Nanoplastics as carriers of organic pollutants in seawater-saturated porous media: a quantitative comparison of transport pathways

Abstract

Nanoplastics (NPs) critically regulate organic pollutant (OP) transport in seawater-saturated porous media through adsorption-driven carrier mechanisms. This study quantifies distinctions between carrier-mediated and non-carried transport pathways by analyzing OP mobility with increasing polarities: nonpolar phenanthrene (PHE), moderately polar triclosan (TCS), and highly polar ibuprofen (IBU), across NPs of varying sizes (300–800 nm) and OPs. In single-component systems, 300 nm NPs (NP300) exhibited a lower effluent-to-influent mass ratio (M_eff = 30.83%) compared to PHE (54.25%), TCS (69.26%), and IBU (99.76%). In binary systems, NP300 reduced PHE, TCS, and IBU by 23.55%, 21.72%, and 12.12%, respectively, correlating with their equilibrium adsorption percentages (θ_t,eq): 85.59% (PHE), 48.96% (TCS), and 4.87% (IBU). Carrier contribution (δ) was the highest for PHE (80.75%), followed by TCS (40.02%) and IBU (4.50%), confirming hydrophobic adsorption as the dominant suppression mechanism for nonpolar OPs. In contrast, IBU's mobility remained non-carrier-dominated due to minimal adsorption and strong electrostatic repulsion. Larger NPs had weaker inhibitory effects: NP500 and NP800 reduced PHE mobility by only 16.62% and 5.54%, respectively, as θ_t,eq declined (60.22% and 30.14%) and NP–OP mobility converged. A predictive model integrating NP migration and adsorption achieved <3% deviation for carried mobility of nonpolar and moderately polar OPs, validating adsorption-driven suppression. These findings establish a quantitative framework to assess NPs as dynamic pollutant sinks in sandy coastal sediments, providing critical insights for managing contamination risks in marine ecosystems dominated by permeable sandy substrates.