Plants from plastics: circular recycling of polymer waste

Plastics have taken the world by storm over the last century, finding applications in virtually every aspect of our lives. However, the rise of these synthetic polymers, which form the basis of plastics, has contributed to many serious environmental issues. The worst of these is the excessive use of petrochemical compounds and the disposal of non-biodegradable materials without recycling; only 14 per cent of all plastic waste is recycled, which hardly puts a dent in the problem.

To solve the plastic conundrum, 'circular' systems need to be developed in which the source materials used to produce the plastics come full circle after disposal and recycling. At Tokyo Institute of Technology, a team of scientists led by assistant Professor Daisuke Aoki and Professor Hideyuki Otsuka is pioneering a novel concept. In their environmentally friendly process, plastics produced using biomass (bioplastics) are chemically recycled back into fertilisers.

The team focused on poly (isosorbide carbonate), or 'PIC', a type of bio-based polycarbonate that has garnered much attention as an alternative to petroleum-based polycarbonates. PIC is produced using a non-toxic material derived from glucose called isosorbide (ISB) as a monomer. The interesting part is that the carbonate links that join the ISB units can be severed using ammonia (NH3) in a process known as ‘ammonolysis’. The process produces urea, a nitrogen-rich molecule that is widely used as a fertiliser. While this chemical reaction was no secret to science, few studies on polymer degradation have focused on the potential uses of all the degradation products instead of only the monomers.

The scientists investigated how well the complete ammonolysis of PIC could be conducted in water at mild conditions (30°C and atmospheric pressure). The rationale behind this decision was to avoid the use of organic solvents and excessive amounts of energy. The team carefully analysed all the reaction products through various means, including nuclear magnetic resonance spectroscopy, the Fourier transform infrared spectroscopy, and gel permeation chromatography.

Although the team managed to produce urea in this way, the degradation of PIC was not complete even after 24 hours, with many ISB derivatives still present. The researchers then tried increasing the temperature and found that complete degradation could be achieved in about six hours at 90°C.

Highlighting the benefits of this approach, Dr. Aoki said: “The reaction occurs without any catalyst, demonstrating that the ammonolysis of PIC can be easily performed using aqueous ammonia and heating. Thus, this procedure is operationally simple and environmentally friendly from the viewpoint of chemical recycling.”

Finally, as a proof-of-concept that all PIC degradation products can be directly used as a fertiliser, the team conducted plant growth experiments with Arabidopsis thaliana, a model organism. They found that plants treated with all PIC degradation products grew better than plants treated with just urea.

The overall results of this study showcase the feasibility of developing fertiliser-from-plastics systems (as shown in the diagram above). The systems can not only help fight pollution and resource depletion, but can also contribute to meeting the world’s increasing food demands.

Dr. Aoki added: “We are convinced that our work represents a milestone toward developing sustainable and recyclable polymer materials in the near future. The era of ‘bread from plastics‘ is just around the corner!”

The study - 'Plastics to Fertilisers: Chemical Recycling of a Bio-based Polycarbonate as a Fertiliser Source' - will be published in Green Chemistry, a journal of the Royal Society of Chemistry focusing on innovative research on sustainable and eco-friendly technologies.