Another form of pollution

Nanoparticle Pollution Could Stunt Crop Growth

Wired,

23 August, 2012

Metallic nanoparticles are ingredients in everything from sunscreen and cosmetics to diesel fuel and odor-eating socks. They may also end up in soil, with uncertain but potentially troublesome consequences.

A new study of soybeans grown in soil mixed with two common nanoparticles — nano-zinc oxide and nano-cerium oxide — suggests they can accumulate in crops and stunt bacteria that naturally fertilize soil.

“Previous studies showed plants can absorb whole nanoparticles, but those were done in hydroponic greenhouses. Real soil is very different,” said environmental microbiologist Patricia Holden of the University of California, Santa Barbara. “We thought most nanoparticles would get stuck to clays in soil, so we were surprised they were biologically available to bacteria and plants.”

“There could be implications for the food supply,” said Holden, who co-authored the study published August 21 in Proceedings of the National Academy of Sciences. “At the same time, however, we shouldn’t be scared of our soybeans. There’s still a lot we don’t know.”

Zinc oxide, or ZnO, and cerium oxide, or CeO₂, are two materials commonly manufactured into nanoparticles. Sunscreens, for example, use nano-zinc oxide particles, each as small as an HIV virus, to absorb ultraviolet light without leaving a white residue. The particles end up running down drains and into sewage sludge, which wastewater treatment plants sell to farmers for use as fertilizer.

Cerium oxide in diesel fuel helps it burn more efficiently, and in catalytic converters it helps cars filter exhaust pollution. Spewed out in exhaust, it ends up falling directly onto soils or washing into sewers, eventually ending up on fields.

To look for possible adverse effects of nanoparticles, Holden and her team grew soybeans in a greenhouse filled with farm soil. “Soybeans are one of the world’s most important food crops, and bacteria in their root nodules fix atmospheric nitrogen to fertilize them,” Holden said.

Some of the soil was mixed with increasing concentrations of either nano-zinc oxide or nano-cerium oxide. Crops grown in the former absorbed far more zinc than plants grown in regular soil. Nano-cerium oxide, meanwhile, accumulated around soybean roots and stunted plant growth.

The pods, roots, leaves and stem of a soybean plant used in nanoparticle growth experiments. Nodules on the roots store soil-fertilizing bacteria. Image: Patricia Holden/UCSB

“The cerium oxide just shut down nitrogen fixation at high concentrations,” Holden said. “The nodules were there, but they were empty with no bacteria. They weren’t functional.”

Holden said her team has yet to determine whether the zinc nanoparticles were absorbed whole or dissolved into a natural, edible state. She also noted that the cerium oxide concentrations were dramatically higher in their experimental soils than in any soils examined in the real world.

“It’s hard to say, but there could be hotspots [of nano-cerium oxide] in soil that are relevant to the concentrations we used,” she said.

Environmental engineer Gregory Lowry of Carnegie Mellon University, who wasn’t involved in the study, described the work as “eye-opening.” But he said that far more realistic conditions are required to say something useful about the environmental threat — or lack thereof — of manufactured nanoparticles.

Not only were the doses high, but “they used nanoparticles straight from the manufacturer,” Lowry said. “That isn’t the form you find in soils. They change a lot before then.”

When nano-zinc oxide enters sewage sludge and soils, for example, Lowry said it sticks to bacteria and organic matter, changing its forms and chemistry. Likewise, cerium oxide might change its properties once mixed into soil.

“The implications of absorption and blocking nitrogen fixation in this study may be largely overstated,” Lowry said.

Duke University’s Mark Wiesner, another environmental engineer not involved in the study, agreed with Lowry. He also noted that farmers probably wouldn’t add treated sludge to their soybean fields.

“Soybeans are legumes, so they can fertilize themselves. You wouldn’t need to add [sludge],” Wiesner said. “The only issue here would be if you’re rotating different kinds of crops.”

To these criticisms, Holden said her team’s study is a first of its kind. “No one really knows all the various forms of these or other [nanomaterials] as they enter soils, but an inaugural study using well-characterized [nanomaterials] as additives was a logical first step,” she wrote in an email to Wired.

Regardless of any disagreements over the study’s approach, Wiesner said any attempt to research nanoparticles in agricultural soil is a major step forward.

“This study moves beyond whether or not [nanoparticles] are simply toxic. It shows they can have some effect on the base of the food chain,” Wiesner said. “That will drive future research, even if this work is not fully realistic.”

All three researchers hope to at least partially answer what Lowry called  “the million-dollar questions:” Are nanoparticles safe, and should they be regulated?

“We’re at a point where, even if we wanted to regulate nanoparticles, we couldn’t. We could only ban their production, which would preclude their numerous beneficial uses,” Wiesner said. “We need to be careful about throwing the baby out with the bathwater.”