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Natural organic small molecules promote the aging of plastic wastes and refractory carbon decomposition in water
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Researchers discovered that natural organic molecules found in rivers accelerate the breakdown of microplastics and nanoplastics by generating chemical radicals on plastic surfaces. As plastics age in water, they release smaller fragments and chemical byproducts like styrene, and the aged plastics actually promote the decomposition of hard-to-break-down carbon in the ecosystem. This study reveals a new connection between microplastic pollution and the carbon cycle in rivers, with implications for how these pollutants affect water quality.
Microplastics and nanoplastics are ubiquitous in rivers and undergo environmental aging. However, the molecular mechanisms of plastic aging and the in-depth effects of aging on ecological functions remain unclear in waters. The synergies of microplastics and nanoplastics (polystyrene as an example) with natural organic small molecules (e.g., natural hyaluronic acid and vitamin C related to biological tissue decomposition) are the key to producing radicals (•OH and •C). The radicals promote the formation of bubbles on plastic surfaces and generate derivatives of plastics such as monomer and dimer styrene. Nanoplastics are easier to age than microplastics. Pristine plastics inhibit the microbial Shannon diversity index and evenness, but the opposite results are observed for aging plastics. Pristine plastics curb pectin decomposition (an indicator of plant-originated refractory carbon), but aging plastics promote pectin decomposition. Microplastics and nanoplastics undergoing aging processes enhance the carbon biogeochemical cycle. For example, the increased carbohydrate active enzyme diversity, especially the related glycoside hydrolase and functional species Pseudomonas and Clostridium, contributes to refractory carbon decomposition. Different from the well-studied toxicity and aging of plastic pollutants, this study connects plastic pollutants with biological tissue decomposition, biodiversity and climate change together in rivers.
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Researchers investigated how dissolved organic matter released from degrading polystyrene and PVC microplastics behaves when exposed to sunlight in water. They found that sunlight breaks down the aromatic compounds in this plastic-derived material and generates reactive chemical species, though at lower rates than natural organic matter. Despite this, these reactive species significantly accelerated the breakdown of co-existing pollutants, suggesting that degrading microplastics may act as unexpected natural catalysts in aquatic environments.
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Researchers used ultra-high-resolution mass spectrometry to identify the dissolved organic molecules that leach from polypropylene, polyethylene, and polystyrene microplastics after prolonged exposure to sunlight. They found that polystyrene released the most diverse array of molecules, many of which could persist in water systems. The study suggests that as microplastics degrade in sunlight, they release non-natural organic compounds that may affect the aquatic carbon cycle from rivers to oceans.
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