The accumulation of plastic waste in the environment is an ecological disaster and will require multiple solutions to tackle the problem. Despite recent initiatives to close the plastics loop, only 9% of plastic was recycled in 2019, with the remaining waste either incinerated or accumulating in landfills or natural environments, posing hazards to both living and non-living systems. Bioplastics, derived from renewable sources, have been investigated as green alternatives to conventional fossil-based plastics. However, costly synthetic routes and low recyclability continue to challenge the growth of bioplastics. Poly(lactic acid) (PLA) is the most popular polymer for commercial bioplastics, but its recycling is limited by challenging mechanical recycling and slow biodegradation. A team of researchers from King’s College London has developed a generalisable biocatalysis engineering strategy to enhance the use of enzymes to depolymerise a broad class of plastics, in a publication recently published in Cell Reports Physical Science. This novel approach is 84 times faster than the 12-week-long industrial composting process currently used for recycling bioplastic materials
The problem with plastic waste
The demand for conventional plastics such as Poly(lactic acid) (PLA) or poly(ethylene terephthalate) (PET) is increasing, with 460 million tons produced in 2019; a 230-fold increase from the 2 million tons produced in 1950. Plastic waste is a significant environmental issue, with millions of tons of plastic ending up in natural environments each year. Traditional recycling methods are often inefficient and unable to produce high-quality reusable materials. Bioplastics, derived from biological sources such as corn starch and sugarcane, are seen as a more sustainable alternative. However, current methods of bioplastic production are costly and compete with food-based agriculture for land use. Furthermore, mechanical recycling methods generate CO2 and are incapable of producing high-quality reusable materials, leading many retailers to revert to using oil and fossil-based materials. As an example, it takes up to 84 days at 60°C in industrial composting to recycle PLA, with very little valorisation possible.
An innovative solution
In this publication, the team of researchers developed a new protocol to recycle PLA. This method involves different component to help depolymerise the material.
First, they used ionic liquids to solubilise the plastic. Ionic liquids are salts in a liquid state that have unique properties, such as low volatility and high thermal stability. Ionic liquids have been shown to have the ability to solubilise polymers used in common plastics such as PET and PLA. Secondly, they used a commercially available enzyme, a lipase from Candida antarctica (CaLB) to degrade the plastic.
As the enzyme may not be stable in ionic liquid, the researchers performed some chemical modifications in three different steps to preserve enzyme activity.
Researchers performed circular dichroism at Diamond Light Source on the B23 beamline to ensure that the secondary structure of the enzyme was intact after the chemical modifications. Measurements realised on B23 also showed that the thermostability of the modified protein was higher in ionic liquid (> 80°C) compared to the unmodified protein in aqueous solution. This parameter is important, as heat is required to help depolymerise plastics.
Read more on Diamond website
Image: Graphical abstract of the publication