1. Non-Biological Pathways for PET Recycling/Upgrading

PET recycling can be achieved through non-biological routes such as mechanical techniques or chemical processes. The mechanical method involves crushing, washing, and heating but has limitations in terms of the purity and quality of the resulting products. Conversely, chemical recycling involves depolymerization and re-polymerization to create high-quality feedstock, promoting closed-loop recycling and the growth of the resource value chain while also cutting down on carbon emissions.

1.1 Mechanical Recycling of PET: Status and Challenges

Mechanical recycling is a mature and widely applied technology in the PET recycling field, and its low cost and ease of operation make it dominant in the recycling of single-material products such as plastic bottles.

Mechanical recycling mainly involves collecting and sorting PET waste, then grinding and milling it into small pieces, followed by washing to remove contaminants, drying, and heating to convert it into molten PET resin, which is then extruded into pellets for manufacturing new products.

However, mechanical recycling also has significant limitations. First, the purity of PET waste must reach a certain standard to ensure the quality of the recycled products. Secondly, the physical recycling process may damage the PET molecular chains, leading to the recycled PET performing poorer than the original material. Moreover, this method is limited in converting PET waste into high-value-added products.

Additionally, the energy consumption and carbon emissions associated with mechanical recycling cannot be overlooked. The crushing, milling, and washing processes in the mechanical recycling workflow require mechanical and thermal energy, which are often derived from the combustion of fossil fuels, resulting in carbon dioxide emissions. The drying and heating stages also require significant energy input, further increasing the overall carbon footprint.

1.2 Chemical Recycling: Pioneering Techniques for PET Upcycling 

Chemical recycling represents a transformative approach to PET waste management, offering a sophisticated alternative to traditional mechanical recycling methods. This process not only yields high-purity feedstocks like terephthalic acid (PTA) and monoethylene glycol (MEG) but also provides the versatility needed to both recycle PET within a closed-loop system and to explore new avenues for resource utilization through open-loop pathways.

Chemical recycling, also known as closed-loop recycling, is a non-biological technology that involves breaking down the long PET polymer chains through thermochemical treatment methods into monomers or other basic petrochemical feedstock, which can then be used to manufacture new PET or other value-added products.

The chemical recycling process includes depolymerization, purification, re-polymerization, and forming. Depolymerization is the critical step, typically achieved by using excess methanol and catalysts to heat and decompose PET into its individual monomers. The purification step involves removing impurities or contaminants from the monomers, while re-polymerization uses heat and catalysts to re-assemble the monomers into new PET polymers.

Compared to the production of virgin PET, chemical recycling can also reduce greenhouse gas emissions and energy consumption by up to 50%. However, chemical recycling is a more complex and expensive process than mechanical recycling, and challenges remain in terms of scalability and commercial viability. Despite these challenges, the potential of PET chemical recycling is immense.

1.2.1 Efficient Closed-Loop Recycling: Converting Waste PET to High-Quality TPA and EG

The closed-loop recycling of waste PET through precise molecular-level treatment can convert it into high-purity TPA and EG, which are crucial monomers for manufacturing brand-new PET products. 

The key to this process lies in the depolymerization, which can be achieved through various methods, including hydrolysis, ammonolysis, alcoholysis, as well as advanced photocatalytic and electrochemical technologies.

Hydrolysis: Under acidic, alkaline, or neutral conditions, PET undergoes hydrolysis at high temperatures and/or pressures, yielding TPA and EG.

Ammonolysis: At high temperatures, PET reacts with ammonia or amine compounds, producing terephthalamide (the amide form of TPA) and EG, which can be further converted to TPA.

Alcoholysis: At specific temperatures, PET reacts with methanol, producing dimethyl terephthalate (DMT) and EG, which is a form of transesterification.

Cutting-edge oxidative techniques for breaking down plastic waste, such as photocatalysis and electrochemistry, are a focus of research in chemical recycling. These approaches offer a promising, eco-friendly, and effective means of transforming and reclaiming valuable materials from waste.

Photocatalysis involves utilizing semiconductor materials as catalysts and subjecting PET to light irradiation to produce TPA and EG, utilizing renewable energy sources such as solar power. This method operates at room temperature and ambient pressure, reducing energy consumption and environmental impact.

Electrochemistry utilizes an electrochemical process in an electrolytic cell to decompose PET into TPA and EG, offering process controllability, high selectivity, and minimal energy consumption. It allows the use of renewable electrical energy to reduce the carbon footprint.

1.2.2 PET Waste Upgrading: From Monomers to Value-Added Chemicals

The application of chemical recycling technologies in the upcycling of PET waste has demonstrated its immense potential, providing new directions for resource recycling and sustainable development.

As mentioned earlier, depolymerization is a crucial step in chemical recycling, and the various depolymerization methods have also opened up possibilities for upgrading PET waste to produce value-added chemicals.

During ammonolysis in chemical recycling, PET polymer can be converted into valuable TPA diamides/amides by reacting with ammonia or amines at high temperatures. These products have various uses such as plasticizers, adhesives, antimicrobial agents, and textile dyes, and can also be further modified into poly(ester-amide)s, terephthalonitrile, and p-xylylenediamine, providing new materials for organic solvents and other chemical industries.

In more advanced oxidation strategies, the photocatalytic process utilizes semiconductor materials as catalysts, generating free radicals or active oxygen species under light irradiation to break down PET plastics into smaller molecules, such as formaldehyde, formic acid, and acetic acid. These products can also serve as solvents, chemical feedstock, and plastic additives, offering a wide range of industrial applications.

The electrochemical process, on the other hand, applies an electric current in an electrolytic cell to decompose PET into TPA and EG, then further converting EG into oxygenated compounds, such as formate esters, formic acid, and glycolic acid. This method can also produce hydrogen, while reducing energy consumption and the carbon footprint.

2. Biological Pathways: Green Innovations in PET Recycling

Enzymatic recycling, also called biological recycling, is a new PET recycling method using enzymes to break down PET products into monomers. It overcomes the drawbacks of mechanical and chemical recycling and reduces greenhouse gas emissions and energy consumption, thereby improving PET recycling efficiency.

The biological recycling process of PET includes multiple steps: first, the collection and sorting of PET waste, then size reduction, followed by enzymatic depolymerization, purification, and finally, the re-polymerization to regenerate PET materials. This method employs microorganisms and enzymes for biodepolymerization, producing a high-quality mixture of monomers and oligomers, providing a green and sustainable recycling pathway.

The primary advantage of biological recycling is its relatively low environmental impact, enabling the recovery of low-quality PET waste and the production of new PET with similar performance to virgin PET. Additionally, biological recycling does not require fossil fuels, thus significantly reducing energy consumption and carbon dioxide emissions.

3. Integrated Non-Biological/Biological Approach: Innovative Recycling of PET Waste

The introduction of biocatalysis has driven the transition of green chemistry towards circular chemistry. The integrated non-biological/biological approach is a staged process that combines the advantages of chemical hydrolysis and biological conversion technologies to achieve the resource utilization of PET waste.

The recycling and upgrading of PET waste can be realized through an innovative integrated non-biological/biological method. This approach first employs chemical pathways, such as hydrolysis, to efficiently convert PET waste into TPA, bis(2-hydroxyethyl) terephthalate (BHET), and EG. These chemically decomposed products then become the ideal substrates for biological conversion, where engineered bacterial strains can further transform these organic monomers into a wider range of useful chemicals, ensuring selective depolymerization and efficient biological conversion.

This integrated chemical-biological approach combines the speed and controllability of chemistry with the selectivity and mild conditions of biology, enabling more efficient production of TPA and EG, as well as promoting the generation of high-value-added chemicals, thereby driving the closed-loop recycling and upgrading of PET. For example, BHET, as the primary product of PET sugar fermentation, can be converted to TPA through enzymatic action, a process that can be completed in a relatively short time and with a high yield.

Furthermore, innovative biocatalytic photoelectrochemical (PEC) systems utilize solar energy to drive biosynthesis, converting compounds such as ethylene glycol into other useful chemicals. These systems employ solid-state catalytic electrodes as solar enzyme activators, providing new perspectives for solar energy conversion and application, further enhancing the sustainability and environmental friendliness of PET recycling.

4. Conclusion

The field of PET recycling has diversified with the emergence of mechanical, chemical, and biological processes, alongside cutting-edge integrated non-biological and biological techniques. Despite their individual benefits and constraints, these technologies unitedly aim to channel PET waste into the circular economy. With future innovations, the effectiveness and sustainability of PET recycling are poised for significant enhancement, further aligning human industry with environmental stewardship.