The effects of organic and inorganic colloids on the aggregation and settling of polystyrene (PS) nanoplastics in mimicked ocean temperature conditions

Microplastics (MPs), as an emerging pollutant, are posing a potential threat to ecosystems and human health. It is critical to understand the fate and transport of microplastics in the marine environment to assess the ecological risks. However, the single and combined effects of minerals and extracellular polysaccharides on microplastic aggregation and sedimentation are not fully understood. This study experimentally demonstrated that nanoplastics could form heteroaggregates with montmorillonite or hematite in artificial seawater, accelerating the settling process. In particular, montmorillonite was able to lead to 70.4 % nanoplastics settled after 120 h of incubation at 28 °C, showing stronger adsorption capacity to nanoplastics. The presence of extracellular polymeric substances (EPS) affected the fate of nanoplastics in binary system and ternary systems. Xanthan gum significantly inhibited the aggregation and settlement of nanoplastics probably due to its strong steric hindrance. The nanoplastics remained evenly dispersed and hardly settled in the presence of montmorillonite or hematite. Comparatively, the inhibition activity of sodium alginate on nanoplastic aggregation and settlement was weak. In the respect of water temperature, the self-aggregation, heteroaggregation and sedimentation rates of nanoplastics were less at 4 °C and 10 °C than at 28 °C. These findings provide new perspectives for understanding the fate and transport of nanoplastics in the marine environment, and also provide a theoretical basis for assessing the potential impacts of mciroplastics on deep-sea ecosystems.