Scientists have long worried about the ecological dangers of releasing powerful nano-antibacterials into waste and water streams. Now New Scientist reports that exposure to nano-silver, commonly used in odour-killing socks and clothing, triggers microbes to release four times the normal levels of nitrous oxide. Nitrous oxide is a potent greenhouse gas. The UNFCC estimates that nitrous oxide has 280 times the global warming potential of carbon dioxide on a 20 year basis.
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Antibacterial socks may boost greenhouse emissions
ANTIBACTERIAL nanoparticles may have more of an impact on the environment than we thought, including potentially raising levels of greenhouse gases.
Silver nanoparticles are used as an antibacterial agent in a wide range of products, from odour-free socks to wound-healing bandages (see diagram). They can find their way into waste water, and have been shown to reduce the activity of bacteria used to remove ammonia when the water is treated.
So far most of the research on the environmental impact of nanoparticles has been carried out on single microbe or plant species within the laboratory. To try to pin down their action in a more realistic setting, Benjamin Colman, a chemist at Duke University in Durham, North Carolina, and colleagues added a high dose of silver nanoparticles – 1.25 milligrams per gram of water – to microbes in a sample of stream water and soil kept within their laboratory. They also set up two outdoor tubs of plants. Treated sludge known to be free of nanoparticles was added to the soil in both tubs, while one tub was also dosed with 55 micrograms of silver nanoparticles per gram of sludge, a concentration similar to levels often found in waste water.
"We are trying to find out what happens when these silver nanoparticles get into the real environment," says Colman. "These particles are developed with the express purpose of killing things."
Two months on, the microbial population in the outdoor tub containing silver had significantly declined relative to the lab sample measured after one week. What’s more, the activity of the enzymes they produce to break down organic matter was 34 per cent lower in the tub that had been dosed with nanoparticles than in the tub to which only sludge had been added.
Given that the outdoor tub containing nanoparticles had a much lower concentration of silver than the lab samples, the drop in its microbial activity is so large that it suggests the lab samples are not a good guide to real-world behaviour, Colman says.
The team also used a gas chromatograph to measure the gases produced by the microbes. They found that the level of nitrous oxide, a greenhouse gas, given off in the tub containing nanoparticles was four times that in the tub in which only sludge was used. Colman presented the research at the annual meeting of the Ecological Society of America in Pittsburgh, Pennsylvania, last week.
Donald Wuebbles, an atmospheric scientist at the University of Illinois at Urbana-Champaign, says that if the results were replicated on a large scale, it could "further contribute to concerns about global changes in climate". He points out that nitrous oxide can also damage the ozone layer if it gets into the stratosphere.
The team is now planning further experiments, including the setting up of a complete wetland ecosystem to measure how it might be affected by waste water containing silver nanoparticles.