Influence of physicochemical factors on the transport behavior of polyethylene nanoplastics in black soil and brown earth

Polyethylene nanoplastics (PENPs) accumulate in soil, presenting significant environmental risks. However, research on the transport behavior of PENPs in soils remains limited. In this study, we employed saturated column experiments to systematically examine the transport of PENPs in two contrasting soil types, black soil (BS) and brown earth (BE), while evaluating the critical effects of ionic strength (IS), pH, and flow velocity. The results indicated that PENP mobility increased with higher flow velocity (1-5 mL min^-1), lower IS (1-10 mM NaCl/CaCl₂), and higher pH (5-9) in both soils. Notably, PENPs exhibited significantly greater transport capacity in BS compared to BE. Under identical IS and pH conditions, BS exhibited higher surface negative charges and more negative zeta potential than BE. DLVO theory calculations confirmed stronger electrostatic repulsion between the negatively charged PENPs and BS, resulting in significantly enhanced transport capacity and reduced deposition compared to BE. These findings demonstrate that soil surface charge critically governs PENP transport and provide a mechanistic foundation for assessing the mobility and ecological risks of nanoplastics in soils with distinct physicochemical properties.