It only takes one day to make PET plastic waste completely “disappear”

The corrosion resistance of plastic makes it one of the most difficult pieces of trash to degrade.

In 2805 AD, due to uncontrolled consumption, the earth was covered with intractable garbage, and almost no life could survive. In order to purify the earth, humans had to collectively migrate to outer space to live, leaving robots to clean up garbage on earth. Hundreds of years have passed, the garbage has piled up into skyscrapers, and only one robot is still in operation.

This is a scene in Pixar’s classic animated film “Robot Story”. Although the background of the story is purely fictional, it also reveals a real problem that humans need to face – garbage disposal. According to statistics from the World Bank, today, the world produces about 2 billion tons of municipal waste every year, of which plastic is the most troublesome.

Due to the characteristics of easy processing, light weight and stable chemical properties, plastics are widely used in food and beverage packaging, building materials, auto parts, home appliances and other aspects, bringing great convenience to people’s lives. In 1950, the world was able to produce 2 million tons of plastic a year. By 2015, more than half a century later, this number had nearly doubled to 381 million tons.

However, the corrosion resistance of plastic also makes it one of the most difficult pieces of trash to degrade, taking about 400 years to degrade naturally. According to statistics, as of 2021, there are about 8.3 billion tons of plastic in the world, of which 6.3 billion tons are not recycled and become garbage. And these unrecycled plastic waste will enter the soil, ocean and other environments, causing long-term and extensive ecological impact.

For decades, researchers have been exploring innovative ways to deal with plastic waste. Among them, biodegradation, as an environmentally friendly treatment method, has received general attention from researchers. In April of this year, a research team from Alper’s lab at the University of Texas at Austin published a new study in the scientific journal Nature, using an important machine learning method in artificial intelligence technology to transform a Enzymes can completely degrade PET plastics within a few days.

PET is polyethylene terephthalate (Polyethylene Terephthalate), the code in the international general resource recycling code is No. 1. PET is a common plastic in daily life. In 2020, the demand for PET plastics in Europe accounted for 8.4% of all plastics. Due to its light weight, airtightness, water resistance, acid and alkali resistance, etc., PET is mostly used to make containers such as mineral water bottles and beverage bottles, and has become a more durable food packaging material instead of glass.

PET is easier to recycle than other non-degradable plastics. According to statistics, the PET recycling rate in the United States is 31%, and that in Europe is 52%. However, the current methods of recycling PET are mostly physical and chemical means, that is, PET is decomposed by electrostatic separation, suspension separation, chain extension modification, chlorination modification and other technologies, and then re-made into new PET bottles, polyester fibers Wait. However, these methods will destroy some of the physical and chemical properties of PET to varying degrees, and the entire process consumes a lot of energy and emits a large amount of greenhouse gases, causing pollution to the environment.

The PET recycling route chosen by the latest research from The University of Texas at Austin is enzymatic degradation. Enzymatic degradation is a kind of biodegradation. Enzymes combine with plastics through active sites and degrade plastics under the catalysis of active sites. This degradation process does not produce harmful gases and is the most promising, green and environmentally friendly plastic. how to handle it.

Since it was first reported in 2005, researchers have discovered nearly 20 different natural PET hydrolases, including esterases, lipases, and cutinases. However, most PET hydrolases can only show considerable hydrolysis activity at a high temperature of 70 degrees Celsius and a certain pH, and have poor activity at room temperature and neutral pH. Leaf compost cutinase (LCC), discovered by researchers in 2012, can degrade 90% of pretreated PET plastic within 10 hours only at 72 degrees Celsius and a pH of 8.0.

Improving the reaction conditions such as temperature and pH or pre-processing PET plastics can easily improve the efficiency of PET degradation, but in the long run, PET hydrolase with high activity and high degradation efficiency has higher utilization value. And this can only be achieved by exploring and discovering new hydrolases, or by protein engineering of known hydrolases.

A team of researchers at the University of Texas at Austin used machine learning to modify the known PET hydrolase PETase.

PETase is the natural hydrolase with the highest efficiency in degrading PET. In 2016, Japanese researchers Tanasupawat and others discovered Osaka Sakai bacteria in a plastic bottle recycling plant in Sakai, Japan, and extracted a hydrolase that degrades PET, called PETase. PETase can degrade PET at room temperature and neutral pH. It can completely degrade 0.2 mm thick low-crystallinity PET film in about 6 weeks, and its degradation products will not pollute the environment.

However, the activity of PETase is not stable, even at 37 degrees Celsius, the activity of PETase will be lost after 24 hours. In addition, it still takes a long time for PETase to degrade high crystallinity PET, which is about 30 times as long as the degradation of low crystallinity PET.

In order to further exploit the properties of PETase and improve its degradation efficiency, many researchers have tried to engineer and optimize PETase through protein engineering, but the results are not satisfactory. The research team at the University of Texas at Austin believes that the previous research on protein engineering was too focused, and it was difficult to balance the stability and activity of enzymes. Therefore, the team introduced the machine learning system MutCompute, which has been tested on over 19,000 samples. After deep learning of the protein structure, it can predict the mutation sites in the protein that can be optimized.

“The machine-learning approach can be applied to any type of protein, including hydrolases, and we ultimately used this approach to identify a new set of mutation sites that can greatly increase the activity and reaction temperature range of PETase.” Dexa Hal Alper, the principal investigator of Alper’s laboratory at the University of Austin at Austin and the author of the research paper, introduced to the “First Financial” magazine.

This group of mutation sites predicted by the algorithm was arranged and combined to form a total of 27 effective PETase variants. The research team analyzed and found that the thermal stability of 23 variants was improved. Next, the research team evaluated the PET hydrolysis activity of these variants using amorphous PET films in the temperature range of 30 to 60 degrees Celsius, and the results showed that the effective temperature range of all variants was extended.

Source: University of Texas at Austin

After comprehensively evaluating and balancing the thermal stability and hydrolytic activity of various PETase variants, the research team finally identified a modified hydrolase with the best PET degradation efficiency and named it FAST-PETase, which means functional, Active, stable, and tolerant PETase. Compared with the original wild-type PETase, FAST-PETase contains a total of 5 mutations and shows superior hydrolytic activity at 30 to 50 degrees Celsius and a certain pH range, far surpassing wild-type PETase and other protein-engineered products. Retrofit variant.

In order to further examine the degradation effect of FAST-PETase, the research team also collected 51 samples of used PET plastic products, which were all from the packaging of food, beverages, medicines, and cosmetics commonly found in stores. The research team degraded the untreated samples directly using FAST-PETase at 50 degrees Celsius and found that, despite differences in crystallinity, molecular weight, thickness and additives contained in the 51 PET plastic samples, all samples were within one week. It is completely degraded, and the fastest only takes 24 hours.

Biodegradation, as an environmentally friendly plastic waste disposal method, has been widely concerned by researchers.

The degradation rate of FAST-PETase is nearly linear if measured by the rate at which PET is depolymerized to produce monomer. In contrast, other hydrolases and other PETase variants exhibited much lower hydrolytic activity under the same conditions. For example, if LCC is used for degradation, even at the optimal reaction temperature of 72 degrees Celsius, its activity is still only about 1/5 of that of FAST-PETase at 50 degrees Celsius.

In addition, the research team also carried out experiments on mineral water bottles made of more difficult to degrade, high crystallinity PET. Although the rate at which FAST-PETase degrades untreated mineral water bottle fragments is greatly reduced, after thermal pretreatment of mineral water bottles, FAST-PETase can also completely degrade them in less than two weeks at 50 degrees Celsius. degradation.

“FAST-PETase degrades plastics at lower temperatures (50 degrees Celsius is the optimum reaction temperature) and a milder pH, reducing energy input and improving the efficiency of decomposing PET monomers,” and in processing ingredients In complex plastic waste, the residues of non-PET components in the mixture will not melt to form a gel, making it easier to filter and further recycle, reducing recycling costs. “Hal Alper said.

However, degrading PET plastic is only the first step. To truly realize a green cycle, it is more critical to recycle and reuse the degraded PET. “The difficulty of plastic disposal is actually collection. If it takes a lot of effort to collect it, and then it is completely biodegraded, it is not necessary.” Li Daoji, director of the Ocean Plastic Research Center of East China Normal University, told ” China Business News magazine said.

Alper’s research team also proposed a closed-loop recycling path for PET: use FAST-PETase to biodegrade the used PET to generate a fully degraded PET solution, and then chemically separate and recycle the decomposed PET monomer. Re-synthesize new PET. Hal Alper said that when the production cost of enzymes is low enough, industrial-scale applications can theoretically be achieved. For example, the team is currently developing engineered strains that can convert PET degradation products into high value-added products.

In addition, the research team also proposed the potential use of FAST-PETase for environmental remediation. Since the reaction conditions of FAST-PETase are consistent with the conditions of cell growth and the conditions of the actual environment, FAST-PETase can be released into the polluted environment to directly realize the restoration of the environment. “But this method requires other microorganisms to help consume the monomers produced after PET degradation, which will be the subject and research direction of our future work.” Hal Alper said.

Currently, the research team is looking for partners to assist in the practice and large-scale promotion of FAST-PETase’s technology. Li Daoji believes that since the cost of recycled plastics is generally higher than that of virgin plastics, in order to promote the degradation and recycling of plastics on a large scale, not only technology, but also the support or constraints of relevant policies are required. The European Union issued relevant documents in 2019, requiring that from 2025, PET beverage bottles need to contain at least 25% recycled plastic; from 2030, all plastic beverage bottles must contain at least 30% recycled plastic.

The establishment of a green and sustainable plastic recycling system is the direction that governments and researchers all over the world are working towards, but the exploration of each step is a long process. At least, thanks to new technologies such as machine learning, scientists have reduced the time it takes to degrade plastic from hundreds of years to a few days or even a day. On the way to a green future, mankind has taken a giant step forward.

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