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Polyethylene fragments in Argentinean horticultural soils: Environmental transformation to a composite material
Summary
Researchers collected weathered polyethylene plastic fragments from agricultural soils in Argentina and found they had transformed into composite materials embedded with soil minerals like clay and iron. These environmentally aged plastics absorbed significantly more heavy metals such as nickel and lead compared to fresh plastic, up to 11 times more for lead. The study suggests that as plastics break down in farm soils, they may become more effective carriers of toxic metals, potentially increasing contamination risks.
Polyethylene macro, meso, and microplastics collected from horticultural soils in Moreno, Buenos Aires, Argentina, were studied to understand their physicochemical transformations after environmental exposure. These plastics contained mechanically stable compounds of Si (1.2-4.0 %), Al (0.91-1.5 %), and Fe (0.64-2.2 %), likely due to soil clay particles adhering to the plastic. The plastics' surfaces were oxidized, with high carbonyl (0.05-0.23) and hydroxyl (1.6-2.7) indices. Weathering led to thinner, rougher surfaces with increased contact angles due to the presence of clay and polar organic groups. Scanning electron microscopy (SEM) showed cracks and particles on the surfaces, while atomic force microscopy (AFM) revealed roughness increased from 0.44 nm in pristine polyethylene (p-PE) to 1.60 nm in weathered samples. Heavy metal sorption experiments showed that the weathered plastics absorbed significantly more Ni and Pb than unweathered PE (4.5 times for Ni and 11 times for Pb). After removing clay with hydrofluoric acid, the plastics still retained the same amount of these metals, suggesting that the sorption occurred mainly on the modified plastic surface rather than the clay. Arsenate tests indicated that the plastics selectively absorbed cationic metals. This study also developed a simple ATR-FTIR method for quantifying silicate on polyethylene surfaces, contributing to an understanding of soil-derived particle interactions with plastic in agricultural settings.