Silver Nanothreads, No Golden Needles: Unintended Consequences?

As the Rev. Jeremiah Wright might say, the chickens are coming home to roost.

On 30 April I attended a talk where I learned of recent disinfection developments using silver nanoparticles. Silver is a very effective bactericide; this fact has been used to create simple ceramic filters to disinfect water in developing countires by groups such as Potters For Peace.

They are even making socks impregnated with silver nanoparticles to kill odor-causing bacteria, and GE even sells a washer that disinfects clothes using silver nanoparticles.

But the silver nanoparticles also kill "good" bacteria, like the kind that remove ammonia from wastewater in treatment plants, says Dr. Zhiqiang Hu of the University of Missouri (MU). This could compromise the ability of wastewater treament plants to function effectively. It could also cause problems in the soil if sewage sludge is used to fertilize plants.

From the University's press release:

Zhiqiang Hu, an MU civil and environmental engineering assistant professor, found in a National Science Foundation (NSF)-sponsored study that silver nanoparticles destroy benign bacteria employed across the country to remove ammonia from the wastewater treatment system. Several products containing silver nanoparticles already are on the market, including nanosilver socks designed to inhibit odor-causing bacteria and wash machines that disinfect clothes with the tiny particles.

“We found that silver nanoparticles are extremely toxic,” Hu said.

Hu said nanosilver particles generate more unique chemicals—known as highly reactive oxygen species—than do larger forms of silver. Those chemicals likely inhibit bacterial growth, he said.

The Water Environment Research Foundation (WERF) recently awarded Hu $150,000 to determine more precisely when silver nanoparticles start to impair wastewater treatment. Work on the follow-up research is slated to start May 1 and be completed by 2010, Hu said.

In that project, Hu will determine how silver nanoparticles affect representative wastewater treatment processes by gradually releasing as well as injecting a shock load of the nanomaterial into wastewater and sludge. Measuring subsequent microbial growth will allow MU researchers to determine the nanosilver levels that will harm wastewater treatment and sludge digestion, Hu said.

With that knowledge, nanoparticles in wastewater can be better managed and regulated, he said.

Hu’s silver nanoparticle research has been published in Water Research and Environmental Science & Technology.

In this picture, courtesy of Professor Hu, arrows point to silver nanparticles attaching themselves to bacteria.

"Inside every silver lining there's dark cloud." -- George Carlin