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Image: Electron microscopy image of the surface of the PBS film after six weeks of incubation in soil: The PBS surface shows clear signs
of degradation due to fungal hyphae and bacterial colonization.
Image credit: ETH Zurich / Michael Zumstein
Modern agriculture uses a lot of plastic, especially the mulch film
that farmers use to cover the soil of their fields.
This keeps the soil moist for crops, suppresses weeds and promotes crop growth
.
However, collecting and processing traditional polyethylene film after use is often very time-consuming and expensive
for farmers.
In addition, it is not possible to recollect all the thin PE films because they tear
easily.
This means that PE fragments remain in the soil and accumulate there because PE does not degrade
.
Biodegradable mulch film is a promising alternative because it ideally does not leave any polymer components
in the soil environment compared to PE film.
The biodegradable polymers it contains are carefully designed so that microorganisms can use them to produce energy and build cellular biomass
.
Biodegradable polymers have predetermined chemical "breakpoints"
in their backbone structure.
Naturally occurring microorganisms, such as those in soil, can release enzymes into the environment that attack these points in the polymer and break them down
.
The released small degradation products are absorbed by microorganisms and eventually form the final productCO2 through respiration.
This is why proving thatCO2 polymer carbon formation is essential
for biodegradation.
Also because in addition to truly biodegradable plastics, there are also PE falsely labeled plastics
containing specific additives.
These films only break down into very small microplastics
that are invisible to the naked eye.
Because they are not degraded by microorganisms, they accumulate in
the environment.
Until now, it has not been possible to track the entire process
of polymer biodegradation based on existing methods.
But over the past few years, ETH Zurich's environmental chemistry group has developed a new way to track and measure whether polymers biodegrade in soil and to what extent
.
Their findings have just been published in Nature Communications.
These results may change the approach to
polymer biodegradation in the future.
The project also includes researchers from ETH Geosciences and Eawag, as well as employees
from the chemical company BASF.
This new method is based on the use of polymers with stable carbon isotope labeling (13C), which allows researchers to selectively track the properties of polymer 13Cduring biodegradation in soil, so they can definitively prove that biodegradation is indeed occurring
.
Until now, the biodegradability of plastics has only been tested
with non-isotope-labeled polymers.
Polymers (or plastic materials consisting of one or more polymers) are certified as biodegradable if part of the added polymer carbon is converted to CO2 for a specific incubation period exceeding a predefined level
.
For example, the standard for biodegradable mulch films requires two years of soil culture in which at least 90% of the mulch film carbon is "mineralized" asCO2.
Electron microscopy image of the surface of the PBS film after six weeks of incubation in soil: The PBS surface shows clear signs
of degradation by fungal hyphae and bacterial colonization.
(Image: Michael Zumstein)
These test methods have been well proven
as a suitable means of detecting polymer mineralization.
However, they do not capture the full extent of biodegradation because they only measureCO2 formation
.
Therefore, using today's standard methods, the researchers could not detect the amount of
polymeric carbon remaining in the soil at the end of the incubation period.
In addition, it is not clear whether this surplus carbon is still in the form of added polymers or whether microbes have absorbed it into their biomass
.
The method developed by ETH researchers and colleagues removes these ambiguities
.
In their tests, they used 13C-labeledpolysuccinate succinate, or PBS, an important commercially biodegradable polyester that is also used in mulch films
.
The researchers were now able to selectively track C in PBS during 13 biodegradation: In addition to determining mineralization to 13 Ltd2, by quantifying the amount of PBS-derived residues, the authors demonstrated that the complete mass balance of PBS carbon 13Cremains in the
soil after culture.
"We are pleased to see that the carbon mass is balanced
over the course of 425 days of soil culture.
This shows that we can pinpoint the final location of the polymer carbon — about two-thirds in CO and 2.
3 in the soil — during these very long incubation periods," explains Taylor Nelson, lead author of the study, who earned his PhD
in the Environmental Chemistry Group.
The researchers also wanted to know in what form the carbon added as PBS remained in the
soil.
How much is absorbed into microbial biomass and how much still exists as surplus PBS?
To answer this question, the authors extracted and quantified residual PBS
from soil at the end of the culture.
They were able to show that while most of the carbon is still in the form of PBS, a significant portion (7%) of the added PBS carbon has been incorporated into the
microbial biomass.
Determining exactly how much polymer is still there, and how much polymer carbon is incorporated into the biomass, is critical
for future research and development of new biodegradable polymers.
"We can now systematically test soil conditions and polymer properties, allowing polymers to fully biodegrade toCO2 as well as microbial biomass – we can assess factors that may slow polymer biodegradation over time," explains Michael Sander
, professor in the ETH Environmental Chemistry Group.
This work is already underway: using new methods, the team is currently investigating further biodegradation of polymers in various agricultural soils, including in the
field.
"In this way, we want to ensure that biodegradable polymers live up to their name and do not remain in the environment," says
Kristopher McNeill, professor of environmental chemistry at ETH Zurich and head of the eponymous research group.
"Replacing traditional polymers with biodegradable polymers can help reduce plastic pollution, especially in applications where the polymer is used directly into the environment, where it is likely to remain after
use," Sander noted.
