We are living in a plastic world. In a little over 70 years, synthetic polymers have transformed modern society. More than 9 billion tonnes of plastic have been produced worldwide, and an estimated 40% of these materials are discarded within a month of use.

It is hard to imagine life without plastic. Lightweight, durable, and versatile, plastics have revolutionised medicine, made space travel possible, and provided materials essential for clean drinking water. However, the properties that make plastic so useful also contribute to the material’s downside, environmental persistence. Unlike organic materials, most plastics do not readily decompose. Instead, they gradually fragment into smaller particles, many of which accumulate in soils and oceans, where they are consumed by wildlife. These particles, known as microplastics, move through the food chain and ultimately reach humans.

Given that synthetic plastics have existed for only a relatively short period compared with the history of life on Earth, their long-term effects on ecosystems and human health are poorly understood. Research into the health impacts of microplastics is still in its infancy. Nevertheless, growing evidence suggests that exposure to them may contribute to digestive inflammation, respiratory problems, and hormone disruption.

With only around 9–10% of plastic currently being recycled, new technologies are needed to support a more circular plastic economy. Finding ways to recover and recycle plastic waste is an important part of environmental remediation. Recycling can return discarded plastics to productive use while reducing demand for new resources, supporting the longer-term goal of a more circular economy in which materials remain in use for as long as possible.

In this article, we highlight recent advances aimed at achieving a circular plastic economy, alongside emerging research into sustainable plastic alternatives, and innovative approaches for removing plastic waste from the environment.

Difficulty recycling plastics

Plastics are far from a homogeneous material. Instead, they comprise a diverse family of polymers, each with specific chemical structures and physical properties to suit their intended applications. For example, different plastic types each require different processing conditions and recycling methods. As a result, there is no universal approach to plastic recycling.

Beyond the polymer itself, many plastic products also contain chemicals known as additives, which are often used to provide beneficial properties to the final plastic product. For example, pigments may be used which can give colour and protect packaged products from degrading in sunlight, and stabilisers may be used to improve durability in plastics that will be exposed to harsh conditions.

While making plastics more functional, additives also complicate recycling. Many can remain in the material throughout the recycling process and reduce the quality of recycled plastics, while others may introduce hazardous substances into the recycling stream.

Achieving a circular plastics economy will require more coordinated regulation and centralised policies that support recycling infrastructure and circular economy objectives.

A systematic review, published in the Open Access journal Sustainability, analysed the barriers that stand in the way of achieving a circular economy in plastic recycling. The authors highlight the following challenges:

• Non-centralised policies slow progress: Different regulations and incentives across countries create uncertainty, making investment in recycling and circular solutions difficult.
• Insufficient collaboration: A circular plastics economy depends on manufacturers, recyclers, policymakers, and consumers all working together.
• Inadequate recycling technologies: Limited recycling infrastructure, contamination, and unequal access to recycling technologies across the world make it difficult to produce high-quality recycled plastics at scale.

While these barriers remain significant, ongoing research and innovation are driving new solutions that are making plastic recycling more efficient, effective, and sustainable.

Mechanical recycling: improving current recycling systems

Mechanical recycling is the process of turning plastic waste into secondary materials without significantly changing the material chemically. Mechanical recycling is the preferred method, and it accounts for more than 99% of European recycled materials.

The process relies on collecting, sorting, washing, shredding, and remelting waste into new materials. However, its success depends heavily on accurately sorting plastics before they are processed. Different polymers cannot be recycled together as they have different properties and melting temperatures.

A review published in the Open Access journal Environments notes that many recycling facilities now use automated sorting technologies; however, the process is still time-consuming and imperfect, representing a major bottleneck to the recycling. Even small amounts of the wrong plastic entering the recycling stream can greatly reduce the overall quality of the recycled material, highlighting the need for more accurate technologies in the sorting of plastic waste.

The implementation of AI into the waste sorting process can overcome difficulties associated with the contamination of different plastic waste. One example of AI implementation involved developing a model for the automatic identification of plastic waste streams. The system was successfully tested in a replica of a real-world recycling facility (Figure 1), demonstrating that this technology can accurately classify waste streams and provide the real-time information needed to improve automated sorting.

Figure 1: Model for sorting plastic waste can identify recyclable plastic products, as well as the type of plastic present on a conveyor belt system.

While advances in AI are improving mechanical recycling, it still depends on clean, well-sorted waste and can reduce plastic quality over repeated recycling. As a result, researchers are increasingly exploring chemical recycling for plastics that cannot be recycled mechanically.

Chemical recycling: regeneration of valuable raw materials

Chemical recycling breaks polymers down into their molecular building blocks. Processes such as depolymerisation and pyrolysis allows plastic waste to be converted back into its monomers or other valuable chemicals that are used to manufacture new plastics.

A review article, published in the Open Access journal Polymers, discusses the advantages of chemical recycling and how it can serve as a complimentary method to other techniques in order to achieve a circular plastic economy. Compared to mechanical recycling it can:

• Process mixed or contaminated plastic waste.
• Remove many impurities.
• Produce recycled materials with properties comparable to virgin plastics.

Although many chemical recycling technologies are still being scaled up, they are widely recognised as a promising route towards a more circular plastic economy, particularly for hard-to-recycle materials.

Depolymerisation

Depolymerisation is a selective chemical recycling process in which catalysts target specific chemical bonds within a polymer, breaking it back down into its original building blocks. This selectivity enables high-purity raw materials to be recovered for the manufacture of new plastics (Figure 2) while minimising the formation of unwanted by-products.

Figure 2: Breaking down polymer chains into their building blocks can allow the recreation of high quality valuable plastics from the same materials.

A study published in Catalysts demonstrates a practical example of this approach. Using N-heterocyclic carbenes as organocatalysts, the researchers broke down polyethylene terephthalate (PET) into bis(hydroxyethyl) terephthalate (BHET), a building block that can be used to produce new PET. The catalyst achieved up to 97% PET conversion and was also successfully tested on post-consumer plastic waste, demonstrating its potential for use in a circular plastic economy.

Recent research has focused on developing catalysts that can operate under milder, more energy-efficient conditions. Looking ahead, key challenges include designing cheaper, highly selective catalysts, scaling laboratory processes for industrial use, and integrating renewable energy and AI to make depolymerisation more sustainable.

Pyrolysis

Unlike depolymerisation, pyrolysis uses high temperatures in an oxygen-free environment to break mixed plastic waste into smaller hydrocarbons. These can be refined into fuels or chemical feedstocks for new plastics, making pyrolysis useful for difficult-to-recycle plastics.

One study highlights how current research is aimed at improving pyrolysis through the development of catalysts that increase the yield of valuable products while reducing energy requirements. This paper combines laboratory experiments with computer modelling, demonstrating how AI-assisted process optimisation could help design more efficient and scalable recycling systems.

Although catalytic depolymerisation and pyrolysis are expanding the scope of chemical recycling, they still require harsh conditions and catalysts that use toxic materials. Biological approaches, in particular enzymes, offer the potential to selectively break down certain plastics under much milder conditions, reducing energy demands while recovering valuable raw materials.

Can enzymes recycle plastics?

Nature often finds its own solutions to the problems we impose on the environment. One example of this is a bacterium which has evolved to effectively eat plastic, Ideonella sakaiensis. Being exposed to plastic in landfill sites, this microorganism evolved two enzymes capable of converting polyethylene terephthalate (PET) into its monomers, which it then uses for its own source of energy.

Isolating these enzymes, in particular PETase which degrades the polymer chain of PET, has given researchers a tool to design and optimise for industrial applications. However, several challenges remain before the technology can be widely adopted. A review published in the Open Access journal Catalysts explores current research focused on engineering more robust enzymes capable of degrading highly crystalline PET, improving enzyme stability and activity, reducing the need for energy-intensive pretreatment, and developing scalable, cost-effective recycling processes for industrial use.

Figure 3: Scheme depicting the crystalline structure of PET plastic which is composed of amorphous and crystalline regions. PET degrading enzymes can only act on amorphous portions of the material, which become more available at higher temperatures. Therefore, increasing the enzyme’s stability at temperatures above its melting temperature is a growing body of research.

Other avenues of research seek to engineer different families of enzymes that naturally break down molecules which are structurally similar to the PET plastic, such as cutinases. Many of these enzymes can be isolated from thermophilic bacteria to overcome the issue of stability observed in PETase enzymes.

Combining knowledge from different areas of enzymatic plastic degradation with advances in AI is providing new insights into how enzymes work and how they can be optimised. Together, these developments make the biodegradation of plastics an increasingly attractive route towards a truly circular plastic economy.

This potential of enzymatic recycling is already being demonstrated by Carbios, which has developed a commercial process using engineered enzymes to recycle PET back into its original monomers.

Looking beyond recycling

Recycling will remain key in efforts to reduce plastic waste, but it cannot solve the problem on its own. Researchers are also developing next-generation plastics that are easier to recycle, contain fewer hazardous additives, or are designed to biodegrade safely under specific conditions. At the same time, advances in materials science are helping manufacturers design products with their entire lifecycle in mind, making them easier to repair, reuse, and recover at the end of their life.

However, scientific innovation can only get us so far. As highlighted throughout this article, achieving a truly circular plastics economy will also require coordinated policies that encourage sustainable practices between industry, researchers, policymakers, and consumers. By combining advances in chemistry, biotechnology, AI, materials science, and policy, researchers are steadily moving towards a future in which plastics remain a valuable resource rather than becoming persistent waste.

Studies on developing new sustainable technologies can be found across the portfolio of MDPI journals covering Chemistry and Materials Science.

Alternatively, you can access the full MDPI journal list here.