A Nanoparticle Primer
Figure 1. Titanium dioxide nanoparticles suspended in ultra-pure water and imaged using transmission electron microscopy.
According to the Woodrow Wilson Institute Project on Emerging Nanotechnology, there are over 1200 consumer products containing engineered nanoparticles currently for sale in the United States . This represents over 500 percent increase in the number of nanoparticle containing products on the market since monitoring began in 2005. As products are rapidly introduced to the market, there is insufficient time to fully understand the ultimate fate of the nanoparticles in these products and potential consequences to human and ecological health.
Engineered nanoparticles are designed in a range of sizes, shapes, and compositions to have at least one dimension less than 100 nanometers (nm). When materials, such as metals, are produced on the nano-scale, many properties are altered compared to the bulk material. These altered properties, such as change in color or antibacterial strength, make nanoparticles advantageous for use in consumer products and medical applications. Nanoparticles are commonly used in personal care products, food storage containers, cleaning supplies, bandages, clothing, and washing machines . An image of nanoparticles in ultra-pure water taken by a transmission electron microscope is presented in Figure 1.
Nanoparticles are likely to enter surface waters during the production, usage, and disposal of nanoparticle containing products. Since 1990, the number of patents for nanoparticle products has doubled every two years . Estimates of nanoparticle production are in the range of 500 tons/year and 50,000 tons/year for silver and titanium dioxide (TiO2), respectively [3-4]. Estimates of nanoparticle concentrations in some natural surface waters are in the nanogram/L - microgram/L range (parts per trillion to parts per billion) [4-5], but the concentrations may increase with greater production and use of nanoparticle - containing products. Using highly sensitive instrumentation, researchers have detected nanoparticles in surface and drinking water samples , but it is unknown if these are naturally occurring or engineered nanoparticles. As instrumentation capabilities improve, researchers will be able to further identify and quantify nanoparticles in water samples with the potential to determine the source.
" As products are rapidly introduced to the market, there is insufficient time to fully understand the ultimate fate of the nanoparticles in these products and potential consequences to human and ecological health. "
Although the likelihood of nanoparticles to enter surface waters, and consequently drinking water sources, is high, only a few studies have investigated the removal of nanoparticles by drinking water treatment processes. These studies have found that nanoparticles can be removed by conventional treatment, but that the removal efficiencies are highly dependent on water characteristics [7-8]. Factors such as pH, natural organic matter (NOM) content, and salt composition can influence the size, aggregation, dissolution, and stability of nanoparticles in the water. These factors will in turn impact the removal of nanoparticles from the drinking water, especially with conventional treatment such as coagulation/flocculation/sedimentation. However, most of these studies were conducted using nanoparticles spiked into ultra-pure, dechlorinated tap water, or synthetic freshwaters at high nanoparticle concentrations. To date, no studies have investigated removal from natural waters or removal through the full drinking water treatment process, including the effects of disinfectants, which may react with nanoparticle surfaces.
But the nanoparticle-water story is not one-sided; nanoparticles are also increasingly used to improve water quality, especially for the developing world. Nanoparticles, such as silver, TiO2, and zinc oxide (ZnO), have been shown in laboratory settings to be anti-bacterial for drinking water treatment [9-10]and could be used in lieu of chemical disinfectants that can cause harmful by-products . Silver has historically been used for disinfection , and silver nanoparticles may be even more effective than bulk silver. Silver nanoparticles can inactivate microbial respiratory enzymes, increase cellular reactive oxygen species generation, and affect DNA replication [10, 12]. To this end, silver nanoparticles are being imbedded into paper and ceramic filters for their antimicrobial properties during water treatment [13-14].
While silver may kill bacteria directly, TiO2 nanoparticles can act as a photocatalyst to inactivate bacteria and viruses and degrade organic chemicals, inorganic chemical contaminants, and heavy metals, especially in the presence of ultraviolet light [15-17]. TiO2 nanoparticles and carbon nanotubes may be incorporated into drinking water treatment technologies for removal of dissolved contaminants, metals, and NOM in order to prevent or delay membrane fouling, therefore, extending the effective life of the membrane [10, 18]. Nanoparticles offer a great potential to create anti-bacterial technologies without great cost and thereby allowing for greater access to safe water in the developing world [10, 15]. Already, many groups are using ceramic filters impregnated with silver nanoparticles to remove pathogens from drinking water in rural areas of developing countries .
" Although the likelihood of nanoparticles to enter surface waters, and consequently drinking water sources, is high, only a few studies have investigated the removal of nanoparticles by drinking water treatment processes. "
Despite these potential advantages of using nanoparticles to disinfect water, it is important to determine if nanoparticles leach through the treatment technology into finished drinking water and to consider the potential health implications of ingestion of nanoparticles in drinking water. Many published studies do not examine nanoparticle leaching [20-21]. A few researchers have measured total metals in treated water after filtration with nanoparticle enhanced treatment technologies, but this is an indirect measure that only indicates potential presence of nanoparticles in water [13-14]. For example, water filtered through silver nanoparticle incorporated paper filters contained detectable levels of silver, although concentrations were below the U.S. Environmental Protection Agency and World Health Organization guideline of 100 ppb silver in drinking water . Ceramic filters coated with silver nanoparticles continued to leach silver into drinking water for the 15 hours of tested filtration . Initial water samples collected within the first two hours of filtration contained silver that exceeded the 100 ppb guidelines, as seen in Figure 2 . These researchers did not determine if the silver that leached through the filters was silver ions or nanoparticles, but the results indicate that the treatment technologies might not be benign. While the filters were effective at removing the microbial contaminants, it is important to weigh the risks of ingesting low levels of nanoparticles in the finished drinking water.
Figure 2. Silver detected in finished drinking water over time from silver nanoparticle painted ceramic filters prepared from three different soil types. Reprinted with permission from Oyanedel-Craver, V. A.; Smith, J. A., Sustainable Colloidal-Silver-Impregnated Ceramic Filter for Point-of-Use Water Treatment. Environmental Science & Technology 2007, 42, (3), 927-933 . Copyright 2007 American Chemical Society.
The effects of ingested nanoparticles are still being investigated, but research indicates that there are adverse health effects from exposure to nanoparticles through in vitro and in vivo experiments. In laboratory experiments at the cellular level, exposure to nanoparticles has lead to cell death, DNA damage, and increased reactive oxygen species [22-24]. Due to their small size, nanoparticles can accumulate inside cells, and, once inside, nanoparticles may release ions that can directly impact cell functioning . In vivo animal studies have investigated the effects of ingestion of nanoparticles. One study found that after 5 days of ingesting TiO2 nanoparticles in drinking water, rats had detectable DNA damage . In other studies, rats and mice that ingested silver or TiO2 nanoparticles resulted in several organs with increased silver or TiO2 compared to the control, including liver, kidneys, brain, and blood [26-28]. The consequences of these increased metal burdens is not fully understood, but the results indicate that exposure to nanoparticles via ingestion can lead to nanoparticles or metal ions in circulation throughout the body.
Our current research at The Johns Hopkins University Bloomberg School of Public Health and The Global Water Program is attempting to address some of the emerging questions of nanoparticles and drinking water.The focus of one such project is on three nanoparticles commonly used in consumer products: silver, ZnO, and TiO2 nanoparticles . Our objectives are to gain an understanding of (i) the dynamics of engineered silver, ZnO, and TiO2 nanoparticles in natural waters for their potential to impact drinking water resources, (ii) the removal efficiency of these nanoparticles from natural waters attainable by conventional and advanced drinking water treatment techniques, and (iii) the toxicity of these nanoparticles ingested in drinking waters.
To meet the first two objectives, we spiked different types of source waters, representing natural drinking water sources, with carefully characterized commercially available nanoparticles. The experimental water sources varied in pH, salt composition, and NOM content to represent the variety of water conditions in the environment. We conducted experiments that mimicked environmental conditions to assess nanoparticle stability. Follow up experiments mimicked drinking water treatment systems and we explored treatment processes, including: coagulation-sedimentation and membrane filtration. In each of these experiments, we asked the question: did nanoparticles remain dispersed in water? Here, a ‘yes’ means that nanoparticles will most likely remain in environmental water sources or in our finished drinking water, leading to human exposure.
" Although nanoparticles are a relatively cheap and effective anti-bacterial agent in drinking water treatment, it is important to develop products that do not leach nanoparticles into the finished water, since this poses additional public health concerns. "
We met the third objective, assessment of toxicity from ingestion to nanoparticles in drinking water, using in vitro cell culture experiments. Two types of human gut epithelial cells (Caco-2 and SW480) were exposed to nanoparticles in cell culture media and synthetic freshwaters. After exposure, cell death and measures of cell stress were assessed to investigate the effects of nanoparticles on the cells compared to the unexposed cells.
We presented the results of this project at the Society of Environmental Toxicology and Chemistry Annual Meeting (http://boston.setac.org). Our results indicate that natural organic matter stabilizes engineered nanoparticles in experimental waters such that some nanoparticles remained in the waters even after conventional treatment or membrane filtration, suggesting human exposure through drinking water. We found that different nanoparticles resulting in varying degrees of cell toxicity – with some exposures leading to cell death and others in more subtle signs of cell stress.
Although nanoparticles are a relatively cheap and effective anti-bacterial agent in drinking water treatment, it is important to develop products that do not leach nanoparticles into the finished water, since this poses additional public health concerns. It is also important to evaluate if current drinking water treatment processes will sufficiently remove these emerging contaminants to protect public health. Despite all of the many positive and transformative applications of nanoparticles, more research is needed to keep pace with the rapid expansion and proliferation of this potentially harmful technology so that we can make decisions that protect public health before we are all exposed.
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