How to solve the problem of plastic degradation

Beijing Bureau of Market Supervision and Administrative Management recently issued a relevant ban on "Ordinance on Restricting Use of Plastic", which prohibited use of non-degradable plastic bags for takeaway food in populated areas from July 1, 2021. and dining in populated areas and picturesque places is prohibited. Use non-degradable, disposable plastic utensils.

Since invention of plastic in 1907, it has not only brought great convenience to people, but also brought all kinds of pollution and harm to environment - plastic, invented over 100 years ago, will not decompose for at least 300 years. years later! The best way to resolve this controversy is to find a way to completely decompose plastic. Scientists have been working on this for decades, so what are latest advances in this area?

1 Light, oxygenation and biodegradation

Currently, main methods of plastics degradation are light, oxygen degradation and biodegradation.

Photodegradable and oxo-degradable plastics are commonly blended with additives to conventional petrochemical plastics to accelerate fragmentation of plastics when exposed to light or heat. Current photodegradable and oxygen-degradable plastics cannot be fully degraded within a controlled time, and some plastics will still be left in environment.

Biodegradation refers to use of microorganisms to decompose plastic in a specific environment and completely convert it into carbon dioxide and water. However, biodegradation suggests that material for production of plastics is biomass, and not plastics made from petrochemical feedstocks. It would be ideal if biodegradation could degrade plastic products made from petrochemical materials as well as plastic products made from biological materials.

Generally speaking, plastics made from biomass are easily biodegradable, but if biodegradable plastics are made from biomass, production cost is more than twice that of petrochemical plastics. Now about two-thirds of world's biodegradable plastic depends on food crops as a raw material, which is obviously not sustainable.

Of course, plants can also be used as raw materials for biodegradable plastics, but various excipients, additives and plasticizers are used in production of biodegradable plastics. A review of biomass plastic products in European market showed that most products contain various chemicals, including significant amounts of toxic chemicals.

It has been reported that 83% of biodegradable plastics are actually "compostable plastics" and require temperatures of up to 50°C and 55% humidity to decompose effectively, but under normal conditions, these are difficult to achieve.words.

2 enzymes that can "eat" PLA plastic

Recently, journal Nature reported on a new method for decomposing plastic, invented by Xu Ting's team at University of California, Berkeley. They wrapped up an enzyme that can "eat" plastic into certain molecules, and in just a few weeks, plastic can completely decompose under influence of slightly higher temperature and water, not only does decomposed plastic become food for soil microorganisms and indeed become "compostable plastics."

One of plastic products that people often use these days is polylactic acid (PLA), a plastic bag that also ends up in landfills and lasts as long as regular plastic bags. Researchers from Xu Ting's team embedded BC-lipase and proteinase K into polylactic acid plastic and added an enzyme-protecting agent. The results showed that in domestic tap water or standard soil composting, plastic created in this way completely decomposes in just three days and eliminates microplastics within days or weeks.

Actually, research on degradation of polylactic acid plastics began in 1981 or even earlier. At that time, enzymes were also considered for degradation, but use of enzymes could not solve problem of high cost. The price of industrial enzymes on market is usually $10 per 1000 grams. In addition to cost issue, for this degradation method to actually work, it must be easy to implement and popularize, so it may be a long time before this enzymatic plastic degradation method is actually used.

3 bacteria that "chew" PET plastic

In 2016, a research team led by Kohei Oda of Kyoto University of Technology and Kenji Miyamoto of Keio University discovered that microorganisms can ingest terephthalate plastic (PET), which is a plastic derived from petrochemicals.

The bacteria that feed on PET are called Sakaibacterium osaka. After careful microbial screening, a Japanese research team found a bacterium that can grow in large numbers on PET films, so bacterium was named Sakaibacterium sakakai. The bacteria were able to start chewing on PET polymers at a moderate temperature of 30°C. Based on this, researchers believe that Sakai's bacteria can be used to degrade PET plastic.

However, further studies have shown that these bacteria like non-crystalline PET, but in fact plastic products produced and used are crystalline PET, so effect of these bacteria on degradation of PET is not ideal. Moreover, under natural environmental conditions, Sakai bacteria slowly decompose plastic, and for complete decompositionIt takes 6 weeks to process a small plastic film. For this bacterium to reach a practical stage, it needs to be further optimized before it can truly be used to clean up world's PET plastic waste. However, Japanese researchers say enzymes used by Sakai bacterium could be engineered to have a faster reaction rate and more convenient use.

In this regard, Wu Bian's group from Institute of Microbiology of Chinese Academy of Sciences conducted a study on optimizing enzymes required for decomposition of PET plastic using Sakaibacterium sakakai, and obtained a processed enzyme with significantly improved stability, which was made by Sakaibacterium sakakai on PET films, degradation efficiency increased by 300 times. The results were published in January 2021 issue of American Chemical Society's Journal of Catalysis.

Four more advanced plastic-eating bacteria

In 2019, a research team from Shiv Nadar University in India discovered two "plastic-edible" bacteria in Greater Noida wetlands, namely strains DR11 and strains DR14, which have ability to degrade polystyrene.

Polystyrene has a high molecular weight and a long-chain polymer structure, and has good resistance to degradation. However, team found that when two isolated bacteria were exposed to polystyrene plastic, they used it as a carbon source and used it to create biofilms. This process changes physical properties of polystyrene, initiating a natural decomposition process, after which bacteria are able to break down polymer chains, releasing hydrolytic enzymes.

In fact, polystyrene is very difficult to degrade and requires some form of pre-treatment before biodegradation, such as chemical, thermal, photo-oxidation, etc. It has now been found that strains DR11 and DR14 can not only form biofilms on untreated polystyrene, but and decompose unmodified plastic, meaning these two bacteria could take on task of decomposing plastic in future. However, research team also stated that metabolic processes of these strains need further evaluation so that they can be used for environmental bioremediation.

April 25, 2021, in an article published by Sun Chaoming's group at Institute of Oceanography of Chinese Academy of Sciences, it is shown that some marine microorganisms can effectively degrade polyethylene (PE) and terephthalate (PET) plastics. The research team has collected thousands of plastic waste from Qingdao's coastal waters since 2016. After extensive screening, it was found that flora attached to plastic waste has an obvious ability to colonize and degrade on PET and PE surfaces..

Researchers suggest that bacteria got extra energy by decomposing plastic. By quantitative analysis of types and abundance of bacterial flora, it was found that dominant populations are 5 species of bacteria, in combination with cultivation technology, pure culture strains of above 5 species of bacteria were successfully obtained, and 3 of them had ability to significantly degrade plastics. Three strains of bacteria were composed according to a certain ratio, and a bacterial group was successfully obtained that can stably coexist and degrade PET and PE plastics significantly. This group of bacteria especially likes to degrade PE plastics, and PE can be decomposed into fragments within two weeks.

Currently, there are no microorganisms and enzymes in polyethylene plastics that could effectively degrade them internationally. Therefore, advances by Chinese researchers mean that in future, marine bacteria could be used to degrade extremely difficult-to-degrade polyethylene plastics.

5. Develop a biodegradable plastic

A higher level strategy for degradable plastics is to develop and manufacture plastics that can degrade on their own. In this regard, Chinese researchers have also made some progress.

Water soluble shopping bag with polyvinyl alcohol as main raw material jointly developed by South China University of Technology, China Guangdong Juyi New Material Co., Ltd. and a Chilean company, will be put into production and enter market. China is responsible for production of raw materials and provides technical support. Chile The company is responsible for marketing and sales of products. In future, raw materials for shopping bags will continue to be produced in Qingyuan, Guangdong, while shopping bags will be produced in Chile or other places and sold all over world.

This type of water-soluble shopping bag has high carrying capacity and cold water solubility, and is suitable for use in anhydrous environment. After use, it "disappears" when it meets water without contamination. The water-soluble polyvinyl alcohol bag only needs 5 minutes to decompose. This is clearly a breakthrough in plastic bag degradation, and many universities and companies around world, including China, are already developing this technology, and as far as actually getting to market, it may take some time.

Also, there are some biodegradable plastics on market, but they are not actually completely biodegradable, so research and application of plastic degradation will be a constant and major challenge for people. (Zhang Tiankang)

Further reading

Currently available on market

What biodegradation existsplayable heads

There are five types of biodegradable plastic on market today, known as green plastic.

One of them is polylactic acid (PLA) plastic, which is a new type of biodegradable material made from renewable plant resources such as raw starch extracted from corn.

The second is poly-3-hydroxyalkanoate (PHA), which is an aliphatic copolyester of various structures synthesized by microorganisms by fermentation of various carbon sources.

The third is polycaprolactone, a low-melting polymer with a melting point of only 62 °C.

The fourth is polyester, which can be produced from petrochemical materials or from renewable crop products such as starch, cellulose and glucose through biofermentation.

The fifth is polyvinyl alcohol (PVA), also known as PVA water-soluble film, which can completely decompose into carbon dioxide and water.

Source: Beijing Daily