ions can easily penetrate ‘non-membrane cell’ [sic] of bacteria or viruses and sup-press their respiration which in turn inhibit [sic] cell growth. On the other hand, Silver Nano is absolutely harmless to the human body” [6]. While Samsung has marketed this antibacterial action as a benef it to customers, some consumers have become concerned about the potential consequences of using © 2009 by Taylor & Francis Group, LLC 158 Nanotechnology and the Environment SilverCare™products.Initialeffortstomarketthewashingmachinemetwithresistance in Germany and the washing machine was taken off the market in Sweden for a brief timeduetoconcernsoverthepotentialtoxiceffectsofdischargingsilvernanoparticles from the use of these machines to wastewater treatment plants [11, 12]. Chapter 4 dis-cusses regulatory actions in the United States regarding such washing machines. Attempts to quantify the discharge of silver from using the washing machine — and thus illuminate the potential effects on a municipal wastewater treatment plant — provide a range of answers based on the available data. In addition to the information provided above regarding the mass and potential form of silver released, the following assumptions about wastewater generation were used to complete a con-servative mass balance: • Each wash cycle uses 12.68 gallons of water [13]. • The typical residence generates approximately 70 gallons of wastewater per person per day [14]. • A four-person household does two loads of laundry per day on average. • All the silver generated in the washing machine enters the sewage. Further, the authors measured the size of a stick of gum at approximately 0.2 by 1.8 by 7.2 cm, assumed that the density of a silver bar was 10.4 g/cm3 [8], and conser-vatively assumed that the entire mass of silver in the two plates would be entirely consumed within the 3000-cycle lifetime. As a f irst approximation, the amount of nanosilver particles that could enter a wastewater treatment plant from the use of SilverCare™ in washing clothes could range from 0.001 micrograms per liter (μg/L) to an extreme upper bound concentra-tion of 9 μg/L. The lowest estimate is based on the reported release of 0.05 g silver per year and the assumption that only 25% ofthe mass wouldcomprise nanoparticles (rather than ions) of silver. The highest estimate is based on complete consumption of the two silver plates during the unit lifetime and the assumption that 75% of the silver was in nanoparticulate form. The actual concentration of nanoparticles would be lower than either of these estimates due to adsorption and agglomeration. Labora-tory experiments with solutions of 25-nm and 130-nm silver particles showed that upon vortex mixing, the silver agglomerated into particles ranging up to 16 μm in diameter, well outside the nanoparticle range [15]. Further, the mass balance calcula-tions do not account for dilution by sources of wastewater other than domestic sew-age from homes using SilverCare™ washing machines. Dilution from other sources would also decrease the concentration of silver nanoparticles. Thus, the upper bound estimate of 9 μg/L should be regarded as an extreme upper bound. What effect could this discharge of silver have on the microorganisms in a wastewater treatment plant? As described previously, silver has antimicrobial prop-erties. At the time this book was written, the authors could not identify published benchmarks that enabled them to directly compare the estimated discharge of silver nanoparticles to levels that are either “safe” or “toxic” to microorganisms at a sew-age treatment plant. The acute ambient water quality criterion for silver, which was not derived specifically for nanoparticles, is 3.2 μg/L [16]. This concentration is comparable to the upper bound estimate of the discharge of silver nanoparticles into © 2009 by Taylor & Francis Group, LLC Treatment of Nanoparticles in Wastewater 159 wastewater from using the SilverCare™ system; however, as noted above, that upper bound estimate was quite conservative. As described below, research on the toxicity of silver nanoparticles provides further relevant information. Rojo et al. [17] assayed the toxicity of colloidal silver nanoparticles in the 5- to 20-nm size range to zebraf ish embryos.They tested solutions containing between 1 and 5000 μg/L silver nanoparticles. Their initial tests showed no effect on devel-opment or survival of the embryos in the first 2 weeks. Subsequent experiments monitored effects on eight selected genes. At the highest nanosilver concentrations tested, the researchers “found a clear effect on gene expression in most cases.” Those concentrations were, however, orders of magnitude higher than the estimated levels of silver nanoparticles in wastewater described above. Other researchers have worked with mammalian cell lines to test the toxicity of silver nanoparticles. Hussain et al. [18] tested the effect of solutions containing 10 to 50 μg/L silver nanoparticles (15 nm) on PC-12 cells. This neuroendocrine cell line originated from Rattus norvegicus (Norwegian rat). The research team observed decreased mitochondrial function in the PC-12 cells upon exposure to the silver nanoparticles. Skebo et al. [15] showed that rat liver cells could internalize silver nanoparticles (25, 80, 130 nm) but that agglomeration of nanoparticles can limit cell penetration. Finally, Braydich-Stolle et al. [19] tested the effects of 15-nm silver nanoparticles on a cell line established from spermatogonia isolated from mice. The nanoparticles reduced mitochondrial function and cell viability at a concentration between 5 and 10 μg/mL (or 5000 and 10,000 μg/L). The researchers estimated the EC50, or the concentration that would provoke a response half-way between the baseline and maximum response, at 8750 μg/L. This level is orders of magnitude higher than the first approximation estimates of silver nanoparticles in wastewater from using the SilverCare™ system. 7.1.2 CASE STUDY: SOCKS WITH NANO SILVER Several manufacturers market socks impregnated with nanosilver particles as an antibacterial agent. Westerhoff’s [20] team at Arizona State University measured the amount of silver that f ive different brands of socks could release when washed. They simulated washing by placing the socks in deionized water for 24 hours (hr) on an orbital mixer, removing, drying, and then rewashing the socks three times (for a total of four wash cycles). Four of the test socks initially contained silver at 2.0 to 1360 μg/g sock. The fifth sock contained no measurable silver. The amount of silver that leached out of the silver-bearing socks after four simulated wash cycles ranged from 0 to 100%. The concentration of silver in the wash water ranged from less than 1 to 600 μg in 500 mL wash water, or up to 300 μg/L. The research team noted that it was difficult to distinguish between silver ions, silver nanoparticles, and aggregated silver nanoparticles in the wash water. These initial laboratory results are difficult to extrapolate to the concentration of silver that might result in sewage from washing socks containing silver nanopar-ticles. As noted above, the typical wash cycle uses more than 12 gallons of water (rather than 500 mL) and runs for much less than 24 hr, suggesting that dilution and © 2009 by Taylor & Francis Group, LLC ... - tailieumienphi.vn
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