“This research directly demonstrates, for the first time, that soil microorganisms mineralise PBAT films in soils and transfer carbon from the polymer into their biomass,” says Michael Sander, Senior Scientist in the Environmental Chemistry Group in the Department of Environmental Systems Science at ETH Zurich.
Like PE, PBAT is a petroleum-based polymer that is used to make various products, including mulch films. Because PBAT was already classified as biodegradable in compost, the ETH and Eawag researchers aimed at assessing whether PBAT also biodegrades in agricultural soils. By comparison, PE does not biodegrade in compost or in soil.
Labelling of polymer with carbon-13
In their experiments, the researchers used PBAT material that was custom-synthesised from monomers to contain a defined amount of the stable carbon-13 isotope. This isotope label enabled the scientists to track the polymer-derived carbon along different biodegradation pathways in soil.
Upon biodegrading PBAT, the soil microorganisms liberated carbon-13 from the polymer.
Using isotope-sensitive analytical equipment, the researchers found that the carbon-13 from PBAT was not only converted into carbon dioxide (CO
2
) as a result of microbial respiration but also incorporated into the biomass of microorganisms colonizing the polymer surface.
True biodegradation
“The beauty of our study is that we used stable isotopes to precisely track PBAT-derived carbon along different biodegradation pathways of the polymer in the soil,” says Michael Zumstein.
The researchers are the first to successfully demonstrate – with high scientific rigor – that a plastic material is effectively biodegraded in soils.
Because not all materials that were labelled “biodegradable” in the past really fulfilled the necessary criteria. “By definition biodegradation demands that microbes metabolically use all carbon in the polymer chains for energy production and biomass formation – as we now demonstrated for PBAT,” says Hans-Peter Kohler, environmental microbiologist at Eawag.
The definition highlights that biodegradable plastics fundamentally differ from those that merely disintegrate into tiny plastic particles, for instance after exposure of the plastic to sunlight, but that do not mineralise. “Many plastic materials simply crumble into tiny fragments that persist in the environment as microplastics – even if this plastic is invisible to the naked eye,” Kohler says.
