Secondary nanoplastic transport in sand and in soil

The widespread use of plastics since the mid-twentieth century has led to their pervasive accumulation in the environment. As plastics progressively weather and fragment, they generate secondary nanoplastics, which constitute the dominant form of nanoplastics in the environment. Despite their relevance, secondary nanoplastics remain largely understudied due to analytical challenges associated with low concentrations, complex matrices, and the diversity of polymers. Consequently, much of the current literature relies on microplastics or model primary nanoplastics that do not adequately represent environmentally formed secondary nanoplastics.

In this work, we introduce a novel analytical method for quantifying secondary nanoplastics in aqueous solutions and leverage these capabilities to study the transport of four distinct, environmentally relevant plastics (PET, PP, LDPE, and HDPE) and landfill-derived nanoplastics that were weathered naturally for more than 20 years (and thus represent real environmental secondary nanoplastics). We then focus on analysis of the mobility of these secondary nanoplastics through sand and soil columns, examining breakthrough curves and size distributions to elucidate the leading transport mechanism(s) of these particles. Our results show a plastic-specific transport that is influenced by plastic chemistry and the type of porous medium. Aliphatic plastics tend to be retained more than aromatic ones, due to higher hydrophobicity. Size distribution analysis indicates that eluted secondary nanoplastics are generally larger, suggesting that smaller particles aggregate or are retained within the media. Despite chemical similarity, secondary HDPE and secondary LDPE differ in their elution patterns, while secondary PET exhibits increased aggregation due to its extended π-orbital system. Landfill-derived nanoplastics showed greater retention owing to inorganic impurities, which promote smaller aggregation. Additional experiments examining secondary HDPE transport across multiple porous media, including three sand grain sizes and a sandy loam soil, showed generally consistent retention behavior, with the notable exception of fine sand. In fine sand, enhanced retention is likely driven by smaller pore throats that promote particle trapping. Size-resolved elution patterns revealed two distinct particle populations in fine sand, whereas medium and coarse sands, as well as soil, exhibited a shift toward larger eluted particles. In soil, a modest delay in secondary HDPE breakthrough further suggests interactions between secondary HDPE and the soil matrix.

Overall, our findings provide new mechanistic insights into secondary nanoplastic transport and represent a significant step toward a realistic assessment of nanoplastic fate in subsurface environments.