Enzymatic recycling of PE and PP lacks evidence, researchers warn
Key takeaways
- NMBU researchers find that enzymatic recycling remains most promising for PET, with limited evidence of effective PE, PP, or polystyrene depolymerization.
- Scientists warn that weight loss, surface damage, or oxidation alone do not prove enzymes have cleaved a plastic polymer’s backbone.
- Hybrid chemical-biological recycling may offer a more practical route for non-hydrolyzable packaging plastics than enzymes alone.
New research explores the potential of enzymatic recycling for certain types of plastic packaging, suggesting that the technology remains limited for non-hydrolyzable polymers, such as PE, PP, and polystyrene.
In biological recycling, enzymes selectively split the chemical bonds that hold plastic together. While the technology has already been developed for PET, research from the Norwegian University of Life Sciences (NMBU) indicates that there are limitations when applying the approach to other types of plastic.
Specifically, the research, published in Nature, assesses the scientific evidence for biological degradation of non-hydrolyzable polymers, which are polymers whose backbones cannot be broken down by hydrolisis, using biocatalysts to cleave apart carbon-carbon bonds.
Packaging Insights speaks to Gustav Vaaje-Kolstad and Anton Stepnov, authors of the study from the Faculty of Chemistry, Biotechnology, and Food Science at NMBU, to weigh the industry potential of enzymatic recycling alongside accurate and validated scientific research into the topic.
Stepnov explains: “Hypothetical microorganisms and enzymes involved in biodegradation of PE and other non-hydrolyzable plastics are expected to rely on oxidative chemistry to cleave the polymer backbone. Such chemistry will degrade products that are chemically distinct from the original monomers.”
Enzymatic recycling misconceptions
Enzymatic recycling is not a new waste management solution, but Vaaje-Kolstad points out that the technology has, at times, been shrouded in misleading efficacy claims that have shaped industry and consumer expectations.
“A common problem is that [enzymatic recycling] claims are based on indirect evidence, such as weight loss, surface damage, or indications of surface oxidation, without providing concrete proof of cleavage of the polymer backbone.”
He adds that additives, contaminants, or pre-oxidation of plastics that can occur during processing can sometimes be consumed by microbes and mistaken for polymer degradation.
Additionally, he observes that the success of enzymatic PET recycling has created an expectation that other polymers, such as PE, can be treated similarly, “but the evidence for this is unfortunately very limited.”
Industry adoption
The research proposes best-practice experimental design principles, including reaction conditions, control experiments, analytical approaches, biochemical assays, and characterization methods, for the continued evaluation of enzymatic recycling of non-hydrolyzable compounds.
However, Stepnov and Vaaje-Kolstad say that scientific gaps limit the industrial use of enzymatic recycling for certain plastics.
“The central gap is still efficient cleavage of the carbon-carbon bonds in common packaging plastics such as PE and PP. Whether this is feasible from a biological point of view is uncertain, but there is certainly a great deal of undiscovered biological diversity,” underscores Vaaje-Kolstad.
Stepnov adds that even if efficient PE-degrading microorganisms and enzymes are identified in the future, developing a cost-effective recycling process “would still be a challenge.”
Vaaje-Kolstad concludes: “Within the current state of the art, the most practical solution would be a hybrid chemical–biological approach, where the non-hydrolyzable packaging waste is chemically converted into products that can subsequently be upgraded to valuable products by microorganisms.”
