|Benjamin Espinasse, CEINT, Duke University|
|Mesocosms provided outdoor labs to study silver nanoparticles.|
The silver from tiny particles used to kill microbes in clothing and other consumer goods may wind up in plants, insects and fish, according to new research. The study is the first to track how silver nanoparticles react, change and move through ecosystems, and it adds to a growing number of studies that raise concerns about their widespread use as anti-microbial agents. Researchers have not yet studied their potential effect on plants and animals.
Manufacturers are putting silver nanoparticles into a growing list of consumer products despite the fact that little is known about their health or environmental impacts. The Project on Emerging Nanotechnologies estimates that as of 2010, more than 300 products use sliver nanoparticles as an antimicrobial agent. These items include clothing, food storage containers, pharmaceuticals, cosmetics, electronics and optical devices.Silver nanoparticles are very small chunks of silver metal made up of thousands of silver atoms. They are so small that 400 million would fit in the period at the end of this sentence. A chemical coat is often added to prevent clumping and protect their silver core.The element silver discourages the growth of bacteria and other pathogens. The U.S. Environmental Protection Agency regulates silver and some related compounds as pesticides. The agency supports health and safety testing of the highly used silver nanoparticles but does not regulate their specific use.Like most nanoparticles, silver varieties have benefits and risks. Their unique size gives them properties different from both large pieces of silver and individual silver ions. As antibacterial agents, the silver nanoparticles are far more effective, cheaper and use less silver than many alternatives.Silver’s ability to kill bacteria has raised concerns that the nanomaterials may affect beneficial bacteria and the plants and animals essential to a healthy ecosystem. While it is not considered toxic to people, invertebrates and fish are far more sensitive to silver.Cell studies suggest silver affects nerve cells, while silver nanoparticles have been shown to interfere with human sperm development. It is known that fish are vulnerable to even low doses of silver, andstudies indicate that silver nanoparticles can cause malformations and death in embryos exposed to the materials (Bar-Ilan et al. 2009). Exposure to silver also affects reproduction in clams (Brown et al. 2003).Silver nanoparticles are released from products during normal use and washing. They enter the environment through wastewater, as water treatment facilities do not always remove them completely (Keagi et al. 2011).Researchers are rushing to understand what happens to nanoparticles after they are released. Initial studies suggest they can clump together to make larger particles, dissolve to release silver ions and react with oxygen and sulfur to form new types of particles.
Silver nanoparticles were sprayed onto soil and water in simulated terrestrial and wetland habitats to determine how the particles might change chemically, move through the ecosystems and interact with plants and animals after they get into the environment.The researchers built the habitats (called mesocosms) in open boxes and left them outside in Duke Forest – a Duke University research area in North Carolina – for 18 months. They added wetland plants typical to the southern United States. Mosquitofish and insects were accidentally introduced with the plants and soils. Other wild insects colonized the ecosystems. Many of the species completed their life cycles during the project.They regularly sampled the soil, water and fish to follow how the silver moved through the constructed environment. At the end, silver levels were analyzed in the soil, water, plants and animals – the fish, fish embryos and insects. They also measured distinct silver compounds to determine how the silver ions reacted with oxygen, sulfur and chlorine in the soils and water.Silver levels measured in the dosed plots were compared to the levels measured in the control plots where silver nanoparticles had not been applied.
Researchers found silver accumulated in both the terrestrial plants (up to 3 percent of the total added) and the aquatic animals.Plants growing in soil that had been dosed with nanoparticles had up to 18 parts per million silver while lower silver levels – ranging from 1 – 7 parts per million – were measured in the plants growing near the water that was dosed with the silver nanoparticles. In all cases, silver was measured in plants that started growing 6 months or more after the application of silver nanoparticles, indicating that the plants could absorb the silver from the soil.They also measured levels of silver in the fish and insects, which included mosquitofish, dragonfly larvae and midges. The mosquitofish had 20 – 30 times higher silver levels than fish in the control plots.More troubling were the high levels (10 – 20 times higher than control) of silver that passed from female mosquitofish to their developing embryos. However, the majority of the silver – about 70 percent of the total put into the systems – was recovered from the soils and the wetland sediments. In addition, for silver nanoparticles applied to the terrestrial environment, erosion and runoff carried some of the metal from the soils into the water where it mostly settled in the sediments.The silver nanoparticles reacted and changed after they were released, but differed between the terrestrial and water habitats. By the end of the study only 18 percent of the silver that was added to the water remained in the original form. The majority – 55 percent – had reacted with sulfur to form silver sulfide, while about 27 percent bound to the organic matter in the bottom sediment.Particles applied to the terrestrial environment were slightly less reactive. Forty-seven percent remained in the original form, while 52 percent reacted to form silver sulfide. These reactions happened more slowly than predicted by laboratory experiments, showing that lab studies do not always accurately predict what happens in the environment.
In terrestrial soil and water, silver nanoparticles can chemically change and contaminate plants and animals with silver. Any health effects from exposures to the metal in this way are not known.The outdoor study from North Carolina is one of the first long term ones to examine how silver nanoparticles change in the environment, specifically in soils, sediments and water. It is also a first step to clarifying where the silver nanoparticles move to after they enter the environment. One surprise was that they tend to settle in the soil and in the sediments underwater.The researchers demonstrated that under realistic conditions the majority of the silver nanoparticles reacted with sulfur and oxygen, changing their structure and function. These newly created silver compounds can be more stable and less toxic than the silver nanoparticles.Over time, though, they may build up in the environment, providing relatively large quantities of silver that can be incorporated into plants and animals. These long-term reservoirs of silver in soils and sediments may lead to increased exposures. For example, silver was found in plants that started growing six months after the nanoparticles were applied.Once changed, the newly formed silver compounds migrated through the soil and water. Runoff and erosion also moved the metal compounds and the nanoparticles. Eventually, the silver was taken up by the plants, insects and fish living in the mock ecosystems.Silver also passed from the mother fish to her embryos. The transfer of contaminants from one generation to the next exposes the embryos early in development and can have long-term effects on health, reproduction and populations (Wu et al. 2010).The effects on plants and other animals in this study are still unknown. Previous research indicates silver can harm fish, clams and other aquatic species.A next step may be to determine if these exposures have any health effects on the species studied. Scientists will also need to determine the levels of silver nanoparticles released into the environment from consumer products. Initial results from a study done on socks containing silver nanoparticles show more than 50 percent of the silver escapes during the first few washings (Geranio et al. 2009). It will also be important to identify the levels of silver nanoparticles that remain intact after release and to understand plant and animal exposures.As more data become available, it may be important to evaluate which products will benefit most from having the silver nanoparticles and which ones may not be worth the risk they may pose to health and environment.(Audrey Bone, Doctoral Student, Duke University, Integrated Toxicology and Environmental Health Program contributed to this synopsis.)
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