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Mechanism of coupled phosphate‑calcium modulation of nanoplastic transport in porous media: Role of solution chemistry and surface interactions
Summary
Scientists used laboratory experiments and molecular simulations to study how phosphate and calcium ions in soil water affect whether polystyrene nanoplastics move freely through the ground or get trapped in soil particles. They found that pH was a key factor: at lower pH levels, phosphate helped nanoplastics travel farther while calcium restricted movement, with both effects linked to how these ions change the surface charge of both the particles and the soil. Understanding nanoplastic mobility in soil is essential for predicting contamination of groundwater and crops.
The transport of nanoplastics in the environment is affected by anions (PO 4 3− , P) and cations (Ca 2+ , Ca). This study investigated the transport of polystyrene nanoplastics (PS) in two media (quartz sand and goethite-coated quartz sand (G-C sand)) under conditions of P/Ca co-existence, and in-depth study of the transport mechanism of PS in combination with techniques such as density function theory (DFT) calculations and molecular dynamics (MD). It was found that pH was an important factor affecting the transport of PS in quartz sand and G-C sand. At pH ≤ 6.0, P promoted the transport of PS in both media, whereas Ca showed inhibition. The adsorption of P and Ca altered the charge distribution on the surface of PS and both media. The extended Derjaguinee-Landauee-Verweyee-Overbeek (XDLVO) theory results revealed that PS transport in quartz sand was mainly subjected to electrostatic interactions, while PS transport in G-C sand was subjected to P/Ca bridging coordination in addition to electrostatic interactions. With pH > 6.0, Ca 5 (PO 4 ) 3 (OH) formed by P and Ca and heterogeneously aggregated with PS, and attached to the surfaces of quartz sand and G-C sand, which altered the charge properties of PS and both media, inhibiting the transport of PS in both media. The XDLVO theory, fourier transform infrared spectroscopy (FTIR), DFT, and MD simulations results suggested that the electrostatic attraction and complexation of Ca 5 (PO 4 ) 3 (OH) with PS and the two media were the main factors inhibiting the transport of PS in both media. This study was the first to illustrate the effect of P/Ca co-existence on the transport of PS in porous media and to analyze the mechanism of their interaction in the context of quantum theory. • P promoted polystyrene nanoplastics (PS) transport while Ca inhibited it. • The pH was an important factor influencing PS transport under P/Ca co-existence. • Ca 5 (PO 4 ) 3 (OH) inhibited PS transport by electrostatic attraction and complexation. • Goethite promoted the inhibition of PS transport by Ca 5 (PO 4 ) 3 (OH). • Study of PS transport under P/Ca coexistence by DFT and MD.