Particles from tires toxic for some water organisms

Video: Nanoparticles from winter tire studs - are they hazardous?



How are nanoparticles emitted?

When cars drive with studded tires, nanoparticles are released from the studs. In Sweden, tungsten carbide cobalt is a common material used in tire studs. Nanoparticles of this material eventually end up in water sheds near the roads. In our research program, the scientists examine what happens to these particles. How do they spread and how do they react with organisms? This is done in order to find out what characteristics are harmful to water organisms and thus, to the environment.

Tungsten containing nanoparticles are used in studded tires, but also in a wide range of machinery and equipment that needs an extremely wear resistant edge. These nanoparticles can also reach surface waters and sediments. The implications of these findings therefore span beyond the geographical borders of Scandinavian countries, where tungsten carbide studs are allowed in winter tires. Professor Sverker Molander, assistant Professor Rikard Arvidsson and PhD-student Anna Furberg at Chalmers University of Technology, have been working with life cycle analysis and investigated the flow of tungsten carbide cobalt.

– We have now really got going with our co-operation and some of the groups have achieved interesting results. We are now entering an important stage where we gather what we have done and look forward, says Sverker Molander.

The researchers recently published an article about the material flow from tire studs. The study shows that the wear from tire studs emit nanoparticles. The emissions to the environment are in the same range as some other commonly emitted nanomaterials. The researchers are now also studying potential effects from tungsten carbide cobalt along the production chain; from the extraction of the metals (mines in China and Congo for example), to Swedish water sheds where the material eventually end up. The results indicate that almost all material disappears as environmental emissions from non-point sources. (close to 98 %). That can be compared to the global use where 10-25 % of the consumed tungsten is recycled.

– The results may lead to an exchange of materials in some applications, and hopefully we can improve the choice of materials and use materials in an economic way, says Professor Sverker Molander.

During the extraction of the metals for the studded tires, many lives are lost. Partly because tungsten is a conflict mineral and different groups fight about controlling it, but also because the extraction is extremely demanding and dangerous.

– Studded tires may not be so good at saving lives, at least not if you include the extraction in the picture, says Sverker Molander.

In order to find out whether emissions of nanoparticles are harmful, we first need ways to measure the emissions. The search for engineered nanomaterials in environmental samples is a very challenging task. Numerous particles of different sizes and compositions are present in natural waters and sediments, which makes the undertaking difficult. The researchers in our program have focused on environmental systems, were they have no control over the input of nanomaterials.

Professor Martin Hassellöv and postdoc Andreas Gondikas at University of Gotheburg have concentrated on finding tungsten carbide nanoparticles emitted from winter tires. Tungsten carbide particles are produced as byproducts, as indicated by visual comparison of new and used winter tire studs. The researchers developed a method of sampling and characterizing particles in creek water and sediments that are likely to receive road runoff. Tungsten is an element that can be easily distinguished from other elements by using appropriate techniques, and this characteristic was used to distinguish the element. Another characteristic is that studs are used during the winter, so the researchers could compare data from winter and summer seasons.

With the help of advanced equipment (electron microscopy and mass spectrometry), they detected higher concentrations during the winter season. The data showed that increased concentrations of tungsten-containing particles were present in creek waters sampled from within the city of Gothenburg, compared to samples collected further away from the city. Most of the particles detected were in the size range of 60 – 140 nanometers. Aggregates of tungsten carbide particles were often observed, indicating that the particles are likely to aggregate with suspended particulate matter in creek waters and are thus more likely to settle on the sediments. The results make it possible to develop a robust methodology for detecting and characterizing particles ranging from a few tens of nanometers, up to several micrometers in natural waters. This approach can be used to develop nanoparticle fate models.

What happens when the nanoparticles are emitted?

After measuring the emissions, the next step is to investigate the behavior of tungsten carbide (WC) and tungsten carbide cobalt (WC-Co) in surface water.

A first study shows minor dissolution of tungsten carbide and rapid aggregation and sedimentation near the source. The particles did not have much interaction with natural organic matter. These interactions were more evident for tungsten carbide cobalt, for which most cobalt in the particles was dissolved into solution. This study was done by Professor Inger Odnevall Wallinder’s group at KTH, the Royal Institute of Technology, together with Tommy Cedervall’s group at LU, Lund University.

The researchers have also worked on what happens to other metal nanoparticles when they are dispersed into surface water. The surface charge of particles, surface oxide characteristics and transformation/dissolution properties are some key factors that need to be characterized in order to understand their fate in the environment. These studies provide useful guidelines on how to characterize particles in solution.


How toxic are the nanoparticles?

The last step in the life cycle of dispersed particles of tungsten carbide and tungsten carbide cobalt in surface water is how aquatic organisms are affected by their presence. The researchers have examined how the small zooplankton Daphnia magna reacts on tungsten carbide nanoparticles on both a short and a long-term perspective.

No acute toxicity (within 24 hours) was observed, but the researchers could detect long-term (two weeks) effects. A prerequisite for showing toxicity after the longer period of exposure was though that the rapidly sedimenting tungsten carbide nanoparticles were manually whirled up every now and then, to get them into solution and in contact with the organisms. Inger Odnevall Wallinder emphasizes that the concentration of particles studied widely exceeds the expected concentrations in the environment.

– The results are nevertheless important, since they clearly show the significance of doing chronic tests, even when acute tests show no negative effect, says Inger Odnevall Wallinder.

Professor Lars-Anders Hanson and PhD Mikael Ekvall have also investigated the influence of tungsten carbide particles on a benthic isopod (Assellus aquaticus), living on the bottom of lakes, but they discovered no toxicity even after two months of exposure. This was despite the fact that tungsten was found within the isopod. These findings show that tungsten carbide can be taken up by organisms living on the bottom of lakes.

The researchers have also studied how Daphnia magna is affected by nanoparticles of tungsten carbide and cobalt that are pre-exposed to organic matter. That is nanoparticles with or without a so-called “eco-corona”. An eco-corona, consisting of adsorbed layers of molecules, forms around nanomaterials immediately upon contact with an environmental compartment (aquatic, sediment or soil) and is dependent on the other organisms present and the surrounding conditions.

Studies are on-going to investigate the influence of adsorbed biomolecules of the eco-corona. More specifically, how the biomolecules affect transformation/dissolution, agglomeration, and toxic effects of the nanoparticles on the zooplankton Daphnia magna.




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