Quartz sand surface-bound rice root exudates decreased the transport of microplastics in porous media†

Plants are widely present in soil ecosystems, and plant root exudates are therefore considered as an important factor that could affect the fate and transport of microplastics (MPs). The effect of quartz sand surface-bound root exudates of rice (long-grained rice) was used to explore its influence on both PS and PET MPs in porous media. 0.51 μm PS MPs and 1.1 μm PS MPs, and 1 μm PET MPs were investigated under 0.1–10 mM NaCl and 0.1–1 mM CaCl₂ solutions. The sand surface-bound root exudates were found to decrease the transport of both PS and PET MPs, with the most obvious difference in the intermediate ionic strengths in both NaCl and CaCl₂ solutions. By performing the column experiment after the removal of sand surface-bound root exudates, it was verified that the role of physical space occupation by the root was not the factor driving the inhibited transport of PS and PET MPs. Further investigations revealed that the surface properties of quartz sand altered by the presence of root exudates was the main factor responsible for the decreased transport of PS and PET MPs. The zeta potentials, excitation–emission–matrix (EEM) spectra, and the components of the root exudates were determined. It was observed that microbial by-product-like substances, fluvic acid-like substances and aromatic protein were the major components of the root exudates. The results indicated that the electrostatic repulsive forces between MPs and quartz sand were expected to be lower in the presence of sand surface bound-root exudates as predicted by the DLVO theory. The findings of this study are essential to shine light on the knowledge of the fate and transport of plastic particles in soil systems with ubiquitous plants.