Natural compounds in freshwater speed up breakdown of 'eco-friendly' microplastics

The Problem with Biodegradable Plastics

Biodegradable plastics are often presented as an answer to the global plastic pollution problem. However, when these materials enter natural environments like rivers and lakes, they do not always break down as intended. Instead, they can fragment into tiny particles known as microplastics. These biodegradable microplastics can persist for long periods, and scientists are working to understand their environmental fate. A new study from researchers at Guangdong University of Technology, the University of Southern Denmark, and the University of Massachusetts examines how these particles degrade under environmentally realistic conditions.

A Focus on Photodegradation

One of the main ways microplastics transform in water is through photodegradation, a process driven by ultraviolet light from the sun. This process can be influenced by other substances present in the water, such as dissolved organic matter, or DOM. DOM is a complex mixture of organic compounds released from the decomposition of plants and animals. While its effect on conventional plastics has been studied, its role in the breakdown of biodegradable microplastics has been less clear.

Natural vs. Pyrogenic Organic Matter

Researchers compared the effects of two different types of DOM on the photodegradation of polylactic acid or PLA, a common biodegradable plastic. The first type was natural DOM, or NDOM, which is typical of what is found in rivers and lakes. The second was biochar-sourced DOM, or BDOM, which comes from pyrogenic or burnt carbon sources, such as after a forest fire or from agricultural biochar application. The team wanted to determine if the source of the DOM changed how quickly PLA microplastics broke down under UV light.

Natural DOM Shows Stronger Effect

The experiments showed that the presence of either type of DOM sped up the breakdown of PLA microplastics compared to pure water. However, the effect was significantly stronger with natural DOM. In the presence of NDOM, the PLA particles showed greater size reduction, more surface cracks and holes, and a more substantial increase in oxygen-containing functional groups on their surface. These chemical changes indicate a more advanced state of degradation, confirming that naturally sourced organic matter is more effective at promoting the breakdown of these plastics.

The Role of Reactive Oxygen Species

The mechanism behind this enhanced degradation involves the chemical structure of the DOM. The study revealed that NDOM contains more aromatic compounds than BDOM. These aromatics act like antennas for UV light, absorbing energy and using it to produce highly reactive chemical molecules called reactive oxygen species or ROS. The research team, led by Jiehong He, Weiwei Ma, and Lanfang Han, found that NDOM produced more of these ROS, particularly hydroxyl radicals, which then aggressively attacked the plastic polymer chains, causing them to break apart.

A Surprising Sorption Mechanism

The study also identified an additional process that boosts degradation. The PLA microplastics were found to preferentially adsorb the non-aromatic components of the DOM onto their surface. This selective binding action effectively concentrates the more photoreactive aromatic components in the surrounding water. With a higher concentration of these light-absorbing compounds in the solution, the generation of plastic-degrading ROS becomes even more efficient, further accelerating the photodegradation of the PLA particles.

Implications for Environmental Assessments

These findings provide a more detailed understanding of the environmental transformation of biodegradable microplastics. The research shows that the rate at which these materials degrade is not constant but depends heavily on the specific chemistry of the aquatic environment they inhabit. The source and composition of dissolved organic matter are key factors that influence the persistence of biodegradable plastics in freshwater systems. This knowledge can help create more accurate models for predicting the fate and potential impact of these materials in the environment.

Corresponding Author:

Lanfang Han

Original Source:

https://doi.org/10.1007/s44246-023-00050-8

Contributions:

Jiehong He: Investigation, Data analysis, Writing—original draft; Weiwei Ma: Methodology, Investigation, Data analysis; Lanfang Han: Investigation, Experimental design, Correspondence, Funding acquisition, Writing—review & editing; Liying Chen: Investigation, Data analysis; Elvis Genbo Xu: Writing—review & editing; Baoshan Xing: Supervision, Writing—review & editing; Zhifeng Yang: Funding acquisition, Supervision, Writing—review & editing. The authors read and approved the final manuscript.