Sedimentation behavior of aggregated microplastics: Influence of particle size and water constituents in environmental waters

Understanding the aggregation and settling behaviors of micro/nanoplastics (M/NPs) is essential for predicting their transport and fate in aquatic environments. However, gaps remain in comprehending how particle size and common environmental water constituents jointly influence M/NPs aggregation and sedimentation. This study investigated the effects of metal cations (Na⁺ and Ca²⁺) and humic acid (HA) on polystyrene particles of 100 nm (PS-100) and 500 nm (PS-500). The resulted indicated that larger PS-500 particles exhibited lower critical coagulation concentration (CCC) and a stronger tendency to aggregate, yet remained suspended even after aggregation ("aggregation without settling") due to their larger secondary energy minima, lower ion adsorption, and a lower density relative to water. In contrast, smaller PS-100 particles displayed a two-stage behavior of "settling following by floating", driven by stronger ion adsorption that increased their relative density and induced sedimentation ("settling without aggregation"). HA modified these processes differently in Na⁺ and Ca²⁺ systems: it increased CCC in Na⁺ solutions by enhancing electrostatic repulsion, but CCC in Ca²⁺ solutions through Ca²⁺-HA complexation and bridging effects. DLVO analysis confirmed that particle size and ion valence jointly controlled energy barriers and secondary minima, determining aggregate stability. Environmental water experiments further revealed that dissolved organic matter (DOM) composition significantly affected aggregation-settling patterns, with protein-like DOM enhancing particle aggregation and floating by competing with ions for adsorption sites. The findings indicate that as microplastics degrade into nanoplastics, their diffusion and settling behaviors change, thereby influencing their distribution in aquatic environments.