Size-dependent transport and retention of micron-sized plastic spheres in natural sand saturated with seawater

A series of one-dimensional column experiments were conducted to investigate the transport and retention of micron-sized plastic spheres (MPs) with diameters of 0.1–2.0 μm in seawater-saturated sand. In seawater with salinity of 35 PSU (practical salinity units), the mass percentages recovered from the effluent (M_eff) of the larger MPs increased from 13.6% to 41.3%, as MP size decreased from 2.0 μm to 0.8 μm. This occurred because of the gradual reduction of physical straining effect of MPs in the pores between sands. The smaller MPs (0.6, 0.4, and 0.1 μm) showed the stronger inhibition of MPs mobility, with M_eff values of 11.5%, 11.9%, and 9.8%, respectively. This was due to the lower energy barriers (from 108 k_BT to 16 k_BT) between the smaller MPs and the sand surface, when compared with the larger MPs (from 296 k_BT to 161 k_BT). In particular, the aggregation of MPs (0.6 or 0.4 μm) triggered a progressive decrease in MP concentration in the effluent. Retention experiments showed that the vertical migration distance of most MP colloids was 0–4 cm at the inlet of column. For 0.6 or 0.4 μm MPs, the particles were concentrated over a 0–2 cm vertical distance. Moreover, the salinity (35–3.5 PSU) did not affect the transport of the larger MPs (2.0–0.8 μm). However, as seawater salinity decreased from 35 PSU to 17.5 or 3.5 PSU, the aggregation of the smaller MPs (0.6–0.1 μm) was dramatically inhibited or completely prevented. Meanwhile, ripening of the sand surface by the MPs (0.6 and 0.4 μm) no longer occurred. By contrast, all MPs in deionized water (0 PSU) achieved complete column breakthroughs because of the strong repulsive energy barrier (from 218 k_BT to 4192 k_BT) between the MPs and the sand surface. Consequently, we find that the transport and retention of MPs in sandy marine environment strongly relies on both the MP size and the salinity levels.