Oxo-degradable plastics: Study highlights biodegradability and sustainability limits

A new study finds that oxo-degradable plastic, promoted as a “more sustainable alternative” to conventional plastic, breaks down poorly in open-air conditions, raising concerns for soil and plant health. 

Oxo-degradable plastic has been said to undergo oxidative degradation through the addition of pro-oxidants, allowing it to break down under conditions of light and heat, followed by biodegradation into carbon dioxide and water by microorganisms. However, newly published scientific research has challenged this perception. 

“Based on the available evidence [generated by my team and others], we would not advocate the use of oxo-degradable plastics as a sustainable alternative material for the packaging industry,” Davey Jones, professor of Soil and Environmental Science at Bangor University, Wales, and study author, tells Packaging Insights.

“Some of the biodegradability claims made by manufacturers do not seem to match the evidence presented in many independent studies. In our view, the use of alternative petroleum-based biodegradable plastics holds much more promise [such as, polybutylene adipate terephthalate (PBAT) — a biodegradable random copolymer — or polylactic acid-based plastics.”

Jones’ study reveals the effects of oxo-degradable plastics of different sizes and concentrations on soil and corn growth and has been published in Frontiers of Agricultural Science and Engineering.

Bioplastic effects on soil and plants

The study includes a “dual-dimensional comparison” of the differences between oxo-degradable microplastics and macroplastics in a range from actual field concentrations to extreme concentrations in soil.

Davey Jones, professor of Soil and Environmental Science at Bangor University, Wales, and study author.The research team selected corn as a model crop and monitored key indicators such as soil pH, electrical conductivity, and nitrate content over six weeks.

“We used maize as it is frequently grown in many regions of the world with plastic mulch films. It is also sensitive to environmental stress, so it is easy to observe the positive or negative effects of plastics on plant growth,” explains Jones. 

The experiment finds that at the low concentrations found in agricultural fields, oxo-degradable plastics have “almost no impact on soil quality and corn growth.” However, exceeding the 1% concentration “significantly” changes the soil properties.

“Soil pH in the microplastic treatment group increased, electrical conductivity doubled, and nitrate content significantly increased, while the impact of macroplastics was much weaker. This may be related to the larger surface area of microplastics, which allows for more extensive contact with soil, thereby altering soil structure and chemical environment more easily.”

High concentrations of such microplastics and macroplastics decreased the corn height and chlorophyll content. The inhibitory effect of microplastics is said to be even more evident. 

Overall, the corn biomass only slightly decreased at the highest concentration, showing a certain level of tolerance.

Degradation limitations 

Furthermore, the study revealed the “degradation limitations” of oxo-degradable plastics. 

Infrared spectroscopy analysis revealed that six weeks after burial in soil resulted in only “mild chain scission of the plastics, with no significant production of oxidation products such as ketones or aldehydes detected.”

Jones argues against the use of “oxo-degradable” plastic for packaging.The result suggests that without light in the soil environment, the degradation process of oxo-degradable plastics is very slow, and can thus lead to accumulation over time.

Jones says that low material degradation can impact soil health and agricultural outcomes. “Like plants, soil microorganisms also respond negatively to microplastics.” 

“However, oxo-degradable plastics can also stimulate microbial activity in soil. As they deliver key ecosystem services [nutrient cycling, pollutant breakdown], they are also a good indicator of how agricultural systems respond to the presence of plastics.”

He adds that Bangor University has the longest-running microplastic field trials in the world with low-density PE and PBAT. 

“However, we have not set one up yet with oxo-degradable plastics, although we could easily do this if funds were available. We acknowledge that sunlight would speed up the breakdown of oxo-degradable plastic, but if the plastics are buried in soil, this won’t happen.”

Jones says: “Oxo-degradable plastic should not be used in ‘eco-friendly’ product lines unless it can be guaranteed that the products can be collected and reprocessed. As this is unlikely, we would not advocate their use, favoring other, more sustainable alternatives.”