Biodegradable plastics built to biodegrade in the open environment should not leave behind micro- and nanoplastics.
Biodegradable plastics may be part of the solution to reduce the accumulation of plastics in the open environment. However, as the biodegradability of such plastics in the environment is not only dependent on the properties of the plastic material, but also the environmental conditions and microorganism present, these plastics should be applied with care and for specific applications only. One focused application that would benefit the environment compared to conventional plastics are products that, to a large degree, release microplastics into the environment due to abrasion during use. Read more about biodegradable plastics, their suggested application and potential ecological risks in the SAPEA report from 2020 Biodegradability of plastics in the open environment – SAPEA.
Biodegradation of biodegradable plastics.
Biodegradation of plastic means that the polymeric plastic material is converted by microorganisms, such as bacteria, fungi and algae, to carbon dioxide and new microbial biomass, and not just broken down into smaller pieces. In anoxic conditions, methane may be produced, in addition to carbon dioxide, by the activity of methanogenic archaea (a group of microorganisms). Biodegradation starts at the surface of the plastic and moves inwards. As the ratio of surface area to volume mass increases with the decreasing size of mass, the biodegradation of smaller fragments proceeds faster per volume mass than do larger fragments. Biodegradable plastics that are built, and correspondingly tested, to biodegrade in the open environment will not leave behind micro- and nanoplastics.
The challenge is that the specific timespan it takes for biodegradable plastics to completely ‘disappear’ is difficult to predict. Environmental biodegradability of plastics is not a mere material property but rather a system property that depends on the type of environment and microorganisms present, as well as a range of other environmental factors. Temperature is believed to be the most important rate-determining factor of the biodegradation process, but recent results from our tank experiments at the Bergen Aquarium, mimicking typical Nordic marine conditions, suggest that other factors such as nutrient concentrations also significantly influence the biodegradation rate (link to the poster).
If something is labelled as ‘biodegradable’, the environmental compartment in which the product is biodegradable (e.g. soil, freshwater, marine) should also be stated. Only labels from certification bodies should be used and accepted. It is only then that we can trust that products are able to completely biodegrade within the stated timeframe under the stated environmental conditions.
Environmental risks associated with biodegradable plastics that need more research.
- Biodegradable plastics that translocate to environments they are not assessed to biodegrade in.
Due to their small size, micro- and nanoplastics are more prone to translocate from one environment to another. If pieces of biodegradable mulch film, intended for biodegradation in agricultural soil, were to escape from the soil to a nearby river for example, the plastic may then biodegrade at a much slower rate. A risk assessment should therefore also include the biodegradability and biodegradation rate in freshwater conditions, and ideally certified for this environmental compartment. If not, there is a risk that freshwater bodies near agricultural soils where biodegradable mulch film has been used will suffer from persistent pollution.
- Micro- and nanoplastics from compostable plastic products.
If compost material from industrial composting (58oC) is used as fertilizer or soil improvement, residual plastic fragments from incomplete biodegradation may be a significant source of micro- and nanoplastics in the open environment. Certain compostable plastics intended for biodegradation at a high temperature, will persist at environmental conditions and while doing so pose the same harms as conventional plastics. Therefore, risk assessment of compost material that includes compostable plastics as waste category should be performed with a focus on biodegradability of residual microplastics before applied as fertilizer or soil improvement.
- Consumption of microplastics from biodegradable and compostable plastics.
Although the rate of biodegradation increases when particles become smaller, micro- and nanoplastics from biodegradable and compostable plastics may still be consumed by organisms in the same manner as micro- and nanoplastics from conventional plastics. For organisms that have gut microorganisms it is here hypothesised that micro- and nanoplastics from biodegradable and compostable plastics may be digested by gut microorganisms after being ingested.
- Leakage of harmful additives during biodegradation.
Recent studies show that products made from biodegradable plastics contain as many harmful chemical compounds as similar products made from conventional plastics. When assessing the toxicity of biodegradable plastic products, toxicological assays should account for realistic leakage rates of leachates, as the biodegradation process itself is likely to accelerate the release rate of non-biodegradable additives to the surrounding environment. It is here hypothesised that the leachates rate of non-biodegradable compounds from biodegradable plastics during biodegradation is inversely proportional to the biodegradation rate.
- Effects associated with the biodegradation process of biodegradable plastics.
When biodegrading in random compartments of the open environment or intentionally in agricultural soil, the biodegradation process may affect microbial activity and ecology. The effects may be related to the increase in available substrate to support microbial growth, which may enrich for specific microbial guilds or even kingdom. Examples are enrichment of sulphate-reducing bacteria and fungus respectively, the latter probably enriched due to the high carbon content of plastic polymers that complement the high carbon to nitrogen ratio of fungal biomass compared to procaryotic biomass. The long-term consequences of the observed effects on microbial ecology and activity are still unknown and thus require further dedicated research.