Plastic bags may have just become almost totally compostable

April 21, 2021
New technique for manufacturing plastic allows it to degrade without creating almost all of the harmful byproducts. (Unsplash/Markus Spiske)

New technique for manufacturing plastic allows it to degrade without creating almost all of the harmful byproducts. (Unsplash/Markus Spiske)

Scientists at the University of California, Berkeley, have developed a new way of producing biodegradable plastics — which often aren't as compostable as advertised — that enables up to 98% of them to break down. 

The study, published Wednesday in Nature, outlines a method of producing plastics embedded with enzymes that break down the material after exposure to heat and water. The researchers found that all but trace amounts of the plastic made using the new technique degrades into small molecules within a few weeks, almost entirely eliminating the harmful microplastics that normally result from the decay of plastics.

This would be a major improvement at composting facilities and everywhere else where the bags become an environmental hazard, explained Ting Xu, a professor of chemistry at UC Berkeley and the corresponding author of the study. "If we can have a biodegradable plastic that is compostable, then the single-use plastic can be sorted out with organic matter, and it will become compost," she told The Academic Times. "The plastic bag on the side of the road can be composted by nature. That's the whole philosophy — if it gets into the landfill or the ocean, we don't need to do anything that nature can't take care of."

Single-use plastics are so prevalent in our environment that they're found strewn across the abyssal plain of the Pacific Ocean and circling at the surface in a famous gyre known as the Great Pacific Garbage Patch. They eventually break down into microplastics, a byproduct that can enter our atmosphere during the degradation process. A global effort is underway to address the crisis through technological innovation: A team of German chemists recently announced a new plant-based plastic that can be recycled nearly 10 times more efficiently than high-density polyethylene, while their Canadian counterparts invented a way to turn fish oil into a cleaner form of polyurethane

But scientific advances haven't yet solved the problem of single-use plastics sticking around practically everywhere. Anybody who's tossed a biodegradable plastic bag from the grocery store into their compost bin knows they usually don't break down as quickly as organic materials. In fact, most single-use plastics that claim to be biodegradable require industrial processing and take more than a year to break down in natural environments, Xu explained.

Not to mention, biodegradable plastics break down into the same microplastics as conventional materials. "These plastics are actually not biodegradable at all," she said. "It takes them years to degrade. It ends up generating microplastic and actually makes the microplastic formation even faster. Those polymers aren't going through 100% degradation."

In a separate study highlighting the questionable claims made by some bag-makers, marine-litter researcher Imogen Napper tested bags with varying degrees of supposed biodegradability in three different environments — soil, undersea and open air. After nine months of air exposure, all of the bags had started to disintegrate and break down into microplastics. But most of the bags, including one labeled explicitly as "biodegradable," were still present in both soil and seawater after three years — and were able to hold shopping items. 

The new technique from UC Berkeley researchers embeds enzymes into the plastic while it is synthesized, acting as a built-in self-destruct sequence. The enzymes are encased in a layer of polymer that ensures the plastic product retains its shape and usefulness throughout its life cycle. "You don't want it to degrade during manufacturing or storage," Xu said. "You don't produce it and use it right away; there has to be a shelf life. By microscopically putting an enzyme in the system, that allows us to control the degradation rate and build in latency." 

When exposed to heat and humidity — inside, say, a compost bin — the enzymes are freed from their polymer casing and begin breaking down the plastic into soil-friendly lactic acid, a product of fermentation, Xu said. Both heat and humidity are required simultaneously to trigger degradation, however. "If you just add water, nothing will happen," she said. "If you just add heat with low humidity, it wouldn't go." That could present storage problems in warm and wet climates, but it's possible to delay the rate of decay by altering the plastic's internal crystalline structure. 

Xu and her colleagues are looking at adapting the technology beyond single-use plastics to bigger and longer-lasting products, such as patio furniture. For now, single-use plastics represent the low-hanging fruit. Xu believes that the solution must be built into the existing system, like a polyester-eating enzyme embedded in a grocery bag. "Consumers are not going to change their behavior," she said. "You can try to get people to bring their cloth bags or charge them to use a paper bag, but at the end of the day [it's about] convenience."  

The study, "Near-complete depolymerization of polyesters with nano-dispersed enzymes," published April 21 in Nature, was authored by Christopher DelRe, Yufeng Jiang, Junpyo Kwon, Le Ma, Robert O. Ritchie and Ting Xu, University of California, Berkeley, and Lawrence Berkeley National Laboratory; Philjun Kang, Aaron Hall, Ivan Jayapurna, Zhiyuan Ruan, Kyle Zolkin and Tim Li, University of California, Berkeley; Corinne D. Scown, Lawrence Berkeley National Laboratory; and Thomas P. Russell, University of California, Berkeley, and University of Massachusetts. 

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