Microplastics in arable soil: tomography with neutrons and X-rays shows where particles are deposited

3D images of plastic particles in the soil
3D images of plastic particles in the soil
A team of researchers from the University of Potsdam and the Helmholtz-Zentrum Berlin (HZB) has developed a measuring method to analyze soil samples with neutrons and X-rays and create 3D tomographies from them: This makes it possible for the first time to precisely localize microplastics in the soil. The 3D tomographies show where the particles are deposited in the soil and how this changes structures in the soil - which in turn can affect water flows and soil properties.

Microplastics are one of the biggest environmental pollutants of our time. Road traffic accounts for a particularly large proportion: in Germany, tire abrasion alone is said to produce around 100,000 tons of microplastics every year, in addition to particles from artificial turf, cosmetics, clothing and washing powders, disposable masks, plastic bags and other waste that end up in nature. But what happens to these particles in different soils? Do they break up into smaller and smaller pieces, are they relocated and transported further?

Of course, this is already being investigated. So far, a soil sample has been floated in a heavy salt solution, whereupon the individual components separate according to density: Plastic and organic particles float to the top, while mineral particles sink. The mixture of organic material and plastic particles is then treated with hydrogen peroxide, for example, whereby the organic components decompose and the microplastic particles should remain. Although this method makes it possible to determine the quantity and type of microplastic in a soil sample, information is lost about where exactly these particles accumulate and are deposited in the soil and whether they change structures in the soil.

3D tomography with neutrons and X-rays

Prof. Sascha Oswald (University of Potsdam) and Christian Tötzke (University of Potsdam and HZB) have developed a method to answer these questions and presented it in a recent study. They worked closely with the team led by Dr. Nikolay Kardjilov (HZB), whose expertise went into setting up a unique measuring station at the Institut Laue-Langevin, Grenoble: there, samples can be analyzed simultaneously with neutrons and X-rays and 3D tomographies can be created using both methods without altering the sample. While neutrons make organic and synthetic particles in particular visible, X-ray tomography shows the mineral particles and the structure they form together.

Method tested on prepared soil samples

To test the method, Tötzke prepared a series of soil samples from sand, organic components such as peat or charcoal and artificial microplastic particles. In a further series of measurements, he investigated how fast-growing lupins penetrate the soil samples with their roots and whether roots react visibly to the microplastics.

In the neutron tomograms, the microplastic particles can be seen, but also some of the organic components. X-ray tomography, on the other hand, reveals the arrangement of the sand grains, while the organic and plastic particles are only visible as diffuse voids. When superimposed, a complete image of the soil sample is obtained. From this, the size and shape of the microplastic particles can be estimated, as well as the change in the soil structure caused by the embedded microplastics.

-This method is of course complex, but it makes it possible for the first time to investigate where microplastics are deposited and how this changes the soil," explains Tötzke. He also analyzed the sandy soil from a field near Beelitz, a typical asparagus-growing area in Brandenburg, into which he mixed pieces of mulch film. Even in practice, it is usually not possible to remove the mulch film from the field without leaving any residue after harvesting. Remaining film residues are then carried into deeper soil layers during plowing. -We have succeeded in showing that fragments of such plastic films can change the water flow in the soil. Microplastic fibers, on the other hand, create small cracks in the soil matrix," says Tötzke. So far, it is not possible to predict how this will affect the hydraulic properties of the soil, for example its ability to store water. -As droughts and heavy rainfall become more likely as climate change progresses, there is an urgent need to answer these questions. We now need to investigate this systematically," says Tötzke.

The study online in Science of the Total Environment (2024):

Christian Tötzke, Boyana Kozhuharova, Nikolay Kardjilov, Nicolas Lenoir, Ingo Manke, Sascha E. Oswald: Non-invasive 3D analysis of microplastic particles in sandy soil - Exploring feasible options and capabilities, https://www.sciencedirect.co­m/science/­article/pi­i/S0048969­723065543?­via%3Dihub

Image: 2023_120_fig5_Christian_Tötzke: A sample of Beelitz sandy soil containing shreds and particles of polyethylene film was examined here. These PET films are used in asparagus cultivation. The neutron tomography (in shades of gray) clearly shows where the PET fragments are located. X-ray tomography of the sample (ochre) reveals the soil structure: superimposed on the neutron tomography, the PET particles (in blue) are visible. Christian Tötzke 17-11-2023 / No. 120