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Exposure Order to Photoaging and Humic Acids Significantly Modifies the Aggregation and Transformation of Nanoplastics in Aqueous Solutions
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Researchers discovered that the order in which nanoplastics are exposed to sunlight and natural organic matter significantly changes how they clump together and behave in water. Nanoplastics aged by sunlight before encountering humic acids behaved differently than those exposed in the reverse order. This finding is important for predicting how nanoplastics actually move and persist in real-world water environments.
The colloidal stability of nanoplastics in aqueous solutions is greatly regulated by photoaging and dissolved organic matter (DOM). However, how the exposure order to sunlight and DOM modifies the environmental behavior of nanoplastics is seldomly determined. Here, with two different exposure orders, we investigated the impact of molecular-weight (MW)-fractionated humic acids (HAs) derived from biochar and the Suwannee River, respectively, on the aggregation of poly(ethylene terephthalate) nanoplastics (PET-NPs) in mono- and divalent electrolyte solutions. For exposure pattern (i) (photoaging followed by HA coating), photoaged PET-NPs had more oxidized surfaces and exhibited 22–320% higher binding affinity to HAs (especially the higher MW fractions) than the pristine counterparts, which greatly improved the dispersion of PET-NPs. For exposure pattern (ii) (HA coating followed by photoaging), HA-PET assemblies were formed, the dispersion of which increased with increasing irradiation time and was significantly higher than that of the samples in the exposure pattern (i) at the end of the experiment. This high dispersion of photoaged HA-PET assemblies was ascribed to the extra oxidation of PET by reactive oxygen species generated in the PET-HA interfaces during photoaging. These findings highlight the “active nature” of HA-PET assemblies, which provide new insight into the reaction of HA with nanoplastics beyond adsorption in the natural environment.
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Natural Organic Matter Stabilizes Pristine Nanoplastics but Destabilizes Photochemical Weathered Nanoplastics in Monovalent Electrolyte Solutions
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This study examined how sunlight weathering and natural organic matter coatings change the behavior of nanoplastics in water. Researchers found that organic matter stabilizes fresh nanoplastics but actually destabilizes sun-weathered ones, meaning aged nanoplastics in natural waters may clump together and settle differently than expected, affecting where they end up in aquatic environments.
Dissolved Organic Matter Enhanced the Aggregation and Oxidation of Nanoplastics under Simulated Sunlight Irradiation in Water
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Dissolved organic matter was found to enhance both the aggregation and photooxidation of nanoplastics under simulated sunlight in water, with humic substances promoting particle clustering and accelerating surface oxidation. The results indicate that organic matter content in natural waters strongly influences nanoplastic fate and transformation.
Combined effects of photoaging and natural organic matter on the colloidal stability of nanoplastics in aquatic environments
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Researchers found that photoaging of polystyrene nanoplastics alters how natural organic matter interacts with their surfaces — reducing humic acid adsorption while increasing protein adsorption — with downstream effects on the nanoplastics' stability and transport in aquatic environments.
Photoaging alters the aggregation behavior of functionalized nanoplastics differently: effects of leached organic matter and surface properties changes
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This study found that UV photoaging of nanoplastics changes their surface chemistry and causes them to release organic compounds, but the downstream effect on how particles clump together (aggregation) differs markedly depending on what chemical groups are on the particle surface. This matters because aggregation behaviour controls whether nanoplastics sink or stay suspended in water, affecting which organisms are exposed and how far the particles travel.
Photoaging-induced variations in heteroaggregation of nanoplastics and suspended sediments in aquatic environments: A case study on nanopolystyrene
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Researchers investigated how photoaging affects the aggregation behavior of polystyrene nanoplastics with suspended sediments in water. They found that 30 days of photoaging retarded aggregation in sodium chloride solutions due to steric hindrance from leached organic matter, but promoted aggregation in calcium chloride solutions through calcium bridging of newly formed oxygen-containing surface groups. The study provides mechanistic insights into how environmental weathering changes the transport and fate of nanoplastics in aquatic systems.
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