Plastic Type Key to Eco-Friendly Breakdown by Soil Microbes

Agriculture field mulch film biodegradable plastic

Swiss research shows that bacteria and fungi in the soil are able to comprehensively break down some types of plastic, but not others.

Thin plastic ‘mulch’ films are commonly used by farmers on fields to prevent weeds, but it is often extremely difficult to collect these films after they have been used.

“While plastic that disintegrates into small pieces…may no longer be visible to the eye, it may persist in the environment in the form of these small pieces for hundreds of years!” Michael Sander, senior researcher at ETH Zurich and research lead on this study, told me.

The most commonly used plastic material for mulch films is polyethylene, but an alternate material called polybutylene adipate terephthalate, or PBAT, can also be used to make these sheets.

Polyethylene is very difficult to breakdown, and therefore potentially very polluting for the environment. “The backbone of polyethylene is exclusively composed of carbon-carbon bonds. We do not know of any microorganisms in the environment that have extracellular enzymes that can readily break these carbon-carbon bonds,” Michael Zumstein, a co-researcher on the study from Cornell University, explained to me.

PBAT on the other hand has more potential for good biodegradability. It contains ester bonds that can be broken down by enzymes produced by bacteria and fungi found in the soil. It can also be digested by these microbes to produce carbon dioxide — a process known as mineralization.

Sander cautioned that not all ester bonds are biodegradable, such as those in another plastic  — polyethylene terephthalate or PET. He noted: “In contrast to PBAT, the ester bonds in PET are less accessible to enzymes due to specific material properties of PET. As a consequence, PET is among the polymers that accumulate in the environment.”

After a few weeks in soils, numerous soil microorganisms colonized the surface of the PBAT films and had begun to biodegrade the polymer. (Electron microscopy image)
After a few weeks in soils, numerous soil microorganisms colonized the surface of the PBAT films and had begun to biodegrade the polymer. (Electron microscopy image)

Although the biodegradable potential of PBAT was already known, the Swiss team showed conclusively, in work published in the journal Science Advances, that it can be digested and broken down in the soil. They achieved this by labelling the carbon molecules and tracing them through the breakdown process in a controlled lab environment.

This news will undoubtedly be welcome to study sponsor BASF, a German company that is one of the few firms manufacturing the more expensive PBAT films. 

While this research focused on agricultural uses for PBAT, it can also be used to make other things such as biodegradable plastic bags, food packaging and water resistant coatings.

“We’ve developed analysis techniques that open the door for industry to test the environmental impact of their plastic products,” commented study co-author Kristopher McNeill. “Thanks to our method, they can switch to using biodegradable materials in the production of thin mulch films instead of non-degradable polyethylene.”

The researchers now plan to test their findings over a longer period of time and directly in the field, to rule out any influence the lab environment may have had on the process.

The team is hopeful about their results, but urges caution on applying them too widely. “Biodegradability is not only polymer-specific but also very much dependent on the environment in which biodegradation is tested. While a polymer may readily biodegrade in soils, it may biodegrade much more slowly in ocean water,” Zumstein noted.

Images via Shutterstock & ETH Zurich / Environmental Chemistry Group

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