Nanoparticles poison soil

23 August 2012
Posted by Chris Smith.

Nanoparticles in cosmetics and exhaust fumes can accumulate in plants and stunt growth, scientists have warned.

In a paper published this week in PNAS, UC Santa Barbara soil microbiologist Varieties of soybeansPatricia Holden and her colleagues investigated the effects of growing soybean plants in soils dosed with zinc oxide and cerium oxide nanoparticles.

Zinc oxide is used in transparent suncreams where the tiny particles soak up harmful UV while leaving visible light untouched so the lotion is invisible. But, inevitably, the stuff ends up going down the drain when people wash it off. If the waste water is then spread on fields, soil can become contaminated. Cerium, on the other hand, is most commonly used as a diesel additive to reduce emissions.

The Santa Barbara team tested plants grown in soils containing a range of concentrations of the particles, which, by definition, measure less than one ten-thousandth of a millimetre across.

Most people had assumed that these sorts of substances would stick to clays or other particles in the soils. Holden and her colleagues were therefore surprised to see significant accumulations of zinc in the leaves and beans of their test plants, which actually also seemed to grow more in the zinc-rich environment.

Since soy is one of the most consumed food staples globally, this could pose a health threat to consumers.

Plants exposed to the cerium, meanwhile, fared less well. Growth of the plants was stunted and the bean yield was reduced.

When the researchers investigated they found that the root nodules of the plants were devoid of the nitrifying bacteria that would normally secure a supply of nitrogen.

The results should be interpreted with caution because the concentrations of cerium used were higher than those one would expect to see in the average field. But as pressure on water and food supplies grows, the practice of using waste-water to fertilise farmland is likely to increase, so these initial observations of the potential "nanohazardicity" of these common chemicals should serve as a warning...

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