Cotransport of nanoplastics (NPs) with fullerene (C60) in saturated sand: Effect of NPs/C60 ratio and seawater salinity

Nanoplastics (NPs) have been identified as newly emerging particulate contaminants. In marine environments, the interaction between NPs and other engineered nanoparticles remains unknown. This study investigated the cotransport of NPs with fullerene (C) in seawater-saturated columns packed with natural sand as affected by the mass concentration ratio of NPs/C and the hydrochemical characteristics. In seawater with 35 practical salinity units (PSU), NPs could remarkably enhance C dispersion with a NPs/C ratio of 1. NPs behaved as a vehicle to facilitate C transport by decreasing colloidal ζ-potential and forming stable primary heteroaggregates. As the NPs/C ratio decreased to 1/3, NPs mobility was progressively restrained because of the formation of large secondary aggregates. When the ratio continuously decreased to 1/10, the stability and transport of colloids were governed by C rather than NPs. Under this condition, the transport trend of binary suspensions was similar to that of single C suspension, which was characterized by a ripening phenomenon. Seawater salinity is another key factor affecting the stability and associated transport of NPs and C. In seawater with 3.5 PSU, NPs and C (1:1) in binary suspension exhibited colloidal dispersion, which was driven by a high-energy barrier. Thus, the profiles of the cotransport and retention of NPs/C resembled those of single NPs suspension. This work demonstrated that the cotransport of NPs/C strongly depended on their mass concentration ratios and seawater salinity.