Dispersion and transport of microplastics in three water-saturated coastal soils

The coastal area is one of the key zones for transport and fate of microplastics (MPs). This study investigated the transport behaviors of different sized MPs in three water-saturated coastal soils, with the aim to explore effects of properties of three different coastal soils on the dispersion and migration of three-sized MPs (0.3, 0.5, and 1 µm). All three-sized MPs had the strongest dispersion in Soil 3 solution, followed by that in Soil 1 solution and then that in Soil 2 solution. The strongest dispersion of MPs in Soil 3 solution was attributed to the lowest ionic strength. Such a high dispersion favored MPs movement in soil solution but readily be sorbed and fixed by rich Fe and Al oxides in Soil 3 solid through strong electrostatic attraction, leading to the lowest transport rate (20.5-41.2%). The high ionic strength in the Soil 1 solution decreased the dispersion of MPs, but the presence of high content of humic acid enhanced the electrostatic repulsion and steric hindrance between MPs and soil particles, resulting in the highest transport ability of MPs in Soil 1 (39.4-72.5%). The large amount of dissolved Ca²⁺ and Mg²⁺ in Soil 2 solution favored MPs bridged with fulvic acid, resulting in the highest aggregation of MPs and relatively lower transport ability (34.1-49.6%). Large-sized MPs had higher electrostatic repulsion between the particles, thus increasing the dispersion and transport capacity of MPs in soil. Modeling showed the experiment-consistent results that Soil 3 had the lowest MPs transport after 600 mm of heavy rainfall, with the maximum migration distance of 7.50-10.5 cm, which was smaller than that in Soil 2 (8.10-12.0 cm) and that in Soil 1 (9.00-18.3 cm). These results indicated that MPs transport in coastal soil is significant and soil solution and solid composition plays an important role in the dispersion and transport of MPs, respectively. These findings afforded a great basis for the assessment of the fate and risk of MPs in coastal areas.