Plastic pollution in water is often imagined as floating bottles or visible fragments. However, the most persistent problem occurs on a microscopic and chemical scale. A new study published in New Contaminants reveals that sunlight transforms microplastics into invisible chemical clouds, capable of contaminating rivers, lakes, and oceans.
Microplastics release substances dissolved in water, and solar radiation accelerates this process. It is not just about physical fragmentation, but a continuous release of invisible compounds that disperse beyond the plastic particle itself.
How chemical release occurs
Once a microplastic enters an aquatic system, contact with water is permanent. Solar radiation breaks chemical bonds on the polymer’s surface, weakening its structure and releasing small molecules into the water.
The study analyzed four common types of plastics:
- Polyethylene (PE) and polyethylene terephthalate (PET), of fossil origin.
- Polylactic acid (PLA) and PBAT, considered biodegradable.
All released dissolved organic carbon, but at different rates. Ultraviolet radiation was decisive: under light, chemical release skyrocketed compared to dark conditions.
The paradox of biodegradables
Biodegradable plastics released more dissolved carbon because their polymer chains are more vulnerable. Designed to break down sooner, they interact more with solar energy, creating an uncomfortable paradox: they chemically pollute faster.
Key study results
- The release rate does not decrease over time: it follows a zero-order kinetics, remaining constant even if the water is already loaded with compounds.
- The limitation is on the plastic’s surface, not in the water concentration.
- Under UV radiation, a water film forms around the microplastic that slows diffusion, but the chemical drip continues.
- The type of polymer and its exposure to light weigh more than the previous accumulation of substances in the environment.
Complex chemical mixtures
Advanced chemical analysis revealed thousands of different molecules in the dissolved organic matter derived from microplastics:
- Industrial additives like phthalates, easily released into the aquatic environment.
- Polymer fragments and products of photochemical reactions.
- Oxygenated compounds (alcohols, acids, ethers, carbonyls) that increase chemical reactivity.
The composition changes over time: protein-like substances decrease, and humic and tannic compounds increase. Natural organic matter remains stable; plastic-derived matter is chemically flexible and mutates quickly.
Ecological and health impacts
- Alteration of microbial networks: some molecules stimulate biological activity, others inhibit it, affecting carbon and oxygen cycles.
- Interaction with heavy metals: copper, cadmium, or lead change their mobility and toxicity.
- Mineral reactions and nutrient transport: reactive oxygen species are generated that transform pollutants and favor nanoparticle formation.
- Drinking water treatment: this invisible chemistry can generate unwanted by-products, complicating processes designed for other contaminants.
A regulatory challenge
Plastics continue to enter aquatic ecosystems with limited regulation. Once there, sunlight guarantees a continuous chemical release that does not stop. The composition of these substances changes over time, and with it, their ecological impacts.
Looking to the future
Machine learning tools are being explored to predict the chemical behavior of this microplastic-derived organic matter and improve risk assessment in ecosystems and water systems.
The research shows that plastic pollution is not only visible: it is also chemical and invisible. Sunlight turns microplastics into constant sources of compounds that alter the balance of aquatic ecosystems and complicate drinking water management. The challenge is twofold: reduce the entry of plastics and better understand their long-term chemical impact.
