Neutrons reveal the biodegradation of plastic

Polyethylene terephthalate (PET) has become an integral part of everyday life: drinking bottles, food packaging, wrapping films – many products are made from this durable plastic. Whilst its durability is desirable from a technical perspective, it becomes an environmental problem when bottles and packaging are discarded improperly after a short period of use.

Enzymes can break down PET, which is used in plastic bottles. Graphic: created using AI, edited by Reiner Müller, FRM II / TUM

Enzymes can break down PET, which is used in plastic bottles. Graphic: created using AI, edited by Reiner Müller, FRM II / TUM

“PET can be chemically degraded under strongly alkaline conditions, but these processes are energy- and resource-intensive and unsustainable,” explains Dr Jean-François Moulin of the Helmholtz-Zentrum Hereon and instrument scientist at the Heinz Maier-Leibnitz Zentrum. Enzymes – biological catalysts produced by living organisms such as bacteria – offer an alternative: some of them are capable of breaking down PET under significantly milder conditions. The researchers view this approach as promising for more sustainable recycling. “However, enzymatic catalysis slows down considerably after a rapid initial phase. This means that complete degradation of PET is not possible,” says Moulin.

Inhibited enzymes

Jean-François Moulin analysed the sample using neutrons, together with his colleagues Dr Gaetano Mangiapia, Dr Rainhard Machatschek and Dr Natalia Tarazona Lizcano. The idea for the study came from Dr Rainhard Machatschek whilst he was working at the Institute for Active Polymers at the Helmholtz-Zentrum Hereon. The neutron reflectometry experiments took place on the Figaro instrument at the Institut Laue Langevin. Here, the researchers tracked the enzymatic degradation of PET directly at the nanometre scale for the first time. They observed that water rapidly penetrates the near-surface layer of the PET, thereby allowing the enzymes access to the polymer chains to be degraded. The initial phase of the degradation therefore proceeds efficiently. After a short time, however, the reaction slows down significantly: degradation products accumulate on the surface, partially inhibiting the enzymes and gradually reducing their effectiveness. The combination of initially good accessibility and increasing inhibition by degradation products explains why the reaction rate declines.

Jean-François Moulin, here at the instrument REFSANS at the Heinz Maier-Leibnitz Zentrum, analyzed the samples with his colleagues using neutrons. © Bernhard Ludewig, FRM II / TUM

Jean-François Moulin, here at the instrument REFSANS at the Heinz Maier-Leibnitz Zentrum, analyzed the samples with his colleagues using neutrons. © Bernhard Ludewig, FRM II / TUM

Working towards biological recycling

The key benefit of the study lies in a better understanding: “We now have a clearer picture of what happens at the nanometre level during PET degradation – in particular, why the reaction slows down significantly after the rapid initial phase,” says Moulin. “This provides us with starting points from which further research can build.”

Which of these starting points can actually be put to use remains to be seen in future studies. Possible avenues of research could include studying the behaviour of the enzymes, the reaction conditions or the handling of the inhibitory degradation products in greater detail.

In the long term, the researchers hope that enzymatic degradation could pave the way to more sustainable PET recycling. Moulin is cautiously optimistic: “Considerable basic research is still needed, however, before such an approach could be transferred from the laboratory to an industrial scale. Our findings are an important first step.”