Engineered nanomaterials and soil ecology

How relevant is ‘nano’ when it comes to the environmental impact of engineered nanomaterials?

The potential environmental impact of engineered nanomaterials weighs heavy on the minds of scientists, regulators and industrialists, and there have been a number of studies carried out on specific materials. Results vary, as one would expect in the real world, and it will be a challenge to integrate and make use of all the information gathered.

English vineyard (© 2007 Francis Sedgemore)

Most of the studies carried out so far are in-vitro, and while these are both useful and necessary, a full understanding of the wider biological effects of nanomaterials will only come with testing of the materials in complex environments. In other words, what are the effects of engineered nanomaterials on whole ecosystems?

One recently published study into the environmental impact of manufactured nanoparticles on ordinary soil showed no negative effects.

The study, carried out by Ron Turco and his colleagues at Purdue University, US, concerns fullerenes, otherwise known as buckyballs. Turco’s team looked at how fullerenes affect soil microorganisms in-situ, including bacteria responsible for breaking down organic material and producing carbon dioxide and other compounds.

Dry buckyballs, and buckyballs suspended in water, were added to soil in various concentrations, and over a six-month period the researchers monitored the size, composition and function of the bacterial community in the soil samples. No significant differences were found between soil containing no fullerene particles, and samples with either low or high levels of buckyballs. And this after some in-vivo studies had shown that fullerenes are in fact toxic to soil microbes.

But another investigation, by Daohui Lin of Zhejiang University in China and Baoshan Xing at the University of Massachusetts Amherst, US, found that the presence of aluminium oxide nanoparticles in soil can stunt root growth in a number of important crop species, albeit at very high particle concentrations.

As if this were not confusing enough, a group of Chinese researchers led by Lei Zheng claims that titanium dioxide nanoparticles can improve the growth of spinach when included at a particular concentration.

Results from these diverse studies will inevitably lead to calls for more research into nanoparticle toxicity, but one has to ask how relevant are the specifically ‘nano’ aspects of the materials in question. This is an important question being asked by Turco and others who have carried out soil ecology studies of engineered nanomaterials.

Says Turco: “The confusion may come from the fact that we call everything a nanomaterial. In fact, what these conflicting studies point out is that we need to consider each type of nanomaterial (carbon, metals, plastics, etc) on its own merits. There needs to be a significant effort to work through both effects of the materials in isolation, and the effects of the materials in complex environmental systems such as soil.”

Mark Wiesner, an environmental engineer at Duke University, US, reinforces this argument and questions the usefulness of the term ‘nanomaterial’ in the context of soil ecology. We need to consider specific materials and their action on soil ecology, he says, with size only one of many factors to consider.

Nevertheless, Wiesner thinks it important to study the effect of nanomaterials in the environment, and comments: “Turco’s study is relevant and original, in that it is looking at one kind of nanomaterial in a complex medium.” Again, this comes back to the point about in-vitro versus in-situ studies, or the importance of looking at the influence of materials on entire organisms and ecosystems.

While the onus must be on those releasing engineered nanomaterials into the environment to show that the materials are not harmful, the results of environmental impact studies of nanomaterials should serve as a warning that nanotechnology does not exist in isolation. Size is only one of many issues to consider when assessing the impact of this new industry on humans, animals and the natural environment.

Photo: What effect, if any, would engineered nanoparticles have if introduced into the soil of this English vineyard? (© 2007 Francis Sedgemore)

References

Impact of Fullerene (C60) on a Soil Microbial Community, Tong et al., Environ. Sci. Technol. 41, 2985 (2007).

Phytotoxicity of nanoparticles: Inhibition of seed germination and root growth, Lin & Xing, Env. Pollution, in press (2007).

Article first published in Nanomaterials News.