Plastic-eating microbes on the rise

Effective solutions

The horror scenario with which the blurb lures readers turns out – 50 years after the book’s first edition and 69 years after the first plastics trade fair in 1952 – to be more of a utopian fantasy than a real danger. For one thing, given what we know today about the capabilities and properties of polymers, one thing can be said with near certainty:

Plastic is not so easily fallen victim to a bacterial feeding frenzy! Although there is something alluring about this idea, given the approximately 150 to 200 million tonnes of plastic waste worldwide that end up in landfill sites and in the environment year after year. If Mother Nature had an organism amongst her many millions of life forms that had a ravenous appetite for plastic and rubber – we wouldn’t need to worry about the consequences of microplastics in the sea, tyre abrasion in the air or the fate of functional polymer plastic packaging for perishable foodstuffs.

In search of

To manage the flood of plastic products that have reached or exceeded the peak of their useful life, or to return the polymers they contain to the material cycle as valuable, high-quality raw materials, an efficient and – from an ecological perspective – sustainable approach is required. Neither traditional (i.e. physical) nor chemical processes have so far enabled complete material or raw material recycling; although the chemical approach offers greater potential than the use of mechanical forces and high temperatures, as outlined in our ‘Topic of the Month’: ‘Focus on Raw Material Recycling’ [Link: https://bit.ly/3rpkXoA].

A major challenge in plastics recycling is the wide variety of polymers available on the market, which differ considerably in their type and composition: materials that have been chemically bonded are, by their very nature, difficult – if not impossible – to separate using physical forces alone. Through shredding, grinding, cutting or the application of heat, one ultimately obtains granules of varying composition that, at best, can be blended with virgin material. Additives, chemicals, colourants and off-odours make it difficult to produce new products of comparable quality from old ones (upcycling). The situation is, of course, different when the colour of the end product is irrelevant and the quality of the material is of secondary importance, as may be the case, for example, with beacon bases, which can be produced in a single mould from various plastics in the recycled mix (downcycling).

It’s all in the mix

Ideally, the recycled material could be used in both directions: for upcycling and for downcycling. However, this requires a recycling process that literally breaks down waste plastics completely into their individual components. Chemical recycling is already a step in the right direction; it now allows certain polymers to be broken down into their monomers and recovered cleanly.

Because this process describes the reversal of polymerisation, it is referred to as depolymerisation, precisely in reference to the manufacturing process: polymerisation takes place by means of suitable chemical reactions, whereby the hydrocarbon-based monomers are linked together in various ways. However, the reverse process requires a more or less extensive and complex set of procedures.

Remarkably simple

How simple, by contrast, the idea described at the outset seems: placing plastic residues of all kinds, origins and compositions into a large vessel, adding a handful of bacteria, and allowing the microbes to do their work with the lid closed, digesting the polymer broth. The monomers are then skimmed off from the brew and neatly separated according to type and size, free from any unwanted material residues, additives or other impurities. What would ultimately result could literally be declared as virgin material and, as such, fed back into the production process time and time again in a virtually endless loop, without any loss of quality.

Bacteria as a source of enzymes

If the microbial, biocatalysed depolymerisation of PET were left to Mother Nature, the process would take quite a long time. There would be no question of the level of efficiency required by industry. Consequently, biotechnological methods are used to stimulate bacteria to produce as much enzyme as possible. ‘Novozymes’, for example, is one of the ‘big players’ in this field. Among other things, the Danish biotechnology company possesses one of the world’s largest collections, comprising more than 8,000 yeast and lactic acid bacteria cultures. With the support of Novozymes, the French company ‘Carbios’ has, in turn, developed and established a process that holds out the prospect of a genuine circular economy for polymer materials.

In April 2020, the scientific journal ** *Nature* published a now widely cited article describing the process by which Carbios’ technology converts plastic waste into “chemical building blocks for the production of new, high-quality plastics”. It is not only the quantity processed that matters here, but also the time required for the process. And, as the article explains, both aspects are impressive in the Carbios process: within just ten hours, 90 per cent of a tonne of shredded plastic had been broken down; the current focus is on PET products (bottles, films, textile fibres, etc.). Carbios describes the mechanism of action of the enzyme, which breaks down polyethylene terephthalate (PET) into monomers via hydrolysis, as a ‘bacterial catalyst’. The resulting chemical degradation surpasses all previously described hydrolysis processes, including those carried out by the aforementioned bacterium Ideonella sakaiensis. It was discovered amongst thousands of microorganisms that had been found and analysed at landfill sites. The authors of the *Nature* article report that it has been demonstrated that biologically recycled PET exhibits the same properties as petrochemical PET, and that enzymatically depolymerised PET waste is suitable for the manufacture of, for example, PET bottles.