Sunlight promotes the release of microplastics, creating a complex and ever-changing mixture of dissolved chemicals in rivers and oceans. This leads to invisible pollution that spreads much farther than the plastic particles themselves. These chemical traces differ from natural organic compounds and can subtly affect aquatic ecosystems and water composition.

How Microplastics Release Chemicals

Scientists have found that microplastics present in rivers, lakes, and seas continuously release complex organic compounds into the water. This process intensifies under sunlight and continues over time. Recent research has allowed a detailed look at the molecular level at how dissolved organic matter released by microplastics (MPs DOM) forms and changes in natural water bodies.

In a study published in the journal New Contaminants, four common types of plastics were examined. Researchers compared the chemicals released by these plastics with natural organic compounds found in rivers. Using kinetic modeling, fluorescence spectroscopy, high-resolution mass spectrometry, and infrared analysis, they showed that each type of plastic emits a unique chemical mixture. These chemical "signatures" change as the plastic surface breaks down under sunlight.

The Impact of Sunlight and Plastic Types

During experiments, microplastics made from polyethylene, polyethylene terephthalate, polylactic acid, and polybutylene adipate-co-terephthalate copolymer were exposed to water both in darkness and under ultraviolet light for 96 hours. Sunlight significantly increased the amount of dissolved organic carbon released by each tested plastic. Biodegradable plastics (PLA and PBAT) released these substances especially intensively, due to their less stable chemical structure.

Kinetic modeling showed that the rate of chemical release was determined by physical and chemical limitations at the plastic surface, not by the amount of material already dissolved. Under ultraviolet light, diffusion through the plastic film was the main limiting factor.

Chemical Composition of Released Substances

Plastics emit complex mixtures of chemicals, including additives, monomers, oligomers, and fragments formed through photo-oxidative reactions. Plastics with aromatic structures, such as PET and PBAT, produced especially complex chemical mixtures. As plastics aged, the content of oxygen-containing functional groups increased, indicating the formation of alcohols, carboxylates, ethers, and carbonyls. Chemical additives like phthalates were also detected, likely due to their weak bonds within the plastic structure.

Fluorescence measurements showed that MPs DOM more closely resembles organic material produced by microbes than organic matter coming from land and soil. This pattern is markedly different from natural dissolved organic matter in rivers. Over time, the ratio of protein-like, lignin-like, and tannin-like substances changed depending on the type of plastic and the level of sunlight exposure.

Ecological Risks of Invisible Pollution

The changing chemical mixtures released by microplastics can affect aquatic ecosystems in various ways. MPs DOM mainly consists of small, biologically available molecules that can stimulate or suppress microbial growth, disrupt nutrient cycles, or interact with metals and other pollutants. Previous studies have shown that MPs DOM can generate reactive oxygen species, influence the formation of disinfection byproducts, and alter how pollutants attach to particles in water.

These findings highlight the importance of considering the full life cycle of microplastics in water, including the invisible dissolved chemicals they release. As global plastic production increases, such dissolved compounds may become increasingly significant for the environment.

Outlook for Prediction and Regulation

Because MPs DOM is chemically complex and constantly changing, researchers believe that machine learning tools could help predict the behavior of these substances in natural waters. Such models could improve risk assessments for ecosystem health, pollutant transport, and carbon cycling.

The authors also note that the flow of microplastics into rivers and oceans is largely unregulated. As plastic continues to fragment and degrade under sunlight, the release of MPs DOM will only increase. Understanding how these chemicals evolve at different stages of plastic breakdown is crucial for assessing their long-term environmental impact.