In the late 2010s, it became clear that plastic pollution is not just a horror story from the speeches of eco-activists: this problem affects everyone. The number of landfills and incinerators is growing, and microparticles of slowly decaying plastic, meanwhile, penetrate everywhere: scientists have already found them in the Arctic ice, at the bottom of the Mariana Trench, and even in human excrement.
The author of the Knife article figured out how scientists invent new ways to dispose of garbage, whether insects can digest plastic, and make a scientific discovery at the nearest landfill.
How and why is garbage disposed of today?
First, there is too much plastic waste. In 2017, researchers estimated that since the 1950s, humanity had produced about 8.3 billion tons of this material – the weight of a billion huge elephants. According to scientists, two-thirds of this volume has already gone to landfills or to the ocean, replenishing the “garbage islands.”
Secondly, plastic pollution has many dangerous consequences. Waste is increasingly becoming food for wild animals and then moving up the food chain. The authors of a recent study found microplastics in the feces of seals living in a UK animal rehabilitation center: the source of the particles was the fish fed to the seals. Another group of scientists found that plastic pollution significantly increases the risk of disease in corals.
The impact of plastic on the body of animals and humans began to be investigated not so long ago, so it is impossible to say with certainty how safe the particles of this material are for us.
In addition, waste is changing the face of the planet: for example, the Great Garbage Patch in the North Pacific is growing every year.
There are several main ways of recycling plastic today. Most often, mechanical recycling is used: plastic waste is crushed, melted, and packaged into small granules suitable for reuse. Less popular methods are high-temperature decomposition in the presence of methanol (methanolysis) or ethylene glycol (glycolysis) and thermal decomposition without air (pyrolysis). Alas, these methods require careful sorting of waste and the use of expensive equipment, so incineration is still cheaper. Researchers and laboratories worldwide are looking for new ways to recycle plastic: today, most of these technologies seem fantastic, but the methods are improving from year to year. Perhaps these approaches will become the backbone of the recycling plants of the future – efficient, green, and inexpensive.
Plastic eaters: recycling with insects
Insects are so diverse that they can eat anything from sour fruits to sweaters from your closet. Scientists have found that plastic can also be fed to certain insect species – the digestive systems of those we used to think of as pests efficiently recycle our garbage, turning it into harmless waste. How is this happening, and can worms and larvae replace plastic sorting plants?
In 2015, researchers from the United States and China found that large flour beetle larvae do well on a plastic diet. Several hundred worms were fed by expanded polystyrene, one of the most popular types of foam plastic, for two weeks while the control larvae ate the bran. The survival rate in both groups was approximately the same; the foam did not harm the future beetles. At the end of the experiment, the researchers sent for analysis the excrement of mealworms that ate foam: it turned out that 48% of the plastic was converted into carbon dioxide by the digestion of the larvae. The other half underwent depolymerization – long polymer chains broke into monomer units. In a day, a hundred larvae destroyed up to 40 milligrams of Styrofoam.
In 2017, a group of European scientists led by Federica Bertocchini discovered another “garbage eaters” type. Bertocchini’s unusual hobby helped the biologists: in her spare time, she breeds bees. The researcher says that the hives need to be regularly cleaned of pests, including the larvae of the large wax moth Galleria mellonella.
These larvae settle on the honeycomb and eat whatever they get to – honey, beebread, and wax. Once Bertocchini, while cleaning the hives, collected the larvae in a plastic bag and soon discovered that the insects had gnawed through the plastic.
The biologist repeated the laboratory experiment and found out that one hundred Galleria mellonella larvae coped with 92 milligrams of polyethylene in 12 hours. However, it remained unclear whether insects can wreck plastic or they can only crush it. To check this, biologists pounded the larvae of moths into a thick paste and then processed polyethylene with it – according to scientists, all substances that could contribute to the decomposition of the material are stored in such a paste. The results were more modest, but about 13% of the plastic still managed to dissolve.
According to Bertocchini and her co-authors, the digestive system of the larvae is well adapted to break the C – C carbon bonds in beeswax, so it can cope with similar structures in polyethylene while releasing the dihydric alcohol ethylene glycol. Researchers do not yet know what mechanisms help insects do this: perhaps it is the enzymes secreted by microorganisms living in the intestines of the larva.
However, some scholars have already questioned Bertocchini’s theory. German researchers repeated the experiment, but instead of mashed larvae, they applied minced pork and egg yolk to the plastic. The result was measured using the same infrared spectroscopy method as in the original experiment, and the spectrogram was very similar to that obtained by Bertocchini. This does not mean that wax moth larvae cannot process garbage: the result of the work of the Germans shows that scientists will be able to talk about a new processing technology only after they figure out which process in the moth’s intestines helped to destroy polyethylene in the first experiments.
Group “Mushrooms”: fungus against the landfill
Another unexpected ally in the fight for a clean planet is fungi. In 2017, scientists from China and Pakistan found it out: in search of organisms that can destroy garbage, they went to a landfill in Islamabad. Scientists have isolated a fungus from landfill soil samples that destroy polyurethane. Although this material is not plastic, recycling technologies are equally important: polyurethane is used to make many things from car tires to boot soles.
The attention of scientists was attracted by the fungus Aspergillus tubingensis, a close relative of the well-known “black mold” Aspergillus niger.
The ability of this fungus to decompose polyurethane was tested first in the laboratory and then in the field – the material was treated with a fungus and buried in the soil.
Aspergillus tubingensis worked best in a jelly-like agar-agar culture medium, but it did well in the soil as well. Infrared spectroscopy showed that the fungus actually breaks down the chemical bonds of the polyurethane.
The authors of this work are not the only ones who discovered the landfill. Scientists from Indonesia managed to do the same: using the fungi Aspergillus nomius and Trichoderma viride, they could destroy low-density polyethylene, which is often used in the production of plastic bags. The disadvantage of this method is that the required reactions are slow; in 45 days, the fungus ate only 5-7% of the plastic samples. Now several groups of scientists worldwide are looking for conditions under which fungi can destroy plastic faster – perhaps reactions will accelerate at a certain temperature or level of acidity of the environment.
Microcosm for purity: bacteria that destroy plastic
All researchers studying processing with the help of insects and fungi conclude that the microflora of “devouring” organisms plays the most critical role in this. If it is possible to find out what these microbes are, the same mechanism can be reproduced without the participation of the fungus or larvae.
In 2016, Japanese biologists managed to obtain such a strain of the bacterium Ideonella sakaiensis. Soil and dirt samples were again sourced from scientists near a recycling plant for PET (polyethylene terephthalate) bottles, one of the world’s most popular packaging materials. Several types of bacteria lived on the plastic remains, but the 201-F6 strain was the only culprit behind PET degradation.
These bacteria secrete special enzymes called PETase and METase, which trigger a series of reactions that break down bottle plastic into two less toxic constituents, terephthalic acid and ethylene glycol alcohol.
According to biologists, the new enzymes destroyed PET more efficiently and faster than derivatives of other “plastic-eating” bacteria, such as Thermobifida fusca, which lives in dung heaps. Scientists say they have not been able to find PETase analogs in bacteria-related Ideonella sakaiensis. Perhaps this suggests that the enzyme arose during the evolution of landfill bacteria. In this way, microorganisms adapted to life in the garbage and received a new food source. The biologists who discovered PETase recognized that the enzyme works very slowly; it took them six weeks to decompose a sample of a thin plastic film, even at an optimal temperature of 29° C.
Several teams of scientists around the world are trying to speed up the work of PETase, and in early 2018, researchers from the University of Portsmouth were suddenly lucky. The British studied the enzyme’s crystal structure to find out exactly how it breaks down plastic into monomers – this process begins the decomposition of the material. It is known that PETase is structurally similar to another enzyme, cutinase, which is secreted by fungi living on plant leaves: cutinase helps fungi destroy the protective shell of the sheet and penetrate inside. PETase is distinguished from cutinase by a more open structure of the active center – the part of the enzyme that retains the molecules of the absorbed material.
To understand how this section of the protein works, the scientists created a “hybrid” – the active center of the new PETase resembled the corresponding section of cutinase. Biologists assumed that an enzyme with a “closed” center would capture fewer molecules and break down plastic less actively, but then the scientists were in for a surprise: the new artificial PETase worked 20% more efficiently. In addition, the enzyme could destroy not only PET but also another polymer, PEF (polyethylene 2,5-furandicarboxylate). Scientists plan to study the structure of PETase further to “accelerate” the enzyme’s work as much as possible: while the degradation reactions are still too slow.
Many, many more times: recycling plastic
It is not easy to completely destroy plastic, but there is another approach to reducing the amount of debris – reusing the material. For example, PET bottles are crushed into small flakes, becoming raw materials for making fabrics, insulation, or new bottles. However, small items will soon have to be thrown away again, and it is not a fact that they will end up in recycling and not in a landfill.
To increase the cycle of plastic use, researchers propose making “long-lasting” objects from waste, for example, adding it to the road surface.
Today, the roadbed is most often covered with asphalt: a mixture of sand and gravel with various bitumen. Engineers suggest replacing some of the bitumen with plastic granules to make the coating stronger and more durable (one manufacturer claims that its development is 60% stronger than traditional asphalt). How these promises correspond to reality can be verified in the coming years: such a road is planned to be built on the territory of the University of California at San Diego.
The new methods look promising, but none of them have yet been put into practice. It will be years before insects, fungi, or artificial enzymes get serious about recycling our garbage. However, we can reduce the amount of plastic pollution today by sorting waste and reducing its volume.
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