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From Plastics to Plants: Transforming Bio-Based Polymers into Fertilisers

New, “circular” fertiliser-from-plastic system shows that bioplastic degradation products can be used to improve plant growth better than urea.

Plastic pollution is one of the biggest environmental threats to our Earth. According to the United Nations Environment Programme, one million plastic drinking bottles are purchased every minute globally and around five trillion single-use plastic bags are used worldwide each year. Despite conversation efforts, only 14 per cent of all plastic waste is recycled, which hardly puts a dent in the problem.

To overcome the plastic conundrum, a circular system is needed to allow source materials used to produce plastics to 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 new, environmentally friendly process of chemically recycling bioplastics back into fertilisers.

In their study, the team focused on poly(isosorbide carbonate) or “PIC,” a type of bio-based polycarbonates that has garnered much attention as an alternative to petroleum-based polycarbonates. PIC is produced using non-toxic material derived from glucose called isosorbide (ISB) as a monomer. Interestingly, the carbonate links that join the ISB units can be severed using ammonia in a process known as “ammonolysis,” one of the end products of which is urea, a nitrogen-rich molecule widely used as a fertiliser. Although this chemical reaction is 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.

To develop an energy-efficient approach that requires no organic solvents, the scientist first investigated how well the complete ammonolysis of PIC could be conducted in water at mild conditions of 30 degrees Celsius and atmospheric pressure. The team then analysed all the reaction products through nuclear magnetic resonance spectroscopy, the Fourier transform infrared spectroscopy, and gel permeation chromatography.

Their findings revealed that they could produce urea using this method. However, the degradation of PIC was slow, incomplete even after 24 hours with many ISB derivatives still present. Therefore, the researchers tried increasing the temperature and discovered that complete degradation could be achieved in about six hours at 90 degrees Celsius.

“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,” said Dr. Aoki.

As a proof-of-concept that all PIC degradation products can be directly used as a fertiliser, the team performed plant growth experiments with the model organism, Arabidopsis thaliana. They observed that plants treated with all PIC degradation products grew better than plants treated with just urea. By showcasing the feasibility of developing fertiliser-from-plastic systems, the team’s findings bring hope to not only fight off pollution and resource depletion but also contribute to meeting the world’s increasing food demands.

“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!” concluded Dr. Aoki excitedly. [APBN]

Source: Abe et al. (2021). Plastics to fertilisers: chemical recycling of a bio-based polycarbonate as a fertilizer source. Green Chemistry.