A new meta-study by Hydra Marine Sciences, a research laboratory, concluded that hydrolysis of polylactic acid (PLA), avoids the formation of persistent microplastics.
The report initially reviewed over 30,000 reports related to PLA degradation, of which 500 were identified by Hydra as relevant and of sufficient quality for deeper review. The meta-study only considered neat PLA, but not specific articles or products made from or with PLA in which the base polymer may have undergone further treatments and modifications.
The research was sponsored by Holland Bioplastics, Futerro, TotalEnergies Corbion, and NatureWorks.
The study investigated what the fate of PLA is in the open environment. Particularly, it asked how fast and to what extent does PLA degrade in the open environment and whether persistent microplastics are formed.
The authors noted that the occurrence of PLA microplastics in nature has been reported ‘only a few times’ from environmental samples. They reported that several short-term field experiments and laboratory tests over several months concluded that PLA does not biodegrade in the open environment.
The study argued back saying that ‘most of these studies seem to neglect or are not aware of the fact that the molecular degradation of PLA by hydrolysis is a bulk erosion process, which for a longer initial time is not measurable as mass loss but only as a decrease in molecular weight until the molecular fragments are small enough to become soluble and diffuse out of the polymer matrix’.
Hydrolysis is an abiotic process that occurs in the presence of moisture or humidity. The study found that as long as these conditions prevail, the molecular weight and size of any PLA objects or fragments will continually decrease via hydrolysis, at a rate determined by temperature, until the polymer chains are so short that the material eventually becomes soluble in water. These soluble substances, oligomers and lactic acid monomers, will subsequently be biodegraded by microorganisms into biomass, water, and carbon dioxide.
The authors nevertheless noted that further research is needed to establish what the hydrolysis rates of PLA at ambient temperatures between 37 C and 0 C are in freshwater, seawater, and soils with different humidity. Studies will have to be conducted in the open environment to show that degradation does not stop at the level of micro- and nanoplastics but continues and that the molecular fragments resulting from PLA hydrolysis eventually become soluble also at moderate to low ambient temperatures, leaving no solid PLA residues.
“The meta-study shows that in contrast to non-biodegradable plastic, which will persist and permanently accumulate as micro- and nanoplastics in the environment, PLA in the environment will not leave persistent pollution as long as humidity and water are present,” said Christian Lott, managing director at Hydra Marine Sciences. “However, we must be aware that it does not belong in the environment, and it is critical that we do not use these attributes to encourage littering or slow the development of global waste infrastructure. Degradation of any material must be balanced with accumulation, or how much material is entering the environment, in order to reduce harm to the environment,” he concluded.