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The scientists noted that the EPA requires pretreatment programs to limit direct industrial metal discharge into publicly owned wastewater treatment plants. However, the research team conveyed that "MNMs – while measurable in the wastewater treatment plant systems – are neither monitored nor regulated, have a high affinity for activated sludge bacteria, and thus concentrate in biosolids."
The authors pointed out that soybean crops are farmed with equipment powered by fossil fuels, and thus MNMs can also be deposited into the soil through exhaust. The study showed that soybean plants grown in soil that contained zinc oxide bioaccumulated zinc; they absorbed it into the stems, leaves, and beans. Food quality was affected, although it may not be harmful to humans to eat the soybeans if the zinc is in the form of ions or salts, in the plants, according to Holden.
In the case of cerium oxide, the nanoparticles did not bioaccumulate, but plant growth was stunted. Changes occurred in the root nodules, where symbiotic bacteria normally accumulate and convert atmospheric nitrogen into ammonium, which fertilizes the plant. The changes in the root nodules indicate that greater use of synthetic fertilizers might be necessary with the buildup of MNMs in the soil.
Image: Soybean stem, leaves, bean pods, and roots. The roots contain nodules where bacteria accumulate and convert atmospheric nitrogen into ammonium, which fertilizes the plant. © Patricia Holden
Holden commented on the likelihood of high concentrations of these nanoparticles in agriculture: "There could be hotspots, places where you have accumulation, including near manufacturing sites where the materials are being made, or if there are spills. We have very limited information about the quantity or state of these synthetic nanomaterials in the environment right now. We know they‘re being used in consumer goods, and we know they‘re going down the drain."
John H. Priester et al.: Soybean susceptibility to manufactured nanomaterials with evidence for food quality and soil fertility interruption, In: PNAS Early View, August 20, 2012, DOI:10.1073/pnas.1205431109 [PNAS Plus: OPEN ACCESS ARTICLE]: http://dx.doi.org/10.1073/pnas.1205431109