Some carbon nanotubes deplete beneficial microbes in certain soils

(Phys.org)—Some types of carbon nanotubes used for strengthening plastics and other materials may have an adverse effect on soil microbiology and soil microbial processes, a Purdue University study shows.

Specifically, these raw, non-functionalized single-walled carbon nanotubes were shown to damage the active microbiology in low-organic soil. Ron Turco, a professor of agronomy, said many of the bacteria affected could be involved in carbon and nitrogen cycling, which are critical processes to ensure a fully functional soil.

"There appears to be more negative potential on the active microbial population than we thought," said Turco, whose findings were published in the journal Environmental Science & Technology. "The as-produced materials could be a negative environmental situation if they are released into low-organic soils that could not absorb them."

Functionalized carbon nanotubes have modifications that create chemical or biological changes to the nanotubes. They're often used in medicines, and Turco's research showed they had no effect in high-organic or low-organic soils.

Non-functionalized single-walled nanotubes - those lacking intentional surface alterations - are being added to a variety of products during manufacturing because they can strengthen the material without adding much weight. Nanotubes contained in manufacturing waste products may find their way into wastewater treatment plants and bio-solids that result from water purification. Those bio-solids cannot be released into water, so they are often discarded by spreading on land, adding critically needed plant nutrients to soil.

"Land application of biosolids is standard procedure now," Turco said. "If any of that contains nanotubes, that could be a problem."

Single-walled nanotubes also didn't affect microbes in high-organic soils, Turco said, likely because organic materials are highly reactive. Organic materials may have reacted with the nanotubes, leaving them unable to affect microbes.

"We want to alert people to the fact that if you're going to apply these as part of a land-treatment program, you may want to focus on high-organic matter soils," he said.

It's also possible, though much less likely, that nanotubes could contaminate soil through accidental spills during a delivery, Turco said.

Next, Turco said he would look at the effects on plants and soils from other nanomaterials and nanometals that are being more widely used in products for different properties they convey, such as nanosilver for its disinfecting properties and nanoindium, which is used in electronics.