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Mechanisms of nanoplastics transport and retention in biochar-blended porous media: Role of surface interactions and hydrodynamic conditions
Summary
Biochar is often added to soil to improve farmland and clean up pollution, but this study found that it can sometimes make tiny plastic particles (nanoplastics) move through soil more easily instead of trapping them—potentially letting them reach groundwater we may eventually drink. The effect depends heavily on the type of biochar used, water acidity, and dissolved salts and organic matter, with some biochars (like cow dung-based) blocking plastic movement while others (like wheat straw-based) let it pass through more freely. This matters because it shows that choosing the wrong soil treatment could unintentionally increase nanoplastic cont
The environmental risks posed by nanoplastics (NPs) are increasingly recognized, particularly their potential to infiltrate groundwater through subsurface transport. However, the role of biochar-a widely applied soil amendment-in modulating NPs fate remains ambiguous. Here, we systematically investigated NPs transport in porous media with different biochar blended under varied hydrogeochemical conditions using column experiments. The breakthrough curves (BTCs) and retention profiles (RPs) were simulated using a two-site kinetic retention model, while the classical Derjaguin-Landau-Verwey-Overbeek (DLVO) theory was employed to semi-quantitatively interpret the underlying retention mechanisms. Results showed that compared with pure quartz sand columns, the transport rate of NPs in biochar-blended sand column significantly facilitated under acidic conditions. In biochar-blended sand columns, biochar facilitated NPs migration within a specific salinity range (1 mM and 10 mM NaCl). Polyvalent cations suppressed NPs mobility more strongly than monovalent cations, with their interaction energy barriers between NPs and porous media of 10.63 KT. While the presence of humic acid (HA) markedly promoted transport through steric effect and electrostatic repulsion. Meanwhile, hydrodynamic conditions also played a key role-higher flow velocities and unsaturated conditions both promoted NPs transport, with the effluent recovery rate of > 86.89%. Importantly, the properties of the biochar itself emerged as a critical factor, in which wheat straw biochar (pyrolyzed at 500 °C, 5% w/w) facilitated NPs migration, whereas cow dung biochar (500 °C) enhanced retention due to its high ash content and greater surface roughness. The dominant retention mechanisms were attributed to heterogeneous aggregation and electrostatic interactions between NPs and the porous medium. This work provides actionable insights for mitigating NPs pollution risks through rational selection of biochar in agricultural and remediation practices.