Secondary plastic nanoparticle transport and retention in sand and soil

Secondary plastic nanoparticles (Sec-PNPs), produced by plastic weathering, are contaminants of emerging concern with poorly understood behavior. Most current understanding of Sec-PNP transport is based mainly on studies of primary PNPs, particularly polystyrene (PS) nanobeads, which differ significantly from environmentally relevant Sec-PNPs. This study addresses this gap by conducting column experiments to benchmark the transport characteristics of various Sec-PNP types, including PS, low-density polyethylene (LDPE), high-density polyethylene (HDPE), polyethylene terephthalate (PET), and environmentally aged landfill-derived PNPs. Column experiments in sand and soil show plastic- and medium-dependent behaviors, as evidenced by breakthrough curves and particle-size data. Aliphatic Sec-PNPs exhibit higher retention than aromatic Sec-PNPs due to hydrophobic interactions, with larger particles enriched in eluted fractions, indicating aggregation or retention of smaller sizes. Despite chemical similarities, Sec-HDPE and Sec-LDPE display different elution patterns, while Sec-PET shows increased aggregation from π-conjugation. Landfill Sec-PNPs exhibit greater retention from inorganic impurities, promoting aggregation. Retention is consistent in all porous media for Sec-HDPE, except in fine sand, where pore constraints cause bimodal elution, and in soil, where matrix interactions delay breakthrough. These results demonstrate that Sec-PNP mobility is governed by the combined effects of polymer structure, particle aggregation, porous-medium characteristics, and contaminant interactions. The findings also highlight the limitations of using primary PS nanobeads as proxies for environmental PNPs and provide a framework for studying Sec-PNP transport under environmentally relevant conditions.